Max Isbey
ec5b2258c9
Every import path now pays only for what it uses, with no public API added or removed: - The protocol models (mcp.types / mcp_types, incl. the JSON-RPC envelopes and the generated per-version wire packages) build their pydantic validators on first use instead of at import (defer_build), through one shared private base class. First-use builds are serialised behind a single process-wide lock, since released pydantic does not make concurrent first use of a deferred model thread-safe; this also fixes a pre-existing concurrent-first-use failure that reproduces on main. - `import mcp` binds the client/server names lazily on first attribute access (PEP 562) instead of importing both stacks eagerly, and the client no longer imports the server, so client entry points stop loading the server, the web stack, httpx2 and cryptography. - The web application stack (starlette's app machinery, sse_starlette, uvicorn) loads with the app builders that use it, and each protocol version's wire package loads on the first message parsed for that version rather than both loading at import. On the fresh-interpreter harness `import mcp` is ~0.4x of v1 (main is ~1.6x), the client entry points ~0.6x of v1, `import mcp.server.mcpserver` ~0.7x, and time-to-ready / stdio cold start land at parity with v1. RSS after `import mcp` is 19 MiB (v1 43.5, main 57). Steady-state per-call latency is unchanged. Observable-but-incidental differences (removed incidental namespace bindings, deeper submodules no longer imported as a side effect of a bare `import mcp`, get_type_hints needing localns= for a documented set of callables, pre-first-use introspection) are catalogued in docs/migration.md; ratchet tests pin the import footprints and the concurrent-first-use safety.
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Advanced
Everything an ordinary server or client needs has a topical home in the sections above.
This section is the escape hatches you reach for when MCPServer's convenience
layer is in the way:
- The low-level Server: the class
MCPServeris built on. Hand-written schemas,on_*handlers, nothing checked for you, and custom JSON-RPC methods of your own. - Pagination and Middleware: two things you
can only do on the low-level
Server. - Extensions and MCP Apps: the protocol's extension surface. Compose extension packages into a server, or write your own.
- Startup cost: what an import loads, and the one-time bills paid on first use instead — for when you are measuring cold start.
A few things you might reasonably look for here live where you'd actually use them instead:
- Authorization is under Running your server because you protect a server where you deploy it.
- OAuth, identity assertion, connecting to multiple servers, and the response cache are all under Clients.
- Multi-round-trip requests and Subscriptions are under Inside your handler because both are things a handler does.
- URI templates is under Servers, next to Resources.
- Protocol versions and Deprecated features each have their own top-level page.
If you're not sure whether you need this section, you don't.