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New in version 2.6.0
Bearer Token authentication is only relevant for HTTP-based transports.
You can configure your FastMCP client to use bearer authentication by supplying a valid access token. This is most appropriate for service accounts, long-lived API keys, CI/CD, applications where authentication is managed separately, or other non-interactive authentication methods. A Bearer token is a JSON Web Token (JWT) that is used to authenticate a request. It is most commonly used in the Authorization header of an HTTP request, using the Bearer scheme: 
Authorization: Bearer <token>

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Client Usage

The most straightforward way to use a pre-existing Bearer token is to provide it as a string to the auth parameter of the fastmcp.Client or transport instance. FastMCP will automatically format it correctly for the Authorization header and bearer scheme.
If you’re using a string token, do not include the Bearer prefix. FastMCP will add it for you.
from fastmcp import Client

async with Client(
    "https://fastmcp.cloud/mcp", 
    auth="<your-token>",
) as client:
    await client.ping()
You can also supply a Bearer token to a transport instance, such as StreamableHttpTransport or SSETransport: 
from fastmcp import Client
from fastmcp.client.transports import StreamableHttpTransport

transport = StreamableHttpTransport(
    "http://fastmcp.cloud/mcp", 
    auth="<your-token>",
)

async with Client(transport) as client:
    await client.ping()

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BearerAuth Helper

If you prefer to be more explicit and not rely on FastMCP to transform your string token, you can use the BearerAuth class yourself, which implements the httpx.Auth interface. 
from fastmcp import Client
from fastmcp.client.auth import BearerAuth

async with Client(
    "https://fastmcp.cloud/mcp", 
    auth=BearerAuth(token="<your-token>"),
) as client:
    await client.ping()

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Custom Headers

If the MCP server expects a custom header or token scheme, you can manually set the client’s headers instead of using the auth parameter by setting them on your transport: 
from fastmcp import Client
from fastmcp.client.transports import StreamableHttpTransport

async with Client(
    transport=StreamableHttpTransport(
        "https://fastmcp.cloud/mcp", 
        headers={"X-API-Key": "<your-token>"},
    ),
) as client:
    await client.ping()