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optimize-bundle-size

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Reduce plugin `page load bundle size` and avoid unnecessary increases in `packages/kbn-optimizer/limits.yml`. Use when proactively optimizing bundles, investigating CI page-load overages, or reviewing PRs that change bundle limits.

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Source SKILL.md: https://github.com/elastic/kibana/blob/HEAD/.agents/skills/optimize-bundle-size/SKILL.md

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Optimize Bundle Size

Use this workflow to keep limits stable by moving non-critical code out of the entry bundle.

1) Set a baseline

  1. Reference docs/extend/ci-metrics.md for metric definitions and limits workflow details.
  2. Build dist metrics for the target plugin and record current page load bundle size.
node scripts/build_kibana_platform_plugins --focus <pluginId> --dist --workers 4
cat <pluginDir>/target/public/metrics.json
  1. If this is a regression investigation, compare plugin limits on branch vs upstream.
git show upstream/main:packages/kbn-optimizer/limits.yml | rg '^\\s{2}<pluginId>:'
rg '^\\s{2}<pluginId>:' packages/kbn-optimizer/limits.yml

2) Identify entry-chunk drivers

  1. Generate a profile build.
node scripts/build_kibana_platform_plugins --focus <pluginId> --dist --profile --no-cache --workers 4
  1. Find the entry chunk id and top modules in that chunk.
entry_id=$(jq -r '.chunks[] | select((.names|index("<pluginId>")) != null) | .id' <pluginDir>/target/public/stats.json)
jq -r --argjson cid "$entry_id" '.modules[] | select((.chunks|index($cid)) != null) | [.size, (.name // .identifier)] | @tsv' <pluginDir>/target/public/stats.json | sort -nr | head -40
  1. Compare with upstream when the delta is unclear.
jq -r .modules[].id <pluginDir>/target/public/stats.json | sort - > moduleids.txt
# generate moduleids.txt on both branches and diff them
  1. Focus on modules imported by the plugin public entry/start contract.

3) Apply high-impact fixes

  1. Replace eager imports in the plugin start contract with lazy boundaries for optional UI.
  2. Avoid importing broad barrel files (index.ts) from entry paths; import only required modules directly.
  3. Split heavy UI into a separate file and load it with React.lazy(() => import(...)).
  4. Keep page load bundle size low; allow async chunks size to grow when code is not needed at initial load.

4) Re-measure and decide limits

  1. Rebuild dist metrics and compare before/after.
  2. Keep or restore the old limit if the new value is below it.
  3. Raise limits only if no meaningful split is possible, then document why.
  4. If a limit increase is required, use --update-limits (sets current size +15kb) and include findings in PR notes.
node scripts/build_kibana_platform_plugins --focus <pluginId> --update-limits

5) Validate changes

  1. Run targeted lint/type checks for touched files.
  2. Run node scripts/check_changes.ts.
  3. Optionally run --validate-limits or --dist --watch while iterating.
  4. If check_changes.ts fails due unrelated pre-existing files, call that out explicitly in PR notes.
node scripts/build_kibana_platform_plugins --validate-limits --focus <pluginId>
node scripts/build_kibana_platform_plugins --dist --watch --focus <pluginId>