Build once and share the output with test shards

Separate the build producer from test consumers and hand the same verified artifact to each shard.

Prompt

Evaluate and apply this CI technique to my repository: Build once and share the output with test shards.

Separate the build producer from test consumers and hand the same verified artifact to each shard.

Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/build-once-for-shards.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 companies used this technique

Read what each company changed and follow the original sources.

Build once, test on every shard

Codex builds one nextest archive per platform, uploads it, and has four test shards download it and run their slice of the same binaries. For Windows ARM64 the archive builds on an x64 host while the tests still run on ARM64.

Built once on x64, tested on native ARM64 shards Source

Use it when: Multiple matrix legs or shards compile the same code before running tests.
Before applying it: Reused artifacts must match the commit, target and toolchain. Artifact transfer can outweigh the build time saved on small jobs.

Build once and hand the artifact to every shard

Stripe React Native separated platform app builds from its end-to-end shards, and each shard downloads the matching built app. On GitHub Actions the same handoff uses build artifacts scoped to the run, platform and configuration.

The full suite went from about 25 to about 14 minutes, together with the move to another CI system. Source

Use it when: Every test shard repeats the same expensive compilation.
Before applying it: The handoff pays off when transfer costs less than the repeated build. Consume only artifacts from the same commit and a compatible platform.

How companies used this technique

Build once, test on every shard

Build once and hand the artifact to every shard

Related techniques

Prepare shared test inputs once

Run shared validation once

Company Playbooks

Related guides

Build once and reuse it across GitHub Actions jobs