[codex] add responses proxy JSON dumps (#16753)

This makes Responses API proxy request/response dumping first-class by
adding an optional `--dump-dir` flag that emits paired JSON files with
shared sequence/timestamp prefixes, captures full request and response
headers and records parsed JSON bodies.

codex-rs/responses-api-proxy/src/lib.rs

@@ -1,5 +1,6 @@
 use std::fs::File;
 use std::fs::{self};
+use std::io::Read;
 use std::io::Write;
 use std::net::SocketAddr;
 use std::net::TcpListener;
@@ -27,7 +28,9 @@ use tiny_http::Response;
 use tiny_http::Server;
 use tiny_http::StatusCode;
 
+mod dump;
 mod read_api_key;
+use dump::ExchangeDumper;
 use read_api_key::read_auth_header_from_stdin;
 
 /// CLI arguments for the proxy.
@@ -49,6 +52,10 @@ pub struct Args {
     /// Absolute URL the proxy should forward requests to (defaults to OpenAI).
     #[arg(long, default_value = "https://api.openai.com/v1/responses")]
     pub upstream_url: String,
+
+    /// Directory where request/response dumps should be written as JSON.
+    #[arg(long, value_name = "DIR")]
+    pub dump_dir: Option<PathBuf>,
 }
 
 #[derive(Serialize)]
@@ -79,6 +86,12 @@ pub fn run_main(args: Args) -> Result<()> {
         upstream_url,
         host_header,
     });
+    let dump_dir = args
+        .dump_dir
+        .map(ExchangeDumper::new)
+        .transpose()
+        .context("creating --dump-dir")?
+        .map(Arc::new);
 
     let (listener, bound_addr) = bind_listener(args.port)?;
     if let Some(path) = args.server_info.as_ref() {
@@ -100,13 +113,20 @@ pub fn run_main(args: Args) -> Result<()> {
     for request in server.incoming_requests() {
         let client = client.clone();
         let forward_config = forward_config.clone();
+        let dump_dir = dump_dir.clone();
         std::thread::spawn(move || {
             if http_shutdown && request.method() == &Method::Get && request.url() == "/shutdown" {
                 let _ = request.respond(Response::new_empty(StatusCode(200)));
                 std::process::exit(0);
             }
 
-            if let Err(e) = forward_request(&client, auth_header, &forward_config, request) {
+            if let Err(e) = forward_request(
+                &client,
+                auth_header,
+                &forward_config,
+                dump_dir.as_deref(),
+                request,
+            ) {
                 eprintln!("forwarding error: {e}");
             }
         });
@@ -144,6 +164,7 @@ fn forward_request(
     client: &Client,
     auth_header: &'static str,
     config: &ForwardConfig,
+    dump_dir: Option<&ExchangeDumper>,
     mut req: Request,
 ) -> Result<()> {
     // Only allow POST /v1/responses exactly, no query string.
@@ -159,8 +180,18 @@ fn forward_request(
 
     // Read request body
     let mut body = Vec::new();
-    let mut reader = req.as_reader();
-    std::io::Read::read_to_end(&mut reader, &mut body)?;
+    let reader = req.as_reader();
+    reader.read_to_end(&mut body)?;
+
+    let exchange_dump = dump_dir.and_then(|dump_dir| {
+        dump_dir
+            .dump_request(&method, &url_path, req.headers(), &body)
+            .map_err(|err| {
+                eprintln!("responses-api-proxy failed to dump request: {err}");
+                err
+            })
+            .ok()
+    });
 
     // Build headers for upstream, forwarding everything from the incoming
     // request except Authorization (we replace it below).
@@ -224,10 +255,17 @@ fn forward_request(
         }
     });
 
+    let response_body: Box<dyn Read + Send> = if let Some(exchange_dump) = exchange_dump {
+        let headers = upstream_resp.headers().clone();
+        Box::new(exchange_dump.tee_response_body(status.as_u16(), &headers, upstream_resp))
+    } else {
+        Box::new(upstream_resp)
+    };
+
     let response = Response::new(
         StatusCode(status.as_u16()),
         response_headers,
-        upstream_resp,
+        response_body,
         content_length,
         None,
     );