diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 8688f3f303..5b143cc89a 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -239,6 +239,11 @@ repos:
       # hooks. Lint/format use the Expo-native toolchain (ESLint flat config +
       # Prettier) rather than web's oxlint/oxfmt: eslint-config-expo gives RN-aware
       # rules and prettier-plugin-tailwindcss sorts NativeWind classes.
+      #
+      # The typecheck/lint/format hooks below each run `bun install --frozen-lockfile`
+      # first: prek runs only on changed files, so a mobile change that doesn't touch
+      # package.json/bun.lock skips the install-check hook, leaving mobile/node_modules
+      # absent in CI (the auto-install hook only fires on post-checkout/merge/rewrite).
       - id: mobile-bun-install
         name: mobile bun install
         description: "Automatically run 'bun install' in mobile/ after a checkout, pull or rebase"
@@ -260,21 +265,21 @@ repos:
       # wired in here.
       - id: mobile-typescript-check
         name: mobile TypeScript type check
-        entry: bash -c 'cd mobile && bunx tsc --noEmit'
+        entry: bash -c 'cd mobile && bun install --frozen-lockfile && bunx tsc --noEmit'
         language: system
         pass_filenames: false
         files: ^mobile/(.*\.(ts|tsx)|package\.json|tsconfig\.json|bun\.lock)$
 
       - id: mobile-lint
         name: mobile lint
-        entry: bash -c 'cd mobile && bunx expo lint'
+        entry: bash -c 'cd mobile && bun install --frozen-lockfile && bunx expo lint'
         language: system
         pass_filenames: false
         files: ^mobile/.*\.(ts|tsx|js|jsx)$
 
       - id: mobile-format
         name: mobile format
-        entry: bash -c 'cd mobile && bunx prettier --write "${@#mobile/}"' _
+        entry: bash -c 'cd mobile && bun install --frozen-lockfile && bunx prettier --write "${@#mobile/}"' _
         language: system
         files: ^mobile/
         types_or: [javascript, jsx, ts, tsx, json, css]
diff --git a/mobile/GETTING_STARTED.md b/mobile/GETTING_STARTED.md
new file mode 100644
index 0000000000..35e57c6273
--- /dev/null
+++ b/mobile/GETTING_STARTED.md
@@ -0,0 +1,69 @@
+# Onyx Mobile — Getting Started (Environment Setup)
+
+One-time setup of the platform tools needed to build Onyx on a Mac. **This doc is setup only** —
+once the tools below are installed, see [`README.md`](./README.md) to install deps and run the app.
+
+> macOS only (iOS needs Xcode). **The JDK and NDK are Android-only** — iOS uses neither.
+
+---
+
+## iOS
+
+1. Install **Xcode** from the **Mac App Store**, then open it once so it finishes installing its
+   components.
+2. **Add a simulator:** Xcode → **Settings → Platforms → iOS Simulator → ＋ Add Additional
+   Simulators** to download a runtime, then **Window → Devices and Simulators → Simulators → ＋**
+   to create one (the default iPhone is usually fine).
+3. RN's iOS build also needs **CocoaPods** — install it if it isn't already (`brew install cocoapods`).
+
+## Android
+
+Android Studio's first run installs the base SDK, and the **build auto-downloads** the rest it
+needs (platform, build-tools, CMake) the first time you build. You only install **four things by
+hand** — a JDK, Android Studio, the NDK, and an emulator.
+
+**1. JDK 17** — install **Eclipse Temurin 17**:
+
+```bash
+brew install --cask temurin@17    # or grab the .pkg installer from https://adoptium.net
+```
+
+> **Why Temurin?** It's the standard, free, vendor-neutral OpenJDK 17 build (Eclipse Adoptium) —
+> no account or license click-through. But **any JDK 17 distribution is fine** (Zulu, Amazon
+> Corretto, Microsoft, or Homebrew's `openjdk@17`) — they're interchangeable; the build only
+> cares that it's version **17**.
+>
+> Why it's a separate step: this is the one piece the build _can't_ fetch for you, and Android
+> Studio's bundled JDK is **21** — which the build rejects. So install **17** yourself. (The
+> build uses it via `JAVA_HOME`; see [`README.md`](./README.md)'s run steps.)
+
+**2. Android Studio** — download from
+**[developer.android.com/studio](https://developer.android.com/studio)** and run the installer.
+Open it once and complete the setup wizard (it installs the SDK to `~/Library/Android/sdk`).
+
+**3. NDK — the exact version** — Android Studio → **Settings → Languages & Frameworks → Android
+SDK → SDK Tools** tab → tick **"Show Package Details"** (bottom-right) → under **NDK (Side by
+side)** check the exact version the build pins, currently **`27.1.12297006`** → **Apply**.
+
+> Use _Show Package Details_ and pick the **exact** version — the plain "NDK (Side by side)"
+> checkbox installs the _latest_, which won't satisfy the build. (The required version is printed
+> in the build log's `ExpoRootProject → ndk:` line and tracks the Expo SDK.)
+
+**4. Create an emulator (AVD)** — Android Studio **Welcome screen → More Actions → Virtual Device
+Manager** (or **View → Tool Windows → Device Manager** inside a project):
+
+1. **Create Device** → pick a recent **Pixel** → **Next**.
+2. **System Image** → click the **⬇** next to a recent Android version to download it → select it
+   → **Next**.
+3. **Verify Configuration → Finish**, then launch it with **▶**.
+
+---
+
+✅ Tools installed. Next → **[`README.md`](./README.md)** to install deps and run the app on the
+simulator / emulator.
+
+### Sources
+
+- [Create & manage Android virtual devices](https://developer.android.com/studio/run/managing-avds)
+- [Expo — Android Studio emulator](https://docs.expo.dev/workflow/android-studio-emulator/)
+- [Apple — install additional Xcode components](https://developer.apple.com/documentation/xcode/downloading-and-installing-additional-xcode-components)
diff --git a/mobile/README.md b/mobile/README.md
index bbf1799870..b642f7673a 100644
--- a/mobile/README.md
+++ b/mobile/README.md
@@ -13,6 +13,9 @@ dependencies, lockfile, and tooling). Scaffolded with `create-expo-app` (Expo Ro
 
 ## Prerequisites
 
+> First-time machine setup (Xcode, Android Studio, JDK 17, NDK, simulator/emulator) is in
+> [`GETTING_STARTED.md`](./GETTING_STARTED.md). The list below is the short version.
+
 - Node LTS on PATH, **Bun**, Xcode + an iOS simulator, CocoaPods.
 - Recommended: `brew install watchman` (faster, more reliable Metro file watching).
 - The project path must contain **no spaces** (RN/Xcode build scripts break otherwise).
@@ -35,7 +38,11 @@ bun run run:ios              # or: bunx expo run:ios --port 8082
 
 > The first iOS build compiles React Native **from source (~15 min)** — RN 0.85 has no prebuilt
 > CocoaPods artifact; this is expected, not a hang. Subsequent builds are incremental.
-> Metro runs on **port 8082** (8081 is commonly taken by Docker). Android: `bun run run:android`.
+> Metro runs on **port 8082** (8081 is commonly taken by Docker).
+>
+> **Android** (`bun run run:android`): the build must compile with **JDK 17** — point `JAVA_HOME`
+> at one first: `export JAVA_HOME=$(/usr/libexec/java_home -v 17)` (Android Studio's bundled
+> JDK 21 won't work). The first Android build compiles native C++ via the NDK (~10–15 min).
 
 ## Scripts
 
diff --git a/mobile/app.json b/mobile/app.json
index afb93c0de0..522f21964d 100644
--- a/mobile/app.json
+++ b/mobile/app.json
@@ -33,6 +33,9 @@
         {
           "ios": {
             "buildReactNativeFromSource": true
+          },
+          "android": {
+            "kotlinVersion": "2.1.20"
           }
         }
       ]