Approval

Human-in-the-loop approval gates for agent actions

Approval adds a human-in-the-loop confirmation step to any server. The LLM presents what it’s about to do, the user approves or rejects via buttons, and the decision flows back into the conversation as a message.

from fastmcp import FastMCP
from fastmcp.apps.approval import Approval

mcp = FastMCP("My Server")
mcp.add_provider(Approval())

This registers a single tool:

Tool	Visibility	Purpose
request_approval	Model	Shows an approval card, sends the user’s decision back as a message

The LLM calls request_approval with a summary (and optional details) whenever it’s about to take a significant action. The user sees a card with Approve and Reject buttons. Clicking either sends a message back into the conversation via SendMessage, which triggers the LLM’s next turn.

The message looks like it came from the user:

"Deploy v3.2 to production" — I selected: Approve

Approval is an advisory gate, not an enforcement mechanism. The conversation isn’t blocked while the card is open — the user can keep typing, and a determined LLM could proceed without waiting. Think of it as a strong UX signal that encourages confirmation, not a security boundary. For hard enforcement, implement approval logic server-side in your tool implementations.

Configuration

The constructor sets defaults; the LLM can override all of these per-call via tool arguments.

Approval(
    name="Approval",              # App name
    title="Approval Required",    # Card heading
    approve_text="Approve",       # Approve button label
    reject_text="Reject",         # Reject button label
    approve_variant="default",    # "default", "destructive", "success", "info"
    reject_variant="outline",     # same options plus "outline"
)

The LLM can customize each invocation:

request_approval(
    summary="Delete 47 files from /tmp",
    details="This cannot be undone.",
    title="Destructive Action",
    approve_text="Delete",
    approve_variant="destructive",
    reject_text="Keep files",
)

How it works

When the user clicks a button, two things happen:

1. SendMessage pushes the decision into the conversation as a user message
2. SetState("decided", True) replaces the buttons with “Response sent.”

The tool description instructs the LLM to stop and wait for the “I selected:” message before proceeding. If approved, it continues. If rejected, it acknowledges and asks how to proceed.