---
schema: rendered-html/v1
title: "Tool Fingerprinting - FastMCP"
source_url: https://gofastmcp.com/servers/tool-fingerprinting
generated_at: 2026-06-11T19:39:51.458702+00:00
---




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Working with Tools

Tool Fingerprinting









  - [Overview](gofastmcp__com__servers__transforms__transforms.md)
  - [Tool Transformation](gofastmcp__com__servers__transforms__tool-transformation.md)
  - [Code Mode

    NEW](gofastmcp__com__servers__transforms__code-mode.md)
  - [Tool Search

    NEW](gofastmcp__com__servers__transforms__tool-search.md)
  - [Namespace](gofastmcp__com__servers__transforms__namespace.md)
  - [Visibility

    NEW](gofastmcp__com__servers__visibility.md)
  - [Resources as Tools

    NEW](gofastmcp__com__servers__transforms__resources-as-tools.md)
  - [Prompts as Tools

    NEW](gofastmcp__com__servers__transforms__prompts-as-tools.md)
  - [Tool Fingerprinting](/servers/tool-fingerprinting)














- [The Recipe](#the-recipe)
- [Why tool.key?](#why-tool-key)
- [Why to\_mcp\_tool()?](#why-to_mcp_tool-)
- [Customizing the Payload](#customizing-the-payload)
- [Detecting Schema Drift in CI](#detecting-schema-drift-in-ci)

Working with Tools

# Tool Fingerprinting


Build stable fingerprints for tool identity and schema change detection


Downstream systems like routers, gateways, and audit loggers often need to detect whether a tool’s schema changed between deployments. Rather than each system inventing its own JSON normalization and hashing logic, you can build stable fingerprints from FastMCP’s existing API surface.
FastMCP does not define a single “contract hash” because the inclusion policy is necessarily application-specific: some systems care only about the input schema, others include the description, metadata, tags, or version. Instead, this recipe shows how to assemble a fingerprint payload from the parts you care about, then hash it deterministically.

## [​](#the-recipe) The Recipe

The two key building blocks are:

- **`tool.key`** — FastMCP’s canonical component identity, encoding type, name, and version (e.g. `tool:greet@1.0` or `tool:greet@`)
- **`tool.to_mcp_tool()`** — the protocol-facing tool object that MCP clients see, including the input schema

Combine them into a payload, serialize deterministically, and hash:

import hashlib
import json


mcp = FastMCP("demo")


@mcp.tool()
def greet(name: str) -> str:
    """Say hello."""
    return f"Hello {name}"


async def fingerprint_tool(server: FastMCP, tool_name: str) -> str:
    tool = await server.get_tool(tool_name)
    if tool is None:
        raise ValueError(f"Tool {tool_name!r} not found")

    mcp_tool = tool.to_mcp_tool()
    dumped = mcp_tool.model_dump(mode="json", by_alias=True, exclude_none=True)

    payload = {
        "key": tool.key,
        "inputSchema": dumped["inputSchema"],

    canonical = json.dumps(payload, sort_keys=True, separators=(",", ":"))
    return hashlib.sha256(canonical.encode("utf-8")).hexdigest()

The fingerprint is stable across process restarts as long as the tool’s name, version, and input schema remain the same.

## [​](#why-tool-key) Why `tool.key`?

`tool.key` is FastMCP’s canonical component identity. It encodes the component type, identifier, and version into a single string:

tool:greet@1.0    # versioned tool
tool:greet@       # unversioned tool

Using `key` rather than just the tool name ensures that two versions of the same tool produce distinct fingerprints, and that a tool and a resource with the same name cannot collide.

## [​](#why-to_mcp_tool-) Why `to_mcp_tool()`?

`to_mcp_tool()` returns the protocol-facing representation — the shape that MCP clients actually receive. This matters because routers and gateways typically operate on the protocol layer, not FastMCP internals. The `model_dump(mode="json", by_alias=True, exclude_none=True)` call produces a clean, serializable dictionary using the MCP protocol field names.

## [​](#customizing-the-payload) Customizing the Payload

You own the inclusion policy. Add or remove fields depending on what constitutes a “contract” in your system:

async def custom_fingerprint(server: FastMCP, tool_name: str) -> str:
    tool = await server.get_tool(tool_name)
    if tool is None:
        raise ValueError(f"Tool {tool_name!r} not found")

    mcp_tool = tool.to_mcp_tool()
    dumped = mcp_tool.model_dump(mode="json", by_alias=True, exclude_none=True)

    # Include description to detect documentation drift
    payload = {
        "key": tool.key,
        "inputSchema": dumped["inputSchema"],
        "description": dumped.get("description"),

    canonical = json.dumps(payload, sort_keys=True, separators=(",", ":"))
    return hashlib.sha256(canonical.encode("utf-8")).hexdigest()

Common variations:

| Field | When to include |
| --- | --- |
| `inputSchema` | Always — this is the core contract |
| `description` | When documentation drift matters (e.g. LLM routing decisions depend on it) |
| `outputSchema` | When downstream consumers validate response shapes |
| `annotations` | When behavioral hints (read-only, destructive) affect routing |
| `_meta` | When custom metadata drives policy decisions |

## [​](#detecting-schema-drift-in-ci) Detecting Schema Drift in CI

Store fingerprints as artifacts and compare between deployments:

import json
import hashlib
from pathlib import Path



async def generate_manifest(server: FastMCP) -> dict[str, str]:
    """Generate a fingerprint manifest for all tools."""
    manifest = {}

    for tool in await server.list_tools():
        mcp_tool = tool.to_mcp_tool()
        dumped = mcp_tool.model_dump(mode="json", by_alias=True, exclude_none=True)

        payload = {
            "key": tool.key,
            "inputSchema": dumped["inputSchema"],

        canonical = json.dumps(payload, sort_keys=True, separators=(",", ":"))
        manifest[tool.key] = hashlib.sha256(canonical.encode("utf-8")).hexdigest()

    return manifest


async def check_drift(server: FastMCP, baseline_path: Path) -> list[str]:
    """Compare current fingerprints against a stored baseline."""
    current = await generate_manifest(server)
    baseline = json.loads(baseline_path.read_text())

    changed = []
    for key, fingerprint in current.items():
        if baseline.get(key) != fingerprint:
            changed.append(key)

    for key in baseline:
        if key not in current:
            changed.append(key)

    return changed

Run `generate_manifest` in CI after each build and compare against the previous run. Any differences indicate a schema change that downstream consumers should be aware of.

[Prompts as Tools

Previous](gofastmcp__com__servers__transforms__prompts-as-tools.md)[Providers

Next](gofastmcp__com__servers__providers__overview.md)






