`edit_file` and `write_file` now capture the file contents before and
after the mutation, generate a unified diff with `similar`, and emit it
at the head of the `ToolResult` body. The TUI's existing
`output_looks_like_diff` detector (history.rs:1335) sees the `@@`
header in the first 5 lines and routes the payload through
`diff_render::render_diff`, which already renders unified diffs with
line numbers and coloured `+`/`-` gutters.
The model also benefits — it sees exactly which lines changed instead
of just `Replaced N occurrence(s)` or `Wrote N bytes`. Identical
content produces an empty diff, in which case the body falls back to
`<summary>\n(no changes)`.
### What's wired
- New `crates/tui/src/tools/diff_format.rs` exposes
`make_unified_diff(path, old, new) -> String` using
`similar::TextDiff::from_lines(...).unified_diff().context_radius(3)`.
- `WriteFileTool::execute` snapshots prior contents (or empty for new
files), writes, then emits `<diff>\n<summary>` where summary is
`Wrote N bytes to PATH` for existing files and
`Created PATH (N bytes)` for new ones.
- `EditFileTool::execute` snapshots, replaces, writes, emits
`<diff>\nReplaced N occurrence(s) in PATH`.
- `similar = "2"` added to `crates/tui/Cargo.toml`. Pure-Rust, no
C deps; v2.7.0 in Cargo.lock.
### Tests
- 4 unit tests in `diff_format::tests` covering identical inputs,
replacement, new-file (against empty), and presence of the `@@`
header in the first 5 lines (so the TUI detector trips).
- Existing `test_write_file_tool` / `test_edit_file_tool` updated to
assert both the summary line and the unified-diff body
(`--- a/`, `-old`, `+new`).
### Verification
cargo fmt --all -- --check ✓
cargo clippy --workspace --all-targets --all-features --locked -- -D warnings ✓
cargo test --workspace --all-features --locked ✓ (1824 + supporting; was 1820)
Closes#505
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
f2cf3843ec