diff --git a/src/renderer/src/components/editor/editor-autosave.ts b/src/renderer/src/components/editor/editor-autosave.ts
index 110f358dd99..c647fbfb779 100644
--- a/src/renderer/src/components/editor/editor-autosave.ts
+++ b/src/renderer/src/components/editor/editor-autosave.ts
@@ -23,6 +23,9 @@ export type EditorPathMutationTarget = {
   relativePath: string
   runtimeEnvironmentId?: string | null
   allowLocalWindowsWslAliases?: true
+  indexedOpenFiles?: {
+    matches: (openFiles: OpenFile[]) => OpenFile[]
+  }
 }
 
 export type EditorSaveQuiesceTarget = { fileId: string } | EditorPathMutationTarget
@@ -127,6 +130,9 @@ export function getOpenFilesForExternalFileChange(
   openFiles: OpenFile[],
   target: EditorPathMutationTarget
 ): OpenFile[] {
+  if (target.indexedOpenFiles) {
+    return target.indexedOpenFiles.matches(openFiles)
+  }
   const absolutePath = joinPath(target.worktreePath, target.relativePath)
   const hasRuntimeOwnerFilter = Object.hasOwn(target, 'runtimeEnvironmentId')
   const targetRuntimeOwner = target.runtimeEnvironmentId?.trim() || null
diff --git a/src/renderer/src/components/editor/editor-external-watch-path-index.test.ts b/src/renderer/src/components/editor/editor-external-watch-path-index.test.ts
new file mode 100644
index 00000000000..68e33bc6baf
--- /dev/null
+++ b/src/renderer/src/components/editor/editor-external-watch-path-index.test.ts
@@ -0,0 +1,185 @@
+import { describe, expect, it } from 'vitest'
+import type { OpenFile } from '@/store/slices/editor'
+import { indexEditorExternalWatchBatchPaths } from './editor-external-watch-path-index'
+
+const wslScope = {
+  worktreeId: 'wt-wsl',
+  worktreePath: '\\\\wsl.localhost\\Ubuntu\\workspace\\repo',
+  runtimeEnvironmentId: null,
+  allowLocalWindowsWslAliases: true as const
+}
+
+function file(overrides: Partial<OpenFile> & Pick<OpenFile, 'id' | 'filePath'>): OpenFile {
+  return {
+    relativePath: 'file.ts',
+    worktreeId: 'wt-wsl',
+    mode: 'edit',
+    isDirty: false,
+    content: '',
+    language: 'typescript',
+    ...overrides
+  } as OpenFile
+}
+
+describe('editor external watch path batch index', () => {
+  it('matches UNC aliases for updates, deletes, and restored tombstones', () => {
+    const restored = file({
+      id: 'restored',
+      filePath: '//wsl.localhost/Ubuntu/workspace/repo/file.ts',
+      externalMutation: 'deleted'
+    })
+    const index = indexEditorExternalWatchBatchPaths(
+      {
+        worktreePath: wslScope.worktreePath,
+        events: [
+          {
+            kind: 'delete',
+            absolutePath: '\\\\wsl$\\Ubuntu\\workspace\\repo\\file.ts'
+          },
+          {
+            kind: 'create',
+            absolutePath: '\\\\wsl.localhost\\Ubuntu\\workspace\\repo\\file.ts'
+          }
+        ]
+      },
+      [restored],
+      wslScope
+    )
+
+    expect(index.deletedOpenEditors).toEqual([
+      {
+        file: restored,
+        normalizedDeletePath: '//wsl/ubuntu/workspace/repo/file.ts'
+      }
+    ])
+    expect(index.matchesCreateOrUpdate(restored)).toBe(true)
+    expect(index.matchingOpenFiles(index.changes[0])).toEqual([restored])
+  })
+
+  it('matches /mnt drive aliases without folding WSL filesystem case', () => {
+    const mounted = file({
+      id: 'mounted',
+      filePath: '//wsl.localhost/Ubuntu/mnt/c/Repo/File.ts'
+    })
+    const nativeScope = { ...wslScope, worktreePath: 'C:\\Repo' }
+    const matching = indexEditorExternalWatchBatchPaths(
+      {
+        worktreePath: nativeScope.worktreePath,
+        events: [{ kind: 'update', absolutePath: 'c:\\repo\\file.ts' }]
+      },
+      [mounted],
+      nativeScope
+    )
+    const wrongLinuxCase = indexEditorExternalWatchBatchPaths(
+      {
+        worktreePath: nativeScope.worktreePath,
+        events: [{ kind: 'update', absolutePath: 'C:\\Repo\\File.ts' }]
+      },
+      [
+        file({
+          id: 'linux-case',
+          filePath: '//wsl.localhost/Ubuntu/home/Alice/File.ts'
+        })
+      ],
+      nativeScope
+    )
+
+    expect(matching.matchingOpenFiles(matching.changes[0])).toEqual([mounted])
+    expect(wrongLinuxCase.matchingOpenFiles(wrongLinuxCase.changes[0])).toEqual([])
+  })
+
+  it('keeps aliases literal for SSH, runtimes, and POSIX scopes', () => {
+    const restored = file({
+      id: 'restored',
+      filePath: '//wsl.localhost/Ubuntu/workspace/repo/file.ts'
+    })
+    const payload = {
+      worktreePath: wslScope.worktreePath,
+      events: [
+        {
+          kind: 'update' as const,
+          absolutePath: '\\\\wsl.localhost\\Ubuntu\\workspace\\repo\\file.ts'
+        }
+      ]
+    }
+
+    for (const scope of [
+      { ...wslScope, allowLocalWindowsWslAliases: undefined },
+      { ...wslScope, runtimeEnvironmentId: 'env-1', allowLocalWindowsWslAliases: undefined }
+    ]) {
+      const index = indexEditorExternalWatchBatchPaths(payload, [restored], scope)
+      expect(index.matchingOpenFiles(index.changes[0])).toEqual([])
+    }
+
+    const posix = indexEditorExternalWatchBatchPaths(
+      {
+        worktreePath: '/srv/repo',
+        events: [{ kind: 'update', absolutePath: '/srv/repo/file.ts' }]
+      },
+      [restored],
+      {
+        ...wslScope,
+        worktreePath: '/srv/repo',
+        allowLocalWindowsWslAliases: undefined
+      }
+    )
+    expect(posix.matchingOpenFiles(posix.changes[0])).toEqual([])
+  })
+
+  it('filters owners and preserves open-file ordering across edit and diff matches', () => {
+    const edit = file({ id: 'edit', filePath: 'C:\\Repo\\file.ts' })
+    const runtime = file({
+      id: 'runtime',
+      filePath: 'C:\\Repo\\file.ts',
+      runtimeEnvironmentId: 'env-1'
+    })
+    const diff = file({
+      id: 'diff',
+      filePath: 'C:\\Repo\\file.ts',
+      mode: 'diff',
+      diffSource: 'unstaged'
+    })
+    const index = indexEditorExternalWatchBatchPaths(
+      {
+        worktreePath: 'C:\\Repo',
+        events: [{ kind: 'update', absolutePath: 'c:\\repo\\file.ts' }]
+      },
+      [diff, runtime, edit],
+      { ...wslScope, worktreePath: 'C:\\Repo' }
+    )
+
+    expect(index.matchingOpenFiles(index.changes[0]).map(({ id }) => id)).toEqual(['diff', 'edit'])
+  })
+
+  it('deduplicates repeated events and detects only working-tree combined diffs', () => {
+    const index = indexEditorExternalWatchBatchPaths(
+      {
+        worktreePath: 'C:\\Repo',
+        events: [
+          { kind: 'create', absolutePath: 'C:\\Repo\\file.ts' },
+          { kind: 'update', absolutePath: 'C:\\Repo\\file.ts' },
+          { kind: 'update', absolutePath: 'C:\\Repo\\folder', isDirectory: true }
+        ]
+      },
+      [
+        file({
+          id: 'combined',
+          filePath: 'C:\\Repo',
+          mode: 'diff',
+          diffSource: 'combined-uncommitted'
+        }),
+        file({
+          id: 'branch',
+          filePath: 'C:\\Repo',
+          mode: 'diff',
+          diffSource: 'combined-branch'
+        })
+      ],
+      { ...wslScope, worktreePath: 'C:\\Repo' }
+    )
+
+    expect(index.changes.map(({ relativePath }) => relativePath)).toEqual(['file.ts'])
+    expect(index.createOrUpdatePaths.size).toBe(1)
+    expect(index.hasCombinedDiffConsumer).toBe(true)
+  })
+})
diff --git a/src/renderer/src/components/editor/editor-external-watch-path-index.ts b/src/renderer/src/components/editor/editor-external-watch-path-index.ts
new file mode 100644
index 00000000000..f1cb7054c67
--- /dev/null
+++ b/src/renderer/src/components/editor/editor-external-watch-path-index.ts
@@ -0,0 +1,256 @@
+import { joinPath } from '@/lib/path'
+import { getExternalFileChangeRelativePath } from '@/components/right-sidebar/useFileExplorerWatch'
+import type { OpenFile } from '@/store/slices/editor'
+import type { FsChangedPayload } from '../../../../shared/types'
+import {
+  getLocalWindowsWslPathIdentity,
+  normalizeRuntimePathForComparison,
+  type LocalWindowsWslPathIdentity
+} from '../../../../shared/cross-platform-path'
+
+type WatchScope = {
+  worktreeId: string
+  worktreePath: string
+  runtimeEnvironmentId: string | null
+  allowLocalWindowsWslAliases?: true
+}
+
+type IndexedPath = {
+  absolutePath: string
+  identity: LocalWindowsWslPathIdentity
+}
+
+type IndexedOpenFile = {
+  file: OpenFile
+  index: number
+  identity: LocalWindowsWslPathIdentity | null
+}
+
+export type IndexedExternalWatchChange = IndexedPath & {
+  relativePath: string
+}
+
+export type EditorExternalWatchBatchPathIndex = {
+  createOrUpdatePaths: ReadonlyMap<string, string>
+  changes: readonly IndexedExternalWatchChange[]
+  deletedOpenEditors: readonly { file: OpenFile; normalizedDeletePath: string }[]
+  hasCombinedDiffConsumer: boolean
+  matchesCreateOrUpdate: (file: OpenFile) => boolean
+  matchingOpenFiles: (
+    change: IndexedExternalWatchChange,
+    currentOpenFiles?: OpenFile[]
+  ) => OpenFile[]
+}
+
+function openFileRuntimeOwner(file: Pick<OpenFile, 'runtimeEnvironmentId'>): string | null {
+  return file.runtimeEnvironmentId?.trim() || null
+}
+
+function addToListMap<T>(map: Map<string, T[]>, key: string, value: T): void {
+  const existing = map.get(key)
+  if (existing) {
+    existing.push(value)
+  } else {
+    map.set(key, [value])
+  }
+}
+
+class IndexedPathLookup<T> {
+  private readonly direct = new Map<string, T>()
+  private readonly aliases = new Map<string, T>()
+  private readonly wslAliases = new Map<string, T>()
+
+  constructor(private readonly allowAliases: boolean) {}
+
+  add(path: IndexedPath, value: T): void {
+    this.direct.set(path.identity.normalizedPath, value)
+    if (!this.allowAliases) {
+      return
+    }
+    this.aliases.set(path.identity.aliasComparisonPath, value)
+    if (path.identity.isWslUnc) {
+      this.wslAliases.set(path.identity.aliasComparisonPath, value)
+    }
+  }
+
+  get(identity: LocalWindowsWslPathIdentity): T | undefined {
+    const direct = this.direct.get(identity.normalizedPath)
+    if (direct !== undefined || !this.allowAliases) {
+      return direct
+    }
+    return identity.isWslUnc
+      ? this.aliases.get(identity.aliasComparisonPath)
+      : this.wslAliases.get(identity.aliasComparisonPath)
+  }
+}
+
+function pathIdentity(value: string, allowAliases: boolean): LocalWindowsWslPathIdentity {
+  if (allowAliases) {
+    return getLocalWindowsWslPathIdentity(value)
+  }
+  const normalizedPath = normalizeRuntimePathForComparison(value)
+  return { normalizedPath, aliasComparisonPath: normalizedPath, isWslUnc: false }
+}
+
+function collectMatchingFiles(
+  direct: readonly IndexedOpenFile[],
+  aliases: readonly IndexedOpenFile[],
+  diffs: readonly IndexedOpenFile[]
+): OpenFile[] {
+  const byIndex = new Map<number, OpenFile>()
+  for (const entry of [...direct, ...aliases, ...diffs]) {
+    byIndex.set(entry.index, entry.file)
+  }
+  return [...byIndex.entries()].sort(([left], [right]) => left - right).map(([, file]) => file)
+}
+
+class IndexedOpenFileLookup {
+  private readonly directEditors = new Map<string, IndexedOpenFile[]>()
+  private readonly aliasEditors = new Map<string, IndexedOpenFile[]>()
+  private readonly wslAliasEditors = new Map<string, IndexedOpenFile[]>()
+  private readonly diffsByRelativePath = new Map<string, IndexedOpenFile[]>()
+  readonly indexedOpenFiles = new Map<string, IndexedOpenFile>()
+  readonly hasCombinedDiffConsumer: boolean
+
+  constructor(
+    openFiles: OpenFile[],
+    scope: WatchScope,
+    private readonly allowAliases: boolean
+  ) {
+    let hasCombinedDiffConsumer = false
+    for (const [index, file] of openFiles.entries()) {
+      if (
+        file.worktreeId !== scope.worktreeId ||
+        openFileRuntimeOwner(file) !== scope.runtimeEnvironmentId
+      ) {
+        continue
+      }
+      if (
+        file.mode === 'diff' &&
+        (file.diffSource === 'combined-uncommitted' || file.diffSource === 'combined-all')
+      ) {
+        hasCombinedDiffConsumer = true
+        continue
+      }
+      if (file.mode === 'diff') {
+        if (file.diffSource === 'unstaged' || file.diffSource === 'staged') {
+          addToListMap(this.diffsByRelativePath, file.relativePath, {
+            file,
+            index,
+            identity: null
+          })
+        }
+        continue
+      }
+      if (file.mode !== 'edit' && file.mode !== 'markdown-preview') {
+        continue
+      }
+      const identity = pathIdentity(file.filePath, allowAliases)
+      const indexedFile = { file, index, identity }
+      this.indexedOpenFiles.set(file.id, indexedFile)
+      addToListMap(this.directEditors, file.filePath, indexedFile)
+      if (allowAliases) {
+        addToListMap(this.aliasEditors, identity.aliasComparisonPath, indexedFile)
+        if (identity.isWslUnc) {
+          addToListMap(this.wslAliasEditors, identity.aliasComparisonPath, indexedFile)
+        }
+      }
+    }
+    this.hasCombinedDiffConsumer = hasCombinedDiffConsumer
+  }
+
+  matchingOpenFiles(change: IndexedExternalWatchChange): OpenFile[] {
+    const aliases = !this.allowAliases
+      ? []
+      : change.identity.isWslUnc
+        ? (this.aliasEditors.get(change.identity.aliasComparisonPath) ?? [])
+        : (this.wslAliasEditors.get(change.identity.aliasComparisonPath) ?? [])
+    return collectMatchingFiles(
+      this.directEditors.get(change.absolutePath) ?? [],
+      aliases,
+      this.diffsByRelativePath.get(change.relativePath) ?? []
+    )
+  }
+}
+
+export function indexEditorExternalWatchBatchPaths(
+  payload: FsChangedPayload,
+  openFiles: OpenFile[],
+  scope: WatchScope
+): EditorExternalWatchBatchPathIndex {
+  const allowAliases = scope.allowLocalWindowsWslAliases === true
+  const createOrUpdateLookup = new IndexedPathLookup<IndexedPath>(allowAliases)
+  const deleteLookup = new IndexedPathLookup<IndexedPath>(allowAliases)
+  const createOrUpdatePaths = new Map<string, string>()
+  const changesByRelativePath = new Map<string, IndexedExternalWatchChange>()
+
+  for (const event of payload.events) {
+    if (event.kind === 'overflow') {
+      continue
+    }
+    const eventPath: IndexedPath = {
+      absolutePath: event.absolutePath,
+      identity: pathIdentity(event.absolutePath, allowAliases)
+    }
+    if (event.kind === 'delete') {
+      deleteLookup.add(eventPath, eventPath)
+      continue
+    }
+    if (event.isDirectory !== true) {
+      createOrUpdatePaths.set(eventPath.identity.normalizedPath, event.absolutePath)
+      createOrUpdateLookup.add(eventPath, eventPath)
+    }
+    const relativePath = getExternalFileChangeRelativePath(
+      scope.worktreePath,
+      event.absolutePath,
+      event.isDirectory
+    )
+    if (relativePath && !changesByRelativePath.has(relativePath)) {
+      const absolutePath = joinPath(scope.worktreePath, relativePath)
+      changesByRelativePath.set(relativePath, {
+        relativePath,
+        absolutePath,
+        identity: eventPath.identity
+      })
+    }
+  }
+
+  const initialOpenFileLookup = new IndexedOpenFileLookup(openFiles, scope, allowAliases)
+  const openFileLookups = new WeakMap<OpenFile[], IndexedOpenFileLookup>()
+  openFileLookups.set(openFiles, initialOpenFileLookup)
+  const getOpenFileLookup = (currentOpenFiles: OpenFile[]): IndexedOpenFileLookup => {
+    const existing = openFileLookups.get(currentOpenFiles)
+    if (existing) {
+      return existing
+    }
+    const indexed = new IndexedOpenFileLookup(currentOpenFiles, scope, allowAliases)
+    openFileLookups.set(currentOpenFiles, indexed)
+    return indexed
+  }
+  const matchesCreateOrUpdate = (file: OpenFile): boolean => {
+    const identity =
+      initialOpenFileLookup.indexedOpenFiles.get(file.id)?.identity ??
+      pathIdentity(file.filePath, allowAliases)
+    return createOrUpdateLookup.get(identity) !== undefined
+  }
+  const deletedOpenEditors: { file: OpenFile; normalizedDeletePath: string }[] = []
+  for (const indexedFile of initialOpenFileLookup.indexedOpenFiles.values()) {
+    const deletedPath = deleteLookup.get(indexedFile.identity!)
+    if (deletedPath) {
+      deletedOpenEditors.push({
+        file: indexedFile.file,
+        normalizedDeletePath: deletedPath.identity.normalizedPath
+      })
+    }
+  }
+
+  return {
+    createOrUpdatePaths,
+    changes: [...changesByRelativePath.values()],
+    deletedOpenEditors,
+    hasCombinedDiffConsumer: initialOpenFileLookup.hasCombinedDiffConsumer,
+    matchesCreateOrUpdate,
+    matchingOpenFiles: (change, currentOpenFiles = openFiles) =>
+      getOpenFileLookup(currentOpenFiles).matchingOpenFiles(change)
+  }
+}
diff --git a/src/renderer/src/hooks/useEditorExternalWatch-complexity.test.ts b/src/renderer/src/hooks/useEditorExternalWatch-complexity.test.ts
new file mode 100644
index 00000000000..cfcb973d968
--- /dev/null
+++ b/src/renderer/src/hooks/useEditorExternalWatch-complexity.test.ts
@@ -0,0 +1,129 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import type * as EditorAutosaveModule from '@/components/editor/editor-autosave'
+import type * as CrossPlatformPathModule from '../../../shared/cross-platform-path'
+import type { FsChangedPayload } from '../../../shared/types'
+
+const pathOperationCounts = vi.hoisted(() => ({
+  aliasComparisons: 0,
+  normalizations: 0,
+  identities: 0
+}))
+
+vi.mock('@/store', () => ({ useAppStore: { getState: vi.fn() } }))
+vi.mock('@/components/editor/editor-autosave', async (importOriginal) => {
+  const actual = await importOriginal<typeof EditorAutosaveModule>()
+  return { ...actual, notifyEditorExternalFileChange: vi.fn() }
+})
+vi.mock('../../../shared/cross-platform-path', async (importOriginal) => {
+  type PathModuleWithIdentity = typeof CrossPlatformPathModule & {
+    getLocalWindowsWslPathIdentity?: (value: string) => unknown
+  }
+  const actual = await importOriginal<PathModuleWithIdentity>()
+  return {
+    ...actual,
+    normalizeRuntimePathForComparison: (value: string) => {
+      pathOperationCounts.normalizations++
+      return actual.normalizeRuntimePathForComparison(value)
+    },
+    areLocalWindowsWslPathAliases: (left: string, right: string) => {
+      pathOperationCounts.aliasComparisons++
+      return actual.areLocalWindowsWslPathAliases(left, right)
+    },
+    ...(actual.getLocalWindowsWslPathIdentity
+      ? {
+          getLocalWindowsWslPathIdentity: (value: string) => {
+            pathOperationCounts.identities++
+            return actual.getLocalWindowsWslPathIdentity!(value)
+          }
+        }
+      : {})
+  }
+})
+
+import { useAppStore } from '@/store'
+import {
+  getOpenFilesForExternalFileChange,
+  notifyEditorExternalFileChange
+} from '@/components/editor/editor-autosave'
+import { createExternalWatchEventHandler } from './useEditorExternalWatch'
+
+const EVENT_COUNT = 5_000
+const OPEN_FILE_COUNT = 100
+const payloadWorktreePath = '\\\\wsl.localhost\\Ubuntu\\workspace\\repo'
+
+describe('external watcher path matching complexity', () => {
+  beforeEach(() => {
+    vi.useFakeTimers({ toFake: ['setTimeout', 'clearTimeout'] })
+    vi.clearAllMocks()
+    pathOperationCounts.aliasComparisons = 0
+    pathOperationCounts.normalizations = 0
+    pathOperationCounts.identities = 0
+    vi.stubGlobal('window', { dispatchEvent: vi.fn() })
+    vi.stubGlobal('navigator', { userAgent: 'Windows' })
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+    vi.unstubAllGlobals()
+  })
+
+  it('bounds a maximum-sized opposite-alias batch by events plus open files', () => {
+    const openFiles = Array.from({ length: OPEN_FILE_COUNT }, (_, index) => ({
+      id: `//wsl.localhost/Ubuntu/workspace/repo/file-${index}.ts`,
+      filePath: `//wsl.localhost/Ubuntu/workspace/repo/file-${index}.ts`,
+      relativePath: `file-${index}.ts`,
+      worktreeId: 'wt-wsl',
+      mode: 'edit' as const,
+      isDirty: false
+    }))
+    const initialOpenFiles = [
+      ...openFiles,
+      {
+        id: 'combined-diff',
+        filePath: payloadWorktreePath,
+        relativePath: '',
+        worktreeId: 'wt-wsl',
+        mode: 'diff' as const,
+        diffSource: 'combined-uncommitted' as const,
+        isDirty: false
+      }
+    ]
+    vi.mocked(useAppStore.getState).mockReturnValue({
+      openFiles: initialOpenFiles,
+      setExternalMutation: vi.fn()
+    } as never)
+    const payload: FsChangedPayload = {
+      worktreePath: payloadWorktreePath,
+      events: Array.from({ length: EVENT_COUNT }, (_, index) => ({
+        kind: 'update' as const,
+        absolutePath: `\\\\wsl.localhost\\Ubuntu\\workspace\\repo\\file-${index}.ts`
+      }))
+    }
+    const { handleFsChanged, dispose } = createExternalWatchEventHandler(() => ({
+      worktreeId: 'wt-wsl',
+      worktreePath: payload.worktreePath,
+      connectionId: undefined,
+      runtimeEnvironmentId: null,
+      allowLocalWindowsWslAliases: true
+    }))
+
+    const startedAt = process.hrtime.bigint()
+    handleFsChanged(payload)
+    const elapsedMs = Number(process.hrtime.bigint() - startedAt) / 1_000_000
+    vi.advanceTimersByTime(100)
+    expect(notifyEditorExternalFileChange).toHaveBeenCalledTimes(EVENT_COUNT)
+    // Why: a tab/store update during debounce must rebuild the index once, not rescan per event.
+    const currentOpenFiles = initialOpenFiles.map((file) => ({ ...file }))
+    for (const [notification] of vi.mocked(notifyEditorExternalFileChange).mock.calls) {
+      getOpenFilesForExternalFileChange(currentOpenFiles as never, notification)
+    }
+    const pathOperations =
+      pathOperationCounts.aliasComparisons +
+      pathOperationCounts.normalizations +
+      pathOperationCounts.identities
+
+    console.info('STA-3942 watcher oracle', { ...pathOperationCounts, pathOperations, elapsedMs })
+    expect(pathOperations).toBeLessThanOrEqual(3 * (EVENT_COUNT + initialOpenFiles.length))
+    dispose()
+  })
+})
diff --git a/src/renderer/src/hooks/useEditorExternalWatch.test.ts b/src/renderer/src/hooks/useEditorExternalWatch.test.ts
index 747babef0e1..891bbc35e4e 100644
--- a/src/renderer/src/hooks/useEditorExternalWatch.test.ts
+++ b/src/renderer/src/hooks/useEditorExternalWatch.test.ts
@@ -287,7 +287,8 @@ describe('createExternalWatchEventHandler tombstone coalescing', () => {
       filePath: 'C:\\Repo\\notes.md',
       relativePath: 'notes.md',
       mode: 'edit' as const,
-      isDirty: false
+      isDirty: false,
+      runtimeEnvironmentId: 'env-1'
     }
     vi.mocked(useAppStore.getState).mockReturnValue({
       openFiles: [file],
@@ -324,7 +325,8 @@ describe('createExternalWatchEventHandler tombstone coalescing', () => {
       filePath: '//Server/Share/Repo/notes.md',
       relativePath: 'notes.md',
       mode: 'edit' as const,
-      isDirty: false
+      isDirty: false,
+      runtimeEnvironmentId: 'env-1'
     }
     vi.mocked(useAppStore.getState).mockReturnValue({
       openFiles: [file],
diff --git a/src/renderer/src/hooks/useEditorExternalWatch.ts b/src/renderer/src/hooks/useEditorExternalWatch.ts
index 5b0236d82f7..c07f9bd25c1 100644
--- a/src/renderer/src/hooks/useEditorExternalWatch.ts
+++ b/src/renderer/src/hooks/useEditorExternalWatch.ts
@@ -2,9 +2,7 @@
 import { useEffect, useRef } from 'react'
 import { useAppStore, type AppState } from '@/store'
 import { basename, joinPath } from '@/lib/path'
-import { getExternalFileChangeRelativePath } from '@/components/right-sidebar/useFileExplorerWatch'
 import {
-  areLocalWindowsWslPathAliases,
   isWindowsAbsolutePathLike,
   normalizeRuntimePathForComparison
 } from '../../../shared/cross-platform-path'
@@ -12,9 +10,9 @@ import {
   canAutoSaveOpenFile,
   getOpenFilesForExternalFileChange,
   isExternalReloadableEditorTab,
-  isWorkingTreeCombinedDiffTab,
   notifyEditorExternalFileChange
 } from '@/components/editor/editor-autosave'
+import { indexEditorExternalWatchBatchPaths } from '@/components/editor/editor-external-watch-path-index'
 import {
   clearSelfWrite,
   getRecentSelfWrite,
@@ -87,6 +85,9 @@ type ExternalWatchNotification = {
   relativePath: string
   runtimeEnvironmentId: string | null
   allowLocalWindowsWslAliases?: true
+  indexedOpenFiles?: {
+    matches: (openFiles: OpenFile[]) => OpenFile[]
+  }
 }
 
 function localWslAliasOption(
@@ -97,26 +98,6 @@ function localWslAliasOption(
     : {}
 }
 
-function findMatchingWatchedPath(
-  watchedPaths: ReadonlyMap<string, string>,
-  filePath: string,
-  allowLocalWslAliases?: true
-): string | undefined {
-  const directMatch = watchedPaths.get(normalizeRuntimePathForComparison(filePath))
-  if (directMatch !== undefined) {
-    return directMatch
-  }
-  if (allowLocalWslAliases !== true || !isLocalWindowsDesktopClient()) {
-    return undefined
-  }
-  for (const watchedPath of watchedPaths.values()) {
-    if (areLocalWindowsWslPathAliases(filePath, watchedPath)) {
-      return watchedPath
-    }
-  }
-  return undefined
-}
-
 function isLocalHostStamp(value: string | null | undefined): boolean {
   return parseExecutionHostId(value)?.kind === 'local'
 }
@@ -525,19 +506,15 @@ export function createExternalWatchEventHandler(
       )
     }
 
-    // Why: collect create/update paths first to cancel any pending same-path delete — this absorbs the macOS atomic-write delete→create split across two payloads.
-    const createOrUpdatePaths = new Map<string, string>()
-    for (const evt of payload.events) {
-      if (evt.isDirectory === true) {
-        continue
-      }
-      if (evt.kind === 'create' || evt.kind === 'update') {
-        createOrUpdatePaths.set(
-          normalizeRuntimePathForComparison(evt.absolutePath),
-          evt.absolutePath
-        )
-      }
-    }
+    // Why: one batch index keeps local WSL alias normalization out of event×tab loops.
+    const openFilesAtStart = useAppStore.getState().openFiles
+    const batchPaths = indexEditorExternalWatchBatchPaths(payload, openFilesAtStart, {
+      worktreeId: target.worktreeId,
+      worktreePath: target.worktreePath,
+      runtimeEnvironmentId: target.runtimeEnvironmentId,
+      ...localWslAliasOption(target)
+    })
+    const createOrUpdatePaths = batchPaths.createOrUpdatePaths
     for (const createdPath of createOrUpdatePaths.keys()) {
       const key = pendingKey(target.worktreeId, target.runtimeEnvironmentId, createdPath)
       const existing = pendingDeletes.get(key)
@@ -548,25 +525,16 @@ export function createExternalWatchEventHandler(
     }
 
     // Why: mark editor tabs deleted/renamed instead of closing them so the user keeps in-memory content; a paired create means rename, a lone delete is hard.
-    // Why: snapshot openFiles once so the delete/rename helpers share a consistent view without N store reads per payload.
-    const openFilesAtStart = useAppStore.getState().openFiles
-    const deletedOpenEditorIdsRaw = collectDeletedOpenEditorIds(
-      payload,
-      target.worktreeId,
-      target.runtimeEnvironmentId,
-      openFilesAtStart,
-      target.allowLocalWindowsWslAliases
-    )
+    // Why: snapshot openFiles once so delete, rename, and update matching share one indexed view.
+    const deletedOpenEditorsRaw = batchPaths.deletedOpenEditors
     // Only pay the per-id lookup to suppress a move's own source-delete while a move is live; else the batch stays O(deletes).
-    const deletedOpenEditorIds = hasActiveEditorPathMoves()
-      ? deletedOpenEditorIdsRaw.filter((fileId) => {
-          const file = openFilesAtStart.find((f) => f.id === fileId)
-          return (
-            !file ||
+    const deletedOpenEditors = hasActiveEditorPathMoves()
+      ? deletedOpenEditorsRaw.filter(
+          ({ file }) =>
             !isActiveMoveSourcePath(target.worktreeId, target.runtimeEnvironmentId, file.filePath)
-          )
-        })
-      : deletedOpenEditorIdsRaw
+        )
+      : deletedOpenEditorsRaw
+    const deletedOpenEditorIds = deletedOpenEditors.map(({ file }) => file.id)
     // Why: correlate creates to deletes by basename to avoid mislabelling unrelated create+delete pairs as "renamed"; default to 'deleted' when we can't correlate.
     const hasPairedCreate =
       deletedOpenEditorIds.length > 0 &&
@@ -580,19 +548,9 @@ export function createExternalWatchEventHandler(
         }
       } else {
         // Why: defer the 'deleted' tombstone so a follow-up same-path create in the next payload can cancel it (macOS atomic write).
-        const deletePathByFileId = buildDeletePathByFileId(
-          payload,
-          target.worktreeId,
-          target.runtimeEnvironmentId,
-          deletedOpenEditorIds,
-          openFilesAtStart,
-          target.allowLocalWindowsWslAliases
-        )
-        for (const fileId of deletedOpenEditorIds) {
-          const absolutePath = deletePathByFileId.get(fileId)
-          if (!absolutePath) {
-            continue
-          }
+        for (const { file, normalizedDeletePath } of deletedOpenEditors) {
+          const fileId = file.id
+          const absolutePath = normalizedDeletePath
           const key = pendingKey(target.worktreeId, target.runtimeEnvironmentId, absolutePath)
           const existing = pendingDeletes.get(key)
           if (existing) {
@@ -622,61 +580,32 @@ export function createExternalWatchEventHandler(
           openFileRuntimeOwner(file) === target.runtimeEnvironmentId &&
           (file.mode === 'edit' || file.mode === 'markdown-preview') &&
           (file.externalMutation === 'deleted' || file.externalMutation === 'renamed') &&
-          findMatchingWatchedPath(
-            createOrUpdatePaths,
-            file.filePath,
-            target.allowLocalWindowsWslAliases
-          ) !== undefined
+          batchPaths.matchesCreateOrUpdate(file)
         ) {
           state.setExternalMutation(file.id, null)
         }
       }
     }
 
-    const changedFiles = new Set<string>()
+    let overflowed = false
     for (const evt of payload.events) {
       if (evt.kind === 'overflow') {
         // Why: overflow omits per-path info, so conservatively clear stale tombstones or a file that reappeared during the overrun stays struck through.
         for (const notification of getOverflowExternalReloadTargets(target)) {
           scheduleDebouncedExternalReload(notification)
         }
-        // Why: `break` not `return` — changedFiles is empty so the rest early-returns anyway, and this is more robust to code added after the loop.
+        overflowed = true
         break
       }
-
-      if (evt.kind === 'update' && evt.isDirectory === true) {
-        continue
-      }
-
-      if (evt.kind === 'delete') {
-        // Why: deletes are tombstoned above; feeding them into reload would read the ENOENT path and replace in-memory content with an error, losing the user's view.
-        continue
-      }
-
-      const relativePath = getExternalFileChangeRelativePath(
-        target.worktreePath,
-        evt.absolutePath,
-        evt.isDirectory
-      )
-      if (relativePath) {
-        changedFiles.add(relativePath)
-      }
     }
 
-    if (changedFiles.size === 0) {
+    if (overflowed || batchPaths.changes.length === 0) {
       return
     }
 
-    // Why: read openFiles once per payload to avoid N store reads on large batches; consumers skip dirty tabs so external writes don't destroy unsaved work.
-    const openFilesSnapshot = useAppStore.getState().openFiles
-    // Why: the combined "Changes" tab is per-worktree not per-path, so compute it once instead of rescanning openFiles per changed file in a large batched payload.
-    const hasCombinedDiffConsumer = openFilesSnapshot.some(
-      (f) =>
-        f.worktreeId === target.worktreeId &&
-        openFileRuntimeOwner(f) === target.runtimeEnvironmentId &&
-        isWorkingTreeCombinedDiffTab(f)
-    )
-    for (const relativePath of changedFiles) {
+    for (const change of batchPaths.changes) {
+      const relativePath = change.relativePath
+      const matching = batchPaths.matchingOpenFiles(change)
       const notification = {
         worktreeId: target.worktreeId,
         worktreePath: target.worktreePath,
@@ -684,11 +613,15 @@ export function createExternalWatchEventHandler(
         runtimeEnvironmentId: target.runtimeEnvironmentId,
         ...localWslAliasOption(target)
       }
-      const absolutePath = joinPath(notification.worktreePath, notification.relativePath)
-      const matching = getOpenFilesForExternalFileChange(openFilesSnapshot, notification)
+      Object.defineProperty(notification, 'indexedOpenFiles', {
+        value: {
+          matches: (openFiles: OpenFile[]) => batchPaths.matchingOpenFiles(change, openFiles)
+        }
+      })
+      const absolutePath = change.absolutePath
       if (matching.length === 0) {
         // Why: combined-diff tab has no in-memory content to clobber and guards its own reload, so notify it directly without self-write suppression.
-        if (hasCombinedDiffConsumer) {
+        if (batchPaths.hasCombinedDiffConsumer) {
           scheduleDebouncedExternalReload(notification)
         }
         continue
@@ -717,7 +650,7 @@ export function createExternalWatchEventHandler(
           scheduleChangedOnDiskMark(target, notification, dirtyIds)
         }
         if (dirtyMatches.length === matching.length) {
-          if (hasCombinedDiffConsumer) {
+          if (batchPaths.hasCombinedDiffConsumer) {
             scheduleDebouncedExternalReload(notification)
           }
           continue
@@ -1033,79 +966,6 @@ export function getOverflowExternalReloadTargets(
   return notifications
 }
 
-function buildDeletePathByFileId(
-  payload: FsChangedPayload,
-  worktreeId: string,
-  runtimeEnvironmentId: string | null,
-  deletedOpenEditorIds: string[],
-  openFiles: OpenFile[],
-  allowLocalWindowsWslAliases?: true
-): Map<string, string> {
-  const deletePaths = new Map<string, string>()
-  for (const evt of payload.events) {
-    if (evt.kind === 'delete') {
-      deletePaths.set(normalizeRuntimePathForComparison(evt.absolutePath), evt.absolutePath)
-    }
-  }
-  const result = new Map<string, string>()
-  if (deletePaths.size === 0) {
-    return result
-  }
-  const deletedIdSet = new Set(deletedOpenEditorIds)
-  for (const file of openFiles) {
-    if (
-      !deletedIdSet.has(file.id) ||
-      file.worktreeId !== worktreeId ||
-      openFileRuntimeOwner(file) !== runtimeEnvironmentId
-    ) {
-      continue
-    }
-    const deletePath = findMatchingWatchedPath(
-      deletePaths,
-      file.filePath,
-      allowLocalWindowsWslAliases
-    )
-    if (deletePath) {
-      result.set(file.id, normalizeRuntimePathForComparison(deletePath))
-    }
-  }
-  return result
-}
-
-function collectDeletedOpenEditorIds(
-  payload: FsChangedPayload,
-  worktreeId: string,
-  runtimeEnvironmentId: string | null,
-  openFiles: OpenFile[],
-  allowLocalWindowsWslAliases?: true
-): string[] {
-  const deletePaths = new Map<string, string>()
-  for (const evt of payload.events) {
-    if (evt.kind === 'delete') {
-      deletePaths.set(normalizeRuntimePathForComparison(evt.absolutePath), evt.absolutePath)
-    }
-  }
-  if (deletePaths.size === 0) {
-    return []
-  }
-  const result: string[] = []
-  for (const file of openFiles) {
-    if (
-      file.worktreeId !== worktreeId ||
-      openFileRuntimeOwner(file) !== runtimeEnvironmentId ||
-      (file.mode !== 'edit' && file.mode !== 'markdown-preview')
-    ) {
-      continue
-    }
-    if (
-      findMatchingWatchedPath(deletePaths, file.filePath, allowLocalWindowsWslAliases) !== undefined
-    ) {
-      result.push(file.id)
-    }
-  }
-  return result
-}
-
 /**
  * Returns true if the batched payload contains at least one file-create event
  * whose basename matches a deleted open editor file.
diff --git a/src/shared/cross-platform-path.ts b/src/shared/cross-platform-path.ts
index becf29236bb..aad0981c3a1 100644
--- a/src/shared/cross-platform-path.ts
+++ b/src/shared/cross-platform-path.ts
@@ -57,18 +57,30 @@ export function normalizeRuntimePathForComparison(rawValue: string): string {
 }
 
 export function areLocalWindowsWslPathAliases(left: string, right: string): boolean {
-  const leftWslPath = parseWslUncPath(left)
-  const rightWslPath = parseWslUncPath(right)
-  if (!leftWslPath && !rightWslPath) {
-    return false
+  const leftIdentity = getLocalWindowsWslPathIdentity(left)
+  const rightIdentity = getLocalWindowsWslPathIdentity(right)
+  return (
+    (leftIdentity.isWslUnc || rightIdentity.isWslUnc) &&
+    leftIdentity.aliasComparisonPath === rightIdentity.aliasComparisonPath
+  )
+}
+
+export type LocalWindowsWslPathIdentity = {
+  normalizedPath: string
+  aliasComparisonPath: string
+  isWslUnc: boolean
+}
+
+export function getLocalWindowsWslPathIdentity(value: string): LocalWindowsWslPathIdentity {
+  const wslPath = parseWslUncPath(value)
+  const normalizedPath = normalizeRuntimePathForComparison(value)
+  return {
+    normalizedPath,
+    aliasComparisonPath: wslPath
+      ? normalizeRuntimePathForComparison(toWindowsWslPath(wslPath.linuxPath, wslPath.distro))
+      : normalizedPath,
+    isWslUnc: wslPath !== null
   }
-  const normalize = (value: string): string => {
-    const wslPath = parseWslUncPath(value)
-    return normalizeRuntimePathForComparison(
-      wslPath ? toWindowsWslPath(wslPath.linuxPath, wslPath.distro) : value
-    )
-  }
-  return normalize(left) === normalize(right)
 }
 
 export function isRuntimePathAbsolute(