diff --git a/packages/typescript/test/async/api.test.ts b/packages/typescript/test/async/api.test.ts
index 4a54dc8ea2a19..85bdddb0c8a08 100644
--- a/packages/typescript/test/async/api.test.ts
+++ b/packages/typescript/test/async/api.test.ts
@@ -110,12 +110,15 @@ import {
 } from "node:test";
 import { fileURLToPath } from "node:url";
 import { isSignatureDeclaration } from "../../src/ast/is.ts";
+import { areTestsFiltered } from "../testUtils.ts";
 import {
     defaultFiles,
     spawnAPI,
 } from "./api.testUtils.ts";
 
-describe("API", () => {
+const concurrency = areTestsFiltered();
+
+describe("API", { concurrency }, () => {
     test("getCurrentLanguageServerSnapshot is LSP-only", () => {
         if (!!false) {
             const standalone = new API();
@@ -718,7 +721,7 @@ declare module "augmentation" {}`,
 });
 
 // @sync-skip-block-start
-describe("API - automatic batching", () => {
+describe("API - automatic batching", { concurrency }, () => {
     test("initializes only once for concurrent first requests", async () => {
         await using api = spawnAPI();
         const client = (api as unknown as {
@@ -761,7 +764,7 @@ describe("API - automatic batching", () => {
     });
 });
 
-describe("API - batchContext", () => {
+describe("API - batchContext", { concurrency }, () => {
     test("transparently paginates batch responses", async () => {
         const api = spawnAPI({ ...defaultFiles }, { maxResponseBytesPerPage: 1 });
         try {
@@ -838,7 +841,7 @@ describe("API - batchContext", () => {
 });
 // @sync-skip-block-end
 
-describe("Checker - getImmediateAliasedSymbol", () => {
+describe("Checker - getImmediateAliasedSymbol", { concurrency }, () => {
     test("resolves one level of alias indirection", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -857,7 +860,7 @@ describe("Checker - getImmediateAliasedSymbol", () => {
     });
 });
 
-describe("Checker - getTargetSymbol", () => {
+describe("Checker - getTargetSymbol", { concurrency }, () => {
     test("gets the target symbol of instantiated symbol", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -895,7 +898,7 @@ test<Bravo>();
     });
 });
 
-describe("Snapshot", () => {
+describe("Snapshot", { concurrency }, () => {
     test("createSnapshot returns snapshot with projects", async () => {
         await using api = spawnAPI();
 
@@ -1049,7 +1052,7 @@ describe("Snapshot", () => {
     });
 });
 
-describe("LanguageService - imports", () => {
+describe("LanguageService - imports", { concurrency }, () => {
     test("getImportEditsForSymbols adds a named import", async () => {
         const source = `const value = foo;\n`;
         await using api = spawnAPI({
@@ -1150,7 +1153,7 @@ describe("LanguageService - imports", () => {
     });
 });
 
-describe("LanguageService - getCompletionsAtPosition", () => {
+describe("LanguageService - getCompletionsAtPosition", { concurrency }, () => {
     test("returns member completions after a dot", async () => {
         const src = `\nconst obj = { name: "hello", age: 42 };\nobj.\n`;
         await using api = spawnAPI({
@@ -1217,7 +1220,7 @@ describe("LanguageService - getCompletionsAtPosition", () => {
     });
 });
 
-describe("LanguageService - getReferencedSymbolsForNode", () => {
+describe("LanguageService - getReferencedSymbolsForNode", { concurrency }, () => {
     test("getReferencedSymbolsForNode", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -1239,7 +1242,7 @@ describe("LanguageService - getReferencedSymbolsForNode", () => {
     });
 });
 
-describe("LanguageService - getSignatureUsage", () => {
+describe("LanguageService - getSignatureUsage", { concurrency }, () => {
     test("getSignatureUsage", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -1259,7 +1262,7 @@ describe("LanguageService - getSignatureUsage", () => {
     });
 });
 
-describe("Checker - getApparentType", () => {
+describe("Checker - getApparentType", { concurrency }, () => {
     test("returns the apparent type of a literal type", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1282,7 +1285,7 @@ describe("Checker - getApparentType", () => {
     });
 });
 
-describe("Checker - getReducedType", () => {
+describe("Checker - getReducedType", { concurrency }, () => {
     test("returns the reduced type", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1319,7 +1322,7 @@ export type Result = RateLimitError | (RateLimitError & QuotaExceededError);`,
     });
 });
 
-describe("Checker - getMemberInModuleExports", () => {
+describe("Checker - getMemberInModuleExports", { concurrency }, () => {
     test("returns a named export when present", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1340,7 +1343,7 @@ describe("Checker - getMemberInModuleExports", () => {
     });
 });
 
-describe("SourceFile", () => {
+describe("SourceFile", { concurrency }, () => {
     test("getSourceFile rejects invalid document identifiers", async () => {
         await using api = spawnAPI();
 
@@ -1530,7 +1533,7 @@ describe("SourceFile", () => {
     });
 });
 
-describe("NodeArray", () => {
+describe("NodeArray", { concurrency }, () => {
     test("hasTrailingComma", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1549,98 +1552,100 @@ describe("NodeArray", () => {
     });
 });
 
-test("unicode escapes", async () => {
-    await using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/1.ts": `"😃"`,
-        "/src/2.ts": `"\\ud83d\\ude03"`,
-        "/src/3.ts": `"\\ud800a\\udc00"`,
+describe("API objects", { concurrency }, () => {
+    test("unicode escapes", async () => {
+        await using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/1.ts": `"😃"`,
+            "/src/2.ts": `"\\ud83d\\ude03"`,
+            "/src/3.ts": `"\\ud800a\\udc00"`,
+        });
+
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const expectedTexts = new Map([
+            ["/src/1.ts", "😃"],
+            ["/src/2.ts", "😃"],
+            ["/src/3.ts", "\ud800a\udc00"],
+        ]);
+
+        for (const file of expectedTexts.keys()) {
+            const sourceFile = await project.program.getSourceFile(file);
+            assert.ok(sourceFile);
+
+            sourceFile.forEachChild(function visit(node) {
+                if (isStringLiteral(node)) {
+                    assert.equal(node.text, expectedTexts.get(file));
+                }
+                node.forEachChild(visit);
+            });
+        }
     });
 
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const expectedTexts = new Map([
-        ["/src/1.ts", "😃"],
-        ["/src/2.ts", "😃"],
-        ["/src/3.ts", "\ud800a\udc00"],
-    ]);
+    test("template unicode escapes", async () => {
+        await using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": "`\\ud800${0}\\udc00`",
+        });
 
-    for (const file of expectedTexts.keys()) {
-        const sourceFile = await project.program.getSourceFile(file);
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = await project.program.getSourceFile("/src/index.ts");
         assert.ok(sourceFile);
 
+        let sawHead = false;
+        let sawTail = false;
         sourceFile.forEachChild(function visit(node) {
-            if (isStringLiteral(node)) {
-                assert.equal(node.text, expectedTexts.get(file));
+            if (isTemplateHead(node)) {
+                assert.equal(node.text, "\ud800");
+                sawHead = true;
+            }
+            else if (isTemplateTail(node)) {
+                assert.equal(node.text, "\udc00");
+                sawTail = true;
             }
             node.forEachChild(visit);
         });
-    }
-});
-
-test("template unicode escapes", async () => {
-    await using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": "`\\ud800${0}\\udc00`",
+        assert.ok(sawHead);
+        assert.ok(sawTail);
     });
 
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = await project.program.getSourceFile("/src/index.ts");
-    assert.ok(sourceFile);
+    test("Object equality", async () => {
+        await using api = spawnAPI();
 
-    let sawHead = false;
-    let sawTail = false;
-    sourceFile.forEachChild(function visit(node) {
-        if (isTemplateHead(node)) {
-            assert.equal(node.text, "\ud800");
-            sawHead = true;
-        }
-        else if (isTemplateTail(node)) {
-            assert.equal(node.text, "\udc00");
-            sawTail = true;
-        }
-        node.forEachChild(visit);
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        // Same symbol returned from same snapshot's checker
+        assert.strictEqual(
+            await project.checker.getSymbolAtPosition("/src/index.ts", 9),
+            await project.checker.getSymbolAtPosition("/src/index.ts", 10),
+        );
     });
-    assert.ok(sawHead);
-    assert.ok(sawTail);
-});
 
-test("Object equality", async () => {
-    await using api = spawnAPI();
-
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    // Same symbol returned from same snapshot's checker
-    assert.strictEqual(
-        await project.checker.getSymbolAtPosition("/src/index.ts", 9),
-        await project.checker.getSymbolAtPosition("/src/index.ts", 10),
-    );
-});
-
-test("Snapshot dispose", async () => {
-    await using api = spawnAPI();
+    test("Snapshot dispose", async () => {
+        await using api = spawnAPI();
 
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const symbol = await project.checker.getSymbolAtPosition("/src/index.ts", 9);
-    assert.ok(symbol);
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const symbol = await project.checker.getSymbolAtPosition("/src/index.ts", 9);
+        assert.ok(symbol);
 
-    // Snapshot dispose should release server-side resources
-    assert.ok(snapshot.isDisposed() === false);
-    await snapshot.dispose();
-    assert.ok(snapshot.isDisposed() === true);
+        // Snapshot dispose should release server-side resources
+        assert.ok(snapshot.isDisposed() === false);
+        await snapshot.dispose();
+        assert.ok(snapshot.isDisposed() === true);
 
-    // After dispose, snapshot methods should throw
-    assert.throws(() => {
-        snapshot.getConfiguredProject("/tsconfig.json");
-    }, {
-        name: "Error",
-        message: "Snapshot is disposed",
+        // After dispose, snapshot methods should throw
+        assert.throws(() => {
+            snapshot.getConfiguredProject("/tsconfig.json");
+        }, {
+            name: "Error",
+            message: "Snapshot is disposed",
+        });
     });
 });
 
-describe("Multiple snapshots", () => {
+describe("Multiple snapshots", { concurrency }, () => {
     test("two snapshots work independently", async () => {
         await using api = spawnAPI();
 
@@ -1846,7 +1851,7 @@ describe("Multiple snapshots", () => {
     });
 });
 
-describe("Source file caching", () => {
+describe("Source file caching", { concurrency }, () => {
     test("same file from same snapshot returns cached object", async () => {
         await using api = spawnAPI();
 
@@ -2014,7 +2019,7 @@ describe("Source file caching", () => {
     });
 });
 
-describe("Snapshot disposal", () => {
+describe("Snapshot disposal", { concurrency }, () => {
     test("dispose is idempotent", async () => {
         await using api = spawnAPI();
 
@@ -2053,7 +2058,7 @@ describe("Snapshot disposal", () => {
     });
 });
 
-describe("Source file cache keying across projects", () => {
+describe("Source file cache keying across projects", { concurrency }, () => {
     // Three projects share the same file (/src/shared.ts).
     // The file sits inside a package.json scope with "type": "module".
     //
@@ -2132,7 +2137,7 @@ describe("Source file cache keying across projects", () => {
     });
 });
 
-describe("Checker - symbol identity across projects", () => {
+describe("Checker - symbol identity across projects", { concurrency }, () => {
     const sharedSymbolFiles = {
         "/projectA/tsconfig.json": JSON.stringify({ files: ["../src/shared.ts"] }),
         "/projectB/tsconfig.json": JSON.stringify({ files: ["../src/shared.ts"] }),
@@ -2186,7 +2191,7 @@ describe("Checker - symbol identity across projects", () => {
     });
 });
 
-describe("Checker - types and signatures", () => {
+describe("Checker - types and signatures", { concurrency }, () => {
     const checkerFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -2614,7 +2619,7 @@ export class Cache {
     });
 });
 
-describe("Symbol - parent, members, exports", () => {
+describe("Symbol - parent, members, exports", { concurrency }, () => {
     const symbolFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/mod.ts": `
@@ -2685,7 +2690,7 @@ export const value = 1;
     });
 });
 
-describe("Type - getSymbol", () => {
+describe("Type - getSymbol", { concurrency }, () => {
     test("getSymbol returns the symbol of a type", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -2711,7 +2716,7 @@ export const instance: Foo = new Foo();
     });
 });
 
-describe("Type - sub-property fetchers", () => {
+describe("Type - sub-property fetchers", { concurrency }, () => {
     const typeFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true, target: "esnext" } }),
         "/src/types.ts": `
@@ -3049,7 +3054,7 @@ export function gh1449<T extends [foo: any, bar?: any]>(a: T): T {
     });
 });
 
-describe("Checker - intrinsic type getters", () => {
+describe("Checker - intrinsic type getters", { concurrency }, () => {
     const intrinsicFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `export const x = 1;`,
@@ -3176,7 +3181,7 @@ describe("Checker - intrinsic type getters", () => {
     });
 });
 
-describe("Checker - multi-project type ID uniqueness", () => {
+describe("Checker - multi-project type ID uniqueness", { concurrency }, () => {
     test("intrinsic types from 3 projects in the same snapshot have non-colliding IDs", async () => {
         await using api = spawnAPI({
             "/proj1/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3282,7 +3287,7 @@ describe("Checker - multi-project type ID uniqueness", () => {
     });
 });
 
-describe("Checker - getBaseTypeOfLiteralType", () => {
+describe("Checker - getBaseTypeOfLiteralType", { concurrency }, () => {
     test("number literal widens to number", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3324,7 +3329,7 @@ describe("Checker - getBaseTypeOfLiteralType", () => {
     });
 });
 
-describe("Checker - getContextualType", () => {
+describe("Checker - getContextualType", { concurrency }, () => {
     test("contextual type from function parameter", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3359,7 +3364,7 @@ foo(42);
     });
 });
 
-describe("Checker - getTypeOfSymbolAtLocation", () => {
+describe("Checker - getTypeOfSymbolAtLocation", { concurrency }, () => {
     test("narrowed type via typeof check", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3414,7 +3419,7 @@ export function check(x: string | number) {
     });
 });
 
-describe("Checker - getShorthandAssignmentValueSymbol", () => {
+describe("Checker - getShorthandAssignmentValueSymbol", { concurrency }, () => {
     test("shorthand property symbol resolves to variable", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3446,7 +3451,7 @@ export const obj = { name };
     });
 });
 
-describe("readFile callback semantics", () => {
+describe("readFile callback semantics", { concurrency }, () => {
     test("readFile: string returns content, null blocks fallback, undefined falls through to real FS", async () => {
         const virtualFiles: Record<string, string> = {
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3496,7 +3501,7 @@ describe("readFile callback semantics", () => {
     });
 });
 
-describe("updateSnapshot file systems", () => {
+describe("updateSnapshot file systems", { concurrency }, () => {
     test("request filesystem factories normalize files and preserve explicit listings", () => {
         const memory = createFileSystem([
             ["/src/index.ts", "posix"],
@@ -4094,7 +4099,7 @@ describe("updateSnapshot file systems", () => {
     // do not model modification times.
 });
 
-describe("Checker - isArrayType / isTupleType", () => {
+describe("Checker - isArrayType / isTupleType", { concurrency }, () => {
     test("number[] is array, not tuple", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4204,7 +4209,7 @@ describe("Checker - isArrayType / isTupleType", () => {
     });
 });
 
-describe("Checker - isReadonlySymbol", () => {
+describe("Checker - isReadonlySymbol", { concurrency }, () => {
     test("properties with a 'readonly' modifier", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4295,7 +4300,7 @@ export type B = InstanceType<typeof Bravo>;
     });
 });
 
-describe("Checker - getReturnTypeOfSignature", () => {
+describe("Checker - getReturnTypeOfSignature", { concurrency }, () => {
     test("returns the return type of a function signature", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4318,7 +4323,7 @@ describe("Checker - getReturnTypeOfSignature", () => {
     });
 });
 
-describe("Checker - getRestTypeOfSignature", () => {
+describe("Checker - getRestTypeOfSignature", { concurrency }, () => {
     test("returns the rest type of a signature with rest parameter", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4341,7 +4346,7 @@ describe("Checker - getRestTypeOfSignature", () => {
     });
 });
 
-describe("Checker - getTypePredicateOfSignature", () => {
+describe("Checker - getTypePredicateOfSignature", { concurrency }, () => {
     test("returns type predicate for 'x is T' guard", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4441,7 +4446,7 @@ export class Dog extends Animal {
     });
 });
 
-describe("Checker - getBaseTypes", () => {
+describe("Checker - getBaseTypes", { concurrency }, () => {
     test("returns base types of a class", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4524,7 +4529,7 @@ export type BoxOfString = Box<string>;
     });
 });
 
-describe("Type - getBaseTypes", () => {
+describe("Type - getBaseTypes", { concurrency }, () => {
     test("returns base types for a class type and undefined for a non-class/interface", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4562,7 +4567,7 @@ export const n: number = 0;
     });
 });
 
-describe("Type - isErrorType", () => {
+describe("Type - isErrorType", { concurrency }, () => {
     test("identifies the error type from an unresolvable annotation", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4592,7 +4597,7 @@ declare const bad: ThisTypeDoesNotExist;
     });
 });
 
-describe("Checker - well-known symbols", () => {
+describe("Checker - well-known symbols", { concurrency }, () => {
     test("isUnknownSymbol identifies the aliased unknown symbol", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4614,7 +4619,7 @@ export type Alias = typeof value;
     });
 });
 
-describe("Checker - well-known signatures", () => {
+describe("Checker - well-known signatures", { concurrency }, () => {
     test("isUnknownSignature identifies an unresolvable call", async () => {
         const src = `
 const ok = (x: number) => x;
@@ -4648,7 +4653,7 @@ notCallable();
     });
 });
 
-describe("Symbol - escaped names and tables", () => {
+describe("Symbol - escaped names and tables", { concurrency }, () => {
     test("getExports/getMembers return a cached Map keyed by escaped name", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4747,7 +4752,7 @@ export { x as '${maliciousName}' };
     });
 });
 
-describe("ast - escapeLeadingUnderscores", () => {
+describe("ast - escapeLeadingUnderscores", { concurrency }, () => {
     test("round-trips display and escaped names", () => {
         assert.equal(escapeLeadingUnderscores("foo"), "foo");
         assert.equal(escapeLeadingUnderscores("_foo"), "_foo");
@@ -4758,7 +4763,7 @@ describe("ast - escapeLeadingUnderscores", () => {
     });
 });
 
-describe("ast - tryGetAmbientModuleNameFromSymbolName", () => {
+describe("ast - tryGetAmbientModuleNameFromSymbolName", { concurrency }, () => {
     test("gets ambient module names from escaped symbol names", () => {
         assert.equal(tryGetAmbientModuleNameFromSymbolName('"pkg"' as __String), "pkg");
         assert.equal(tryGetAmbientModuleNameFromSymbolName('__"*.css"pattern@1234' as __String), "*.css");
@@ -4768,7 +4773,7 @@ describe("ast - tryGetAmbientModuleNameFromSymbolName", () => {
     });
 });
 
-describe("ast - getJSDocTags", () => {
+describe("ast - getJSDocTags", { concurrency }, () => {
     test("returns a node's own tags, and inherited @param / @template tags", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4910,7 +4915,7 @@ const cast = /** @type {number} */ (someValue);
     });
 });
 
-describe("Checker - getPropertiesOfType", () => {
+describe("Checker - getPropertiesOfType", { concurrency }, () => {
     test("returns properties of an object type", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4941,7 +4946,7 @@ export declare const p: Person;
     });
 });
 
-describe("Checker - getIndexInfosOfType", () => {
+describe("Checker - getIndexInfosOfType", { concurrency }, () => {
     test("returns index signatures of an indexed type", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4996,7 +5001,7 @@ export declare const m: ReadonlyMap;
     });
 });
 
-describe("Checker - TypeScript API parity", () => {
+describe("Checker - TypeScript API parity", { concurrency }, () => {
     const files = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -5085,7 +5090,7 @@ export type Exported = number;
     });
 });
 
-describe("Checker - getConstraintOfTypeParameter", () => {
+describe("Checker - getConstraintOfTypeParameter", { concurrency }, () => {
     test("returns constraint of a type parameter", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5110,7 +5115,7 @@ describe("Checker - getConstraintOfTypeParameter", () => {
     });
 });
 
-describe("Checker - TypeParameter getters", () => {
+describe("Checker - TypeParameter getters", { concurrency }, () => {
     test("getConstraint() and getDefault() return the constraint and default types", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5165,7 +5170,7 @@ describe("Checker - TypeParameter getters", () => {
     });
 });
 
-describe("Checker - getTypeArguments", () => {
+describe("Checker - getTypeArguments", { concurrency }, () => {
     test("returns type arguments of a generic instantiation", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5214,7 +5219,7 @@ describe("Checker - getTypeArguments", () => {
     });
 });
 
-describe("Checker - getBaseConstraintOfType", () => {
+describe("Checker - getBaseConstraintOfType", { concurrency }, () => {
     test("returns the base constraint of a type parameter", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5255,7 +5260,7 @@ describe("Checker - getBaseConstraintOfType", () => {
     });
 });
 
-describe("Checker - getPropertyOfType", () => {
+describe("Checker - getPropertyOfType", { concurrency }, () => {
     test("returns a named property symbol of a type", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5284,7 +5289,7 @@ export declare const p: Person;
     });
 });
 
-describe("Checker - getConstantValue", () => {
+describe("Checker - getConstantValue", { concurrency }, () => {
     test("returns numeric value of an enum member", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5363,7 +5368,7 @@ describe("Checker - getConstantValue", () => {
     });
 });
 
-describe("Checker - getSignatureFromDeclaration", () => {
+describe("Checker - getSignatureFromDeclaration", { concurrency }, () => {
     test("returns the signature of a function declaration", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5389,7 +5394,7 @@ describe("Checker - getSignatureFromDeclaration", () => {
     });
 });
 
-describe("Checker - getExportSpecifierLocalTargetSymbol", () => {
+describe("Checker - getExportSpecifierLocalTargetSymbol", { concurrency }, () => {
     test("resolves the local target of an export specifier", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5415,7 +5420,7 @@ export { value as renamed };
     });
 });
 
-describe("Checker - getAliasedSymbol", () => {
+describe("Checker - getAliasedSymbol", { concurrency }, () => {
     test("resolves an import alias to its target symbol", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5436,7 +5441,7 @@ describe("Checker - getAliasedSymbol", () => {
     });
 });
 
-describe("Checker - getFullyQualifiedName", () => {
+describe("Checker - getFullyQualifiedName", { concurrency }, () => {
     test("returns module-qualified names for exported symbols and dotted names for members", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5466,7 +5471,7 @@ export class Standalone {}
     });
 });
 
-describe("Checker - getExportsOfModule", () => {
+describe("Checker - getExportsOfModule", { concurrency }, () => {
     test("returns all exports including re-exports via 'export *'", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5490,7 +5495,7 @@ export * from "./inner";
     });
 });
 
-describe("Checker - getSymbolsInScope", () => {
+describe("Checker - getSymbolsInScope", { concurrency }, () => {
     const scopeFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -5551,7 +5556,7 @@ function f() {
     });
 });
 
-describe("Symbol - getDocumentationComment and getJsDocTags", () => {
+describe("Symbol - getDocumentationComment and getJsDocTags", { concurrency }, () => {
     const docFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -5595,7 +5600,7 @@ export function add(a: number, b: number): number { return a + b; }
     });
 });
 
-describe("TypeParameter - isThisType", () => {
+describe("TypeParameter - isThisType", { concurrency }, () => {
     test("isThisType is true for the polymorphic 'this' type in a class method", async () => {
         const src = `\nexport class Builder {\n    setName(name: string): this { return this; }\n}\n`;
         await using api = spawnAPI({
@@ -5635,7 +5640,7 @@ describe("TypeParameter - isThisType", () => {
     });
 });
 
-describe("Type - getAliasTypeArguments", () => {
+describe("Type - getAliasTypeArguments", { concurrency }, () => {
     test("returns the type arguments of a single-param generic type alias", async () => {
         const src = `\ntype Box<T> = { value: T };\nexport const x: Box<string> = { value: "hi" };\n`;
         await using api = spawnAPI({
@@ -5694,7 +5699,7 @@ describe("Type - getAliasTypeArguments", () => {
     });
 });
 
-describe("Type - getAliasSymbol", () => {
+describe("Type - getAliasSymbol", { concurrency }, () => {
     test("returns the symbol for a non-generic type alias", async () => {
         // Object-type aliases preserve aliasSymbol; primitive aliases (type Foo = string) do not.
         const src = `\ntype Point = { x: number; y: number };\nexport const p: Point = { x: 1, y: 2 };\n`;
@@ -5753,7 +5758,7 @@ describe("Type - getAliasSymbol", () => {
     });
 });
 
-describe("IntrinsicType - intrinsicName", () => {
+describe("IntrinsicType - intrinsicName", { concurrency }, () => {
     test("intrinsicName matches the primitive type name", async () => {
         const src = `\nexport const x: string = "hello";\n`;
         await using api = spawnAPI({
@@ -5779,7 +5784,7 @@ describe("IntrinsicType - intrinsicName", () => {
     });
 });
 
-describe("FreshableType - getFreshType and getRegularType", () => {
+describe("FreshableType - getFreshType and getRegularType", { concurrency }, () => {
     test("LiteralType.value is empty string for the empty-string literal type", async () => {
         const src = `\nexport const empty: "" = "";\n`;
         await using api = spawnAPI({
@@ -5962,7 +5967,7 @@ describe("FreshableType - getFreshType and getRegularType", () => {
     });
 });
 
-describe("Checker - isContextSensitive", () => {
+describe("Checker - isContextSensitive", { concurrency }, () => {
     test("arrow function with no type annotation is context sensitive", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5987,7 +5992,7 @@ describe("Checker - isContextSensitive", () => {
     });
 });
 
-describe("Checker - isTypeAssignableTo", () => {
+describe("Checker - isTypeAssignableTo", { concurrency }, () => {
     test("returns true when source is assignable to target", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -6040,7 +6045,7 @@ describe("Checker - isTypeAssignableTo", () => {
     });
 });
 
-describe("Printer", () => {
+describe("Printer", { concurrency }, () => {
     const emitterFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -6283,7 +6288,7 @@ export const obj = { m: 1, s: "hi", b: true };
     });
 });
 
-describe("Program - selected file emit", () => {
+describe("Program - selected file emit", { concurrency }, () => {
     const files = {
         "/tsconfig.json": JSON.stringify({
             compilerOptions: {
@@ -6345,7 +6350,7 @@ describe("Program - selected file emit", () => {
     });
 });
 
-describe("SnapshotInternalAPI - formatNodeForInsertion", () => {
+describe("SnapshotInternalAPI - formatNodeForInsertion", { concurrency }, () => {
     test("formats a synthesized statement with correct indentation for insertion inside a function body", async () => {
         const files = {
             "/tsconfig.json": "{}",
@@ -6423,7 +6428,7 @@ describe("SnapshotInternalAPI - formatNodeForInsertion", () => {
     });
 });
 
-describe("modifierFlags", () => {
+describe("modifierFlags", { concurrency }, () => {
     test("export async function has Export | Async flags", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -6471,7 +6476,7 @@ describe("modifierFlags", () => {
     });
 });
 
-describe("Checker - getResolvedSymbol", () => {
+describe("Checker - getResolvedSymbol", { concurrency }, () => {
     test("resolves variable reference to its declaration symbol", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -6500,7 +6505,7 @@ describe("Checker - getResolvedSymbol", () => {
     });
 });
 
-describe("VariableDeclarationList - BlockScoped flags", () => {
+describe("VariableDeclarationList - BlockScoped flags", { concurrency }, () => {
     test("let declaration has Let flag", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -6546,161 +6551,163 @@ describe("VariableDeclarationList - BlockScoped flags", () => {
     });
 });
 
-test("TypeOperator operator kind", async () => {
-    await using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `function test(arg: readonly number[]) { }\n`,
-    });
-
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = await project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    const param = (sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").FunctionDeclaration).parameters[0];
-    assert(param);
-    const type = param.type as import("@typescript/typescript/unstable/ast").TypeOperatorNode;
-    assert(type);
-    assert.equal(type.kind, SyntaxKind.TypeOperator);
-    assert.equal(type.operator, SyntaxKind.ReadonlyKeyword);
-    const printed = await api.printer.printFile(sourceFile);
-    assert.equal(sourceFile.text, printed);
-});
-
-test("SpreadAssignment roundtrip", async () => {
-    await using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `var thing = { ...other };\n`,
-    });
-
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = await project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    const stmt = sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
-    const object = stmt.declarationList.declarations[0].initializer as import("@typescript/typescript/unstable/ast").ObjectLiteralExpression;
-    const assignment = object.properties[0] as import("@typescript/typescript/unstable/ast").SpreadAssignment;
-    assert(assignment);
-    assert.equal(assignment.kind, SyntaxKind.SpreadAssignment);
-    const expr = assignment.expression;
-    assert(expr);
-    assert.equal(expr.kind, SyntaxKind.Identifier);
-    const printed = await api.printer.printFile(sourceFile);
-    assert.equal(sourceFile.text, printed);
-});
+describe("AST roundtrips", { concurrency }, () => {
+    test("TypeOperator operator kind", async () => {
+        await using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `function test(arg: readonly number[]) { }\n`,
+        });
 
-test("VariableDeclarationList const flag clone", async () => {
-    await using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `const thing = 123;\n`,
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = await project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
+        const param = (sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").FunctionDeclaration).parameters[0];
+        assert(param);
+        const type = param.type as import("@typescript/typescript/unstable/ast").TypeOperatorNode;
+        assert(type);
+        assert.equal(type.kind, SyntaxKind.TypeOperator);
+        assert.equal(type.operator, SyntaxKind.ReadonlyKeyword);
+        const printed = await api.printer.printFile(sourceFile);
+        assert.equal(sourceFile.text, printed);
     });
 
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = await project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    {
+    test("SpreadAssignment roundtrip", async () => {
+        await using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `var thing = { ...other };\n`,
+        });
+
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = await project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
         const stmt = sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
-        const list = stmt.declarationList;
-        assert(list.flags & NodeFlags.Const);
-    }
-    const cloned = getSynthesizedDeepClone(sourceFile);
-    {
-        const stmt = cloned.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
-        const list = stmt.declarationList;
-        assert(list.flags & NodeFlags.Const);
-    }
-    const printed = await api.printer.printFile(cloned);
-    assert.equal(sourceFile.text, printed);
-});
+        const object = stmt.declarationList.declarations[0].initializer as import("@typescript/typescript/unstable/ast").ObjectLiteralExpression;
+        const assignment = object.properties[0] as import("@typescript/typescript/unstable/ast").SpreadAssignment;
+        assert(assignment);
+        assert.equal(assignment.kind, SyntaxKind.SpreadAssignment);
+        const expr = assignment.expression;
+        assert(expr);
+        assert.equal(expr.kind, SyntaxKind.Identifier);
+        const printed = await api.printer.printFile(sourceFile);
+        assert.equal(sourceFile.text, printed);
+    });
 
-test("JSDoc before ExpressionStatement allowed", async () => {
-    await using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `
+    test("VariableDeclarationList const flag clone", async () => {
+        await using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `const thing = 123;\n`,
+        });
+
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = await project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
+        {
+            const stmt = sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
+            const list = stmt.declarationList;
+            assert(list.flags & NodeFlags.Const);
+        }
+        const cloned = getSynthesizedDeepClone(sourceFile);
+        {
+            const stmt = cloned.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
+            const list = stmt.declarationList;
+            assert(list.flags & NodeFlags.Const);
+        }
+        const printed = await api.printer.printFile(cloned);
+        assert.equal(sourceFile.text, printed);
+    });
+
+    test("JSDoc before ExpressionStatement allowed", async () => {
+        await using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `
 /**
  * A doc.
  */
 doThing();
         `,
-    });
-
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = await project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    const printed = await api.printer.printFile(sourceFile);
-    assert.equal(sourceFile.text.trim(), printed.trim());
-});
+        });
 
-test("Factory ModifierList auto-conversion", async () => {
-    await using api = spawnAPI();
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = await project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
+        const printed = await api.printer.printFile(sourceFile);
+        assert.equal(sourceFile.text.trim(), printed.trim());
+    });
 
-    const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const node = createTypeAliasDeclaration(
-        [createToken(SyntaxKind.ExportKeyword)],
-        createIdentifier("Test"),
-        undefined,
-        createKeywordTypeNode(SyntaxKind.AnyKeyword),
-    );
+    test("Factory ModifierList auto-conversion", async () => {
+        await using api = spawnAPI();
 
-    assert.equal(await api.printer.printNode(node), "export type Test = any;");
+        const snapshot = await api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const node = createTypeAliasDeclaration(
+            [createToken(SyntaxKind.ExportKeyword)],
+            createIdentifier("Test"),
+            undefined,
+            createKeywordTypeNode(SyntaxKind.AnyKeyword),
+        );
 
-    const cloned = getSynthesizedDeepClone(node);
-    assert.equal(await api.printer.printNode(cloned), "export type Test = any;");
-});
+        assert.equal(await api.printer.printNode(node), "export type Test = any;");
 
-test("Parse-clone-emit roundtrip", async () => {
-    const tsSource = fileURLToPath(new URL("../../../../tsc/testdata/fixtures", import.meta.url).toString());
-    await using api = new API({
-        cwd: tsSource,
+        const cloned = getSynthesizedDeepClone(node);
+        assert.equal(await api.printer.printNode(cloned), "export type Test = any;");
     });
-    const target = {
-        cloneCrashed: 0,
-        printCrashed: 0,
-        clonePrintCrashed: 0,
-    };
-    const errors = { ...target };
-
-    for (const tsconfig of globSync("**/tsconfig.json", { cwd: tsSource })) {
-        const snapshot = await api.createSnapshot({ openProject: resolve(tsSource, tsconfig) });
-        const project = snapshot.getConfiguredProject(tsconfig);
-        assert(project);
-        for (const file of project.rootFiles) {
-            const source = await project.program.getSourceFile(file);
-            assert(source);
-            let clone: typeof source;
-
-            try {
-                await api.printer.printNode(source);
-            }
-            catch {
-                errors.printCrashed++;
-                continue;
-            }
 
-            try {
-                clone = getSynthesizedDeepClone(source);
-            }
-            catch {
-                errors.cloneCrashed++;
-                continue;
-            }
+    test("Parse-clone-emit roundtrip", async () => {
+        const tsSource = fileURLToPath(new URL("../../../../tsc/testdata/fixtures", import.meta.url).toString());
+        await using api = new API({
+            cwd: tsSource,
+        });
+        const target = {
+            cloneCrashed: 0,
+            printCrashed: 0,
+            clonePrintCrashed: 0,
+        };
+        const errors = { ...target };
+
+        for (const tsconfig of globSync("**/tsconfig.json", { cwd: tsSource })) {
+            const snapshot = await api.createSnapshot({ openProject: resolve(tsSource, tsconfig) });
+            const project = snapshot.getConfiguredProject(tsconfig);
+            assert(project);
+            for (const file of project.rootFiles) {
+                const source = await project.program.getSourceFile(file);
+                assert(source);
+                let clone: typeof source;
+
+                try {
+                    await api.printer.printNode(source);
+                }
+                catch {
+                    errors.printCrashed++;
+                    continue;
+                }
 
-            try {
-                await api.printer.printNode(clone);
-            }
-            catch {
-                errors.clonePrintCrashed++;
-                continue;
+                try {
+                    clone = getSynthesizedDeepClone(source);
+                }
+                catch {
+                    errors.cloneCrashed++;
+                    continue;
+                }
+
+                try {
+                    await api.printer.printNode(clone);
+                }
+                catch {
+                    errors.clonePrintCrashed++;
+                    continue;
+                }
             }
         }
-    }
 
-    assert.deepEqual(errors, target);
+        assert.deepEqual(errors, target);
+    });
 });
 
-describe("Program - diagnostics", () => {
+describe("Program - diagnostics", { concurrency }, () => {
     test("getSyntacticDiagnostics", async () => {
         const source = `const x: = 1;`;
         await using api = spawnAPI({
@@ -7030,7 +7037,7 @@ describe("Program - diagnostics", () => {
     });
 });
 
-describe("getDefaultProjectForFile", () => {
+describe("getDefaultProjectForFile", { concurrency }, () => {
     test("snapshot opens reject unreadable virtual files without panicking", async () => {
         const fileName = "/src/App.vue.ts";
         const source = `export const component = 1;`;
@@ -7212,7 +7219,7 @@ describe("getDefaultProjectForFile", () => {
     });
 });
 
-describe("Program - emit", () => {
+describe("Program - emit", { concurrency }, () => {
     const files = {
         "/tsconfig.json": `{
                 "compilerOptions": {
@@ -7472,7 +7479,7 @@ describe("Program - emit", () => {
     // @sync-skip-block-end
 });
 
-describe("Timing", () => {
+describe("Timing", { concurrency }, () => {
     test("collects combined client, server, and transport timing info", async () => {
         await using api = new API({
             cwd: fileURLToPath(new URL("../../../../", import.meta.url).toString()),
@@ -7608,7 +7615,7 @@ describe("Timing", () => {
     });
 });
 
-describe("runWithTemporaryFileUpdate", () => {
+describe("runWithTemporaryFileUpdate", { concurrency }, () => {
     test("temporary file update is visible in the callback and reverted afterward", async () => {
         await using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
diff --git a/packages/typescript/test/async/astnav.test.ts b/packages/typescript/test/async/astnav.test.ts
index 0200611cd8727..f551056711dc0 100644
--- a/packages/typescript/test/async/astnav.test.ts
+++ b/packages/typescript/test/async/astnav.test.ts
@@ -24,6 +24,7 @@ import {
     test,
 } from "node:test";
 import { fileURLToPath } from "node:url";
+import { areTestsFiltered } from "../testUtils.ts";
 
 // ---------------------------------------------------------------------------
 // Go JSON baseline format
@@ -76,7 +77,7 @@ const baselineDir = resolve(repoRoot, "tsc/testdata/baselines/reference/astnav")
 // Tests
 // ---------------------------------------------------------------------------
 
-describe("astnav", () => {
+describe("astnav", { concurrency: areTestsFiltered() }, () => {
     let fileText: string;
 
     try {
diff --git a/packages/typescript/test/diagnosticFormatter.test.ts b/packages/typescript/test/diagnosticFormatter.test.ts
index 4cefa0b845f8b..8c4b62651052d 100644
--- a/packages/typescript/test/diagnosticFormatter.test.ts
+++ b/packages/typescript/test/diagnosticFormatter.test.ts
@@ -9,8 +9,9 @@ import {
     describe,
     test,
 } from "node:test";
+import { areTestsFiltered } from "./testUtils.ts";
 
-describe("diagnosticFormatter", () => {
+describe("diagnosticFormatter", { concurrency: areTestsFiltered() }, () => {
     test("formats diagnostics with a configured program host", async () => {
         const source = `const x: number = "oops";\n`;
         const api = spawnAPI({
diff --git a/packages/typescript/test/encoder.test.ts b/packages/typescript/test/encoder.test.ts
index ec90992619a17..7570d240300a4 100644
--- a/packages/typescript/test/encoder.test.ts
+++ b/packages/typescript/test/encoder.test.ts
@@ -47,6 +47,9 @@ import {
     NODE_OFFSET_DATA,
 } from "../src/api/node/protocol.ts";
 import { Wtf8Decoder } from "../src/api/node/wtf8.ts";
+import { areTestsFiltered } from "./testUtils.ts";
+
+const concurrency = areTestsFiltered();
 
 function makeSF(text: string, fileName: string, statements: readonly Statement[]): SourceFile {
     const endOfFileToken = createToken(SyntaxKind.EndOfFile);
@@ -57,7 +60,7 @@ function decode(data: Uint8Array): RemoteSourceFile {
     return new RemoteSourceFile(data, new Wtf8Decoder());
 }
 
-describe("Encoder", () => {
+describe("Encoder", { concurrency }, () => {
     test("encodes empty source file", () => {
         const sf = makeSF("", "/test.ts", []);
 
@@ -348,7 +351,7 @@ describe("Encoder", () => {
     });
 });
 
-describe("UTF-8 vs UTF-16 position encoding", () => {
+describe("UTF-8 vs UTF-16 position encoding", { concurrency }, () => {
     // Positions in the encoded AST must be UTF-16 code unit offsets so that
     // file.text.slice(node.pos, node.end) works correctly on JS strings.
     // This is the same convention TypeScript uses.
@@ -409,7 +412,7 @@ describe("UTF-8 vs UTF-16 position encoding", () => {
     });
 });
 
-describe("Line and character mapping", () => {
+describe("Line and character mapping", { concurrency }, () => {
     function makeSourceFile(text: string): RemoteSourceFile {
         return decode(encodeSourceFile(makeSF(text, "/test.ts", [])));
     }
diff --git a/packages/typescript/test/spanMap.test.ts b/packages/typescript/test/spanMap.test.ts
index c0dc8314185c7..29e80e7a87bd6 100644
--- a/packages/typescript/test/spanMap.test.ts
+++ b/packages/typescript/test/spanMap.test.ts
@@ -9,8 +9,9 @@ import {
     describe,
     test,
 } from "node:test";
+import { areTestsFiltered } from "./testUtils.ts";
 
-describe("SpanMap", () => {
+describe("SpanMap", { concurrency: areTestsFiltered() }, () => {
     const map = new SpanMap([
         { virtualStart: 2, virtualEnd: 6, originalStart: 10, originalEnd: 14, kind: SpanMapKind.Verbatim },
         { virtualStart: 8, virtualEnd: 11, originalStart: 20, originalEnd: 27, kind: SpanMapKind.Atom },
diff --git a/packages/typescript/test/sync/api-generators.test.ts b/packages/typescript/test/sync/api-generators.test.ts
index f9bacf9a96206..5eccff4568fa0 100644
--- a/packages/typescript/test/sync/api-generators.test.ts
+++ b/packages/typescript/test/sync/api-generators.test.ts
@@ -67,6 +67,7 @@ import {
     type APIRequestGenerator,
     executeRequestGenerators,
 } from "../../src/api/sync/generatorSupport.ts";
+import { areTestsFiltered } from "../testUtils.ts";
 import { spawnAPI } from "./api.testUtils.ts";
 
 const parityFiles = {
@@ -415,7 +416,7 @@ function assertPublicGeneratorCoverage(owners: readonly { readonly name: string;
     assert.deepEqual(missing, [], `Uncovered public generator getters: ${missing.join(", ")}`);
 }
 
-describe("API - generator batching", () => {
+describe("API - generator batching", { concurrency: areTestsFiltered() }, () => {
     test("batches source file requests", context => {
         const api = spawnAPI(parityFiles);
         context.after(() => api.close());
diff --git a/packages/typescript/test/sync/api.test.ts b/packages/typescript/test/sync/api.test.ts
index 969c2885ceb35..3817add5a0a0d 100644
--- a/packages/typescript/test/sync/api.test.ts
+++ b/packages/typescript/test/sync/api.test.ts
@@ -118,12 +118,15 @@ import {
 } from "node:test";
 import { fileURLToPath } from "node:url";
 import { isSignatureDeclaration } from "../../src/ast/is.ts";
+import { areTestsFiltered } from "../testUtils.ts";
 import {
     defaultFiles,
     spawnAPI,
 } from "./api.testUtils.ts";
 
-describe("API", () => {
+const concurrency = areTestsFiltered();
+
+describe("API", { concurrency }, () => {
     test("getCurrentLanguageServerSnapshot is LSP-only", () => {
         if (!!false) {
             const standalone = new API();
@@ -706,7 +709,7 @@ declare module "augmentation" {}`,
     });
 });
 
-describe("Checker - getImmediateAliasedSymbol", () => {
+describe("Checker - getImmediateAliasedSymbol", { concurrency }, () => {
     test("resolves one level of alias indirection", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -725,7 +728,7 @@ describe("Checker - getImmediateAliasedSymbol", () => {
     });
 });
 
-describe("Checker - getTargetSymbol", () => {
+describe("Checker - getTargetSymbol", { concurrency }, () => {
     test("gets the target symbol of instantiated symbol", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -763,7 +766,7 @@ test<Bravo>();
     });
 });
 
-describe("Snapshot", () => {
+describe("Snapshot", { concurrency }, () => {
     test("createSnapshot returns snapshot with projects", () => {
         using api = spawnAPI();
 
@@ -917,7 +920,7 @@ describe("Snapshot", () => {
     });
 });
 
-describe("LanguageService - imports", () => {
+describe("LanguageService - imports", { concurrency }, () => {
     test("getImportEditsForSymbols adds a named import", () => {
         const source = `const value = foo;\n`;
         using api = spawnAPI({
@@ -1018,7 +1021,7 @@ describe("LanguageService - imports", () => {
     });
 });
 
-describe("LanguageService - getCompletionsAtPosition", () => {
+describe("LanguageService - getCompletionsAtPosition", { concurrency }, () => {
     test("returns member completions after a dot", () => {
         const src = `\nconst obj = { name: "hello", age: 42 };\nobj.\n`;
         using api = spawnAPI({
@@ -1085,7 +1088,7 @@ describe("LanguageService - getCompletionsAtPosition", () => {
     });
 });
 
-describe("LanguageService - getReferencedSymbolsForNode", () => {
+describe("LanguageService - getReferencedSymbolsForNode", { concurrency }, () => {
     test("getReferencedSymbolsForNode", () => {
         using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -1107,7 +1110,7 @@ describe("LanguageService - getReferencedSymbolsForNode", () => {
     });
 });
 
-describe("LanguageService - getSignatureUsage", () => {
+describe("LanguageService - getSignatureUsage", { concurrency }, () => {
     test("getSignatureUsage", () => {
         using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -1127,7 +1130,7 @@ describe("LanguageService - getSignatureUsage", () => {
     });
 });
 
-describe("Checker - getApparentType", () => {
+describe("Checker - getApparentType", { concurrency }, () => {
     test("returns the apparent type of a literal type", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1150,7 +1153,7 @@ describe("Checker - getApparentType", () => {
     });
 });
 
-describe("Checker - getReducedType", () => {
+describe("Checker - getReducedType", { concurrency }, () => {
     test("returns the reduced type", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1187,7 +1190,7 @@ export type Result = RateLimitError | (RateLimitError & QuotaExceededError);`,
     });
 });
 
-describe("Checker - getMemberInModuleExports", () => {
+describe("Checker - getMemberInModuleExports", { concurrency }, () => {
     test("returns a named export when present", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1208,7 +1211,7 @@ describe("Checker - getMemberInModuleExports", () => {
     });
 });
 
-describe("SourceFile", () => {
+describe("SourceFile", { concurrency }, () => {
     test("getSourceFile rejects invalid document identifiers", () => {
         using api = spawnAPI();
 
@@ -1398,7 +1401,7 @@ describe("SourceFile", () => {
     });
 });
 
-describe("NodeArray", () => {
+describe("NodeArray", { concurrency }, () => {
     test("hasTrailingComma", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -1417,98 +1420,100 @@ describe("NodeArray", () => {
     });
 });
 
-test("unicode escapes", () => {
-    using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/1.ts": `"😃"`,
-        "/src/2.ts": `"\\ud83d\\ude03"`,
-        "/src/3.ts": `"\\ud800a\\udc00"`,
+describe("API objects", { concurrency }, () => {
+    test("unicode escapes", () => {
+        using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/1.ts": `"😃"`,
+            "/src/2.ts": `"\\ud83d\\ude03"`,
+            "/src/3.ts": `"\\ud800a\\udc00"`,
+        });
+
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const expectedTexts = new Map([
+            ["/src/1.ts", "😃"],
+            ["/src/2.ts", "😃"],
+            ["/src/3.ts", "\ud800a\udc00"],
+        ]);
+
+        for (const file of expectedTexts.keys()) {
+            const sourceFile = project.program.getSourceFile(file);
+            assert.ok(sourceFile);
+
+            sourceFile.forEachChild(function visit(node) {
+                if (isStringLiteral(node)) {
+                    assert.equal(node.text, expectedTexts.get(file));
+                }
+                node.forEachChild(visit);
+            });
+        }
     });
 
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const expectedTexts = new Map([
-        ["/src/1.ts", "😃"],
-        ["/src/2.ts", "😃"],
-        ["/src/3.ts", "\ud800a\udc00"],
-    ]);
+    test("template unicode escapes", () => {
+        using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": "`\\ud800${0}\\udc00`",
+        });
 
-    for (const file of expectedTexts.keys()) {
-        const sourceFile = project.program.getSourceFile(file);
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = project.program.getSourceFile("/src/index.ts");
         assert.ok(sourceFile);
 
+        let sawHead = false;
+        let sawTail = false;
         sourceFile.forEachChild(function visit(node) {
-            if (isStringLiteral(node)) {
-                assert.equal(node.text, expectedTexts.get(file));
+            if (isTemplateHead(node)) {
+                assert.equal(node.text, "\ud800");
+                sawHead = true;
+            }
+            else if (isTemplateTail(node)) {
+                assert.equal(node.text, "\udc00");
+                sawTail = true;
             }
             node.forEachChild(visit);
         });
-    }
-});
-
-test("template unicode escapes", () => {
-    using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": "`\\ud800${0}\\udc00`",
+        assert.ok(sawHead);
+        assert.ok(sawTail);
     });
 
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = project.program.getSourceFile("/src/index.ts");
-    assert.ok(sourceFile);
+    test("Object equality", () => {
+        using api = spawnAPI();
 
-    let sawHead = false;
-    let sawTail = false;
-    sourceFile.forEachChild(function visit(node) {
-        if (isTemplateHead(node)) {
-            assert.equal(node.text, "\ud800");
-            sawHead = true;
-        }
-        else if (isTemplateTail(node)) {
-            assert.equal(node.text, "\udc00");
-            sawTail = true;
-        }
-        node.forEachChild(visit);
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        // Same symbol returned from same snapshot's checker
+        assert.strictEqual(
+            project.checker.getSymbolAtPosition("/src/index.ts", 9),
+            project.checker.getSymbolAtPosition("/src/index.ts", 10),
+        );
     });
-    assert.ok(sawHead);
-    assert.ok(sawTail);
-});
 
-test("Object equality", () => {
-    using api = spawnAPI();
-
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    // Same symbol returned from same snapshot's checker
-    assert.strictEqual(
-        project.checker.getSymbolAtPosition("/src/index.ts", 9),
-        project.checker.getSymbolAtPosition("/src/index.ts", 10),
-    );
-});
-
-test("Snapshot dispose", () => {
-    using api = spawnAPI();
+    test("Snapshot dispose", () => {
+        using api = spawnAPI();
 
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const symbol = project.checker.getSymbolAtPosition("/src/index.ts", 9);
-    assert.ok(symbol);
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const symbol = project.checker.getSymbolAtPosition("/src/index.ts", 9);
+        assert.ok(symbol);
 
-    // Snapshot dispose should release server-side resources
-    assert.ok(snapshot.isDisposed() === false);
-    snapshot.dispose();
-    assert.ok(snapshot.isDisposed() === true);
+        // Snapshot dispose should release server-side resources
+        assert.ok(snapshot.isDisposed() === false);
+        snapshot.dispose();
+        assert.ok(snapshot.isDisposed() === true);
 
-    // After dispose, snapshot methods should throw
-    assert.throws(() => {
-        snapshot.getConfiguredProject("/tsconfig.json");
-    }, {
-        name: "Error",
-        message: "Snapshot is disposed",
+        // After dispose, snapshot methods should throw
+        assert.throws(() => {
+            snapshot.getConfiguredProject("/tsconfig.json");
+        }, {
+            name: "Error",
+            message: "Snapshot is disposed",
+        });
     });
 });
 
-describe("Multiple snapshots", () => {
+describe("Multiple snapshots", { concurrency }, () => {
     test("two snapshots work independently", () => {
         using api = spawnAPI();
 
@@ -1714,7 +1719,7 @@ describe("Multiple snapshots", () => {
     });
 });
 
-describe("Source file caching", () => {
+describe("Source file caching", { concurrency }, () => {
     test("same file from same snapshot returns cached object", () => {
         using api = spawnAPI();
 
@@ -1882,7 +1887,7 @@ describe("Source file caching", () => {
     });
 });
 
-describe("Snapshot disposal", () => {
+describe("Snapshot disposal", { concurrency }, () => {
     test("dispose is idempotent", () => {
         using api = spawnAPI();
 
@@ -1921,7 +1926,7 @@ describe("Snapshot disposal", () => {
     });
 });
 
-describe("Source file cache keying across projects", () => {
+describe("Source file cache keying across projects", { concurrency }, () => {
     // Three projects share the same file (/src/shared.ts).
     // The file sits inside a package.json scope with "type": "module".
     //
@@ -2000,7 +2005,7 @@ describe("Source file cache keying across projects", () => {
     });
 });
 
-describe("Checker - symbol identity across projects", () => {
+describe("Checker - symbol identity across projects", { concurrency }, () => {
     const sharedSymbolFiles = {
         "/projectA/tsconfig.json": JSON.stringify({ files: ["../src/shared.ts"] }),
         "/projectB/tsconfig.json": JSON.stringify({ files: ["../src/shared.ts"] }),
@@ -2054,7 +2059,7 @@ describe("Checker - symbol identity across projects", () => {
     });
 });
 
-describe("Checker - types and signatures", () => {
+describe("Checker - types and signatures", { concurrency }, () => {
     const checkerFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -2482,7 +2487,7 @@ export class Cache {
     });
 });
 
-describe("Symbol - parent, members, exports", () => {
+describe("Symbol - parent, members, exports", { concurrency }, () => {
     const symbolFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/mod.ts": `
@@ -2553,7 +2558,7 @@ export const value = 1;
     });
 });
 
-describe("Type - getSymbol", () => {
+describe("Type - getSymbol", { concurrency }, () => {
     test("getSymbol returns the symbol of a type", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -2579,7 +2584,7 @@ export const instance: Foo = new Foo();
     });
 });
 
-describe("Type - sub-property fetchers", () => {
+describe("Type - sub-property fetchers", { concurrency }, () => {
     const typeFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true, target: "esnext" } }),
         "/src/types.ts": `
@@ -2917,7 +2922,7 @@ export function gh1449<T extends [foo: any, bar?: any]>(a: T): T {
     });
 });
 
-describe("Checker - intrinsic type getters", () => {
+describe("Checker - intrinsic type getters", { concurrency }, () => {
     const intrinsicFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `export const x = 1;`,
@@ -3044,7 +3049,7 @@ describe("Checker - intrinsic type getters", () => {
     });
 });
 
-describe("Checker - multi-project type ID uniqueness", () => {
+describe("Checker - multi-project type ID uniqueness", { concurrency }, () => {
     test("intrinsic types from 3 projects in the same snapshot have non-colliding IDs", () => {
         using api = spawnAPI({
             "/proj1/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3150,7 +3155,7 @@ describe("Checker - multi-project type ID uniqueness", () => {
     });
 });
 
-describe("Checker - getBaseTypeOfLiteralType", () => {
+describe("Checker - getBaseTypeOfLiteralType", { concurrency }, () => {
     test("number literal widens to number", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3192,7 +3197,7 @@ describe("Checker - getBaseTypeOfLiteralType", () => {
     });
 });
 
-describe("Checker - getContextualType", () => {
+describe("Checker - getContextualType", { concurrency }, () => {
     test("contextual type from function parameter", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3227,7 +3232,7 @@ foo(42);
     });
 });
 
-describe("Checker - getTypeOfSymbolAtLocation", () => {
+describe("Checker - getTypeOfSymbolAtLocation", { concurrency }, () => {
     test("narrowed type via typeof check", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3282,7 +3287,7 @@ export function check(x: string | number) {
     });
 });
 
-describe("Checker - getShorthandAssignmentValueSymbol", () => {
+describe("Checker - getShorthandAssignmentValueSymbol", { concurrency }, () => {
     test("shorthand property symbol resolves to variable", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3314,7 +3319,7 @@ export const obj = { name };
     });
 });
 
-describe("readFile callback semantics", () => {
+describe("readFile callback semantics", { concurrency }, () => {
     test("readFile: string returns content, null blocks fallback, undefined falls through to real FS", () => {
         const virtualFiles: Record<string, string> = {
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -3364,7 +3369,7 @@ describe("readFile callback semantics", () => {
     });
 });
 
-describe("updateSnapshot file systems", () => {
+describe("updateSnapshot file systems", { concurrency }, () => {
     test("request filesystem factories normalize files and preserve explicit listings", () => {
         const memory = createFileSystem([
             ["/src/index.ts", "posix"],
@@ -3962,7 +3967,7 @@ describe("updateSnapshot file systems", () => {
     // do not model modification times.
 });
 
-describe("Checker - isArrayType / isTupleType", () => {
+describe("Checker - isArrayType / isTupleType", { concurrency }, () => {
     test("number[] is array, not tuple", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4072,7 +4077,7 @@ describe("Checker - isArrayType / isTupleType", () => {
     });
 });
 
-describe("Checker - isReadonlySymbol", () => {
+describe("Checker - isReadonlySymbol", { concurrency }, () => {
     test("properties with a 'readonly' modifier", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4163,7 +4168,7 @@ export type B = InstanceType<typeof Bravo>;
     });
 });
 
-describe("Checker - getReturnTypeOfSignature", () => {
+describe("Checker - getReturnTypeOfSignature", { concurrency }, () => {
     test("returns the return type of a function signature", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4186,7 +4191,7 @@ describe("Checker - getReturnTypeOfSignature", () => {
     });
 });
 
-describe("Checker - getRestTypeOfSignature", () => {
+describe("Checker - getRestTypeOfSignature", { concurrency }, () => {
     test("returns the rest type of a signature with rest parameter", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4209,7 +4214,7 @@ describe("Checker - getRestTypeOfSignature", () => {
     });
 });
 
-describe("Checker - getTypePredicateOfSignature", () => {
+describe("Checker - getTypePredicateOfSignature", { concurrency }, () => {
     test("returns type predicate for 'x is T' guard", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4309,7 +4314,7 @@ export class Dog extends Animal {
     });
 });
 
-describe("Checker - getBaseTypes", () => {
+describe("Checker - getBaseTypes", { concurrency }, () => {
     test("returns base types of a class", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4392,7 +4397,7 @@ export type BoxOfString = Box<string>;
     });
 });
 
-describe("Type - getBaseTypes", () => {
+describe("Type - getBaseTypes", { concurrency }, () => {
     test("returns base types for a class type and undefined for a non-class/interface", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4430,7 +4435,7 @@ export const n: number = 0;
     });
 });
 
-describe("Type - isErrorType", () => {
+describe("Type - isErrorType", { concurrency }, () => {
     test("identifies the error type from an unresolvable annotation", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4460,7 +4465,7 @@ declare const bad: ThisTypeDoesNotExist;
     });
 });
 
-describe("Checker - well-known symbols", () => {
+describe("Checker - well-known symbols", { concurrency }, () => {
     test("isUnknownSymbol identifies the aliased unknown symbol", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4482,7 +4487,7 @@ export type Alias = typeof value;
     });
 });
 
-describe("Checker - well-known signatures", () => {
+describe("Checker - well-known signatures", { concurrency }, () => {
     test("isUnknownSignature identifies an unresolvable call", () => {
         const src = `
 const ok = (x: number) => x;
@@ -4516,7 +4521,7 @@ notCallable();
     });
 });
 
-describe("Symbol - escaped names and tables", () => {
+describe("Symbol - escaped names and tables", { concurrency }, () => {
     test("getExports/getMembers return a cached Map keyed by escaped name", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4615,7 +4620,7 @@ export { x as '${maliciousName}' };
     });
 });
 
-describe("ast - escapeLeadingUnderscores", () => {
+describe("ast - escapeLeadingUnderscores", { concurrency }, () => {
     test("round-trips display and escaped names", () => {
         assert.equal(escapeLeadingUnderscores("foo"), "foo");
         assert.equal(escapeLeadingUnderscores("_foo"), "_foo");
@@ -4626,7 +4631,7 @@ describe("ast - escapeLeadingUnderscores", () => {
     });
 });
 
-describe("ast - tryGetAmbientModuleNameFromSymbolName", () => {
+describe("ast - tryGetAmbientModuleNameFromSymbolName", { concurrency }, () => {
     test("gets ambient module names from escaped symbol names", () => {
         assert.equal(tryGetAmbientModuleNameFromSymbolName('"pkg"' as __String), "pkg");
         assert.equal(tryGetAmbientModuleNameFromSymbolName('__"*.css"pattern@1234' as __String), "*.css");
@@ -4636,7 +4641,7 @@ describe("ast - tryGetAmbientModuleNameFromSymbolName", () => {
     });
 });
 
-describe("ast - getJSDocTags", () => {
+describe("ast - getJSDocTags", { concurrency }, () => {
     test("returns a node's own tags, and inherited @param / @template tags", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4778,7 +4783,7 @@ const cast = /** @type {number} */ (someValue);
     });
 });
 
-describe("Checker - getPropertiesOfType", () => {
+describe("Checker - getPropertiesOfType", { concurrency }, () => {
     test("returns properties of an object type", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4809,7 +4814,7 @@ export declare const p: Person;
     });
 });
 
-describe("Checker - getIndexInfosOfType", () => {
+describe("Checker - getIndexInfosOfType", { concurrency }, () => {
     test("returns index signatures of an indexed type", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4864,7 +4869,7 @@ export declare const m: ReadonlyMap;
     });
 });
 
-describe("Checker - TypeScript API parity", () => {
+describe("Checker - TypeScript API parity", { concurrency }, () => {
     const files = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -4953,7 +4958,7 @@ export type Exported = number;
     });
 });
 
-describe("Checker - getConstraintOfTypeParameter", () => {
+describe("Checker - getConstraintOfTypeParameter", { concurrency }, () => {
     test("returns constraint of a type parameter", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -4978,7 +4983,7 @@ describe("Checker - getConstraintOfTypeParameter", () => {
     });
 });
 
-describe("Checker - TypeParameter getters", () => {
+describe("Checker - TypeParameter getters", { concurrency }, () => {
     test("getConstraint() and getDefault() return the constraint and default types", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5033,7 +5038,7 @@ describe("Checker - TypeParameter getters", () => {
     });
 });
 
-describe("Checker - getTypeArguments", () => {
+describe("Checker - getTypeArguments", { concurrency }, () => {
     test("returns type arguments of a generic instantiation", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5082,7 +5087,7 @@ describe("Checker - getTypeArguments", () => {
     });
 });
 
-describe("Checker - getBaseConstraintOfType", () => {
+describe("Checker - getBaseConstraintOfType", { concurrency }, () => {
     test("returns the base constraint of a type parameter", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5123,7 +5128,7 @@ describe("Checker - getBaseConstraintOfType", () => {
     });
 });
 
-describe("Checker - getPropertyOfType", () => {
+describe("Checker - getPropertyOfType", { concurrency }, () => {
     test("returns a named property symbol of a type", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5152,7 +5157,7 @@ export declare const p: Person;
     });
 });
 
-describe("Checker - getConstantValue", () => {
+describe("Checker - getConstantValue", { concurrency }, () => {
     test("returns numeric value of an enum member", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5231,7 +5236,7 @@ describe("Checker - getConstantValue", () => {
     });
 });
 
-describe("Checker - getSignatureFromDeclaration", () => {
+describe("Checker - getSignatureFromDeclaration", { concurrency }, () => {
     test("returns the signature of a function declaration", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5257,7 +5262,7 @@ describe("Checker - getSignatureFromDeclaration", () => {
     });
 });
 
-describe("Checker - getExportSpecifierLocalTargetSymbol", () => {
+describe("Checker - getExportSpecifierLocalTargetSymbol", { concurrency }, () => {
     test("resolves the local target of an export specifier", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5283,7 +5288,7 @@ export { value as renamed };
     });
 });
 
-describe("Checker - getAliasedSymbol", () => {
+describe("Checker - getAliasedSymbol", { concurrency }, () => {
     test("resolves an import alias to its target symbol", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5304,7 +5309,7 @@ describe("Checker - getAliasedSymbol", () => {
     });
 });
 
-describe("Checker - getFullyQualifiedName", () => {
+describe("Checker - getFullyQualifiedName", { concurrency }, () => {
     test("returns module-qualified names for exported symbols and dotted names for members", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5334,7 +5339,7 @@ export class Standalone {}
     });
 });
 
-describe("Checker - getExportsOfModule", () => {
+describe("Checker - getExportsOfModule", { concurrency }, () => {
     test("returns all exports including re-exports via 'export *'", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5358,7 +5363,7 @@ export * from "./inner";
     });
 });
 
-describe("Checker - getSymbolsInScope", () => {
+describe("Checker - getSymbolsInScope", { concurrency }, () => {
     const scopeFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -5419,7 +5424,7 @@ function f() {
     });
 });
 
-describe("Symbol - getDocumentationComment and getJsDocTags", () => {
+describe("Symbol - getDocumentationComment and getJsDocTags", { concurrency }, () => {
     const docFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -5463,7 +5468,7 @@ export function add(a: number, b: number): number { return a + b; }
     });
 });
 
-describe("TypeParameter - isThisType", () => {
+describe("TypeParameter - isThisType", { concurrency }, () => {
     test("isThisType is true for the polymorphic 'this' type in a class method", () => {
         const src = `\nexport class Builder {\n    setName(name: string): this { return this; }\n}\n`;
         using api = spawnAPI({
@@ -5503,7 +5508,7 @@ describe("TypeParameter - isThisType", () => {
     });
 });
 
-describe("Type - getAliasTypeArguments", () => {
+describe("Type - getAliasTypeArguments", { concurrency }, () => {
     test("returns the type arguments of a single-param generic type alias", () => {
         const src = `\ntype Box<T> = { value: T };\nexport const x: Box<string> = { value: "hi" };\n`;
         using api = spawnAPI({
@@ -5562,7 +5567,7 @@ describe("Type - getAliasTypeArguments", () => {
     });
 });
 
-describe("Type - getAliasSymbol", () => {
+describe("Type - getAliasSymbol", { concurrency }, () => {
     test("returns the symbol for a non-generic type alias", () => {
         // Object-type aliases preserve aliasSymbol; primitive aliases (type Foo = string) do not.
         const src = `\ntype Point = { x: number; y: number };\nexport const p: Point = { x: 1, y: 2 };\n`;
@@ -5621,7 +5626,7 @@ describe("Type - getAliasSymbol", () => {
     });
 });
 
-describe("IntrinsicType - intrinsicName", () => {
+describe("IntrinsicType - intrinsicName", { concurrency }, () => {
     test("intrinsicName matches the primitive type name", () => {
         const src = `\nexport const x: string = "hello";\n`;
         using api = spawnAPI({
@@ -5647,7 +5652,7 @@ describe("IntrinsicType - intrinsicName", () => {
     });
 });
 
-describe("FreshableType - getFreshType and getRegularType", () => {
+describe("FreshableType - getFreshType and getRegularType", { concurrency }, () => {
     test("LiteralType.value is empty string for the empty-string literal type", () => {
         const src = `\nexport const empty: "" = "";\n`;
         using api = spawnAPI({
@@ -5830,7 +5835,7 @@ describe("FreshableType - getFreshType and getRegularType", () => {
     });
 });
 
-describe("Checker - isContextSensitive", () => {
+describe("Checker - isContextSensitive", { concurrency }, () => {
     test("arrow function with no type annotation is context sensitive", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
@@ -5855,7 +5860,7 @@ describe("Checker - isContextSensitive", () => {
     });
 });
 
-describe("Checker - isTypeAssignableTo", () => {
+describe("Checker - isTypeAssignableTo", { concurrency }, () => {
     test("returns true when source is assignable to target", () => {
         using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -5908,7 +5913,7 @@ describe("Checker - isTypeAssignableTo", () => {
     });
 });
 
-describe("Printer", () => {
+describe("Printer", { concurrency }, () => {
     const emitterFiles = {
         "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
         "/src/main.ts": `
@@ -6151,7 +6156,7 @@ export const obj = { m: 1, s: "hi", b: true };
     });
 });
 
-describe("Program - selected file emit", () => {
+describe("Program - selected file emit", { concurrency }, () => {
     const files = {
         "/tsconfig.json": JSON.stringify({
             compilerOptions: {
@@ -6213,7 +6218,7 @@ describe("Program - selected file emit", () => {
     });
 });
 
-describe("SnapshotInternalAPI - formatNodeForInsertion", () => {
+describe("SnapshotInternalAPI - formatNodeForInsertion", { concurrency }, () => {
     test("formats a synthesized statement with correct indentation for insertion inside a function body", () => {
         const files = {
             "/tsconfig.json": "{}",
@@ -6291,7 +6296,7 @@ describe("SnapshotInternalAPI - formatNodeForInsertion", () => {
     });
 });
 
-describe("modifierFlags", () => {
+describe("modifierFlags", { concurrency }, () => {
     test("export async function has Export | Async flags", () => {
         using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -6339,7 +6344,7 @@ describe("modifierFlags", () => {
     });
 });
 
-describe("Checker - getResolvedSymbol", () => {
+describe("Checker - getResolvedSymbol", { concurrency }, () => {
     test("resolves variable reference to its declaration symbol", () => {
         using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -6368,7 +6373,7 @@ describe("Checker - getResolvedSymbol", () => {
     });
 });
 
-describe("VariableDeclarationList - BlockScoped flags", () => {
+describe("VariableDeclarationList - BlockScoped flags", { concurrency }, () => {
     test("let declaration has Let flag", () => {
         using api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -6414,161 +6419,163 @@ describe("VariableDeclarationList - BlockScoped flags", () => {
     });
 });
 
-test("TypeOperator operator kind", () => {
-    using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `function test(arg: readonly number[]) { }\n`,
-    });
-
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    const param = (sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").FunctionDeclaration).parameters[0];
-    assert(param);
-    const type = param.type as import("@typescript/typescript/unstable/ast").TypeOperatorNode;
-    assert(type);
-    assert.equal(type.kind, SyntaxKind.TypeOperator);
-    assert.equal(type.operator, SyntaxKind.ReadonlyKeyword);
-    const printed = api.printer.printFile(sourceFile);
-    assert.equal(sourceFile.text, printed);
-});
-
-test("SpreadAssignment roundtrip", () => {
-    using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `var thing = { ...other };\n`,
-    });
-
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    const stmt = sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
-    const object = stmt.declarationList.declarations[0].initializer as import("@typescript/typescript/unstable/ast").ObjectLiteralExpression;
-    const assignment = object.properties[0] as import("@typescript/typescript/unstable/ast").SpreadAssignment;
-    assert(assignment);
-    assert.equal(assignment.kind, SyntaxKind.SpreadAssignment);
-    const expr = assignment.expression;
-    assert(expr);
-    assert.equal(expr.kind, SyntaxKind.Identifier);
-    const printed = api.printer.printFile(sourceFile);
-    assert.equal(sourceFile.text, printed);
-});
+describe("AST roundtrips", { concurrency }, () => {
+    test("TypeOperator operator kind", () => {
+        using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `function test(arg: readonly number[]) { }\n`,
+        });
 
-test("VariableDeclarationList const flag clone", () => {
-    using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `const thing = 123;\n`,
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
+        const param = (sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").FunctionDeclaration).parameters[0];
+        assert(param);
+        const type = param.type as import("@typescript/typescript/unstable/ast").TypeOperatorNode;
+        assert(type);
+        assert.equal(type.kind, SyntaxKind.TypeOperator);
+        assert.equal(type.operator, SyntaxKind.ReadonlyKeyword);
+        const printed = api.printer.printFile(sourceFile);
+        assert.equal(sourceFile.text, printed);
     });
 
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    {
+    test("SpreadAssignment roundtrip", () => {
+        using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `var thing = { ...other };\n`,
+        });
+
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
         const stmt = sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
-        const list = stmt.declarationList;
-        assert(list.flags & NodeFlags.Const);
-    }
-    const cloned = getSynthesizedDeepClone(sourceFile);
-    {
-        const stmt = cloned.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
-        const list = stmt.declarationList;
-        assert(list.flags & NodeFlags.Const);
-    }
-    const printed = api.printer.printFile(cloned);
-    assert.equal(sourceFile.text, printed);
-});
+        const object = stmt.declarationList.declarations[0].initializer as import("@typescript/typescript/unstable/ast").ObjectLiteralExpression;
+        const assignment = object.properties[0] as import("@typescript/typescript/unstable/ast").SpreadAssignment;
+        assert(assignment);
+        assert.equal(assignment.kind, SyntaxKind.SpreadAssignment);
+        const expr = assignment.expression;
+        assert(expr);
+        assert.equal(expr.kind, SyntaxKind.Identifier);
+        const printed = api.printer.printFile(sourceFile);
+        assert.equal(sourceFile.text, printed);
+    });
 
-test("JSDoc before ExpressionStatement allowed", () => {
-    using api = spawnAPI({
-        "/tsconfig.json": "{}",
-        "/src/index.ts": `
+    test("VariableDeclarationList const flag clone", () => {
+        using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `const thing = 123;\n`,
+        });
+
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
+        {
+            const stmt = sourceFile.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
+            const list = stmt.declarationList;
+            assert(list.flags & NodeFlags.Const);
+        }
+        const cloned = getSynthesizedDeepClone(sourceFile);
+        {
+            const stmt = cloned.statements[0] as import("@typescript/typescript/unstable/ast").VariableStatement;
+            const list = stmt.declarationList;
+            assert(list.flags & NodeFlags.Const);
+        }
+        const printed = api.printer.printFile(cloned);
+        assert.equal(sourceFile.text, printed);
+    });
+
+    test("JSDoc before ExpressionStatement allowed", () => {
+        using api = spawnAPI({
+            "/tsconfig.json": "{}",
+            "/src/index.ts": `
 /**
  * A doc.
  */
 doThing();
         `,
-    });
-
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const sourceFile = project.program.getSourceFile("/src/index.ts");
-    assert(sourceFile);
-    const printed = api.printer.printFile(sourceFile);
-    assert.equal(sourceFile.text.trim(), printed.trim());
-});
+        });
 
-test("Factory ModifierList auto-conversion", () => {
-    using api = spawnAPI();
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const sourceFile = project.program.getSourceFile("/src/index.ts");
+        assert(sourceFile);
+        const printed = api.printer.printFile(sourceFile);
+        assert.equal(sourceFile.text.trim(), printed.trim());
+    });
 
-    const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
-    const project = snapshot.getConfiguredProject("/tsconfig.json")!;
-    const node = createTypeAliasDeclaration(
-        [createToken(SyntaxKind.ExportKeyword)],
-        createIdentifier("Test"),
-        undefined,
-        createKeywordTypeNode(SyntaxKind.AnyKeyword),
-    );
+    test("Factory ModifierList auto-conversion", () => {
+        using api = spawnAPI();
 
-    assert.equal(api.printer.printNode(node), "export type Test = any;");
+        const snapshot = api.createSnapshot({ openProject: "/tsconfig.json" });
+        const project = snapshot.getConfiguredProject("/tsconfig.json")!;
+        const node = createTypeAliasDeclaration(
+            [createToken(SyntaxKind.ExportKeyword)],
+            createIdentifier("Test"),
+            undefined,
+            createKeywordTypeNode(SyntaxKind.AnyKeyword),
+        );
 
-    const cloned = getSynthesizedDeepClone(node);
-    assert.equal(api.printer.printNode(cloned), "export type Test = any;");
-});
+        assert.equal(api.printer.printNode(node), "export type Test = any;");
 
-test("Parse-clone-emit roundtrip", () => {
-    const tsSource = fileURLToPath(new URL("../../../../tsc/testdata/fixtures", import.meta.url).toString());
-    using api = new API({
-        cwd: tsSource,
+        const cloned = getSynthesizedDeepClone(node);
+        assert.equal(api.printer.printNode(cloned), "export type Test = any;");
     });
-    const target = {
-        cloneCrashed: 0,
-        printCrashed: 0,
-        clonePrintCrashed: 0,
-    };
-    const errors = { ...target };
-
-    for (const tsconfig of globSync("**/tsconfig.json", { cwd: tsSource })) {
-        const snapshot = api.createSnapshot({ openProject: resolve(tsSource, tsconfig) });
-        const project = snapshot.getConfiguredProject(tsconfig);
-        assert(project);
-        for (const file of project.rootFiles) {
-            const source = project.program.getSourceFile(file);
-            assert(source);
-            let clone: typeof source;
-
-            try {
-                api.printer.printNode(source);
-            }
-            catch {
-                errors.printCrashed++;
-                continue;
-            }
 
-            try {
-                clone = getSynthesizedDeepClone(source);
-            }
-            catch {
-                errors.cloneCrashed++;
-                continue;
-            }
+    test("Parse-clone-emit roundtrip", () => {
+        const tsSource = fileURLToPath(new URL("../../../../tsc/testdata/fixtures", import.meta.url).toString());
+        using api = new API({
+            cwd: tsSource,
+        });
+        const target = {
+            cloneCrashed: 0,
+            printCrashed: 0,
+            clonePrintCrashed: 0,
+        };
+        const errors = { ...target };
+
+        for (const tsconfig of globSync("**/tsconfig.json", { cwd: tsSource })) {
+            const snapshot = api.createSnapshot({ openProject: resolve(tsSource, tsconfig) });
+            const project = snapshot.getConfiguredProject(tsconfig);
+            assert(project);
+            for (const file of project.rootFiles) {
+                const source = project.program.getSourceFile(file);
+                assert(source);
+                let clone: typeof source;
+
+                try {
+                    api.printer.printNode(source);
+                }
+                catch {
+                    errors.printCrashed++;
+                    continue;
+                }
 
-            try {
-                api.printer.printNode(clone);
-            }
-            catch {
-                errors.clonePrintCrashed++;
-                continue;
+                try {
+                    clone = getSynthesizedDeepClone(source);
+                }
+                catch {
+                    errors.cloneCrashed++;
+                    continue;
+                }
+
+                try {
+                    api.printer.printNode(clone);
+                }
+                catch {
+                    errors.clonePrintCrashed++;
+                    continue;
+                }
             }
         }
-    }
 
-    assert.deepEqual(errors, target);
+        assert.deepEqual(errors, target);
+    });
 });
 
-describe("Program - diagnostics", () => {
+describe("Program - diagnostics", { concurrency }, () => {
     test("getSyntacticDiagnostics", () => {
         const source = `const x: = 1;`;
         using api = spawnAPI({
@@ -6898,7 +6905,7 @@ describe("Program - diagnostics", () => {
     });
 });
 
-describe("getDefaultProjectForFile", () => {
+describe("getDefaultProjectForFile", { concurrency }, () => {
     test("snapshot opens reject unreadable virtual files without panicking", () => {
         const fileName = "/src/App.vue.ts";
         const source = `export const component = 1;`;
@@ -7080,7 +7087,7 @@ describe("getDefaultProjectForFile", () => {
     });
 });
 
-describe("Program - emit", () => {
+describe("Program - emit", { concurrency }, () => {
     const files = {
         "/tsconfig.json": `{
                 "compilerOptions": {
@@ -7321,7 +7328,7 @@ describe("Program - emit", () => {
     });
 });
 
-describe("Timing", () => {
+describe("Timing", { concurrency }, () => {
     test("collects combined client, server, and transport timing info", () => {
         using api = new API({
             cwd: fileURLToPath(new URL("../../../../", import.meta.url).toString()),
@@ -7457,7 +7464,7 @@ describe("Timing", () => {
     });
 });
 
-describe("runWithTemporaryFileUpdate", () => {
+describe("runWithTemporaryFileUpdate", { concurrency }, () => {
     test("temporary file update is visible in the callback and reverted afterward", () => {
         using api = spawnAPI({
             "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
diff --git a/packages/typescript/test/sync/ast.test.ts b/packages/typescript/test/sync/ast.test.ts
index f5284f5b223e7..fad091bbb7732 100644
--- a/packages/typescript/test/sync/ast.test.ts
+++ b/packages/typescript/test/sync/ast.test.ts
@@ -56,8 +56,11 @@ import {
     test,
 } from "node:test";
 import { fileURLToPath } from "node:url";
+import { areTestsFiltered } from "../testUtils.ts";
 import { runBenchmarks } from "./ast.bench.ts";
 
+const concurrency = areTestsFiltered();
+
 // ---------------------------------------------------------------------------
 // Helpers
 // ---------------------------------------------------------------------------
@@ -71,7 +74,7 @@ function collectKinds(node: Node): SyntaxKind[] {
     return kinds;
 }
 
-describe("NodeObject + childrenIter", () => {
+describe("NodeObject + childrenIter", { concurrency }, () => {
     test("skips absent optional child lists", () => {
         const node = createMissingDeclaration();
         assert.deepStrictEqual([...node.childrenIter()], []);
@@ -86,7 +89,7 @@ test("Benchmarks", async () => {
 // cloneNode
 // ---------------------------------------------------------------------------
 
-describe("cloneNode", () => {
+describe("cloneNode", { concurrency }, () => {
     test("clones an identifier", () => {
         const id = createIdentifier("hello");
         const clone = cloneNode(id);
@@ -156,7 +159,7 @@ describe("cloneNode", () => {
 // visitNode / visitNodes
 // ---------------------------------------------------------------------------
 
-describe("visitNode", () => {
+describe("visitNode", { concurrency }, () => {
     test("returns undefined for undefined input", () => {
         const nothing: Node | undefined = undefined;
         const result = visitNode(nothing, () => undefined);
@@ -177,7 +180,7 @@ describe("visitNode", () => {
     });
 });
 
-describe("visitNodes", () => {
+describe("visitNodes", { concurrency }, () => {
     test("returns undefined for undefined input", () => {
         const nothing: NodeArray<Node> | undefined = undefined;
         const result = visitNodes(nothing, () => undefined);
@@ -222,7 +225,7 @@ describe("visitNodes", () => {
 // visitEachChild
 // ---------------------------------------------------------------------------
 
-describe("visitEachChild", () => {
+describe("visitEachChild", { concurrency }, () => {
     test("returns same node if nothing changed (identity visitor)", () => {
         const left = createIdentifier("a");
         const right = createIdentifier("b");
@@ -307,7 +310,7 @@ describe("visitEachChild", () => {
 // getSynthesizedDeepClone
 // ---------------------------------------------------------------------------
 
-describe("getSynthesizedDeepClone", () => {
+describe("getSynthesizedDeepClone", { concurrency }, () => {
     test("deeply clones identifier", () => {
         const id = createIdentifier("hello");
         const clone = getSynthesizedDeepClone(id);
@@ -440,7 +443,7 @@ describe("getSynthesizedDeepClone", () => {
 // getSynthesizedDeepClones (NodeArray)
 // ---------------------------------------------------------------------------
 
-describe("getSynthesizedDeepClones", () => {
+describe("getSynthesizedDeepClones", { concurrency }, () => {
     test("deeply clones a NodeArray", () => {
         const a = createIdentifier("a");
         const b = createIdentifier("b");
@@ -475,7 +478,7 @@ describe("getSynthesizedDeepClones", () => {
 // Type-only import use sites
 // ---------------------------------------------------------------------------
 
-describe("isValidTypeOnlyAliasUseSite", () => {
+describe("isValidTypeOnlyAliasUseSite", { concurrency }, () => {
     test("classifies syntactic type-only import use sites", () => {
         const source = `
 type TypeUse = TypeOnlyName;
@@ -527,7 +530,7 @@ class JSDocAugmentsUse {}
 // Integration: visitor transformation
 // ---------------------------------------------------------------------------
 
-describe("visitor transformation", () => {
+describe("visitor transformation", { concurrency }, () => {
     test("rename all identifiers via recursive visitor", () => {
         const a = createIdentifier("oldName");
         const b = createIdentifier("oldName");
@@ -602,7 +605,7 @@ function getRemoteSourceFile(api: API, configPath: string, filePath: string) {
     return getRemoteSourceFileAndChecker(api, configPath, filePath)[0];
 }
 
-describe("RemoteNode + cloneNode", () => {
+describe("RemoteNode + cloneNode", { concurrency }, () => {
     test("does not read a sibling as an invalid JSDoc link name", () => {
         const api = spawnAPI({
             "/tsconfig.json": "{}",
@@ -718,7 +721,7 @@ interface I extends Parent<boolean> {}
     });
 });
 
-describe("RemoteNode + visitEachChild", () => {
+describe("RemoteNode + visitEachChild", { concurrency }, () => {
     test("identity visitor returns same remote node", () => {
         const api = spawnAPI();
         try {
@@ -760,7 +763,7 @@ describe("RemoteNode + visitEachChild", () => {
     });
 });
 
-describe("RemoteNodeList inherited array methods", () => {
+describe("RemoteNodeList inherited array methods", { concurrency }, () => {
     test("filter/map/slice return plain arrays without throwing", () => {
         const api = spawnAPI();
         try {
@@ -793,7 +796,7 @@ describe("RemoteNodeList inherited array methods", () => {
     });
 });
 
-describe("RemoteNode + getSynthesizedDeepClone", () => {
+describe("RemoteNode + getSynthesizedDeepClone", { concurrency }, () => {
     test("deep clones a remote import declaration", () => {
         const api = spawnAPI();
         try {
@@ -907,7 +910,7 @@ function assertGetterInvariants(node: Node, sf: SourceFile) {
     });
 }
 
-describe("RemoteNode + position/text getters", () => {
+describe("RemoteNode + position/text getters", { concurrency }, () => {
     const source = "/* lead */ const value = 123;";
     const files = {
         "/tsconfig.json": "{}",
@@ -1060,7 +1063,7 @@ describe("RemoteNode + position/text getters", () => {
 // RemoteNode: child/token getters
 // ---------------------------------------------------------------------------
 
-describe("RemoteNode + child/token getters", () => {
+describe("RemoteNode + child/token getters", { concurrency }, () => {
     function withFirstStatement(source: string, fn: (stmt: Node, sf: SourceFile) => void) {
         const api = spawnAPI({ "/tsconfig.json": "{}", "/src/children.ts": source });
         try {
diff --git a/packages/typescript/test/sync/astnav.test.ts b/packages/typescript/test/sync/astnav.test.ts
index 3747e638d013b..206bc9936a5d0 100644
--- a/packages/typescript/test/sync/astnav.test.ts
+++ b/packages/typescript/test/sync/astnav.test.ts
@@ -29,6 +29,7 @@ import {
     test,
 } from "node:test";
 import { fileURLToPath } from "node:url";
+import { areTestsFiltered } from "../testUtils.ts";
 
 // ---------------------------------------------------------------------------
 // Go JSON baseline format
@@ -81,7 +82,7 @@ const baselineDir = resolve(repoRoot, "tsc/testdata/baselines/reference/astnav")
 // Tests
 // ---------------------------------------------------------------------------
 
-describe("astnav", () => {
+describe("astnav", { concurrency: areTestsFiltered() }, () => {
     let fileText: string;
 
     try {
diff --git a/packages/typescript/test/testUtils.ts b/packages/typescript/test/testUtils.ts
new file mode 100644
index 0000000000000..3760c6ed098b7
--- /dev/null
+++ b/packages/typescript/test/testUtils.ts
@@ -0,0 +1,3 @@
+export function areTestsFiltered(): boolean {
+    return process.execArgv.some(arg => arg === "--test-name-pattern" || arg.startsWith("--test-name-pattern="));
+}
diff --git a/packages/typescript/test/wtf8.test.ts b/packages/typescript/test/wtf8.test.ts
index 60cc878b0fd52..01998913edade 100644
--- a/packages/typescript/test/wtf8.test.ts
+++ b/packages/typescript/test/wtf8.test.ts
@@ -11,8 +11,11 @@ import {
     encodeWtf8,
     Wtf8Decoder,
 } from "../src/api/node/wtf8.ts";
+import { areTestsFiltered } from "./testUtils.ts";
 
-describe("encodeWtf8", () => {
+const concurrency = areTestsFiltered();
+
+describe("encodeWtf8", { concurrency }, () => {
     test("matches UTF-8 for scalar values", () => {
         assert.deepEqual(encodeWtf8("hello 🦀"), new TextEncoder().encode("hello 🦀"));
     });
@@ -48,7 +51,7 @@ describe("encodeWtf8", () => {
     });
 });
 
-describe("Wtf8Decoder", () => {
+describe("Wtf8Decoder", { concurrency }, () => {
     test("decodes standard UTF-8", () => {
         const decoder = new Wtf8Decoder();
         assert.strictEqual(decoder.decode(new TextEncoder().encode("hello 🦀")), "hello 🦀");
@@ -86,7 +89,7 @@ describe("Wtf8Decoder", () => {
     });
 });
 
-describe("Msgpack strings", () => {
+describe("Msgpack strings", { concurrency }, () => {
     test("round-trip lone surrogates", () => {
         const text = `high:${String.fromCharCode(0xD800)} low:${String.fromCharCode(0xDFFF)}`;
         const writer = new MsgpackWriter();
diff --git a/packages/vscode-typescript/test/contentMapperContributions.test.ts b/packages/vscode-typescript/test/contentMapperContributions.test.ts
index 74c4dd07116f9..60db78fb9d403 100644
--- a/packages/vscode-typescript/test/contentMapperContributions.test.ts
+++ b/packages/vscode-typescript/test/contentMapperContributions.test.ts
@@ -1,5 +1,5 @@
 import assert from "node:assert/strict";
-import test from "node:test";
+import test, { describe } from "node:test";
 import {
     type ContentMapperContribution,
     documentMatchesContentMapperContributions,
@@ -20,35 +20,37 @@ const documentedContribution = {
     },
 } satisfies ContentMapperContribution;
 
-test("content mapper extensions match document paths case-insensitively", () => {
-    const registrations = new Map<string, readonly ContentMapperContribution[]>([[
-        "publisher.extension",
-        [{ extensions: [".vue"] }],
-    ]]);
-    const document = { uri: { path: "/workspace/Component.VUE" } };
+describe("content mapper contributions", { concurrency: true }, () => {
+    test("content mapper extensions match document paths case-insensitively", () => {
+        const registrations = new Map<string, readonly ContentMapperContribution[]>([[
+            "publisher.extension",
+            [{ extensions: [".vue"] }],
+        ]]);
+        const document = { uri: { path: "/workspace/Component.VUE" } };
 
-    assert.equal(documentMatchesContentMapperContributions(document, registrations), true);
-});
+        assert.equal(documentMatchesContentMapperContributions(document, registrations), true);
+    });
 
-test("serializes the documented inferred project contribution", () => {
-    const registrations = new Map<string, readonly ContentMapperContribution[]>([[
-        "publisher.extension",
-        [documentedContribution],
-    ]]);
+    test("serializes the documented inferred project contribution", () => {
+        const registrations = new Map<string, readonly ContentMapperContribution[]>([[
+            "publisher.extension",
+            [documentedContribution],
+        ]]);
 
-    assert.deepEqual(serializeContentMapperContributions(registrations), [{
-        contributorId: "publisher.extension",
-        extensions: [".vue"],
-        inferredProjectContribution: {
-            options: { strictTemplates: true },
-            manifest: {
-                name: "Vue mapper",
-                version: "1.2.3",
-                exec: ["node", "mapper.js"],
-                cwd: undefined,
-                compilerOptions: ["strict"],
-                dynamicConfig: true,
+        assert.deepEqual(serializeContentMapperContributions(registrations), [{
+            contributorId: "publisher.extension",
+            extensions: [".vue"],
+            inferredProjectContribution: {
+                options: { strictTemplates: true },
+                manifest: {
+                    name: "Vue mapper",
+                    version: "1.2.3",
+                    exec: ["node", "mapper.js"],
+                    cwd: undefined,
+                    compilerOptions: ["strict"],
+                    dynamicConfig: true,
+                },
             },
-        },
-    }]);
+        }]);
+    });
 });
diff --git a/packages/vscode-typescript/test/tsdkPackage.test.ts b/packages/vscode-typescript/test/tsdkPackage.test.ts
index ab633ed55fc98..5a96fa1246110 100644
--- a/packages/vscode-typescript/test/tsdkPackage.test.ts
+++ b/packages/vscode-typescript/test/tsdkPackage.test.ts
@@ -2,7 +2,7 @@ import assert from "node:assert/strict";
 import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
-import test from "node:test";
+import test, { describe } from "node:test";
 import { resolvePackageExecutable } from "../src/tsdkPackage";
 
 const platformPackage = `typescript-${process.platform}-${process.arch}`;
@@ -59,69 +59,71 @@ function testResolution(name: string, resolutionCase: ResolutionCase): void {
     });
 }
 
-testResolution("resolves the TypeScript 7 package", {
-    packagePath: "node_modules/typescript",
-    platformPath: `node_modules/@typescript/${platformPackage}`,
-});
+describe("tsdk package resolution", { concurrency: true }, () => {
+    testResolution("resolves the TypeScript 7 package", {
+        packagePath: "node_modules/typescript",
+        platformPath: `node_modules/@typescript/${platformPackage}`,
+    });
 
-testResolution("resolves the TypeScript 7 scoped alias recommended in the 7.0 release post", {
-    packagePath: "node_modules/@typescript/native",
-    platformPath: `node_modules/@typescript/${platformPackage}`,
-});
+    testResolution("resolves the TypeScript 7 scoped alias recommended in the 7.0 release post", {
+        packagePath: "node_modules/@typescript/native",
+        platformPath: `node_modules/@typescript/${platformPackage}`,
+    });
 
-testResolution("resolves an unscoped TypeScript 7 alias", {
-    packagePath: "node_modules/typescript-next",
-    platformPath: `node_modules/@typescript/${platformPackage}`,
-});
+    testResolution("resolves an unscoped TypeScript 7 alias", {
+        packagePath: "node_modules/typescript-next",
+        platformPath: `node_modules/@typescript/${platformPackage}`,
+    });
 
-const nativePlatformPackage = `native-preview-${process.platform}-${process.arch}`;
-testResolution("resolves the native-preview package", {
-    packagePath: "node_modules/@typescript/typescript",
-    platformPath: `node_modules/@typescript/${nativePlatformPackage}`,
-    platformPackage: nativePlatformPackage,
-    exeName: nativeExeName,
-});
+    const nativePlatformPackage = `native-preview-${process.platform}-${process.arch}`;
+    testResolution("resolves the native-preview package", {
+        packagePath: "node_modules/@typescript/typescript",
+        platformPath: `node_modules/@typescript/${nativePlatformPackage}`,
+        platformPackage: nativePlatformPackage,
+        exeName: nativeExeName,
+    });
 
-testResolution("resolves a non-hoisted platform package", {
-    packagePath: "node_modules/@typescript/native",
-    platformPath: `node_modules/@typescript/native/node_modules/@typescript/${platformPackage}`,
-});
+    testResolution("resolves a non-hoisted platform package", {
+        packagePath: "node_modules/@typescript/native",
+        platformPath: `node_modules/@typescript/native/node_modules/@typescript/${platformPackage}`,
+    });
 
-testResolution("resolves a platform package hoisted above a workspace", {
-    packagePath: "packages/app/node_modules/@typescript/native",
-    platformPath: `node_modules/@typescript/${platformPackage}`,
-});
+    testResolution("resolves a platform package hoisted above a workspace", {
+        packagePath: "packages/app/node_modules/@typescript/native",
+        platformPath: `node_modules/@typescript/${platformPackage}`,
+    });
 
-const pnpmStore = "node_modules/.pnpm/typescript@7.0.2/node_modules";
-testResolution("resolves a pnpm virtual-store package", {
-    packagePath: `${pnpmStore}/typescript`,
-    platformPath: `node_modules/.pnpm/@typescript+${platformPackage}@7.0.2/node_modules/@typescript/${platformPackage}`,
-    dependencyLink: `${pnpmStore}/@typescript/${platformPackage}`,
-});
+    const pnpmStore = "node_modules/.pnpm/typescript@7.0.2/node_modules";
+    testResolution("resolves a pnpm virtual-store package", {
+        packagePath: `${pnpmStore}/typescript`,
+        platformPath: `node_modules/.pnpm/@typescript+${platformPackage}@7.0.2/node_modules/@typescript/${platformPackage}`,
+        dependencyLink: `${pnpmStore}/@typescript/${platformPackage}`,
+    });
 
-const nativePreviewVersion = "7.0.0-dev.20260707.2";
-const pnpmNativePreviewStore = `node_modules/.pnpm/@typescript+native-preview@${nativePreviewVersion}/node_modules`;
-testResolution("resolves a pnpm native-preview virtual-store package", {
-    packagePath: `${pnpmNativePreviewStore}/@typescript/typescript`,
-    platformPath: `node_modules/.pnpm/@typescript+${nativePlatformPackage}@${nativePreviewVersion}/node_modules/@typescript/${nativePlatformPackage}`,
-    dependencyLink: `${pnpmNativePreviewStore}/@typescript/${nativePlatformPackage}`,
-    platformPackage: nativePlatformPackage,
-    exeName: nativeExeName,
-});
+    const nativePreviewVersion = "7.0.0-dev.20260707.2";
+    const pnpmNativePreviewStore = `node_modules/.pnpm/@typescript+native-preview@${nativePreviewVersion}/node_modules`;
+    testResolution("resolves a pnpm native-preview virtual-store package", {
+        packagePath: `${pnpmNativePreviewStore}/@typescript/typescript`,
+        platformPath: `node_modules/.pnpm/@typescript+${nativePlatformPackage}@${nativePreviewVersion}/node_modules/@typescript/${nativePlatformPackage}`,
+        dependencyLink: `${pnpmNativePreviewStore}/@typescript/${nativePlatformPackage}`,
+        platformPackage: nativePlatformPackage,
+        exeName: nativeExeName,
+    });
 
-const npmStore = "node_modules/.store/typescript@7.0.2-hash/node_modules";
-testResolution("resolves an npm linked-store package", {
-    packagePath: `${npmStore}/typescript`,
-    platformPath: `node_modules/.store/@typescript+${platformPackage}@7.0.2-hash/node_modules/@typescript/${platformPackage}`,
-    dependencyLink: `${npmStore}/@typescript/${platformPackage}`,
-});
+    const npmStore = "node_modules/.store/typescript@7.0.2-hash/node_modules";
+    testResolution("resolves an npm linked-store package", {
+        packagePath: `${npmStore}/typescript`,
+        platformPath: `node_modules/.store/@typescript+${platformPackage}@7.0.2-hash/node_modules/@typescript/${platformPackage}`,
+        dependencyLink: `${npmStore}/@typescript/${platformPackage}`,
+    });
 
-test("throws when the platform package is missing", t => {
-    const root = createFixture(t);
-    const packageJsonPath = createPackage(root, "node_modules/@typescript/native");
+    test("throws when the platform package is missing", t => {
+        const root = createFixture(t);
+        const packageJsonPath = createPackage(root, "node_modules/@typescript/native");
 
-    assert.throws(
-        () => resolvePackageExecutable(packageJsonPath, platformPackage, exeName),
-        { code: "MODULE_NOT_FOUND" },
-    );
+        assert.throws(
+            () => resolvePackageExecutable(packageJsonPath, platformPackage, exeName),
+            { code: "MODULE_NOT_FOUND" },
+        );
+    });
 });