[Core][Logging] Add log_color and log_stream to LoggingConfig

[errmakov]

Purpose

Follow-up to #57205 for the RFC in #57200. This moves two items from its checklist into LoggingConfig:

• VLLM_LOGGING_COLOR and standard NO_COLOR (plus FORCE_COLOR from [CI] Support FORCE_COLOR env var; enable colors in CPU CI output #56913) → log_color (auto / always / never)
• VLLM_LOGGING_STREAM → log_stream

As @markmc asked in the issue, they are fields of LoggingConfig, not top-level flags. They can be set with --logging-config '{"log_color": "never"}', --logging-config.log_color never, --logging-config.log_stream ext://sys.stderr, or through --config YAML. They reach child processes the same way as the rest of LoggingConfig.

The environment variables still supply the defaults, and the old behaviour is unchanged:

env	log_color default
NO_COLOR set, or VLLM_LOGGING_COLOR=0	never
else VLLM_LOGGING_COLOR=1 or FORCE_COLOR set	always
otherwise	auto (color only if log_stream is a TTY)

Precedence: an explicit CLI/YAML value beats every env var, FORCE_COLOR and NO_COLOR included. This follows the rule the RFC sets for the other fields, and the no-color.org guidance that command-line arguments should override NO_COLOR. The issue left open whether FORCE_COLOR should be a special case. I kept one consistent rule, and I'm happy to change it if you'd prefer otherwise.

Like log_level, both fields apply only to the built-in dictConfig. A custom pylogging_config_file stays authoritative.

The (ProcName pid=N) stdout/stderr prefix in vllm.utils.system_utils._add_prefix still reads the env vars. I left it alone on purpose: #57957 removes _add_prefix and moves that prefix into ColoredFormatter, so once it lands the prefix follows log_color with no further change.

Not a duplicate: I searched open and closed PRs for log_color, log-color, log_stream, VLLM_LOGGING_COLOR, VLLM_LOGGING_STREAM, NO_COLOR, FORCE_COLOR, LoggingConfig, and 57200. The only hit that touches these settings is #57205, which this builds on. I claimed these two checklist items in the issue first.

Test Plan

.venv/bin/python -m pytest tests/test_logger.py tests/entrypoints/launchers/test_cli_args.py tests/engine/test_arg_utils.py -q
pre-commit run --files vllm/logger.py vllm/config/logging.py tests/test_logger.py tests/entrypoints/launchers/test_cli_args.py examples/features/logging_configuration.md
pre-commit run mypy-3.12 --hook-stage manual --files vllm/logger.py vllm/config/logging.py

New tests:

• test_log_color_defaults_from_env: env → log_color default mapping, including NO_COLOR / VLLM_LOGGING_COLOR=0 beating FORCE_COLOR. This replaces test_use_color_force_color.
• test_log_stream_defaults_from_env
• test_use_color: auto follows the TTY state of the selected stream; always and never ignore it.
• test_log_color_and_stream_config_override_env: an explicit config beats NO_COLOR, FORCE_COLOR, and VLLM_LOGGING_STREAM on the real handler.
• test_logging_config_color_and_stream_cli_args: dotted CLI args parse into LoggingConfig.

Manual check: NO_COLOR=1 vllm serve facebook/opt-125m --logging-config.log_stream ext://sys.stderr --logging-config.log_color always

Test Result

• Without the implementation, all 14 new test cases fail. With it: tests/test_logger.py + tests/entrypoints/launchers/test_cli_args.py give 74 passed, and tests/engine/test_arg_utils.py gives 110 passed.
• pre-commit (ruff, mypy 3.10, mypy 3.12, test tethering) passes.
• Manual check: stdout was empty, and all log lines went to stderr with ANSI colors even though NO_COLOR=1.

No model evaluation: this change only affects logging and does not change model output.

---

AI assistance (Claude) was used to write this change. I reviewed every changed line and ran the tests above myself.

[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.

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[mergify]

Documentation preview: https://vllm--58712.org.readthedocs.build/en/58712/

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @errmakov.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork