PrefectHQ/fastmcp · error · ImportError

FormInput requires prefab-ui. Install with: pip install 'fas

Error message

FormInput requires prefab-ui. Install with: pip install 'fastmcp[apps]'

What it means

FastMCP's FormInput component in fastmcp/apps/form.py depends on the third-party prefab-ui package for UI primitives (Form, Muted, If, reactive STATE/RESULT). The module intentionally raises this ImportError at import time when prefab-ui is missing, telling you exactly which extra to install. FastMCP treats app/UI support as an optional dependency group so the core library stays lightweight.

Source

Thrown at fastmcp_slim/fastmcp/apps/form.py:51

try:
    import prefab_ui
    from prefab_ui.actions import SetState
    from prefab_ui.actions.mcp import CallTool, SendMessage
    from prefab_ui.app import PrefabApp
    from prefab_ui.components import (
        H3,
        Card,
        CardContent,
        CardFooter,
        CardHeader,
        Column,
        Form,
        Muted,
    )
    from prefab_ui.components.control_flow import If
    from prefab_ui.rx import RESULT, STATE
except ImportError as _exc:
    raise ImportError(
        "FormInput requires prefab-ui. Install with: pip install 'fastmcp[apps]'"
    ) from _exc

# `defaults` kwarg on Form.from_model was added in prefab-ui 0.19.1. Gate on
# version so that older prefab-ui keeps working — `default` silently no-ops.
try:
    _FORM_SUPPORTS_DEFAULTS = Version(prefab_ui.__version__) >= Version("0.19.1")
except InvalidVersion:
    _FORM_SUPPORTS_DEFAULTS = False

import pydantic

from fastmcp.apps.app import FastMCPApp


def _backfill_boolean_defaults(
    model: type[pydantic.BaseModel],
    data: dict[str, Any],

View on GitHub (pinned to 1f02114297)

Solutions

Run pip install 'fastmcp[apps]' (or add fastmcp[apps] to requirements/pyproject)
If using fastmcp-slim, install the equivalent extra that pulls in prefab-ui
Verify with python -c 'import prefab_ui' after installing

Example fix

# before
from fastmcp.apps.form import FormInput  # ImportError

# after
# pip install 'fastmcp[apps]'
from fastmcp.apps.form import FormInput

Defensive patterns

Strategy: fallback

Validate before calling

import importlib.util
if importlib.util.find_spec('prefab_ui') is None:
    raise SystemExit("Install with: pip install 'fastmcp[apps]'")

Try / catch

try:
    from fastmcp.apps.form import FormInput
except ImportError as exc:
    if 'prefab-ui' in str(exc):
        print("Run: pip install 'fastmcp[apps]'")
    else:
        raise

Prevention

Add fastmcp[apps] to your dependency manifest, not bare fastmcp
Smoke-test imports in CI with the same extras you use locally
Use uv sync so extras stay consistent across environments

When it happens

Trigger: Importing fastmcp.apps.form (or anything importing it, e.g. FormInput / Form.from_model) in an environment where prefab_ui is not installed.

Common situations: Installed fastmcp or fastmcp-slim without the 'apps' extra; deployed to a fresh container/CI env from a requirements.txt that predates apps support; a dependency stripped prefab-ui.

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/0b77d2db63d1a536. Report an issue: GitHub.