fix(gemini): keep tool_call JSON valid across streaming SSE replays

[Alex-Electron]

Summary

• The native Gemini adapter emits json.dumps(args) on every SSE event that carries a functionCall. Gemini re-sends the same call across repeated events and may mutate args mid-stream, so the consumer (which concatenates delta.tool_calls[].function.arguments by index per OpenAI streaming protocol) ends up with invalid JSON like {"q": "A"}{"q": "AB"}, silently discarding the tool call.
• The previous args_str.startswith(last_arguments) prefix-grow guard rarely fires on JSON: the trailing " / } mean a longer args dict almost never has the shorter one as a textual prefix (e.g. {"q":"abcdef"} doesn't start with {"q":"abc"}), so the fallback path emits the full new args.
• Switch to a flush-on-finish strategy: on first sighting of a (part_index, name, signature) slot, emit a header chunk with name/id and empty arguments; cache the latest args across subsequent events; when finishReason arrives, emit one chunk per slot with the final full args, then the finish chunk. Concatenation by the consumer yields a single valid JSON string per slot, regardless of how often Gemini re-emits or mutates the call.

Test plan

• tests/agent/test_gemini_native_adapter.py — full file passes (13/13)
• New test_stream_event_translation_handles_non_prefix_args_change — reproduces the bug (would FAIL on main: json.decoder.JSONDecodeError: Extra data: line 1 column 11 (char 10) on '{"q": "A"}{"q": "AB"}') and PASSES with this fix
• New test_stream_event_translation_handles_prefix_grow_args — final concat equals final args for the growing-args case
• test_stream_event_translation_emits_tool_call_delta_with_stable_index rewritten to assert on protocol-level concat instead of fragile per-chunk ordering — still passes
• test_stream_event_translation_keeps_identical_calls_in_distinct_parts — unchanged, still passes
• End-to-end smoke test on a live Telegram-bot deployment: a single user turn that triggers mcp_context7_resolve_library_id + web_search + delegate_task in one assistant response. Pre-fix: tool-call args garbled into concatenated JSON, agent silently fell back to default behaviour. Post-fix: all three calls dispatched correctly and the structured answer returned.

Notes

The header chunk carries arguments: "" so consumers can open the tool-call slot (and UIs that surface "agent is calling X…" still get the signal), while the actual args land in a single chunk just before the finish chunk. This sidesteps both the dedup and the partial-args cases without needing the consumer to handle non-prefix-preserving fragments.

[teknium1]

Thanks for the focused provider-boundary fix. The underlying failure is present on current main: agent/gemini_native_adapter.py:704-720 can emit two full argument objects for a changed replay, while agent/chat_completion_helpers.py:2319-2320 concatenates argument fragments for the same slot.

Problems

• Commit 44aa08a's non-prefix regression test accepts either the initial or final argument object. That is weaker than the stated flush-on-finish contract, which caches and emits the latest arguments.

Suggested changes

• Require the final replayed value ({"q": "AB"}) in test_stream_event_translation_handles_non_prefix_args_change; this prevents a future implementation from preserving valid JSON while dispatching stale arguments.

GitHub reports this branch as conflicting, but the target streaming logic is still present on current main. This is an automated hermes-sweeper review.