Linear's CI rework: the specific dozen changes that absorbed a 4x test-suite growth
Linear published the engineering detail behind rebuilding CI once coding agents made verification, not authoring, the expensive step. The concrete moves: third-party runners instead of GitHub Actions (34% faster jobs on average, 52% on TypeScript compilation), tsgo replacing tsc (73% off the typecheck median, removing it as the bottleneck), a change-detection job cut from 26s to 8s median via capped fetch depth and sparse blobless checkouts, a custom checkout composite action with retry and connection timeouts, pre-installed Postgres client in the base image (7-8s per test shard), pnpm workspace installs scoped to one package (44-73s down to 16-18s), seven checks consolidated into two jobs (~87,000 runner-minutes per month), four shards to eight, and an opt-in `isolate: false` Vitest project sharing module state for ~17% monthly savings. PR wait went from over 6 minutes to just over 5 while the suite nearly quadrupled.
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