Add OAuth authentication client for HTTPX (#751)

Co-authored-by: Paul Carleton <paulc@anthropic.com>

README.md

@@ -796,6 +796,60 @@ async def main():
             tool_result = await session.call_tool("echo", {"message": "hello"})
 ```
 
+### OAuth Authentication for Clients
+
+The SDK includes [authorization support](https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization) for connecting to protected MCP servers:
+
+```python
+from mcp.client.auth import OAuthClientProvider, TokenStorage
+from mcp.client.session import ClientSession
+from mcp.client.streamable_http import streamablehttp_client
+from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
+
+
+class CustomTokenStorage(TokenStorage):
+    """Simple in-memory token storage implementation."""
+
+    async def get_tokens(self) -> OAuthToken | None:
+        pass
+
+    async def set_tokens(self, tokens: OAuthToken) -> None:
+        pass
+
+    async def get_client_info(self) -> OAuthClientInformationFull | None:
+        pass
+
+    async def set_client_info(self, client_info: OAuthClientInformationFull) -> None:
+        pass
+
+
+async def main():
+    # Set up OAuth authentication
+    oauth_auth = OAuthClientProvider(
+        server_url="https://api.example.com",
+        client_metadata=OAuthClientMetadata(
+            client_name="My Client",
+            redirect_uris=["http://localhost:3000/callback"],
+            grant_types=["authorization_code", "refresh_token"],
+            response_types=["code"],
+        ),
+        storage=CustomTokenStorage(),
+        redirect_handler=lambda url: print(f"Visit: {url}"),
+        callback_handler=lambda: ("auth_code", None),
+    )
+
+    # Use with streamable HTTP client
+    async with streamablehttp_client(
+        "https://api.example.com/mcp", auth=oauth_auth
+    ) as (read, write, _):
+        async with ClientSession(read, write) as session:
+            await session.initialize()
+            # Authenticated session ready
+```
+
+For a complete working example, see [`examples/clients/simple-auth-client/`](examples/clients/simple-auth-client/).
+
+
 ### MCP Primitives
 
 The MCP protocol defines three core primitives that servers can implement: