Build Tooling Vite Webpack Review

Build Tooling (Vite/Webpack) Review

Purpose

Bundler config advice that doesn't match the installed major version is worse than no advice — it produces a config diff that silently no-ops, throws at build time, or (worst case) applies to the wrong bundler engine entirely. Vite ships two build engines depending on version and config shape (Rollup-backed rollupOptions, or Rolldown-backed rolldownOptions), and Vite 8 removed the object form of manualChunks outright. This skill reviews Vite/Webpack build and chunking configuration itself — the config file, not the resulting bundle bytes — and treats the target bundler's exact major version as a required input, not an assumption, before proposing any config diff.

When to use

Use this skill when the user asks to:

• review or tune Vite build.rollupOptions/build.rolldownOptions or Webpack optimization.splitChunks chunking configuration,
• diagnose a duplicate-dependency-across-chunks defect or an unsplit vendor chunk caused by chunking config (not by application-level import boundaries),
• evaluate a proposed bundler migration (Webpack to Vite, Rollup-backed Vite to Rolldown-backed Vite, or a Vite major-version upgrade) for build-time and config-compatibility impact,
• diagnose a tree-shaking failure that traces back to bundler/build config (module format, sideEffects field interaction with the bundler, barrel-file re-exports defeating static analysis) rather than to source-code side effects,
• set up or review a CI bundle-size budget gate at the build-tool level (build script, CI job config).

When NOT to use

• Setting or auditing the numeric size budget itself, ranking analyzer contributors by byte weight vs. execution cost, or deciding route/component-level code-splitting boundaries — hand off to bundle-budget-code-splitting-review for that; this skill reviews the bundler config mechanism, not the budget methodology or split-boundary decision.
• Verifying that tree-shaking actually eliminated dead code via a before/after byte diff, or auditing sideEffects: false correctness on a specific package — hand off to tree-shaking-dead-code-review for that; this skill only flags build-config patterns (barrel imports, module-format mismatches) that are known to defeat tree-shaking, it does not verify the outcome.
• Non-bundler build tooling (Babel-only transpilation pipelines with no bundling step, TypeScript tsc-only builds, esbuild/Rolldown used as a library outside Vite/Webpack) — different config surface, not in scope here.

Context7 Documentation Protocol

Vite's build-engine and chunking API surface changed between major versions — do not prescribe a config from memorized training data.

1. Call ToolSearch with query "context7" (or "select:mcp__Context7__resolve-library-id,mcp__Context7__query-docs") to load the Context7 tools if not already loaded this session.
2. Call mcp__Context7__resolve-library-id for each bundler in scope (/vitejs/vite, Webpack, /rollup/rollup, and /rolldown/rolldown if the project's Vite build is Rolldown-backed) before prescribing any chunking configuration.
3. Call mcp__Context7__query-docs for the specific mechanism — e.g. "rollupOptions.output.manualChunks vs rolldownOptions.output.codeSplitting", "splitChunks cacheGroups defaults", "manualChunks object form vs function form" — before ruling on it. Do this per review; do not reuse a prior session's memory of bundler internals.
4. Known version-sensitive facts verified via Context7 as of this skill's updated date:
   • Vite's migration guide documents that the object form of build.rollupOptions.output.manualChunks was removed, and the function form was deprecated; the replacement is build.rolldownOptions.output.codeSplitting, which takes a groups array of { name, test } entries (Rolldown's declarative chunk-grouping API). Do not hand a project on that config path a manualChunks: { vendor: [...] } object-form snippet — it will not apply.
   • Rollup itself (used directly, or via Vite versions/configs that still route through rollupOptions) continues to support both the object form and function form of output.manualChunks per Rollup's own configuration docs — the removal is specific to Vite's option surface and its migration path toward Rolldown, not a Rollup-wide deprecation. Confirm which bundler engine actually processes the config (rollupOptions vs rolldownOptions key) before applying either party's rules.
   • Vite ships a built-in splitVendorChunkPlugin that composes with function-form manualChunks but explicitly logs a warning and no-ops when combined with the object form — treat "vendor chunk isn't splitting and there's a console warning about splitVendorChunk" as a symptom of this specific interaction, not a generic bug report.
5. Webpack's optimization.splitChunks surface (cacheGroups, chunks: 'all' | 'async' | 'initial', minSize, maxInitialRequests) is comparatively stable across recent majors per Context7-grounded docs, including its documented defaults (minSize: 20000, defaultVendors/default cache groups with priority: -10/-20) — still confirm the installed Webpack major before asserting a specific default, since defaults have shifted across major versions historically.
6. If Context7 is unavailable or returns no relevant match for the installed bundler, fall back to official_docs and mark the claim documentation-based (Context7 unavailable) rather than presenting it as freshly verified.
7. Never invent a bundler config key, CLI flag, or plugin option that no queried source confirms.

Lean operating rules

• Confirm the installed Vite major version and which output-options key (rollupOptions vs rolldownOptions) the project's own config already uses before proposing a diff — these are two different bundler engines with two different chunking APIs, and giving the wrong era's config is a concrete, verified failure, not a style choice.
• Distinguish Vite's dev-server behavior (native ESM, no bundling, chunking config irrelevant) from its production build behavior (bundled via Rollup or Rolldown depending on version/config); every recommendation in this skill applies only to the production build path.
• Flag barrel-file (index.ts re-export) imports and wildcard imports of large libraries (icon sets, date libraries, utility libraries) as build-config-relevant tree-shaking risks when the project's module format or bundler settings can't statically resolve them — name the specific named/subpath-import fix, and hand off to tree-shaking-dead-code-review to verify the byte-level outcome.
• Treat a duplicate-dependency-across-chunks finding as a chunk-grouping config defect (fix manualChunks/codeSplitting/cacheGroups), not an application-code defect — do not propose restructuring import statements to solve what a config change solves more directly.
• Require a stated measured baseline (current bundle output, build time) before endorsing any bundler or bundler-major migration, plus a rollback path (pinned lockfile entry, ability to revert the config key).
• Recommend a CI bundle-size budget gate at the build-tool/CI-job level for any budget-critical route that lacks one, rather than treating build-config review as a one-time audit — but define the numeric budget itself only via bundle-budget-code-splitting-review.
• Do not evaluate bundle-size numbers as if they were field/RUM Core Web Vitals data; a build's stats output is lab data from a CI build unless explicitly stated otherwise.
• Never accept "it built without errors" as evidence a chunking config change is correct — a removed or renamed option key can silently no-op (see the Vite 8 manualChunks object-form removal) rather than error.

References

Load these only when needed:

• Vite chunking config reference — use when reviewing or writing rollupOptions.output.manualChunks or rolldownOptions.output.codeSplitting, or diagnosing which bundler engine a Vite config actually invokes.
• Webpack splitChunks reference — use when reviewing or writing optimization.splitChunks, cacheGroups, or runtimeChunk configuration.
• Migration and config-diff safety — use before endorsing a bundler migration or a chunking-config change, to confirm baseline capture, rollback path, and the verification steps that must precede calling a config change complete.

Response minimum

Return, at minimum:

• the bundler and confirmed major version (and, for Vite, the confirmed output-options key: rollupOptions or rolldownOptions) the guidance targets,
• build-config findings (duplicate-dependency chunk-grouping defects, config keys that no longer apply for the confirmed version, barrel-file/module-format patterns defeating tree-shaking) with file/line evidence,
• proposed chunking/config diff (not applied), labeled lab vs field for any performance number cited,
• evidence level (live evidence, user-provided sanitized evidence, documentation-based, or inference),
• migration caveat (baseline + rollback) if a bundler or bundler-major change is recommended, and a handoff note to bundle-budget-code-splitting-review or tree-shaking-dead-code-review if the user's actual question is budget-setting or tree-shaking verification rather than config review.