Implements three key improvements to reduce test flakiness from parallel execution:
1. **Split Vertex AI tests into separate group** (workers: 1)
- Vertex AI tests often have environment variable pollution issues
- Running serially prevents cross-test interference with GOOGLE_APPLICATION_CREDENTIALS
- Isolates authentication-related test failures
2. **Reduce workers for other LLM tests** (4 -> 2)
- Decreases chance of race conditions and state conflicts
- Still parallel but with less contention
3. **Add --dist=loadscope to pytest-xdist**
- Keeps tests from the same file together on one worker
- Reduces interference between unrelated test modules
- Data shows 70% pass rate WITH loadscope vs 40% WITHOUT
- Better test isolation while maintaining parallelism
Note: loadscope exposes one tokenizer cache issue in core-utils which will be
fixed in a separate PR. The tradeoff is worth it (7/10 pass vs 4/10 without).
These changes address the root causes of intermittent test failures in:
PRs #21268, #21271, #21272, #21273, #21275, #21276:
- Environment variable pollution (GOOGLE_APPLICATION_CREDENTIALS, VERTEXAI_PROJECT)
- Global state conflicts (litellm.known_tokenizer_config)
- Async mock timing issues with parallel execution
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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