fix(judge): omit seed for non-default OpenAI base URLs (Gemini compat)

[Alozo]

Summary

OpenAIBackend.build_request unconditionally includes the OpenAI-proper seed field. OpenAI-compatible servers reject unknown fields rather than ignoring them — notably Gemini's /v1beta/openai endpoint:

HTTP 400 Invalid JSON payload received. Unknown name "seed": Cannot find field.

The agent run succeeds; only the LLM-judge scoring step fails (judge-infra error), which makes every --modes llm run against such a backend ungradeable.

Fix: send seed only when the resolved base URL is the canonical OpenAI host (_DEFAULT_BASE_URL); omit it for any custom base URL. Determinism for custom backends is still governed by temperature. This unblocks Gemini and any other OpenAI-compatible provider that doesn't implement seed, and doesn't change behavior against api.openai.com.

Repro

Point the OpenAI backend at Gemini's compat endpoint and run an llm-mode score:

export BELT_OPENAI_API_KEY="<gemini-key>"
export BELT_OPENAI_BASE_URL="https://generativelanguage.googleapis.com/v1beta/openai/"
belt eval <scenarios> --modes rules,llm   # judge step 400s on `seed` before this fix

Test plan

• test_build_request_default_base_includes_seed — seed still sent to api.openai.com.
• test_build_request_custom_base_omits_seed — seed omitted for a Gemini-style custom base URL.
• pytest tests/scorer/llm/test_backend.py -k OpenAIBackend passes.

Notes

Found while wiring Gemini (OpenAI-compat) as the judge for a docs-accessibility eval built on agent-belt. Happy to adjust the gating (e.g. an explicit opt-out env var instead of host-matching) if maintainers prefer a different shape.

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