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Python: [Bug][Python][AG-UI] MCP tool results containing TextContent list are incorrectly serialized to object repr string #2509
Description
Describe the bug
When using MCP tools with the AG-UI protocol, tool results containing list[TextContent] are incorrectly serialized. Instead of
extracting the text content, the serialization produces Python object repr strings like:
[<agent_framework._types.TextContent object at 0x7f8e7c1cc730>]
This results in the frontend displaying raw object addresses instead of actual tool output data.
To Reproduce
- Set up an AG-UI endpoint using
AgentFrameworkAgentwith MCP tools - Invoke any MCP tool that returns text content
- Observe the
MessagesSnapshotEventor tool result display
Expected behavior
Tool results should be properly serialized to display the actual text content, e.g.:
{"role": "tool", "content": "Actual tool output text here..."}Root cause analysis
The issue is in agent_framework_ag_ui/_events.py around line 415-418:
if isinstance(content.result, dict):
result_content = json.dumps(content.result)
elif content.result is not None:
result_content = str(content.result) # <- Bug here
else:
result_content = ""When MCP tools return results, they go through _mcp_call_tool_result_to_ai_contents() in agent_framework/_mcp.py:69-73, which
converts them to list[Contents] (typically list[TextContent]).
The current serialization logic doesn't handle list[TextContent] - it just calls str() on the list, producing the object repr
string.
Suggested fix
Update the serialization logic to handle list[Contents]:
if isinstance(content.result, dict):
result_content = json.dumps(content.result)
elif isinstance(content.result, list):
# Handle list of Contents (e.g., from MCP tools)
texts = []
for item in content.result:
if hasattr(item, 'text'): # TextContent
texts.append(item.text)
elif hasattr(item, 'model_dump'):
texts.append(json.dumps(item.model_dump(mode='json')))
else:
texts.append(str(item))
result_content = '\n'.join(texts) if len(texts) == 1 else json.dumps(texts)
elif content.result is not None:
result_content = str(content.result)
else:
result_content = ""Environment
- OS: macOS Darwin 25.1.0
- Python: 3.10
- agent-framework-ag-ui version: 1.0.0b251120
- agent-framework-core version: 1.0.0b251120
Additional context
This bug affects all MCP tool integrations when using the AG-UI protocol. It prevents proper display of tool results in frontend
applications like CopilotKit.
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[-][Bug][Python][AG-UI] MCP tool results containing TextContent list are incorrectly serialized to object repr string[/-][+]Python: [Bug][Python][AG-UI] MCP tool results containing TextContent list are incorrectly serialized to object repr string[/+]Fix MCP tool result serialization for list[TextContent]
claude89757 commentedon Nov 28, 2025
I've submitted a fix for this issue in PR #2523.
The fix properly handles
list[TextContent]results from MCP tools by:textattributemodel_dump()for items that support itstr()for other typesThis ensures that MCP tool results are correctly serialized and displayed in AG-UI frontends.
Fix MCP tool result serialization for list[TextContent]
Python: Fix MCP tool result serialization for list[TextContent] (#2523)
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Python: Fix MCP tool result serialization for list[TextContent] (micr…