feat(agent): make Gemini and MLflow-route models usable through databricks_v2

[atishpatel]

Summary

Makes Gemini — and every other non-Claude, non-GPT-5 model on the Databricks MLflow route (databricks_v2) — usable in an agent loop. These are the non-benchmarks/ changes from benchmark/harness-accounting-and-solo, lifted onto a clean base off main so they can land independently while the harness work continues.

Two defects made these models unusable, one fatal and one silent. Both live only in openai_body / parse_openai, which is the least-exercised of the three databricks_v2 sub-routes — the luna/sol conditions run the Responses route and the opus conditions run the Anthropic route, so this change is inert for every model already in use and only lights up the MLflow path.

Why a third route at all

databricks_v2_route_for_model buckets by model family: claude* → Anthropic Messages, gpt-5/code-names → OpenAI Responses, everything else → MLflow chat-completions. Gemini, Qwen, gpt-oss, and friends all fall through to that third pair — and both bugs below live only there.

D1 — dropped thought signatures (fatal)

Gemini returns a thoughtSignature on every tool call and requires it echoed back. openai_body reserialized each call as {id, type, function} only, dropping the field, so the next request 400'd:

HTTP 400  Function call is missing a thought_signature in functionCall parts.

For a coding agent this fires on the first tool call, so the model never completes a single turn.

Position is load-bearing. A four-shape replay probe against the live gateway established that the signature must sit as a sibling of function — nesting it inside function{} fails with the same 400 as omitting it. A fix that "preserves the field" without preserving its position passes a unit test and still 400s.

The fix: ToolCall gains provider_extra: Map<String, Value>. parse_openai captures every top-level wire key except the three we model (id, type, function); openai_body re-emits them beside function. Keeping whatever we did not model, rather than naming thoughtSignature, means the next provider with an opaque per-call token needs no change here. The Responses and Anthropic replay shapes are fully modelled, so they pass Default::default() and stay byte-identical to before.

D1b — duplicate tool-call ids (same root cause)

Gemini returns the function name as the id, so two parallel calls to one function arrive sharing an id — and that id is what pairs a role:"tool" result back to its call, making two results indistinguishable. dedupe_provider_ids suffixes collisions (get_weather, get_weather-2). Safe because both halves of the pairing (the assistant tool_calls[].id and the result's tool_call_id) are re-emitted from this same value; the provider never sees its original id again.

D2 — block-array content discarded (silent, worse than a crash)

parse_openai read content with as_str(), which returns "" for anything that isn't a JSON string. Gemini (and Qwen35, gpt-oss) send an array of typed blocks:

"content": [
  {"type": "reasoning", "summary": [{"type": "summary_text", "text": "…"}]},
  {"type": "text", "text": "391"}
]

So the model answered and the answer was thrown away — no error, no warning, just a turn that looked like the model had said nothing. On a benchmark this reads as "Gemini is bad at the task" rather than "buzz dropped the reply."

openai_content_parts now accepts either shape — string as before, or a block array where text blocks concatenate into text and reasoning blocks into reasoning (Gemini nests the prose one level down under summary). Message-level reasoning_content / reasoning still win when present, so DeepSeek and vLLM-style hosts are unchanged; block reasoning is the last fallback.

Also: a turn-start log line (buzz-acp pool.rs)

Small, independent observability change that also rides in the non-benchmark delta: run_prompt_task now emits a pool::prompt "turn starting" line, labelled by the same prompt_label helper as log_stop_reason, so a log reads as start/stop pairs. An unpaired start is the only durable evidence that a turn was entered and never returned — without it, a stalled agent and an agent nobody woke leave identical (zero-completion) logs.

Interaction with #3538

#3538 (already merged) rewrote databricks_v2_route_for_model to route by boundary-aware model-family segments. That change and this one touch different functions in llm.rs — routing vs. body/parse — and compose cleanly; the family routing decides which pair runs, and this fixes the MLflow pair it can now select.

Testing

• cargo fmt --all -- --check, cargo clippy -p buzz-agent -p buzz-acp --all-targets -- -D warnings — clean.
• cargo test -p buzz-agent -p buzz-acp — all green (304 + 632 lib tests plus integration suites, 0 failures). Five new tests cover: block-array text extraction, plain-string regression, passthrough capture (and non-duplication of the modelled keys), replay position (thoughtSignature beside function, not inside it), and id de-duplication.
• Wire evidence: the four-shape replay table and the reasoning-effort probe were run against block-lakehouse-staging (recorded in the design doc).

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