Summary

Fixes #1392 — DeepSeek V4 thinking mode rejects evalscope's multi-turn requests with HTTP 400 "The reasoning_content in the thinking mode must be passed back to the API.", and Qwen3 thinking silently pollutes context by parsing <think>...</think> as plain text.

Root cause: openai_assistant_content encodes ContentReasoning as <think>...</think> inside the content string and never writes a top-level reasoning_content field. This worked for OpenAI/Together/Groq but breaks modern OpenAI-compatible providers that expect reasoning_content as an independent field.

Fix: redefine GenerateConfig.reasoning_history literal from inspect_ai-style filter values ('none'|'all'|'last'|'auto') to wire-format values ('none'|'think_tag'|'reasoning_field'), and consume it in OpenAICompatibleAPI.generate / generate_async to control the per-request wire shape.

⚠ Breaking Change

R1 users must explicitly set --generation-config '{\"reasoning_history\":\"none\"}' because R1's protocol forbids reasoning_content echo-back. Documented in docs/{zh,en}/get_started/parameters.md.

Affected benchmarks before the fix

• bfcl_v3 (the one reported in DeepSeek模型多轮对话时reasoning_content字 段处理不当导致评测失败 #1392), gaia, swe_bench_*_agentic, swe_bench_pro_agentic — all route prior-turn assistant messages through openai_chat_messages and hit the bug
• tau_bench / tau2 / tau3 / terminal_bench already workaround via response-side openai_chat_choices(include_reasoning=False); their behavior is unchanged

Drive-by cleanups in GenerateConfig

• Remove the cargo-culted migrate_reasoning bool→literal validator (zero consumers since v1.0)
• Remove 4 zombie fields with zero repo-wide consumers: best_of, cache_prompt, internal_tools, max_tool_output
• Document batch_size as an internal field (synced from TaskConfig.eval_batch_size, not user-facing)
• Document 6 previously-undocumented but actively-consumed fields: stop_seqs, repetition_penalty, response_schema, reasoning_effort, reasoning_tokens, reasoning_summary. Fix timeout type (int → int/float)

Design notes

• Last ContentReasoning wins when multiple are present on an assistant message (matches litellm's behavior; see _extract_last_reasoning).
• Blank/whitespace reasoning falls back to a single-space \" \" placeholder with a debug log (DeepSeek V4 rejects empty reasoning_content).
• No capability flag / model-name sniffing; users opt in/out explicitly via reasoning_history.
• litellm_compatible.py path is intentionally out of scope (litellm has its own schema validation that may strip extra top-level keys — deferred to a follow-up PR).

Prior art: BerriAI/litellm#28080 — the same fix for the DeepSeek path in litellm, including the single-space fallback design.

Test plan

• make lint passes
• pytest tests/models/test_openai_reasoning.py -v — 12 new unit tests covering 3 modes × 5 message shapes + schema invariants (legacy literal rejection, new literal acceptance, default behavior)
• pytest tests/agent/external/test_openai_chat_round_trip.py -v — 6 round-trip tests still green (regression check)
• pytest tests/cli/test_all.py::TestRun::test_ci_lite -v -s -p no:warnings — CI smoke green (no regression on the happy-path benchmark run)
• End-to-end against qwen3-max (DashScope, thinking enabled): all 3 modes produce the expected wire shape (<think> in content / top-level reasoning_content / clean drop) and all 3 are accepted by the server
• Reviewer: verify against DeepSeek V4 (deepseek-v4-pro + extra_body={\"thinking\":{\"type\":\"enabled\"}}) returns 200 instead of 400
• Reviewer: verify DeepSeek R1 (deepseek-reasoner) regression — default config should now 400; reasoning_history='none' should return 200