fix: propagate error messages in /mcp endpoint responses

Previously, exceptions in tool invocation were caught and an empty list was
returned, hiding error details from clients. Now errors are re-raised to let
the MCP SDK properly convert them to JSON-RPC error responses.

This ensures clients see actual error messages (e.g., '401 Unauthorized')
instead of empty responses.

Fixes #2570

Signed-off-by: Jonathan Fulton <jonathan@jonathanfulton.com>

Author: jonathan-fulton

mcpgateway/transports/streamablehttp_transport.py

@@ -639,7 +639,9 @@ def _convert_meta(meta: Any) -> dict[str, Any] | None:
             return unstructured
     except Exception as e:
         logger.exception(f"Error calling tool '{name}': {e}")
-        return []
+        # Re-raise the exception so the MCP SDK can properly convert it to an error response
+        # This ensures error details are propagated to the client instead of returning empty results
+        raise
 
 
 @mcp_app.list_tools()

tests/unit/mcpgateway/transports/test_streamablehttp_transport.py

@@ -261,7 +261,7 @@ async def fake_get_db():
 
 @pytest.mark.asyncio
 async def test_call_tool_exception(monkeypatch, caplog):
-    """Test call_tool returns [] and logs exception on error."""
+    """Test call_tool re-raises exception after logging for proper MCP SDK error handling."""
     # First-Party
     from mcpgateway.transports.streamablehttp_transport import call_tool, tool_service
 
@@ -275,8 +275,8 @@ async def fake_get_db():
     monkeypatch.setattr(tool_service, "invoke_tool", AsyncMock(side_effect=Exception("fail!")))
 
     with caplog.at_level("ERROR"):
-        result = await call_tool("mytool", {"foo": "bar"})
-        assert result == []
+        with pytest.raises(Exception, match="fail!"):
+            await call_tool("mytool", {"foo": "bar"})
         assert "Error calling tool 'mytool': fail!" in caplog.text