Form Input

Collect structured data from users via Pydantic models

FormInput generates a validated form from a Pydantic model. The user fills it out, and the submission is validated against the model before being returned. Structured elicitation that can’t be hallucinated.

from typing import Literal

from pydantic import BaseModel, Field
from fastmcp import FastMCP
from fastmcp.apps.form import FormInput

class BugReport(BaseModel):
    title: str = Field(description="Brief summary")
    severity: Literal["low", "medium", "high", "critical"]
    description: str = Field(
        description="Detailed description",
        json_schema_extra={"ui": {"type": "textarea"}},
    )

mcp = FastMCP("My Server")
mcp.add_provider(FormInput(model=BugReport))

This registers two tools:

Tool	Visibility	Purpose
collect_bugreport	Model	Opens the form UI
submit_form	App only	Validates and processes the submission

The tool name is derived from the model class name, lowercased: collect_{modelname}. So BugReport becomes collect_bugreport, ShippingAddress becomes collect_shippingaddress. Use tool_name to override if needed. The LLM calls it with a prompt explaining what it needs, and the user gets a form with fields matching the model.

Field mapping

FormInput uses Prefab’s Form.from_model(), which maps Pydantic types to form components:

Python type	Form component
str	Text input
int, float	Number input
bool	Checkbox
datetime.date	Date picker
Literal[...]	Select dropdown
SecretStr	Password input

Use Field() metadata to control labels (title), placeholders (description), and validation (min_length, max_length, ge, le). Use json_schema_extra={"ui": {"type": "textarea"}} for multiline text.

Callback

By default, the validated model is returned as JSON. Provide an on_submit callback to process the data server-side:

def save_report(report: BugReport) -> str:
    db.insert(report.model_dump())
    return f"Bug #{db.last_id} filed: {report.title}"

mcp.add_provider(FormInput(model=BugReport, on_submit=save_report))

The callback receives a validated model instance and returns a string that becomes the tool result.

Configuration

FormInput(
    model=BugReport,             # Required: the Pydantic model
    name="BugTracker",           # App name (default: model name)
    title="File a Bug",          # Card heading (default: model name)
    tool_name="file_bug",        # Tool name (default: collect_{model})
    submit_text="Submit Report", # Button label (default: "Submit")
    on_submit=save_report,       # Optional callback
    send_message=True,           # Push result as a chat message
)

Set send_message=True to push the result back into the conversation via SendMessage, triggering the LLM’s next turn. Without it, the result is just the tool return value.

Multiple forms

Add multiple providers for different models — each gets its own tool:

mcp = FastMCP(
    "My Server",
    providers=[
        FormInput(model=ShippingAddress),
        FormInput(model=BugReport),
        FormInput(model=ContactInfo),
    ],
)