next-bundle-analyzer

name

next-bundle-analyzer

description

Analyze and quality-check this project's Next.js 16 (Turbopack) bundle sizes using `vp run analyze:output` (i.e. `next experimental-analyze --output`). Use when the user wants to: inspect bundle sizes, find large dependencies, detect duplicate modules, quality-check production builds, or get optimization recommendations. Triggers on requests like "analyze the bundle", "check bundle size", "find large packages", "bundle quality check", "バンドル分析", "バンドルサイズ確認", or when the user mentions `next experimental-analyze`.

Next.js Bundle Analyzer (project-scoped)

Analyze Turbopack bundle output from next experimental-analyze --output for this repository.

Workflow

Step 1: Generate analyze output

Use the pre-defined scripts (see package.json):

vp run analyze:output

This runs next experimental-analyze --output and writes results to .next/diagnostics/analyze/.

Note: Next.js 16 is a breaking-change release. If the command is missing or the output path changes, consult node_modules/next/dist/docs/ before adjusting.

Step 2: Run the analysis script

uv run .claude/skills/next-bundle-analyzer/scripts/analyze_bundle.py .next/diagnostics/analyze

The script has PEP 723 inline metadata (requires-python = ">=3.11"), so uv provisions its own interpreter automatically — no venv needed.

Step 3: Interpret results

Section	What to look for
Total bundle size	Compressed size matters for network; raw size for parse time
Top output files	Files >100KB (raw) are candidates for investigation
Top npm packages	User-land packages unexpectedly large (not next/react)
Duplicate packages	Same package at multiple versions = wasted bytes
Recommendations	Auto-detected optimization opportunities

The script is pnpm-aware and correctly attributes packages from node_modules/.pnpm/<name>@<version>/.

Output file markers (in "TOP OUTPUT FILES" listing)

• [client-fs] — shipped to the browser. Focus optimization here.
• [output] — server-only artifacts (SSR / RSC runtime).
• [pkg] — internal next/dist/compiled/*. A gzip size of 2 B is a marker that the file is not a shipped compressed chunk; usually framework overhead, not actionable.

When the ">100KB candidate" rule from the table above triggers on [pkg] / [output] entries, it's almost always framework weight. Real wins come from trimming [client-fs] entries.

When "Recommendations" is empty

The script's auto-recommendations only fire on user-land packages crossing a size threshold. Empty = no obviously large user-land package. This is not a signal that nothing can be optimized — cross-reference package.json and manually flag icon / UI / utility libraries (e.g. lucide-react, @base-ui/react) that may still benefit from optimizePackageImports.

Step 4: Apply optimizations

Reference: node_modules/next/dist/docs/01-app/02-guides/package-bundling.md (read this before editing config to confirm API names for Next.js 16).

Large packages with many named exports → Add to optimizePackageImports in next.config.ts:

experimental: {
  optimizePackageImports: ['icon-library', 'utility-library'],
}

Candidates in this project are typically: lucide-react, @base-ui/react, or similar icon/utility-heavy packages.

Heavy client-side libraries that could run server-side → Move logic to a Server Component (e.g. markdown parsing, date formatting, syntax highlighting).

Duplicate packages → Run pnpm dedupe.

Server-only packages leaking into client → Add to serverExternalPackages in next.config.ts:

serverExternalPackages: ['package-name'],

Comparing before/after

# Save current state
cp -r .next/diagnostics/analyze ./analyze-before

# Make changes, then re-analyze
vp run analyze:output

# Compare
uv run .claude/skills/next-bundle-analyzer/scripts/analyze_bundle.py ./analyze-before
uv run .claude/skills/next-bundle-analyzer/scripts/analyze_bundle.py .next/diagnostics/analyze

Key diff signals to compare:

• Total compressed increased → user-facing network cost went up.
• Top npm packages gained an unexpected entry → a refactor pulled a new heavy dep into the client bundle.
• Duplicate packages went from none to some → run pnpm dedupe.
• A new [client-fs] output file crossed 100KB → candidate for code-splitting or optimizePackageImports.

Interactive mode (visual treemap)

For browser-based exploration (no --output flag):

vp run analyze
# Opens http://localhost:4000