[BUG] [v0.1.0] generate_diff_preview() produces double-prefixed diff lines (--old / ++new) for Edit tool previews

[climax-dev-1]

Bug Description

generate_diff_preview() in cortex-tui/src/runner/event_loop/streaming.rs produces malformed unified diff output for the Edit tool. The first line of old_str gets a -- prefix and the first line of new_str gets a ++ prefix, because the format string template already includes - and + prefixes, and then the .map() closures add another - or + to every line.

Location

src/cortex-tui/src/runner/event_loop/streaming.rs, lines 889–903

Root Cause

The format string on line 889-890 is:

"--- a/{}\n+++ b/{}\n@@ Edit Preview @@\n-{}\n+{}"

The \n-{} and \n+{} already prefix the first interpolated block with - and +. But the interpolated values are built by mapping each line with an additional prefix:

old_str
    .lines()
    .map(|l| format!("-{}", l))   // adds - to EVERY line including the first
    .collect::<Vec<_>>()
    .join("\n"),
new_str
    .lines()
    .map(|l| format!("+{}", l))   // adds + to EVERY line including the first
    .collect::<Vec<_>>()
    .join("\n"),

For a single-line edit like old_str = "foo", new_str = "bar", the output is:

--- a/file.rs
+++ b/file.rs
@@ Edit Preview @@
--foo
++bar

The correct output should be:

--- a/file.rs
+++ b/file.rs
@@ Edit Preview @@
-foo
+bar

Fix

Remove the - and + from the format template so the .map() closures are the sole source of line prefixes:

let preview = format!(
    "--- a/{}\n+++ b/{}\n@@ Edit Preview @@\n{}\n{}",
    file_path,
    file_path,
    old_str.lines().map(|l| format!("-{}", l)).collect::<Vec<_>>().join("\n"),
    new_str.lines().map(|l| format!("+{}", l)).collect::<Vec<_>>().join("\n"),
);

Impact

Every Edit tool diff preview shown in the approval UI has malformed diff syntax. The first line of removed and added content is double-prefixed, making the diff harder to read and potentially confusing diff parsers or syntax highlighters that consume this output.

Version

v0.1.0