OAuth Authentication

import { VersionBadge } from "/snippets/version-badge.mdx"

<VersionBadge version="2.6.0" /> <Tip> OAuth authentication is only relevant for HTTP-based transports and requires user interaction via a web browser. </Tip>

When your FastMCP client needs to access an MCP server protected by OAuth 2.1, and the process requires user interaction (like logging in and granting consent), you should use the Authorization Code Flow. FastMCP provides the fastmcp.client.auth.OAuth helper to simplify this entire process.

This flow is common for user-facing applications where the application acts on behalf of the user.

Client Usage

Default Configuration

The simplest way to use OAuth is to pass the string "oauth" to the auth parameter of the Client or transport instance. FastMCP will automatically configure the client to use OAuth with default settings:

python

from fastmcp import Client

# Uses default OAuth settings
async with Client("https://your-server.fastmcp.app/mcp", auth="oauth") as client:
    await client.ping()

`OAuth` Helper

To fully configure the OAuth flow, use the OAuth helper and pass it to the auth parameter of the Client or transport instance. OAuth manages the complexities of the OAuth 2.1 Authorization Code Grant with PKCE (Proof Key for Code Exchange) for enhanced security, and implements the full httpx.Auth interface.

python

from fastmcp import Client
from fastmcp.client.auth import OAuth

oauth = OAuth(scopes=["user"])

async with Client("https://your-server.fastmcp.app/mcp", auth=oauth) as client:
    await client.ping()

<Note> You don't need to pass `mcp_url` when using `OAuth` with `Client(auth=...)` — the transport provides the server URL automatically. </Note>

`OAuth` Parameters

scopes (str | list[str], optional): OAuth scopes to request. Can be space-separated string or list of strings
client_name (str, optional): Client name for dynamic registration. Defaults to "FastMCP Client"
client_id (str, optional): Pre-registered OAuth client ID. When provided, skips Dynamic Client Registration entirely. See Pre-Registered Clients
client_secret (str, optional): OAuth client secret for pre-registered clients. Optional — public clients that rely on PKCE can omit this
client_metadata_url (str, optional): URL-based client identity (CIMD). See CIMD Authentication for details
token_storage (AsyncKeyValue, optional): Storage backend for persisting OAuth tokens. Defaults to in-memory storage (tokens lost on restart). See Token Storage for encrypted storage options
additional_client_metadata (dict[str, Any], optional): Extra metadata for client registration
callback_port (int, optional): Fixed port for OAuth callback server. If not specified, uses a random available port
httpx_client_factory (McpHttpClientFactory, optional): Factory for creating httpx clients

OAuth Flow

The OAuth flow is triggered when you use a FastMCP Client configured to use OAuth.

<Steps> <Step title="Token Check"> The client first checks the configured `token_storage` backend for existing, valid tokens for the target server. If one is found, it will be used to authenticate the client. </Step> <Step title="OAuth Server Discovery"> If no valid tokens exist, the client attempts to discover the OAuth server's endpoints using a well-known URI (e.g., `/.well-known/oauth-authorization-server`) based on the `mcp_url`. </Step> <Step title="Client Registration"> If a `client_id` is provided, the client uses those pre-registered credentials directly and skips this step entirely. Otherwise, if a `client_metadata_url` is configured and the server supports CIMD, the client uses its metadata URL as its identity. As a fallback, the client performs Dynamic Client Registration (RFC 7591) if the server supports it. </Step> <Step title="Local Callback Server"> A temporary local HTTP server is started on an available port (or the port specified via `callback_port`). This server's address (e.g., `http://127.0.0.1:<port>/callback`) acts as the `redirect_uri` for the OAuth flow. </Step> <Step title="Browser Interaction"> The user's default web browser is automatically opened, directing them to the OAuth server's authorization endpoint. The user logs in and grants (or denies) the requested `scopes`. </Step> <Step title="Authorization Code & Token Exchange"> Upon approval, the OAuth server redirects the user's browser to the local callback server with an `authorization_code`. The client captures this code and exchanges it with the OAuth server's token endpoint for an `access_token` (and often a `refresh_token`) using PKCE for security. </Step> <Step title="Token Caching"> The obtained tokens are saved to the configured `token_storage` backend for future use, eliminating the need for repeated browser interactions. </Step> <Step title="Authenticated Requests"> The access token is automatically included in the `Authorization` header for requests to the MCP server. </Step> <Step title="Refresh Token"> If the access token expires, the client will automatically use the refresh token to get a new access token. </Step> </Steps>

Token Storage

By default, tokens are stored in memory and lost when your application restarts. For persistent storage, pass an AsyncKeyValue-compatible storage backend to the token_storage parameter.

<Warning> **Security Consideration**: Use encrypted storage for production. MCP clients can accumulate OAuth credentials for many servers over time, and a compromised token store could expose access to multiple services. </Warning>

python

from fastmcp import Client
from fastmcp.client.auth import OAuth
from key_value.aio.stores.disk import DiskStore
from key_value.aio.wrappers.encryption import FernetEncryptionWrapper
from cryptography.fernet import Fernet
import os

# Create encrypted disk storage
encrypted_storage = FernetEncryptionWrapper(
    key_value=DiskStore(directory="~/.fastmcp/oauth-tokens"),
    fernet=Fernet(os.environ["OAUTH_STORAGE_ENCRYPTION_KEY"])
)

oauth = OAuth(token_storage=encrypted_storage)

async with Client("https://your-server.fastmcp.app/mcp", auth=oauth) as client:
    await client.ping()

You can use any AsyncKeyValue-compatible backend from the key-value library including Redis, DynamoDB, and more. Wrap your storage in FernetEncryptionWrapper for encryption.

<Note> When selecting a storage backend, review the [py-key-value documentation](https://github.com/strawgate/py-key-value) to understand the maturity level and limitations of your chosen backend. Some backends may be in preview or have constraints that affect production suitability. </Note>

CIMD Authentication

Client ID Metadata Documents (CIMD) provide an alternative to Dynamic Client Registration. Instead of registering with each server, your client hosts a static JSON document at an HTTPS URL. That URL becomes your client's identity, and servers can verify who you are through your domain ownership.

python

from fastmcp import Client
from fastmcp.client.auth import OAuth

async with Client(
    "https://mcp-server.example.com/mcp",
    auth=OAuth(
        client_metadata_url="https://myapp.example.com/oauth/client.json",
    ),
) as client:
    await client.ping()

See the CIMD Authentication page for complete documentation on creating, hosting, and validating CIMD documents.

Pre-Registered Clients

Some OAuth servers don't support Dynamic Client Registration — the MCP spec explicitly makes DCR optional. If your client has been pre-registered with the server (you already have a client_id and optionally a client_secret), you can provide them directly to skip DCR entirely.

python

from fastmcp import Client
from fastmcp.client.auth import OAuth

async with Client(
    "https://mcp-server.example.com/mcp",
    auth=OAuth(
        client_id="my-registered-client-id",
        client_secret="my-client-secret",
    ),
) as client:
    await client.ping()

Public clients that rely on PKCE for security can omit client_secret:

python

oauth = OAuth(client_id="my-public-client-id")

<Note> When using pre-registered credentials, the client will not attempt Dynamic Client Registration. If the server rejects the credentials, the error is surfaced immediately rather than falling back to DCR. </Note>

Client Usage

Default Configuration

OAuth Helper

OAuth Parameters

OAuth Flow

Token Storage

CIMD Authentication

Pre-Registered Clients

`OAuth` Helper

`OAuth` Parameters