fix: address @ochafik comments

- update test comment
- log warning if no protocol version is set
- nits

Author: felixweinberger

src/mcp/client/streamable_http.py

@@ -146,6 +146,8 @@ def _maybe_extract_protocol_version_from_message(
             if "protocolVersion" in result:
                 self.protocol_version = result["protocolVersion"]
                 logger.info(f"Negotiated protocol version: {self.protocol_version}")
+            else:
+                logger.warning(f"Initialization response does not contain protocolVersion: {result}")
 
     async def _handle_sse_event(
         self,
@@ -286,9 +288,7 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
             content_type = response.headers.get(CONTENT_TYPE, "").lower()
 
             if content_type.startswith(JSON):
-                await self._handle_json_response(
-                    response, ctx.read_stream_writer, is_initialization
-                )
+                await self._handle_json_response(response, ctx.read_stream_writer, is_initialization)
             elif content_type.startswith(SSE):
                 await self._handle_sse_response(response, ctx, is_initialization)
             else:
@@ -331,11 +331,7 @@ async def _handle_sse_response(
                 is_complete = await self._handle_sse_event(
                     sse,
                     ctx.read_stream_writer,
-                    resumption_callback=(
-                        ctx.metadata.on_resumption_token_update
-                        if ctx.metadata
-                        else None
-                    ),
+                    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

src/mcp/server/streamable_http.py

@@ -27,6 +27,7 @@
 from mcp.shared.message import ServerMessageMetadata, SessionMessage
 from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 from mcp.types import (
+    DEFAULT_PROTOCOL_VERSION,
     INTERNAL_ERROR,
     INVALID_PARAMS,
     INVALID_REQUEST,
@@ -295,7 +296,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
             has_json, has_sse = self._check_accept_headers(request)
             if not (has_json and has_sse):
                 response = self._create_error_response(
-                    ("Not Acceptable: Client must accept both application/json and " "text/event-stream"),
+                    ("Not Acceptable: Client must accept both application/json and text/event-stream"),
                     HTTPStatus.NOT_ACCEPTABLE,
                 )
                 await response(scope, receive, send)
@@ -696,9 +697,9 @@ async def _validate_protocol_version(self, request: Request, send: Send) -> bool
         # Get the protocol version from the request headers
         protocol_version = request.headers.get(MCP_PROTOCOL_VERSION_HEADER)
 
-        # If no protocol version provided, assume version 2025-03-26
-        if not protocol_version:
-            protocol_version = "2025-03-26"
+        # If no protocol version provided, assume default version
+        if protocol_version is None:
+            protocol_version = DEFAULT_PROTOCOL_VERSION
 
         # Check if the protocol version is supported
         if protocol_version not in SUPPORTED_PROTOCOL_VERSIONS:
@@ -711,9 +712,7 @@ async def _validate_protocol_version(self, request: Request, send: Send) -> bool
 
         return True
 
-    async def _replay_events(
-        self, last_event_id: str, request: Request, send: Send
-    ) -> None:
+    async def _replay_events(self, last_event_id: str, request: Request, send: Send) -> None:
         """
         Replays events that would have been sent after the specified event ID.
         Only used when resumability is enabled.

src/mcp/types.py

@@ -23,6 +23,7 @@
 """
 
 LATEST_PROTOCOL_VERSION = "2025-03-26"
+DEFAULT_PROTOCOL_VERSION = "2025-03-26"
 
 ProgressToken = str | int
 Cursor = str

tests/shared/test_streamable_http.py

@@ -1412,9 +1412,7 @@ async def test_streamablehttp_request_context_isolation(context_aware_server: No
 
 
 @pytest.mark.anyio
-async def test_client_includes_protocol_version_header_after_init(
-    context_aware_server, basic_server_url
-):
+async def test_client_includes_protocol_version_header_after_init(context_aware_server, basic_server_url):
     """Test that client includes mcp-protocol-version header after initialization."""
     async with streamablehttp_client(f"{basic_server_url}/mcp") as (
         read_stream,
@@ -1439,7 +1437,7 @@ async def test_client_includes_protocol_version_header_after_init(
 
 
 def test_server_validates_protocol_version_header(basic_server, basic_server_url):
-    """Test that server returns 400 Bad Request version if header missing or invalid."""
+    """Test that server returns 400 Bad Request version if header unsupported or invalid."""
     # First initialize a session to get a valid session ID
     init_response = requests.post(
         f"{basic_server_url}/mcp",
@@ -1464,10 +1462,7 @@ def test_server_validates_protocol_version_header(basic_server, basic_server_url
         json={"jsonrpc": "2.0", "method": "tools/list", "id": "test-2"},
     )
     assert response.status_code == 400
-    assert (
-        MCP_PROTOCOL_VERSION_HEADER in response.text
-        or "protocol version" in response.text.lower()
-    )
+    assert MCP_PROTOCOL_VERSION_HEADER in response.text or "protocol version" in response.text.lower()
 
     # Test request with unsupported protocol version (should fail)
     response = requests.post(
@@ -1481,10 +1476,7 @@ def test_server_validates_protocol_version_header(basic_server, basic_server_url
         json={"jsonrpc": "2.0", "method": "tools/list", "id": "test-3"},
     )
     assert response.status_code == 400
-    assert (
-        MCP_PROTOCOL_VERSION_HEADER in response.text
-        or "protocol version" in response.text.lower()
-    )
+    assert MCP_PROTOCOL_VERSION_HEADER in response.text or "protocol version" in response.text.lower()
 
     # Test request with valid protocol version (should succeed)
     negotiated_version = extract_protocol_version_from_sse(init_response)
@@ -1502,9 +1494,7 @@ def test_server_validates_protocol_version_header(basic_server, basic_server_url
     assert response.status_code == 200
 
 
-def test_server_backwards_compatibility_no_protocol_version(
-    basic_server, basic_server_url
-):
+def test_server_backwards_compatibility_no_protocol_version(basic_server, basic_server_url):
     """Test server accepts requests without protocol version header."""
     # First initialize a session to get a valid session ID
     init_response = requests.post(