diff --git a/src/bus/message.rs b/src/bus/message.rs
index b0980dd..10db019 100644
--- a/src/bus/message.rs
+++ b/src/bus/message.rs
@@ -468,21 +468,10 @@ impl OutboundMessage {
 }
 
 pub(crate) fn format_tool_call_content(tool_name: &str, tool_arguments: &serde_json::Value) -> String {
-    let mut lines = vec![format!("### {}", tool_name)];
-
     match tool_arguments {
-        serde_json::Value::Object(map) if !map.is_empty() => {
-            let mut entries: Vec<_> = map.iter().collect();
-            entries.sort_by(|(left, _), (right, _)| left.cmp(right));
-            for (key, value) in entries {
-                lines.push(format!("- {}: {}", key, format_tool_argument_value(value)));
-            }
-        }
-        serde_json::Value::Object(_) => {}
-        other => lines.push(format!("- args: {}", format_tool_argument_value(other))),
+        serde_json::Value::Object(map) if map.is_empty() => tool_name.to_string(),
+        other => format!("{}\nargs: {}", tool_name, format_tool_arguments_json(other)),
     }
-
-    lines.join("\n")
 }
 
 fn format_tool_result_content(tool_name: &str, content: &str) -> String {
@@ -497,6 +486,28 @@ fn format_tool_argument_value(value: &serde_json::Value) -> String {
     }
 }
 
+fn format_tool_arguments_json(value: &serde_json::Value) -> String {
+    match value {
+        serde_json::Value::Object(map) => {
+            let mut entries: Vec<_> = map.iter().collect();
+            entries.sort_by(|(left, _), (right, _)| left.cmp(right));
+            let body = entries
+                .into_iter()
+                .map(|(key, value)| {
+                    format!(
+                        "{}:{}",
+                        serde_json::to_string(key).unwrap_or_else(|_| format!("\"{}\"", key)),
+                        serde_json::to_string(value).unwrap_or_else(|_| value.to_string()),
+                    )
+                })
+                .collect::<Vec<_>>()
+                .join(",");
+            format!("{{{}}}", body)
+        }
+        other => format_tool_argument_value(other),
+    }
+}
+
 // ============================================================================
 // Helpers
 // ============================================================================
@@ -545,9 +556,9 @@ mod tests {
         assert_eq!(outbound[0].event_kind, OutboundEventKind::ToolCall);
         assert_eq!(outbound[0].tool_name.as_deref(), Some("calculator"));
         assert_eq!(outbound[0].tool_arguments.as_ref().unwrap()["expression"], "1 + 1");
-        assert_eq!(outbound[0].content, "### calculator\n- expression: 1 + 1");
+        assert_eq!(outbound[0].content, "calculator\nargs: {\"expression\":\"1 + 1\"}");
         assert_eq!(outbound[1].tool_name.as_deref(), Some("file_read"));
-        assert_eq!(outbound[1].content, "### file_read\n- path: README.md");
+        assert_eq!(outbound[1].content, "file_read\nargs: {\"path\":\"README.md\"}");
     }
 
     #[test]