[Frontend] Log output token IDs at DEBUG level

[ruirui6946]

Purpose

Allow operators to keep human-readable generated output logs without emitting output token IDs at the default INFO level.

Following maintainer feedback, this now mirrors the existing request-input logging split instead of adding a new CLI flag:

• INFO keeps generated text and the finish reason.
• DEBUG additionally logs output token IDs.
• --max-log-len continues to truncate both output text and token IDs at their respective levels.
• Streaming delta, streaming-complete, and non-streaming output logs share the same behavior.

Fixes #51912.

Duplicate check: I rechecked the issue discussion and open PRs by issue number and output-token-ID logging keywords. No other open PR implements this behavior.

No model behavior or evaluation output changes. Model evals are not applicable because this only changes request log formatting and CLI help text.

AI assistance disclosure: OpenAI Codex assisted with implementation, review-feedback handling, and test execution. The human submitter must review every changed line, understand the end-to-end change, and take responsibility for the PR before merge.

Test Plan

• Execute the existing tests/entrypoints/serve/utils/test_request_logger.py unit tests, covering INFO/DEBUG separation, streaming and non-streaming logs, truncation, and edge cases.
• Run Ruff check and format validation on all three net-changed files.
• Run the repository Python 3.10 mypy hook on all three net-changed files.
• Run validate-config, applicable commit hooks, and git diff --check.

Test Result

• Request logger unit tests: 9 passed in 0.12s. The local Windows environment did not contain the full vLLM runtime/test dependency set, so the existing test file was run with --noconftest and minimal import stubs; the real RequestLogger implementation and all tests in that file were executed.
• Ruff check and format: passed.
• Python 3.10 mypy on the three net-changed files: passed.
• validate-config: passed with UTF-8 console mode.
• Applicable commit hooks passed, including typos, SPDX, forbidden imports, configuration validation, and DCO sign-off.
• git diff --check: passed.
• actionlint and update-dockerfile-graph were skipped because no workflow or Dockerfile changed.

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test commands.
• The test results, such as pasting the results comparison before and after, or e2e results.
• Documentation impact considered. CLI help text now describes the INFO/DEBUG split; no separate documentation page is needed.

[github-actions]

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[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[ruirui6946]

The pre-run check is currently blocked by the new-contributor CI authorization gate (0 merged PRs), so the pre-commit job was skipped rather than failing on code. Could a maintainer please add the appropriate ready/verified label or trigger /ci run? Local validation completed successfully: the focused pytest suite passed (41 tests), all applicable pre-commit hooks passed, the manual Python 3.12 mypy hook passed, and git diff --check passed.

[DarkLight1337]

I think it would be better to handle this like log_inputs, i.e., the output tokens should only be logged at DEBUG level

[ruirui6946]

Thanks, addressed in 58eeb18. I removed the new CLI flag and changed output logging to follow the existing request-input split: generated text and finish reason remain at INFO, while output_token_ids are emitted only at DEBUG. I also updated the CLI help and extended the existing request-logger tests for the INFO/DEBUG contract and truncation. Local results: 9 request-logger tests passed; Ruff, format, mypy on all net-changed files, validate-config, applicable commit hooks, DCO, and git diff --check passed.

[DarkLight1337]

Thanks!

[DarkLight1337]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83731 for commit 58eeb18aaf8e.