Move unit tests to a parallel-by-default framework, gated on ten clean runs

Migrate serial tests to a framework that runs them in parallel, and ship each package after ten green runs.

Prompt

Evaluate and apply this CI technique to my repository: Move unit tests to a parallel-by-default framework, gated on ten clean runs.

Migrate serial tests to a framework that runs them in parallel, and ship each package after ten green runs.

Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/parallel-test-framework.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.

View all playbooks

How DoorDash used this technique

Read what DoorDash changed and follow the original sources.

Move unit tests to a parallel-by-default framework, gated on ten clean runs

Mostly serialized XCTest runs had stretched DoorDash's iOS CI test step into double-digit minutes. DoorDash migrated its shared iOS packages to Swift Testing, which runs synchronous and asynchronous tests in parallel by default. A Cursor rule made the mechanical edits, and the migration prompt told the agent to run each package's tests with Bazel after every file and fix failures before moving on. A package counted as migrated once its tests passed ten times in a row, which surfaced tests that depended on order, shared state or timing. The unit test step in CI got about 60% faster, and overall builds for shared iOS packages about 40% faster.

The unit test step in CI became roughly 60% faster. Source

This contributed to an average of about 40% faster overall builds for shared iOS packages. Source

Use it when: A test step runs mostly serially, and the test framework or a newer one can run the same tests in parallel.
Before applying it: Parallel runs expose hidden order, shared-state and timing assumptions; fix each root cause, and leave tests on the old framework where the new one lacks a needed feature.

How DoorDash used this technique

Move unit tests to a parallel-by-default framework, gated on ten clean runs

Related techniques

Build the test database once for all workers

Isolate state before running tests in parallel

Match test concurrency to the runner

Company Playbooks

Related guides

Shard tests across parallel jobs

Cut repeated test setup

Diagnose slow tests in CI

Optimize Playwright tests

Make Vitest run mode explicit

Parallelize independent jobs