diff --git a/actions/setup/js/messages_core.cjs b/actions/setup/js/messages_core.cjs
index 2150e9fd9d..a92c0dd5f0 100644
--- a/actions/setup/js/messages_core.cjs
+++ b/actions/setup/js/messages_core.cjs
@@ -21,6 +21,8 @@
  * Both camelCase and snake_case placeholder formats are supported.
  */
 
+const { getErrorMessage } = require("./error_helpers.cjs");
+
 /**
  * @typedef {Object} SafeOutputMessages
  * @property {string} [footer] - Custom footer message template
@@ -53,8 +55,6 @@ function getMessages() {
     return null;
   }
 
-  const { getErrorMessage } = require("./error_helpers.cjs");
-
   try {
     // Parse JSON with camelCase keys from Go struct (using json struct tags)
     return JSON.parse(messagesEnv);
@@ -79,21 +79,23 @@ function renderTemplate(template, context) {
 }
 
 /**
- * Convert context object keys to snake_case for template rendering
+ * Convert context object keys to snake_case for template rendering.
+ * Also keeps original camelCase keys for backwards compatibility.
  * @param {Record<string, any>} obj - Object with camelCase keys
- * @returns {Record<string, any>} Object with snake_case keys
+ * @returns {Record<string, any>} Object with both snake_case and original keys
  */
 function toSnakeCase(obj) {
-  /** @type {Record<string, any>} */
-  const result = {};
-  for (const [key, value] of Object.entries(obj)) {
-    // Convert camelCase to snake_case
-    const snakeKey = key.replace(/([A-Z])/g, "_$1").toLowerCase();
-    result[snakeKey] = value;
-    // Also keep original key for backwards compatibility
-    result[key] = value;
-  }
-  return result;
+  return Object.fromEntries(
+    Object.entries(obj).flatMap(([key, value]) => {
+      const snakeKey = key.replace(/([A-Z])/g, "_$1").toLowerCase();
+      return snakeKey === key
+        ? [[key, value]]
+        : [
+            [snakeKey, value],
+            [key, value],
+          ];
+    })
+  );
 }
 
 module.exports = {
diff --git a/actions/setup/js/messages_core.test.cjs b/actions/setup/js/messages_core.test.cjs
new file mode 100644
index 0000000000..bebd52c610
--- /dev/null
+++ b/actions/setup/js/messages_core.test.cjs
@@ -0,0 +1,168 @@
+/**
+ * Test Suite: messages_core.cjs
+ *
+ * Tests for the core message utilities module including:
+ * - Template rendering with placeholder replacement
+ * - Snake_case conversion with camelCase compatibility
+ * - Messages config parsing from environment variable
+ */
+import { describe, it, expect, beforeEach, vi } from "vitest";
+
+const mockCore = {
+  debug: vi.fn(),
+  info: vi.fn(),
+  warning: vi.fn(),
+  error: vi.fn(),
+};
+
+global.core = mockCore;
+
+describe("messages_core.cjs", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.resetModules();
+    delete process.env.GH_AW_SAFE_OUTPUT_MESSAGES;
+  });
+
+  describe("renderTemplate", () => {
+    it("should replace a single placeholder", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("Hello, {name}!", { name: "World" });
+      expect(result).toBe("Hello, World!");
+    });
+
+    it("should replace multiple placeholders", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("{greeting}, {name}! Run: {run_url}", {
+        greeting: "Hello",
+        name: "Alice",
+        run_url: "https://github.com/actions/runs/123",
+      });
+      expect(result).toBe("Hello, Alice! Run: https://github.com/actions/runs/123");
+    });
+
+    it("should keep placeholder unchanged when key is missing from context", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("Hello, {name}! {unknown}", { name: "World" });
+      expect(result).toBe("Hello, World! {unknown}");
+    });
+
+    it("should keep placeholder unchanged when value is undefined", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("{key}", { key: undefined });
+      expect(result).toBe("{key}");
+    });
+
+    it("should coerce numeric values to strings", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("Issue #{number}", { number: 42 });
+      expect(result).toBe("Issue #42");
+    });
+
+    it("should coerce boolean values to strings", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("Active: {active}", { active: true });
+      expect(result).toBe("Active: true");
+    });
+
+    it("should return template unchanged when no placeholders present", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("No placeholders here.", {});
+      expect(result).toBe("No placeholders here.");
+    });
+
+    it("should handle empty template", async () => {
+      const { renderTemplate } = await import("./messages_core.cjs?" + Date.now());
+      const result = renderTemplate("", { key: "value" });
+      expect(result).toBe("");
+    });
+  });
+
+  describe("toSnakeCase", () => {
+    it("should convert camelCase keys to snake_case", async () => {
+      const { toSnakeCase } = await import("./messages_core.cjs?" + Date.now());
+      const result = toSnakeCase({ workflowName: "test" });
+      expect(result.workflow_name).toBe("test");
+    });
+
+    it("should preserve original camelCase keys for backwards compatibility", async () => {
+      const { toSnakeCase } = await import("./messages_core.cjs?" + Date.now());
+      const result = toSnakeCase({ workflowName: "test" });
+      expect(result.workflowName).toBe("test");
+    });
+
+    it("should not duplicate snake_case keys that are already snake_case", async () => {
+      const { toSnakeCase } = await import("./messages_core.cjs?" + Date.now());
+      const result = toSnakeCase({ run_url: "https://example.com" });
+      // Only one entry for already-snake_case keys
+      expect(result.run_url).toBe("https://example.com");
+      expect(Object.keys(result).filter(k => k === "run_url")).toHaveLength(1);
+    });
+
+    it("should handle multi-word camelCase keys", async () => {
+      const { toSnakeCase } = await import("./messages_core.cjs?" + Date.now());
+      const result = toSnakeCase({ newDiscussionNumber: 42, newDiscussionUrl: "https://github.com" });
+      expect(result.new_discussion_number).toBe(42);
+      expect(result.new_discussion_url).toBe("https://github.com");
+      // Original keys also preserved
+      expect(result.newDiscussionNumber).toBe(42);
+    });
+
+    it("should handle empty object", async () => {
+      const { toSnakeCase } = await import("./messages_core.cjs?" + Date.now());
+      const result = toSnakeCase({});
+      expect(result).toEqual({});
+    });
+
+    it("should handle multiple fields mixed camelCase and snake_case", async () => {
+      const { toSnakeCase } = await import("./messages_core.cjs?" + Date.now());
+      const result = toSnakeCase({ workflowName: "my-workflow", run_url: "https://example.com" });
+      expect(result.workflow_name).toBe("my-workflow");
+      expect(result.workflowName).toBe("my-workflow");
+      expect(result.run_url).toBe("https://example.com");
+    });
+  });
+
+  describe("getMessages", () => {
+    it("should return null when env var is not set", async () => {
+      const { getMessages } = await import("./messages_core.cjs?" + Date.now());
+      const result = getMessages();
+      expect(result).toBeNull();
+    });
+
+    it("should return null when env var is empty", async () => {
+      process.env.GH_AW_SAFE_OUTPUT_MESSAGES = "";
+      const { getMessages } = await import("./messages_core.cjs?" + Date.now());
+      const result = getMessages();
+      expect(result).toBeNull();
+    });
+
+    it("should parse valid JSON config", async () => {
+      process.env.GH_AW_SAFE_OUTPUT_MESSAGES = JSON.stringify({ footer: "Custom footer" });
+      const { getMessages } = await import("./messages_core.cjs?" + Date.now());
+      const result = getMessages();
+      expect(result).toEqual({ footer: "Custom footer" });
+    });
+
+    it("should parse config with multiple message fields", async () => {
+      const config = {
+        footer: "Custom footer",
+        runStarted: "Workflow started",
+        runSuccess: "Workflow succeeded",
+        appendOnlyComments: true,
+      };
+      process.env.GH_AW_SAFE_OUTPUT_MESSAGES = JSON.stringify(config);
+      const { getMessages } = await import("./messages_core.cjs?" + Date.now());
+      const result = getMessages();
+      expect(result).toEqual(config);
+    });
+
+    it("should return null and warn on invalid JSON", async () => {
+      process.env.GH_AW_SAFE_OUTPUT_MESSAGES = "not-valid-json";
+      const { getMessages } = await import("./messages_core.cjs?" + Date.now());
+      const result = getMessages();
+      expect(result).toBeNull();
+      expect(mockCore.warning).toHaveBeenCalledWith(expect.stringContaining("Failed to parse GH_AW_SAFE_OUTPUT_MESSAGES"));
+    });
+  });
+});