Next Bundle Optimizer

# Bundle optimizer

## 1. Set the scope

Analyze the whole app by default, covering all routes and shared client dependencies. Narrow the scope only when the user specifies a route, dependency, feature or other subset. Choose **audit** or **fix** mode. Audit is the default: generate analyzer artifacts and report candidates while leaving application source, dependencies and lockfiles unchanged. Fix mode requires an explicit request to change the app. Ask before changing visible behavior, timing, compatibility or a trust boundary. For a route's static App Shell, use [`next-cache-components-optimizer`](https://github.com/vercel/next.js/tree/canary/skills/next-cache-components-optimizer); install it first if unavailable (`npx skills add https://github.com/vercel/next.js/tree/canary/skills/next-cache-components-optimizer`). For navigation prefetch work, use [`next-partial-prefetching-optimizer`](https://github.com/vercel/next.js/tree/canary/skills/next-partial-prefetching-optimizer); install it first if unavailable (`npx skills add https://github.com/vercel/next.js/tree/canary/skills/next-partial-prefetching-optimizer`).

**Done:** the whole-app or user-specified scope, mode and intended behavior are recorded. A request to capture or export data is audit mode, not permission to fix.

## 2. Capture and export a baseline

Run from the app directory with its package manager. Read `next analyze --help` for capture options and `next analyze export --help` for replay options in the installed CLI. Reuse a selected saved snapshot, or capture with a distinctive name first; then export it through gzip at its default compression level. Keep before/after `.jsonl.gz` files separate.

**Sandbox requirement:** If the agent’s sandbox blocks TCP port binding (as Codex’s does), it MUST run the `next analyze --output` capture outside the sandbox. Do not attempt the capture inside that sandbox, even though `--output` does not serve the analyzer UI.

Run the pipeline in Bash:

```bash set -o pipefail # Capture only when a new baseline is needed pnpm exec next analyze --output --snapshot 'audit-before-unique-1' pnpm exec next analyze export --snapshot 'audit-before-unique-1' | gzip > /tmp/analyze-app-before.jsonl.gz ```

`--output` builds and saves binary/UI artifacts without serving. `next analyze export` reads a saved snapshot without building; stdout is one typed JSON record per line, with errors on stderr. Omit `--route` for whole-app analysis; add it for a user-specified route. The route filter keeps the whole-app module graph, so scope it in the next step.

For a custom `distDir`, replay with `--dist-dir <configured-directory>` (relative or absolute): capture loads the app config, replay does not. `--snapshot <name>` selects a retained name; omission selects the newest snapshot. Capture generates a unique timestamp name when omitted, and **replaces** an existing capture when an explicit name is reused. Use distinct before/after names, including names with spaces, to preserve both baselines. For interactive exploration, capture without `--output` to serve the UI; `next build --analyze` also produces replayable data.

**Done:** the chosen capture is identified, export succeeded, and the baseline file, snapshot name and analysis scope are recorded. Check the whole pipeline's exit status before processing the compressed file: `pipefail` prevents gzip from hiding a failed export. Export validates one route at a time, so a failure can leave a partial archive even if `gzip -t` accepts it. Discard output when export or gzip fails.

## 3. Interpret the evidence

Resolve the schema from the app's **installed Next.js**, using its Node launcher:

```bash pnpm exec node -p "require.resolve('next/analyze/graph-v1.schema.json')" ```

Read its descriptions for record meanings, joins, attribution and coverage. Stream decompression with `gzip -dc /tmp/analyze-app-before.jsonl.gz` into a line-oriented analysis script rather than loading the whole dump into context. Use the schema to interpret client/server contributions across every route in scope and choose the metric.

**Done:** the baseline's route-attributed client/server contributions and the metric are identified. Treat this as **build evidence**: claims about observed browser requests, timing or transfer savings need separate evidence.

## 4. Explain a candidate

Inspect actual project source and exact importers. Rank client-output contributions across the whole app, or within the user-specified scope, by attributed size, repetition, likely runtime cost, need before interaction and correctness risk.

### Community → min-cut pass

For broad audits seeking opportunities across multiple features or a large contributor list, run **community detection → target selection → directed min-cut → source validation** after the initial ranking. Read [Graph methods](references/graph-methods.md) before constructing the graph or running solvers.

1. Rank detected communities by scoped client-output attri