diff --git a/.eslint-plugin-local/code-no-declare-const-enum.ts b/.eslint-plugin-local/code-no-declare-const-enum.ts
new file mode 100644
index 00000000000..b448adee89c
--- /dev/null
+++ b/.eslint-plugin-local/code-no-declare-const-enum.ts
@@ -0,0 +1,52 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as eslint from 'eslint';
+
+/**
+ * Disallows `declare const enum` declarations. esbuild does not inline
+ * `declare const enum` values, leaving the enum identifier in the output
+ * which causes a ReferenceError at runtime.
+ *
+ * Use `const enum` (without `declare`) instead.
+ *
+ * See https://github.com/evanw/esbuild/issues/4394
+ */
+export default new class NoDeclareConstEnum implements eslint.Rule.RuleModule {
+
+	readonly meta: eslint.Rule.RuleMetaData = {
+		messages: {
+			noDeclareConstEnum: '"declare const enum" is not supported by esbuild. Use "const enum" instead. See https://github.com/evanw/esbuild/issues/4394',
+		},
+		schema: false,
+		fixable: 'code',
+	};
+
+	create(context: eslint.Rule.RuleContext): eslint.Rule.RuleListener {
+		return {
+			TSEnumDeclaration(node: any) {
+				if (node.const && node.declare) {
+					context.report({
+						node,
+						messageId: 'noDeclareConstEnum',
+						fix: (fixer) => {
+							// Remove "declare " from "declare const enum"
+							const sourceCode = context.sourceCode;
+							const text = sourceCode.getText(node);
+							const declareIndex = text.indexOf('declare');
+							if (declareIndex !== -1) {
+								return fixer.removeRange([
+									node.range[0] + declareIndex,
+									node.range[0] + declareIndex + 'declare '.length
+								]);
+							}
+							return null;
+						}
+					});
+				}
+			}
+		};
+	}
+};
diff --git a/eslint.config.js b/eslint.config.js
index dfb489e0f0b..a0bcfc05560 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -78,6 +78,7 @@ export default tseslint.config(
 			'semi': 'warn',
 			'local/code-translation-remind': 'warn',
 			'local/code-no-native-private': 'warn',
+			'local/code-no-declare-const-enum': 'warn',
 			'local/code-parameter-properties-must-have-explicit-accessibility': 'warn',
 			'local/code-no-nls-in-standalone-editor': 'warn',
 			'local/code-no-potentially-unsafe-disposables': 'warn',
diff --git a/src/vs/workbench/contrib/chat/common/tools/promptTsxTypes.ts b/src/vs/workbench/contrib/chat/common/tools/promptTsxTypes.ts
index 20de711820c..53617f61395 100644
--- a/src/vs/workbench/contrib/chat/common/tools/promptTsxTypes.ts
+++ b/src/vs/workbench/contrib/chat/common/tools/promptTsxTypes.ts
@@ -7,9 +7,12 @@
  * This is a subset of the types export from jsonTypes.d.ts in @vscode/prompt-tsx.
  * It's just the types needed to stringify prompt-tsx tool results.
  * It should be kept in sync with the types in that file.
+ *
+ * Note: do NOT use `declare` with const enums, esbuild doesn't inline them.
+ * See https://github.com/evanw/esbuild/issues/4394
  */
 
-export declare const enum PromptNodeType {
+export const enum PromptNodeType {
 	Piece = 1,
 	Text = 2
 }
@@ -23,7 +26,7 @@ export interface TextJSON {
  * less descriptive than the actual constructor, as we only care to preserve
  * the element data that the renderer cares about.
  */
-export declare const enum PieceCtorKind {
+export const enum PieceCtorKind {
 	BaseChatMessage = 1,
 	Other = 2,
 	ImageChatMessage = 3