DRAFT: Rust: negotiate 2026-07-28, call the Messages API directly, and bump rmcp to 3.4

[a-akimov]

Motivation and Context

Three problems in the Rust examples, all found while reviewing #200.

1. The client could not reach protocol 2026-07-28. rmcp's serve only performs the legacy
initialize handshake, and ProtocolVersion::LATEST is still 2025-11-25 even in rmcp 3.4, so bumping
the SDK does not fix this on its own. Under 2025-11-25 a server cannot offer this era's features, so the
client could not call any tool with an array-rooted outputSchema — including get_alerts, which every
weather server in this repo declares that way. Against the Python server it failed at tools/list;
against the TypeScript server and the test mock it failed at tools/call.

2. The genai crate could not express three things the client needs. The first was raised by Copilot
on #200:

• Signed thinking blocks. genai 0.4.2 keeps Anthropic's thinking / redacted_thinking blocks out of
  ChatResponse.content and does not retain the signature, so the follow-up request after a thinking turn
  could not echo them back as the API requires.
• Tool errors. ToolResponse has no is_error field, so a failed tool call reached the model looking
  like a successful one.
• A configurable endpoint. It reads no environment variable for the API base URL, which is why the Rust
  client was the one client excluded from the tool-loop smoke test added in Fix the tool-use loop in the Go, Rust, and Ruby clients and add a tool-loop smoke test #200.

I checked whether a genai bump would solve any of this. It does not: as of 0.6.5 the Anthropic adapter
still drops ThoughtSignature parts when sending, and ToolResponse still has only call_id and
content.

3. Both examples were several rmcp releases behind, 3.1.2 against 3.4.0, and each carried tracing and
tracing-subscriber dependencies that no source file references.

What changed

mcp-client-rust

• Negotiates with serve_with_lifecycle and ClientLifecycleMode::Auto: one server/discover probe for
  2026-07-28, falling back to the 2025-11-25 initialize handshake. This is rmcp's equivalent of the
  auto mode the Python and TypeScript clients already use. The negotiated revision is printed on
  connect, as the Python client does.
• Sends its own clientInfo via ClientConfig. It previously identified itself as rmcp 3.4.0.
• Replaces genai with a direct reqwest call to the Messages API. There is no official Anthropic SDK
  for Rust, and calling the endpoint directly is what makes the other two fixes possible: an assistant
  turn is kept as raw content blocks and fed back byte-for-byte, signatures included, and a tool result
  carries a real is_error. ANTHROPIC_BASE_URL now overrides the endpoint, matching the official SDKs.
• Bumps rmcp to 3.4, jsonschema to 0.56, reqwest to 0.13, tokio to 1.53, and drops the tracing,
  tracing-subscriber and server-feature dependencies, none of which were referenced. Dropping genai
  takes the lockfile from 312 crates to 256.
• Prints the full error chain on a failed query ({:#}), so a wrapped cause is no longer hidden.

weather-server-rust

• Bumps rmcp to 3.4, tokio to 1.53, reqwest to 0.13, and drops the unreferenced tracing and
  tracing-subscriber dependencies.
• Moves off the ServerInfo alias, which rmcp 3.4 deprecates in favour of ServerConfig.

Tests

• Adds the Rust client to the tool-loop smoke test, so all five clients are now covered.
• Corrects a stale comment above those tests: the helper sends three queries, not one.

Note that rmcp 3.4's MSRV is 1.88, which is exactly the "Rust 1.88+" the READMEs already asked for. That
requirement is now justified rather than arbitrary.

How Has This Been Tested?

• The Rust client passes the tool-loop smoke test, all 91 checks, including the two checks that assert
  thinking blocks come back with their signature and the one that requires is_error. It could not have
  passed any of those before this change.
• It now reports Connected over protocol 2026-07-28 against the Python, TypeScript and Rust weather
  servers. The Python server previously rejected its tools/list outright.
• ./tests/smoke-test.sh passes every step my machine has a toolchain for, which is all of them except
  Go and Ruby; those two were exercised in golang:1.25 and ruby:3.4 containers and are unaffected by
  this PR.
• cargo build is clean, no warnings, for both examples in debug and release.

Breaking Changes

None for anyone running the examples. The client's command line, output and no-key behaviour are
unchanged. Worth flagging for reviewers: this swaps the Rust client's LLM library, so the diff in
src/main.rs is larger than a dependency bump would suggest.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

AI assistance

The investigation, code changes and this description were prepared with Claude Code and
reviewed by me.

[Copilot]

Copilot review overview

🔵 Needs a closer look

The direct LLM protocol implementation and lifecycle negotiation warrant final human validation despite strong smoke-test coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Updates the Rust MCP examples for modern protocol negotiation and direct Anthropic Messages API integration.

Changes:

• Adds MCP 2026-07-28 negotiation with legacy fallback.
• Replaces genai with reqwest, preserving thinking blocks and tool errors.
• Upgrades Rust dependencies and adds tool-loop smoke coverage.

File	Description
mcp-client-rust/​src/​main.rs	Implements negotiation and direct Messages API calls.
mcp-client-rust/​Cargo.toml	Updates and simplifies dependencies.
mcp-client-rust/​Cargo.lock	Records client dependency changes.
mcp-client-rust/​README.md	Documents API access and negotiation.
weather-server-rust/​src/​main.rs	Migrates to ServerConfig.
weather-server-rust/​Cargo.toml	Upgrades and removes unused dependencies.
weather-server-rust/​Cargo.lock	Records server dependency changes.
tests/​smoke-test.sh	Adds Rust tool-loop testing.
tests/​README.md	Documents complete client coverage.

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