tests

• Test real objects on the lightest real path. Construct vLLM objects with their real constructors; do not stand them in with SimpleNamespace or MagicMock, set private attributes, or patch methods to reach a state. Mock only external services. Pick the cheapest setup that exercises the change: a layer or kernel test does not need vllm_runner, and single-rank parallel state is enough when parallelism is irrelevant. A fix to user-visible behavior needs at least one regression test through LLM (vllm_runner) or RemoteOpenAIServer, with real weights when loading is involved. A test that only asserts assert_called* or "no exception" guards nothing.
• Show the failure before the fix. When the bug is reproducible, run the new test on the base commit (rebuild if the fix touches csrc/) and confirm it fails for the reported reason, then passes with the fix. Paste both commands and the relevant output in the PR; "fails before" without output is not evidence.
• Never buy a green CI. Do not add flaky/reruns, retry loops, skip, xfail, lowered thresholds, wider atol/rtol, or deletions to make a test pass. Find out whether the failure is caused by your change or also happens on main (.buildkite/scripts/rerun-test.sh, docs/contributing/ci/failures.md); the only exception is xfail(strict=True) linking an open issue for a bug on main.
• Deterministic and exact by construction. Use greedy or seeded sampling. Assert equality where the result is exact (a change that must not alter output is compared against the code it replaces, not another reference), and keep tolerances as tight as the math allows. vLLM is not batch invariant by default: batch size, chunked prefill, prefix caching, backend, and TP size can flip greedy tokens, so compare against a reference with check_logprobs_close / check_embeddings_close, or require VLLM_BATCH_INVARIANT=1 for exact equality. Never synchronize with a fixed sleep; poll an explicit condition with a timeout, and force race orderings with events.
• Make sure CI runs it. A test no step in .buildkite/test_areas/*.yaml selects never runs, which looks like passing. Check that the step's commands and pytest markers (core_model, slow_test, distributed) select the test and its source_file_dependencies cover the changed files; update the step if not.