fix(scheduler): skip lookahead allocation for running prefill chunks

[voipmonitor]

Problem

The scheduler allocated speculative lookahead slots to requests that were still executing a running/chunked prefill. Those slots cannot be consumed by speculative decode in that iteration and reduce usable KV capacity.

Fix

Allocate lookahead only once the scheduled request is actually in decode. New requests retain the existing behavior.

This is a standalone current-FF extraction of the scheduler fix previously carried inside #88.

Validation

• Targeted running chunked-prefill scheduler test: 1 passed
• Existing scheduler assertions extended to cover zero lookahead during running prefill
• ruff check and ruff format --check on changed files
• git diff --check

Summary by CodeRabbit

• Bug Fixes
  • Improved chunked prefill scheduling by ensuring no lookahead tokens are allocated during prefill steps.
  • Preserved lookahead token allocation for other scheduling operations.

[coderabbitai]

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: 5f383f0d-5383-4c30-82ac-1dbec3a3231e

📥 Commits

Reviewing files that changed from the base of the PR and between e4879e1 and b1cd154.

📒 Files selected for processing (2)

• tests/v1/core/test_scheduler.py
• vllm/v1/core/sched/scheduler.py

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📝 Walkthrough

Walkthrough

The scheduler now passes zero lookahead tokens to KV cache allocation for prefill chunks. A regression test patches allocation, records the argument, and verifies the value is zero.

Changes

Chunked prefill scheduling

Layer / File(s)	Summary
Effective lookahead allocation and regression coverage vllm/v1/core/sched/scheduler.py, tests/v1/core/test_scheduler.py	Scheduler.schedule() uses zero lookahead tokens for prefill chunks, while other requests retain the configured value; a test verifies the chunked-prefill argument.

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

Suggested reviewers: njhill, kl527, omerpaz95

🚥 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 and accurately summarizes the scheduler fix to skip lookahead allocation for running prefill chunks.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/ff-prefill-lookahead-allocation-20260717

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