diff --git a/.changeset/fix-tsdown-extension.md b/.changeset/fix-tsdown-extension.md
new file mode 100644
index 00000000..57dca877
--- /dev/null
+++ b/.changeset/fix-tsdown-extension.md
@@ -0,0 +1,6 @@
+---
+"perstack": patch
+"create-expert": patch
+---
+
+Fix .mjs output extension caused by tsdown 0.20 defaulting fixedExtension to true for node platform
diff --git a/tsdown.config.ts b/tsdown.config.ts
index 41e03cfc..5b0026a7 100644
--- a/tsdown.config.ts
+++ b/tsdown.config.ts
@@ -4,6 +4,7 @@ export const baseConfig: UserConfig = {
   cwd: process.cwd(),
   dts: true,
   entry: { "src/index": "src/index.ts" },
+  fixedExtension: false,
   format: "es",
   inlineOnly: false,
   sourcemap: true,