A tool call's extra_content rides back to the endpoint within the turn

[gedaiu]

What happened

Against Gemini's OpenAI-compatible endpoint (gemini-3.8-flash), every chat question that used a tool ended with answer_failed on its second turn, while the model connection test passed. Reproduced with curl: Gemini 3 attaches a signature to each function call,

"tool_calls":[{"extra_content":{"google":{"thought_signature":"Eq0C…"}},"function":{…},"id":"call_170866","type":"function"}]

and the turn that returns the tool result without it is refused:

400 INVALID_ARGUMENT: Function call is missing a thought_signature in functionCall parts.
This is required for tools to work correctly … https://ai.google.dev/gemini-api/docs/thought-signatures

The same turn with the signature echoed answers normally. Function calls from earlier, completed turns are accepted without one (also verified), so this is about the in-turn replay only.

What changed

• utopia_llm::ToolCall gains extra_content: Option<Value>, the endpoint's object kept opaque, read in both the unary and the streaming parser, written back by to_message. Not an object (absent, null, a string) reads as none and adds no key.
• rig_model: a streamed call carries it in RawStreamingToolCall::additional_params, a unary one in ToolCall::additional_params (rig preserves both into history), and push_message writes it back as extra_content.

Tests: both transports keep the Gemini shape and echo it; absent or malformed extra_content adds no key; the turn → rig → wire round trip carries it; a call without one stays as before.

cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test -p utopia-llm -p utopia-server pass (49 + 603).

Related: #1135 (embedding index), the other half of running rc8 against Gemini.

[WaylandYang]

Thank you for this, and for the curl reproduction: it made the cause plain. The change reads correctly. extra_content is kept opaque, read in both the unary and the streaming parser, and written back only when the endpoint sent an object, so other endpoints see the same requests as before. Checked here: the utopia-llm tests and the adapter tests pass, and the branch merges cleanly with dev. Merging now. Thanks again.