--- title: "Declare the inputs and outputs of cached builds" description: "Make build-cache contracts cover the actual files and configuration consumed and produced by the task." url: https://starsling.dev/ci-playbooks/techniques/declare-cache-inputs canonicalUrl: https://starsling.dev/ci-playbooks/techniques/declare-cache-inputs --- # Declare the inputs and outputs of cached builds Make build-cache contracts cover the actual files and configuration consumed and produced by the task. ## How to choose an approach Compare the examples by repository setup, prerequisites, and risks before adopting one. Each company implemented this in its own environment; measure each change against your own baseline. - **[Mastra](https://starsling.dev/ci-playbooks/mastra#build-inputs)**: Use it when A remote build cache misses after edits that leave the emitted artifact unchanged. Check before adopting: Exclusions must match actual tools, generators and imported assets. - **[OpenAI](https://starsling.dev/ci-playbooks/openai#cache-transform-outputs)**: Use it when A build task reruns an expensive transformation on unchanged inputs. Check before adopting: An undeclared input serves stale output. Toolchain paths can limit reuse across machines, and the cache needs a trusted writer. ## Apply this technique ````text Evaluate and apply this CI technique to my repository: Declare the inputs and outputs of cached builds. Make build-cache contracts cover the actual files and configuration consumed and produced by the task. Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/declare-cache-inputs.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 companies used this technique Read what each company changed and follow the original sources. ### Mastra: Make build cache keys describe the build Mastra declared source, package and compiler configuration inputs for library builds, excluding test files and Markdown where they were not build inputs. Composite tasks received their own input declarations. Evidence: Source PR/commit evidence Use it when: A remote build cache misses after edits that leave the emitted artifact unchanged. Before applying it: Exclusions must match actual tools, generators and imported assets. [Read more in the Mastra playbook](https://starsling.dev/ci-playbooks/mastra#build-inputs) [Primary source](https://github.com/mastra-ai/mastra/pull/14432) ### OpenAI: Declare every input and output of expensive transforms The Java SDK declared the R8 program, rule and JDK inputs and both the transformed JAR and mapping outputs, so Gradle restores the result when inputs match. Both executable compatibility smoke tests still run. Evidence: Source PR/commit evidence Fixture command on local JDK 21: 45s to 3s with the outputs restored ([Source](https://github.com/openai/openai-java/pull/1014)) Use it when: A build task reruns an expensive transformation on unchanged inputs. Before applying it: An undeclared input serves stale output. Toolchain paths can limit reuse across machines, and the cache needs a trusted writer. [Read more in the OpenAI playbook](https://starsling.dev/ci-playbooks/openai#cache-transform-outputs) [Primary source](https://github.com/openai/openai-java/pull/1014) ## 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 ### [Bake stable dependencies into runner images](https://starsling.dev/ci-playbooks/techniques/prebuild-runner-images) Prepare versioned system dependencies and build downloads once in the runner image instead of installing them on every job. 2 implementations · 2 companies (Anthropic, Linear) ### [Cache package downloads and dependency stores](https://starsling.dev/ci-playbooks/techniques/reuse-package-downloads) Restore package-manager downloads or dependency stores while keeping a locked install that validates the requested dependency graph. 7 implementations · 6 companies (DoorDash, Linear, Shopify, Spotify, Stripe, Uber) ### [Cache the directories the build actually uses](https://starsling.dev/ci-playbooks/techniques/cache-real-output-paths) Align restored and saved cache paths with the tool’s real output directories so the next run can use the cached work. 2 implementations · 2 companies (OpenAI, Uber) ## 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. ### [Cache workflow dependencies](https://starsling.dev/best-practices/github-actions/cache-dependencies) Choose dependency cache paths and keys for your package manager. ### [Cut repeated test setup](https://starsling.dev/best-practices/github-actions/cut-repeated-test-setup) Share expensive setup at the scope your tests need. ### [Share setup steps across jobs](https://starsling.dev/github-actions/optimizations/avoid-duplicated-setup) Centralize repeated setup definitions and remove duplicate or unused steps. ### [Optimize npm installs](https://starsling.dev/github-actions/optimizations/optimize-npm-install) Cache npm dependencies and large downloads and remove unused tool installs. ### [Optimize pnpm installs](https://starsling.dev/github-actions/optimizations/optimize-pnpm-install) Set up the pnpm store cache in the right order and scope recursive commands.