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* Fix the tool-use loop in the Go, Rust, and Ruby clients and add a tool-loop smoke test ## Motivation and Context Only the Python and TypeScript clients implemented a correct tool-use loop. The other three had bugs that break ordinary multi-step requests: - **Go, Rust, Ruby** did a single tool round and then called Claude once more *without* `tools`, so any request that needs a second tool call stalled. Go silently dropped `tool_use` blocks from that final response. - **Ruby** called Claude once per `tool_use` block, each time with a single `tool_result`. Parallel tool calls produced a malformed message sequence and an API 400. - **max_tokens** was 1000 in TypeScript and Ruby, 1024 in Go, and unset in Rust. Sonnet 5 thinks adaptively, so 1000 tokens is regularly exhausted mid-thought. Python already used 10000 with a comment explaining why. - **Ruby** never forwarded MCP `isError` as `is_error`, so the model could not tell a failed tool call from a successful one. Rust cannot set the flag either (see below). - **TypeScript** forwarded raw MCP content blocks to Anthropic. The two block shapes differ, so a non-text result would have failed. The existing smoke test never noticed because it only checks that each client calls `tools/list`. None of these bugs was reachable without an API key. ### What changed All five clients now run the same loop: call tools until Claude answers without one, send every `tool_result` of a turn in one user message, pass `tools` on every call, and stop after `MAX_TOOL_TURNS = 10`. `max_tokens` is 10000 everywhere, with the same comment. Python and TypeScript also keep the last response's text when the turn cap is hit; it used to be dropped. Rust keeps the `genai` crate. Its tool-response type has no `is_error` field, so an error result is marked with an `Error:` prefix in the text and the limitation is commented. ### New smoke test `tests/helpers/tool-loop-test.ts` starts a fake Anthropic Messages API on a loopback port, points a client at it through `ANTHROPIC_BASE_URL`, pipes in one query, and checks every request the client sends. It scripts two parallel tool calls, then a call to a tool the mock server lacks (an `isError` result), then a final answer, and asserts that tools are passed each time, `max_tokens` is 10000, all `tool_result`s arrive in one message with matching ids, and the error is forwarded. `smoke-test.sh` runs it for Python, TypeScript, Go, and Ruby. The Rust client is excluded for now: `genai` reads no endpoint variable, and the client negotiates protocol `2025-11-25`, under which the mock's array-rooted tool is refused at call time. Both will be addressed in a follow-up that brings all examples to current SDK versions. ## How Has This Been Tested? - `./tests/smoke-test.sh` passes for every example. - The new tool-loop test passes all 19 checks for Python, TypeScript, Go, and Ruby. - The Rust client was driven through the same scripted conversation with a temporary endpoint resolver (not committed) and passes. - As a negative check, the Ruby client from `main` was run through the new test. It fails 10 of 19 checks, on exactly the bugs listed above. - Go and Ruby were built and run in `golang:1.25` and `ruby:3.4` containers. ## Breaking Changes None. The clients' command lines, output format, and no-key behavior are unchanged. ## Types of changes - [x] 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) - [x] Documentation update ## Checklist - [x] I have read the [MCP Documentation](https://modelcontextprotocol.io) - [x] My code follows the repository's style guidelines - [x] New and existing tests pass locally - [x] I have added appropriate error handling - [x] I have added or updated documentation as needed ## Additional context Two pre-existing issues surfaced while testing and are left for a follow-up: - The Rust client (rmcp 3.1.2) negotiates `2025-11-25`. The TypeScript mock server and the Python weather server both reject its calls to array-rooted tools under that revision. Even current rmcp defaults to `2025-11-25`, so the client must request `2026-07-28` explicitly. - The Go SDK's tool-result helper sends content as a list of text blocks rather than a string. The API accepts both, so it is unchanged. AI assistance: the analysis, code changes, test helper, and this description were prepared partially manually, partially with Claude Code and reviewed by me. * Improvements * Address comments |
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An LLM-Powered Chatbot MCP Client written in Rust
An interactive chatbot MCP client: it launches an MCP server over stdio, lists its tools, and hands them to Claude, which can call them while answering your questions.
This example accompanies the Build an MCP client tutorial.
Prerequisites
- Rust 1.88+ and Cargo
- An Anthropic API key (optional — see below)
Setup
Export your API key, or put it in a .env file in this directory (the .env file is optional):
export ANTHROPIC_API_KEY=your-api-key-here
Run the client
Pass the command that starts the server — a binary or a script — after --:
cargo run -- ../weather-server-rust/target/release/weather
(Build the server first with cargo build --release in its directory.)
Type a question (for example, "What's the weather in Sacramento?") and Claude answers using the server's tools. Type quit to exit.
Without an ANTHROPIC_API_KEY, the client still connects, prints the server's tools, and exits — useful for verifying the MCP wiring without credentials.
Structured content
rmcp does not validate tool output, so this client compiles each declared outputSchema at connect time and checks results against it — the spec's client-side SHOULD. It uses the jsonschema crate, which rmcp's own documentation recommends.
The two channels go to different readers: content is forwarded to the model, while structured_content is used as data — the client counts the items it returns. See Structured Content.