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• Have you read the docs? Agents SDK docs
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• Have you searched for related issues? Others may have faced similar issues.
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Describe the bug

If an MCP tool is registered but returns an empty value (e.g.: None, [], {}), the MCPServer.call_tool() response does not create a ContentBlock in the CallToolResult object. As a result, MCPUtil.invoke_mcp_tool() in agents.mcp.utils.py logs the error Errored MCP tool result:

Errored MCP tool result: meta=None content=[] structuredContent={'result': None} isError=False

Debug information

• Agents SDK version: v0.1.0
• Python version Python 3.11

Repro steps

This is what a sample MCP tool to fetch a DB may look like. If no results are found (an empty list is returned), the tool logs an Errored MCP tool result instead of returning the empty list.

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("my_db")


@mcp.tool()
def fetch_items_by_id(ids: list[int]) -> list[dict]:
    """
    Fetch the items in the database by their IDs.

    Returns:
        A list of items in dict (JSON) format.
    """
    results: list[dict] = []

    # ...some app logic

    return results

Expected behavior

The tool call returns a string with an empty object (e.g., en empty list) to let the LLM know the tool returned no results, without it being considered an error. This would improve tool-call reliability and prevent false negatives when no data is legitimately found.

Non-MCP tools work as expected.