diff --git a/dialect/agentscript/src/tests/list-type-parameters.test.ts b/dialect/agentscript/src/tests/list-type-parameters.test.ts
new file mode 100644
index 00000000..475525d9
--- /dev/null
+++ b/dialect/agentscript/src/tests/list-type-parameters.test.ts
@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2026, Salesforce, Inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ * For full license text, see the LICENSE file in the repo root or https://www.apache.org/licenses/LICENSE-2.0
+ */
+
+import { expect, test } from 'vitest';
+import {
+  AGENTSCRIPT_PRIMITIVE_TYPES,
+  SubscriptExpression,
+} from '@agentscript/language';
+import { DiagnosticSeverity } from '@agentscript/types';
+import { AgentScriptSchema } from '../schema.js';
+import { parseWithDiagnostics } from './test-utils.js';
+
+function parseListType(parameter: string) {
+  const result = parseWithDiagnostics(
+    `variables:
+    items: mutable list[${parameter}]
+`,
+    AgentScriptSchema
+  );
+  expect(result.value.variables?.get('items')?.type).toBeInstanceOf(
+    SubscriptExpression
+  );
+  expect(result.diagnostics.filter(d => d.code === 'syntax-error')).toEqual([]);
+  return result.diagnostics;
+}
+
+test.each(['1', 'True', '"string"', '@variables.other', 'string + number'])(
+  'rejects a non-type list parameter: %s',
+  parameter => {
+    const errors = parseListType(parameter).filter(
+      d => d.severity === DiagnosticSeverity.Error
+    );
+    expect(errors).toHaveLength(1);
+    expect(errors[0].code).toBe('invalid-type-parameter');
+    expect(errors[0].message).toContain('Expected a primitive type');
+    expect(errors[0].range).toEqual({
+      start: { line: 1, character: 24 },
+      end: { line: 1, character: 24 + parameter.length },
+    });
+  }
+);
+
+test.each(AGENTSCRIPT_PRIMITIVE_TYPES.map(type => type.keyword))(
+  'accepts a supported list element type: %s',
+  parameter => {
+    expect(
+      parseListType(parameter).filter(
+        d => d.severity === DiagnosticSeverity.Error
+      )
+    ).toEqual([]);
+  }
+);
+
+test('still rejects an unknown list element type', () => {
+  const errors = parseListType('missing').filter(
+    d => d.severity === DiagnosticSeverity.Error
+  );
+  expect(errors).toHaveLength(1);
+  expect(errors[0].code).toBe('unknown-type');
+});
+
+test('still rejects a nested list element type', () => {
+  const errors = parseListType('list[string]').filter(
+    d => d.severity === DiagnosticSeverity.Error
+  );
+  expect(errors).toHaveLength(1);
+  expect(errors[0].code).toBe('nested-list-type');
+});
diff --git a/packages/language/src/core/typed-map-parser.ts b/packages/language/src/core/typed-map-parser.ts
index b692fe0e..6de405c7 100644
--- a/packages/language/src/core/typed-map-parser.ts
+++ b/packages/language/src/core/typed-map-parser.ts
@@ -775,6 +775,15 @@ export class TypedMapParser<T extends TypedDeclarationBase> {
       } else {
         this.emitDeprecatedTypeDiagnostic(elemType, idx as Parsed<object>);
       }
+    } else {
+      this.dc.add(
+        createDiagnostic(
+          idx as Parsed<object>,
+          `Invalid list element type for ${this.blockLabel} ${element.name}. Expected a primitive type (e.g., list[string]).`,
+          DiagnosticSeverity.Error,
+          'invalid-type-parameter'
+        )
+      );
     }
   }
 }