feat(diff): Add custom diff tool support for fs_write via Q_DIFF_TOOL

crates/chat-cli/src/cli/chat/cli/diff_tool.rs

@@ -0,0 +1,60 @@
+use std::path::Path;
+use uuid::Uuid;
+
+use crate::cli::chat::ChatError;
+use crate::util::env_var::try_get_diff_tool;
+
+/// Check if custom diff tool is configured
+pub fn has_diff_tool() -> bool {
+    try_get_diff_tool().is_ok()
+}
+
+/// Launch custom diff tool with two file paths
+pub fn launch_diff_tool(before_path: &Path, after_path: &Path) -> Result<(), ChatError> {
+    let diff_tool_cmd = try_get_diff_tool()
+        .map_err(|_| ChatError::Custom("Q_DIFF_TOOL not configured".into()))?;
+
+    let mut parts = shlex::split(&diff_tool_cmd)
+        .ok_or_else(|| ChatError::Custom("Failed to parse Q_DIFF_TOOL command".into()))?;
+
+    if parts.is_empty() {
+        return Err(ChatError::Custom("Q_DIFF_TOOL is empty".into()));
+    }
+
+    let tool_bin = parts.remove(0);
+    let mut cmd = std::process::Command::new(tool_bin);
+
+    for arg in parts {
+        cmd.arg(arg);
+    }
+
+    cmd
+        .arg(before_path)
+        .arg(after_path)
+        .status()
+        .map_err(|e| ChatError::Custom(format!("Failed to launch diff tool: {}", e).into()))?;
+
+    Ok(())
+}
+
+/// Create temporary files and launch diff tool with content
+pub fn diff_with_tool(before_content: &str, after_content: &str, label: &str) -> Result<(), ChatError> {
+    let temp_dir = std::env::temp_dir();
+    let uuid = Uuid::new_v4();
+
+    // Sanitize label to create valid filename
+    let safe_label = label.replace(['/', '\\', ':'], "_");
+
+    let before_file = temp_dir.join(format!("q_diff_before_{}_{}.txt", safe_label, uuid));
+    let after_file = temp_dir.join(format!("q_diff_after_{}_{}.txt", safe_label, uuid));
+
+    std::fs::write(&before_file, before_content)
+        .map_err(|e| ChatError::Custom(format!("Failed to create before file: {}", e).into()))?;
+
+    std::fs::write(&after_file, after_content)
+        .map_err(|e| ChatError::Custom(format!("Failed to create after file: {}", e).into()))?;
+
+    // Don't cleanup - let OS temp directory cleanup handle it
+    // Files need to persist for async tools and approval flow
+    launch_diff_tool(&before_file, &after_file)
+}

crates/chat-cli/src/cli/chat/cli/mod.rs

@@ -4,6 +4,7 @@ pub mod checkpoint;
 pub mod clear;
 pub mod compact;
 pub mod context;
+pub mod diff_tool;
 pub mod editor;
 pub mod experiment;
 pub mod hooks;

crates/chat-cli/src/cli/chat/tools/fs_write.rs

@@ -316,7 +316,7 @@ impl FsWrite {
                     Default::default()
                 };
                 let new = stylize_output_if_able(&relative_path, &file_text);
-                print_diff(output, &prev, &new, 1)?;
+                print_diff_or_launch_tool(output, &prev, &new, 1, &relative_path.to_string_lossy())?;
 
                 // Display summary as purpose if available after the diff
                 super::display_purpose(self.get_summary(), output)?;
@@ -345,7 +345,7 @@ impl FsWrite {
 
                 let old = stylize_output_if_able(&relative_path, &old);
                 let new = stylize_output_if_able(&relative_path, &new);
-                print_diff(output, &old, &new, start_line)?;
+                print_diff_or_launch_tool(output, &old, &new, start_line, &relative_path.to_string_lossy())?;
 
                 // Display summary as purpose if available after the diff
                 super::display_purpose(self.get_summary(), output)?;
@@ -364,7 +364,7 @@ impl FsWrite {
                 };
                 let old_str = stylize_output_if_able(&relative_path, old_str);
                 let new_str = stylize_output_if_able(&relative_path, new_str);
-                print_diff(output, &old_str, &new_str, start_line)?;
+                print_diff_or_launch_tool(output, &old_str, &new_str, start_line, &relative_path.to_string_lossy())?;
 
                 // Display summary as purpose if available after the diff
                 super::display_purpose(self.get_summary(), output)?;
@@ -376,7 +376,7 @@ impl FsWrite {
                 let relative_path = format_path(cwd, &path);
                 let start_line = os.fs.read_to_string_sync(&path)?.lines().count() + 1;
                 let file = stylize_output_if_able(&relative_path, new_str);
-                print_diff(output, &Default::default(), &file, start_line)?;
+                print_diff_or_launch_tool(output, &Default::default(), &file, start_line, &relative_path.to_string_lossy())?;
 
                 // Display summary as purpose if available after the diff
                 super::display_purpose(self.get_summary(), output)?;
@@ -633,6 +633,37 @@ fn get_lines_with_context(
     )
 }
 
+/// Prints diff using custom tool if configured, otherwise falls back to inline diff
+fn print_diff_or_launch_tool(
+    output: &mut impl Write,
+    old_str: &StylizedFile,
+    new_str: &StylizedFile,
+    start_line: usize,
+    label: &str,
+) -> Result<()> {
+    use crate::cli::chat::cli::diff_tool;
+
+    if diff_tool::has_diff_tool() {
+        // Strip all ANSI codes for external tools
+        let old_content = String::from_utf8_lossy(&strip_ansi_escapes::strip(&old_str.content)).to_string();
+        let new_content = String::from_utf8_lossy(&strip_ansi_escapes::strip(&new_str.content)).to_string();
+
+        match diff_tool::diff_with_tool(&old_content, &new_content, label) {
+            Ok(()) => {
+                writeln!(output, "Diff displayed in external tool.")?;
+                Ok(())
+            },
+            Err(e) => {
+                writeln!(output, "Failed to launch diff tool: {}", e)?;
+                writeln!(output, "Falling back to inline diff...\n")?;
+                print_diff(output, old_str, new_str, start_line)
+            },
+        }
+    } else {
+        print_diff(output, old_str, new_str, start_line)
+    }
+}
+
 /// Prints a git-diff style comparison between `old_str` and `new_str`.
 /// - `start_line` - 1-indexed line number that `old_str` and `new_str` start at.
 fn print_diff(

crates/chat-cli/src/util/consts.rs

@@ -102,6 +102,9 @@ pub mod env_var {
         /// Editor environment variable
         EDITOR = "EDITOR",
 
+        /// Custom diff tool command
+        Q_DIFF_TOOL = "Q_DIFF_TOOL",
+
         /// Terminal type
         TERM = "TERM",
 

crates/chat-cli/src/util/env_var.rs

@@ -66,6 +66,11 @@ pub fn try_get_editor() -> Result<String, std::env::VarError> {
     Env::new().get(EDITOR)
 }
 
+/// Try to get diff tool without fallback
+pub fn try_get_diff_tool() -> Result<String, std::env::VarError> {
+    Env::new().get(Q_DIFF_TOOL)
+}
+
 /// Get terminal type
 pub fn get_term() -> Option<String> {
     Env::new().get(TERM).ok()