diff --git a/docs/social-preview/generate.mjs b/docs/social-preview/generate.ts
similarity index 96%
rename from docs/social-preview/generate.mjs
rename to docs/social-preview/generate.ts
index 7b021ca..e434d62 100644
--- a/docs/social-preview/generate.mjs
+++ b/docs/social-preview/generate.ts
@@ -1,7 +1,7 @@
 // Renders social-preview.html to docs/social-preview.png (exactly 1280x640).
 // Renders at 2x (2560x1280) first, then downsamples via Chromium's high-quality
 // image smoothing, so text and the embedded screenshot stay sharp.
-// Usage: node docs/social-preview/generate.mjs (or: npm run social-preview)
+// Usage: node docs/social-preview/generate.ts (or: npm run social-preview)
 import { chromium } from 'playwright-core';
 import { homedir } from 'node:os';
 import { fileURLToPath, pathToFileURL } from 'node:url';
diff --git a/package.json b/package.json
index 404e145..d1bda28 100644
--- a/package.json
+++ b/package.json
@@ -40,7 +40,7 @@
     "test": "npm run typecheck && vitest run",
     "release": "bash scripts/publish.sh",
     "hooks": "bash scripts/install-hooks.sh",
-    "social-preview": "node docs/social-preview/generate.mjs"
+    "social-preview": "node docs/social-preview/generate.ts"
   },
   "dsh": {
     "bundle": {
diff --git a/tests/assistant-blocks.spec.mjs b/tests/assistant-blocks.spec.ts
similarity index 99%
rename from tests/assistant-blocks.spec.mjs
rename to tests/assistant-blocks.spec.ts
index ba52b83..98b25cb 100644
--- a/tests/assistant-blocks.spec.mjs
+++ b/tests/assistant-blocks.spec.ts
@@ -9,7 +9,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { renderView, snapshot } = bed
diff --git a/tests/browser-autoload.spec.mjs b/tests/browser-autoload.spec.ts
similarity index 97%
rename from tests/browser-autoload.spec.mjs
rename to tests/browser-autoload.spec.ts
index 1bdb563..a432c8a 100644
--- a/tests/browser-autoload.spec.mjs
+++ b/tests/browser-autoload.spec.ts
@@ -5,7 +5,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { renderView, snapshot } = bed
diff --git a/tests/browser.spec.mjs b/tests/browser.spec.ts
similarity index 99%
rename from tests/browser.spec.mjs
rename to tests/browser.spec.ts
index 95e94d3..1a165de 100644
--- a/tests/browser.spec.mjs
+++ b/tests/browser.spec.ts
@@ -6,7 +6,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, catRowOf, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, catRowOf, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { hookStates, renderView, snapshot } = bed
diff --git a/tests/chart.spec.mjs b/tests/chart.spec.ts
similarity index 99%
rename from tests/chart.spec.mjs
rename to tests/chart.spec.ts
index d9ba5db..94ef490 100644
--- a/tests/chart.spec.mjs
+++ b/tests/chart.spec.ts
@@ -7,7 +7,7 @@
 import assert from 'node:assert/strict'
 import { readFile } from 'node:fs/promises'
 import { test } from 'vitest'
-import { bootViewBed, byClass, plainText, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, plainText, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { hookStates, renderView, snapshot } = bed
diff --git a/tests/client.spec.mjs b/tests/client.spec.ts
similarity index 99%
rename from tests/client.spec.mjs
rename to tests/client.spec.ts
index fe490c5..e704d33 100644
--- a/tests/client.spec.mjs
+++ b/tests/client.spec.ts
@@ -8,7 +8,7 @@
 import assert from 'node:assert/strict'
 import { readFile } from 'node:fs/promises'
 import { test } from 'vitest'
-import { bootViewBed, FAKE_PRIMITIVES, makeFakeDoc } from './helpers/viewBed.mjs'
+import { bootViewBed, FAKE_PRIMITIVES, makeFakeDoc } from './helpers/viewBed'
 
 const bundle = await readFile(new URL('../lib/client.js', import.meta.url), 'utf8')
 
diff --git a/tests/composition-link.spec.mjs b/tests/composition-link.spec.ts
similarity index 99%
rename from tests/composition-link.spec.mjs
rename to tests/composition-link.spec.ts
index c69651f..3db838e 100644
--- a/tests/composition-link.spec.mjs
+++ b/tests/composition-link.spec.ts
@@ -5,7 +5,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, catRowOf, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, catRowOf, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { renderView, snapshot } = bed
diff --git a/tests/diff-fold.mjs b/tests/diff-fold.ts
similarity index 97%
rename from tests/diff-fold.mjs
rename to tests/diff-fold.ts
index 106bc27..1b13c2a 100644
--- a/tests/diff-fold.mjs
+++ b/tests/diff-fold.ts
@@ -25,7 +25,7 @@
  * canonicalHeader is identity — stored headers are already canonical).
  *
  * Usage:
- *   bun tests/diff-fold.mjs <session.jsonl> [more.jsonl...]
+ *   bun tests/diff-fold.ts <session.jsonl> [more.jsonl...]
  *   zstd -dc ~/.dsh/sessions/<ws>/<sid>/session.jsonl.zstd > /tmp/s.jsonl
  *
  * Exit code 0 when every log matches, 1 otherwise.
@@ -89,7 +89,7 @@ function imageDeltaOf(blocks) {
 
 const files = process.argv.slice(2)
 if (files.length === 0) {
-  console.error('usage: bun tests/diff-fold.mjs <session.jsonl> [...]')
+  console.error('usage: bun tests/diff-fold.ts <session.jsonl> [...]')
   process.exit(2)
 }
 
diff --git a/tests/helpers/viewBed.mjs b/tests/helpers/viewBed.ts
similarity index 100%
rename from tests/helpers/viewBed.mjs
rename to tests/helpers/viewBed.ts
diff --git a/tests/hmr-safety.client.spec.mjs b/tests/hmr-safety.client.spec.ts
similarity index 98%
rename from tests/hmr-safety.client.spec.mjs
rename to tests/hmr-safety.client.spec.ts
index 79cd693..bd16cad 100644
--- a/tests/hmr-safety.client.spec.mjs
+++ b/tests/hmr-safety.client.spec.ts
@@ -11,7 +11,7 @@ import assert from 'node:assert/strict'
 import { readFile } from 'node:fs/promises'
 import { join } from 'node:path'
 import { test } from 'vitest'
-import { FAKE_PRIMITIVES } from './helpers/viewBed.mjs'
+import { FAKE_PRIMITIVES } from './helpers/viewBed'
 
 test('client HMR safety: fiber dispose removes every registration', async () => {
   const bundle = await readFile(join(process.cwd(), 'lib/client.js'), 'utf8')
diff --git a/tests/host.spec.mjs b/tests/host.spec.ts
similarity index 100%
rename from tests/host.spec.mjs
rename to tests/host.spec.ts
diff --git a/tests/image-tokens.spec.mjs b/tests/image-tokens.spec.ts
similarity index 100%
rename from tests/image-tokens.spec.mjs
rename to tests/image-tokens.spec.ts
diff --git a/tests/images.spec.mjs b/tests/images.spec.ts
similarity index 98%
rename from tests/images.spec.mjs
rename to tests/images.spec.ts
index c33301f..5f2fa44 100644
--- a/tests/images.spec.mjs
+++ b/tests/images.spec.ts
@@ -5,7 +5,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { hookStates, renderView, snapshot } = bed
diff --git a/tests/linkage.spec.mjs b/tests/linkage.spec.ts
similarity index 97%
rename from tests/linkage.spec.mjs
rename to tests/linkage.spec.ts
index f31a393..77d9900 100644
--- a/tests/linkage.spec.mjs
+++ b/tests/linkage.spec.ts
@@ -5,7 +5,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { renderView, snapshot } = bed
diff --git a/tests/modal.spec.mjs b/tests/modal.spec.ts
similarity index 98%
rename from tests/modal.spec.mjs
rename to tests/modal.spec.ts
index fd71a2c..04980df 100644
--- a/tests/modal.spec.mjs
+++ b/tests/modal.spec.ts
@@ -5,7 +5,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { bailCalls, evaluate, modalComponent, modalSource, snapshot } = bed
diff --git a/tests/multimodal.spec.mjs b/tests/multimodal.spec.ts
similarity index 100%
rename from tests/multimodal.spec.mjs
rename to tests/multimodal.spec.ts
diff --git a/tests/real-react.client.spec.mjs b/tests/real-react.client.spec.ts
similarity index 100%
rename from tests/real-react.client.spec.mjs
rename to tests/real-react.client.spec.ts
diff --git a/tests/resilience.spec.mjs b/tests/resilience.spec.ts
similarity index 98%
rename from tests/resilience.spec.mjs
rename to tests/resilience.spec.ts
index 7a0ddad..b3ba62c 100644
--- a/tests/resilience.spec.mjs
+++ b/tests/resilience.spec.ts
@@ -6,7 +6,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { classInstances, evaluate, modalComponent, renderView, snapshot, viewComponent } = bed
diff --git a/tests/tool-chip-bridge.spec.mjs b/tests/tool-chip-bridge.spec.ts
similarity index 98%
rename from tests/tool-chip-bridge.spec.mjs
rename to tests/tool-chip-bridge.spec.ts
index a4bc617..b90ba60 100644
--- a/tests/tool-chip-bridge.spec.mjs
+++ b/tests/tool-chip-bridge.spec.ts
@@ -5,7 +5,7 @@
  */
 import assert from 'node:assert/strict'
 import { test } from 'vitest'
-import { bootViewBed, byClass, textOf } from './helpers/viewBed.mjs'
+import { bootViewBed, byClass, textOf } from './helpers/viewBed'
 
 const bed = await bootViewBed()
 const { renderView, snapshot } = bed
diff --git a/vitest.config.mjs b/vitest.config.mjs
deleted file mode 100644
index 2eab4b0..0000000
--- a/vitest.config.mjs
+++ /dev/null
@@ -1,10 +0,0 @@
-import { defineConfig } from 'vitest/config'
-
-export default defineConfig({
-  test: {
-    // Specs exercise the BUILT artifacts (lib/), so `pnpm run build` first.
-    // `*.client.spec.mjs` is the browser lane (jsdom pragma per file); the
-    // plain `*.spec.mjs` files run in node with their own fakes.
-    include: ['tests/**/*.spec.mjs'],
-  },
-})
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 0000000..b1c5120
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    // Specs exercise the BUILT artifacts (lib/), so `pnpm run build` first.
+    // `*.client.spec.ts` is the browser lane (jsdom pragma per file); the
+    // plain `*.spec.ts` files run in node with their own fakes.
+    include: ['tests/**/*.spec.ts'],
+  },
+})