fix(spec-decode): separate AR draft Philox stream

[malaiwah]

Summary

Move probabilistic proposal noise in the V2 autoregressive speculator onto the documented salted draft Philox range, so rejection recovery cannot reuse the exact Gumbel field that selected the rejected draft token.

Fixes #205.

Corrected scope

The issue's re-review established the exact live surface:

• affected: V2 AutoRegressiveSpeculator with probabilistic drafts and sampled requests;
• unaffected: greedy drafts, the V1 proposer, DFlash/DSpark, and the all-accepted bonus draw;
• shipped live preset: plain glm52 / glm52-hybrid with MTP>0, V2, default probabilistic draft sampling;
• r12 EXL3 release gate is greedy and the production turnkey uses V1, so neither executes this line.

The first-order defect is recovery after rejection. The verifier reuses the rejected draft token's unsalted (seed, offset) and therefore the exact per-vocabulary Gumbel vector conditioned on that draft having won. Acceptance-word correlation is negligible; the bonus offset is already disjoint.

Change

Replace:

positions + 1

with:

draft_gumbel_pos(positions)

in AutoRegressiveSpeculator.sample_draft.

The override remains intact. In particular, it still forwards output_processed_logits_active_rows=self.active_num_reqs, which prevents padded FULL-graph rows from overwriting real request state.

Invariants

• Draft proposal offsets become positions + 1 + 2**30, exactly disjoint from verifier offsets.
• The existing int64 tensor shape and one elementwise add are unchanged; only the immediate changes.
• FULL-graph active-row protection is unchanged.
• Greedy behavior, bonus offsets, acceptance logic, capture operation count, allocations, public ABI, and non-AR speculators are unchanged.
• Output tokens can change bit-for-bit on the affected probabilistic path; distributionally this restores the rejection sampler's independence contract.

Verification

CPU-isolated r12 container:

3 passed

The new regression constructs the real AR override with probabilistic draft logits, intercepts gumbel_sample, and asserts the exact passed position tensor is:

positions + 1 + 2**30

Additional checks:

• Ruff lint and format: pass
• git diff --check: pass
• independent adversarial review: APPROVE, confidence 0.99, no findings

The reviewer re-derived the draft/rejection offset algebra and confirmed the rejected-token collision is removed while bonus range, FULL-graph safeguards, dtype/shape, and greedy behavior remain unchanged.

No acceptance-rate or user-visible distribution magnitude claim is made without the optional statistical A/B. Production was not rebuilt or restarted.

Summary by CodeRabbit

• Bug Fixes

  • Improved speculative decoding’s probabilistic draft sampling to use separate random-number generator offsets, preventing overlap with acceptance and recovery sampling.
  • Deterministic draft sampling behavior remains unchanged.

• Tests

  • Added coverage verifying that probabilistic draft sampling uses disjoint random-number generator offsets.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: f29cb261-2ae6-476b-aad7-d952b28b7678

📥 Commits

Reviewing files that changed from the base of the PR and between f978d00 and a62944f.

📒 Files selected for processing (2)

• tests/v1/worker/test_gpu_autoregressive_speculator.py
• vllm/v1/worker/gpu/spec_decode/autoregressive/speculator.py

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📝 Walkthrough

Walkthrough

Autoregressive probabilistic draft sampling now uses draft_gumbel_pos(positions) instead of positions + 1. A unit test verifies the resulting +1 + (1 << 30) offset.

Changes

Speculative Gumbel offset handling

Layer / File(s)	Summary
Use and validate draft-specific Gumbel positions vllm/v1/worker/gpu/spec_decode/autoregressive/speculator.py, tests/v1/worker/test_gpu_autoregressive_speculator.py	sample_draft uses salted draft Gumbel positions, and the new test verifies the captured offset.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: voipmonitor

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: separating AR draft Philox streams for speculative decoding.
Linked Issues check	✅ Passed	The code and regression test implement the required switch to salted draft Gumbel positions, addressing issue #205.
Out of Scope Changes check	✅ Passed	The changes stay focused on the Philox offset fix and its test coverage, with no unrelated additions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[voipmonitor]

Superseded by #211. The replacement keeps the same salted Philox correctness fix, centralizes probabilistic draft sampling in the base speculator, routes the GG autoregressive and DSpark paths through it, and preserves the autoregressive FULL-graph active-row safeguard. Merge #211 only.