diff --git a/packages/client/package.json b/packages/client/package.json
index caa6be2..d1ad262 100644
--- a/packages/client/package.json
+++ b/packages/client/package.json
@@ -50,7 +50,7 @@
   "scripts": {
     "build": "tsup",
     "prepack": "node ../../scripts/assert-pnpm.mjs",
-    "prepublishOnly": "pnpm build",
+    "prepublishOnly": "node ../../scripts/assert-built.mjs",
     "test": "vitest run",
     "typecheck": "tsc --noEmit"
   },
diff --git a/packages/protocol/package.json b/packages/protocol/package.json
index 783a92a..9cfc95d 100644
--- a/packages/protocol/package.json
+++ b/packages/protocol/package.json
@@ -48,7 +48,7 @@
   "scripts": {
     "build": "tsup",
     "prepack": "node ../../scripts/assert-pnpm.mjs",
-    "prepublishOnly": "pnpm build",
+    "prepublishOnly": "node ../../scripts/assert-built.mjs",
     "typecheck": "tsc --noEmit"
   },
   "devDependencies": {
diff --git a/packages/react/package.json b/packages/react/package.json
index 2dfb2d0..4c175d8 100644
--- a/packages/react/package.json
+++ b/packages/react/package.json
@@ -49,7 +49,7 @@
   "scripts": {
     "build": "tsup",
     "prepack": "node ../../scripts/assert-pnpm.mjs",
-    "prepublishOnly": "pnpm build",
+    "prepublishOnly": "node ../../scripts/assert-built.mjs",
     "test": "vitest run",
     "typecheck": "tsc --noEmit"
   },
diff --git a/packages/widget-embed/package.json b/packages/widget-embed/package.json
index 7522768..b783ca9 100644
--- a/packages/widget-embed/package.json
+++ b/packages/widget-embed/package.json
@@ -44,7 +44,7 @@
   "scripts": {
     "build": "tsup",
     "prepack": "node ../../scripts/assert-pnpm.mjs",
-    "prepublishOnly": "pnpm build",
+    "prepublishOnly": "node ../../scripts/assert-built.mjs",
     "test": "echo \"covered by @fishaudio/agent-widget\" && exit 0",
     "typecheck": "tsc --noEmit"
   },
diff --git a/packages/widget/package.json b/packages/widget/package.json
index 58a3355..83840b0 100644
--- a/packages/widget/package.json
+++ b/packages/widget/package.json
@@ -67,7 +67,7 @@
   "scripts": {
     "build": "tsup",
     "prepack": "node ../../scripts/assert-pnpm.mjs",
-    "prepublishOnly": "pnpm build",
+    "prepublishOnly": "node ../../scripts/assert-built.mjs",
     "test": "vitest run",
     "typecheck": "tsc --noEmit",
     "demo": "vite -c demo/vite.config.ts"
diff --git a/scripts/assert-built.mjs b/scripts/assert-built.mjs
new file mode 100644
index 0000000..3a891b7
--- /dev/null
+++ b/scripts/assert-built.mjs
@@ -0,0 +1,21 @@
+// prepublishOnly guard: the package must already be built.
+//
+// Publishing used to rebuild each package, but `changeset publish` publishes
+// packages concurrently and every build starts by emptying its dist/, so a
+// sibling's types build could find a dependency's dist/ half-written. The
+// release script builds everything once up front; this only checks that it
+// happened.
+import { existsSync } from "node:fs";
+import { join } from "node:path";
+
+const manifest = JSON.parse(
+  await import("node:fs/promises").then((fs) => fs.readFile("package.json", "utf8")),
+);
+const entry = manifest.main ?? manifest.module ?? manifest.types;
+if (!entry || !existsSync(join(process.cwd(), entry))) {
+  console.error(
+    `${manifest.name}: ${entry ?? "dist/"} is missing. Run \`pnpm build\` at the repo root ` +
+      "(or `pnpm release`, which builds first) before publishing.",
+  );
+  process.exit(1);
+}