diff --git a/src/agent/agent_loop.rs b/src/agent/agent_loop.rs
index 26fd7d8..ffbedec 100644
--- a/src/agent/agent_loop.rs
+++ b/src/agent/agent_loop.rs
@@ -1055,8 +1055,8 @@ impl AgentLoop {
                     let error = result.error.unwrap_or_default();
                     tracing::error!(
                         tool = %tool_call.name,
-                        args = %truncate_args(&tool_call.arguments, 2_000),
-                        normalized_args = %truncate_args(&normalized_arguments, 2_000),
+                        args = %truncate_args(&tool_call.arguments, 4_000),
+                        normalized_args = %truncate_args(&normalized_arguments, 4_000),
                         error = %error,
                         output = %result.output,
                         "Tool returned an error result"
@@ -1067,8 +1067,8 @@ impl AgentLoop {
             Err(e) => {
                 tracing::error!(
                     tool = %tool_call.name,
-                    args = %truncate_args(&tool_call.arguments, 2_000),
-                    normalized_args = %truncate_args(&normalized_arguments, 2_000),
+                    args = %truncate_args(&tool_call.arguments, 4_000),
+                    normalized_args = %truncate_args(&normalized_arguments, 4_000),
                     error = %e,
                     error_details = %format!("{:#}", e),
                     "Tool execution failed"
diff --git a/src/observability/mod.rs b/src/observability/mod.rs
index 11521e9..4e06af5 100644
--- a/src/observability/mod.rs
+++ b/src/observability/mod.rs
@@ -187,7 +187,14 @@ pub fn truncate_args(args: &serde_json::Value, max_len: usize) -> String {
     if args_str.len() <= max_len {
         return args_str;
     }
-    format!("{}...truncated", &args_str[..max_len])
+    // Find the last valid char boundary before max_len to avoid splitting UTF-8 chars
+    let truncate_at = args_str
+        .char_indices()
+        .take_while(|(idx, _)| *idx < max_len)
+        .last()
+        .map(|(idx, c)| idx + c.len_utf8())
+        .unwrap_or(0);
+    format!("{}...truncated", &args_str[..truncate_at])
 }
 
 #[cfg(test)]
@@ -288,4 +295,15 @@ mod tests {
         assert!(truncated.ends_with("...truncated"));
         assert!(truncated.len() < long_args.to_string().len());
     }
+
+    #[test]
+    fn test_truncate_args_utf8_boundary() {
+        // Test that truncation respects UTF-8 character boundaries
+        // Each Chinese character is 3 bytes in UTF-8
+        let long_args = serde_json::json!({"key": "测试测试测试测试测试测试测试测试测试测试测试测试测试测试"});
+        let truncated = truncate_args(&long_args, 50);
+        assert!(truncated.ends_with("...truncated"));
+        // Verify the truncated string is valid UTF-8 (no panic occurred)
+        assert!(truncated.is_char_boundary(truncated.len()));
+    }
 }