[Frontend] Gate scale-out endpoints behind opt-in flag

[franciscojavierarceo]

Purpose

The scale-out routes were registered on every standard generation server. This
change makes that surface explicit and consistent across the Python and Rust
frontends:

• VLLM_ENABLE_SCALE_OUT_ENDPOINTS accepts only 0 or 1; unset and empty
  values preserve the mode-specific default;
• ordinary vllm serve processes omit /render, /derender, and
  /inference/v1/generate unless the flag is 1, and log when the routes are
  skipped;
• dedicated vllm launch render and vllm serve --tokens-only modes enable
  their required routes when the flag is unset or 1, and reject an explicit
  0 instead of starting in a broken state;
• the Rust frontend omits /inference/v1/generate unless the flag is 1;
• /cohere/v2/chat/render requires both VLLM_ENABLE_COHERE_API=1 and
  VLLM_ENABLE_SCALE_OUT_ENDPOINTS=1, while Cohere chat remains available with
  only its own flag; and
• scale-out integration fixtures, examples, serving docs, and security docs now
  state or apply the opt-in explicitly.

This is not a duplicate of an existing open PR. It follows the security review
feedback on #50195 and is intentionally split out so that PR remains focused on
Responses rendering.

Test Plan

PYTHONPATH=$PWD .venv/bin/python -m pytest \
  tests/entrypoints/scale_out/test_factories.py \
  tests/test_envs.py \
  tests/entrypoints/cohere/test_api_router.py -q

cd rust
cargo test -p vllm-server --lib --no-fail-fast

pre-commit run --files <all changed files>

Test Result

• Pytest: 100 passed, with one pre-existing Starlette deprecation warning
• Rust server library tests: 366 passed
• Pre-commit: all applicable hooks passed, including Ruff, markdownlint, mypy,
  SPDX checks, and Rust formatting
• The GPU-backed scale-out integration suites were not runnable locally. Their
  server fixtures now set VLLM_ENABLE_SCALE_OUT_ENDPOINTS=1; CI remains the
  end-to-end verification.
• Model evaluation: not applicable; this changes route registration only and
  does not affect model execution or output.

This change was developed with assistance from OpenAI Codex. The human
submitter reviewed every changed line, reran the relevant checks, and confirmed
end-to-end understanding.

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[mergify]

Documentation preview: https://vllm--54579.org.readthedocs.build/en/54579/

[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.

[DarkLight1337]

@claude review

[chaunceyjiang]

/ci run

[github-actions]

✅ Triggered Buildkite CI #86484 for commit c581690b57a7.

[hickeyma]

Thanks @franciscojavierarceo for pushing this PR so quickly after feedback on your other PR. Overall it looks good. Here are some things (and also some inline) that probably should be addressed:

• The Rust frontend ignores the new env var. rust/src/server/src/routes.rs registers /inference/v1/generate unconditionally, so VLLM_USE_RUST_FRONTEND=1 still exposes it. Need to handle this frontend also as otherwise there is a hole in this scenario
• Without the new env var set vllm serve --tokens-only now has no /inference/v1/generate and no /abort_requests but it starts up quietly anyway
• /cohere/v2/chat/render is still open. cohere/api_router.py returns the same GenerateRequest through the same ServingRender, gated only on VLLM_ENABLE_COHERE_API
• It would be good to log when you skip with the env var. Right now an operator upgrading into this just gets a 404 with nothing in the logs naming the flag. Follow the pattern in cohere/api_router.py
• Update the following as they state that it still works on vllm serve without the env var set: examples/scale_out/example_mm_serve.py:16 and token_generation_client.py:6, docs/serving/online_serving/README.md:122-145 anddocs/serving/online_serving/derenderer.md:57-64
• Would the check be better placed in scale_out/factories.py? I'm asking because usually the opt-in is inside the feature's own attach function rather than the shared dispatcher

[DarkLight1337]

Looks like you need to set the flag for all relevant tests

[franciscojavierarceo]

Looks like you need to set the flag for all relevant tests

done, thanks!

[DarkLight1337]

/ci run

[github-actions]

✅ Triggered Buildkite CI #86611 for commit 061a468ac3ed.

[franciscojavierarceo]

fyi @chaunceyjiang tests all pass

@hickeyma can you update your review now that I believe I've addressed your feedback?

[chaunceyjiang]

I tested it locally. Great work!

[hickeyma]

@hickeyma can you update your review now that I believe I've addressed your feedback?

Sorry @franciscojavierarceo but I miseed it before the PR was merged. LGTM bar point I raised in #54579 (comment) about launching explicitly in render mode.