Empty mcp tool result error (#1142)

### Summary

Fixed `MCPUtil.invoke_mcp_tool()` to return `"[]"` instead of logging an
error when MCP tools return empty content. Empty results (`None, []`)
are valid outputs and should be treated consistently with
function tools. The `None -> "[]"` and `[] -> "[]"` behavior comes from
the MCP Python SDK's[ FastMCP implementation](
https://github.com/modelcontextprotocol/python-sdk/blob/6566c08446ad37b66f1457532c7f5a14f243ae10/src/mcp/server/fastmcp/utilities/func_metadata.py#L500).


  ### Test plan

- Added comprehensive test coverage in
`test_mcp_fastmcp_behavior_verification()` that verifies all FastMCP
edge cases
  - Enhanced FakeMCPServer to support custom content testing scenarios
  - Ran full test suite - all tests pass
  - Verified the fix handles `None, [], {}, [{}], [[]]` cases correctly

  ### Issue number

  Closes #1035

  ### Checks

  - [x] I've added new tests (if relevant)
  - [ ] I've added/updated the relevant documentation
  - [x] I've run `make lint` and `make format`
  - [x] I've made sure tests pass

Co-authored-by: thein <[email protected]>

Author: thoo

src/agents/mcp/util.py

@@ -209,8 +209,8 @@ async def invoke_mcp_tool(
         elif server.use_structured_content and result.structuredContent:
             tool_output = json.dumps(result.structuredContent)
         else:
-            logger.error(f"Errored MCP tool result: {result}")
-            tool_output = "Error running tool."
+            # Empty content is a valid result (e.g., "no results found")
+            tool_output = "[]"
 
         current_span = get_current_span()
         if current_span:

tests/mcp/helpers.py

@@ -4,7 +4,14 @@
 from typing import Any
 
 from mcp import Tool as MCPTool
-from mcp.types import CallToolResult, GetPromptResult, ListPromptsResult, PromptMessage, TextContent
+from mcp.types import (
+    CallToolResult,
+    Content,
+    GetPromptResult,
+    ListPromptsResult,
+    PromptMessage,
+    TextContent,
+)
 
 from agents.mcp import MCPServer
 from agents.mcp.server import _MCPServerWithClientSession
@@ -67,6 +74,7 @@ def __init__(
         self.tool_results: list[str] = []
         self.tool_filter = tool_filter
         self._server_name = server_name
+        self._custom_content: list[Content] | None = None
 
     def add_tool(self, name: str, input_schema: dict[str, Any]):
         self.tools.append(MCPTool(name=name, inputSchema=input_schema))
@@ -91,6 +99,11 @@ async def list_tools(self, run_context=None, agent=None):
     async def call_tool(self, tool_name: str, arguments: dict[str, Any] | None) -> CallToolResult:
         self.tool_calls.append(tool_name)
         self.tool_results.append(f"result_{tool_name}_{json.dumps(arguments)}")
+
+        # Allow testing custom content scenarios
+        if self._custom_content is not None:
+            return CallToolResult(content=self._custom_content)
+
         return CallToolResult(
             content=[TextContent(text=self.tool_results[-1], type="text")],
         )

tests/mcp/test_mcp_util.py

@@ -234,6 +234,60 @@ async def test_agent_convert_schemas_false():
     assert baz_tool.strict_json_schema is False, "Shouldn't be converted unless specified"
 
 
+@pytest.mark.asyncio
+async def test_mcp_fastmcp_behavior_verification():
+    """Test that verifies the exact FastMCP _convert_to_content behavior we observed.
+
+    Based on our testing, FastMCP's _convert_to_content function behaves as follows:
+    - None → content=[] → MCPUtil returns "[]"
+    - [] → content=[] → MCPUtil returns "[]"
+    - {} → content=[TextContent(text="{}")] → MCPUtil returns full JSON
+    - [{}] → content=[TextContent(text="{}")] → MCPUtil returns full JSON (flattened)
+    - [[]] → content=[] → MCPUtil returns "[]" (recursive empty)
+    """
+
+    from mcp.types import TextContent
+
+    server = FakeMCPServer()
+    server.add_tool("test_tool", {})
+
+    ctx = RunContextWrapper(context=None)
+    tool = MCPTool(name="test_tool", inputSchema={})
+
+    # Case 1: None -> "[]".
+    server._custom_content = []
+    result = await MCPUtil.invoke_mcp_tool(server, tool, ctx, "")
+    assert result == "[]", f"None should return '[]', got {result}"
+
+    # Case 2: [] -> "[]".
+    server._custom_content = []
+    result = await MCPUtil.invoke_mcp_tool(server, tool, ctx, "")
+    assert result == "[]", f"[] should return '[]', got {result}"
+
+    # Case 3: {} -> {"type":"text","text":"{}","annotations":null}.
+    server._custom_content = [TextContent(text="{}", type="text")]
+    result = await MCPUtil.invoke_mcp_tool(server, tool, ctx, "")
+    expected = '{"type":"text","text":"{}","annotations":null}'
+    assert result == expected, f"{{}} should return {expected}, got {result}"
+
+    # Case 4: [{}] -> {"type":"text","text":"{}","annotations":null}.
+    server._custom_content = [TextContent(text="{}", type="text")]
+    result = await MCPUtil.invoke_mcp_tool(server, tool, ctx, "")
+    expected = '{"type":"text","text":"{}","annotations":null}'
+    assert result == expected, f"[{{}}] should return {expected}, got {result}"
+
+    # Case 5: [[]] -> "[]".
+    server._custom_content = []
+    result = await MCPUtil.invoke_mcp_tool(server, tool, ctx, "")
+    assert result == "[]", f"[[]] should return '[]', got {result}"
+
+    # Case 6: String values work normally.
+    server._custom_content = [TextContent(text="hello", type="text")]
+    result = await MCPUtil.invoke_mcp_tool(server, tool, ctx, "")
+    expected = '{"type":"text","text":"hello","annotations":null}'
+    assert result == expected, f"String should return {expected}, got {result}"
+
+
 @pytest.mark.asyncio
 async def test_agent_convert_schemas_unset():
     """Test that leaving convert_schemas_to_strict unset (defaulting to False) leaves tool schemas