diff --git a/apps/web/package.json b/apps/web/package.json
index 02e1632..b3dce32 100644
--- a/apps/web/package.json
+++ b/apps/web/package.json
@@ -9,7 +9,9 @@
     "lint": "eslint .",
     "test": "playwright test",
     "test:a11y": "playwright test tests/a11y.spec.ts",
-    "test:ui": "playwright test --ui"
+    "test:ui": "playwright test --ui",
+    "check:metadata": "node scripts/check-metadata.mjs",
+    "check": "pnpm run lint && pnpm run build && pnpm run check:metadata"
   },
   "dependencies": {
     "@marsidev/react-turnstile": "^1.4.1",
diff --git a/apps/web/scripts/check-metadata.mjs b/apps/web/scripts/check-metadata.mjs
new file mode 100644
index 0000000..58626ff
--- /dev/null
+++ b/apps/web/scripts/check-metadata.mjs
@@ -0,0 +1,93 @@
+#!/usr/bin/env node
+/**
+ * Assert the social and canonical tags on the pages we actually built.
+ *
+ * Metadata in Next is composed at build time from a parent object and whatever each page
+ * exports, and it merges shallowly: a page that declares its own `openGraph` block replaces the
+ * parent's, image included. That is not visible in a diff, in a type error, or in lint. It is
+ * only visible in the HTML, which is why this reads the HTML.
+ *
+ * Seventeen pages had done it before `pageMetadata()` landed, twelve of them with no
+ * `opengraph-image.tsx` to fall back on, so those shared to Slack and LinkedIn as a bare grey
+ * rectangle for however long it had been true. This is the check that would have said so.
+ *
+ *   node scripts/check-metadata.mjs        # after `pnpm build`
+ */
+import { readdirSync, readFileSync, existsSync, statSync } from "node:fs";
+import { join, relative } from "node:path";
+
+const BUILD = join(process.cwd(), ".next", "server", "app");
+
+/** Every tag a page must carry, and why it is not optional. */
+const REQUIRED = [
+  {
+    name: "og:image",
+    test: /property="og:image"/,
+    why: "a shared link renders as a blank card without it",
+  },
+  {
+    name: "twitter:image",
+    test: /name="twitter:image"/,
+    why: "X and iMessage read this one, not og:image",
+  },
+  {
+    name: "canonical",
+    test: /rel="canonical"/,
+    why: "without it a page competes with its own query-string variants",
+  },
+  {
+    name: "description",
+    test: /name="description"/,
+    why: "the search snippet is otherwise chosen for us",
+  },
+];
+
+function htmlFiles(dir, out = []) {
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    const full = join(dir, entry.name);
+    if (entry.isDirectory()) htmlFiles(full, out);
+    else if (entry.name.endsWith(".html")) out.push(full);
+  }
+  return out;
+}
+
+if (!existsSync(BUILD) || !statSync(BUILD).isDirectory()) {
+  console.error(
+    `check-metadata: no build output at ${relative(process.cwd(), BUILD)}. ` +
+      "This reads the HTML Next actually emitted, so it needs `pnpm build` to have run. " +
+      "Refusing rather than reporting a pass over zero pages.",
+  );
+  process.exit(2);
+}
+
+const pages = htmlFiles(BUILD);
+if (pages.length === 0) {
+  console.error(
+    `check-metadata: ${relative(process.cwd(), BUILD)} exists but contains no .html, so the ` +
+      "build produced nothing to check. Refusing rather than passing vacuously.",
+  );
+  process.exit(2);
+}
+
+let failed = 0;
+for (const file of pages.sort()) {
+  const html = readFileSync(file, "utf8");
+  const route = "/" + relative(BUILD, file).replace(/\.html$/, "").replace(/^index$/, "");
+  for (const { name, test, why } of REQUIRED) {
+    if (!test.test(html)) {
+      console.error(`check-metadata: ${route} has no ${name}: ${why}`);
+      failed += 1;
+    }
+  }
+}
+
+if (failed) {
+  console.error(
+    `\ncheck-metadata: ${failed} missing tag(s) across ${pages.length} built pages. ` +
+      "Build the page's metadata with `pageMetadata()` from src/lib/page-metadata.ts, which " +
+      "fills these in, rather than hand-writing an openGraph block that drops them.",
+  );
+  process.exit(1);
+}
+
+console.log(`check-metadata: ${pages.length} built pages carry og:image, twitter:image, canonical and description`);
diff --git a/package.json b/package.json
index c40bccb..4528b8f 100644
--- a/package.json
+++ b/package.json
@@ -7,7 +7,8 @@
     "start": "turbo run start",
     "lint": "turbo run lint",
     "clean": "turbo run clean && rm -rf node_modules",
-    "format": "prettier --write \"**/*.{ts,tsx,md}\""
+    "format": "prettier --write \"**/*.{ts,tsx,md}\"",
+    "check": "turbo run check"
   },
   "devDependencies": {
     "eslint": "^9.0.0",
@@ -39,4 +40,4 @@
       "fast-uri": "^3.1.2"
     }
   }
-}
\ No newline at end of file
+}
diff --git a/turbo.json b/turbo.json
index 9983fed..6090e0a 100644
--- a/turbo.json
+++ b/turbo.json
@@ -1,14 +1,27 @@
 {
   "$schema": "https://turbo.build/schema.json",
-  "globalDependencies": ["**/.env.*local"],
+  "globalDependencies": [
+    "**/.env.*local"
+  ],
   "tasks": {
     "build": {
-      "dependsOn": ["^build"],
-      "outputs": [".next/**", "!.next/cache/**", "dist/**", ".docusaurus/**"],
-      "env": ["NEXT_PUBLIC_TURNSTILE_SITE_KEY"]
+      "dependsOn": [
+        "^build"
+      ],
+      "outputs": [
+        ".next/**",
+        "!.next/cache/**",
+        "dist/**",
+        ".docusaurus/**"
+      ],
+      "env": [
+        "NEXT_PUBLIC_TURNSTILE_SITE_KEY"
+      ]
     },
     "lint": {
-      "dependsOn": ["^lint"]
+      "dependsOn": [
+        "^lint"
+      ]
     },
     "dev": {
       "cache": false,
@@ -17,6 +30,12 @@
     "start": {
       "cache": false,
       "persistent": true
+    },
+    "check": {
+      "dependsOn": [
+        "^build"
+      ],
+      "cache": false
     }
   }
-}
\ No newline at end of file
+}