Buffer per-request StreamableHTTP streams to avoid serial-router head-of-line block (#2934)

src/mcp/server/streamable_http.py

@@ -12,8 +12,9 @@ from abc import ABC, abstractmethod
 from collections.abc import AsyncGenerator, Awaitable, Callable
 from contextlib import asynccontextmanager
 from dataclasses import dataclass
+from functools import partial
 from http import HTTPStatus
-from typing import Any
+from typing import Any, Final
 
 import anyio
 import pydantic_core
@@ -59,6 +60,11 @@ CONTENT_TYPE_SSE = "text/event-stream"
 # Special key for the standalone GET stream
 GET_STREAM_KEY = "_GET_stream"
 
+# Buffer for the per-request `_request_streams` so the serial `message_router`
+# can deposit a response and move on instead of head-of-line blocking the
+# whole session on a lazily-started `sse_writer`. See #1764.
+REQUEST_STREAM_BUFFER_SIZE: Final = 16
+
 # Session ID validation pattern (visible ASCII characters ranging from 0x21 to 0x7E)
 # Pattern ensures entire string contains only valid characters by using ^ and $ anchors
 SESSION_ID_PATTERN = re.compile(r"^[\x21-\x7E]+$")
@@ -66,6 +72,8 @@ SESSION_ID_PATTERN = re.compile(r"^[\x21-\x7E]+$")
 # Type aliases
 StreamId = str
 EventId = str
+# An SSE event-dict as accepted by sse-starlette (`event`, `data`, `id`, `retry`).
+SSEEvent = dict[str, Any]
 
 
 @dataclass
@@ -169,7 +177,7 @@ class StreamableHTTPServerTransport:
                 MemoryObjectReceiveStream[EventMessage],
             ],
         ] = {}
-        self._sse_stream_writers: dict[RequestId, MemoryObjectSendStream[dict[str, str]]] = {}
+        self._sse_stream_writers: dict[RequestId, MemoryObjectSendStream[SSEEvent]] = {}
         self._terminated = False
         # Idle timeout cancel scope; managed by the session manager.
         self.idle_scope: anyio.CancelScope | None = None
@@ -256,31 +264,48 @@ class StreamableHTTPServerTransport:
 
         return SessionMessage(message, metadata=metadata)
 
-    async def _maybe_send_priming_event(
-        self,
-        request_id: RequestId,
-        sse_stream_writer: MemoryObjectSendStream[dict[str, Any]],
-        protocol_version: str,
-    ) -> None:
-        """Send priming event for SSE resumability if event_store is configured.
+    async def _mint_priming_event(self, stream_id: StreamId, protocol_version: str) -> SSEEvent | None:
+        """Store the priming cursor for `stream_id` and return its SSE wire form.
 
-        Only sends priming events to clients with protocol version >= 2025-11-25,
-        which includes the fix for handling empty SSE data. Older clients would
-        crash trying to parse empty data as JSON.
+        Called before the request is dispatched so the priming row precedes
+        anything `message_router` can store for this stream. Returns `None`
+        when no event store is configured or the client predates 2025-11-25
+        (older clients cannot parse the empty-data event).
         """
         if not self._event_store:
-            return
-        # Priming events have empty data which older clients cannot handle.
+            return None
         if not is_version_at_least(protocol_version, "2025-11-25"):
-            return
-        priming_event_id = await self._event_store.store_event(
-            str(request_id),  # Convert RequestId to StreamId (str)
-            None,  # Priming event has no payload
-        )
-        priming_event: dict[str, str | int] = {"id": priming_event_id, "data": ""}
+            return None
+        priming_event_id = await self._event_store.store_event(stream_id, None)
+        priming_event: SSEEvent = {"id": priming_event_id, "data": ""}
         if self._retry_interval is not None:
             priming_event["retry"] = self._retry_interval
-        await sse_stream_writer.send(priming_event)
+        return priming_event
+
+    async def _run_sse_writer(
+        self,
+        request_id: RequestId,
+        sse_stream_writer: MemoryObjectSendStream[SSEEvent],
+        request_stream_reader: MemoryObjectReceiveStream[EventMessage],
+        priming_event: SSEEvent | None,
+    ) -> None:
+        """Forward `_request_streams[request_id]` onto the SSE wire for one POST."""
+        try:
+            async with sse_stream_writer, request_stream_reader:
+                if priming_event is not None:
+                    await sse_stream_writer.send(priming_event)
+                async for event_message in request_stream_reader:
+                    await sse_stream_writer.send(self._create_event_data(event_message))
+                    if isinstance(event_message.message, JSONRPCResponse | JSONRPCError):
+                        break
+        except anyio.ClosedResourceError:  # pragma: lax no cover
+            logger.debug("SSE stream closed by close_sse_stream()")
+        except Exception:  # pragma: lax no cover
+            logger.exception("Error in SSE writer")
+        finally:
+            logger.debug("Closing SSE writer")
+            self._sse_stream_writers.pop(request_id, None)
+            await self._clean_up_memory_streams(request_id)
 
     def _create_error_response(
         self,
@@ -334,7 +359,7 @@ class StreamableHTTPServerTransport:
         """Extract the session ID from request headers."""
         return request.headers.get(MCP_SESSION_ID_HEADER)
 
-    def _create_event_data(self, event_message: EventMessage) -> dict[str, str]:
+    def _create_event_data(self, event_message: EventMessage) -> SSEEvent:
         """Create event data dictionary from an EventMessage."""
         event_data = {
             "event": "message",
@@ -521,13 +546,13 @@ class StreamableHTTPServerTransport:
                 else request.headers.get(MCP_PROTOCOL_VERSION_HEADER, DEFAULT_NEGOTIATED_VERSION)
             )
 
-            # Extract the request ID outside the try block for proper scope
             request_id = str(message.id)
-            # Register this stream for the request ID
-            self._request_streams[request_id] = anyio.create_memory_object_stream[EventMessage](0)
-            request_stream_reader = self._request_streams[request_id][1]
 
             if self.is_json_response_enabled:
+                self._request_streams[request_id] = anyio.create_memory_object_stream[EventMessage](
+                    REQUEST_STREAM_BUFFER_SIZE
+                )
+                request_stream_reader = self._request_streams[request_id][1]
                 # Process the message
                 metadata = ServerMessageMetadata(request_context=request)
                 session_message = SessionMessage(message, metadata=metadata)
@@ -571,41 +596,19 @@ class StreamableHTTPServerTransport:
                 finally:
                     await self._clean_up_memory_streams(request_id)
             else:
-                # Create SSE stream
-                sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[dict[str, str]](0)
+                # Mint the priming event before any per-request state exists:
+                # `EventStore.store_event` is user code and may raise, in which
+                # case the outer handler returns a 500 with nothing to clean up.
+                # Still strictly precedes dispatch, so storage order == wire order.
+                priming_event = await self._mint_priming_event(request_id, protocol_version)
 
-                # Store writer reference so close_sse_stream() can close it
+                sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[SSEEvent](0)
                 self._sse_stream_writers[request_id] = sse_stream_writer
+                self._request_streams[request_id] = anyio.create_memory_object_stream[EventMessage](
+                    REQUEST_STREAM_BUFFER_SIZE
+                )
+                request_stream_reader = self._request_streams[request_id][1]
 
-                async def sse_writer():
-                    # Get the request ID from the incoming request message
-                    try:
-                        async with sse_stream_writer, request_stream_reader:
-                            # Send priming event for SSE resumability
-                            await self._maybe_send_priming_event(request_id, sse_stream_writer, protocol_version)
-
-                            # Process messages from the request-specific stream
-                            async for event_message in request_stream_reader:
-                                # Build the event data
-                                event_data = self._create_event_data(event_message)
-                                await sse_stream_writer.send(event_data)
-
-                                # If response, remove from pending streams and close
-                                if isinstance(event_message.message, JSONRPCResponse | JSONRPCError):
-                                    break
-                    except anyio.ClosedResourceError:  # pragma: lax no cover
-                        # Expected when close_sse_stream() is called
-                        logger.debug("SSE stream closed by close_sse_stream()")
-                    except Exception:  # pragma: lax no cover
-                        logger.exception("Error in SSE writer")
-                    finally:
-                        logger.debug("Closing SSE writer")
-                        self._sse_stream_writers.pop(request_id, None)
-                        await self._clean_up_memory_streams(request_id)
-
-                # Create and start EventSourceResponse
-                # SSE stream mode (original behavior)
-                # Set up headers
                 headers = {
                     "Cache-Control": "no-cache, no-transform",
                     "Connection": "keep-alive",
@@ -614,7 +617,9 @@ class StreamableHTTPServerTransport:
                 }
                 response = EventSourceResponse(
                     content=sse_stream_reader,
-                    data_sender_callable=sse_writer,
+                    data_sender_callable=partial(
+                        self._run_sse_writer, request_id, sse_stream_writer, request_stream_reader, priming_event
+                    ),
                     headers=headers,
                 )
 
@@ -633,20 +638,16 @@ class StreamableHTTPServerTransport:
                 finally:
                     await sse_stream_reader.aclose()
 
-        except Exception as err:  # pragma: lax no cover
-            # Reached only when something raises during POST handling outside
-            # the per-SSE-stream guard above; whether tests reach this depends
-            # on client teardown timing.
+        except Exception as err:
             logger.exception("Error handling POST request")
             response = self._create_error_response(
-                f"Error handling POST request: {err}",
+                "Error handling POST request",
                 HTTPStatus.INTERNAL_SERVER_ERROR,
                 INTERNAL_ERROR,
             )
             await response(scope, receive, send)
-            if writer:
-                await writer.send(Exception(err))
-            return  # pragma: no cover
+            await writer.send(Exception(err))
+            return
 
     async def _handle_get_request(self, request: Request, send: Send) -> None:
         """Handle GET request to establish SSE.
@@ -697,13 +698,15 @@ class StreamableHTTPServerTransport:
             return
 
         # Create SSE stream
-        sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[dict[str, str]](0)
+        sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[SSEEvent](0)
 
         async def standalone_sse_writer():
             try:
                 # Create a standalone message stream for server-initiated messages
 
-                self._request_streams[GET_STREAM_KEY] = anyio.create_memory_object_stream[EventMessage](0)
+                self._request_streams[GET_STREAM_KEY] = anyio.create_memory_object_stream[EventMessage](
+                    REQUEST_STREAM_BUFFER_SIZE
+                )
                 standalone_stream_reader = self._request_streams[GET_STREAM_KEY][1]
 
                 async with sse_stream_writer, standalone_stream_reader:
@@ -871,7 +874,7 @@ class StreamableHTTPServerTransport:
             replay_protocol_version = request.headers.get(MCP_PROTOCOL_VERSION_HEADER, DEFAULT_NEGOTIATED_VERSION)
 
             # Create SSE stream for replay
-            sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[dict[str, str]](0)
+            sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[SSEEvent](0)
 
             async def replay_sender():
                 try:
@@ -886,22 +889,32 @@ class StreamableHTTPServerTransport:
 
                         # If stream ID not in mapping, create it
                         if stream_id and stream_id not in self._request_streams:  # pragma: no branch
-                            # Register SSE writer so close_sse_stream() can close it
-                            self._sse_stream_writers[stream_id] = sse_stream_writer
+                            try:
+                                # Register SSE writer so close_sse_stream() can close it
+                                self._sse_stream_writers[stream_id] = sse_stream_writer
 
-                            # Send priming event for this new connection
-                            await self._maybe_send_priming_event(stream_id, sse_stream_writer, replay_protocol_version)
+                                # Prime the resumed connection so the client sees the stream
+                                # is re-registered. The replay→live-tail ordering window here
+                                # is pre-existing and tracked separately.
+                                priming_event = await self._mint_priming_event(stream_id, replay_protocol_version)
+                                if priming_event is not None:
+                                    await sse_stream_writer.send(priming_event)
 
-                            # Create new request streams for this connection
-                            self._request_streams[stream_id] = anyio.create_memory_object_stream[EventMessage](0)
-                            msg_reader = self._request_streams[stream_id][1]
+                                # Create new request streams for this connection
+                                self._request_streams[stream_id] = anyio.create_memory_object_stream[EventMessage](
+                                    REQUEST_STREAM_BUFFER_SIZE
+                                )
+                                msg_reader = self._request_streams[stream_id][1]
 
-                            # Forward messages to SSE
-                            async with msg_reader:
-                                async for event_message in msg_reader:
-                                    event_data = self._create_event_data(event_message)
+                                # Forward messages to SSE
+                                async with msg_reader:
+                                    async for event_message in msg_reader:
+                                        event_data = self._create_event_data(event_message)
 
-                                    await sse_stream_writer.send(event_data)
+                                        await sse_stream_writer.send(event_data)
+                            finally:
+                                self._sse_stream_writers.pop(stream_id, None)
+                                await self._clean_up_memory_streams(stream_id)
                 except anyio.ClosedResourceError:  # pragma: lax no cover
                     # Expected when close_sse_stream() is called
                     logger.debug("Replay SSE stream closed by close_sse_stream()")

tests/interaction/transports/test_hosting_resume.py

@@ -113,6 +113,43 @@ async def test_a_post_sse_stream_begins_with_a_priming_event_and_stamps_every_ev
     )
 
 
+@requirement("hosting:resume:priming")
+async def test_the_priming_row_is_stored_before_any_handler_output_for_that_stream() -> None:
+    """The priming cursor is the first row the event store records for a request's stream.
+
+    The POST handler stores the priming row before dispatching the request, so by construction
+    it precedes anything `message_router` can store for that stream id.
+    """
+    store = SequencedEventStore()
+    mcp = MCPServer("resumable")
+
+    @mcp.tool()
+    async def burst(ctx: Context) -> str:
+        await ctx.info("a")  # pyright: ignore[reportDeprecated]
+        await ctx.info("b")  # pyright: ignore[reportDeprecated]
+        await ctx.info("c")  # pyright: ignore[reportDeprecated]
+        return "done"
+
+    async with mounted_app(mcp, event_store=store) as (http, _):
+        session_id = await initialize_via_http(http)
+        with anyio.fail_after(5):
+            async with http.stream(  # pragma: no branch
+                "POST", "/mcp", content=_tools_call(2, "burst", {}), headers=base_headers(session_id=session_id)
+            ) as response:
+                await _read_events(response, 5)
+
+    # initialize wrote two rows (its own priming + response); everything after is this call.
+    call_rows = store._events[2:]
+    stream_id = call_rows[0][0]
+    assert [(s, None if m is None else type(m).__name__) for s, m in call_rows] == [
+        (stream_id, None),
+        (stream_id, "JSONRPCNotification"),
+        (stream_id, "JSONRPCNotification"),
+        (stream_id, "JSONRPCNotification"),
+        (stream_id, "JSONRPCResponse"),
+    ]
+
+
 @requirement("hosting:resume:replay")
 @requirement("hosting:resume:stream-scoped")
 @requirement("hosting:resume:buffered-replay")
@@ -182,6 +219,46 @@ async def test_get_with_last_event_id_replays_only_that_streams_missed_events()
     )
 
 
+@requirement("hosting:resume:priming")
+async def test_a_pre_2025_11_25_reconnect_replays_without_minting_a_priming_event() -> None:
+    """A pre-2025-11-25 client reconnecting via Last-Event-ID gets the replay with no priming row.
+
+    The store-length assertion is the load-bearing proof that no priming cursor was minted.
+    """
+    release = anyio.Event()
+    store = SequencedEventStore()
+    mcp = MCPServer("resumable")
+
+    @mcp.tool()
+    async def count(ctx: Context) -> str:
+        await ctx.info("tick 1")  # pyright: ignore[reportDeprecated]
+        await release.wait()
+        await ctx.info("tick 2")  # pyright: ignore[reportDeprecated]
+        return "counted"
+
+    async with mounted_app(mcp, event_store=store, retry_interval=0) as (http, _):
+        session_id = await initialize_via_http(http)
+        with anyio.fail_after(5):
+            async with http.stream(
+                "POST", "/mcp", content=_tools_call(1, "count", {}), headers=base_headers(session_id=session_id)
+            ) as response:
+                _, first = await _read_events(response, 2)
+            release.set()
+            await store.wait_until_stored(6)
+            old_client_headers = base_headers(session_id=session_id) | {
+                "mcp-protocol-version": "2025-06-18",
+                "last-event-id": first.id,
+            }
+            async with http.stream("GET", "/mcp", headers=old_client_headers) as replay:  # pragma: no branch
+                assert replay.status_code == 200
+                missed = await _read_events(replay, 2)
+
+    assert [(event.id, bool(event.data)) for event in missed] == snapshot([("5", True), ("6", True)])
+    # No priming cursor was minted on reconnect: the store still holds only the six rows
+    # written before the GET (init priming+response, POST priming, tick 1, tick 2, result).
+    assert len(store._events) == 6
+
+
 @requirement("hosting:resume:bad-event-id")
 async def test_an_unknown_last_event_id_yields_an_empty_replay_stream() -> None:
     """A Last-Event-ID the event store cannot map produces an empty SSE stream rather than an error.

tests/server/test_streamable_http_router.py

@@ -0,0 +1,116 @@
+"""Regression coverage for the StreamableHTTP per-session response router."""
+
+import anyio
+import pytest
+from starlette.types import Message, Scope
+
+from mcp.server.streamable_http import (
+    REQUEST_STREAM_BUFFER_SIZE,
+    EventCallback,
+    EventId,
+    EventMessage,
+    EventStore,
+    StreamableHTTPServerTransport,
+    StreamId,
+)
+from mcp.shared.message import SessionMessage
+from mcp.types import JSONRPCMessage, JSONRPCResponse
+
+
+class _PrimingFailingStore(EventStore):
+    async def store_event(self, stream_id: StreamId, message: JSONRPCMessage | None) -> EventId:
+        raise RuntimeError("backend unavailable")
+
+    async def replay_events_after(self, last_event_id: EventId, send_callback: EventCallback) -> StreamId | None:
+        raise NotImplementedError
+
+
+@pytest.mark.anyio
+async def test_router_unconsumed_request_stream_does_not_block_siblings() -> None:
+    """A response whose `sse_writer` is not yet receiving must not park the router (#1764).
+
+    Drives the routing layer directly (the production race does not reproduce
+    on loopback), so this pins the router semantics, not the call sites.
+    """
+    transport = StreamableHTTPServerTransport(mcp_session_id="sid", is_json_response_enabled=False)
+    streams = transport._request_streams
+    async with transport.connect() as (_read_stream, write_stream):
+        # Model two concurrent POSTs at the point _handle_post_request has
+        # registered the per-request stream but A's sse_writer has not yet
+        # reached its first receive().
+        streams["A"] = anyio.create_memory_object_stream[EventMessage](REQUEST_STREAM_BUFFER_SIZE)
+        streams["B"] = anyio.create_memory_object_stream[EventMessage](REQUEST_STREAM_BUFFER_SIZE)
+        a_send, a_recv = streams["A"]
+        b_reader = streams["B"][1]
+        b_received = anyio.Event()
+
+        async def consume_b() -> None:
+            async with b_reader:
+                await b_reader.receive()
+                b_received.set()
+
+        async def server_writes() -> None:
+            await write_stream.send(SessionMessage(JSONRPCResponse(jsonrpc="2.0", id="A", result={})))
+            await write_stream.send(SessionMessage(JSONRPCResponse(jsonrpc="2.0", id="B", result={})))
+
+        async with anyio.create_task_group() as tg:
+            tg.start_soon(consume_b)
+            tg.start_soon(server_writes)
+            with anyio.fail_after(5):
+                await b_received.wait()
+            # A's response was buffered for its (late) consumer, not dropped.
+            assert a_send.statistics().current_buffer_used == 1
+            await a_recv.aclose()
+            await a_send.aclose()
+
+
+@pytest.mark.anyio
+async def test_priming_store_failure_leaves_no_per_request_state() -> None:
+    """`EventStore.store_event` raising on the priming row must not leak per-request entries."""
+    transport = StreamableHTTPServerTransport(
+        mcp_session_id=None,
+        is_json_response_enabled=False,
+        event_store=_PrimingFailingStore(),
+    )
+
+    body = b'{"jsonrpc":"2.0","id":"req-1","method":"tools/list","params":{}}'
+    scope: Scope = {
+        "type": "http",
+        "method": "POST",
+        "path": "/",
+        "query_string": b"",
+        "headers": [
+            (b"accept", b"application/json, text/event-stream"),
+            (b"content-type", b"application/json"),
+            (b"mcp-protocol-version", b"2025-11-25"),
+        ],
+    }
+    body_sent = False
+
+    async def receive() -> Message:
+        nonlocal body_sent
+        if not body_sent:
+            body_sent = True
+            return {"type": "http.request", "body": body, "more_body": False}
+        raise NotImplementedError
+
+    sent: list[Message] = []
+
+    async def asgi_send(message: Message) -> None:
+        sent.append(message)
+
+    async with transport.connect() as (read_stream, _write_stream):
+        async with anyio.create_task_group() as tg:
+            tg.start_soon(transport.handle_request, scope, receive, asgi_send)
+            with anyio.fail_after(5):
+                forwarded = await read_stream.receive()
+            assert isinstance(forwarded, Exception)
+        # handle_request has returned; connect()'s finally (which clears
+        # _request_streams unconditionally) has not yet run.
+        assert transport._request_streams == {}
+        assert transport._sse_stream_writers == {}
+
+    assert sent[0]["type"] == "http.response.start"
+    assert sent[0]["status"] == 500
+    body = b"".join(m.get("body", b"") for m in sent if m["type"] == "http.response.body")
+    assert b"backend unavailable" not in body

tests/shared/test_streamable_http.py

@@ -1634,84 +1634,6 @@ async def test_handle_sse_event_skips_empty_data() -> None:
         await read_stream.aclose()
 
 
-@pytest.mark.anyio
-async def test_priming_event_not_sent_for_old_protocol_version() -> None:
-    """_maybe_send_priming_event skips for old protocol versions (backwards compat)."""
-    # Create a transport with an event store
-    transport = StreamableHTTPServerTransport(
-        "/mcp",
-        event_store=SimpleEventStore(),
-    )
-
-    # Create a mock stream writer
-    write_stream, read_stream = anyio.create_memory_object_stream[dict[str, Any]](1)
-
-    try:
-        # Call _maybe_send_priming_event with OLD protocol version - should NOT send
-        await transport._maybe_send_priming_event("test-request-id", write_stream, "2025-06-18")
-
-        # Nothing should have been written to the stream
-        assert write_stream.statistics().current_buffer_used == 0
-
-        # Now test with NEW protocol version - should send
-        await transport._maybe_send_priming_event("test-request-id-2", write_stream, "2025-11-25")
-
-        # Should have written a priming event
-        assert write_stream.statistics().current_buffer_used == 1
-    finally:
-        await write_stream.aclose()
-        await read_stream.aclose()
-
-
-@pytest.mark.anyio
-async def test_priming_event_not_sent_without_event_store() -> None:
-    """_maybe_send_priming_event returns early when no event_store is configured."""
-    # Create a transport WITHOUT an event store
-    transport = StreamableHTTPServerTransport("/mcp")
-
-    # Create a mock stream writer
-    write_stream, read_stream = anyio.create_memory_object_stream[dict[str, Any]](1)
-
-    try:
-        # Call _maybe_send_priming_event - should return early without sending
-        await transport._maybe_send_priming_event("test-request-id", write_stream, "2025-11-25")
-
-        # Nothing should have been written to the stream
-        assert write_stream.statistics().current_buffer_used == 0
-    finally:
-        await write_stream.aclose()
-        await read_stream.aclose()
-
-
-@pytest.mark.anyio
-async def test_priming_event_includes_retry_interval() -> None:
-    """_maybe_send_priming_event includes the retry field when retry_interval is set."""
-    # Create a transport with an event store AND retry_interval
-    transport = StreamableHTTPServerTransport(
-        "/mcp",
-        event_store=SimpleEventStore(),
-        retry_interval=5000,
-    )
-
-    # Create a mock stream writer
-    write_stream, read_stream = anyio.create_memory_object_stream[dict[str, Any]](1)
-
-    try:
-        # Call _maybe_send_priming_event with new protocol version
-        await transport._maybe_send_priming_event("test-request-id", write_stream, "2025-11-25")
-
-        # Should have written a priming event with retry field
-        assert write_stream.statistics().current_buffer_used == 1
-
-        # Read the event and verify it has retry field
-        event = await read_stream.receive()
-        assert "retry" in event
-        assert event["retry"] == 5000
-    finally:
-        await write_stream.aclose()
-        await read_stream.aclose()
-
-
 @pytest.mark.anyio
 async def test_close_sse_stream_callback_not_provided_for_old_protocol_version() -> None:
     """close_sse_stream callbacks are only provided for protocol versions that support polling."""
@@ -1744,30 +1666,6 @@ async def test_close_sse_stream_callback_not_provided_for_old_protocol_version()
     assert session_msg_new.metadata.close_standalone_sse_stream is not None
 
 
-@pytest.mark.anyio
-async def test_priming_event_not_sent_for_unknown_protocol_version() -> None:
-    """_maybe_send_priming_event treats unrecognized version strings conservatively.
-
-    A garbage version must not be mistaken for a future one (lexicographically
-    "zzz" sorts after every date-shaped revision).
-    """
-    transport = StreamableHTTPServerTransport(
-        "/mcp",
-        event_store=SimpleEventStore(),
-    )
-
-    write_stream, read_stream = anyio.create_memory_object_stream[dict[str, Any]](1)
-
-    try:
-        await transport._maybe_send_priming_event("test-request-id", write_stream, "zzz")
-
-        # Nothing should have been written to the stream
-        assert write_stream.statistics().current_buffer_used == 0
-    finally:
-        await write_stream.aclose()
-        await read_stream.aclose()
-
-
 @pytest.mark.anyio
 async def test_close_sse_stream_callback_not_provided_for_unknown_protocol_version() -> None:
     """close_sse_stream callbacks are withheld when the client's version is unrecognized."""