diff --git a/src/types.ts b/src/types.ts
index 9870c07..07499f2 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -73,10 +73,14 @@ export interface LLMOptions<
 export type ChatMessage = openai.ChatMessage
 export type ChatMessageRole = openai.ChatMessageRole
 
+export type ChatMessageContent<TInput extends TaskInput = void> =
+  | string
+  | ((input: TInput | any) => string)
+
 export type ChatMessageInput<TInput extends TaskInput = void> =
   | ChatMessage
   | {
-      content: (input: TInput | any) => string
+      content: ChatMessageContent<TInput>
     }
 
 export interface ChatModelOptions<
diff --git a/src/utils.ts b/src/utils.ts
index 18c74c7..3b4cec0 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -215,3 +215,7 @@ export function throttleKy(
 export function isFunction(value: any): value is Function {
   return typeof value === 'function'
 }
+
+export function isString(value: any): value is string {
+  return typeof value === 'string'
+}
diff --git a/test/utils.test.ts b/test/utils.test.ts
index 59079f3..d664000 100644
--- a/test/utils.test.ts
+++ b/test/utils.test.ts
@@ -35,7 +35,8 @@ test('sleep should delay execution', async (t) => {
   const start = Date.now()
   await sleep(1000) // for example, 1000ms / 1sec
   const end = Date.now()
-  t.true(end - start >= 1000)
+  const duration = end - start
+  t.true(duration >= 1000 - 10)
 })
 
 test('defaultIDGeneratorFn should generate URL-safe string', (t) => {