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

packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts

@@ -181,6 +181,44 @@ export function buildStartManifest(options: {
   }
 }
 
+/**
+ * Serializes the manifest as a JS module that declares each unique asset
+ * once and references it by variable name, preserving object-reference
+ * sharing so downstream serializers can dedupe shared assets in the SSR HTML.
+ */
+export function serializeStartManifestAsModule(manifest: {
+  routes: Record<string, RouteTreeRoute>
+  clientEntry: string
+}): string {
+  const varNameByIdentity = new Map<string, string>()
+  const assetDecls: Array<string> = []
+
+  for (const route of Object.values(manifest.routes)) {
+    for (const asset of route.assets ?? []) {
+      const identity = getAssetIdentity(asset)
+      if (varNameByIdentity.has(identity)) continue
+      const varName = `__tsr_a${varNameByIdentity.size}`
+      varNameByIdentity.set(identity, varName)
+      assetDecls.push(`const ${varName}=${JSON.stringify(asset)};`)
+    }
+  }
+
+  // Replace each asset with a sentinel string, then strip the quotes so the
+  // generated source references the shared const by identifier.
+  const body = JSON.stringify(manifest, (key, value) => {
+    if (key === 'assets' && Array.isArray(value)) {
+      return (value as Array<RouterManagedTag>).map(
+        (asset) =>
+          `@@TSR_ASSET_REF@@${varNameByIdentity.get(getAssetIdentity(asset))!}`,
+      )
+    }
+    return value
+  }).replace(/"@@TSR_ASSET_REF@@(__tsr_a\d+)"/g, '$1')
+
+  return `${assetDecls.join('\n')}
+export const tsrStartManifest = () => (${body});`
+}
+
 export function scanClientChunks(
   clientBuild: NormalizedClientBuild,
 ): ScannedClientChunks {

packages/start-plugin-core/src/vite/start-manifest-plugin/plugin.ts

@@ -2,7 +2,10 @@ import { joinURL } from 'ufo'
 import { VIRTUAL_MODULES } from '@tanstack/start-server-core'
 import { resolveViteId } from '../../utils'
 import { ENTRY_POINTS, START_ENVIRONMENT_NAMES } from '../../constants'
-import { buildStartManifest } from '../../start-manifest-plugin/manifestBuilder'
+import {
+  buildStartManifest,
+  serializeStartManifestAsModule,
+} from '../../start-manifest-plugin/manifestBuilder'
 import type { GetConfigFn, NormalizedClientBuild } from '../../types'
 import type { PluginOption } from 'vite'
 
@@ -59,7 +62,7 @@ export function startManifestPlugin(opts: {
             basePath: resolvedStartConfig.basePaths.publicBase,
           })
 
-          return `export const tsrStartManifest = () => (${JSON.stringify(startManifest)})`
+          return serializeStartManifestAsModule(startManifest)
         }
 
         return undefined

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

@@ -10,6 +10,7 @@ import {
   normalizeViteClientBuild,
   normalizeViteClientChunk,
   scanClientChunks,
+  serializeStartManifestAsModule,
 } from '../../src/start-manifest-plugin/manifestBuilder'
 import type { Rollup } from 'vite'
 
@@ -1237,3 +1238,115 @@ describe('buildStartManifest route pruning', () => {
     expect(manifest.routes['/about']).toBeUndefined()
   })
 })
+
+// Evaluates the generated module source via `new Function` so each test
+// gets its own scope for the `__tsr_aN` consts.
+function evaluateManifestModule(source: string) {
+  const body = `${source.replace(
+    'export const tsrStartManifest',
+    'const tsrStartManifest',
+  )}\nreturn tsrStartManifest();`
+  return new Function(body)() as {
+    routes: Record<string, any>
+    clientEntry: string
+  }
+}
+
+describe('serializeStartManifestAsModule', () => {
+  test('preserves object-reference identity across routes that share an asset', () => {
+    const sharedAsset = {
+      tag: 'link' as const,
+      attrs: {
+        rel: 'stylesheet',
+        href: '/assets/shared.css',
+        type: 'text/css',
+      },
+    }
+    const uniqueToA = {
+      tag: 'link' as const,
+      attrs: {
+        rel: 'stylesheet',
+        href: '/assets/only-a.css',
+        type: 'text/css',
+      },
+    }
+
+    const source = serializeStartManifestAsModule({
+      routes: {
+        __root__: { assets: [sharedAsset] },
+        '/a': {
+          filePath: '/routes/a.tsx',
+          assets: [sharedAsset, uniqueToA],
+        },
+        '/b': {
+          filePath: '/routes/b.tsx',
+          assets: [sharedAsset],
+        },
+      },
+      clientEntry: '/assets/entry.js',
+    })
+
+    const manifest = evaluateManifestModule(source)
+
+    // Shared assets must be the same object across routes so downstream
+    // TSON serialization can emit back-references instead of re-inlining.
+    expect(manifest.routes.__root__.assets[0]).toBe(
+      manifest.routes['/a'].assets[0],
+    )
+    expect(manifest.routes['/a'].assets[0]).toBe(
+      manifest.routes['/b'].assets[0],
+    )
+    // Non-shared assets remain distinct objects.
+    expect(manifest.routes['/a'].assets[1]).not.toBe(
+      manifest.routes['/a'].assets[0],
+    )
+  })
+
+  test('preserves filePath, children, preloads, and clientEntry fields', () => {
+    const asset = {
+      tag: 'link' as const,
+      attrs: {
+        rel: 'stylesheet',
+        href: '/assets/main.css',
+        type: 'text/css',
+      },
+    }
+
+    const source = serializeStartManifestAsModule({
+      routes: {
+        __root__: { children: ['/a'], assets: [asset] },
+        '/a': {
+          filePath: '/routes/a.tsx',
+          assets: [asset],
+          preloads: ['/assets/a.js', '/assets/vendor.js'],
+        },
+      },
+      clientEntry: '/assets/entry.js',
+    })
+
+    const manifest = evaluateManifestModule(source)
+    expect(manifest.clientEntry).toBe('/assets/entry.js')
+    expect(manifest.routes.__root__.children).toEqual(['/a'])
+    expect(manifest.routes['/a'].filePath).toBe('/routes/a.tsx')
+    expect(manifest.routes['/a'].preloads).toEqual([
+      '/assets/a.js',
+      '/assets/vendor.js',
+    ])
+  })
+
+  test('handles manifests where no route carries any assets', () => {
+    const source = serializeStartManifestAsModule({
+      routes: {
+        __root__: { children: ['/a'] },
+        '/a': { filePath: '/routes/a.tsx' },
+      },
+      clientEntry: '/assets/entry.js',
+    })
+
+    const manifest = evaluateManifestModule(source)
+    expect(manifest.clientEntry).toBe('/assets/entry.js')
+    expect(manifest.routes.__root__.children).toEqual(['/a'])
+    expect(manifest.routes['/a'].filePath).toBe('/routes/a.tsx')
+    expect(manifest.routes['/a'].assets).toBeUndefined()
+  })
+})