--- title: "Share module initialization only for explicitly safe tests" description: "Linear added an opt-in Vitest project with isolate: false, so safe files share a module registry within each worker." url: https://starsling.dev/ci-playbooks/techniques/share-module-state canonicalUrl: https://starsling.dev/ci-playbooks/techniques/share-module-state --- # Share module initialization only for explicitly safe tests Linear added an opt-in Vitest project with isolate: false, so safe files share a module registry within each worker. ## How to choose an approach This technique currently has one documented company example. Validate its prerequisites, tradeoffs and original CI environment against your repository before adapting it. - **[Linear](https://starsling.dev/ci-playbooks/linear#share-module-state)**: Use it when Import and transform overhead dominate eligible Vitest files. Check before adopting: This was Linear’s highest correctness risk. Files that use fake timers or shared state stay in the isolated project. ## Apply this technique ````text Evaluate and apply this CI technique to my repository: Share module initialization only for explicitly safe tests. Linear added an opt-in Vitest project with isolate: false, so safe files share a module registry within each worker. Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/share-module-state.md Inspect this repository’s CI workflows, build and test commands, dependencies, and recent run timings. Identify where this technique applies and explain the expected benefit using this repository’s evidence. Measure a baseline from recent comparable runs: elapsed time, queue time, runner minutes and failures where available. If data is unavailable, say what cannot be measured. Compare relevant company approaches, checking prerequisites, platform differences and tradeoffs. Rank applicable options by evidence, fit and measurable opportunity; do not present company results as predictions for this repo. Recommend the smallest safe change, show what checks and fallback paths remain, and ASK ME WHICH CHANGE TO APPROVE BEFORE EDITING ANY FILES. Only after approval, work on a new feature branch, make one reversible change, and open a reviewable pull request. Never merge or push to a default or protected branch. Keep required checks, test coverage, deployment and release behavior intact. Validate the change and compare equivalent runs where possible; report limitations and what should be monitored after merge. ```` ## How Linear used this technique Read what Linear changed and follow the original sources. ### Linear: Share module initialization only for explicitly safe tests Linear added an opt-in Vitest project with isolate: false, so safe files share a module registry within each worker. Each eligible file carries an opt-in comment, shared state gets explicit teardown, and because agents now write most of Linear’s tests, its agent skills follow the same opt-in by default. Linear calls it its largest single performance improvement, worth roughly 17% in monthly savings. Evidence: Documented technique Use it when: Import and transform overhead dominate eligible Vitest files. Before applying it: This was Linear’s highest correctness risk. Files that use fake timers or shared state stay in the isolated project. [Read more in the Linear playbook](https://starsling.dev/ci-playbooks/linear#share-module-state) [Primary source](https://linear.app/now/ci-bottleneck-reworked#share-module-state-only-with-strict-isolation-rules) ## CI optimization is continuous work. Apply this technique yourself, or use StarSling Runners for faster GitHub Actions and optional agent-driven optimization PRs, which teams on eligible paid plans can turn on. [Get self-improving CI](https://starsling.dev/products/runners) ## Related techniques ### [Let an agent break import cycles and make the app load lazily](https://starsling.dev/ci-playbooks/techniques/agent-import-cycles) Point a coding agent at the import cycles and eager imports behind a slow app boot, and merge what it lands. 1 implementation · 1 company (Ramp) ### [Reduce repeated process and build-graph startup](https://starsling.dev/ci-playbooks/techniques/reduce-test-process-overhead) Batch compatible test work or share its build graph to avoid paying process startup and initialization for every file or target. 3 implementations · 3 companies (Anthropic, OpenAI, Stripe) ### [Balance test work across shards](https://starsling.dev/ci-playbooks/techniques/balance-test-shards) Use measured runtimes or smaller test files to reduce the slowest shard, which determines when the suite finishes. 4 implementations · 4 companies (Linear, OpenAI, Ramp, Stripe) ## Company Playbooks ### [Anthropic](https://starsling.dev/ci-playbooks/anthropic) Run only affected tests ### [DoorDash](https://starsling.dev/ci-playbooks/doordash) Run build and test in parallel, sharded across CI agents ### [Linear](https://starsling.dev/ci-playbooks/linear) Benchmark the same job on faster hardware ### [Mastra](https://starsling.dev/ci-playbooks/mastra) StarSling runners for the slowest test workflow ### [OpenAI](https://starsling.dev/ci-playbooks/openai) Rank CI fixes by the minutes they save ### [Ramp](https://starsling.dev/ci-playbooks/ramp) Move a CPU-bound test suite to a faster CPU ### [Shopify](https://starsling.dev/ci-playbooks/shopify) Let an agent iterate against one CI metric ### [Spotify](https://starsling.dev/ci-playbooks/spotify) Build arm64 on native arm64 runners ### [Stripe](https://starsling.dev/ci-playbooks/stripe) Run only the tests a change can reach ### [Uber](https://starsling.dev/ci-playbooks/uber) Build the merge-queue speculations most likely to be needed ## Related guides Implementation guides for this technique. ### [Cut repeated test setup](https://starsling.dev/best-practices/github-actions/cut-repeated-test-setup) Move shared fixture work out of individual test cases and keep necessary cleanup. ### [Shard tests across parallel jobs](https://starsling.dev/best-practices/github-actions/shard-tests) Split a long suite with a matrix and your test framework’s sharding support. ### [Diagnose slow tests in CI](https://starsling.dev/github-actions/problems/slow-tests) Check common causes in order and confirm each with workflow evidence. ### [Optimize Playwright tests](https://starsling.dev/github-actions/optimizations/optimize-playwright) Choose trace and video retention and install the browsers your suite uses. ### [Make Vitest run mode explicit](https://starsling.dev/github-actions/optimizations/optimize-vitest) Pin run mode to the test command and bound the job with a timeout.