feat: add lifespan context manager support #203
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Adds a context manager based lifespan API in mcp.server.lowlevel.server to manage server lifecycles in a type-safe way. This enables servers to: - Initialize resources on startup and clean them up on shutdown - Pass context data from startup to request handlers - Support async startup/shutdown operations
Adds support for the lifespan API to FastMCP server, enabling: - Simple setup with FastMCP constructor - Type-safe context passing to tools and handlers - Configuration via Settings class
Adds comprehensive tests for lifespan functionality: - Tests for both low-level Server and FastMCP classes - Coverage for startup, shutdown, and context access - Verifies context passing to request handlers
Add comprehensive documentation for lifespan support: - Add usage examples for both Server and FastMPC classes - Document startup/shutdown patterns - Show context access in tools and handlers - Clean up spacing in test files
Replace nested context managers with AsyncExitStack to ensure proper cleanup order during server shutdown and make the code more maintainable.
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jspahrsummers
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Feb 12, 2025
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| async with AsyncExitStack() as stack: | ||
| lifespan_context = await stack.enter_async_context(self.lifespan(self)) | ||
| session = await stack.enter_async_context( | ||
| ServerSession(read_stream, write_stream, initialization_options) | ||
| ) |
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Is this different from doing async with lifespan_context, session?
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The AsyncExitStack() version provides more granular control over multiple context managers. If an error occurs during the second enter_async_context, the first one will still be properly cleaned up.
Chatted more with Claude about it, we came to the. conclusion that AsyncExitStack is better here.
src/mcp/server/lowlevel/server.py
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| ) | ||
|
|
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| async def _handle_request( | ||
| self, | ||
| message: RequestResponder, | ||
| req: Any, | ||
| session: ServerSession, | ||
| lifespan_context: object, |
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Suggested change
| lifespan_context: object, | |
| lifespan_context: LifespanResultT, |
- Add proper generic parameter for lifespan context type - Update README with TypedDict example for strong typing - Fix context variable initialization in server - Improve property return type safety - Remove redundant documentation - Ensure compatibility with existing tests
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Summary
Test plan