Resolve protocol version per request and expose it as ctx.protocol_version (#2886)

docs/migration.md

@@ -674,7 +674,7 @@ ctx: ClientRequestContext
 server_ctx: ServerRequestContext[LifespanContextT, RequestT]
 ```
 
-`ServerRequestContext` is now a standalone dataclass — it no longer subclasses `RequestContext[ServerSession]`. It carries the same fields (`session`, `request_id`, `meta`, `lifespan_context`, `request`, `close_sse_stream`, `close_standalone_sse_stream`), so handler code is unaffected, but `isinstance(ctx, RequestContext)` checks and `RequestContext[ServerSession]` annotations need updating to `ServerRequestContext`.
+`ServerRequestContext` is now a standalone dataclass — it no longer subclasses `RequestContext[ServerSession]`. It carries the same fields (`session`, `request_id`, `meta`, `lifespan_context`, `request`, `close_sse_stream`, `close_standalone_sse_stream`) plus a new `protocol_version: str` field, so handler code is unaffected, but `isinstance(ctx, RequestContext)` checks and `RequestContext[ServerSession]` annotations need updating to `ServerRequestContext`.
 
 The high-level `Context` class (injected into `@mcp.tool()` etc.) similarly dropped its `ServerSessionT` parameter: `Context[ServerSessionT, LifespanContextT, RequestT]` → `Context[LifespanContextT, RequestT]`. Both remaining parameters have defaults, so bare `Context` is usually sufficient:
 

src/mcp/server/connection.py

@@ -87,7 +87,7 @@ class Connection:
     """The protocol version negotiated during `initialize`; `None` before
     initialization. Stateless connections don't require the handshake, so this
     normally stays `None` there (a client that sends `initialize` anyway still
-    commits it). Handlers read this as `ServerSession.protocol_version`."""
+    commits it). For the per-request value, read `ctx.protocol_version`."""
 
     initialized: anyio.Event
     """Set when `notifications/initialized` arrives (matches TS `oninitialized`);

src/mcp/server/context.py

@@ -31,6 +31,7 @@ class ServerRequestContext(Generic[LifespanContextT, RequestT]):
 
     session: ServerSession
     lifespan_context: LifespanContextT
+    protocol_version: str
     request_id: RequestId | None = None
     meta: RequestParamsMeta | None = None
     request: RequestT | None = None

src/mcp/server/runner.py

@@ -34,7 +34,7 @@ from mcp.shared._otel import extract_trace_context, otel_span
 from mcp.shared.dispatcher import DispatchContext, DispatchMiddleware, OnRequest
 from mcp.shared.exceptions import MCPError
 from mcp.shared.jsonrpc_dispatcher import JSONRPCDispatcher
-from mcp.shared.message import ServerMessageMetadata
+from mcp.shared.message import MessageMetadata, ServerMessageMetadata
 from mcp.shared.transport_context import TransportContext
 from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 from mcp.types import (
@@ -42,6 +42,7 @@ from mcp.types import (
     INVALID_PARAMS,
     LATEST_PROTOCOL_VERSION,
     METHOD_NOT_FOUND,
+    PROTOCOL_VERSION_META_KEY,
     ErrorData,
     Implementation,
     InitializeRequestParams,
@@ -79,6 +80,30 @@ def _extract_meta(params: Mapping[str, Any] | None) -> RequestParamsMeta | None:
         return None
 
 
+def _resolve_protocol_version(
+    negotiated: str | None,
+    meta: RequestParamsMeta | None,
+    md: MessageMetadata,
+) -> str:
+    """Resolve the protocol version for this inbound message.
+
+    Handshake-committed value wins; else per-request `_meta`, else the
+    transport hint. Unsupported values fall through so surface validation
+    never sees them.
+    """
+    if negotiated is not None:
+        return negotiated
+    if meta is not None:
+        v = meta.get(PROTOCOL_VERSION_META_KEY)
+        if isinstance(v, str) and v in SUPPORTED_PROTOCOL_VERSIONS:
+            return v
+    if isinstance(md, ServerMessageMetadata):
+        hint = md.protocol_version
+        if hint is not None and hint in SUPPORTED_PROTOCOL_VERSIONS:
+            return hint
+    return "2025-11-25"
+
+
 def otel_middleware(next_on_request: OnRequest) -> OnRequest:
     """Dispatch-tier middleware that wraps each request in an OpenTelemetry span.
 
@@ -218,11 +243,9 @@ class ServerRunner(Generic[LifespanT]):
         method: str,
         params: Mapping[str, Any] | None,
     ) -> dict[str, Any]:
-        ctx = self._make_context(dctx, _extract_meta(params))
+        meta = _extract_meta(params)
-        # Literal, not LATEST_PROTOCOL_VERSION: the fallback covers the initialize
+        version = _resolve_protocol_version(self.connection.protocol_version, meta, dctx.message_metadata)
-        # handshake (which only exists at <=2025) and stateless until the header
+        ctx = self._make_context(dctx, meta, version)
-        # is plumbed; its meaning is fixed regardless of LATEST bumps.
-        version = self.connection.protocol_version or "2025-11-25"
         is_spec_method = method in _methods.SPEC_CLIENT_METHODS
 
         async def _inner() -> HandlerResult:
@@ -289,9 +312,9 @@ class ServerRunner(Generic[LifespanT]):
         method: str,
         params: Mapping[str, Any] | None,
     ) -> None:
-        ctx = self._make_context(dctx, _extract_meta(params))
+        meta = _extract_meta(params)
-        # Same fallback as `_on_request`: covers pre-handshake and stateless.
+        version = _resolve_protocol_version(self.connection.protocol_version, meta, dctx.message_metadata)
-        version = self.connection.protocol_version or "2025-11-25"
+        ctx = self._make_context(dctx, meta, version)
 
         async def _inner() -> None:
             if method in _methods.SPEC_CLIENT_NOTIFICATION_METHODS:
@@ -349,7 +372,7 @@ class ServerRunner(Generic[LifespanT]):
         return call
 
     def _make_context(
-        self, dctx: DispatchContext[TransportContext], meta: RequestParamsMeta | None
+        self, dctx: DispatchContext[TransportContext], meta: RequestParamsMeta | None, protocol_version: str
     ) -> ServerRequestContext[LifespanT, Any]:
         # TODO(maxisbey): remove for Context rework. Reads the SHTTP per-request
         # data off the raw `dctx.message_metadata` carrier; replace with the
@@ -366,6 +389,7 @@ class ServerRunner(Generic[LifespanT]):
             lifespan_context=self.lifespan_state,
             request_id=dctx.request_id,
             meta=meta,
+            protocol_version=protocol_version,
             request=request,
             close_sse_stream=close_sse_stream,
             close_standalone_sse_stream=close_standalone_sse_stream,

src/mcp/server/session.py

@@ -55,10 +55,8 @@ class ServerSession:
     def protocol_version(self) -> str | None:
         """The protocol version negotiated during `initialize`.
 
-        `None` before initialization completes. Stateless connections don't
+        `None` before initialization, and normally `None` on stateless
-        require the handshake, so this is normally `None` there (on streamable
+        connections. For the per-request value, read `ctx.protocol_version`.
-        HTTP the per-request version is the `MCP-Protocol-Version` header,
-        available via `ctx.request.headers`).
         """
         return self._connection.protocol_version
 

src/mcp/server/streamable_http.py

@@ -248,11 +248,12 @@ class StreamableHTTPServerTransport:
 
             metadata = ServerMessageMetadata(
                 request_context=request,
+                protocol_version=protocol_version,
                 close_sse_stream=close_stream_callback,
                 close_standalone_sse_stream=close_standalone_stream_callback,
             )
         else:
-            metadata = ServerMessageMetadata(request_context=request)
+            metadata = ServerMessageMetadata(request_context=request, protocol_version=protocol_version)
 
         return SessionMessage(message, metadata=metadata)
 
@@ -506,7 +507,10 @@ class StreamableHTTPServerTransport:
                 await response(scope, receive, send)
 
                 # Process the message after sending the response
-                metadata = ServerMessageMetadata(request_context=request)
+                metadata = ServerMessageMetadata(
+                    request_context=request,
+                    protocol_version=request.headers.get(MCP_PROTOCOL_VERSION_HEADER, DEFAULT_NEGOTIATED_VERSION),
+                )
                 session_message = SessionMessage(message, metadata=metadata)
                 await writer.send(session_message)
 
@@ -529,7 +533,7 @@ class StreamableHTTPServerTransport:
 
             if self.is_json_response_enabled:
                 # Process the message
-                metadata = ServerMessageMetadata(request_context=request)
+                metadata = ServerMessageMetadata(request_context=request, protocol_version=protocol_version)
                 session_message = SessionMessage(message, metadata=metadata)
                 await writer.send(session_message)
                 try:

src/mcp/shared/message.py

@@ -35,6 +35,9 @@ class ServerMessageMetadata:
     # transports, None for stdio). Typed as Any because the server layer is
     # transport-agnostic.
     request_context: Any = None
+    # Per-message protocol version observed by the transport (e.g. the
+    # validated MCP-Protocol-Version header).
+    protocol_version: str | None = None
     # Callback to close SSE stream for the current request without terminating
     close_sse_stream: CloseSSEStreamCallback | None = None
     # Callback to close the standalone GET SSE stream (for unsolicited notifications)

tests/issues/test_176_progress_token.py

@@ -21,6 +21,7 @@ async def test_progress_token_zero_first_call():
         session=mock_session,
         meta={"progress_token": 0},
         lifespan_context=None,
+        protocol_version="2025-11-25",
     )
 
     # Create context with our mocks

tests/server/mcpserver/test_server.py

@@ -1501,6 +1501,7 @@ async def test_report_progress_passes_related_request_id():
         session=mock_session,
         meta={"progress_token": "tok-1"},
         lifespan_context=None,
+        protocol_version="2025-11-25",
     )
 
     ctx = Context(request_context=request_context, mcp_server=MagicMock())

tests/server/test_runner.py

@@ -18,11 +18,12 @@ import mcp.server.runner
 from mcp.server.context import ServerRequestContext
 from mcp.server.lowlevel.server import NotificationOptions, Server
 from mcp.server.models import InitializationOptions
-from mcp.server.runner import ServerRunner, _extract_meta, otel_middleware
+from mcp.server.runner import ServerRunner, _extract_meta, _resolve_protocol_version, otel_middleware
 from mcp.server.session import ServerSession
 from mcp.shared.dispatcher import DispatchContext, DispatchMiddleware, OnRequest
 from mcp.shared.exceptions import MCPError
 from mcp.shared.jsonrpc_dispatcher import JSONRPCDispatcher
+from mcp.shared.message import ClientMessageMetadata, ServerMessageMetadata
 from mcp.shared.peer import dump_params
 from mcp.shared.transport_context import TransportContext
 from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
@@ -31,6 +32,7 @@ from mcp.types import (
     INVALID_PARAMS,
     LATEST_PROTOCOL_VERSION,
     METHOD_NOT_FOUND,
+    PROTOCOL_VERSION_META_KEY,
     CallToolRequestParams,
     ClientCapabilities,
     ErrorData,
@@ -41,6 +43,7 @@ from mcp.types import (
     PaginatedRequestParams,
     ProgressNotificationParams,
     RequestParams,
+    RequestParamsMeta,
     SetLevelRequestParams,
     Tool,
 )
@@ -218,6 +221,7 @@ async def test_runner_routes_to_handler_and_builds_context(server: SrvT):
     assert isinstance(ctx.session, ServerSession)
     assert ctx.session is runner.session
     assert ctx.request_id is not None
+    assert ctx.protocol_version == LATEST_PROTOCOL_VERSION
 
 
 @pytest.mark.anyio
@@ -650,6 +654,65 @@ async def test_runner_server_middleware_wraps_notifications(server: SrvT):
     ]
 
 
+def test_resolve_protocol_version_handshake_committed_value_wins():
+    md = ServerMessageMetadata(protocol_version="2025-03-26")
+    meta: RequestParamsMeta = {PROTOCOL_VERSION_META_KEY: "2025-03-26"}
+    assert _resolve_protocol_version("2025-06-18", meta, md) == "2025-06-18"
+
+
+def test_resolve_protocol_version_reads_per_request_meta_when_no_handshake():
+    md = ServerMessageMetadata(protocol_version="2025-03-26")
+    meta: RequestParamsMeta = {PROTOCOL_VERSION_META_KEY: "2025-06-18"}
+    assert _resolve_protocol_version(None, meta, md) == "2025-06-18"
+
+
+def test_resolve_protocol_version_skips_unsupported_meta_value():
+    md = ServerMessageMetadata(protocol_version="2025-03-26")
+    meta: RequestParamsMeta = {PROTOCOL_VERSION_META_KEY: "1900-01-01"}
+    assert _resolve_protocol_version(None, meta, md) == "2025-03-26"
+
+
+def test_resolve_protocol_version_skips_non_string_meta_value():
+    md = ServerMessageMetadata(protocol_version="2025-03-26")
+    meta: RequestParamsMeta = {PROTOCOL_VERSION_META_KEY: 42}
+    assert _resolve_protocol_version(None, meta, md) == "2025-03-26"
+
+
+def test_resolve_protocol_version_reads_transport_hint_when_no_handshake_or_meta():
+    md = ServerMessageMetadata(protocol_version="2025-06-18")
+    assert _resolve_protocol_version(None, None, md) == "2025-06-18"
+    assert _resolve_protocol_version(None, {}, md) == "2025-06-18"
+
+
+def test_resolve_protocol_version_skips_unsupported_transport_hint():
+    """The `initialize` params version reaches the metadata unvalidated; surface validation must never see it."""
+    md = ServerMessageMetadata(protocol_version="1900-01-01")
+    assert _resolve_protocol_version(None, None, md) == "2025-11-25"
+
+
+def test_resolve_protocol_version_terminal_default_with_no_signals():
+    assert _resolve_protocol_version(None, None, None) == "2025-11-25"
+    assert _resolve_protocol_version(None, None, ServerMessageMetadata()) == "2025-11-25"
+    assert _resolve_protocol_version(None, None, ClientMessageMetadata()) == "2025-11-25"
+
+
+@pytest.mark.anyio
+async def test_runner_ctx_protocol_version_is_terminal_default_on_stateless_in_memory(server: SrvT):
+    async with connected_runner(server, initialized=False, stateless=True) as (client, runner):
+        await client.send_raw_request("tools/list", None)
+    ctx = _seen_ctx[0]
+    assert ctx.protocol_version == "2025-11-25"
+    assert ctx.session.protocol_version is None
+    assert runner.connection.protocol_version is None
+
+
+@pytest.mark.anyio
+async def test_runner_ctx_protocol_version_tracks_per_request_meta_on_stateless(server: SrvT):
+    async with connected_runner(server, initialized=False, stateless=True) as (client, _):
+        await client.send_raw_request("tools/list", {"_meta": {PROTOCOL_VERSION_META_KEY: "2025-06-18"}})
+    assert _seen_ctx[0].protocol_version == "2025-06-18"
+
+
 def test_extract_meta_returns_none_for_absent_or_malformed():
     """Context construction is independent of `_meta` validity; the params
     validation inside `call_next()` is what surfaces the error."""

tests/shared/test_streamable_http.py

@@ -49,6 +49,7 @@ from mcp.shared._context_streams import create_context_streams
 from mcp.shared.message import ClientMessageMetadata, ServerMessageMetadata, SessionMessage
 from mcp.shared.session import RequestResponder
 from mcp.types import (
+    DEFAULT_NEGOTIATED_VERSION,
     CallToolRequestParams,
     CallToolResult,
     InitializeResult,
@@ -81,14 +82,18 @@ BASE_URL = "http://127.0.0.1:8000"
 
 
 # Helper functions
-def extract_protocol_version_from_sse(response: httpx.Response) -> str:
+def first_sse_data(response: httpx.Response) -> dict[str, Any]:
-    """Extract the negotiated protocol version from an SSE initialization response."""
+    """Return the first SSE `data:` payload of a response, parsed as JSON."""
     assert response.headers.get("Content-Type") == "text/event-stream"
     for line in response.text.splitlines():
         if line.startswith("data: "):
-            init_data = json.loads(line[6:])
+            return json.loads(line.removeprefix("data: "))
-            return init_data["result"]["protocolVersion"]
+    raise ValueError("No data event in SSE response")  # pragma: no cover
-    raise ValueError("Could not extract protocol version from SSE response")  # pragma: no cover
+
+
+def extract_protocol_version_from_sse(response: httpx.Response) -> str:
+    """Extract the negotiated protocol version from an SSE initialization response."""
+    return first_sse_data(response)["result"]["protocolVersion"]
 
 
 # Simple in-memory event store for testing
@@ -1318,13 +1323,14 @@ async def _handle_context_call_tool(ctx: ServerRequestContext, params: CallToolR
         "headers": dict(ctx.request.headers),
         "method": ctx.request.method,
         "path": ctx.request.url.path,
+        "protocol_version": ctx.protocol_version,
+        "session_protocol_version": ctx.session.protocol_version,
     }
     return CallToolResult(content=[TextContent(type="text", text=json.dumps(context_data))])
 
 
-@pytest.fixture
+@asynccontextmanager
-async def context_app() -> AsyncIterator[Starlette]:
+async def _run_context_app(*, stateless: bool) -> AsyncIterator[Starlette]:
-    """An app whose server echoes request context, served in process."""
     server = Server(
         "ContextAwareServer",
         on_list_tools=_handle_context_list_tools,
@@ -1332,6 +1338,7 @@ async def context_app() -> AsyncIterator[Starlette]:
     )
     session_manager = StreamableHTTPSessionManager(
         app=server,
+        stateless=stateless,
         security_settings=TransportSecuritySettings(enable_dns_rebinding_protection=False),
     )
     app = Starlette(routes=[Mount("/mcp", app=session_manager.handle_request)])
@@ -1339,6 +1346,51 @@ async def context_app() -> AsyncIterator[Starlette]:
         yield app
 
 
+@pytest.fixture
+async def context_app() -> AsyncIterator[Starlette]:
+    """An app whose server echoes request context, served in process."""
+    async with _run_context_app(stateless=False) as app:
+        yield app
+
+
+@pytest.fixture
+async def stateless_context_app() -> AsyncIterator[Starlette]:
+    async with _run_context_app(stateless=True) as app:
+        yield app
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    ("header_value", "expected"),
+    [
+        ("2025-06-18", "2025-06-18"),
+        ("2025-11-25", "2025-11-25"),
+        (None, DEFAULT_NEGOTIATED_VERSION),
+    ],
+)
+async def test_streamablehttp_stateless_ctx_protocol_version_tracks_the_header(
+    stateless_context_app: Starlette, header_value: str | None, expected: str
+) -> None:
+    """No handshake on stateless: the header (or the spec's 2025-03-26 default) reaches `ctx.protocol_version`."""
+    body = JSONRPCRequest(
+        jsonrpc="2.0",
+        id=1,
+        method="tools/call",
+        params={"name": "echo_context", "arguments": {"request_id": "r"}},
+    )
+    headers = {"Accept": "application/json, text/event-stream", "Content-Type": "application/json"}
+    if header_value is not None:
+        headers[MCP_PROTOCOL_VERSION_HEADER] = header_value
+    async with make_client(stateless_context_app) as client:
+        response = await client.post(
+            f"{BASE_URL}/mcp", json=body.model_dump(by_alias=True, exclude_none=True), headers=headers
+        )
+    assert response.status_code == 200
+    echoed = json.loads(first_sse_data(response)["result"]["content"][0]["text"])
+    assert echoed["protocol_version"] == expected
+    assert echoed["session_protocol_version"] is None
+
+
 @pytest.mark.anyio
 async def test_streamablehttp_request_context_propagation(context_app: Starlette) -> None:
     """Custom HTTP headers on the connection are visible to server handlers via ctx.request."""