diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..de2e361
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,23 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  format:
+    name: Check formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+
+      - run: bun install --frozen-lockfile
+
+      - run: bun run format:check
diff --git a/.gitignore b/.gitignore
index c2658d7..504afef 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 node_modules/
+package-lock.json
diff --git a/bun.lock b/bun.lock
index 1cf2542..8b25f1a 100644
--- a/bun.lock
+++ b/bun.lock
@@ -11,6 +11,7 @@
       "devDependencies": {
         "@types/bun": "1.3.9",
         "@types/react": "19.2.14",
+        "prettier": "3.5.3",
       },
     },
   },
@@ -77,6 +78,8 @@
 
     "patch-console": ["patch-console@2.0.0", "", {}, "sha512-0YNdUceMdaQwoKce1gatDScmMo5pu/tfABfnzEqeG0gtTmd7mh/WcwgUjtAeOU7N8nFFlbQBnFK2gXW5fGvmMA=="],
 
+    "prettier": ["prettier@3.5.3", "", { "bin": { "prettier": "bin/prettier.cjs" } }, "sha512-QQtaxnoDJeAkDvDKWCLiwIXkTgRhwYDEQCghU9Z6q03iyek/rxRh/2lC3HB7P8sWT2xC/y5JDctPLBIGzHKbhw=="],
+
     "pure-rand": ["pure-rand@6.1.0", "", {}, "sha512-bVWawvoZoBYpp6yIoQtQXHZjmz35RSVHnUOTefl8Vcjr8snTPY1wnpSPMWekcFwbxI6gtmT7rSYPFvz71ldiOA=="],
 
     "react": ["react@19.2.4", "", {}, "sha512-9nfp2hYpCwOjAN+8TZFGhtWEwgvWHXqESH8qT89AT/lWklpLON22Lc8pEtnpsZz7VmawabSU0gCjnj8aC0euHQ=="],
diff --git a/docs/adr-001-react-effect.md b/docs/adr-001-react-effect.md
index ee78576..5fc53e2 100644
--- a/docs/adr-001-react-effect.md
+++ b/docs/adr-001-react-effect.md
@@ -64,11 +64,11 @@ Wrap `SubscriptionRef` updates with a Node `EventEmitter`. React subscribes via
 
 The mapping is natural:
 
-| `useSyncExternalStore` contract | Effect primitive |
-|---|---|
-| `subscribe(callback)` | Fork a fiber consuming `ref.changes`, call `callback` on each emission |
-| `getSnapshot()` | `Runtime.runSync(SubscriptionRef.get(ref))` |
-| unsubscribe (returned by subscribe) | `Fiber.interrupt(fiber)` |
+| `useSyncExternalStore` contract     | Effect primitive                                                       |
+| ----------------------------------- | ---------------------------------------------------------------------- |
+| `subscribe(callback)`               | Fork a fiber consuming `ref.changes`, call `callback` on each emission |
+| `getSnapshot()`                     | `Runtime.runSync(SubscriptionRef.get(ref))`                            |
+| unsubscribe (returned by subscribe) | `Fiber.interrupt(fiber)`                                               |
 
 ## Implementation Sketch
 
@@ -89,9 +89,7 @@ export function EffectProvider<R>({
   children: ReactNode;
 }) {
   return (
-    <RuntimeContext.Provider value={runtime as AnyRuntime}>
-      {children}
-    </RuntimeContext.Provider>
+    <RuntimeContext.Provider value={runtime as AnyRuntime}>{children}</RuntimeContext.Provider>
   );
 }
 
@@ -108,17 +106,13 @@ export function useRuntime<R>(): ManagedRuntime.ManagedRuntime<R, never> {
 import { Effect, Fiber, Stream, SubscriptionRef } from "effect";
 import { useSyncExternalStore } from "react";
 
-export function useSubscriptionRef<A>(
-  ref: SubscriptionRef.SubscriptionRef<A>,
-): A {
+export function useSubscriptionRef<A>(ref: SubscriptionRef.SubscriptionRef<A>): A {
   const runtime = useRuntime();
 
   return useSyncExternalStore(
     (onStoreChange) => {
       const fiber = runtime.runFork(
-        ref.changes.pipe(
-          Stream.runForEach(() => Effect.sync(onStoreChange)),
-        ),
+        ref.changes.pipe(Stream.runForEach(() => Effect.sync(onStoreChange))),
       );
       return () => {
         runtime.runFork(Fiber.interrupt(fiber));
@@ -135,24 +129,18 @@ export function useSubscriptionRef<A>(
 // main.ts
 const ShuffleLayer = Layer.succeed(Shuffle, { shuffle: fisherYatesShuffle });
 const runtime = ManagedRuntime.make(ShuffleLayer);
-const gameRef = runtime.runSync(
-  newGame().pipe(Effect.andThen(SubscriptionRef.make)),
-);
+const gameRef = runtime.runSync(newGame().pipe(Effect.andThen(SubscriptionRef.make)));
 
 render(
   <EffectProvider runtime={runtime}>
     <App gameRef={gameRef} />
-  </EffectProvider>
+  </EffectProvider>,
 );
 ```
 
 ```tsx
 // ui.tsx
-export function App({
-  gameRef,
-}: {
-  gameRef: SubscriptionRef.SubscriptionRef<Game>;
-}) {
+export function App({ gameRef }: { gameRef: SubscriptionRef.SubscriptionRef<Game> }) {
   const game = useSubscriptionRef(gameRef); // rerenders automatically
   const runtime = useRuntime();
   const [error, setError] = useState<string | null>(null);
@@ -175,10 +163,10 @@ export function App({
 
 ## State Ownership Boundary
 
-| State | Owner | Rationale |
-|---|---|---|
-| Game (domain) | `SubscriptionRef` in Effect | Single source of truth for engine logic |
-| Phase, error message (UI) | React `useState` | Local to the view, not an engine concern |
+| State                     | Owner                       | Rationale                                |
+| ------------------------- | --------------------------- | ---------------------------------------- |
+| Game (domain)             | `SubscriptionRef` in Effect | Single source of truth for engine logic  |
+| Phase, error message (UI) | React `useState`            | Local to the view, not an engine concern |
 
 This mirrors the Redux convention: shared/domain state in the store, local UI state
 in components.
diff --git a/package.json b/package.json
index 9ee2857..af630ad 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,7 @@
 {
   "scripts": {
-    "start": "bun run src/main.ts"
+    "start": "bun run src/main.ts",
+    "format:check": "prettier --check ."
   },
   "dependencies": {
     "effect": "3.19.19",
@@ -9,6 +10,7 @@
   },
   "devDependencies": {
     "@types/bun": "1.3.9",
-    "@types/react": "19.2.14"
+    "@types/react": "19.2.14",
+    "prettier": "3.5.3"
   }
 }