fix: surface MCP structuredContent-only results

libs/agno/agno/utils/mcp.py

@@ -146,6 +146,9 @@ async def _call_with_session(active_session: ClientSession) -> ToolResult:
                     # Handle other content types
                     response_str += f"[Unsupported content type: {content_item.type}]\n"
 
+            if not response_str.strip():
+                response_str = _structured_content_as_tool_output(result) or ""
+
             return ToolResult(
                 content=response_str.strip(),
                 metadata=_build_mcp_metadata(result),
@@ -214,6 +217,19 @@ def _build_mcp_metadata(result: "CallToolResult") -> Optional[Dict[str, Any]]:
     return metadata or None
 
 
+def _structured_content_as_tool_output(result: "CallToolResult") -> Optional[str]:
+    """Serialize structuredContent so structured-only MCP responses reach the model loop."""
+
+    structured_content = getattr(result, "structuredContent", None)
+    if structured_content is None:
+        return None
+
+    try:
+        return json.dumps(structured_content, ensure_ascii=False, sort_keys=True, default=str)
+    except (TypeError, ValueError):
+        return str(structured_content)
+
+
 def prepare_command(command: str) -> list[str]:
     """Sanitize a command and split it into parts before using it to run a MCP server."""
     import os

libs/agno/tests/unit/tools/test_mcp.py

@@ -1,3 +1,4 @@
+import json
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -1053,9 +1054,33 @@ async def test_mcp_tool_result_preserves_structured_content():
     entrypoint = get_entrypoint_for_tool(mock_tool, session)
     result = await entrypoint()
 
+    assert result.content == "hello"
     assert result.metadata["structured_content"] == {"id": "u1", "name": "Ada"}
 
 
+@pytest.mark.asyncio
+async def test_mcp_tool_result_uses_structured_content_when_content_is_empty():
+    mock_tool = MagicMock()
+    mock_tool.name = "get_data"
+    structured_content = {"id": "u1", "name": "Ada", "role": "EMPLOYEE"}
+
+    session = AsyncMock()
+    session.send_ping = AsyncMock()
+    session.call_tool = AsyncMock(
+        return_value=CallToolResult(
+            content=[],
+            isError=False,
+            structuredContent=structured_content,
+        )
+    )
+
+    entrypoint = get_entrypoint_for_tool(mock_tool, session)
+    result = await entrypoint()
+
+    assert json.loads(result.content) == structured_content
+    assert result.metadata["structured_content"] == structured_content
+
+
 @pytest.mark.asyncio
 async def test_mcp_tool_error_result_preserves_structured_content():
     mock_tool = MagicMock()