fix(server): Server.__init__ compatibility with mcp>=1.12.0

[Daniyal0100101]

Describe your changes

Fix Server.__init__ compatibility with mcp>=1.12.0 (issue #322).

Root Cause

In mcp>=1.12.0, the Server.__init__ signature changed:

# mcp 1.12.0+
def __init__(
    self,
    name: str,
    version: str | None = None,           # was `description` in older versions
    instructions: str | None = None,
    ...
):

The old code Server(self.name, self.description) passed self.description as the second positional argument, which now maps to version instead of instructions. This caused:

TypeError: Server.__init__() takes 2 positional arguments but 3 were given

Fix

1. fastapi_mcp/server.py (line 144): Changed Server(self.name, self.description) to Server(name=self.name, instructions=self.description) — uses keyword arguments so the mapping is explicit and version-agnostic.

2. pyproject.toml: Pinned mcp dependency to >=1.12.0,<2.0.0 to prevent future breaking changes from mcp 2.x.

Testing

• All 117 existing tests pass (83.04% coverage)
• Manual verification: FastApiMCP instantiation, mount_http(), mount_sse() all work without TypeError
• Ruff, mypy, pre-commit all pass

Related

Closes #322

[K4bain]

Hi @Daniyal0100101 - since this PR has been open a while, I rebased your patch onto current main and verified it (69/69 unit tests pass against mcp 1.29.1): see #331. Your original diff was applied as-is with attribution in the commit message. Noted there: with mcp 2.x now released, the cap in this patch is the critical part - uncapped installs currently break at import time.

[Daniyal0100101]

Superseded by #331, which carries this patch forward on current main and verifies it against the current MCP 1.x release. Closing this one to avoid keeping duplicate PRs open.