diff --git a/.gitattributes b/.gitattributes
index 1b447a9189e..145c06043bd 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -41,3 +41,10 @@
 # Generated method->params catalog: compared byte-for-byte by
 # verify:rpc-params-catalog, so a CRLF checkout would fail the gate.
 /src/shared/rpc-contract/rpc-params-catalog.generated.ts linguist-generated=true text eol=lf
+# Mobile web bundle source. Every text byte here is hashed into an asset digest and
+# from there into buildId, so a CRLF checkout produces a different bundle id for the
+# same commit (91af2897 vs 9d78435e). The PNG is -text because it must not be touched.
+/src/mobile-web/index.html text eol=lf
+/src/mobile-web/src/*.ts text eol=lf
+/src/mobile-web/src/*.css text eol=lf
+/src/mobile-web/src/*.png -text
diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index 27e14c930e3..e2112de521b 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -780,6 +780,12 @@ jobs:
       - name: Project web client from renderer build
         run: pnpm run build:web-from-renderer
 
+      # Why here and not inside "Build package inputs": this job assembles packaging inputs step by
+      # step instead of calling build:release, and electron-builder's beforePack guard hard-fails
+      # without out/mobile-web.
+      - name: Build mobile web bundle
+        run: pnpm run build:mobile-web
+
       - name: Build native components
         run: pnpm run build:native
 
diff --git a/config/electron-builder.config.cjs b/config/electron-builder.config.cjs
index c26d1b4858a..72efa691dbf 100644
--- a/config/electron-builder.config.cjs
+++ b/config/electron-builder.config.cjs
@@ -14,6 +14,10 @@ const {
 } = require('./packaged-runtime-node-modules.cjs')
 const { verifyLinuxGlibcFloor } = require('./scripts/verify-linux-glibc-floor.cjs')
 const { writeMacBuildCompatibility } = require('./scripts/mac-build-compatibility.cjs')
+const {
+  MOBILE_WEB_BUNDLE_DIR,
+  assertMobileWebBundleBuilt
+} = require('./scripts/verify-packaged-mobile-web-bundle.cjs')
 const { verifyPackagedPluginResources } = require('./scripts/verify-packaged-plugin-resources.cjs')
 const {
   verifyPackagedWindowsNodePty
@@ -177,6 +181,9 @@ module.exports = {
     // Why: these repo-only inputs are either bundled into out/ or copied via
     // extraResources. Shipping them in app.asar bloats the desktop bundle.
     '!src{,/**/*}',
+    // Redundant under !src above, kept explicit: the built bundle ships from out/mobile-web via the
+    // out rules exactly as out/web does, and the source tree must never be mistaken for it.
+    '!src/mobile-web{,/**/*}',
     '!config{,/**/*}',
     '!docs{,/**/*}',
     '!mobile{,/**/*}',
@@ -289,8 +296,11 @@ module.exports = {
       verifyStaticAppImagePackage(file, arch)
     }
   },
-  beforePack: (context) => {
+  // electron-builder calls this with the context alone. The second parameter is the bundle root,
+  // so a test can point the guard at a scratch bundle instead of needing the repo's out/ built.
+  beforePack: (context, mobileWebBundleDir = MOBILE_WEB_BUNDLE_DIR) => {
     assertPackagedNativeVariantsInstalled(context.electronPlatformName, context.arch)
+    assertMobileWebBundleBuilt(mobileWebBundleDir)
   },
   afterPack: async (context) => {
     const resourcesDir =
diff --git a/config/knip.json b/config/knip.json
index 9af0b8d1a74..e5a014b32e1 100644
--- a/config/knip.json
+++ b/config/knip.json
@@ -16,6 +16,7 @@
     "src/main/hang-watchdog/main-thread-hang-watchdog-entry.ts",
     "src/main/agent-hooks/managed-agent-hook-controls.ts",
     "src/main/claude-accounts/keychain.ts",
+    "src/mobile-web/src/bootstrap.ts",
     "src/renderer/src/main.tsx",
     "src/renderer/src/popout.tsx",
     "src/renderer/src/web/main.tsx",
diff --git a/config/scripts/build-mobile-web-bundle.mjs b/config/scripts/build-mobile-web-bundle.mjs
new file mode 100644
index 00000000000..52957858784
--- /dev/null
+++ b/config/scripts/build-mobile-web-bundle.mjs
@@ -0,0 +1,230 @@
+import { createHash } from 'node:crypto'
+import { realpathSync } from 'node:fs'
+import { mkdir, readFile, rm, writeFile } from 'node:fs/promises'
+import { join } from 'node:path'
+import { fileURLToPath, pathToFileURL } from 'node:url'
+import * as esbuild from 'esbuild'
+
+const projectDir = fileURLToPath(new URL('../..', import.meta.url))
+const sourceDir = join(projectDir, 'src', 'mobile-web')
+const defaultOutDir = join(projectDir, 'out', 'mobile-web')
+
+export const MOBILE_WEB_BUNDLE_SCHEMA_VERSION = 1
+export const MOBILE_WEB_BUNDLE_ENTRYPOINT = 'index.html'
+
+const CONTENT_TYPE_BY_EXTENSION = {
+  css: 'text/css; charset=utf-8',
+  html: 'text/html; charset=utf-8',
+  js: 'text/javascript; charset=utf-8',
+  png: 'image/png'
+}
+
+/**
+ * Canonical serialization the buildId hashes. Key order is fixed and the list is sorted by path,
+ * so the id is a pure function of content. Must stay byte-identical to the contract module's
+ * serializer in src/shared/mobile-web-bundle/.
+ */
+export function serializeMobileWebBundleAssets(assets) {
+  return JSON.stringify(
+    [...assets]
+      .sort((left, right) => (left.path < right.path ? -1 : left.path > right.path ? 1 : 0))
+      .map(({ path, sha256, byteLength, contentType }) => ({
+        path,
+        sha256,
+        byteLength,
+        contentType
+      }))
+  )
+}
+
+export function computeMobileWebBundleBuildId(assets) {
+  return createHash('sha256').update(serializeMobileWebBundleAssets(assets), 'utf8').digest('hex')
+}
+
+function sha256Hex(bytes) {
+  return createHash('sha256').update(bytes).digest('hex')
+}
+
+function contentTypeForExtension(extension) {
+  const contentType = CONTENT_TYPE_BY_EXTENSION[extension]
+  if (!contentType) {
+    throw new Error(`[build-mobile-web-bundle] no content type registered for .${extension}`)
+  }
+  return contentType
+}
+
+function readIntegerConstant(source, name) {
+  const match = new RegExp(`export const ${name} = (\\d+)`).exec(source)
+  if (!match) {
+    throw new Error(`[build-mobile-web-bundle] ${name} not found in src/shared/protocol-version.ts`)
+  }
+  return Number.parseInt(match[1], 10)
+}
+
+/**
+ * Parsed rather than imported because protocol-version.ts is TypeScript and this script runs on
+ * bare node during packaging, before any build output exists.
+ */
+async function readProtocolWindow() {
+  const source = await readFile(join(projectDir, 'src', 'shared', 'protocol-version.ts'), 'utf8')
+  return {
+    runtimeProtocolVersion: readIntegerConstant(source, 'RUNTIME_PROTOCOL_VERSION'),
+    // The bundle is a client: the floor it cares about is the oldest host protocol it can talk to.
+    minCompatibleRuntimeProtocolVersion: readIntegerConstant(
+      source,
+      'MIN_COMPATIBLE_RUNTIME_SERVER_VERSION'
+    )
+  }
+}
+
+async function readDesktopVersion() {
+  const packageJson = JSON.parse(await readFile(join(projectDir, 'package.json'), 'utf8'))
+  if (typeof packageJson.version !== 'string' || packageJson.version.length === 0) {
+    throw new Error('[build-mobile-web-bundle] root package.json has no version')
+  }
+  return packageJson.version
+}
+
+async function transformEntries(protocolWindow, desktopVersion) {
+  const result = await esbuild.build({
+    absWorkingDir: sourceDir,
+    entryPoints: [join(sourceDir, 'src', 'bootstrap.ts'), join(sourceDir, 'src', 'bootstrap.css')],
+    bundle: true,
+    minify: true,
+    // Virtual: write is false, so outdir only names the emitted files esbuild hands back.
+    outdir: 'dist',
+    write: false,
+    format: 'iife',
+    target: ['es2022'],
+    charset: 'utf8',
+    legalComments: 'none',
+    // Why no sourcemap and no metafile: both embed absolute paths, which would break reproducibility.
+    sourcemap: false,
+    logLevel: 'silent',
+    define: {
+      ORCA_MOBILE_WEB_DESKTOP_VERSION: JSON.stringify(desktopVersion),
+      ORCA_MOBILE_WEB_RUNTIME_PROTOCOL_VERSION: JSON.stringify(
+        protocolWindow.runtimeProtocolVersion
+      ),
+      ORCA_MOBILE_WEB_MIN_COMPATIBLE_RUNTIME_PROTOCOL_VERSION: JSON.stringify(
+        protocolWindow.minCompatibleRuntimeProtocolVersion
+      )
+    }
+  })
+  const byExtension = new Map()
+  for (const file of result.outputFiles) {
+    const extension = file.path.endsWith('.css') ? 'css' : 'js'
+    byExtension.set(extension, Buffer.from(file.contents))
+  }
+  const script = byExtension.get('js')
+  const stylesheet = byExtension.get('css')
+  if (!script || !stylesheet) {
+    throw new Error('[build-mobile-web-bundle] esbuild did not emit both a script and a stylesheet')
+  }
+  return { script, stylesheet }
+}
+
+function hashedAsset(bytes, extension) {
+  const sha256 = sha256Hex(bytes)
+  return {
+    bytes,
+    path: `assets/${sha256}.${extension}`,
+    sha256,
+    byteLength: bytes.byteLength,
+    contentType: contentTypeForExtension(extension)
+  }
+}
+
+export async function buildMobileWebBundle({ outDir = defaultOutDir } = {}) {
+  const [desktopVersion, protocolWindow] = await Promise.all([
+    readDesktopVersion(),
+    readProtocolWindow()
+  ])
+  const { script, stylesheet } = await transformEntries(protocolWindow, desktopVersion)
+  const mark = await readFile(join(sourceDir, 'src', 'orca-mark.png'))
+
+  const hashed = [
+    hashedAsset(script, 'js'),
+    hashedAsset(stylesheet, 'css'),
+    hashedAsset(mark, 'png')
+  ]
+  const [scriptAsset, stylesheetAsset, markAsset] = hashed
+
+  const template = await readFile(join(sourceDir, MOBILE_WEB_BUNDLE_ENTRYPOINT), 'utf8')
+  const substitutions = {
+    __ORCA_BOOTSTRAP_JS__: scriptAsset.path,
+    __ORCA_BOOTSTRAP_CSS__: stylesheetAsset.path,
+    __ORCA_MARK_PNG__: markAsset.path
+  }
+  let html = template
+  for (const [token, value] of Object.entries(substitutions)) {
+    if (!html.includes(token)) {
+      throw new Error(`[build-mobile-web-bundle] ${MOBILE_WEB_BUNDLE_ENTRYPOINT} lacks ${token}`)
+    }
+    html = html.replaceAll(token, value)
+  }
+  const indexBytes = Buffer.from(html, 'utf8')
+  const indexAsset = {
+    bytes: indexBytes,
+    path: MOBILE_WEB_BUNDLE_ENTRYPOINT,
+    sha256: sha256Hex(indexBytes),
+    byteLength: indexBytes.byteLength,
+    contentType: contentTypeForExtension('html')
+  }
+
+  const written = [indexAsset, ...hashed]
+  const assets = written
+    .map(({ path, sha256, byteLength, contentType }) => ({ path, sha256, byteLength, contentType }))
+    .sort((left, right) => (left.path < right.path ? -1 : left.path > right.path ? 1 : 0))
+  const manifest = {
+    schemaVersion: MOBILE_WEB_BUNDLE_SCHEMA_VERSION,
+    buildId: computeMobileWebBundleBuildId(assets),
+    desktopVersion,
+    minCompatibleRuntimeProtocolVersion: protocolWindow.minCompatibleRuntimeProtocolVersion,
+    runtimeProtocolVersion: protocolWindow.runtimeProtocolVersion,
+    entrypoint: MOBILE_WEB_BUNDLE_ENTRYPOINT,
+    totalBytes: assets.reduce((total, asset) => total + asset.byteLength, 0),
+    assets
+  }
+
+  // Why a full clear: a stale asset left from an earlier build would ship unreferenced inside asar.
+  await rm(outDir, { recursive: true, force: true })
+  await mkdir(join(outDir, 'assets'), { recursive: true })
+  for (const asset of written) {
+    await writeFile(join(outDir, asset.path), asset.bytes)
+  }
+  await writeFile(join(outDir, 'manifest.json'), `${JSON.stringify(manifest, null, 2)}\n`, 'utf8')
+  return { manifest, outDir }
+}
+
+/**
+ * Whether this module was run as the entry script. Two ways to get this wrong, both of which end
+ * with the builder exiting 0 having written nothing: `file://${path}` never matches on Windows,
+ * where import.meta.url is `file:///C:/...`; and Node resolves symlinks in import.meta.url but not
+ * in argv[1], so `node /tmp/...` against a /private/tmp realpath compares two different strings.
+ * Both seams are injectable so win32 and a missing path can be exercised from a posix runner.
+ */
+export function isDirectInvocation(
+  moduleUrl,
+  scriptPath,
+  { toFileUrl = pathToFileURL, realpath = realpathSync } = {}
+) {
+  if (!scriptPath) {
+    return false
+  }
+  let resolved = scriptPath
+  try {
+    resolved = realpath(scriptPath)
+  } catch {
+    // A path that cannot be resolved cannot be this module; fall through to the literal compare.
+  }
+  return moduleUrl === toFileUrl(resolved).href
+}
+
+if (isDirectInvocation(import.meta.url, process.argv[1])) {
+  const { manifest, outDir } = await buildMobileWebBundle()
+  console.log(
+    `[build-mobile-web-bundle] OK — ${String(manifest.assets.length)} asset(s), ` +
+      `${String(manifest.totalBytes)} bytes, buildId ${manifest.buildId} -> ${outDir}`
+  )
+}
diff --git a/config/scripts/build-mobile-web-bundle.test.mjs b/config/scripts/build-mobile-web-bundle.test.mjs
new file mode 100644
index 00000000000..ca2f74bd1d0
--- /dev/null
+++ b/config/scripts/build-mobile-web-bundle.test.mjs
@@ -0,0 +1,261 @@
+import { execFileSync } from 'node:child_process'
+import { createHash } from 'node:crypto'
+import { mkdtemp, readFile, readdir, rm, symlink, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { describe, expect, it } from 'vitest'
+import {
+  buildMobileWebBundle,
+  computeMobileWebBundleBuildId,
+  isDirectInvocation,
+  serializeMobileWebBundleAssets
+} from './build-mobile-web-bundle.mjs'
+import {
+  MOBILE_WEB_BUNDLE_PHASE_A_MAX_ASSETS,
+  MOBILE_WEB_BUNDLE_PHASE_A_MAX_TOTAL_BYTES,
+  assertNoCarriageReturnsInSource
+} from './verify-mobile-web-bundle.mjs'
+
+async function buildIntoScratch() {
+  const scratch = await mkdtemp(join(tmpdir(), 'orca-mobile-web-build-'))
+  const bundleDir = join(scratch, 'mobile-web')
+  const { manifest } = await buildMobileWebBundle({ outDir: bundleDir })
+  return { scratch, bundleDir, manifest }
+}
+
+describe('buildMobileWebBundle', () => {
+  it('emits a content-addressed bundle whose only stable name is the entrypoint', async () => {
+    const { scratch, bundleDir, manifest } = await buildIntoScratch()
+    try {
+      const root = await readdir(bundleDir)
+      expect(root.sort()).toEqual(['assets', 'index.html', 'manifest.json'])
+      for (const name of await readdir(join(bundleDir, 'assets'))) {
+        const [digest, extension] = name.split('.')
+        expect(digest).toMatch(/^[0-9a-f]{64}$/)
+        const bytes = await readFile(join(bundleDir, 'assets', name))
+        expect(createHash('sha256').update(bytes).digest('hex')).toBe(digest)
+        expect(extension).toMatch(/^(js|css|png)$/)
+      }
+      const html = await readFile(join(bundleDir, 'index.html'), 'utf8')
+      for (const asset of manifest.assets) {
+        if (asset.path !== 'index.html') {
+          expect(html).toContain(asset.path)
+        }
+      }
+      expect(html).not.toContain('__ORCA_')
+    } finally {
+      await rm(scratch, { recursive: true, force: true })
+    }
+  })
+
+  it('carries every manifest field the Phase A contract names', async () => {
+    const { scratch, manifest } = await buildIntoScratch()
+    try {
+      expect(Object.keys(manifest)).toEqual([
+        'schemaVersion',
+        'buildId',
+        'desktopVersion',
+        'minCompatibleRuntimeProtocolVersion',
+        'runtimeProtocolVersion',
+        'entrypoint',
+        'totalBytes',
+        'assets'
+      ])
+      expect(manifest.schemaVersion).toBe(1)
+      expect(manifest.entrypoint).toBe('index.html')
+      const packageJson = JSON.parse(
+        await readFile(new URL('../../package.json', import.meta.url), 'utf8')
+      )
+      expect(manifest.desktopVersion).toBe(packageJson.version)
+      const protocolSource = await readFile(
+        new URL('../../src/shared/protocol-version.ts', import.meta.url),
+        'utf8'
+      )
+      expect(protocolSource).toContain(
+        `export const RUNTIME_PROTOCOL_VERSION = ${String(manifest.runtimeProtocolVersion)}`
+      )
+      expect(protocolSource).toContain(
+        `export const MIN_COMPATIBLE_RUNTIME_SERVER_VERSION = ${String(manifest.minCompatibleRuntimeProtocolVersion)}`
+      )
+      expect(manifest.totalBytes).toBe(
+        manifest.assets.reduce((total, asset) => total + asset.byteLength, 0)
+      )
+    } finally {
+      await rm(scratch, { recursive: true, force: true })
+    }
+  })
+
+  it('produces the same buildId from two independent builds', async () => {
+    const first = await buildIntoScratch()
+    const second = await buildIntoScratch()
+    try {
+      expect(second.manifest.buildId).toBe(first.manifest.buildId)
+      expect(second.manifest).toEqual(first.manifest)
+    } finally {
+      await rm(first.scratch, { recursive: true, force: true })
+      await rm(second.scratch, { recursive: true, force: true })
+    }
+  })
+
+  it('embeds no absolute path from the machine that built it', async () => {
+    const { scratch, bundleDir } = await buildIntoScratch()
+    try {
+      const names = [
+        'index.html',
+        'manifest.json',
+        ...(await readdir(join(bundleDir, 'assets'))).map((name) => join('assets', name))
+      ]
+      for (const name of names) {
+        const text = (await readFile(join(bundleDir, name))).toString('latin1')
+        expect(text).not.toContain(scratch)
+        expect(text).not.toContain(process.cwd())
+      }
+    } finally {
+      await rm(scratch, { recursive: true, force: true })
+    }
+  })
+
+  it('stays inside the Phase A budget', async () => {
+    const { scratch, manifest } = await buildIntoScratch()
+    try {
+      expect(manifest.assets.length).toBeLessThanOrEqual(MOBILE_WEB_BUNDLE_PHASE_A_MAX_ASSETS)
+      expect(manifest.totalBytes).toBeLessThanOrEqual(MOBILE_WEB_BUNDLE_PHASE_A_MAX_TOTAL_BYTES)
+    } finally {
+      await rm(scratch, { recursive: true, force: true })
+    }
+  })
+})
+
+describe('computeMobileWebBundleBuildId', () => {
+  const assets = [
+    { path: 'index.html', sha256: 'a'.repeat(64), byteLength: 3, contentType: 'text/html' },
+    { path: 'assets/b.js', sha256: 'b'.repeat(64), byteLength: 5, contentType: 'text/javascript' }
+  ]
+
+  it('sorts by path, so input order cannot change the id', () => {
+    expect(computeMobileWebBundleBuildId(assets.toReversed())).toBe(
+      computeMobileWebBundleBuildId(assets)
+    )
+  })
+
+  it('serializes a fixed key order regardless of the input object key order', () => {
+    const reordered = assets.map(({ contentType, byteLength, sha256, path }) => ({
+      contentType,
+      byteLength,
+      sha256,
+      path
+    }))
+    expect(serializeMobileWebBundleAssets(reordered)).toBe(serializeMobileWebBundleAssets(assets))
+  })
+
+  it('changes when any hashed field changes', () => {
+    const baseline = computeMobileWebBundleBuildId(assets)
+    for (const field of ['sha256', 'byteLength', 'contentType', 'path']) {
+      const mutated = assets.map((asset, index) =>
+        index === 0 ? { ...asset, [field]: field === 'byteLength' ? 4 : `${asset[field]}x` } : asset
+      )
+      expect(computeMobileWebBundleBuildId(mutated)).not.toBe(baseline)
+    }
+  })
+})
+
+describe('isDirectInvocation', () => {
+  const thisFile = import.meta.filename
+
+  it('matches the path this module was loaded from', () => {
+    expect(isDirectInvocation(import.meta.url, thisFile)).toBe(true)
+  })
+
+  it('does not match a different script', () => {
+    expect(isDirectInvocation(import.meta.url, join(thisFile, '..', 'other.mjs'))).toBe(false)
+  })
+
+  it('tolerates an absent argv[1]', () => {
+    expect(isDirectInvocation(import.meta.url, undefined)).toBe(false)
+    expect(isDirectInvocation(import.meta.url, '')).toBe(false)
+  })
+
+  // Why an injected converter: a win32 path cannot be exercised through node:url's pathToFileURL
+  // on a posix runner, and CI is ubuntu.
+  const toWin32FileUrl = (windowsPath) => new URL(`file:///${windowsPath.replaceAll('\\', '/')}`)
+
+  it('matches a Windows entry path, which the file:// template form never does', () => {
+    const scriptPath = 'C:\\orca\\config\\scripts\\build-mobile-web-bundle.mjs'
+    const moduleUrl = 'file:///C:/orca/config/scripts/build-mobile-web-bundle.mjs'
+    const keepAsIs = (path) => path
+    expect(
+      isDirectInvocation(moduleUrl, scriptPath, {
+        toFileUrl: toWin32FileUrl,
+        realpath: keepAsIs
+      })
+    ).toBe(true)
+    // The regression this guards: `file://${argv[1]}` yields file://C:\orca\... on Windows,
+    // so the builder exited 0 having written nothing and packaging failed downstream.
+    expect(`file://${scriptPath}`).not.toBe(moduleUrl)
+  })
+
+  it('is not written with the file:// template form', async () => {
+    const source = await readFile(new URL('./build-mobile-web-bundle.mjs', import.meta.url), 'utf8')
+    expect(source).not.toMatch(/file:\/\/\$\{process\.argv\[1\]\}/)
+    expect(source).toContain('pathToFileURL')
+  })
+})
+
+describe('mobile web source line endings', () => {
+  it('accepts the committed source tree', async () => {
+    await expect(assertNoCarriageReturnsInSource()).resolves.toBeUndefined()
+  })
+
+  it('rejects a CRLF source file, because CRLF changes every asset hash and the buildId', async () => {
+    const scratch = await mkdtemp(join(tmpdir(), 'orca-mobile-web-eol-'))
+    try {
+      await writeFile(join(scratch, 'bootstrap.ts'), 'const a = 1\r\nconst b = 2\r\n', 'utf8')
+      await expect(assertNoCarriageReturnsInSource(scratch)).rejects.toThrow(
+        /CRLF in mobile web source/
+      )
+    } finally {
+      await rm(scratch, { recursive: true, force: true })
+    }
+  })
+
+  it('pins eol=lf for every committed text source and -text for the binary', () => {
+    const files = execFileSync('git', ['ls-files', 'src/mobile-web'], { encoding: 'utf8' })
+      .split('\n')
+      .filter(Boolean)
+    expect(files.length).toBeGreaterThanOrEqual(4)
+    for (const file of files) {
+      const attributes = execFileSync('git', ['check-attr', 'text', 'eol', '--', file], {
+        encoding: 'utf8'
+      })
+      if (file.endsWith('.png')) {
+        expect(attributes).toContain('text: unset')
+      } else {
+        expect(attributes).toContain('eol: lf')
+      }
+    }
+  })
+})
+
+describe('running the builder through a symlink', () => {
+  // Node resolves symlinks in import.meta.url but not in argv[1]. Before the guard realpath'd the
+  // entry path, `node /tmp/<link>` compared /tmp against /private/tmp and the builder exited 0
+  // having written nothing — a green packaging job with no bundle in it.
+  it('still recognises the entry module', async () => {
+    const scratch = await mkdtemp(join(tmpdir(), 'orca-mobile-web-link-'))
+    try {
+      const builderUrl = new URL('./build-mobile-web-bundle.mjs', import.meta.url).href
+      const real = join(scratch, 'entry.mjs')
+      await writeFile(
+        real,
+        `import { isDirectInvocation } from ${JSON.stringify(builderUrl)}\n` +
+          'process.stdout.write(String(isDirectInvocation(import.meta.url, process.argv[1])))\n',
+        'utf8'
+      )
+      const link = join(scratch, 'entry-link.mjs')
+      await symlink(real, link)
+      expect(execFileSync(process.execPath, [link], { encoding: 'utf8' })).toBe('true')
+    } finally {
+      await rm(scratch, { recursive: true, force: true })
+    }
+  })
+})
diff --git a/config/scripts/electron-builder-config.test.mjs b/config/scripts/electron-builder-config.test.mjs
index b7d5f16400b..f88967e5132 100644
--- a/config/scripts/electron-builder-config.test.mjs
+++ b/config/scripts/electron-builder-config.test.mjs
@@ -3,7 +3,8 @@ import { chmod, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises'
 import { createRequire } from 'node:module'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
-import { describe, expect, it } from 'vitest'
+import { afterAll, beforeAll, describe, expect, it } from 'vitest'
+import { buildMobileWebBundle } from './build-mobile-web-bundle.mjs'
 
 const REPO_ROOT = join(import.meta.dirname, '..', '..')
 const SRC_MAIN_DIR = join(REPO_ROOT, 'src', 'main')
@@ -442,8 +443,23 @@ describe('arch-aware packaging guard', () => {
   const OTHER_ARCH_NAME = process.arch === 'arm64' ? 'x64' : 'arm64'
   const SHERPA_PLATFORM = process.platform === 'win32' ? 'win' : process.platform
   const otherSherpa = `sherpa-onnx-${SHERPA_PLATFORM}-${OTHER_ARCH_NAME}`
+
+  // beforePack also hash-verifies the mobile web bundle, which the unit-test job never builds.
+  // Point it at a real bundle built into a temp dir: these tests are about the native-variant
+  // guard, and the bundle guard has its own suite.
+  let scratch
+  let bundleDir
+  beforeAll(async () => {
+    scratch = await mkdtemp(join(tmpdir(), 'orca-electron-builder-guard-'))
+    bundleDir = join(scratch, 'mobile-web')
+    await buildMobileWebBundle({ outDir: bundleDir })
+  })
+  afterAll(async () => {
+    await rm(scratch, { recursive: true, force: true })
+  })
+
   const packHost = (arch) =>
-    electronBuilderConfig.beforePack({ electronPlatformName: process.platform, arch })
+    electronBuilderConfig.beforePack({ electronPlatformName: process.platform, arch }, bundleDir)
 
   it('allows packaging the host platform and architecture', () => {
     expect(() => packHost(HOST_ARCH)).not.toThrow()
@@ -471,7 +487,7 @@ describe('arch-aware packaging guard', () => {
       (resource) => resource.to === join('node_modules', '@vscode', 'windows-process-tree')
     )
     const packWindows = () =>
-      electronBuilderConfig.beforePack({ electronPlatformName: 'win32', arch: 1 })
+      electronBuilderConfig.beforePack({ electronPlatformName: 'win32', arch: 1 }, bundleDir)
     if (process.platform === 'win32' || windowsAddon) {
       expect(packWindows).not.toThrow()
     } else {
diff --git a/config/scripts/mobile-web-bundle-packaging-workflow-contract.test.mjs b/config/scripts/mobile-web-bundle-packaging-workflow-contract.test.mjs
new file mode 100644
index 00000000000..203bea1d87e
--- /dev/null
+++ b/config/scripts/mobile-web-bundle-packaging-workflow-contract.test.mjs
@@ -0,0 +1,159 @@
+import { readFileSync, readdirSync } from 'node:fs'
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { describe, expect, it } from 'vitest'
+import { parseDocument } from 'yaml'
+
+const workflowsDir = fileURLToPath(new URL('../../.github/workflows', import.meta.url))
+
+// Every script whose chain reaches build:mobile-web. build:unpack -> build -> build:desktop, and
+// build:mac/linux/win each call build:desktop, so all of them produce out/mobile-web. The chain
+// itself is not an assumption here: 'the build scripts' below resolves each one for real.
+const BUNDLE_PRODUCING_SCRIPTS = [
+  'build',
+  'build:desktop',
+  'build:release',
+  'build:release:parallel',
+  'build:unpack',
+  'build:mobile-web',
+  'build:mac',
+  'build:mac:release',
+  'build:linux',
+  'build:win'
+]
+
+const BUNDLE_PRODUCER = new RegExp(
+  `pnpm (?:run )?(?:${BUNDLE_PRODUCING_SCRIPTS.join('|')})(?=$|[\\s'"&|;])`,
+  'm'
+)
+
+const packageScripts = JSON.parse(
+  readFileSync(fileURLToPath(new URL('../../package.json', import.meta.url)), 'utf8')
+).scripts
+
+const SCRIPT_INVOCATION = /pnpm (?:run )?([\w:-]+)(?=$|[\s'"&|;])/g
+
+/** Whether `pnpm run <name>` eventually runs build:mobile-web. */
+function reachesBundleBuild(name, seen = new Set()) {
+  if (name === 'build:mobile-web') {
+    return true
+  }
+  if (seen.has(name)) {
+    return false
+  }
+  seen.add(name)
+  const body = packageScripts[name]
+  if (typeof body !== 'string') {
+    return false
+  }
+  return [...body.matchAll(SCRIPT_INVOCATION)].some((match) => reachesBundleBuild(match[1], seen))
+}
+
+/**
+ * Whether `pnpm run <name>` eventually runs electron-builder without --prepackaged, i.e. runs
+ * beforePack. A workflow job that packs through such a script is a packaging job even though the
+ * literal electron-builder line lives in package.json (daemon-relocation-spike's build:unpack).
+ */
+function reachesElectronBuilder(name, seen = new Set()) {
+  if (seen.has(name)) {
+    return false
+  }
+  seen.add(name)
+  const body = packageScripts[name]
+  if (typeof body !== 'string') {
+    return false
+  }
+  if (packsWithBeforePack(body)) {
+    return true
+  }
+  return [...body.matchAll(SCRIPT_INVOCATION)].some((match) =>
+    reachesElectronBuilder(match[1], seen)
+  )
+}
+
+/** Whether text invokes electron-builder in a way that reaches beforePack. */
+function packsWithBeforePack(text) {
+  const invocations = [...text.matchAll(/[^\n]*electron-builder --config[^\n]*/g)].map(
+    (match) => match[0]
+  )
+  // --prepackaged short-circuits doPack before emitBeforePack, so those jobs never run the guard.
+  return (
+    invocations.length > 0 &&
+    !invocations.every((invocation) => invocation.includes('--prepackaged'))
+  )
+}
+
+// Every job that packs an app and therefore runs beforePack. Listed so that a new packaging
+// workflow has to be added here deliberately, with its bundle step, rather than slipping in.
+const EXPECTED_PACKAGING_JOBS = [
+  'adhoc-mac-build.yml build-adhoc-mac',
+  'daemon-relocation-spike.yml spike',
+  'daily-mac-build.yml build-daily-mac',
+  'dev-channel-win-build.yml build-win',
+  'hourly-mac-build.yml build-hourly-mac',
+  'pr.yml package',
+  'pr.yml package_windows',
+  'release-cut.yml build',
+  'release-mac-build.yml build-mac',
+  'win-crash-survival-e2e.yml crash-survival',
+  'win-update-survival-e2e.yml survival',
+  'windows-signing-rehearsal.yml rehearse'
+]
+
+/**
+ * Raw source text per job, sliced by the parsed job boundaries. Why not yaml.stringify(job):
+ * re-serializing folds long lines, and the fold in dev-channel-win-build's build-win landed
+ * between `electron-builder` and `--config`, hiding a whole packaging job from this census.
+ */
+function packagingJobs() {
+  const jobs = []
+  for (const file of readdirSync(workflowsDir).filter((name) => name.endsWith('.yml'))) {
+    const source = readFileSync(join(workflowsDir, file), 'utf8')
+    const jobsNode = parseDocument(source).get('jobs', true)
+    const items = jobsNode?.items ?? []
+    for (const [index, pair] of items.entries()) {
+      const end = index + 1 < items.length ? items[index + 1].key.range[0] : jobsNode.range[2]
+      const text = source.slice(pair.key.range[0], end)
+      const packsViaScript = [...text.matchAll(SCRIPT_INVOCATION)].some((match) =>
+        reachesElectronBuilder(match[1])
+      )
+      if (!packsWithBeforePack(text) && !packsViaScript) {
+        continue
+      }
+      jobs.push({ label: `${file} ${String(pair.key.value)}`, text })
+    }
+  }
+  return jobs
+}
+
+describe('mobile web bundle packaging coverage', () => {
+  it('finds every packaging job', () => {
+    // A rename or a restructure that shrank this list would make every assertion below vacuous.
+    const labels = packagingJobs().map((job) => job.label)
+    expect(labels.length).toBeGreaterThanOrEqual(EXPECTED_PACKAGING_JOBS.length)
+    expect(labels.toSorted()).toEqual(EXPECTED_PACKAGING_JOBS.toSorted())
+  })
+
+  it.each(packagingJobs().map((job) => [job.label, job]))(
+    'produces out/mobile-web before electron-builder packs: %s',
+    (_label, job) => {
+      // Job granularity, not step ordering: the failure this exists for is a job that never builds
+      // the bundle at all, which is what beforePack turns into a hard packaging failure.
+      expect(job.text).toMatch(BUNDLE_PRODUCER)
+    }
+  )
+})
+
+describe('the build scripts the census trusts', () => {
+  // The census only checks that a packaging job invokes one of these. If a chain stopped calling
+  // build:mobile-web, every job would still look covered while packaging failed at beforePack.
+  it.each(BUNDLE_PRODUCING_SCRIPTS)('%s runs build:mobile-web', (name) => {
+    expect(packageScripts[name]).toBeTypeOf('string')
+    expect(reachesBundleBuild(name)).toBe(true)
+  })
+
+  it('pr.yml package builds the bundle by hand, because it never calls build:release', () => {
+    const source = readFileSync(join(workflowsDir, 'pr.yml'), 'utf8')
+    expect(source).toMatch(/- name: Build mobile web bundle\n\s+run: pnpm run build:mobile-web\n/)
+  })
+})
diff --git a/config/scripts/run-typecheck-projects-in-parallel.mjs b/config/scripts/run-typecheck-projects-in-parallel.mjs
index 13047989db5..09b0a8e1d1f 100644
--- a/config/scripts/run-typecheck-projects-in-parallel.mjs
+++ b/config/scripts/run-typecheck-projects-in-parallel.mjs
@@ -2,9 +2,14 @@ import { spawn } from 'node:child_process'
 import { availableParallelism } from 'node:os'
 import { fileURLToPath } from 'node:url'
 
-// The three projects overlap heavily in src/shared but have no build dependency on
+// These projects overlap heavily in src/shared but have no build dependency on
 // each other, so tsc can check them concurrently instead of in a `&&` chain.
-const projects = ['tsconfig.node.json', 'tsconfig.tc.cli.json', 'tsconfig.tc.web.json']
+const projects = [
+  'tsconfig.node.json',
+  'tsconfig.tc.cli.json',
+  'tsconfig.tc.web.json',
+  'tsconfig.mobile-web.json'
+]
 const repoRoot = fileURLToPath(new URL('../..', import.meta.url))
 const tsc = fileURLToPath(new URL('../../node_modules/typescript/bin/tsc', import.meta.url))
 
diff --git a/config/scripts/verify-mobile-web-bundle.mjs b/config/scripts/verify-mobile-web-bundle.mjs
new file mode 100644
index 00000000000..0fedaac11c1
--- /dev/null
+++ b/config/scripts/verify-mobile-web-bundle.mjs
@@ -0,0 +1,117 @@
+import { mkdtemp, readFile, readdir, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { buildMobileWebBundle, isDirectInvocation } from './build-mobile-web-bundle.mjs'
+import { assertMobileWebBundleBuilt } from './verify-packaged-mobile-web-bundle.cjs'
+
+const projectDir = fileURLToPath(new URL('../..', import.meta.url))
+const bundleDir = join(projectDir, 'out', 'mobile-web')
+const sourceDir = join(projectDir, 'src', 'mobile-web')
+
+// Phase A budget, not the contract ceiling: a bootstrap page past a quarter-megabyte has stopped
+// being a bootstrap. Phase C raises these deliberately.
+export const MOBILE_WEB_BUNDLE_PHASE_A_MAX_ASSETS = 16
+export const MOBILE_WEB_BUNDLE_PHASE_A_MAX_TOTAL_BYTES = 256 * 1024
+
+class VerificationError extends Error {}
+
+function fail(message) {
+  throw new VerificationError(message)
+}
+
+async function buildIntoScratch() {
+  const scratch = await mkdtemp(join(tmpdir(), 'orca-mobile-web-verify-'))
+  try {
+    const { manifest } = await buildMobileWebBundle({ outDir: join(scratch, 'mobile-web') })
+    return manifest
+  } finally {
+    await rm(scratch, { recursive: true, force: true })
+  }
+}
+
+async function listSourceFiles(directory) {
+  const entries = await readdir(directory, { withFileTypes: true })
+  const files = []
+  for (const entry of entries) {
+    const entryPath = join(directory, entry.name)
+    if (entry.isDirectory()) {
+      files.push(...(await listSourceFiles(entryPath)))
+    } else if (entry.isFile()) {
+      files.push(entryPath)
+    }
+  }
+  return files.sort()
+}
+
+/**
+ * A CRLF checkout changes the bytes of every text source, which changes every asset hash and so
+ * the buildId. .gitattributes pins eol=lf; this is what notices when that pin stops working.
+ */
+export async function assertNoCarriageReturnsInSource(directory = sourceDir) {
+  const offenders = []
+  for (const file of await listSourceFiles(directory)) {
+    // Binary assets are pinned -text and may legitimately contain 0x0d.
+    if (file.endsWith('.png')) {
+      continue
+    }
+    if ((await readFile(file)).includes(0x0d)) {
+      offenders.push(file.slice(directory.length + 1))
+    }
+  }
+  if (offenders.length > 0) {
+    fail(
+      `CRLF in mobile web source, which would change every asset hash and the buildId: ` +
+        `${offenders.join(', ')}. Check the .gitattributes eol=lf pin for src/mobile-web.`
+    )
+  }
+}
+
+export async function verifyMobileWebBundle() {
+  await assertNoCarriageReturnsInSource()
+
+  // The packaging guard owns manifest integrity (safe paths, recomputed buildId, totalBytes, hashes,
+  // no stray files); a manifest edited after the build fails here exactly as it would at beforePack.
+  const manifest = assertMobileWebBundleBuilt(bundleDir)
+
+  if (manifest.assets.length > MOBILE_WEB_BUNDLE_PHASE_A_MAX_ASSETS) {
+    fail(
+      `bundle has ${String(manifest.assets.length)} assets, over the Phase A budget of ` +
+        `${String(MOBILE_WEB_BUNDLE_PHASE_A_MAX_ASSETS)}`
+    )
+  }
+  if (manifest.totalBytes > MOBILE_WEB_BUNDLE_PHASE_A_MAX_TOTAL_BYTES) {
+    fail(
+      `bundle is ${String(manifest.totalBytes)} bytes, over the Phase A budget of ` +
+        `${String(MOBILE_WEB_BUNDLE_PHASE_A_MAX_TOTAL_BYTES)}`
+    )
+  }
+
+  // Two fresh builds into scratch dirs: a timestamp, an absolute path, or an unstable ordering
+  // anywhere in the pipeline shows up here as a buildId mismatch rather than as a phone cache miss.
+  const first = await buildIntoScratch()
+  const second = await buildIntoScratch()
+  if (first.buildId !== second.buildId) {
+    fail(`buildId is not reproducible: ${first.buildId} then ${second.buildId}`)
+  }
+  if (first.buildId !== manifest.buildId) {
+    fail(
+      `${bundleDir} is stale: it carries buildId ${manifest.buildId}, a fresh build produces ${first.buildId}`
+    )
+  }
+  return manifest
+}
+
+if (isDirectInvocation(import.meta.url, process.argv[1])) {
+  try {
+    const manifest = await verifyMobileWebBundle()
+    console.log(
+      `[verify-mobile-web-bundle] OK — ${String(manifest.assets.length)} asset(s), ` +
+        `${String(manifest.totalBytes)}/${String(MOBILE_WEB_BUNDLE_PHASE_A_MAX_TOTAL_BYTES)} bytes, ` +
+        `reproducible buildId ${manifest.buildId}`
+    )
+  } catch (error) {
+    console.error(`[verify-mobile-web-bundle] ${error.message}`)
+    process.exit(1)
+  }
+}
diff --git a/config/scripts/verify-packaged-mobile-web-bundle.cjs b/config/scripts/verify-packaged-mobile-web-bundle.cjs
new file mode 100644
index 00000000000..13cf71b008b
--- /dev/null
+++ b/config/scripts/verify-packaged-mobile-web-bundle.cjs
@@ -0,0 +1,191 @@
+const { createHash } = require('node:crypto')
+const { readFileSync, readdirSync, statSync } = require('node:fs')
+const { join, resolve } = require('node:path')
+
+const projectDir = resolve(__dirname, '..', '..')
+const MOBILE_WEB_BUNDLE_DIR = join(projectDir, 'out', 'mobile-web')
+const REMEDY = 'Run pnpm build:mobile-web (build:desktop and build:release already do).'
+const ENTRYPOINT = 'index.html'
+const SHA256_PATTERN = /^[0-9a-f]{64}$/
+
+function failure(message) {
+  return new Error(`[verify-packaged-mobile-web-bundle] ${message}`)
+}
+
+function assertSafeRelativePath(path) {
+  if (typeof path !== 'string' || path.length === 0) {
+    throw failure('manifest asset has a missing or empty path')
+  }
+  const segments = path.split('/')
+  if (
+    path.includes('\\') ||
+    path.startsWith('/') ||
+    /^[a-zA-Z]:/.test(path) ||
+    segments.some((segment) => segment === '' || segment === '.' || segment === '..')
+  ) {
+    throw failure(`manifest asset path is not a safe relative path: ${path}`)
+  }
+}
+
+function assertInteger(value, field) {
+  if (!Number.isSafeInteger(value) || value < 0) {
+    throw failure(`manifest field ${field} is not a non-negative integer: ${String(value)}`)
+  }
+}
+
+/**
+ * Canonical serialization of the asset list. Must stay byte-identical to
+ * serializeMobileWebBundleAssets in config/scripts/build-mobile-web-bundle.mjs; a divergence here
+ * would reject every honest bundle, so the two move together.
+ */
+function serializeAssets(assets) {
+  return JSON.stringify(
+    [...assets]
+      .sort((left, right) => (left.path < right.path ? -1 : left.path > right.path ? 1 : 0))
+      .map(({ path, sha256, byteLength, contentType }) => ({
+        path,
+        sha256,
+        byteLength,
+        contentType
+      }))
+  )
+}
+
+function parseManifest(bundleDir) {
+  const manifestPath = join(bundleDir, 'manifest.json')
+  let raw
+  try {
+    raw = readFileSync(manifestPath, 'utf8')
+  } catch (error) {
+    throw failure(
+      `no bundle manifest at ${manifestPath} (${error.code ?? error.message}). ${REMEDY}`
+    )
+  }
+  let manifest
+  try {
+    manifest = JSON.parse(raw)
+  } catch (error) {
+    throw failure(`${manifestPath} is not valid JSON: ${error.message}. ${REMEDY}`)
+  }
+  if (typeof manifest !== 'object' || manifest === null || Array.isArray(manifest)) {
+    throw failure(`${manifestPath} is not a JSON object. ${REMEDY}`)
+  }
+  if (manifest.schemaVersion !== 1) {
+    throw failure(`unsupported manifest schemaVersion: ${String(manifest.schemaVersion)}`)
+  }
+  if (typeof manifest.buildId !== 'string' || !SHA256_PATTERN.test(manifest.buildId)) {
+    throw failure(`manifest buildId is not a sha256 digest: ${String(manifest.buildId)}`)
+  }
+  if (typeof manifest.desktopVersion !== 'string' || manifest.desktopVersion.length === 0) {
+    throw failure('manifest desktopVersion is missing')
+  }
+  assertInteger(manifest.minCompatibleRuntimeProtocolVersion, 'minCompatibleRuntimeProtocolVersion')
+  assertInteger(manifest.runtimeProtocolVersion, 'runtimeProtocolVersion')
+  assertInteger(manifest.totalBytes, 'totalBytes')
+  if (manifest.entrypoint !== ENTRYPOINT) {
+    throw failure(`manifest entrypoint must be ${ENTRYPOINT}, got ${String(manifest.entrypoint)}`)
+  }
+  if (!Array.isArray(manifest.assets) || manifest.assets.length === 0) {
+    throw failure('manifest lists no assets')
+  }
+  for (const asset of manifest.assets) {
+    if (typeof asset !== 'object' || asset === null) {
+      throw failure('manifest asset entry is not an object')
+    }
+    assertSafeRelativePath(asset.path)
+    if (typeof asset.sha256 !== 'string' || !SHA256_PATTERN.test(asset.sha256)) {
+      throw failure(`manifest asset ${asset.path} has no sha256 digest`)
+    }
+    assertInteger(asset.byteLength, `assets[${asset.path}].byteLength`)
+    if (typeof asset.contentType !== 'string' || asset.contentType.length === 0) {
+      throw failure(`manifest asset ${asset.path} has no contentType`)
+    }
+  }
+  if (!manifest.assets.some((asset) => asset.path === manifest.entrypoint)) {
+    throw failure(`manifest entrypoint ${manifest.entrypoint} is not one of its assets`)
+  }
+  const declaredTotal = manifest.assets.reduce((total, asset) => total + asset.byteLength, 0)
+  if (declaredTotal !== manifest.totalBytes) {
+    throw failure(
+      `manifest totalBytes is ${String(manifest.totalBytes)}, its assets sum to ${String(declaredTotal)}`
+    )
+  }
+  const recomputed = createHash('sha256')
+    .update(serializeAssets(manifest.assets), 'utf8')
+    .digest('hex')
+  if (recomputed !== manifest.buildId) {
+    throw failure(
+      `manifest buildId ${manifest.buildId} does not match its asset list (expected ${recomputed}). ${REMEDY}`
+    )
+  }
+  return manifest
+}
+
+/** Every file under the bundle directory, as a manifest-shaped relative path. */
+function listBundleFiles(directory, prefix = '') {
+  const found = []
+  for (const entry of readdirSync(directory, { withFileTypes: true })) {
+    const relativePath = prefix === '' ? entry.name : `${prefix}/${entry.name}`
+    if (entry.isDirectory()) {
+      found.push(...listBundleFiles(join(directory, entry.name), relativePath))
+    } else {
+      found.push(relativePath)
+    }
+  }
+  return found
+}
+
+/**
+ * Nothing in the bundle directory may be unaccounted for. An asset dropped from the manifest but
+ * left on disk by an interrupted build ships inside asar, unreachable and unverified, and grows
+ * the installer; content-addressed names mean stale copies never get overwritten.
+ */
+function assertNoUnlistedFiles(bundleDir, manifest) {
+  const listed = new Set(['manifest.json', ...manifest.assets.map((asset) => asset.path)])
+  const strays = listBundleFiles(bundleDir).filter((path) => !listed.has(path))
+  if (strays.length > 0) {
+    throw failure(
+      `${bundleDir} holds ${String(strays.length)} file(s) the manifest does not list: ` +
+        `${strays.sort().join(', ')}. ${REMEDY}`
+    )
+  }
+}
+
+/**
+ * Packaging guard: electron-builder only warns about a missing input, so without this a release
+ * would ship an app that advertises the bundle capability and then errors on every request. The
+ * hash check, not the existence check, is what catches a half-written or stale out/.
+ */
+function assertMobileWebBundleBuilt(bundleDir = MOBILE_WEB_BUNDLE_DIR) {
+  const manifest = parseManifest(bundleDir)
+  assertNoUnlistedFiles(bundleDir, manifest)
+  for (const asset of manifest.assets) {
+    const assetPath = join(bundleDir, asset.path)
+    let size
+    try {
+      size = statSync(assetPath).size
+    } catch (error) {
+      throw failure(
+        `manifest lists ${asset.path}, which is missing from ${bundleDir} (${error.code ?? error.message}). ${REMEDY}`
+      )
+    }
+    if (size !== asset.byteLength) {
+      throw failure(
+        `${asset.path} is ${String(size)} bytes on disk, manifest says ${String(asset.byteLength)}. ${REMEDY}`
+      )
+    }
+    const sha256 = createHash('sha256').update(readFileSync(assetPath)).digest('hex')
+    if (sha256 !== asset.sha256) {
+      throw failure(
+        `${asset.path} hashes to ${sha256} on disk, manifest says ${asset.sha256}. ${REMEDY}`
+      )
+    }
+  }
+  console.log(
+    `[verify-packaged-mobile-web-bundle] OK — buildId ${manifest.buildId}, ` +
+      `${String(manifest.assets.length)} asset(s), ${String(manifest.totalBytes)} bytes`
+  )
+  return manifest
+}
+
+module.exports = { MOBILE_WEB_BUNDLE_DIR, assertMobileWebBundleBuilt }
diff --git a/config/scripts/verify-packaged-mobile-web-bundle.test.mjs b/config/scripts/verify-packaged-mobile-web-bundle.test.mjs
new file mode 100644
index 00000000000..40acb8fb144
--- /dev/null
+++ b/config/scripts/verify-packaged-mobile-web-bundle.test.mjs
@@ -0,0 +1,214 @@
+import { mkdtemp, readFile, readdir, rm, writeFile } from 'node:fs/promises'
+import { createRequire } from 'node:module'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { buildMobileWebBundle } from './build-mobile-web-bundle.mjs'
+
+const require = createRequire(import.meta.url)
+const {
+  MOBILE_WEB_BUNDLE_DIR,
+  assertMobileWebBundleBuilt
+} = require('./verify-packaged-mobile-web-bundle.cjs')
+const electronBuilderConfig = require('../electron-builder.config.cjs')
+const REPO_ROOT = join(import.meta.dirname, '..', '..')
+
+async function withBundle(run) {
+  const scratch = await mkdtemp(join(tmpdir(), 'orca-mobile-web-guard-'))
+  const bundleDir = join(scratch, 'mobile-web')
+  try {
+    const { manifest } = await buildMobileWebBundle({ outDir: bundleDir })
+    await run({ bundleDir, manifest })
+  } finally {
+    await rm(scratch, { recursive: true, force: true })
+  }
+}
+
+async function rewriteManifest(bundleDir, mutate) {
+  const manifestPath = join(bundleDir, 'manifest.json')
+  const manifest = JSON.parse(await readFile(manifestPath, 'utf8'))
+  mutate(manifest)
+  await writeFile(manifestPath, JSON.stringify(manifest, null, 2), 'utf8')
+}
+
+describe('assertMobileWebBundleBuilt', () => {
+  beforeEach(() => {
+    vi.spyOn(console, 'log').mockImplementation(() => {})
+  })
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('accepts a freshly built bundle', async () => {
+    await withBundle(({ bundleDir, manifest }) => {
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).not.toThrow()
+      expect(manifest.entrypoint).toBe('index.html')
+      expect(manifest.assets.length).toBeGreaterThanOrEqual(3)
+      expect(
+        new Set(manifest.assets.map((asset) => asset.contentType)).size
+      ).toBeGreaterThanOrEqual(2)
+    })
+  })
+
+  it('fails on a file the manifest does not list, so no stale asset ships inside asar', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      // An asset dropped from the manifest keeps its content-addressed name, so nothing ever
+      // overwrites it; without this check it packs unreachable and unverified.
+      await writeFile(join(bundleDir, 'assets', 'stale.js'), '// from an earlier build\n', 'utf8')
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(
+        /does not list: assets\/stale\.js/
+      )
+    })
+  })
+
+  it('accepts exactly the manifest, the entrypoint and the listed assets', async () => {
+    await withBundle(async ({ bundleDir, manifest }) => {
+      const onDisk = (await readdir(bundleDir, { recursive: true, withFileTypes: true }))
+        .filter((entry) => entry.isFile())
+        .map((entry) => join(entry.parentPath, entry.name).slice(bundleDir.length + 1))
+      expect(onDisk.toSorted()).toEqual(
+        ['manifest.json', ...manifest.assets.map((asset) => asset.path)].toSorted()
+      )
+    })
+  })
+
+  it('fails when the manifest is missing', async () => {
+    const scratch = await mkdtemp(join(tmpdir(), 'orca-mobile-web-guard-'))
+    try {
+      expect(() => assertMobileWebBundleBuilt(scratch)).toThrow(/no bundle manifest/)
+    } finally {
+      await rm(scratch, { recursive: true, force: true })
+    }
+  })
+
+  it('fails when the manifest is not JSON', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      await writeFile(join(bundleDir, 'manifest.json'), 'not json', 'utf8')
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/not valid JSON/)
+    })
+  })
+
+  it('fails when an asset is tampered with on disk', async () => {
+    await withBundle(async ({ bundleDir, manifest }) => {
+      const asset = manifest.assets.find((entry) => entry.path.endsWith('.js'))
+      const bytes = await readFile(join(bundleDir, asset.path))
+      // Same length, different content: only the hash check can catch this.
+      bytes[bytes.length - 1] = bytes.at(-1) === 0x20 ? 0x09 : 0x20
+      await writeFile(join(bundleDir, asset.path), bytes)
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/hashes to .* on disk/)
+    })
+  })
+
+  it('fails when an asset is truncated', async () => {
+    await withBundle(async ({ bundleDir, manifest }) => {
+      const asset = manifest.assets.find((entry) => entry.path.endsWith('.css'))
+      await writeFile(join(bundleDir, asset.path), 'truncated', 'utf8')
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/bytes on disk, manifest says/)
+    })
+  })
+
+  it('fails when a listed asset was never written', async () => {
+    await withBundle(async ({ bundleDir, manifest }) => {
+      const asset = manifest.assets.find((entry) => entry.path.endsWith('.png'))
+      await rm(join(bundleDir, asset.path))
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/which is missing from/)
+    })
+  })
+
+  it('fails when the manifest buildId no longer matches its asset list', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      await rewriteManifest(bundleDir, (manifest) => {
+        manifest.buildId = 'f'.repeat(64)
+      })
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/does not match its asset list/)
+    })
+  })
+
+  it('fails on an unknown schemaVersion', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      await rewriteManifest(bundleDir, (manifest) => {
+        manifest.schemaVersion = 2
+      })
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(
+        /unsupported manifest schemaVersion/
+      )
+    })
+  })
+
+  it('fails when a required field is dropped', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      await rewriteManifest(bundleDir, (manifest) => {
+        delete manifest.desktopVersion
+      })
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/desktopVersion is missing/)
+    })
+  })
+
+  it('fails when totalBytes disagrees with the asset list', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      await rewriteManifest(bundleDir, (manifest) => {
+        manifest.totalBytes += 1
+      })
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/its assets sum to/)
+    })
+  })
+
+  it('refuses an asset path that escapes the bundle directory', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      await rewriteManifest(bundleDir, (manifest) => {
+        manifest.assets[0].path = '../outside.js'
+      })
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/not a safe relative path/)
+    })
+  })
+
+  it('refuses a manifest whose entrypoint is not one of its assets', async () => {
+    await withBundle(async ({ bundleDir }) => {
+      await rewriteManifest(bundleDir, (manifest) => {
+        manifest.entrypoint = 'index.html'
+        manifest.assets = manifest.assets.filter((asset) => asset.path !== 'index.html')
+      })
+      expect(() => assertMobileWebBundleBuilt(bundleDir)).toThrow(/is not one of its assets/)
+    })
+  })
+})
+
+describe('electron-builder packaging wiring', () => {
+  it('excludes the mobile-web source tree from app.asar', () => {
+    expect(electronBuilderConfig.files).toContain('!src/mobile-web{,/**/*}')
+    // The source tree lives under src/, which is excluded wholesale; the explicit entry above
+    // only survives as a marker, so assert the broad rule is still what does the work.
+    expect(electronBuilderConfig.files).toContain('!src{,/**/*}')
+  })
+
+  it('does not exclude the built bundle, so out/mobile-web ships like out/web', () => {
+    const excludesBuiltBundle = electronBuilderConfig.files.some(
+      (entry) => typeof entry === 'string' && entry.startsWith('!out/mobile-web')
+    )
+    expect(excludesBuiltBundle).toBe(false)
+  })
+
+  it('runs the bundle guard in beforePack', () => {
+    expect(String(electronBuilderConfig.beforePack)).toContain('assertMobileWebBundleBuilt')
+  })
+
+  it('defaults the bundle root to out/mobile-web when electron-builder calls it', () => {
+    expect(MOBILE_WEB_BUNDLE_DIR).toBe(join(REPO_ROOT, 'out', 'mobile-web'))
+    // electron-builder passes the context alone, so the default is what ships.
+    expect(electronBuilderConfig.beforePack.length).toBe(1)
+  })
+
+  it('verifies the bundle root it is given, not the repo one', async () => {
+    // The seam exists so unit tests need no built out/; it would be worthless if the root were
+    // accepted and then ignored.
+    await withBundle(async ({ bundleDir }) => {
+      await rm(join(bundleDir, 'manifest.json'))
+      expect(() =>
+        electronBuilderConfig.beforePack(
+          { electronPlatformName: process.platform, arch: process.arch === 'arm64' ? 3 : 1 },
+          bundleDir
+        )
+      ).toThrow(/no bundle manifest/)
+    })
+  })
+})
diff --git a/config/tsconfig.mobile-web.json b/config/tsconfig.mobile-web.json
new file mode 100644
index 00000000000..93436c7a4b3
--- /dev/null
+++ b/config/tsconfig.mobile-web.json
@@ -0,0 +1,8 @@
+{
+  "extends": "@electron-toolkit/tsconfig/tsconfig.web.json",
+  "include": ["../src/mobile-web/src/**/*"],
+  "compilerOptions": {
+    "composite": true,
+    "types": []
+  }
+}
diff --git a/package.json b/package.json
index 9d37cb13cba..23d1ecf3e34 100644
--- a/package.json
+++ b/package.json
@@ -93,10 +93,11 @@
     "build:electron-vite:parallel": "node config/scripts/run-electron-vite-targets-in-parallel.mjs",
     "build:web": "node config/scripts/run-vite-web-build.mjs && node config/scripts/verify-web-build.mjs",
     "build:web-from-renderer": "node config/scripts/project-renderer-web-client.mjs && node config/scripts/verify-web-build.mjs",
-    "build:desktop": "pnpm run typecheck && pnpm run build:relay && pnpm run build:cli && pnpm run build:electron-vite && pnpm run verify:built-skills-cli && pnpm run build:web-from-renderer",
+    "build:mobile-web": "node config/scripts/build-mobile-web-bundle.mjs && node config/scripts/verify-mobile-web-bundle.mjs",
+    "build:desktop": "pnpm run typecheck && pnpm run build:relay && pnpm run build:cli && pnpm run build:electron-vite && pnpm run verify:built-skills-cli && pnpm run build:web-from-renderer && pnpm run build:mobile-web",
     "build": "pnpm run build:desktop && pnpm run build:native",
-    "build:release": "pnpm run build:relay && pnpm run build:native && pnpm run verify:computer-native && pnpm run build:cli && pnpm run build:electron-vite && pnpm run verify:built-skills-cli && pnpm run build:web-from-renderer",
-    "build:release:parallel": "pnpm run build:relay && pnpm run build:native && pnpm run verify:computer-native && pnpm run build:cli && pnpm run build:electron-vite:parallel && pnpm run verify:built-skills-cli && pnpm run build:web-from-renderer",
+    "build:release": "pnpm run build:relay && pnpm run build:native && pnpm run verify:computer-native && pnpm run build:cli && pnpm run build:electron-vite && pnpm run verify:built-skills-cli && pnpm run build:web-from-renderer && pnpm run build:mobile-web",
+    "build:release:parallel": "pnpm run build:relay && pnpm run build:native && pnpm run verify:computer-native && pnpm run build:cli && pnpm run build:electron-vite:parallel && pnpm run verify:built-skills-cli && pnpm run build:web-from-renderer && pnpm run build:mobile-web",
     "postinstall": "node config/scripts/rebuild-native-deps.mjs",
     "rebuild:electron": "node config/scripts/rebuild-native-deps.mjs",
     "reclaim:electron-dists": "node config/scripts/reclaim-electron-dists.mjs",
diff --git a/src/mobile-web/index.html b/src/mobile-web/index.html
new file mode 100644
index 00000000000..9e91c0e3e0b
--- /dev/null
+++ b/src/mobile-web/index.html
@@ -0,0 +1,17 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />
+    <title>Orca mobile bundle</title>
+    <link rel="stylesheet" href="__ORCA_BOOTSTRAP_CSS__" />
+  </head>
+  <body>
+    <main class="bootstrap" id="bootstrap">
+      <img class="bootstrap__mark" src="__ORCA_MARK_PNG__" width="48" height="48" alt="" />
+      <h1 class="bootstrap__title">Orca mobile bundle</h1>
+      <dl class="bootstrap__facts" id="bootstrap-facts"></dl>
+    </main>
+    <script src="__ORCA_BOOTSTRAP_JS__"></script>
+  </body>
+</html>
diff --git a/src/mobile-web/src/bootstrap.css b/src/mobile-web/src/bootstrap.css
new file mode 100644
index 00000000000..3fc2529db97
--- /dev/null
+++ b/src/mobile-web/src/bootstrap.css
@@ -0,0 +1,54 @@
+:root {
+  color-scheme: dark light;
+  --bootstrap-fg: #e6edf3;
+  --bootstrap-muted: #8b98a5;
+  --bootstrap-bg: #0d1117;
+}
+
+body {
+  margin: 0;
+  background: var(--bootstrap-bg);
+  color: var(--bootstrap-fg);
+  font-family:
+    ui-sans-serif,
+    system-ui,
+    -apple-system,
+    'Segoe UI',
+    sans-serif;
+}
+
+.bootstrap {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  gap: 12px;
+  padding: 24px;
+}
+
+.bootstrap__mark {
+  image-rendering: pixelated;
+}
+
+.bootstrap__title {
+  margin: 0;
+  font-size: 18px;
+  font-weight: 600;
+}
+
+.bootstrap__facts {
+  display: grid;
+  grid-template-columns: max-content 1fr;
+  gap: 4px 12px;
+  margin: 0;
+  font-size: 13px;
+}
+
+.bootstrap__facts dt {
+  color: var(--bootstrap-muted);
+}
+
+.bootstrap__facts dd {
+  margin: 0;
+  overflow-wrap: anywhere;
+  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
+}
diff --git a/src/mobile-web/src/bootstrap.ts b/src/mobile-web/src/bootstrap.ts
new file mode 100644
index 00000000000..805ee5381df
--- /dev/null
+++ b/src/mobile-web/src/bootstrap.ts
@@ -0,0 +1,65 @@
+// Build-time constants, substituted by config/scripts/build-mobile-web-bundle.mjs via esbuild define.
+declare const ORCA_MOBILE_WEB_DESKTOP_VERSION: string
+declare const ORCA_MOBILE_WEB_RUNTIME_PROTOCOL_VERSION: number
+declare const ORCA_MOBILE_WEB_MIN_COMPATIBLE_RUNTIME_PROTOCOL_VERSION: number
+
+// Why a runtime read and not a define: buildId is the hash of the asset list that index.html
+// belongs to, so injecting it into a hashed asset would make the hash depend on itself.
+const MANIFEST_URL = './manifest.json'
+
+function isBuildId(value: unknown): value is string {
+  return typeof value === 'string' && /^[0-9a-f]{64}$/.test(value)
+}
+
+async function readBuildId(): Promise<string> {
+  const response = await fetch(MANIFEST_URL, { cache: 'no-store' })
+  if (!response.ok) {
+    throw new Error(`manifest request failed with ${String(response.status)}`)
+  }
+  const manifest: unknown = await response.json()
+  // `in` narrows without an assertion; the manifest is untrusted JSON either way.
+  if (typeof manifest !== 'object' || manifest === null || !('buildId' in manifest)) {
+    throw new Error('manifest has no buildId')
+  }
+  const { buildId } = manifest
+  if (!isBuildId(buildId)) {
+    throw new Error('manifest buildId is not a sha256 digest')
+  }
+  return buildId
+}
+
+function renderFacts(facts: readonly (readonly [string, string])[]): void {
+  const list = document.getElementById('bootstrap-facts')
+  if (!(list instanceof HTMLDListElement)) {
+    return
+  }
+  list.replaceChildren()
+  for (const [term, description] of facts) {
+    const dt = document.createElement('dt')
+    dt.textContent = term
+    const dd = document.createElement('dd')
+    dd.textContent = description
+    dd.dataset.fact = term
+    list.append(dt, dd)
+  }
+}
+
+async function start(): Promise<void> {
+  let buildId: string
+  try {
+    buildId = await readBuildId()
+  } catch (error) {
+    buildId = `unavailable (${error instanceof Error ? error.message : String(error)})`
+  }
+  renderFacts([
+    ['buildId', buildId],
+    ['desktopVersion', ORCA_MOBILE_WEB_DESKTOP_VERSION],
+    ['runtimeProtocolVersion', String(ORCA_MOBILE_WEB_RUNTIME_PROTOCOL_VERSION)],
+    [
+      'minCompatibleRuntimeProtocolVersion',
+      String(ORCA_MOBILE_WEB_MIN_COMPATIBLE_RUNTIME_PROTOCOL_VERSION)
+    ]
+  ])
+}
+
+void start()
diff --git a/src/mobile-web/src/orca-mark.png b/src/mobile-web/src/orca-mark.png
new file mode 100644
index 00000000000..274fd7bf782
Binary files /dev/null and b/src/mobile-web/src/orca-mark.png differ