[Roydon Sequeira (roydonsequeira)]

Description

Fixes #3668.

With fail_on_missing_function=False, _execute_call_section_async returns a malformed_arguments function output for arguments that are not valid JSON or not a JSON object, so the model can correct the call. Arguments that are a JSON object but don't match a FunctionTool's signature (a wrong type, a missing required argument or an unexpected key) were only checked inside FunctionTool.execute_async, and the ValidationError failed the whole send.

Tool now has a validate_arguments method, which _execute_call_section_async calls before execute_async. It accepts any arguments by default, since MCP servers report input errors in the tool result, and FunctionTool implements it with its argument model. When it raises, the dispatcher returns the same malformed_arguments output with invoked=False, or re-raises when fail_on_missing_function=True, which is what that mode already did. Only the validation call is guarded, so an exception raised by the function itself is not reported as an argument error. MessageToolCallScorer keeps counting these calls as not executed.

Tests and Documentation

Added tests in tests/unit/prompt_target/target/test_tool.py:

• FunctionTool.validate_arguments raises ValidationError for a wrong type, a missing required argument and an unexpected key.
• Through send_prompt_async, the same three cases reach the model as a malformed_arguments output in the next request. The function is never called, the output is recorded with invoked=False, and the stored history still validates for replay. With fail_on_missing_function=True the send raises ValidationError after one request.

No docs or notebooks changed, so JupyText was not needed.

[Roydon Sequeira (roydonsequeira)]

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