diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index 97590bd251a..f791a59c63e 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -53,6 +53,15 @@ pnpm install
 pnpm dev
 ```
 
+`mobile/` is a separate pnpm project with its own lockfile, so the root `pnpm install` leaves its
+dependencies uninstalled. `pnpm build` (`build:mobile-web` fails without them) and the lint
+passes that cover `mobile/` need them. Install them before building or linting, and again
+when `mobile/pnpm-lock.yaml` changes:
+
+```bash
+pnpm -C mobile install --frozen-lockfile
+```
+
 ### Dev Profiles and Parallel Instances
 
 `pnpm dev` in a linked Git worktree uses its own userData profile, renderer port, and remote
@@ -170,7 +179,7 @@ Avoid vague names like `test`, `misc`, or `changes`.
 
 ## Before Opening a PR
 
-Run the same checks that CI runs:
+Run the same checks that CI runs, after the `mobile/` install from [Local Setup](#local-setup):
 
 ```bash
 pnpm lint
diff --git a/config/scripts/build-mobile-web-app-bundle.mjs b/config/scripts/build-mobile-web-app-bundle.mjs
index 4f2d0a32b17..d7ec945614d 100644
--- a/config/scripts/build-mobile-web-app-bundle.mjs
+++ b/config/scripts/build-mobile-web-app-bundle.mjs
@@ -1,6 +1,7 @@
 import { readFile } from 'node:fs/promises'
 import { readRouteSnapshot } from './mobile-web-app/mobile-web-app-route-snapshot.mjs'
-import { realpathSync } from 'node:fs'
+import { MOBILE_WEB_APP_DEPENDENCY_PATH } from './mobile-web-app/mobile-web-app-bundle-dependencies.mjs'
+import { existsSync, realpathSync } from 'node:fs'
 import { basename, extname, join, resolve } from 'node:path'
 import { createRequire } from 'node:module'
 import { fileURLToPath } from 'node:url'
@@ -503,6 +504,12 @@ export async function buildMobileWebAppBundle({
 }
 
 if (isDirectInvocation(import.meta.url, process.argv[1])) {
+  if (!existsSync(MOBILE_WEB_APP_DEPENDENCY_PATH)) {
+    console.error(
+      '[build-mobile-web-app-bundle] mobile/ dependencies are not installed; run `pnpm -C mobile install --frozen-lockfile`'
+    )
+    process.exit(1)
+  }
   try {
     const { manifest, outDir, routeKeys, entryStaticBytes, chunkCount } =
       await buildMobileWebAppBundle()
diff --git a/config/scripts/mobile-web-app/mobile-web-app-bundle-dependencies.mjs b/config/scripts/mobile-web-app/mobile-web-app-bundle-dependencies.mjs
index 36465dbfa1b..4d6dd232805 100644
--- a/config/scripts/mobile-web-app/mobile-web-app-bundle-dependencies.mjs
+++ b/config/scripts/mobile-web-app/mobile-web-app-bundle-dependencies.mjs
@@ -7,14 +7,19 @@ const projectDir = fileURLToPath(new URL('../../..', import.meta.url))
 // Mobile CI requires dependencies so broken installs fail instead of silently skipping their tests.
 export const MOBILE_WEB_APP_DEPENDENCIES_REQUIRED_ENV = 'ORCA_MOBILE_WEB_APP_DEPS_REQUIRED'
 
+export const MOBILE_WEB_APP_DEPENDENCY_PATH = join(
+  projectDir,
+  'mobile',
+  'node_modules',
+  'react-native-web'
+)
+
 const SKIP_NOTICE =
   '[mobile-web-app] skipping the bundling tests: mobile/node_modules/react-native-web is absent. ' +
   'They run for real in pr.yml, in the mobile_web_app job, which installs mobile dependencies.'
 
 // Sharded root tests omit mobile/node_modules; check availability before bundling react-native-web pages.
-export function mobileWebAppDependenciesPresent(
-  modulePath = join(projectDir, 'mobile', 'node_modules', 'react-native-web')
-) {
+export function mobileWebAppDependenciesPresent(modulePath = MOBILE_WEB_APP_DEPENDENCY_PATH) {
   if (existsSync(modulePath)) {
     return true
   }