Address @ihrpr comments

- Use InitializeResult type for parsing server init response
- Add explanatory comment for DEFAULT_NEGOTIATED_VERSION constant
- Include supported protocol versions in error response when version mismatch occurs

Author: felixweinberger

src/mcp/client/streamable_http.py

@@ -23,6 +23,7 @@
 from mcp.shared.message import ClientMessageMetadata, SessionMessage
 from mcp.types import (
     ErrorData,
+    InitializeResult,
     JSONRPCError,
     JSONRPCMessage,
     JSONRPCNotification,
@@ -141,13 +142,14 @@ def _maybe_extract_protocol_version_from_message(
     ) -> None:
         """Extract protocol version from initialization response message."""
         if isinstance(message.root, JSONRPCResponse) and message.root.result:
-            # Check if result has protocolVersion field
-            result = message.root.result
-            if "protocolVersion" in result:
-                self.protocol_version = result["protocolVersion"]
+            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}")
-            else:
-                logger.warning(f"Initialization response does not contain protocolVersion: {result}")
+            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,

src/mcp/server/streamable_http.py

@@ -27,7 +27,7 @@
 from mcp.shared.message import ServerMessageMetadata, SessionMessage
 from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 from mcp.types import (
-    DEFAULT_PROTOCOL_VERSION,
+    DEFAULT_NEGOTIATED_VERSION,
     INTERNAL_ERROR,
     INVALID_PARAMS,
     INVALID_REQUEST,
@@ -699,12 +699,14 @@ async def _validate_protocol_version(self, request: Request, send: Send) -> bool
 
         # If no protocol version provided, assume default version
         if protocol_version is None:
-            protocol_version = DEFAULT_PROTOCOL_VERSION
+            protocol_version = DEFAULT_NEGOTIATED_VERSION
 
         # Check if the protocol version is supported
         if protocol_version not in SUPPORTED_PROTOCOL_VERSIONS:
+            supported_versions = ", ".join(SUPPORTED_PROTOCOL_VERSIONS)
             response = self._create_error_response(
-                f"Bad Request: Unsupported protocol version: {protocol_version}",
+                f"Bad Request: Unsupported protocol version: {protocol_version}. "
+                + f"Supported versions: {supported_versions}",
                 HTTPStatus.BAD_REQUEST,
             )
             await response(request.scope, request.receive, send)

src/mcp/types.py

@@ -23,7 +23,14 @@
 """
 
 LATEST_PROTOCOL_VERSION = "2025-03-26"
-DEFAULT_PROTOCOL_VERSION = "2025-03-26"
+
+"""
+The default negotiated version of the Model Context Protocol when no version is specified.
+We need this to satisfy the MCP specification, which requires the server to assume a
+specific version if none is provided by the client. See section "Protocol Version Header" at
+https://modelcontextprotocol.io/specification
+"""
+DEFAULT_NEGOTIATED_VERSION = "2025-03-26"
 
 ProgressToken = str | int
 Cursor = str