Truncate tool use. (#9)

Truncate tool use to 10 lines by default.

Reviewed-on: #9
Co-authored-by: Drew Galbraith <drew@tiramisu.one>
Co-committed-by: Drew Galbraith <drew@tiramisu.one>

src/tui/tool_display.rs

@@ -8,6 +8,9 @@ use similar::{ChangeTag, TextDiff};
 
 use crate::core::types::ToolDisplay;
 
+/// Maximum number of output lines shown for a tool result before truncation.
+const MAX_RESULT_LINES: usize = 10;
+
 /// Format a tool that is currently executing (or awaiting approval).
 pub fn format_executing(name: &str, display: &ToolDisplay) -> String {
     match display {
@@ -31,9 +34,12 @@ pub fn format_executing(name: &str, display: &ToolDisplay) -> String {
 }
 
 /// Format a tool result after execution completes.
+///
+/// Output is truncated to [`MAX_RESULT_LINES`] lines to keep the TUI
+/// responsive for commands that produce large output (e.g. verbose test runs).
 pub fn format_result(name: &str, display: &ToolDisplay, is_error: bool) -> String {
     let prefix = if is_error { "error" } else { "result" };
-    match display {
+    let raw = match display {
         ToolDisplay::WriteFile {
             path,
             old_content,
@@ -54,7 +60,19 @@ pub fn format_result(name: &str, display: &ToolDisplay, is_error: bool) -> Strin
         ToolDisplay::ListDirectory { path } => format!("ls {path}"),
         ToolDisplay::ReadFile { path } => format!("read {path}"),
         ToolDisplay::Generic { summary } => format!("[{name} {prefix}] {summary}"),
+    };
+    truncate_lines(&raw, MAX_RESULT_LINES)
 }
+
+/// Keep at most `max` lines, appending a summary if truncated.
+fn truncate_lines(s: &str, max: usize) -> String {
+    let total = s.lines().count();
+    if total <= max {
+        return s.to_string();
+    }
+    let mut out: String = s.lines().take(max).collect::<Vec<_>>().join("\n");
+    out.push_str(&format!("\n... ({} lines truncated)", total - max));
+    out
 }
 
 /// Produce a unified diff between `old` and `new` text.
@@ -130,4 +148,33 @@ mod tests {
         assert!(diff.contains("+line1"));
         assert!(diff.contains("+line2"));
     }
+
+    #[test]
+    fn truncate_lines_under_limit() {
+        assert_eq!(truncate_lines("a\nb\nc", 5), "a\nb\nc");
+    }
+
+    #[test]
+    fn truncate_lines_over_limit() {
+        let input: String = (0..300)
+            .map(|i| format!("line {i}"))
+            .collect::<Vec<_>>()
+            .join("\n");
+        let output = truncate_lines(&input, MAX_RESULT_LINES);
+        assert_eq!(output.lines().count(), MAX_RESULT_LINES + 1); // +1 for summary
+        assert!(output.contains("290 lines truncated"));
+    }
+
+    #[test]
+    fn format_result_shell_truncates_large_output() {
+        let big_output = (0..500)
+            .map(|i| format!("line {i}"))
+            .collect::<Vec<_>>()
+            .join("\n");
+        let display = ToolDisplay::ShellExec {
+            command: format!("cargo test\n{big_output}"),
+        };
+        let result = format_result("shell_exec", &display, false);
+        assert!(result.contains("lines truncated"));
+    }
 }