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Copilot CLI exposes both MCP content and structuredContent #4515
Description
Describe the bug
Summary
When an MCP tool result contains both content and structuredContent,
Copilot CLI appears to add both fields to the conversation context.
When structuredContent is present, the client should use it as the
structured tool result instead of exposing an equivalent or unrelated
content representation to the model.
For backward compatibility MCP tools which return structured content, should also set the unstructured content
For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.
see: MCP spec about structured content
Example
server.py
Here is a simple MCP server (based on the MCP Python SDK) to demonstrate the issue:
from pydantic import BaseModel
from mcp.server.mcpserver import MCPServer
class HelloOutput(BaseModel):
message: str
server = MCPServer(name="hello", version="0.1.0")
@server.tool(structured_output=True)
def hello() -> HelloOutput:
"""Return a simple JSON hello object."""
return HelloOutput(message="Hello World")
if __name__ == "__main__":
server.run("stdio")This is what the MCP server returns for the hello tool call.
{
"_meta": null,
"content": [
{
"type": "text",
"text": "{\n \"message\": \"Hello World\"\n}",
"annotations": null,
"_meta": null
}
],
"structuredContent": {
"message": "Hello World"
},
"isError": false,
"resultType": "complete"
}Actual Copilot CLI context
Copilot CLI exposes both representations:
{
"message": "Hello World"
}
{"message":"Hello World"}
Expected result
Only the structured result should be provided to the model:
{"message":"Hello World"}The content field may remain available for UI rendering or backwards
compatibility, but it should not be concatenated into the model context when it
duplicates structuredContent.
Impact
The behavior unnecessarily increases context size and may cause the model to
interpret one tool result as two JSON objects.
Related issue
The reporter of issue #3258 expects the opposite behavior.
Anyway, this contradicts the intention of the MCP spec:
Tool results may contain structured or unstructured content.
For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.
That means:
- a MCP tool must either return structured or unstructured content (but not both)
- but they should return a serialized representation in the unstructured content block for backward compatibility (i.e. for legacy clients which don't support structured content)
Clients which support structured content must therefore silently ignore the unstructured content if there is structured content.
Affected version
GitHub Copilot CLI 1.0.80.
Activity
teguh281295-arch commentedon Aug 17, 2026
mcodilla commentedon Sep 29, 2026
still reproduces in 1.0.89-7
kerobbi commentedon Oct 15, 2026
Hey @rroesch1, a fix for this is rolling out on
1.0.97and later -- it hasn't reached everyone yet, so you may still see duplication after updating.The fix avoids sending the same data from
contentandstructuredContenttwice, while preserving both when they contain different information. I'm going to keep this open until the rollout is complete.copilot-cli-bot commentedon Nov 12, 2026
This appears to be resolved as of GitHub Copilot CLI v1.0.109. Please update to that release or a newer stable version. We're closing this as completed. If it still happens, please comment with your version and reproduction steps.