[Core][Frontend] Add weight version tagging for RL rollouts by ShuoleiWang · Pull Request #49040 · vllm-project/vllm

ShuoleiWang · 2026-07-18T14:58:30Z

Purpose

Part of #48306, this PR implements the part of Section 2.2 (Weight Version Tagging) that records a weight version on each request and exposes it through APIs and rollout responses. It builds on the weight-update lifecycle introduced in #39212.

For online RL inference:

Each EngineCore starts with weight_version=0. After a target-model finish_weight_update completes successfully, that EngineCore increments its counter. Draft-model updates and a failed finish on that EngineCore do not increment it.
Immediately before Scheduler.add_request, EngineCore copies the current counter value into Request.weight_version.
The current counter can be read through LLM.get_weight_version(), AsyncLLM.get_weight_version(), or the dev-mode GET /weight_info endpoint.
The version assigned to the request is returned in RequestOutput and in Python and Rust token-in/token-out (TITO) /inference/v1/generate responses.

The request first follows one common path. After EngineCoreOutput is created, the same value continues through either the Python or Rust output path:

A generation request
  -> EngineCore.add_request(Request)
     -> Request.weight_version = EngineCore._weight_version
     -> Scheduler.add_request(Request)
  -> scheduling and model execution
  -> Scheduler creates EngineCoreOutput
     -> EngineCoreOutput.weight_version = Request.weight_version
  -> one of the two output paths below

     +-- Python path
     |     -> OutputProcessor
     |     -> RequestOutput
     |        +-- normal Python API output
     |        `-- Python TITO streaming/non-streaming response
     |
     `-- Rust TITO path
           -> MessagePack transport
           -> Rust EngineCoreOutput
           -> GenerateOutput
              +-- Rust TITO streaming response
              `-- collect -> CollectedGenerateOutput
                           -> Rust TITO non-streaming response

Reading the current version is separate from generating a response:

LLM.get_weight_version()
  -> LLMEngine.get_weight_version()
  -> EngineCoreClient.get_weight_version()
  -> EngineCore.get_weight_version()

AsyncLLM.get_weight_version()
  -> EngineCoreClient.get_weight_version_async()
  -> EngineCore.get_weight_version()

GET /weight_info calls the server's EngineClient.get_weight_version() and uses the same async client path shown above.

The tag records the counter value assigned when the request enters the scheduler; it does not freeze the model weights for that request. A request kept with pause(mode="keep") can retain its original tag even if later tokens use newer weights.

For n > 1, vLLM combines the child samples into one RequestOutput. That aggregated output reports weight_version=None because its children may have been assigned different versions.

The counter belongs to one EngineCore and resets when that core restarts. In multi-DP deployments, the existing weight-update fan-out is unchanged and each EngineCore keeps its own counter. This PR does not add distributed rollback, the RFC's multi-replica (dp_rank, version) identity, or router-side version selection.

Most of the diff carries the same optional field through the existing request and response types shown above. It does not change model execution, logits, token selection, scheduling policy, or weight-transfer mechanics.

Test Plan

Verify that only a successful target-model finish advances an EngineCore's counter.
Verify that a request receives the current version before Scheduler.add_request.
Verify propagation through EngineCoreOutput and RequestOutput.
Verify the dev-mode GET /weight_info endpoint.
Verify Python and Rust TITO streaming and non-streaming propagation.
Run pre-commit and diff checks.

Test Result

Core, API, and output tests (6 passed):

.venv/bin/pytest -q tests/v1/engine/test_weight_version.py tests/entrypoints/serve/dev/test_rlhf.py tests/test_outputs.py

Scheduler and Python TITO regression tests (4 passed):

HF_HUB_OFFLINE=1 .venv/bin/pytest -q tests/v1/core/test_scheduler.py::test_finish_request tests/v1/core/test_scheduler.py::test_scheduler_stats_route_to_existing_output_client tests/entrypoints/scale_out/token_in_token_out/test_generate_stream.py::test_serve_tokens_skips_mm_cache_for_remote_engine_execution tests/entrypoints/scale_out/token_in_token_out/test_generate_stream.py::test_stream_basic

Pre-commit passed for all modified files, and git diff --check upstream/main reported no errors.
Rust tests were not run locally because Cargo is unavailable. They will be validated by the Rust Frontend Cargo Tests CI job.
tests/v1/engine/test_output_processor.py::test_incremental_detokenization did not reach its assertions locally because meta-llama/Llama-3.2-1B was unavailable from Hugging Face in this environment.

AI assistance disclosure

OpenAI Codex was used for code exploration, implementation assistance and code review. I reviewed and understood every changed line and ran the tests reported above.

github-actions · 2026-07-18T14:58:37Z

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mergify · 2026-07-18T14:59:04Z

Documentation preview: https://vllm--49040.org.readthedocs.build/en/49040/

aoshen02 · 2026-07-18T15:10:32Z


 The key insight is that requests paused with `mode="keep"` will produce tokens from the **old** weights before the pause and tokens from the **new** weights after resume. The `clear_cache` parameter controls whether the KV cache is invalidated during the pause. When `clear_cache=True`, previously cached key-value entries are discarded, so all tokens generated after resume will be computed entirely with the new weights. When `clear_cache=False`, existing KV cache entries are retained, meaning some tokens in context may still reflect the old weights (stale KV cache).

+## Weight Version Tagging


Can be simpler, no need to have a separate paragraph.

OK, Thanks!

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.

mergify · 2026-07-18T17:56:09Z

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

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

Track successful target-policy weight updates in EngineCore and bind requests to the current version at scheduler admission time. Expose the current version through LLM, AsyncLLM, and HTTP APIs, and propagate admitted versions through Python and Rust generation outputs. Part of vllm-project#48306, Section 2.2. Signed-off-by: Shuolei Wang <shuoleiwang123@gmail.com>

aoshen02 · 2026-07-19T14:59:21Z

+
+from vllm.engine.protocol import EngineClient
+from vllm.entrypoints.serve.dev.rlhf.api_router import attach_router
+


no need to have a test file here.

aoshen02 · 2026-07-19T15:01:02Z

+
+
+def _make_engine_core() -> EngineCore:
+    engine_core = object.__new__(EngineCore)


we only need to add some e2e test, ai-generated unit test is not recommended to add if without any human involved

I see, I'll remove them.

mergify Bot added documentation Improvements or additions to documentation frontend rust v1 labels Jul 18, 2026

aoshen02 reviewed Jul 18, 2026

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ShuoleiWang force-pushed the feature/weight-version-tagging branch from aaa3db0 to 9dc0b86 Compare July 18, 2026 16:29

ShuoleiWang marked this pull request as ready for review July 18, 2026 17:03

ShuoleiWang requested review from AndreasKaratzas, ApostaC, BugenZhao, DarkLight1337, NickLucche, WoosukKwon, aarnphm, alexm-redhat, heheda12345, ivanium, njhill, orozery, robertgshaw2-redhat and ywang96 as code owners July 18, 2026 17:03

claude Bot reviewed Jul 18, 2026

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mergify Bot added the needs-rebase label Jul 18, 2026

ShuoleiWang force-pushed the feature/weight-version-tagging branch from 9dc0b86 to f5aae57 Compare July 18, 2026 18:25

mergify Bot removed the needs-rebase label Jul 18, 2026

aoshen02 reviewed Jul 19, 2026

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[Core][Frontend] Add weight version tagging for RL rollouts#49040

[Core][Frontend] Add weight version tagging for RL rollouts#49040
ShuoleiWang wants to merge 1 commit into
vllm-project:mainfrom
ShuoleiWang:feature/weight-version-tagging

ShuoleiWang commented Jul 18, 2026 •

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		The key insight is that requests paused with `mode="keep"` will produce tokens from the old weights before the pause and tokens from the new weights after resume. The `clear_cache` parameter controls whether the KV cache is invalidated during the pause. When `clear_cache=True`, previously cached key-value entries are discarded, so all tokens generated after resume will be computed entirely with the new weights. When `clear_cache=False`, existing KV cache entries are retained, meaning some tokens in context may still reflect the old weights (stale KV cache).

		## Weight Version Tagging


		from vllm.engine.protocol import EngineClient
		from vllm.entrypoints.serve.dev.rlhf.api_router import attach_router



		def _make_engine_core() -> EngineCore:
		engine_core = object.__new__(EngineCore)

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Purpose

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ShuoleiWang commented Jul 18, 2026 •

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