diff --git a/.changeset/void-ipc-acks.md b/.changeset/void-ipc-acks.md
new file mode 100644
index 00000000000..85caba20d35
--- /dev/null
+++ b/.changeset/void-ipc-acks.md
@@ -0,0 +1,5 @@
+---
+"@trigger.dev/core": patch
+---
+
+Fix a ~6-second delay between a task finishing and its run completing (and a ~31-second delay when cancelling a run) in projects that use zod 4.4 or newer. Run cost and billed usage was not impacted by this issue.
diff --git a/packages/core/src/v3/zodIpc.ts b/packages/core/src/v3/zodIpc.ts
index ebd5a685df7..e2b5028b376 100644
--- a/packages/core/src/v3/zodIpc.ts
+++ b/packages/core/src/v3/zodIpc.ts
@@ -124,7 +124,9 @@ const Packet = discriminatedUnion("type", [
   }),
   z.object({
     type: z.literal("ACK"),
-    message: z.any(),
+    // Optional: a void callback acks with `message: undefined`, and `process.send`
+    // drops undefined keys when serializing, so the key is absent on arrival.
+    message: z.any().optional(),
     id: z.number(),
   }),
   z.object({
diff --git a/packages/core/test/fixtures/zodIpcCatalog.ts b/packages/core/test/fixtures/zodIpcCatalog.ts
new file mode 100644
index 00000000000..6f24288b049
--- /dev/null
+++ b/packages/core/test/fixtures/zodIpcCatalog.ts
@@ -0,0 +1,14 @@
+import { z } from "zod/v4";
+
+export const ParentToChild = {
+  FLUSH: {
+    message: z.object({ timeoutInMs: z.number() }),
+    callback: z.void(),
+  },
+  PING: {
+    message: z.object({ value: z.string() }),
+    callback: z.object({ echoed: z.string() }),
+  },
+};
+
+export const ChildToParent = {};
diff --git a/packages/core/test/fixtures/zodIpcChild.ts b/packages/core/test/fixtures/zodIpcChild.ts
new file mode 100644
index 00000000000..721b534ddb9
--- /dev/null
+++ b/packages/core/test/fixtures/zodIpcChild.ts
@@ -0,0 +1,13 @@
+import { ZodIpcConnection } from "../../src/v3/zodIpc.js";
+import { ChildToParent, ParentToChild } from "./zodIpcCatalog.js";
+
+// Forked by zodIpc.test.ts: answers the parent's messages over the real IPC channel.
+new ZodIpcConnection({
+  listenSchema: ParentToChild,
+  emitSchema: ChildToParent,
+  process,
+  handlers: {
+    FLUSH: async () => {},
+    PING: async ({ value }) => ({ echoed: value }),
+  },
+});
diff --git a/packages/core/test/zodIpc.test.ts b/packages/core/test/zodIpc.test.ts
new file mode 100644
index 00000000000..7b5d2669723
--- /dev/null
+++ b/packages/core/test/zodIpc.test.ts
@@ -0,0 +1,44 @@
+import { fork, type ChildProcess } from "node:child_process";
+import { fileURLToPath } from "node:url";
+import { afterEach, describe, expect, it } from "vitest";
+import { ZodIpcConnection } from "../src/v3/zodIpc.js";
+import { ChildToParent, ParentToChild } from "./fixtures/zodIpcCatalog.js";
+
+const childPath = fileURLToPath(new URL("./fixtures/zodIpcChild.ts", import.meta.url));
+
+let child: ChildProcess | undefined;
+
+afterEach(() => {
+  child?.kill();
+  child = undefined;
+});
+
+// Node's default IPC serialization is JSON, so an ack with `message: undefined`
+// arrives without the `message` key, exactly as it does between a worker and its task run process.
+function forkChild() {
+  child = fork(childPath, { execArgv: ["--import", "tsx"], stdio: "inherit" });
+
+  return new ZodIpcConnection({
+    listenSchema: ChildToParent,
+    emitSchema: ParentToChild,
+    process: child,
+  });
+}
+
+describe("ZodIpcConnection", () => {
+  it("resolves sendWithAck for a void callback", async () => {
+    const connection = forkChild();
+
+    await expect(connection.sendWithAck("FLUSH", { timeoutInMs: 1000 }, 2000)).resolves.toBe(
+      undefined
+    );
+  });
+
+  it("resolves sendWithAck with the callback payload", async () => {
+    const connection = forkChild();
+
+    await expect(connection.sendWithAck("PING", { value: "hello" }, 2000)).resolves.toEqual({
+      echoed: "hello",
+    });
+  });
+});