From 3d86a6fe7aea6bec4fc08a8526a9c12b5279d694 Mon Sep 17 00:00:00 2001
From: Sakana <15715093608@163.com>
Date: Thu, 5 Nov 2026 16:20:54 +0800
Subject: [PATCH] fix(rolldown): preserve missing and zero comparison metrics

---
 .../src/app/components/compare/DeltaValue.vue |  9 ++-
 .../app/components/compare/SingleSideRow.vue  |  2 +-
 .../src/app/components/compare/SplitRow.vue   | 10 +--
 .../src/app/components/compare/TabView.vue    | 61 +++++++++++--------
 .../app/components/modules/BuildMetrics.vue   |  6 +-
 .../__tests__/reader-index-cache.test.ts      | 14 +++--
 .../rolldown/src/node/rolldown/log-cache.ts   | 11 ++--
 .../node/rpc/__tests__/module-compare.test.ts | 50 +++++++++++++++
 .../rolldown-get-session-compare-details.ts   | 13 ++--
 packages/rolldown/src/shared/types/data.ts    | 12 ++--
 .../utils/__tests__/compare-metrics.test.ts   | 22 +++++++
 .../src/shared/utils/compare-metrics.ts       | 25 ++++++++
 12 files changed, 177 insertions(+), 58 deletions(-)
 create mode 100644 packages/rolldown/src/node/rpc/__tests__/module-compare.test.ts
 create mode 100644 packages/rolldown/src/shared/utils/__tests__/compare-metrics.test.ts
 create mode 100644 packages/rolldown/src/shared/utils/compare-metrics.ts

diff --git a/packages/rolldown/src/app/components/compare/DeltaValue.vue b/packages/rolldown/src/app/components/compare/DeltaValue.vue
index aa5a0770..1389fac8 100644
--- a/packages/rolldown/src/app/components/compare/DeltaValue.vue
+++ b/packages/rolldown/src/app/components/compare/DeltaValue.vue
@@ -3,7 +3,7 @@ import { bytesToHumanSize } from '@vitejs/devtools-ui/utils/format'
 import { computed } from 'vue'
 
 const props = withDefaults(defineProps<{
-  value: number
+  value: number | null
   format?: 'bytes' | 'duration' | 'number'
   signed?: boolean
 }>(), {
@@ -12,18 +12,21 @@ const props = withDefaults(defineProps<{
 })
 
 const sign = computed(() => {
-  if (!props.signed || props.value === 0)
+  if (!props.signed || props.value == null || props.value === 0)
     return ''
   return props.value > 0 ? '+' : '-'
 })
 
 const colorClass = computed(() => {
-  if (!props.signed || props.value === 0)
+  if (!props.signed || props.value == null || props.value === 0)
     return 'text-gray-500'
   return props.value > 0 ? 'text-red-500' : 'text-green-500'
 })
 
 const formatted = computed(() => {
+  if (props.value == null)
+    return { amount: 'Unavailable', unit: '' }
+
   const value = props.signed ? Math.abs(props.value) : props.value
 
   if (props.format === 'bytes') {
diff --git a/packages/rolldown/src/app/components/compare/SingleSideRow.vue b/packages/rolldown/src/app/components/compare/SingleSideRow.vue
index 1d075457..9809dc0c 100644
--- a/packages/rolldown/src/app/components/compare/SingleSideRow.vue
+++ b/packages/rolldown/src/app/components/compare/SingleSideRow.vue
@@ -11,7 +11,7 @@ withDefaults(defineProps<{
   sessionLabel: string
   title: string
   titleMeta?: string
-  delta: number
+  delta: number | null
   format: 'bytes' | 'duration' | 'number'
   subtitle?: string
   link?: string
diff --git a/packages/rolldown/src/app/components/compare/SplitRow.vue b/packages/rolldown/src/app/components/compare/SplitRow.vue
index 29156864..e2a5ced3 100644
--- a/packages/rolldown/src/app/components/compare/SplitRow.vue
+++ b/packages/rolldown/src/app/components/compare/SplitRow.vue
@@ -22,9 +22,9 @@ const props = withDefaults(defineProps<{
   currentStats?: SplitDiffStat[]
   previousBadges?: string[]
   currentBadges?: string[]
-  previous: number
-  current: number
-  delta: number
+  previous: number | null
+  current: number | null
+  delta: number | null
   format: 'bytes' | 'duration' | 'number'
   ratioText?: string
   selectable?: boolean
@@ -42,9 +42,9 @@ const emit = defineEmits<{
 }>()
 
 const ratioClass = computed(() => {
-  if (props.delta > 0)
+  if (props.delta != null && props.delta > 0)
     return 'text-red-500'
-  if (props.delta < 0)
+  if (props.delta != null && props.delta < 0)
     return 'text-green-500'
   return 'text-gray-500'
 })
diff --git a/packages/rolldown/src/app/components/compare/TabView.vue b/packages/rolldown/src/app/components/compare/TabView.vue
index 241d2e29..5da85b6b 100644
--- a/packages/rolldown/src/app/components/compare/TabView.vue
+++ b/packages/rolldown/src/app/components/compare/TabView.vue
@@ -7,10 +7,11 @@ import type {
 } from '~~/shared/types'
 import { bytesToHumanSize } from '@vitejs/devtools-ui/utils/format'
 import { computed, ref, watch } from 'vue'
+import { formatMetricRatio, sumMetrics } from '~~/shared/utils/compare-metrics'
 import { useRoute, useRouter } from '#app/composables/router'
 
 type CompareTab = 'overview' | 'plugins' | 'chunks' | 'modules' | 'assets' | 'packages'
-type DiffMode = 'changed' | 'added' | 'removed'
+type DiffMode = 'changed' | 'added' | 'removed' | 'unavailable'
 type DiffFormat = 'bytes' | 'duration' | 'number'
 type SummaryTone = 'increase' | 'decrease'
 
@@ -29,7 +30,7 @@ interface SplitDiffStat {
   hidden?: boolean
 }
 
-interface SplitDiffRow extends SessionCompareMetricValue {
+interface SplitDiffRow extends SessionCompareMetricValue<number | null> {
   key: string
   status: SessionCompareChangeStatus
   previousTitle?: string
@@ -253,7 +254,7 @@ const sessionSummaries = computed(() => {
           },
           {
             label: 'Calls',
-            value: pluginStats.calls.toLocaleString(),
+            value: pluginStats.calls?.toLocaleString() ?? 'Unavailable',
             icon: 'i-ph-arrow-counter-clockwise-duotone',
             tone: getSummaryTone(index, pluginStats.calls, pluginStatsA.calls),
           },
@@ -416,12 +417,16 @@ const activeRows = computed(() => {
   }
 })
 
+const unavailableRows = computed(() => activeRows.value.filter(item => item.status === 'unavailable'))
+
 const changedRows = computed(() => activeRows.value.filter(item => item.status === 'changed'))
 const addedRows = computed(() => activeRows.value.filter(item => item.status === 'added'))
 const removedRows = computed(() => activeRows.value.filter(item => item.status === 'removed'))
 
 const activeDiffRows = computed(() => {
   switch (activeDiffMode.value) {
+    case 'unavailable':
+      return unavailableRows.value
     case 'added':
       return addedRows.value
     case 'removed':
@@ -438,7 +443,7 @@ const diffModeOptions = computed<Array<{
   description: string
   icon: string
   count: number
-  delta: number
+  delta: number | null
   rows: SplitDiffRow[]
 }>>(() => {
   return [
@@ -469,6 +474,17 @@ const diffModeOptions = computed<Array<{
       delta: sumDelta(removedRows.value),
       rows: removedRows.value.slice(0, TABLE_LIMIT),
     },
+    ...(props.tab === 'modules'
+      ? [{
+          id: 'unavailable' as const,
+          label: 'Unavailable',
+          icon: 'i-ph-question-duotone',
+          description: 'Matched modules without a size measurement on one or both sides.',
+          count: unavailableRows.value.length,
+          delta: null,
+          rows: unavailableRows.value.slice(0, TABLE_LIMIT),
+        }]
+      : []),
   ]
 })
 
@@ -487,6 +503,8 @@ const activeDiffFormat = computed(() => activeRows.value[0]?.format || 'bytes')
 
 const activeDiffEmptyText = computed(() => {
   switch (activeDiffMode.value) {
+    case 'unavailable':
+      return 'No unavailable measurements'
     case 'added':
       return 'No added entries'
     case 'removed':
@@ -607,20 +625,13 @@ function getStatTone(value: number, baseline: number | undefined): SplitDiffStat
 function createSplitRow(row: Omit<SplitDiffRow, 'ratioText'>): SplitDiffRow {
   return {
     ...row,
-    ratioText: formatRatio(row),
+    ratioText: formatMetricRatio(row),
   }
 }
 
-function formatRatio(item: SessionCompareMetricValue) {
-  if (item.deltaRatio == null)
-    return item.previous === 0 && item.current > 0 ? 'new' : '-'
-  if (item.deltaRatio === 0)
-    return '0%'
-  const sign = item.deltaRatio > 0 ? '+' : ''
-  return `${sign}${(item.deltaRatio * 100).toFixed(2)}%`
-}
-
-function formatValue(value: number, format: DiffFormat) {
+function formatValue(value: number | null, format: DiffFormat) {
+  if (value == null)
+    return 'Unavailable'
   if (format === 'duration') {
     if (value >= 1000)
       return `${+(value / 1000).toFixed(2)}s`
@@ -630,14 +641,16 @@ function formatValue(value: number, format: DiffFormat) {
   return value.toLocaleString()
 }
 
-function formatBytesSummary(value: number) {
+function formatBytesSummary(value: number | null) {
+  if (value == null)
+    return 'Unavailable'
   if (!value)
     return '0 B'
   return bytesToHumanSize(value).join(' ')
 }
 
-function getSummaryTone(index: number, value: number, baseline: number | undefined): SummaryTone | undefined {
-  if (index === 0 || baseline == null || value === baseline)
+function getSummaryTone(index: number, value: number | null, baseline: number | null | undefined): SummaryTone | undefined {
+  if (index === 0 || value == null || baseline == null || value === baseline)
     return undefined
   return value > baseline ? 'increase' : 'decrease'
 }
@@ -662,12 +675,12 @@ function getPluginSummaryStats(index: number, session: NormalizedSessionCompareC
   }
 }
 
-function sumCompareSide<T>(items: T[] | undefined, index: number, getPrevious: (item: T) => number, getCurrent: (item: T) => number) {
-  return (items ?? []).reduce((total, item) => total + (index === 0 ? getPrevious(item) : getCurrent(item)), 0)
+function sumCompareSide<T>(items: T[] | undefined, index: number, getPrevious: (item: T) => number | null, getCurrent: (item: T) => number | null) {
+  return sumMetrics((items ?? []).map(item => index === 0 ? getPrevious(item) : getCurrent(item)))
 }
 
 function sumDelta(rows: SplitDiffRow[]) {
-  return rows.reduce((total, row) => total + row.delta, 0)
+  return sumMetrics(rows.map(row => row.delta))
 }
 </script>
 
@@ -712,7 +725,7 @@ function sumDelta(rows: SplitDiffRow[]) {
             </button>
           </div>
 
-          <div class="grid grid-cols-3 gap-3">
+          <div class="grid gap-3" :class="props.tab === 'modules' ? 'grid-cols-4' : 'grid-cols-3'">
             <button
               v-for="item of diffModeOptions"
               :key="item.id"
@@ -728,13 +741,13 @@ function sumDelta(rows: SplitDiffRow[]) {
               </div>
               <div class="flex items-end gap-2">
                 <span class="text-3xl font-mono font-600">{{ item.count }}</span>
-                <CompareDeltaValue v-if="item.delta" :value="item.delta" :format="activeDiffFormat" signed />
+                <CompareDeltaValue v-if="item.delta !== 0" :value="item.delta" :format="activeDiffFormat" signed />
               </div>
             </button>
           </div>
 
           <section v-if="activeDiffOption" class="flex flex-col gap-2">
-            <template v-if="activeDiffMode === 'changed'">
+            <template v-if="activeDiffMode === 'changed' || activeDiffMode === 'unavailable'">
               <CompareSplitRow
                 v-for="row of activeDiffOption.rows"
                 :key="row.key"
diff --git a/packages/rolldown/src/app/components/modules/BuildMetrics.vue b/packages/rolldown/src/app/components/modules/BuildMetrics.vue
index ca8364fa..c3cbdeb8 100644
--- a/packages/rolldown/src/app/components/modules/BuildMetrics.vue
+++ b/packages/rolldown/src/app/components/modules/BuildMetrics.vue
@@ -37,8 +37,8 @@ const sourceCodeSize = computed(() => {
 })
 
 const transformedCodeSize = computed(() => {
-  const data = props.metrics?.transforms.filter(t => t.transformed_code_size)
-  return data.at(-1)?.transformed_code_size
+  const data = props.metrics?.transforms
+  return data?.at(-1)?.transformed_code_size
 })
 </script>
 
@@ -69,7 +69,7 @@ const transformedCodeSize = computed(() => {
     >
       <span class="i-ph-clock-duotone inline-block" />
     </DisplayDuration>
-    <template v-if="sourceCodeSize && transformedCodeSize">
+    <template v-if="sourceCodeSize != null && transformedCodeSize != null">
       <span class="op40">|</span>
       <div class="flex gap-1 items-center">
         <DisplayFileSizeBadge title="Source code size" :bytes="sourceCodeSize" />
diff --git a/packages/rolldown/src/node/rolldown/__tests__/reader-index-cache.test.ts b/packages/rolldown/src/node/rolldown/__tests__/reader-index-cache.test.ts
index 9cae4637..f20b8013 100644
--- a/packages/rolldown/src/node/rolldown/__tests__/reader-index-cache.test.ts
+++ b/packages/rolldown/src/node/rolldown/__tests__/reader-index-cache.test.ts
@@ -58,7 +58,7 @@ describe('reader index codec', () => {
 })
 
 describe('rolldown log cache', () => {
-  function writeLog() {
+  function writeLog(finalContent = 'b') {
     const dir = mkdtempSync(join(tmpdir(), 'rolldown-log-cache-'))
     dirs.push(dir)
     const base = { build_id: 'bid_1', session_id: 'sid_1' }
@@ -66,8 +66,10 @@ describe('rolldown log cache', () => {
       { timestamp: 1, action: 'BuildStart', ...base },
       { timestamp: 2, action: 'HookResolveIdCallStart', importer: null, module_request: '/src/main.ts', import_kind: 'import-statement', plugin_name: 'p', plugin_id: 0, trigger: 'automatic', call_id: 'c1', ...base },
       { timestamp: 3, action: 'HookResolveIdCallEnd', resolved_id: '/src/main.ts', is_external: false, plugin_name: 'p', plugin_id: 0, trigger: 'automatic', call_id: 'c1', ...base },
+      { timestamp: 3.2, action: 'HookTransformCallStart', module_id: '/src/main.ts', content: 'aa', plugin_name: 'first', plugin_id: 1, trigger: 'automatic', call_id: 'first', ...base },
+      { timestamp: 3.3, action: 'HookTransformCallEnd', module_id: '/src/main.ts', content: 'a', plugin_name: 'first', plugin_id: 1, trigger: 'automatic', call_id: 'first', ...base },
       { timestamp: 4, action: 'HookTransformCallStart', module_id: '/src/main.ts', content: 'a', plugin_name: 'p', plugin_id: 0, trigger: 'automatic', call_id: 'c2', ...base },
-      { timestamp: 5, action: 'HookTransformCallEnd', module_id: '/src/main.ts', content: 'b', plugin_name: 'p', plugin_id: 0, trigger: 'automatic', call_id: 'c2', ...base },
+      { timestamp: 5, action: 'HookTransformCallEnd', module_id: '/src/main.ts', content: finalContent, plugin_name: 'p', plugin_id: 0, trigger: 'automatic', call_id: 'c2', ...base },
       { timestamp: 6, action: 'BuildEnd', ...base },
     ]
     const filepath = join(dir, 'logs.json')
@@ -80,8 +82,8 @@ describe('rolldown log cache', () => {
     return [state.moduleEventIndex, state.stringRefIndex, state.pendingHookCallStarts, state.assetsReadyLocations]
   }
 
-  it('restores the summary with module metrics but without the index, then restores the same index on a full read', async () => {
-    const filepath = writeLog()
+  it.each(['b', ''])('preserves the final transform %j through summary, full and detail cache reads', async (content) => {
+    const filepath = writeLog(content)
     const cold = RolldownEventsReader.get(filepath)
     await cold.read()
     expect((cold as unknown as { moduleEventIndex: Map<string, unknown> }).moduleEventIndex.size).toBeGreaterThan(0)
@@ -89,7 +91,7 @@ describe('rolldown log cache', () => {
     const coldEvents = cold.manager.eventCount
     const coldModules = cold.manager.module_build_metrics.size
     const coldMetrics = cold.manager.module_build_metrics.get('/src/main.ts')!
-    expect(coldMetrics.transforms).toHaveLength(1)
+    expect(coldMetrics.transforms).toHaveLength(2)
     cold.dispose()
 
     const warm = RolldownEventsReader.get(filepath)
@@ -106,6 +108,8 @@ describe('rolldown log cache', () => {
       await warm.read()
       expect(indexOf(warm)).toEqual(coldIndex)
       expect(warm.manager.eventCount).toBe(coldEvents)
+      const detail = await warm.readModuleBuildMetrics('/src/main.ts')
+      expect(detail.transforms.at(-1)?.transformed_code_size).toBe(content.length)
     }
     finally {
       warm.dispose()
diff --git a/packages/rolldown/src/node/rolldown/log-cache.ts b/packages/rolldown/src/node/rolldown/log-cache.ts
index e91b4e7e..e99af3b9 100644
--- a/packages/rolldown/src/node/rolldown/log-cache.ts
+++ b/packages/rolldown/src/node/rolldown/log-cache.ts
@@ -12,7 +12,7 @@ export const ROLLDOWN_LOG_READER_INDEX_CACHE_FILENAME = 'reader-index.bin'
 export const ROLLDOWN_LOG_PACKAGE_SUMMARY_CACHE_FILENAME = 'package-summary.json'
 export const ROLLDOWN_LOG_PLUGIN_SUMMARY_CACHE_FILENAME = 'plugin-summary.json'
 export const ROLLDOWN_LOG_MODULE_METRICS_SUMMARY_CACHE_FILENAME = 'module-metrics-summary.json'
-const ROLLDOWN_LOG_CACHE_VERSION = 3
+const ROLLDOWN_LOG_CACHE_VERSION = 4
 
 type ManagerSnapshot = ReturnType<RolldownEventsManager['snapshot']>
 
@@ -232,12 +232,9 @@ function compactModuleBuildMetrics(metrics: ModuleBuildMetrics): ModuleBuildMetr
     : []
 
   const transformsByPlugin = new Map<number, ModuleBuildMetrics['transforms'][number]>()
-  let finalTransformedCodeSize = 0
+  const finalTransformedCodeSize = metrics.transforms.at(-1)?.transformed_code_size
   for (const transform of metrics.transforms) {
-    if (transform.transformed_code_size)
-      finalTransformedCodeSize = transform.transformed_code_size
-
-    if (!transform.transformed_code_size || transform.source_code_size === transform.transformed_code_size)
+    if (transform.source_code_size === transform.transformed_code_size)
       continue
 
     const item = transformsByPlugin.get(transform.plugin_id)
@@ -275,7 +272,7 @@ function compactModuleBuildMetrics(metrics: ModuleBuildMetrics): ModuleBuildMetr
     })
   }
 
-  if (finalTransformedCodeSize && metrics.transforms.length) {
+  if (finalTransformedCodeSize != null && metrics.transforms.length) {
     transforms.push({
       ...metrics.transforms.at(-1)!,
       id: '',
diff --git a/packages/rolldown/src/node/rpc/__tests__/module-compare.test.ts b/packages/rolldown/src/node/rpc/__tests__/module-compare.test.ts
new file mode 100644
index 00000000..99674bfb
--- /dev/null
+++ b/packages/rolldown/src/node/rpc/__tests__/module-compare.test.ts
@@ -0,0 +1,50 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { RolldownEventsReader } from '../../rolldown/events-reader'
+import { rolldownGetSessionCompareDetails } from '../functions/rolldown-get-session-compare-details'
+
+const { manager } = vi.hoisted(() => ({ manager: { loadAssetSession: vi.fn() } }))
+vi.mock('../utils', () => ({ getLogsManager: () => manager }))
+
+const readers: RolldownEventsReader[] = []
+afterEach(() => {
+  readers.splice(0).forEach(reader => reader.dispose())
+  vi.restoreAllMocks()
+  vi.resetAllMocks()
+})
+
+function fixture(session: string, sizes: Record<string, number | null>) {
+  const reader = RolldownEventsReader.get(`/module-compare-test/${session}/logs.json`)
+  readers.push(reader)
+  vi.spyOn(reader, 'readPluginBuildMetricsSummary').mockResolvedValue(new Map())
+  reader.manager.chunks.set(0, { chunk_id: 0, name: 'main', imports: [], modules: Object.keys(sizes) } as any)
+  for (const [id, size] of Object.entries(sizes)) {
+    reader.manager.modules.set(id, {
+      id,
+      imports: [],
+      importers: [],
+      build_metrics: { resolve_ids: [], loads: [], transforms: size == null ? [] : [{ transformed_code_size: size }] },
+    } as any)
+  }
+  return reader
+}
+
+describe('module comparison RPC', () => {
+  it('preserves presence, missing metrics and measured zero through JSON transport', async () => {
+    const a = fixture('a', { unknown: null, zero: 0, stable: 12, removed: 9, removedUnknown: null })
+    const b = fixture('b', { unknown: 42, zero: 42, stable: 12, added: 9, addedUnknown: null })
+    manager.loadAssetSession.mockImplementation(async id => id === 'a' ? a : b)
+    const { handler } = await rolldownGetSessionCompareDetails.setup!({ cwd: '/' } as any)
+    const result = JSON.parse(JSON.stringify(await handler!({ sessions: ['a', 'b'] })))
+    const byId = Object.fromEntries(result.modules.map((item: any) => [item.key, item]))
+    expect(byId.unknown).toMatchObject({ status: 'unavailable', previous: null, current: 42, delta: null })
+    expect(byId.zero).toMatchObject({ status: 'changed', previous: 0, current: 42, delta: 42 })
+    expect(byId.added.previousId).toBeUndefined()
+    expect(byId.removed.currentId).toBeUndefined()
+    expect(byId.stable).toMatchObject({ status: 'unchanged', delta: 0 })
+    expect(byId.added).toMatchObject({ status: 'added', delta: 9 })
+    expect(byId.removed).toMatchObject({ status: 'removed', delta: -9 })
+    expect(byId.addedUnknown).toMatchObject({ status: 'added', current: null, delta: null })
+    expect(byId.removedUnknown).toMatchObject({ status: 'removed', previous: null, delta: null })
+    expect(result.modules.slice(-3).map((item: any) => item.key).sort()).toEqual(['addedUnknown', 'removedUnknown', 'unknown'])
+  })
+})
diff --git a/packages/rolldown/src/node/rpc/functions/rolldown-get-session-compare-details.ts b/packages/rolldown/src/node/rpc/functions/rolldown-get-session-compare-details.ts
index 9b8f28d4..f6ae3246 100644
--- a/packages/rolldown/src/node/rpc/functions/rolldown-get-session-compare-details.ts
+++ b/packages/rolldown/src/node/rpc/functions/rolldown-get-session-compare-details.ts
@@ -15,6 +15,7 @@ import type {
 import type { RolldownEventsReader } from '../../rolldown/events-reader'
 import { defineRpcFunction } from '@vitejs/devtools-kit'
 import { extname, isAbsolute, relative } from 'pathe'
+import { createModuleMetric } from '../../../shared/utils/compare-metrics'
 import { getLogsManager } from '../utils'
 import { getPackageMeta } from './rolldown-get-packages'
 
@@ -74,8 +75,10 @@ function compareMaps<TPrevious, TCurrent, TResult>(
     .map(key => map(key, previous.get(key), current.get(key)))
 }
 
-function sortByDeltaImpact<T extends { delta: number, name?: string, key: string }>(items: T[]) {
+function sortByDeltaImpact<T extends { delta: number | null, name?: string, key: string }>(items: T[]) {
   return items.toSorted((a, b) => {
+    if (a.delta == null || b.delta == null)
+      return a.delta == null ? (b.delta == null ? a.key.localeCompare(b.key) : 1) : -1
     const impact = Math.abs(b.delta) - Math.abs(a.delta)
     if (impact)
       return impact
@@ -138,7 +141,7 @@ function getChunkSize(reader: RolldownEventsReader, chunk: RolldownChunkInfo | u
 }
 
 function getModuleSize(reader: RolldownEventsReader, id: string) {
-  return reader.manager.modules.get(id)?.build_metrics?.transforms?.at(-1)?.transformed_code_size ?? 0
+  return reader.manager.modules.get(id)?.build_metrics?.transforms?.at(-1)?.transformed_code_size ?? null
 }
 
 function getModuleName(id: string, cwd: string) {
@@ -383,7 +386,9 @@ function compareModules(previous: SessionCompareSource, current: SessionCompareS
 
     return {
       key,
-      status: getStatus(previousId, currentId, previousSize !== currentSize || moved),
+      status: previousId && currentId && (previousSize == null || currentSize == null)
+        ? 'unavailable' as const
+        : getStatus(previousId, currentId, previousSize !== currentSize || moved),
       name: getModuleName(key, cwd),
       package: getModulePackage(key),
       previousId,
@@ -393,7 +398,7 @@ function compareModules(previous: SessionCompareSource, current: SessionCompareS
       previousChunkKeys,
       currentChunkKeys,
       moved,
-      ...createMetric(previousSize, currentSize),
+      ...createModuleMetric(previousSize, currentSize),
     }
   }))
 }
diff --git a/packages/rolldown/src/shared/types/data.ts b/packages/rolldown/src/shared/types/data.ts
index 08076f74..ef1b3b28 100644
--- a/packages/rolldown/src/shared/types/data.ts
+++ b/packages/rolldown/src/shared/types/data.ts
@@ -64,12 +64,12 @@ export interface SessionCompareContext {
   initial_js: number
 }
 
-export type SessionCompareChangeStatus = 'added' | 'removed' | 'changed' | 'unchanged'
+export type SessionCompareChangeStatus = 'added' | 'removed' | 'changed' | 'unchanged' | 'unavailable'
 
-export interface SessionCompareMetricValue {
-  previous: number
-  current: number
-  delta: number
+export interface SessionCompareMetricValue<T extends number | null = number> {
+  previous: T
+  current: T
+  delta: T
   deltaRatio: number | null
 }
 
@@ -99,7 +99,7 @@ export interface SessionCompareChunkDiff extends SessionCompareMetricValue {
   currentInitial: boolean
 }
 
-export interface SessionCompareModuleDiff extends SessionCompareMetricValue {
+export interface SessionCompareModuleDiff extends SessionCompareMetricValue<number | null> {
   key: string
   status: SessionCompareChangeStatus
   name: string
diff --git a/packages/rolldown/src/shared/utils/__tests__/compare-metrics.test.ts b/packages/rolldown/src/shared/utils/__tests__/compare-metrics.test.ts
new file mode 100644
index 00000000..5377f438
--- /dev/null
+++ b/packages/rolldown/src/shared/utils/__tests__/compare-metrics.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest'
+import { createModuleMetric, formatMetricRatio, sumMetrics } from '../compare-metrics'
+
+describe('module comparison measurements', () => {
+  it('keeps unavailable measurements distinct from measured zero', () => {
+    expect(createModuleMetric(null, 42)).toEqual({ previous: null, current: 42, delta: null, deltaRatio: null })
+    expect(createModuleMetric(0, 42)).toEqual({ previous: 0, current: 42, delta: 42, deltaRatio: null })
+    expect(createModuleMetric(42, null).delta).toBeNull()
+    expect(createModuleMetric(null, null).delta).toBeNull()
+    expect(createModuleMetric(42, 0)).toMatchObject({ delta: -42, deltaRatio: -1 })
+  })
+
+  it('does not present incomplete totals or zero baselines as new modules', () => {
+    expect(sumMetrics([12, null, 30])).toBeNull()
+    expect(sumMetrics([12, 0, 30])).toBe(42)
+    expect(sumMetrics([])).toBe(0)
+    expect(formatMetricRatio(createModuleMetric(0, 42))).toBe('—')
+    expect(formatMetricRatio(createModuleMetric(null, 42))).toBe('Unavailable')
+    expect(formatMetricRatio(createModuleMetric(0, 0))).toBe('0%')
+    expect(formatMetricRatio(createModuleMetric(20, 30))).toBe('+50.00%')
+  })
+})
diff --git a/packages/rolldown/src/shared/utils/compare-metrics.ts b/packages/rolldown/src/shared/utils/compare-metrics.ts
new file mode 100644
index 00000000..77c742d9
--- /dev/null
+++ b/packages/rolldown/src/shared/utils/compare-metrics.ts
@@ -0,0 +1,25 @@
+import type { SessionCompareMetricValue } from '../types'
+
+export function createModuleMetric(previous: number | null, current: number | null): SessionCompareMetricValue<number | null> {
+  const delta = previous == null || current == null ? null : current - previous
+  return {
+    previous,
+    current,
+    delta,
+    deltaRatio: delta == null || previous === 0 ? (delta === 0 ? 0 : null) : delta / previous!,
+  }
+}
+
+export function sumMetrics(values: (number | null)[]): number | null {
+  return values.reduce<number | null>((total, value) => total == null || value == null ? null : total + value, 0)
+}
+
+export function formatMetricRatio(item: SessionCompareMetricValue<number | null>) {
+  if (item.delta == null)
+    return 'Unavailable'
+  if (item.deltaRatio == null)
+    return '—'
+  if (item.deltaRatio === 0)
+    return '0%'
+  return `${item.deltaRatio > 0 ? '+' : ''}${(item.deltaRatio * 100).toFixed(2)}%`
+}