diff --git a/__tests__/action.test.ts b/__tests__/action.test.ts
index 32a93306..f81848ca 100644
--- a/__tests__/action.test.ts
+++ b/__tests__/action.test.ts
@@ -137,6 +137,44 @@ describe('Action', () => {
     });
   });
 
+  describe('nested metadata validation', () => {
+    const metadata = {
+      name: 'Fixture action',
+      description: 'Fixture description',
+      runs: { using: 'node24', main: 'index.js' },
+    };
+
+    it.each([
+      { inputs: { option: null } },
+      { inputs: { option: 42 } },
+      { inputs: { option: { description: 42 } } },
+      { outputs: { result: null } },
+      { outputs: { result: { description: false } } },
+      { inputs: 'not a mapping' },
+    ])('rejects malformed nested metadata through YAML loading: %j', async (nested) => {
+      vi.mocked(fs.readFileSync).mockReturnValueOnce(YAML.stringify({ ...metadata, ...nested }));
+      const { default: Action } = await import('../src/Action.js');
+      expect(() => new Action(actTestYmlPath, mockLogTask)).toThrow('Invalid action metadata syntax');
+    });
+
+    it('accepts scalar defaults and nullish descriptions through YAML loading', async () => {
+      vi.mocked(fs.readFileSync).mockReturnValueOnce(YAML.stringify({
+        ...metadata,
+        inputs: {
+          enabled: { description: 'Enabled', default: false, required: false },
+          retries: { description: 'Retries', default: 3 },
+          optional: { default: null, description: null },
+        },
+        outputs: { result: { description: null } },
+      }));
+      const { default: Action } = await import('../src/Action.js');
+      const action = new Action(actTestYmlPath, mockLogTask);
+      expect(action.inputs?.enabled?.default).toBe(false);
+      expect(action.inputs?.retries?.default).toBe(3);
+      expect(action.outputs?.result?.description).toBeNull();
+    });
+  });
+
   describe('inputDefault', () => {
     it('should return the default value for an input', async () => {
       const { default: Action } = await import('../src/Action.js');
diff --git a/src/Action.ts b/src/Action.ts
index d55dad20..c54d3647 100644
--- a/src/Action.ts
+++ b/src/Action.ts
@@ -128,6 +128,19 @@ export default class Action implements ActionYaml {
       return false;
     }
     const record = obj as Record<string, unknown>;
+    const isRecord = (value: unknown): value is Record<string, unknown> =>
+      typeof value === 'object' && value !== null && !Array.isArray(value);
+    const validDescriptions = (value: unknown): boolean =>
+      value === undefined ||
+      (isRecord(value) &&
+        Object.values(value).every(
+          (entry) =>
+            isRecord(entry) &&
+            (entry.description == null || typeof entry.description === 'string'),
+        ));
+    if (!validDescriptions(record.inputs) || !validDescriptions(record.outputs)) {
+      return false;
+    }
     if ('name' in record && 'description' in record && 'runs' in record) {
       const runs = record.runs;
       if (typeof runs === 'object' && runs !== null && !Array.isArray(runs) && 'using' in runs) {