feat: implement MCP-Protocol-Version header requirement for HTTP transport (#898)

src/mcp/client/auth.py

@@ -17,7 +17,13 @@ from urllib.parse import urlencode, urljoin
 import anyio
 import httpx
 
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata, OAuthToken
+from mcp.client.streamable_http import MCP_PROTOCOL_VERSION
+from mcp.shared.auth import (
+    OAuthClientInformationFull,
+    OAuthClientMetadata,
+    OAuthMetadata,
+    OAuthToken,
+)
 from mcp.types import LATEST_PROTOCOL_VERSION
 
 logger = logging.getLogger(__name__)
@@ -121,7 +127,7 @@ class OAuthClientProvider(httpx.Auth):
         # Extract base URL per MCP spec
         auth_base_url = self._get_authorization_base_url(server_url)
         url = urljoin(auth_base_url, "/.well-known/oauth-authorization-server")
-        headers = {"MCP-Protocol-Version": LATEST_PROTOCOL_VERSION}
+        headers = {MCP_PROTOCOL_VERSION: LATEST_PROTOCOL_VERSION}
 
         async with httpx.AsyncClient() as client:
             try:

src/mcp/client/streamable_http.py

@@ -22,6 +22,7 @@ from mcp.shared._httpx_utils import McpHttpClientFactory, create_mcp_http_client
 from mcp.shared.message import ClientMessageMetadata, SessionMessage
 from mcp.types import (
     ErrorData,
+    InitializeResult,
     JSONRPCError,
     JSONRPCMessage,
     JSONRPCNotification,
@@ -39,6 +40,7 @@ StreamReader = MemoryObjectReceiveStream[SessionMessage]
 GetSessionIdCallback = Callable[[], str | None]
 
 MCP_SESSION_ID = "mcp-session-id"
+MCP_PROTOCOL_VERSION = "mcp-protocol-version"
 LAST_EVENT_ID = "last-event-id"
 CONTENT_TYPE = "content-type"
 ACCEPT = "Accept"
@@ -97,17 +99,20 @@ class StreamableHTTPTransport:
         )
         self.auth = auth
         self.session_id = None
+        self.protocol_version = None
         self.request_headers = {
             ACCEPT: f"{JSON}, {SSE}",
             CONTENT_TYPE: JSON,
             **self.headers,
         }
 
-    def _update_headers_with_session(self, base_headers: dict[str, str]) -> dict[str, str]:
-        """Update headers with session ID if available."""
+    def _prepare_request_headers(self, base_headers: dict[str, str]) -> dict[str, str]:
+        """Update headers with session ID and protocol version if available."""
         headers = base_headers.copy()
         if self.session_id:
             headers[MCP_SESSION_ID] = self.session_id
+        if self.protocol_version:
+            headers[MCP_PROTOCOL_VERSION] = self.protocol_version
         return headers
 
     def _is_initialization_request(self, message: JSONRPCMessage) -> bool:
@@ -128,12 +133,28 @@ class StreamableHTTPTransport:
             self.session_id = new_session_id
             logger.info(f"Received session ID: {self.session_id}")
 
+    def _maybe_extract_protocol_version_from_message(
+        self,
+        message: JSONRPCMessage,
+    ) -> None:
+        """Extract protocol version from initialization response message."""
+        if isinstance(message.root, JSONRPCResponse) and message.root.result:
+            try:
+                # Parse the result as InitializeResult for type safety
+                init_result = InitializeResult.model_validate(message.root.result)
+                self.protocol_version = str(init_result.protocolVersion)
+                logger.info(f"Negotiated protocol version: {self.protocol_version}")
+            except Exception as exc:
+                logger.warning(f"Failed to parse initialization response as InitializeResult: {exc}")
+                logger.warning(f"Raw result: {message.root.result}")
+
     async def _handle_sse_event(
         self,
         sse: ServerSentEvent,
         read_stream_writer: StreamWriter,
         original_request_id: RequestId | None = None,
         resumption_callback: Callable[[str], Awaitable[None]] | None = None,
+        is_initialization: bool = False,
     ) -> bool:
         """Handle an SSE event, returning True if the response is complete."""
         if sse.event == "message":
@@ -141,6 +162,10 @@ class StreamableHTTPTransport:
                 message = JSONRPCMessage.model_validate_json(sse.data)
                 logger.debug(f"SSE message: {message}")
 
+                # Extract protocol version from initialization response
+                if is_initialization:
+                    self._maybe_extract_protocol_version_from_message(message)
+
                 # If this is a response and we have original_request_id, replace it
                 if original_request_id is not None and isinstance(message.root, JSONRPCResponse | JSONRPCError):
                     message.root.id = original_request_id
@@ -174,7 +199,7 @@ class StreamableHTTPTransport:
             if not self.session_id:
                 return
 
-            headers = self._update_headers_with_session(self.request_headers)
+            headers = self._prepare_request_headers(self.request_headers)
 
             async with aconnect_sse(
                 client,
@@ -194,7 +219,7 @@ class StreamableHTTPTransport:
 
     async def _handle_resumption_request(self, ctx: RequestContext) -> None:
         """Handle a resumption request using GET with SSE."""
-        headers = self._update_headers_with_session(ctx.headers)
+        headers = self._prepare_request_headers(ctx.headers)
         if ctx.metadata and ctx.metadata.resumption_token:
             headers[LAST_EVENT_ID] = ctx.metadata.resumption_token
         else:
@@ -227,7 +252,7 @@ class StreamableHTTPTransport:
 
     async def _handle_post_request(self, ctx: RequestContext) -> None:
         """Handle a POST request with response processing."""
-        headers = self._update_headers_with_session(ctx.headers)
+        headers = self._prepare_request_headers(ctx.headers)
         message = ctx.session_message.message
         is_initialization = self._is_initialization_request(message)
 
@@ -256,9 +281,9 @@ class StreamableHTTPTransport:
             content_type = response.headers.get(CONTENT_TYPE, "").lower()
 
             if content_type.startswith(JSON):
-                await self._handle_json_response(response, ctx.read_stream_writer)
+                await self._handle_json_response(response, ctx.read_stream_writer, is_initialization)
             elif content_type.startswith(SSE):
-                await self._handle_sse_response(response, ctx)
+                await self._handle_sse_response(response, ctx, is_initialization)
             else:
                 await self._handle_unexpected_content_type(
                     content_type,
@@ -269,18 +294,29 @@ class StreamableHTTPTransport:
         self,
         response: httpx.Response,
         read_stream_writer: StreamWriter,
+        is_initialization: bool = False,
     ) -> None:
         """Handle JSON response from the server."""
         try:
             content = await response.aread()
             message = JSONRPCMessage.model_validate_json(content)
+
+            # Extract protocol version from initialization response
+            if is_initialization:
+                self._maybe_extract_protocol_version_from_message(message)
+
             session_message = SessionMessage(message)
             await read_stream_writer.send(session_message)
         except Exception as exc:
             logger.error(f"Error parsing JSON response: {exc}")
             await read_stream_writer.send(exc)
 
-    async def _handle_sse_response(self, response: httpx.Response, ctx: RequestContext) -> None:
+    async def _handle_sse_response(
+        self,
+        response: httpx.Response,
+        ctx: RequestContext,
+        is_initialization: bool = False,
+    ) -> None:
         """Handle SSE response from the server."""
         try:
             event_source = EventSource(response)
@@ -289,6 +325,7 @@ class StreamableHTTPTransport:
                     sse,
                     ctx.read_stream_writer,
                     resumption_callback=(ctx.metadata.on_resumption_token_update if ctx.metadata else None),
+                    is_initialization=is_initialization,
                 )
                 # If the SSE event indicates completion, like returning respose/error
                 # break the loop
@@ -385,7 +422,7 @@ class StreamableHTTPTransport:
             return
 
         try:
-            headers = self._update_headers_with_session(self.request_headers)
+            headers = self._prepare_request_headers(self.request_headers)
             response = await client.delete(self.url, headers=headers)
 
             if response.status_code == 405: