fix: preserve thought_signature in functionCall roundtrip to fix multi-turn tool use

[Ken Chau (kenotron-ms)]

Summary

Closes / related to: microsoft-amplifier/amplifier-shared#41

Gemini's API now returns a thought_signature field on functionCall parts in responses. When those parts are included as conversation history in subsequent request turns, the thought_signature must be present — otherwise the API returns 400 INVALID_ARGUMENT. The provider was discarding this field during parsing and never re-attaching it during serialization, breaking all multi-turn tool use for any Gemini-routed agent (browser-tester, any vision-role agent) after the first tool call.

Root Cause

Two locations in amplifier_module_provider_gemini/__init__.py:

Parse (drop site) — _convert_to_chat_response() (~line 964): ToolCallBlock and ToolCall were built without preserving fc.thought_signature.

Serialize (re-submit site) — _convert_messages() (~line 1121): function_call dicts were built with only name and args, never re-attaching any signature field.

Changes

amplifier_module_provider_gemini/__init__.py

• _convert_to_chat_response(): When a functionCall part contains thought_signature, store it as an extra field on both ToolCallBlock and ToolCall. Both already carry model_config = ConfigDict(extra="allow"), so no model changes needed.
• _convert_messages(): When building a function_call dict for a subsequent request turn, re-attach thought_signature if the stored ToolCall has the field.
• Absence handled gracefully: responses without thought_signature pass through unchanged (backward-compat).

New: tests/test_thought_signature.py — 8 tests

Test	Covers
test_parse_preserves_thought_signature_in_tool_call	Parse: field stored on ToolCall
test_parse_preserves_thought_signature_in_tool_call_block	Parse: field stored on ToolCallBlock
test_serialize_reattaches_thought_signature	Serialize: field re-attached in function_call dict
test_serialize_without_thought_signature_still_works	Backward-compat: no field when absent
test_thought_signature_full_roundtrip	Full parse → model_dump() → serialize roundtrip
test_multiple_tool_calls_each_thought_signature_preserved	Each tool call keeps its own signature
test_mixed_tool_calls_only_signed_ones_get_signature	Mixed: only signed calls re-attach
test_second_tool_call_turn_does_not_raise_with_thought_signature	Integration: second complete() with prior history

Test evidence

• 104 tests pass (2 pre-existing failures in test_pricing_and_vision.py require live GOOGLE_API_KEY — failing on main before this branch, unrelated)
• DTU validated (instance issue-41-gemini):
  • Provider module loads correctly ✅
  • 8 new tests all pass ✅
  • Live round-trip JSON confirms thought_signature survives parse → serialize:

    {"function_call": {"name": "search", "args": {"q": "amplifier"}, "thought_signature": "abc123-secret-signature-XYZ"}}

  • Backward-compat (absent signature → no field on output) ✅

Affected agents (all fixed by this PR)

• browser-tester:browser-operator (confirmed reproducing on chore: add CI workflow #41)
• browser-tester:browser-researcher (same routing)
• browser-tester:visual-documenter (same routing)
• Any other agent using model_role=vision or direct Gemini config with tool use

Reference

• https://ai.google.dev/gemini-api/docs/thought-signatures