diff --git a/.changeset/toolkit-lib-metadata-null-guard.md b/.changeset/toolkit-lib-metadata-null-guard.md
new file mode 100644
index 0000000000..ce1f45cef0
--- /dev/null
+++ b/.changeset/toolkit-lib-metadata-null-guard.md
@@ -0,0 +1,5 @@
+---
+'@aws-amplify/backend-deployer': patch
+---
+
+Map the `@aws-cdk/toolkit-lib` `countAssemblyResults` crash (`TypeError: Cannot convert undefined or null to object` when a synthesized stack has no metadata) to a fault instead of a misleading backend `SyntaxError`.
diff --git a/packages/backend-deployer/src/cdk_error_mapper.test.ts b/packages/backend-deployer/src/cdk_error_mapper.test.ts
index 165c07b0ff..eaf1c71ac8 100644
--- a/packages/backend-deployer/src/cdk_error_mapper.test.ts
+++ b/packages/backend-deployer/src/cdk_error_mapper.test.ts
@@ -545,3 +545,49 @@ void describe('Node version error per deployment type', () => {
     );
   });
 });
+
+void describe('toolkit-lib countAssemblyResults metadata crash', () => {
+  const cdkErrorMapper = new CdkErrorMapper(formatterStub);
+
+  // The TypeError @aws-cdk/toolkit-lib throws when a stack has no metadata.
+  // Refs aws-amplify/amplify-backend#3316.
+  const buildCountAssemblyResultsError = () => {
+    const error = new TypeError('Cannot convert undefined or null to object');
+    error.stack = [
+      'TypeError: Cannot convert undefined or null to object',
+      '    at Object.values (<anonymous>)',
+      '    at node_modules/@aws-cdk/toolkit-lib/lib/toolkit/private/count-assembly-results.js:12:30',
+      '    at Array.flatMap (<anonymous>)',
+      '    at countAssemblyResults (node_modules/@aws-cdk/toolkit-lib/lib/toolkit/private/count-assembly-results.js:12:10)',
+      '    at synthAndMeasure (node_modules/@aws-cdk/toolkit-lib/lib/toolkit/toolkit.js:1709:59)',
+    ].join('\n');
+    return error;
+  };
+
+  void it('classifies the crash as a fault, not a user error', () => {
+    const error = cdkErrorMapper.getAmplifyError(
+      buildCountAssemblyResultsError(),
+    );
+    assert.equal(error.name, 'CDKSynthAssemblyMetadataFault');
+    // Must be classified as a FAULT (internal/library bug), not a user ERROR
+    // that would tell the customer to fix their own backend code.
+    assert.equal(error.classification, 'FAULT');
+    assert.match(error.message, /@aws-cdk\/toolkit-lib/);
+  });
+
+  void it('preserves the original TypeError as the cause', () => {
+    const original = buildCountAssemblyResultsError();
+    const error = cdkErrorMapper.getAmplifyError(original);
+    assert.equal(error.cause, original);
+  });
+
+  void it('does not intercept unrelated TypeErrors (still a backend SyntaxError)', () => {
+    const unrelated = new TypeError('foo is not a function');
+    unrelated.stack =
+      'TypeError: foo is not a function\n    at amplify/backend.ts:3:1';
+    assert.throws(
+      () => cdkErrorMapper.getAmplifyError(unrelated),
+      (thrown: Error) => thrown.name === 'SyntaxError',
+    );
+  });
+});
diff --git a/packages/backend-deployer/src/cdk_error_mapper.ts b/packages/backend-deployer/src/cdk_error_mapper.ts
index 41cec7f2cb..c5caf79981 100644
--- a/packages/backend-deployer/src/cdk_error_mapper.ts
+++ b/packages/backend-deployer/src/cdk_error_mapper.ts
@@ -167,6 +167,34 @@ export class CdkErrorMapper {
         },
         error,
       );
+    } else if (
+      error.name === 'TypeError' &&
+      /Cannot convert undefined or null to object/.test(error.message) &&
+      /countAssemblyResults/.test(error.stack ?? '')
+    ) {
+      // @aws-cdk/toolkit-lib bug: countAssemblyResults calls
+      // Object.values(stack.metadata) with no null guard and throws when a
+      // synthesized stack has no metadata. This is a library fault, not a
+      // customer backend error, so it must not fall through to the generic
+      // TypeError branch below. Refs aws-amplify/amplify-backend#3316.
+      // This match relies on the TypeError reaching the mapper without being
+      // wrapped by an intermediate layer: the countAssemblyResults stack frame
+      // is the stable anchor, while the error.name check would silently miss
+      // if a layer wraps the error into a new type before it gets here.
+      // TODO(aws-amplify/amplify-backend#3316): the upstream null guard is
+      // merged (aws/aws-cdk-cli#1952). Once a @aws-cdk/toolkit-lib release ships
+      // it and this package bumps its pin, countAssemblyResults no longer throws
+      // and this branch becomes dead code — remove it as part of that bump.
+      return new AmplifyFault(
+        'CDKSynthAssemblyMetadataFault',
+        {
+          message:
+            'Unable to synthesize the Amplify backend due to a bug in the CDK toolkit library (@aws-cdk/toolkit-lib) that fails when a synthesized stack has no metadata.',
+          resolution:
+            'This is a known upstream bug in @aws-cdk/toolkit-lib, not an issue in your backend code; retrying is unlikely to help. Track aws-amplify/amplify-backend#3316 for a fix.',
+        },
+        error,
+      );
     } else if (
       ['TypeError', 'TransformError', 'ReferenceError', 'SyntaxError'].some(
         (errorName) => errorName === error.name,
@@ -628,6 +656,7 @@ export type CDKDeploymentError =
   | 'CDKAssetBundleError'
   | 'CDKNotFoundError'
   | 'CDKResolveAWSAccountError'
+  | 'CDKSynthAssemblyMetadataFault'
   | 'CDKVersionMismatchError'
   | 'CFNUpdateNotSupportedError'
   | 'CloudformationResourceCircularDependencyError'