diff --git a/packages/@aws-cdk/toolkit-lib/lib/toolkit/toolkit.ts b/packages/@aws-cdk/toolkit-lib/lib/toolkit/toolkit.ts
index 3007f92a2..5ea9cf232 100644
--- a/packages/@aws-cdk/toolkit-lib/lib/toolkit/toolkit.ts
+++ b/packages/@aws-cdk/toolkit-lib/lib/toolkit/toolkit.ts
@@ -757,13 +757,23 @@ export class Toolkit extends CloudAssemblySourceBuilder implements AsyncDisposab
         default: return 'CDK_ASSEMBLY_I9999';
       }
     };
-    await stacks.validateMetadata(this.props.assemblyFailureAt, async (level, msg) => ioHost.notify({
-      time: new Date(),
-      level,
-      code: code(level),
-      message: `[${level} at ${msg.id}] ${msg.entry.data}`,
-      data: msg,
-    }));
+
+    await stacks.validateMetadata(this.props.assemblyFailureAt, async (level, msg) => {
+      // Data comes from Annotations and the data can be of object type containing 'Fn::Join' or 'Ref' when tokens are included in Annotations.
+      // Therefore, we use JSON.stringify to convert it to a string when the data is of object type.
+      // see: https://github.com/aws/aws-cdk/issues/33527
+      const data = msg.entry.data !== null && typeof msg.entry.data === 'object'
+        ? JSON.stringify(msg.entry.data)
+        : msg.entry.data;
+
+      await ioHost.notify({
+        time: new Date(),
+        level,
+        code: code(level),
+        message: `[${level} at ${msg.id}] ${data}`,
+        data: msg,
+      });
+    });
   }
 
   /**
diff --git a/packages/@aws-cdk/toolkit-lib/test/toolkit/toolkit.test.ts b/packages/@aws-cdk/toolkit-lib/test/toolkit/toolkit.test.ts
index 156b428b8..42154487a 100644
--- a/packages/@aws-cdk/toolkit-lib/test/toolkit/toolkit.test.ts
+++ b/packages/@aws-cdk/toolkit-lib/test/toolkit/toolkit.test.ts
@@ -7,7 +7,7 @@
 
 import * as chalk from 'chalk';
 import { Toolkit } from '../../lib';
-import { TestIoHost } from '../_helpers';
+import { TestCloudAssemblySource, TestIoHost } from '../_helpers';
 
 describe('message formatting', () => {
   test('emojis can be stripped from message', async () => {
@@ -70,3 +70,79 @@ describe('message formatting', () => {
     }));
   });
 });
+
+describe('metadata message formatting', () => {
+  test('converts object data for log message to string', async () => {
+    const ioHost = new TestIoHost();
+    const toolkit = new Toolkit({ ioHost });
+
+    const source = new TestCloudAssemblySource({
+      stacks: [{
+        stackName: 'test-stack',
+        metadata: {
+          'test-stack': [{
+            type: 'aws:cdk:warning',
+            data: {
+              'Fn::Join': [
+                '',
+                [
+                  'stackId: ',
+                  {
+                    Ref: 'AWS::StackId',
+                  },
+                ],
+              ],
+            } as any,
+          }],
+        },
+      }],
+    });
+
+    await toolkit.synth(source);
+
+    expect(ioHost.notifySpy).toHaveBeenCalledWith(expect.objectContaining({
+      level: 'warn',
+      message: expect.stringContaining('{"Fn::Join":["",["stackId: ",{"Ref":"AWS::StackId"}]]}'),
+      data: {
+        entry: {
+          type: 'aws:cdk:warning',
+          data: { 'Fn::Join': ['', ['stackId: ', { Ref: 'AWS::StackId' }]] },
+        },
+        id: 'test-stack',
+        level: 'warning',
+      },
+    }));
+  });
+
+  test('keeps non-object data for log message as-is', async () => {
+    const ioHost = new TestIoHost();
+    const toolkit = new Toolkit({ ioHost });
+
+    const source = new TestCloudAssemblySource({
+      stacks: [{
+        stackName: 'test-stack',
+        metadata: {
+          'test-stack': [{
+            type: 'aws:cdk:info',
+            data: 'simple string message',
+          }],
+        },
+      }],
+    });
+
+    await toolkit.synth(source);
+
+    expect(ioHost.notifySpy).toHaveBeenCalledWith(expect.objectContaining({
+      level: 'info',
+      message: expect.stringContaining('simple string message'),
+      data: {
+        entry: {
+          type: 'aws:cdk:info',
+          data: 'simple string message',
+        },
+        id: 'test-stack',
+        level: 'info',
+      },
+    }));
+  });
+});