Your matrix expands to 12 jobs but the external test service allows only 4 concurrent sessions. How do you limit concurrency of matrix jobs to 4?
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Short Explanation and Infographic
strategy.max-parallel caps how many matrix jobs run at once inside that job strategy. fail-fast is boolean cancel behavior; concurrency groups work across runs differently.
Full explanation below image
Full Explanation
max-parallel under strategy limits simultaneous matrix jobs for that matrix. With 12 combinations and max-parallel: 4, Actions schedules four at a time until all complete. fail-fast accepts a boolean, not an integer concurrency limit. The concurrency key uses group and cancel-in-progress for cross-run or job serialization and does not take max-parallel. permissions has no parallel field. Combining max-parallel with fail-fast: false is common for large matrices that must respect external rate limits while still finishing all combinations. Note that organization and account runner concurrency and billing minutes still apply on top of this matrix throttle.