[nxphi47]

Capture action-independent sampler probability heads for prime-rl #3927 and prime-rl #4012, paired with verifiers #2804.

A positive integer sampling_params["logprobs"] requests a probability head. Without replay, request two extra candidates, exclude cutoff ties, and retain the sampler's original full-vocabulary normalization. With a recorded sampling mask, retain its complete support, including boundary ties, and require probability mass one over that support. Accept native list masks and prime-rl's packed int32 CSR masks; reject missing probabilities or malformed support. Boolean and sampled-only requests keep their existing request path.

Compact numeric transport avoids per-token JSON overhead and accepts native JSON fallback. The response parser detects the payload format directly. Native logprobs accept both numeric token_id fields and token_id:N strings. Validate buffers, probability mass, and sampled-token agreement, including conflicting repeated token IDs.

Validation on 2026-10-19: 103 focused client tests passed, including native/compact and packed/list replay-support cases; seven unrelated model/multimodal cases were deselected. The paired prime-rl stack passed 19 vLLM 0.32 response-format tests, including complete replay support and native/compact equivalence. Renderer Ruff and formatting checks passed. These checks did not launch a GPU inference engine.

Updated onto main without rewriting the existing PR history. The replay-support compatibility follow-up is 5ffade9.

Note

Add compact sampler-head transport for top-logprob score centering

• Adds binary compact-logprob decoding for generation responses in renderers/client.py, including format, offset, candidate-count, and sampled-token validation
• generate now requests the head size plus 2 candidates, enables compact transport through extra_args by default, and returns completion_top_ids and completion_top_logprobs
• Adds a shared head selector in _select_sampler_head that ranks candidates deterministically, is independent of the sampled action, and preserves all candidates covered by a recorded sampling support mask
• _logprob_token_id accepts numeric token_id fields alongside the existing textual token form; _strip_base64_fields now extracts all matching base64 fields so multi-choice responses keep their buffers
• Behavioral Change: responses without a positive requested head return None for the new top-logprob fields; callers can override compact transport via extra_args

^{Macroscope summarized 5ffade9.}

[macroscopeapp]

Approvability

Verdict: Needs Human Approval

Macroscope's review found this PR requires human approval — This adds a substantial opt-in sampler-head capability with new request semantics, compact binary transport, replay-mask handling, and additional response fields in the production client. Its cross-component protocol behavior and sizable parsing/selection logic merit human review.

Notes:

• Macroscope's correctness review did not run, so approvability was decided on eligibility alone.

^{You can add or adjust custom eligibility rules. Learn more.}

[hallerite]

lgtm