diff --git a/.git-blame-ignore-revs b/.git-blame-ignore-revs
new file mode 100644
index 0000000..84b6f10
--- /dev/null
+++ b/.git-blame-ignore-revs
@@ -0,0 +1,2 @@
+# Bulk reformats. `git config blame.ignoreRevsFile .git-blame-ignore-revs`
+6e12c84d059bb4f261a25a27a57c44e58f658bca  # prettier, 12 files
diff --git a/.github/dependabot.yml b/.github/dependabot.yml
index a646416..554fa37 100644
--- a/.github/dependabot.yml
+++ b/.github/dependabot.yml
@@ -11,7 +11,7 @@
 version: 2
 updates:
   - package-ecosystem: npm
-    directory: "/"
+    directory: '/'
     schedule:
       interval: weekly
     open-pull-requests-limit: 5
@@ -20,7 +20,7 @@ updates:
         update-types: [minor, patch]
 
   - package-ecosystem: github-actions
-    directory: "/"
+    directory: '/'
     schedule:
       interval: weekly
     open-pull-requests-limit: 3
diff --git a/.prettierignore b/.prettierignore
new file mode 100644
index 0000000..b698663
--- /dev/null
+++ b/.prettierignore
@@ -0,0 +1,31 @@
+# Build output and vendored trees — formatting these is noise.
+node_modules
+.next
+dist
+build
+out
+coverage
+.turbo
+.vercel
+*.min.js
+*.min.css
+
+# Generated during a build, so it is absent locally and present in CI — which
+# makes a clean local --check no evidence at all. Contentlayer's output also
+# uses import assertions, which prettier's parser rejects outright.
+.contentlayer
+.astro
+.svelte-kit
+storybook-static
+test-results
+playwright-report
+
+# Lockfiles are generated; prettier would rewrite them wholesale.
+package-lock.json
+pnpm-lock.yaml
+yarn.lock
+
+# Markdown is deliberately out of scope for now. Prettier rewraps prose, which
+# is where it is most opinionated and least useful, and it would bury the real
+# diff. Remove this line when you want docs formatted too.
+*.md
diff --git a/.prettierrc b/.prettierrc
new file mode 100644
index 0000000..616247a
--- /dev/null
+++ b/.prettierrc
@@ -0,0 +1,9 @@
+{
+  "semi": true,
+  "singleQuote": true,
+  "printWidth": 100,
+  "tabWidth": 2,
+  "trailingComma": "all",
+  "arrowParens": "always",
+  "endOfLine": "lf"
+}
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 25dc93d..428e6f2 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -2,9 +2,9 @@
 // library code, and a bespoke rule set would be a second opinion to maintain
 // for no benefit. The floor is "lint runs and can fail", not "lint encodes
 // taste".
-import js from '@eslint/js'
-import globals from 'globals'
-import tseslint from 'typescript-eslint'
+import js from '@eslint/js';
+import globals from 'globals';
+import tseslint from 'typescript-eslint';
 
 export default tseslint.config(
   {
@@ -22,4 +22,4 @@ export default tseslint.config(
     files: ['test/**/*.js', 'scripts/**/*.{js,mjs}'],
     languageOptions: { globals: { ...globals.node, ...globals.nodeBuiltin } },
   },
-)
+);
diff --git a/package-lock.json b/package-lock.json
index e1e72fb..5fd5a26 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -13,11 +13,12 @@
         "@types/node": "^22.10.2",
         "eslint": "^9.39.5",
         "globals": "^15.15.0",
+        "prettier": "3.9.6",
         "typescript": "^5.8.2",
         "typescript-eslint": "^8.67.0"
       },
       "engines": {
-        "node": ">=18"
+        "node": ">=20"
       }
     },
     "node_modules/@eslint-community/eslint-utils": {
@@ -1308,6 +1309,22 @@
         "node": ">= 0.8.0"
       }
     },
+    "node_modules/prettier": {
+      "version": "3.9.6",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.9.6.tgz",
+      "integrity": "sha512-OpN0zzVdiaiAhxpuuj5efpIS4sY9j7bY6uR5mnj5yPzGkdkjNKSJeUThPb60Jw29QuAZgA4o+/iB49kFiaBX6g==",
+      "dev": true,
+      "license": "MIT",
+      "bin": {
+        "prettier": "bin/prettier.cjs"
+      },
+      "engines": {
+        "node": ">=14"
+      },
+      "funding": {
+        "url": "https://github.com/prettier/prettier?sponsor=1"
+      }
+    },
     "node_modules/punycode": {
       "version": "2.3.1",
       "resolved": "https://registry.npmjs.org/punycode/-/punycode-2.3.1.tgz",
diff --git a/package.json b/package.json
index 1ebec7d..a192faa 100644
--- a/package.json
+++ b/package.json
@@ -44,14 +44,17 @@
     "lint": "eslint .",
     "typecheck": "tsc -p tsconfig.json --noEmit",
     "test": "node --test test/*.test.js",
-    "verify": "npm run lint && npm run typecheck && npm run build && npm test",
-    "prepare": "npm run build"
+    "verify": "npm run format:check && npm run lint && npm run typecheck && npm run build && npm test",
+    "prepare": "npm run build",
+    "format": "prettier --write .",
+    "format:check": "prettier --check ."
   },
   "devDependencies": {
     "@eslint/js": "^9.39.5",
     "@types/node": "^22.10.2",
     "eslint": "^9.39.5",
     "globals": "^15.15.0",
+    "prettier": "3.9.6",
     "typescript": "^5.8.2",
     "typescript-eslint": "^8.67.0"
   }
diff --git a/src/ai.ts b/src/ai.ts
index 03996c6..d75a277 100644
--- a/src/ai.ts
+++ b/src/ai.ts
@@ -67,11 +67,11 @@ export function whenMentioned(aliases: readonly string[]): RespondPolicy {
   // addressed to it.
   const patterns = aliases
     .filter(Boolean)
-    .map(a => new RegExp(`(^|\\W)${a.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')}(\\W|$)`, 'i'));
+    .map((a) => new RegExp(`(^|\\W)${a.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')}(\\W|$)`, 'i'));
   return ({ visible, self }) => {
     const last = visible[visible.length - 1];
     if (!last || last.authorId === self.actorId) return false;
-    return patterns.some(re => re.test(last.body));
+    return patterns.some((re) => re.test(last.body));
   };
 }
 
@@ -84,7 +84,7 @@ export function whenMentioned(aliases: readonly string[]): RespondPolicy {
  * clinical thread unusable the moment an assistant joined.
  */
 export function defaultRespondPolicy(ctx: AiTurnContext): boolean {
-  const active = ctx.thread.participants.filter(p => !p.leftAt);
+  const active = ctx.thread.participants.filter((p) => !p.leftAt);
   if (active.length <= 2) return afterEveryMessage(ctx);
   return whenMentioned([ctx.self.role ?? 'assistant', 'ai'])(ctx);
 }
@@ -98,12 +98,9 @@ function trailingSelfMessages(visible: readonly Message[], actorId: string): num
   return n;
 }
 
-function renderTranscript(
-  ctx: AiTurnContext,
-  label: (p: Participant) => string
-): string {
+function renderTranscript(ctx: AiTurnContext, label: (p: Participant) => string): string {
   return ctx.visible
-    .map(m => {
+    .map((m) => {
       const author = findParticipant(ctx.thread, m.authorId);
       const who = author ? label(author) : 'unknown';
       return `${who}: ${m.body}`;
@@ -124,7 +121,7 @@ function renderTranscript(
 export async function runAiTurn(
   thread: Thread,
   allMessages: readonly Message[],
-  config: AiParticipantConfig
+  config: AiParticipantConfig,
 ): Promise<AiTurnResult> {
   const self = findParticipant(thread, config.actorId);
   if (!self) return { status: 'skipped', reason: 'not a participant in this thread' };
diff --git a/src/index.ts b/src/index.ts
index 584cac4..951f77d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -22,15 +22,5 @@ export { unreadCount, unreadMessages, unreadThreadCount } from './unread.js';
 
 export { compareMessages, isPending, mergeMessages } from './merge.js';
 
-export {
-  afterEveryMessage,
-  defaultRespondPolicy,
-  runAiTurn,
-  whenMentioned,
-} from './ai.js';
-export type {
-  AiParticipantConfig,
-  AiTurnContext,
-  AiTurnResult,
-  RespondPolicy,
-} from './ai.js';
+export { afterEveryMessage, defaultRespondPolicy, runAiTurn, whenMentioned } from './ai.js';
+export type { AiParticipantConfig, AiTurnContext, AiTurnResult, RespondPolicy } from './ai.js';
diff --git a/src/merge.ts b/src/merge.ts
index 1d9ee1f..494ac15 100644
--- a/src/merge.ts
+++ b/src/merge.ts
@@ -26,7 +26,7 @@ export function compareMessages(a: Message, b: Message): number {
  */
 export function mergeMessages(
   existing: readonly Message[],
-  incoming: readonly Message[]
+  incoming: readonly Message[],
 ): Message[] {
   const byId = new Map<string, Message>();
   const idByClientId = new Map<string, string>();
diff --git a/src/participants.ts b/src/participants.ts
index 7f72be9..89aecb7 100644
--- a/src/participants.ts
+++ b/src/participants.ts
@@ -10,11 +10,8 @@ import type { Message, Participant, Thread } from './types.js';
  * thread does not have that failure mode.
  */
 
-export function findParticipant(
-  thread: Thread,
-  actorId: string
-): Participant | undefined {
-  return thread.participants.find(p => p.actorId === actorId);
+export function findParticipant(thread: Thread, actorId: string): Participant | undefined {
+  return thread.participants.find((p) => p.actorId === actorId);
 }
 
 /** Has this actor ever been part of the thread? Necessary, never sufficient — see `canSeeMessage`. */
@@ -63,11 +60,11 @@ export function canSeeMessage(p: Participant, message: Message): boolean {
 export function visibleMessages(
   thread: Thread,
   actorId: string,
-  messages: readonly Message[]
+  messages: readonly Message[],
 ): Message[] {
   const p = findParticipant(thread, actorId);
   if (!p) return [];
-  return messages.filter(m => m.threadId === thread.id && canSeeMessage(p, m));
+  return messages.filter((m) => m.threadId === thread.id && canSeeMessage(p, m));
 }
 
 /**
@@ -78,7 +75,7 @@ export function visibleMessages(
  * access that does not exist.
  */
 export function readersOf(thread: Thread, message: Message): Participant[] {
-  return thread.participants.filter(p => {
+  return thread.participants.filter((p) => {
     if (p.actorId === message.authorId) return false;
     if (!canSeeMessage(p, message)) return false;
     if (!p.lastReadAt) return false;
diff --git a/src/unread.ts b/src/unread.ts
index 13d03ef..9673885 100644
--- a/src/unread.ts
+++ b/src/unread.ts
@@ -15,12 +15,12 @@ import { findParticipant, canSeeMessage } from './participants.js';
 export function unreadMessages(
   thread: Thread,
   actorId: string,
-  messages: readonly Message[]
+  messages: readonly Message[],
 ): Message[] {
   const p = findParticipant(thread, actorId);
   if (!p) return [];
   const since = p.lastReadAt?.getTime() ?? null;
-  return messages.filter(m => {
+  return messages.filter((m) => {
     if (m.threadId !== thread.id) return false;
     if (m.authorId === actorId) return false;
     if (!canSeeMessage(p, m)) return false;
@@ -30,11 +30,7 @@ export function unreadMessages(
   });
 }
 
-export function unreadCount(
-  thread: Thread,
-  actorId: string,
-  messages: readonly Message[]
-): number {
+export function unreadCount(thread: Thread, actorId: string, messages: readonly Message[]): number {
   return unreadMessages(thread, actorId, messages).length;
 }
 
@@ -46,7 +42,7 @@ export function unreadCount(
  */
 export function unreadThreadCount(
   entries: readonly { thread: Thread; messages: readonly Message[] }[],
-  actorId: string
+  actorId: string,
 ): number {
-  return entries.filter(e => unreadCount(e.thread, actorId, e.messages) > 0).length;
+  return entries.filter((e) => unreadCount(e.thread, actorId, e.messages) > 0).length;
 }
diff --git a/test/ai.test.js b/test/ai.test.js
index 95d7e7c..d0d40b8 100644
--- a/test/ai.test.js
+++ b/test/ai.test.js
@@ -18,7 +18,7 @@ function msg(id, authorId, createdAt, body = 'hello') {
 /** Records what the model was shown, so the test can assert on the prompt itself. */
 function spyComplete(reply = 'a reply') {
   const seen = [];
-  const complete = async input => {
+  const complete = async (input) => {
     seen.push(input);
     return reply;
   };
@@ -47,7 +47,7 @@ test('the AI sees only what it was granted, not the whole transcript', async ()
   assert.equal(seen.length, 1);
   assert.ok(
     !seen[0].prompt.includes('private history'),
-    'a model added later must not be handed the history it was not granted'
+    'a model added later must not be handed the history it was not granted',
   );
   assert.ok(seen[0].prompt.includes('summarise my sleep'));
 });
diff --git a/test/authorization.test.js b/test/authorization.test.js
index f85bc44..69f3d10 100644
--- a/test/authorization.test.js
+++ b/test/authorization.test.js
@@ -1,13 +1,7 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 
-import {
-  canRead,
-  canWrite,
-  readersOf,
-  unreadCount,
-  visibleMessages,
-} from '../dist/index.js';
+import { canRead, canWrite, readersOf, unreadCount, visibleMessages } from '../dist/index.js';
 
 const T0 = new Date('2026-01-01T09:00:00Z');
 const T1 = new Date('2026-01-01T10:00:00Z');
@@ -40,7 +34,7 @@ test('a clinician added later does not see what was said before they joined', ()
   const t = thread([patient, drA, drB]);
   const messages = [msg('m1', 'patient-1', T1), msg('m2', 'dr-a', T3)];
 
-  const seen = visibleMessages(t, 'dr-b', messages).map(m => m.id);
+  const seen = visibleMessages(t, 'dr-b', messages).map((m) => m.id);
   assert.deepEqual(seen, ['m2'], 'history before joining is not granted by default');
 });
 
@@ -55,7 +49,7 @@ test('granting thread-start hands over the whole history, on purpose', () => {
   const t = thread([patient, drA, drB]);
   const messages = [msg('m1', 'patient-1', T1), msg('m2', 'dr-a', T3)];
 
-  const seen = visibleMessages(t, 'dr-b', messages).map(m => m.id);
+  const seen = visibleMessages(t, 'dr-b', messages).map((m) => m.id);
   assert.deepEqual(seen, ['m1', 'm2']);
 });
 
@@ -70,7 +64,10 @@ test('someone who left keeps their history and stops receiving what comes next',
   const t = thread([patient, locum]);
   const messages = [msg('m1', 'patient-1', T1), msg('m2', 'patient-1', T3)];
 
-  assert.deepEqual(visibleMessages(t, 'locum', messages).map(m => m.id), ['m1']);
+  assert.deepEqual(
+    visibleMessages(t, 'locum', messages).map((m) => m.id),
+    ['m1'],
+  );
   assert.equal(canWrite(t, 'locum', T3), false, 'a departed participant loses their voice');
   assert.equal(canRead(t, 'locum'), true, 'but not what they already saw');
 });
@@ -112,5 +109,8 @@ test('a read receipt never implies access the reader does not have', () => {
   const early = msg('m1', 'patient-1', T1);
 
   // dr-b's clock says they have read up to T3, but m1 predates their window.
-  assert.deepEqual(readersOf(t, early).map(p => p.actorId), []);
+  assert.deepEqual(
+    readersOf(t, early).map((p) => p.actorId),
+    [],
+  );
 });
diff --git a/test/merge.test.js b/test/merge.test.js
index a07abab..fe7bc66 100644
--- a/test/merge.test.js
+++ b/test/merge.test.js
@@ -41,8 +41,8 @@ test('messages sharing a timestamp keep a stable order', () => {
   const a = { id: 'b', threadId: 't1', authorId: 'x', body: 'a', createdAt: T1 };
   const b = { id: 'a', threadId: 't1', authorId: 'x', body: 'b', createdAt: T1 };
 
-  const once = mergeMessages([a, b], []).map(m => m.id);
-  const twice = mergeMessages([b, a], []).map(m => m.id);
+  const once = mergeMessages([a, b], []).map((m) => m.id);
+  const twice = mergeMessages([b, a], []).map((m) => m.id);
   assert.deepEqual(once, twice, 'insertion order must not change what the user sees');
   assert.deepEqual(once, ['a', 'b']);
 });
@@ -51,7 +51,10 @@ test('merging is ordered by time, not by arrival', () => {
   const late = { id: 'm2', threadId: 't1', authorId: 'x', body: 'second', createdAt: T2 };
   const early = { id: 'm1', threadId: 't1', authorId: 'x', body: 'first', createdAt: T1 };
 
-  assert.deepEqual(mergeMessages([late], [early]).map(m => m.id), ['m1', 'm2']);
+  assert.deepEqual(
+    mergeMessages([late], [early]).map((m) => m.id),
+    ['m1', 'm2'],
+  );
 });
 
 test('a pending message is distinguishable from a confirmed one', () => {
diff --git a/test/package.test.js b/test/package.test.js
index 1f3a122..d2a998b 100644
--- a/test/package.test.js
+++ b/test/package.test.js
@@ -52,7 +52,7 @@ before(() => {
     cwd: process.cwd(),
     stdio: ['ignore', 'ignore', 'inherit'],
   });
-  const tarball = readdirSync(workspace).find(f => f.endsWith('.tgz'));
+  const tarball = readdirSync(workspace).find((f) => f.endsWith('.tgz'));
   assert.ok(tarball, 'npm pack produced no tarball');
 
   execFileSync('tar', ['-xzf', join(workspace, tarball), '-C', installed, '--strip-components=1']);
@@ -67,14 +67,14 @@ before(() => {
       'try { out.resolved = import.meta.resolve("threadkit"); } catch { out.resolved = null; }',
       'try { out.exports = Object.keys(await import("threadkit")).sort(); } catch { out.exports = null; }',
       'console.log(JSON.stringify(out));',
-    ].join('\n')
+    ].join('\n'),
   );
 
   probe = JSON.parse(
     execFileSync('node', [join(workspace, 'probe.mjs')], {
       cwd: workspace,
       encoding: 'utf8',
-    })
+    }),
   );
 });
 
@@ -94,12 +94,12 @@ test('the type declarations it advertises are actually in the tarball', () => {
     execFileSync('node', ['-p', 'JSON.stringify(require("./package.json"))'], {
       cwd: installed,
       encoding: 'utf8',
-    })
+    }),
   );
   const types = pkg.exports['.'].types;
   assert.ok(
     existsSync(join(installed, types)),
-    `the package advertises types at ${types}, which is not in the tarball`
+    `the package advertises types at ${types}, which is not in the tarball`,
   );
 });
 
diff --git a/test/unread.test.js b/test/unread.test.js
index fa1d78e..ebfb84e 100644
--- a/test/unread.test.js
+++ b/test/unread.test.js
@@ -41,8 +41,8 @@ test('a participant who has never read owes every visible message from someone e
   const messages = [msg('m1', 'patient-1', T1), msg('m2', 'patient-1', T2)];
 
   assert.deepEqual(
-    unreadMessages(thread, 'dr-a', messages).map(m => m.id),
-    ['m1', 'm2']
+    unreadMessages(thread, 'dr-a', messages).map((m) => m.id),
+    ['m1', 'm2'],
   );
 });
 
@@ -55,14 +55,14 @@ test('unread never reaches back past the day you joined', () => {
   assert.equal(
     unreadCount(thread, 'dr-b', messages),
     0,
-    'a message from before dr-b joined is not theirs to owe'
+    'a message from before dr-b joined is not theirs to owe',
   );
 });
 
 test('the nav badge counts threads that want you, not messages to read', () => {
   const me = { actorId: 'me', kind: 'human', joinedAt: T0, lastReadAt: null };
   const other = { actorId: 'other', kind: 'human', joinedAt: T0 };
-  const mk = id => ({ id, participants: [me, other], createdAt: T0 });
+  const mk = (id) => ({ id, participants: [me, other], createdAt: T0 });
 
   const entries = [
     {
@@ -75,5 +75,9 @@ test('the nav badge counts threads that want you, not messages to read', () => {
     { thread: mk('t2'), messages: [] },
   ];
 
-  assert.equal(unreadThreadCount(entries, 'me'), 1, 'two unread messages in one thread is one badge');
+  assert.equal(
+    unreadThreadCount(entries, 'me'),
+    1,
+    'two unread messages in one thread is one badge',
+  );
 });