Form Input

New in version 3.2.0 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.

The FormInput provider shown in Goose, with a bug report form

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. The LLM can also pass title and submit_text to override the card heading and button label, and default, a partial dict of field values that pre-fills the form for the user to confirm or edit. default requires prefab-ui>=0.19.1 and is ignored on older versions.

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:

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

class BugReport(BaseModel):
    title: str
    description: str

reports: list[BugReport] = []

def save_report(report: BugReport) -> str:
    reports.append(report)
    return f"Bug #{len(reports)} filed: {report.title}"

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

The callback receives a validated model instance and returns a string that becomes the result of the submit_form call.

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 goes only to the form UI, which shows “Submitted.”, and the LLM does not see the submission.

Multiple forms

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

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

class ShippingAddress(BaseModel):
    street: str
    city: str

class BugReport(BaseModel):
    title: str
    description: str

class ContactInfo(BaseModel):
    name: str
    email: str

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

Get Started

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Field mapping

Callback

Configuration

Multiple forms