Absorb #2926 (SEP-2577 deprecations); fix client 4xx id-correlation

After rebasing onto #2926:
- Re-add # pyright: ignore[reportDeprecated] on the deprecated method calls in the
  reworked test_connection.py / test_server_context.py / test_stateless_mode.py
  (the rewrites had dropped main's suppressions)

Fix to the client 4xx body-parsing patch (bb9b134):
- A server's 4xx JSON-RPC error body may carry id:null (request rejected before its
  id was parsed). Surfacing that verbatim broke response correlation (client waits
  for id=N, gets id=null, hangs). Now use the parsed error data with the client's
  own request id.
- Only surface the body when it's a JSONRPCError; anything else falls through to
  the status-derived stand-in.
- Relax the two 'Session terminated' message-text assertions in test_streamable_http.py
  (the client now surfaces the server's actual message).

Author: maxisbey

src/mcp/client/streamable_http.py

@@ -324,10 +324,15 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
                         try:
                             body = await response.aread()
                             parsed = jsonrpc_message_adapter.validate_json(body, by_name=False)
-                            await ctx.read_stream_writer.send(SessionMessage(parsed))
-                            return
+                            if isinstance(parsed, JSONRPCError):
+                                # The server may have set `id: null` (request rejected before its
+                                # id was parsed); use this request's id so correlation works.
+                                reply = JSONRPCError(jsonrpc="2.0", id=message.id, error=parsed.error)
+                                await ctx.read_stream_writer.send(SessionMessage(reply))
+                                return
                         except (httpx.StreamError, ValidationError):
-                            logger.debug("Non-2xx body was not a valid JSON-RPC message; using fallback error")
+                            pass
+                        logger.debug("Non-2xx body was not a JSON-RPC error; using fallback")
                     if response.status_code == 404:
                         error_data = ErrorData(code=INVALID_REQUEST, message="Session terminated")
                     else:

tests/server/test_connection.py

@@ -274,7 +274,7 @@ async def test_connection_ping_sends_ping_on_standalone():
 async def test_connection_log_sends_logging_message_notification():
     out = StubOutbound()
     conn = Connection.for_loop(out)
-    await conn.log("info", {"k": "v"}, logger="my.logger")
+    await conn.log("info", {"k": "v"}, logger="my.logger")  # pyright: ignore[reportDeprecated]
     method, params = out.notifications[0]
     assert method == "notifications/message"
     assert params is not None
@@ -287,7 +287,7 @@ async def test_connection_log_sends_logging_message_notification():
 async def test_connection_log_with_meta_includes_meta_in_params():
     out = StubOutbound()
     conn = Connection.for_loop(out)
-    await conn.log("info", "x", meta={"traceId": "abc"})
+    await conn.log("info", "x", meta={"traceId": "abc"})  # pyright: ignore[reportDeprecated]
     _, params = out.notifications[0]
     assert params is not None
     assert params["_meta"] == {"traceId": "abc"}

tests/server/test_server_context.py

@@ -58,7 +58,7 @@ async def test_context_log_sends_request_scoped_message_notification():
 
     async def server_on_request(dctx: DCtx, method: str, params: Mapping[str, Any] | None) -> dict[str, Any]:
         ctx: Context[_Lifespan] = Context(dctx, lifespan=_Lifespan("app"), connection=Connection.for_loop(dctx))
-        await ctx.log("debug", "hello")
+        await ctx.log("debug", "hello")  # pyright: ignore[reportDeprecated]
         return {}
 
     async with running_pair(direct_pair, server_on_request=server_on_request, client_on_notify=c_notify) as (
@@ -80,7 +80,7 @@ async def test_context_log_includes_logger_and_meta_when_supplied():
 
     async def server_on_request(dctx: DCtx, method: str, params: Mapping[str, Any] | None) -> dict[str, Any]:
         ctx: Context[_Lifespan] = Context(dctx, lifespan=_Lifespan("app"), connection=Connection.for_loop(dctx))
-        await ctx.log("info", "x", logger="my.log", meta={"traceId": "t"})
+        await ctx.log("info", "x", logger="my.log", meta={"traceId": "t"})  # pyright: ignore[reportDeprecated]
         return {}
 
     async with running_pair(direct_pair, server_on_request=server_on_request, client_on_notify=c_notify) as (

tests/server/test_stateless_mode.py

@@ -61,7 +61,7 @@ async def test_list_roots_raises_no_back_channel(no_channel_session: ServerSessi
     """SDK-defined: `list_roots` has no `related_request_id` so it always rides
     the standalone channel, which raises here."""
     with pytest.raises(NoBackChannelError) as exc:
-        await no_channel_session.list_roots()
+        await no_channel_session.list_roots()  # pyright: ignore[reportDeprecated]
     assert exc.value.method == "roots/list"
 
 
@@ -77,7 +77,7 @@ async def test_send_ping_raises_no_back_channel(no_channel_session: ServerSessio
 async def test_create_message_raises_no_back_channel_without_related_id(no_channel_session: ServerSession):
     """SDK-defined: `create_message` without a related id rides the standalone channel and raises."""
     with pytest.raises(NoBackChannelError) as exc:
-        await no_channel_session.create_message(
+        await no_channel_session.create_message(  # pyright: ignore[reportDeprecated]
             messages=[types.SamplingMessage(role="user", content=types.TextContent(type="text", text="hi"))],
             max_tokens=100,
         )
@@ -144,7 +144,7 @@ async def test_loop_connection_outbound_does_not_raise_no_back_channel():
     conn = Connection.for_loop(standalone)
     assert conn.has_standalone_channel is True
     session = ServerSession(StubOutbound(), conn, standalone_outbound=conn.outbound)
-    result = await session.list_roots()
+    result = await session.list_roots()  # pyright: ignore[reportDeprecated]
     assert isinstance(result, types.ListRootsResult)
     assert standalone.requests[0][0] == "roots/list"
 

tests/shared/test_streamable_http.py

@@ -1045,7 +1045,7 @@ async def test_streamable_http_client_session_termination(basic_app: Starlette)
         ):
             async with ClientSession(read_stream, write_stream) as session:  # pragma: no branch
                 # Attempt to make a request after termination
-                with pytest.raises(MCPError, match="Session terminated"):  # pragma: no branch
+                with pytest.raises(MCPError, match="[Ss]ession.*terminated"):  # pragma: no branch
                     await session.list_tools()
 
 
@@ -1106,7 +1106,7 @@ async def mock_delete(self: httpx.AsyncClient, *args: Any, **kwargs: Any) -> htt
         ):
             async with ClientSession(read_stream, write_stream) as session:  # pragma: no branch
                 # Attempt to make a request after termination
-                with pytest.raises(MCPError, match="Session terminated"):  # pragma: no branch
+                with pytest.raises(MCPError, match="[Ss]ession.*terminated"):  # pragma: no branch
                     await session.list_tools()