Monorepo CI Optimisation Plan

Run only what changed and keep builds fast.

Added October 15, 2026. Written with AI assistance and checked for clarity and placeholders; not reviewed by a lawyer, accountant, clinician or other professional. How these prompts are made

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Plan CI optimisation for my monorepo. Repository layout and packages: [REPO_LAYOUT]. Build tool and CI platform: [TOOLS]. Current pipeline duration and queue times: [DURATIONS]. Dependencies between packages: [DEPENDENCIES].

Describe how to detect affected packages from a diff (path filters or a dependency graph), how to compute the transitive set, and how to schedule only those jobs. Include remote caching, test sharding, and artifact reuse with specific configuration concepts for my CI platform. Build a before-and-after table of estimated durations using my numbers and state assumptions. Identify risks: skipped tests that should have run, cache key mistakes, and release coordination. Provide a safe rollout plan that runs the old and new pipelines side by side for two weeks and compares results.

Add a section that lists the metrics I should capture for two weeks before changing anything (queue time, build time per package, cache hit rate, rerun rate), with the command or dashboard concept for each. Finish with a decision rule for when the monorepo tooling cost outweighs the benefit, using my team size and package count, so I can judge honestly rather than assume.

Add a worked example of affected-package detection for an invented repository of eight packages: a change in the shared utilities package, the transitive set of six dependents, the jobs scheduled, and the jobs skipped, with the reasoning for each. Show the arithmetic of the before-and-after duration table, including queue time. Add a safety net section: a nightly full build that catches anything the affected-package logic missed, an alert when the full build finds a failure the pull-request build did not, and a monthly count of those misses used as the quality measure of the filter. Provide a short guide to cache keys covering what to include (lock file hash, tool version, package path) and what to exclude, and a checklist for diagnosing a stale cache.

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Layout: 12 packages, shared lib. Current build: 38 min, queue 9 min.
Plan: detect affected packages from the diff, include dependants of the shared lib, remote cache, shard tests.
Before and after (assumptions stated): average pull request 38 min -> about 14 min if 3 packages change; worst case (shared lib change) unchanged at about 32 min.
Risks: a skipped test that should run (mitigate by running everything nightly); cache keys missing a config file; coordinating releases.
Rollout: run old and new pipelines side by side for two weeks; compare pass/fail results; switch when mismatches are zero.
Measure first: queue time, cache hit rate, rerun rate.

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