fix(vllm): declare the ModelExpress startup weight version

[c2w-sea]

Overview:

A Python vLLM worker whose initial weights come from the ModelExpress RL loader now declares that version at startup, so get_weight_version reports what the worker actually serves (MX_REFIT_DESIRED_VERSION_UID) instead of version_declared: false.

Details:

• With --load-format modelexpress (or mx), MX_LOAD_STRATEGY_CHAIN=RL, and MX_REFIT_DESIRED_VERSION_UID set, ModelExpress fails engine startup unless every rank loaded the desired version (MX_REFIT_DESIRED_VERSION_UID). A constructed handler therefore serves it, and the constructor declares it.
• Any other configuration keeps the existing undeclared state. That includes the default INFERENCE chain, which ignores the desired version and loads the base weights, and any non-ModelExpress load format.
• This complements fix(vllm): distinguish undeclared RL weight versions #13041. A controller that scales out or restarts a worker already knows the version it asked ModelExpress to load, but without this the new worker reports undeclared and the controller re-applies the same update. It mirrors the startup reconciliation ModelExpress performs for native vLLM (modelexpress_rl/inference/engines/vllm/startup_probe.py), which records the desired version through vllm.Control after load.

Where should the reviewer start?

_modelexpress_startup_weight_version in components/src/dynamo/vllm/handlers.py, then the two new tests in TestRLAdminRouteHardening in components/src/dynamo/vllm/tests/test_vllm_worker_handler.py.

Validation

• Pre-commit passes on the changed files (isort, black, flake8, ruff, codespell).
• The new tests construct the real handler and cover both load-format aliases, case and whitespace in the chain value, and each condition that must leave the version undeclared. They were not run locally because the environment lacks torch, vLLM and the Dynamo bindings; the helper's logic was exercised in isolation against the same cases.

Related Issues

🚫 This PR is NOT linked to an issue:

• Confirmed — no related issue

Summary by CodeRabbit

• New Features
  • ModelExpress workers configured for the RL load strategy can now declare the requested version at startup when every rank has loaded it. Startup fails if the requested version is unavailable on any rank.
• Documentation
  • Updated the reinforcement learning integration reference with the startup version declaration requirements and behavior. Under the default inference strategy, the requested version is ignored and the worker starts undeclared.

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🌿 Fern Docs Preview: https://nvidia-preview-01a0ef66-84ee-70d5-b0bc-c210e57aaf02.docs.buildwithfern.com/dynamo/dev

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Walkthrough

The vLLM worker now declares a startup weight version from the requested UID when the load format is ModelExpress and the strategy chain is RL. Tests and integration documentation describe the supported configuration and cases that leave the version undeclared.

Changes

Startup weight version

Layer / File(s)	Summary
Select and initialize startup version components/src/dynamo/vllm/constants.py, components/src/dynamo/vllm/main.py, components/src/dynamo/vllm/handlers.py	The supported ModelExpress load formats are defined in constants.py and imported by main.py and handlers.py. The worker uses the trimmed desired version only when the strategy chain is RL and the load format is supported.
Test and document startup behavior components/src/dynamo/vllm/tests/test_vllm_worker_handler.py, docs/fern/pages/use-cases/reinforcement-learning/integration-reference.md	Tests cover supported formats, trimmed configuration values, and cases that leave the version undeclared. The reference documents the ModelExpress and default INFERENCE behavior.

Priority: ⬇️ Low

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

Merge Risk: 🟠 High · up to d2a2b

ModelExpress RL workers can report a requested version without verifying that they loaded its weights. Fix the startup declaration before merging; also correct the attribute access and documentation.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 4 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: declaring the ModelExpress startup weight version for vLLM workers.
Description check	✅ Passed	The description covers the overview, implementation details, reviewer starting points, validation status, and the required Related Issues section. It also clearly identifies the configurations that de…
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.

Full details: Docstring Coverage

Explanation

Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 4 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 3

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@components/src/dynamo/vllm/handlers.py`:
- Around line 178-180: Remove the `_weight_version` inference based on
`MX_LOAD_STRATEGY_CHAIN` and `MX_REFIT_DESIRED_VERSION_UID`; `load_format` does
not confirm which weights the loader serves. Set `_weight_version` only when an
authoritative loader-owned startup signal is available, and otherwise leave it
undeclared.
- Line 179: In the handler, read the declared load_format attribute directly
from config.engine_args instead of using getattr with a default. This lets
malformed engine argument objects raise the expected missing-attribute error
rather than selecting the fallback path.

In `@docs/fern/pages/use-cases/reinforcement-learning/integration-reference.md`:
- Line 244: Update the earlier version-declaration rule to include ModelExpress
startup with the RL load strategy when the desired version is loaded
successfully. Keep the existing weight-update route and set_weight_version
cases, and clarify that the default INFERENCE chain still starts undeclared.

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📥 Commits

Reviewing files that changed from the base of the PR and between db998b5 and d2a2bad.

📒 Files selected for processing (5)

• components/src/dynamo/vllm/constants.py
• components/src/dynamo/vllm/handlers.py
• components/src/dynamo/vllm/main.py
• components/src/dynamo/vllm/tests/test_vllm_worker_handler.py
• docs/fern/pages/use-cases/reinforcement-learning/integration-reference.md

Included review availability: Your plan provides up to 12 included reviews per hour; 11 remain after this review.

[KrishnanPrash]

/ok to test 24316e3

[KrishnanPrash]

Pre-approving please address/verify review comments

[rmccorm4]

approving docs change - thanks for the contribution @c2w-sea !

[c2w-sea]

Hmm, not sure what is the blocker to merge this

[GuanLuo]

/ok to test a9c5f54

[c2w-sea]

@GuanLuo thanks for triggering the CI. However, it looks like the CI infra failed?

[c2w-sea]

/ok to test a9c5f54