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Tool Behavior

When you pass a function to tool_use_behavior, tools are called first, and then your function is called with the tool results to determine if execution should stop or continue.

The execution flow works as follows:

  1. Tools execute first: All function tools are executed in parallel using asyncio.gather()

  2. Your function is called with results: After tools complete, the system calls _check_for_final_output_from_tools() which evaluates your custom function, passing it the RunContextWrapper and list of FunctionToolResult objects

  3. Decision based on your function's return: Your function returns a ToolsToFinalOutputResult that determines whether to stop execution (using tool output as final result) or continue the conversation loop

Notes

This behavior is specific to FunctionTool objects. Hosted tools (like file search, web search) are always processed by the LLM regardless of the tool_use_behavior setting.