PrefectHQ/fastmcp · error · ImportError

FastMCP server support is not installed. Install `fastmcp` o

Error message

FastMCP server support is not installed. Install `fastmcp` or `fastmcp-slim[server]`.

What it means

`fastmcp.server` is a lazy loader: accessing `Context` triggers a deferred import of fastmcp.server.context, which only exists when full server dependencies are installed. In a slim install (fastmcp-slim without the [server] extra) the import fails with ImportError, which is re-raised as ImportError with the message 'FastMCP server support is not installed. Install `fastmcp` or `fastmcp-slim[server]`.'

Source

Thrown at fastmcp_slim/fastmcp/server/__init__.py:19

import importlib
from typing import TYPE_CHECKING

from fastmcp import _install_hints

if TYPE_CHECKING:
    from .context import Context as Context
    from .server import FastMCP as FastMCP
    from .server import create_proxy as create_proxy


def __getattr__(name: str) -> object:
    if name in {"context", "dependencies"}:
        return importlib.import_module(f"fastmcp.server.{name}")
    if name == "Context":
        try:
            from .context import Context
        except ImportError as exc:
            raise ImportError(_install_hints.SERVER_SUPPORT) from exc

        return Context
    if name in {"FastMCP", "create_proxy"}:
        try:
            from .server import FastMCP, create_proxy
        except ImportError as exc:
            raise ImportError(_install_hints.SERVER_SUPPORT) from exc

        return FastMCP if name == "FastMCP" else create_proxy
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")


__all__ = ["Context", "FastMCP", "create_proxy"]

View on GitHub (pinned to 1f02114297)

Solutions

1. Install server support: `pip install fastmcp-slim[server]` (or plain `pip install fastmcp`).
2. Guard server-only imports so client-only code doesn't touch fastmcp.server attributes.
3. Pin the full `fastmcp` package in server-related environments (requirements/pyproject).
4. If you maintain the slim package, verify the optional extra name in install hints matches your install command.

Example fix

// before
pip install fastmcp-slim
from fastmcp.server import Context  # ImportError

// after
pip install "fastmcp-slim[server]"
from fastmcp.server import Context

import importlib.util
if importlib.util.find_spec('fastmcp.server.context') is None:
    raise ImportError("Install server support: pip install 'fastmcp-slim[server]'")

try:
    from fastmcp.server import Context
except ImportError as e:
    raise SystemExit(f'server deps missing: {e}')

When it happens

Trigger: In a slim installation, execute `from fastmcp.server import Context` or `import fastmcp.server; fastmcp.server.Context` — the __getattr__ path at fastmcp_slim/fastmcp/server/__init__.py:19 converts the inner ImportError into the install-hint error.

Common situations: A project installed fastmcp-slim for client usage but shared code (or a tutorial) imports Context/FastMCP from fastmcp.server; CI uses the slim package while local dev uses full fastmcp; a dependency was slimmed down to shrink installs and server imports broke.

Understand the failure class

Background: "X is not installed. Please install it with pip install Y": missing optional dependency errors — ImportError/ValueError raised when a library's optional extra was never installed — this error's family across 22 libraries.

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AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/7c52005c364a2f58. Report an issue: GitHub.