diff --git a/src/providers/anthropic.rs b/src/providers/anthropic.rs
index 249e0e6..cf3778f 100644
--- a/src/providers/anthropic.rs
+++ b/src/providers/anthropic.rs
@@ -614,15 +614,20 @@ mod tests {
         assert_eq!(chain, "single error");
     }
 
+    /// 用 thiserror 构造真正的嵌套 source 链，验证 "caused by" 拼接
+    #[derive(Debug, thiserror::Error)]
+    enum OuterError {
+        #[error("outer wrapper")]
+        Wrapped(#[source] std::io::Error),
+    }
+
     #[test]
     fn test_format_error_chain_nested() {
         let inner = std::io::Error::new(std::io::ErrorKind::Other, "root cause");
-        let outer = std::io::Error::new(std::io::ErrorKind::Other, "outer error");
-        // std::io::Error 的 source 链需要手动构造，这里用 anyhow 风格的 context 模拟
-        // 简化：验证无 source 时不拼接 "caused by"
+        let outer = OuterError::Wrapped(inner);
         let chain = format_error_chain(&outer);
-        assert!(chain.contains("outer error"));
-        assert!(!chain.contains("caused by"));
-        let _ = inner; // 抑制未使用警告
+        assert!(chain.contains("outer wrapper"));
+        assert!(chain.contains("caused by"));
+        assert!(chain.contains("root cause"));
     }
 }