[Bugfix] Fall back to native sampler when FlashInfer sampling kernel fails to build

[Dhruv235]

AI assistance was used on this PR. The fix and its design are mine; an AI assistant
(Claude Opus 5) helped with the rebase onto current main, running and interpreting the
test/control runs, and drafting this description. Attribution is in the commit trailer.
I have reviewed every changed line and run the tests myself.

Purpose

Fixes #49497.

FlashInfer JIT-compiles its top-k/top-p sampling kernels on first use. When that build fails at call time, the exception propagates out of the sampler and takes down engine startup, with no fallback to the PyTorch-native sampler.

This wraps the FlashInfer sampling call in both runner paths — TopKTopPSampler.forward_cuda (V1) and vllm/v1/worker/gpu/sample/sampler.py (V2 model runner) — so that a build failure warns once and permanently degrades to the native sampler. When the user has explicitly opted in with VLLM_USE_FLASHINFER_SAMPLER=1, it raises a clear RuntimeError naming that variable instead of silently falling back.

Relationship to the neighbouring fixes

There are three distinct failure modes in this area, and this PR is deliberately scoped to the third:

	Failure mode	Where it's caught
#49314	FlashInfer not discoverable at all (no nvcc, no flashinfer_cubin)	backend gate, via has_flashinfer()
#48956	FlashInfer present but cannot target the current GPU arch	eligibility check
this PR	FlashInfer selected, but the sampling kernel fails to build at call time	call site, try/except → native

The first two decide whether to select FlashInfer. This one handles the build failing after it has been selected, which no gate can predict. It applies cleanly to main standalone and does not depend on either of the others landing first.

Why this is not a duplicate

Per AGENTS.md, duplicate-work checks run before proposing:

gh issue view 49497 --repo vllm-project/vllm --comments
gh pr list --repo vllm-project/vllm --state open --search "49497 in:body"
gh pr list --repo vllm-project/vllm --state open --search "flashinfer sampler fallback"

No open PR addresses the JIT-build-failure case. The only neighbours the keyword search surfaces in this area are #48956 (FlashInfer cannot target the GPU arch) and #49314 (backend discovery gate via has_flashinfer()), both distinct per the table above. @Aliiiqbp noted in the issue that they were planning work here; as of opening, no PR exists, and I flagged my implementation in the thread so we don't write it twice.

Model evaluation

Not applicable, and stating that rather than leaving it for review. On the success path the generated code is byte-identical — the try returns exactly what the previous call returned, so sampling output and accuracy are unchanged. The new path is reached only when the FlashInfer kernel fails to build, where the current behaviour is a crashed engine rather than different output. If a reviewer would still like numbers, tests/evals/gsm8k is the obvious target and I'll run it on request.

On sequencing: I said in the issue thread that I'd hold this until #49314 merged so the gate change went in first. Opening now instead because @Aliiiqbp mentioned planning work in the same area, and having the implementation visible seemed better than two people writing the same fix. Happy to rebase, re-scope, or close in favour of another approach if maintainers would rather sequence it differently.

Test Plan

Two regression tests added to tests/v1/sample/test_topk_topp_sampler.py, both monkeypatching flashinfer_sample to raise a representative build error:

• test_forward_cuda_falls_back_to_native_on_flashinfer_build_failure — asserts valid token ids are still returned, and that sampler.forward is permanently rebound to forward_native so later calls skip the failing build.
• test_forward_cuda_raises_when_flashinfer_explicitly_forced — asserts a RuntimeError matching VLLM_USE_FLASHINFER_SAMPLER=1 when the user opted in explicitly.

pytest tests/v1/sample/test_topk_topp_sampler.py

Test Result

Both new tests pass:

tests/v1/sample/test_topk_topp_sampler.py::test_forward_cuda_falls_back_to_native_on_flashinfer_build_failure PASSED
tests/v1/sample/test_topk_topp_sampler.py::test_forward_cuda_raises_when_flashinfer_explicitly_forced PASSED
2 passed, 131 deselected

Full file, run with and without this commit on the same machine to separate pre-existing failures from anything introduced here:

	failed	passed
without this commit (239fc7355)	34	95
with this commit	34	97

The 34 failures are identical in both runs. They are the TestFlashInferTopkToppRobustness / TestFlashInferDistributionMatch cases, which require a working FlashInfer build and therefore cannot pass on a box without a discoverable CUDA toolkit — the exact environment this bug is about. This change introduces no new failures and adds the two passing tests above.

Environment: RTX 4060 Ti, WSL2 (kernel 6.18.33), driver 596.49, torch 2.11.0+cu130, no system CUDA toolkit (nvcc not on PATH, flashinfer_cubin not importable) — the same environment as the issue report.

Also verified end-to-end on that box before rebasing: both runner paths warn once and generate normally instead of crashing engine startup.

Rebased onto current main; the only conflict was an import union in vllm/v1/worker/gpu/sample/sampler.py. pre-commit passes clean (ruff check, ruff format, mypy, SPDX, forbidden-imports) with no files modified.

cc @hclsys @Aliiiqbp

[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]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Dhruv235.

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