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.
Max Isbey
ec5b2258c9