fix(start): preserve asset object identity when serializing manifest by ZacharyL2 · Pull Request #7147 · TanStack/router

ZacharyL2 · 2026-04-11T03:23:03Z

The problem

The Start manifest's serialization step uses JSON.stringify(startManifest)
in startManifestPlugin. A single CSS asset referenced by many routes is a
shared object in memory at build time — but JSON.stringify produces a
structurally-cloned tree, so every route ends up with its own independent
copy of the asset.

Downstream, the SSR serializer dedupes repeated values using reference
identity (seenMap.has(obj)). Because the post-JSON.stringify objects
are all distinct, the serializer can't recognize them as repeats and inlines
each copy in full into the SSR HTML.

On a medium-sized app, this caused the same CSS asset object to be inlined
dozens of times into the root document. Observed impact: root HTML went
from ~310 KB → ~57 KB (~81% reduction) after the fix.

This complements #7030, which deduped the manifest at the data layer. The
data structure is already deduped (each shared asset is the same object
in memory), but the serialization step used to discard that sharing before
the downstream serializer saw it.

The fix

Introduce serializeStartManifestAsModule in manifestBuilder.ts. Instead
of JSON.stringify(manifest), the manifest is emitted as a JS module:

const __tsr_a0 = {
  tag: 'link',
  attrs: { rel: 'stylesheet', href: '/assets/foo.css' /* ... */ },
}
const __tsr_a1 = {
  tag: 'link',
  attrs: { rel: 'stylesheet', href: '/assets/bar.css' /* ... */ },
}

export const tsrStartManifest = () => ({
  routes: {
    '/a': { assets: [__tsr_a0, __tsr_a1] },
    '/b': { assets: [__tsr_a0] }, // same __tsr_a0 object
    '/c': { assets: [__tsr_a0] }, // same __tsr_a0 object
    // ...
  },
  clientEntry: '...',
})

Each unique asset is declared once as a module-level const; every route
references it by variable name. When the generated module is loaded, all
routes that share an asset hold the same JS object reference in memory,
so the SSR serializer can collapse them as designed.

Asset uniqueness is determined by the existing getAssetIdentity() helper,
so the identity logic (which already respects tag, href, src, rel,
type, etc.) is reused without change.

Implementation is a single JSON.stringify pass with a replacer that swaps
each assets[] item for a marker string, then a final .replace() over
the produced string converts markers into bare variable references. No
runtime cost — pure build-time codegen.

How I tested it

New unit tests (`manifestBuilder.test.ts`, 3 tests)

All three are observation-based, using new Function(...) to evaluate the
generated module source and inspect the resulting object graph:

preserves object-reference identity across routes that share an
asset — 3 routes share 1 asset; the eval'd module must satisfy
routes.a.assets[0] === routes.b.assets[0] === routes.c.assets[0].
preserves filePath, children, preloads, and clientEntry fields —
the JSON replacer only intercepts assets; all other fields must be
passed through unchanged.
handles manifests where no route carries any assets — empty-assets
edge case doesn't throw or emit stray declarations.

End-to-end verification in a real app

Measured on a medium TanStack Start app:

metric	before	after
root HTML size	~310 KB	~57 KB
manifest JS chunk	~310 KB	~88 KB
unique shared asset vars	—	148
max CSS href repeats in HTML	30+	2

The 2 remaining "repeats" per CSS href are the legitimate <link> tag in
<head> plus a single declaration in the embedded SSR payload. Each
CSS href now appears exactly where it needs to, once as a <link> and
once as a data field.

Summary by CodeRabbit

Release Notes

Improvements
- Enhanced manifest serialization to deduplicate and reuse asset references across routes, resulting in more efficient module generation and smaller output sizes.
Tests
- Added comprehensive test coverage for manifest serialization, verifying asset identity preservation, route field handling, and optional asset support.

…ifest `JSON.stringify(startManifest)` produces a structurally-cloned tree, which breaks the identity-based dedup used by the downstream SSR serializer. As a result, CSS assets referenced by many routes end up inlined once per route in the SSR HTML payload. Introduce `serializeStartManifestAsModule`, which emits each unique asset as a top-level `const __tsr_aN = {...}` and has every route reference the shared variable by name. When the generated module is loaded, routes that share an asset resolve to the same JS object, so the SSR serializer can dedupe them as intended. On a medium app, the root HTML drops from ~310 KB to ~57 KB.

coderabbitai · 2026-04-11T03:23:23Z

📝 Walkthrough

Walkthrough

The changes introduce a new serializeStartManifestAsModule function that optimizes start manifest serialization by deduplicating asset objects across routes and emitting them as shared constants, replacing the previous inline JSON stringification approach.

Changes

Cohort / File(s)	Summary
Manifest Serialization Enhancement `packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts`	Added `serializeStartManifestAsModule` function that generates a JavaScript module string by scanning routes for assets, computing stable identities, emitting deduplicated asset constants, and post-processing JSON to reference them by identifier rather than inline.
Plugin Integration `packages/start-plugin-core/src/vite/start-manifest-plugin/plugin.ts`	Updated imports and replaced inline `JSON.stringify` module generation with a call to `serializeStartManifestAsModule` to produce optimized manifest output.
Test Coverage `packages/start-plugin-core/tests/start-manifest-plugin/manifestBuilder.test.ts`	Added test suite for `serializeStartManifestAsModule` with helper function to evaluate generated module, verifying asset identity preservation across routes, field integrity, and optional asset handling.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 A rabbit hops through manifests with glee,
Deduplicating assets wild and free!
No more repeated JSON bloat in sight,
Constants share the burden, shiny-bright.
Module strings dance with newfound grace,
Hopping faster through this codebase space! 🚀

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title 'fix(start): preserve asset object identity when serializing manifest' clearly and specifically describes the main change—preserving asset object identity during manifest serialization to fix a deduplication bug. This directly aligns with the changeset's primary objective.

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github-actions · 2026-04-11T03:26:03Z

Bundle Size Benchmarks

Commit: 13ec8a305bbd
Measured at: 2026-04-11T03:25:22.686Z
Baseline source: history:e61c49ce31af
Dashboard: bundle-size history

Scenario	Current (gzip)	Delta vs baseline	Raw	Brotli	Trend
`react-router.minimal`	87.51 KiB	0 B (0.00%)	275.69 KiB	76.08 KiB	▁▁▂▃▃▅██▆▆▆
`react-router.full`	90.77 KiB	0 B (0.00%)	286.88 KiB	78.95 KiB	▁▁▃▆▆▅██▃▃▃
`solid-router.minimal`	35.60 KiB	0 B (0.00%)	107.36 KiB	31.94 KiB	▁▁▁▅▅▆█████
`solid-router.full`	40.07 KiB	0 B (0.00%)	120.90 KiB	35.96 KiB	▁▁▁▄▄▆█████
`vue-router.minimal`	53.46 KiB	0 B (0.00%)	153.16 KiB	48.00 KiB	▁▁▁▂▂▆█████
`vue-router.full`	58.36 KiB	0 B (0.00%)	168.62 KiB	52.25 KiB	▁▁▁▂▂▆█████
`react-start.minimal`	101.90 KiB	0 B (0.00%)	323.57 KiB	88.17 KiB	▇▇▇▇▇▆██▇▁▁
`react-start.full`	105.35 KiB	0 B (0.00%)	333.92 KiB	90.99 KiB	▃▃▄▅▅▆██▇▁▁
`solid-start.minimal`	49.62 KiB	0 B (0.00%)	153.24 KiB	43.76 KiB	▁▁▁▅▅▆███▂▂
`solid-start.full`	55.15 KiB	0 B (0.00%)	169.48 KiB	48.46 KiB	▁▁▂▅▅▆███▄▄

Trend sparkline is historical gzip bytes ending with this PR measurement; lower is better.

coderabbitai

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In
`@packages/start-plugin-core/tests/start-manifest-plugin/manifestBuilder.test.ts`:
- Around line 1249-1252: The helper that evaluates the manifest currently casts
the result to a loose type with "any" (the return of new Function(body)() is
typed as { routes: Record<string, any>; clientEntry: string }); replace that
with a strict manifest type: define or import a Manifest interface (e.g.,
interface Manifest { routes: Record<string, RouteDefinition>; clientEntry:
string }) or use ReturnType from the real manifest builder, then cast the
evaluated result as Manifest (not any) and adjust RouteDefinition to capture the
expected route shape so test assertions are type-checked at compile time; update
the typed symbols "routes" and "clientEntry" accordingly.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

Push a commit to this branch (recommended)
Create a new PR with the fixes

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: d8fa3269-e161-4983-9f37-2b3b6597d9e4

📥 Commits

Reviewing files that changed from the base of the PR and between 13ec8a3 and 7932f79.

📒 Files selected for processing (3)

packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts
packages/start-plugin-core/src/vite/start-manifest-plugin/plugin.ts
packages/start-plugin-core/tests/start-manifest-plugin/manifestBuilder.test.ts

packages/start-plugin-core/tests/start-manifest-plugin/manifestBuilder.test.ts

schiller-manuel · 2026-04-11T07:57:14Z

please provide a complete reproducer project as a git repo that shows this issue clearly.

github-actions bot added the package: start-plugin-core label Apr 11, 2026

coderabbitai bot reviewed Apr 11, 2026

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fix(start): preserve asset object identity when serializing manifest#7147

fix(start): preserve asset object identity when serializing manifest#7147
ZacharyL2 wants to merge 1 commit intoTanStack:mainfrom
ZacharyL2:fix/manifest-asset-identity

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ZacharyL2 commented Apr 11, 2026 • edited by coderabbitai bot Loading Uh oh! There was an error while loading. Please reload this page.

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The problem

The fix

How I tested it

New unit tests (manifestBuilder.test.ts, 3 tests)

End-to-end verification in a real app

Summary by CodeRabbit

Release Notes

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ZacharyL2 commented Apr 11, 2026 •

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New unit tests (`manifestBuilder.test.ts`, 3 tests)

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