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Title: Don't block the event loop on sync resource and prompt functions by maxisbey · Pull Request #2380 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: Don't block the event loop on sync resource and prompt functions by maxisbey · Pull Request #2380 · modelcontextprotocol/python-sdk

X Title: Don't block the event loop on sync resource and prompt functions by maxisbey · Pull Request #2380 · modelcontextprotocol/python-sdk

Description: Extends the #1909 fix to resources and prompts — sync @mcp.resource and @mcp.prompt handlers now run in worker threads instead of blocking the event loop. Motivation and Context #1909 fixed tools by routing sync functions through anyio.to_thread.run_sync, but the same blocking pattern existed in three other places: FunctionResource.read() ResourceTemplate.create_resource() Prompt.render() All three called self.fn(...) directly and checked inspect.iscoroutine(result) afterward. A blocking sync handler (file I/O, HTTP request, CPU-bound work) would freeze the entire event loop. This applies the same fix: check inspect.iscoroutinefunction(self.fn) up front and dispatch sync functions to a worker thread. Related: #1646, #1839 How Has This Been Tested? Added thread-identity regression tests for each of the three call sites. Each test captures threading.get_ident() inside a sync handler and asserts it differs from the event loop's thread. Verified that pydantic.validate_call (which wraps the stored self.fn in templates and prompts) preserves async-ness — inspect.iscoroutinefunction(validate_call(async_fn)) returns True, so the dispatch check works correctly on the wrapped function. All 39 tests in the affected test files pass; pyright, ruff, and strict-no-cover are clean. Breaking Changes None. Sync handlers that were previously starving the event loop now run concurrently. 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 Additional context The previous implementation happened to support callable objects with async __call__ (by checking iscoroutine(result) after calling). That edge case is not preserved here — matching the approach taken in #1909, which relies on _is_async_callable being evaluated at registration time for tools. Resources and prompts have no equivalent pre-computed field; if that edge case matters, it's worth a separate discussion. AI Disclaimer

Open Graph Description: Extends the #1909 fix to resources and prompts — sync @mcp.resource and @mcp.prompt handlers now run in worker threads instead of blocking the event loop. Motivation and Context #1909 fixed tools b...

X Description: Extends the #1909 fix to resources and prompts — sync @mcp.resource and @mcp.prompt handlers now run in worker threads instead of blocking the event loop. Motivation and Context #1909 fixed tools b...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2380

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