PrefectHQ/fastmcp · error · ImportError

FastMCP CLI support is not installed. Install `fastmcp` or `

Error message

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

What it means

fastmcp/cli/__init__.py wraps the import of the CLI app in try/except ImportError and re-raises with an install hint: either install the full `fastmcp` package or fastmcp-slim with the `[server]` extra. The slim distribution ships without CLI dependencies to stay minimal.

Source

Thrown at fastmcp_slim/fastmcp/cli/__init__.py:8

"""FastMCP CLI package."""

try:
    from .cli import app
except ImportError as exc:
    from fastmcp import _install_hints

    raise ImportError(_install_hints.CLI_SUPPORT) from exc

View on GitHub (pinned to 1f02114297)

Solutions

pip install fastmcp (full package) to get CLI support
Or pip install 'fastmcp-slim[server]' to add CLI deps to the slim build
Confirm the CLI entrypoint resolves: fastmcp --help

Defensive patterns

Strategy: fallback

Validate before calling

import importlib.util
try:
    importlib.util.find_spec('fastmcp.cli')
except Exception:
    raise SystemExit('Install `fastmcp` or `fastmcp-slim[server]` for CLI support')

Try / catch

try:
    from fastmcp.cli import app
except ImportError as exc:
    if 'CLI support' in str(exc):
        print('Install `fastmcp` or `fastmcp-slim[server]`')
    else:
        raise

Prevention

Use the full fastmcp package wherever the CLI is needed
Document that slim installs need fastmcp-slim[server] for CLI commands
Verify `fastmcp --help` works in your container image build

When it happens

Trigger: `from fastmcp.cli import app` (or importing fastmcp.cli at all) in a fastmcp-slim install without the server extra; invoking the CLI entrypoint from a slim install.

Common situations: Using fastmcp-slim in production images and then attempting fastmcp run/dev/install CLI commands; CI scripts assuming full fastmcp; partially-installed package where CLI deps were skipped.

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.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/dfd4e319654fe644. Report an issue: GitHub.