Add client doc

docs/clients/auth/bearer.mdx

@@ -0,0 +1,88 @@
+---
+title: Bearer Authentication
+sidebarTitle: Bearer Auth
+description: Authenticate your FastMCP client using pre-existing OAuth 2.0 Bearer tokens.
+icon: key
+---
+
+import { VersionBadge } from "/snippets/version-badge.mdx"
+
+<VersionBadge version="2.6.0" />
+
+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:
+
+```http
+Authorization: Bearer <token>
+```
+
+
+## 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.
+
+<Tip>
+If you're using a string token, do not include the `Bearer` prefix. FastMCP will add it for you.
+</Tip>
+
+```python {6}
+from fastmcp import Client
+
+async def main():
+    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`:
+
+```python {6}
+from fastmcp import Client
+from fastmcp.client.transports import StreamableHttpTransport
+
+transport = StreamableHttpTransport(
+    "http://fastmcp.cloud/mcp",
+    auth="<your-token>",
+)
+
+async def main():
+    async with Client(transport) as client:
+        await client.ping()
+```
+
+## `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.
+
+```python {7}
+from fastmcp import Client
+from fastmcp.client.auth import BearerAuth
+
+async def main():
+    async with Client(
+        "https://fastmcp.cloud/mcp",
+        auth=BearerAuth(token="<your-token>"),
+    ) as client:
+        await client.ping()
+```
+
+## 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:
+
+```python {6-8}
+from fastmcp import Client
+
+async def main():
+    async with Client(
+        "https://fastmcp.cloud/mcp",
+        headers={
+            'X-API-Key': '<your-token>',
+        },
+    ) as client:
+        await client.ping()
+```