diff --git a/src/memory/query_rewriter.rs b/src/memory/query_rewriter.rs
index da9ce47..8051e29 100644
--- a/src/memory/query_rewriter.rs
+++ b/src/memory/query_rewriter.rs
@@ -94,7 +94,7 @@ fn format_history(messages: &[ChatMessage]) -> String {
         .iter()
         .filter_map(|m| {
             m.content.as_ref().map(|c| {
-                let snippet = crate::utils::str::truncate_chars(c, 200);
+                let snippet = crate::utils::strings::truncate_chars(c, 200);
                 format!("{}: {}", m.role, snippet)
             })
         })
diff --git a/src/memory/rag.rs b/src/memory/rag.rs
index c11a834..c5eca1d 100644
--- a/src/memory/rag.rs
+++ b/src/memory/rag.rs
@@ -42,7 +42,7 @@ pub async fn auto_retrieve_context(
     for msg in &results {
         if let Some(content) = &msg.content {
             let role = &msg.role;
-            let snippet = crate::utils::str::truncate_chars(content, 300);
+            let snippet = crate::utils::strings::truncate_chars(content, 300);
             block.push_str(&format!("[{}] {}\n", role, snippet));
         }
     }
@@ -136,8 +136,8 @@ mod tests {
     async fn test_auto_retrieve_truncates_long_snippets() {
         // Verify the 300-char truncation logic via truncate_chars
         let content = "x".repeat(500);
-        let snippet = crate::utils::str::truncate_chars(&content, 300);
-        assert_eq!(snippet.len(), 303); // 300 + "..."
+        let snippet = crate::utils::strings::truncate_chars(&content, 300);
+        assert_eq!(snippet.chars().count(), 303); // 300 chars + "..."
         assert!(snippet.ends_with("..."));
     }
 
@@ -147,7 +147,7 @@ mod tests {
         // content longer than 300 bytes with Chinese characters.
         // Old &content[..300] would panic here.
         let long_chinese = "每日論文摘要（香港時間）人工智能".repeat(25); // ~400 chars, >1200 bytes
-        let result = crate::utils::str::truncate_chars(&long_chinese, 300);
+        let result = crate::utils::strings::truncate_chars(&long_chinese, 300);
         assert!(result.ends_with("..."), "should be truncated");
         assert!(
             std::str::from_utf8(result.as_bytes()).is_ok(),
diff --git a/src/platform/tool_notifier.rs b/src/platform/tool_notifier.rs
index 11baa25..7a7124d 100644
--- a/src/platform/tool_notifier.rs
+++ b/src/platform/tool_notifier.rs
@@ -29,14 +29,14 @@ pub fn format_args_preview(args_json: &str) -> String {
                         serde_json::Value::String(s) => s.clone(),
                         other => other.to_string(),
                     };
-                    let truncated = crate::utils::str::truncate_chars(&s, 60);
+                    let truncated = crate::utils::strings::truncate_chars(&s, 60);
                     return format!("\"{}\"", truncated);
                 }
             }
         }
     }
     // Fallback: truncate raw JSON
-    crate::utils::str::truncate_chars(args_json, 60)
+    crate::utils::strings::truncate_chars(args_json, 60)
 }
 
 /// Manages the live-edited Telegram status message during agent tool execution.
diff --git a/src/utils/mod.rs b/src/utils/mod.rs
index 3bb9df5..e8dfd78 100644
--- a/src/utils/mod.rs
+++ b/src/utils/mod.rs
@@ -1 +1 @@
-pub mod str;
+pub mod strings;
diff --git a/src/utils/str.rs b/src/utils/strings.rs
similarity index 86%
rename from src/utils/str.rs
rename to src/utils/strings.rs
index 0cf4039..408eca6 100644
--- a/src/utils/str.rs
+++ b/src/utils/strings.rs
@@ -60,4 +60,12 @@ mod tests {
         assert!(result.ends_with("..."));
         assert!(std::str::from_utf8(result.as_bytes()).is_ok());
     }
+
+    #[test]
+    fn test_truncate_chars_zero_max() {
+        // max_chars=0: every non-empty string is truncated immediately to "..."
+        assert_eq!(truncate_chars("hello", 0), "...");
+        // Empty string is never truncated (loop body never entered)
+        assert_eq!(truncate_chars("", 0), "");
+    }
 }