diff --git a/actions/setup/js/error_codes.test.cjs b/actions/setup/js/error_codes.test.cjs
new file mode 100644
index 00000000000..fbd589095e2
--- /dev/null
+++ b/actions/setup/js/error_codes.test.cjs
@@ -0,0 +1,90 @@
+// @ts-check
+import { describe, it, expect } from "vitest";
+const { ERR_VALIDATION, ERR_PERMISSION, ERR_API, ERR_CONFIG, ERR_NOT_FOUND, ERR_PARSE, ERR_SYSTEM, SAFE_OUTPUT_E001, SAFE_OUTPUT_E099 } = require("./error_codes.cjs");
+
+describe("error_codes", () => {
+  describe("module exports", () => {
+    it("exports all expected error codes", () => {
+      expect(ERR_VALIDATION).toBeDefined();
+      expect(ERR_PERMISSION).toBeDefined();
+      expect(ERR_API).toBeDefined();
+      expect(ERR_CONFIG).toBeDefined();
+      expect(ERR_NOT_FOUND).toBeDefined();
+      expect(ERR_PARSE).toBeDefined();
+      expect(ERR_SYSTEM).toBeDefined();
+      expect(SAFE_OUTPUT_E001).toBeDefined();
+      expect(SAFE_OUTPUT_E099).toBeDefined();
+    });
+
+    it("exports string values only", () => {
+      const codes = [ERR_VALIDATION, ERR_PERMISSION, ERR_API, ERR_CONFIG, ERR_NOT_FOUND, ERR_PARSE, ERR_SYSTEM, SAFE_OUTPUT_E001, SAFE_OUTPUT_E099];
+      for (const code of codes) {
+        expect(typeof code).toBe("string");
+      }
+    });
+  });
+
+  describe("primary error codes", () => {
+    it("ERR_VALIDATION is 'ERR_VALIDATION'", () => {
+      expect(ERR_VALIDATION).toBe("ERR_VALIDATION");
+    });
+
+    it("ERR_PERMISSION is 'ERR_PERMISSION'", () => {
+      expect(ERR_PERMISSION).toBe("ERR_PERMISSION");
+    });
+
+    it("ERR_API is 'ERR_API'", () => {
+      expect(ERR_API).toBe("ERR_API");
+    });
+
+    it("ERR_CONFIG is 'ERR_CONFIG'", () => {
+      expect(ERR_CONFIG).toBe("ERR_CONFIG");
+    });
+
+    it("ERR_NOT_FOUND is 'ERR_NOT_FOUND'", () => {
+      expect(ERR_NOT_FOUND).toBe("ERR_NOT_FOUND");
+    });
+
+    it("ERR_PARSE is 'ERR_PARSE'", () => {
+      expect(ERR_PARSE).toBe("ERR_PARSE");
+    });
+
+    it("ERR_SYSTEM is 'ERR_SYSTEM'", () => {
+      expect(ERR_SYSTEM).toBe("ERR_SYSTEM");
+    });
+  });
+
+  describe("legacy safe-output codes", () => {
+    it("SAFE_OUTPUT_E001 is 'E001'", () => {
+      expect(SAFE_OUTPUT_E001).toBe("E001");
+    });
+
+    it("SAFE_OUTPUT_E099 is 'E099'", () => {
+      expect(SAFE_OUTPUT_E099).toBe("E099");
+    });
+  });
+
+  describe("usage as error message prefixes", () => {
+    it("can be used as a prefix in an Error message", () => {
+      const err = new Error(`${ERR_VALIDATION}: Missing required field: title`);
+      expect(err.message).toBe("ERR_VALIDATION: Missing required field: title");
+    });
+
+    it("can be used as a prefix in a setFailed-style string", () => {
+      const msg = `${ERR_CONFIG}: GH_AW_PROMPT environment variable is not set`;
+      expect(msg).toBe("ERR_CONFIG: GH_AW_PROMPT environment variable is not set");
+    });
+
+    it("all primary codes are distinct from each other", () => {
+      const primary = [ERR_VALIDATION, ERR_PERMISSION, ERR_API, ERR_CONFIG, ERR_NOT_FOUND, ERR_PARSE, ERR_SYSTEM];
+      const unique = new Set(primary);
+      expect(unique.size).toBe(primary.length);
+    });
+
+    it("legacy codes are distinct from primary codes", () => {
+      const all = [ERR_VALIDATION, ERR_PERMISSION, ERR_API, ERR_CONFIG, ERR_NOT_FOUND, ERR_PARSE, ERR_SYSTEM, SAFE_OUTPUT_E001, SAFE_OUTPUT_E099];
+      const unique = new Set(all);
+      expect(unique.size).toBe(all.length);
+    });
+  });
+});