diff --git a/src/tool/fs/helpers.rs b/src/tool/fs/helpers.rs
index d766395..9df5d97 100644
--- a/src/tool/fs/helpers.rs
+++ b/src/tool/fs/helpers.rs
@@ -39,6 +39,22 @@ pub fn not_found_help(ctx: &super::super::ToolCtx, path: &Path, rel: &str) -> St
     }
 }
 
+/// Maximum number of lines a diff block may contain before being truncated.
+pub const MAX_DIFF_LINES: usize = 200;
+
+/// Cap a unified diff at `MAX_DIFF_LINES` lines, appending a truncation note.
+///
+/// Return: `diff` unchanged if it's within the limit; otherwise the first
+/// `MAX_DIFF_LINES` lines followed by `"... ({N} more lines truncated)"`.
+pub fn truncate_diff(diff: &str) -> String {
+    let lines: Vec<&str> = diff.lines().collect();
+    if lines.len() <= MAX_DIFF_LINES {
+        return diff.to_string();
+    }
+    let remaining = lines.len() - MAX_DIFF_LINES;
+    format!("{}\n... ({remaining} more lines truncated)", lines[..MAX_DIFF_LINES].join("\n"))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -73,4 +89,18 @@ mod tests {
         let args = json!({"key": null});
         assert!(arg_str(&args, "key").is_err());
     }
+
+    #[test]
+    fn test_truncate_diff_under_limit_unchanged() {
+        let diff = "line1\nline2\nline3";
+        assert_eq!(truncate_diff(diff), diff);
+    }
+
+    #[test]
+    fn test_truncate_diff_over_limit_truncates() {
+        let diff = (0..250).map(|i| format!("line{i}")).collect::<Vec<_>>().join("\n");
+        let result = truncate_diff(&diff);
+        assert!(result.contains("... (50 more lines truncated)"));
+        assert_eq!(result.lines().count(), MAX_DIFF_LINES + 1);
+    }
 }