An MCP server exposes `token_verifier`. That hook is the auth boundary. A missing or rejected Authdog bearer never reaches a tool.

Getting started, including the runnable sample, is [MCP agents](/docs/agent-frameworks/mcp). [Connect an MCP client](/docs/ai/mcp-development/connect-client) is how a client obtains a token. This page is the check after the client presents one.

This is your MCP server. [Authdog's management MCP](/docs/ai/mcp-server) is a different host.

## The verifier

`verify_token` calls `fetch_user_data`, the same userinfo check as `require_auth`. It returns an MCP `AccessToken` only when the payload is authenticated and has a subject. `None` refuses the bearer.

```python
return AccessToken(
    token=token,
    client_id=subject,
    scopes=["agent:invoke"],
    subject=subject,
)
```

`subject_of` reads `id`, then `sub`. Pass the verifier on `MCPServer`:

```python
server = MCPServer(
    name="authdog-mcp",
    token_verifier=AuthdogTokenVerifier(payload),
    auth=AuthSettings(
        issuer_url=issuer,
        resource_server_url=resource_url,
        required_scopes=["agent:invoke"],
        validate_token_resource=False,
    ),
)
```

`validate_token_resource=False` because this verifier checks the token by userinfo. It does not require an `aud` claim equal to the MCP resource URL. Set the audience on the Authdog client when you need that binding. The sample does not add a second JWT parser.

`PK_AUTHDOG` selects the identity host. The server never sees the client secret.

## What you should see

`POST /mcp` with no bearer returns `401`. The `initialize` method does not run as an authenticated session. Repeat with a bearer userinfo accepts, then call `whoami`. The text is the Authdog subject.

The tool reads that subject from `get_access_token()`, which is the context the verifier filled in. It does not parse the header again.

## Related

| Read | To learn how to |
| --- | --- |
| [MCP agents](/docs/agent-frameworks/mcp) | Run the sample |
| [API keys and M2M for MCP](/docs/ai/mcp/api-keys-and-m2m) | Choose the token the client presents |
| [Authorizing tool calls for MCP](/docs/ai/mcp/authorize-tool-calls) | Allow a tool after the verifier returns |
| [Build an MCP server](/docs/ai/mcp-development/build-server) | Add the runtime guard beside this verifier |