fix: send JSONRPCError instead of raw Exception for failed requests

When a JSONRPCRequest fails with a connection error in post_writer, the
error must be sent as a JSONRPCError wrapped in a SessionMessage so
the response router in BaseSession can match it to the waiting
send_request call. Sending a raw Exception causes the request to hang
indefinitely because _receive_loop forwards it to _handle_incoming
(a no-op by default) while send_request blocks on its response stream.

Notification errors are still forwarded as raw exceptions since nothing
waits on a response stream for them.

Author: omar-y-abdi

src/mcp/client/streamable_http.py

@@ -476,7 +476,17 @@ async def handle_request_async() -> None:
                             raise
                         except Exception as exc:
                             with contextlib.suppress(Exception):
-                                await read_stream_writer.send(exc)
+                                if isinstance(message, JSONRPCRequest):
+                                    error_data = ErrorData(
+                                        code=INTERNAL_ERROR,
+                                        message=str(exc) or "Connection error",
+                                    )
+                                    error_msg = SessionMessage(
+                                        JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data)
+                                    )
+                                    await read_stream_writer.send(error_msg)
+                                else:
+                                    await read_stream_writer.send(exc)
 
                     # If this is a request, start a new task to handle it
                     if isinstance(message, JSONRPCRequest):

tests/shared/test_streamable_http.py

@@ -56,6 +56,7 @@
     CallToolRequestParams,
     CallToolResult,
     InitializeResult,
+    JSONRPCError,
     JSONRPCRequest,
     ListToolsResult,
     PaginatedRequestParams,
@@ -2353,7 +2354,10 @@ async def test_connection_error_forwarded_to_read_stream():
             )
             await write_stream.send(init_message)
 
-            # The connection error should be forwarded to the read stream
+            # The connection error should be forwarded as a JSONRPCError
+            # so that send_request can route it to the correct response stream
             with anyio.fail_after(5):
                 result = await read_stream.receive()
-            assert isinstance(result, Exception)
+            assert isinstance(result, SessionMessage)
+            assert isinstance(result.message, JSONRPCError)
+            assert result.message.id == "init-err"