Redirect debug client output to a file (#17234)

In the app-server debug client, allow redirecting output to a file in
addition to just stdout. Shell redirecting works OK but is a bit weird
with the interactive mode of the debug client since a bunch of newlines
get dumped into the shell. With async messages from MCPs starting it's
also tricky to actually type in a prompt.

codex-rs/debug-client/src/client.rs

@@ -299,8 +299,8 @@ impl AppServerClient {
             }
 
             let line = buffer.trim_end_matches(['\n', '\r']);
-            if !line.is_empty() && !self.filtered_output {
-                let _ = output.server_line(line);
+            if !line.is_empty() {
+                let _ = output.server_json_line(line, self.filtered_output);
             }
 
             let message = match serde_json::from_str::<JSONRPCMessage>(line) {

codex-rs/debug-client/src/main.rs

@@ -4,8 +4,10 @@ mod output;
 mod reader;
 mod state;
 
+use std::fs::File;
 use std::io;
 use std::io::BufRead;
+use std::path::PathBuf;
 use std::sync::mpsc;
 
 use anyhow::Context;
@@ -50,6 +52,10 @@ struct Cli {
     #[arg(long, default_value_t = false)]
     final_only: bool,
 
+    /// Write raw server JSONL to this file instead of stdout.
+    #[arg(long, value_name = "PATH")]
+    output_file: Option<PathBuf>,
+
     /// Optional model override when starting/resuming a thread.
     #[arg(long)]
     model: Option<String>,
@@ -65,7 +71,18 @@ struct Cli {
 
 fn main() -> Result<()> {
     let cli = Cli::parse();
-    let output = Output::new();
+    let jsonl_file = cli
+        .output_file
+        .as_ref()
+        .map(File::create)
+        .transpose()
+        .with_context(|| {
+            let Some(path) = cli.output_file.as_ref() else {
+                return "open output file".to_string();
+            };
+            format!("open output file {}", path.display())
+        })?;
+    let output = Output::new(jsonl_file);
     let approval_policy = parse_approval_policy(&cli.approval_policy)?;
 
     let mut client = AppServerClient::spawn(

codex-rs/debug-client/src/output.rs

@@ -1,4 +1,5 @@
 #![allow(clippy::expect_used)]
+use std::fs::File;
 use std::io;
 use std::io::IsTerminal;
 use std::io::Write;
@@ -24,19 +25,41 @@ pub struct Output {
     lock: Arc<Mutex<()>>,
     prompt: Arc<Mutex<PromptState>>,
     color: bool,
+    jsonl_file: Option<Arc<Mutex<File>>>,
 }
 
 impl Output {
-    pub fn new() -> Self {
+    pub fn new(jsonl_file: Option<File>) -> Self {
         let no_color = std::env::var_os("NO_COLOR").is_some();
         let color = !no_color && io::stdout().is_terminal() && io::stderr().is_terminal();
         Self {
             lock: Arc::new(Mutex::new(())),
             prompt: Arc::new(Mutex::new(PromptState::default())),
             color,
+            jsonl_file: jsonl_file.map(|file| Arc::new(Mutex::new(file))),
         }
     }
 
+    pub fn server_json_line(&self, line: &str, filtered_output: bool) -> io::Result<()> {
+        let _guard = self.lock.lock().expect("output lock poisoned");
+
+        if let Some(file) = self.jsonl_file.as_ref() {
+            let mut file = file.lock().expect("jsonl file lock poisoned");
+            writeln!(file, "{line}")?;
+            file.flush()?;
+        }
+
+        if self.jsonl_file.is_none() && !filtered_output {
+            self.clear_prompt_line_locked()?;
+            let mut stdout = io::stdout();
+            writeln!(stdout, "{line}")?;
+            stdout.flush()?;
+            self.redraw_prompt_locked()?;
+        }
+
+        Ok(())
+    }
+
     pub fn server_line(&self, line: &str) -> io::Result<()> {
         let _guard = self.lock.lock().expect("output lock poisoned");
         self.clear_prompt_line_locked()?;
@@ -120,3 +143,33 @@ impl Output {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::fs;
+
+    use super::*;
+
+    #[test]
+    fn server_json_line_writes_to_configured_file() {
+        let path = std::env::temp_dir().join(format!(
+            "codex-debug-client-output-{}.jsonl",
+            std::process::id()
+        ));
+        let file = File::create(&path).expect("create output file");
+        let output = Output::new(Some(file));
+
+        output
+            .server_json_line(r#"{"id":1}"#, false)
+            .expect("write unfiltered line");
+        output
+            .server_json_line(r#"{"id":2}"#, true)
+            .expect("write filtered line");
+
+        assert_eq!(
+            fs::read_to_string(&path).expect("read output file"),
+            "{\"id\":1}\n{\"id\":2}\n"
+        );
+        let _ = fs::remove_file(path);
+    }
+}

codex-rs/debug-client/src/reader.rs

@@ -67,8 +67,8 @@ pub fn start_reader(
             }
 
             let line = buffer.trim_end_matches(['\n', '\r']);
-            if !line.is_empty() && !filtered_output {
-                let _ = output.server_line(line);
+            if !line.is_empty() {
+                let _ = output.server_json_line(line, filtered_output);
             }
 
             let Ok(message) = serde_json::from_str::<JSONRPCMessage>(line) else {