[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/dump.rs

@@ -0,0 +1,360 @@
+use std::fs;
+use std::io;
+use std::io::Read;
+use std::path::PathBuf;
+use std::sync::atomic::AtomicU64;
+use std::sync::atomic::Ordering;
+use std::time::SystemTime;
+use std::time::UNIX_EPOCH;
+
+use reqwest::header::HeaderMap;
+use serde::Serialize;
+use serde_json::Value;
+use tiny_http::Header;
+use tiny_http::Method;
+
+const AUTHORIZATION_HEADER_NAME: &str = "authorization";
+const REDACTED_HEADER_VALUE: &str = "[REDACTED]";
+
+pub(crate) struct ExchangeDumper {
+    dump_dir: PathBuf,
+    next_sequence: AtomicU64,
+}
+
+impl ExchangeDumper {
+    pub(crate) fn new(dump_dir: PathBuf) -> io::Result<Self> {
+        fs::create_dir_all(&dump_dir)?;
+
+        Ok(Self {
+            dump_dir,
+            next_sequence: AtomicU64::new(1),
+        })
+    }
+
+    pub(crate) fn dump_request(
+        &self,
+        method: &Method,
+        url: &str,
+        headers: &[Header],
+        body: &[u8],
+    ) -> io::Result<ExchangeDump> {
+        let sequence = self.next_sequence.fetch_add(1, Ordering::Relaxed);
+        let timestamp_ms = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .map_or(0, |duration| duration.as_millis());
+        let prefix = format!("{sequence:06}-{timestamp_ms}");
+
+        let request_path = self.dump_dir.join(format!("{prefix}-request.json"));
+        let response_path = self.dump_dir.join(format!("{prefix}-response.json"));
+
+        let request_dump = RequestDump {
+            method: method.as_str().to_string(),
+            url: url.to_string(),
+            headers: headers.iter().map(HeaderDump::from).collect(),
+            body: dump_body(body),
+        };
+
+        write_json_dump(&request_path, &request_dump)?;
+
+        Ok(ExchangeDump { response_path })
+    }
+}
+
+pub(crate) struct ExchangeDump {
+    response_path: PathBuf,
+}
+
+impl ExchangeDump {
+    pub(crate) fn tee_response_body<R: Read>(
+        self,
+        status: u16,
+        headers: &HeaderMap,
+        response_body: R,
+    ) -> ResponseBodyDump<R> {
+        ResponseBodyDump {
+            response_body,
+            response_path: self.response_path,
+            status,
+            headers: headers.iter().map(HeaderDump::from).collect(),
+            body: Vec::new(),
+            dump_written: false,
+        }
+    }
+}
+
+pub(crate) struct ResponseBodyDump<R> {
+    response_body: R,
+    response_path: PathBuf,
+    status: u16,
+    headers: Vec<HeaderDump>,
+    body: Vec<u8>,
+    dump_written: bool,
+}
+
+impl<R> ResponseBodyDump<R> {
+    fn write_dump_if_needed(&mut self) {
+        if self.dump_written {
+            return;
+        }
+
+        self.dump_written = true;
+
+        let response_dump = ResponseDump {
+            status: self.status,
+            headers: std::mem::take(&mut self.headers),
+            body: dump_body(&self.body),
+        };
+
+        if let Err(err) = write_json_dump(&self.response_path, &response_dump) {
+            eprintln!(
+                "responses-api-proxy failed to write {}: {err}",
+                self.response_path.display()
+            );
+        }
+    }
+}
+
+impl<R: Read> Read for ResponseBodyDump<R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let bytes_read = self.response_body.read(buf)?;
+        if bytes_read == 0 {
+            self.write_dump_if_needed();
+            return Ok(0);
+        }
+
+        self.body.extend_from_slice(&buf[..bytes_read]);
+        Ok(bytes_read)
+    }
+}
+
+impl<R> Drop for ResponseBodyDump<R> {
+    fn drop(&mut self) {
+        self.write_dump_if_needed();
+    }
+}
+
+#[derive(Serialize)]
+struct RequestDump {
+    method: String,
+    url: String,
+    headers: Vec<HeaderDump>,
+    body: Value,
+}
+
+#[derive(Serialize)]
+struct ResponseDump {
+    status: u16,
+    headers: Vec<HeaderDump>,
+    body: Value,
+}
+
+#[derive(Debug, Serialize)]
+struct HeaderDump {
+    name: String,
+    value: String,
+}
+
+impl From<&Header> for HeaderDump {
+    fn from(header: &Header) -> Self {
+        let name = header.field.as_str().to_string();
+        let value = if should_redact_header(&name) {
+            REDACTED_HEADER_VALUE.to_string()
+        } else {
+            header.value.as_str().to_string()
+        };
+
+        Self { name, value }
+    }
+}
+
+impl From<(&reqwest::header::HeaderName, &reqwest::header::HeaderValue)> for HeaderDump {
+    fn from(header: (&reqwest::header::HeaderName, &reqwest::header::HeaderValue)) -> Self {
+        let name = header.0.as_str();
+        let value = if should_redact_header(name) {
+            REDACTED_HEADER_VALUE.to_string()
+        } else {
+            String::from_utf8_lossy(header.1.as_bytes()).into_owned()
+        };
+
+        Self {
+            name: name.to_string(),
+            value,
+        }
+    }
+}
+
+fn should_redact_header(name: &str) -> bool {
+    name.eq_ignore_ascii_case(AUTHORIZATION_HEADER_NAME)
+        || name.to_ascii_lowercase().contains("cookie")
+}
+
+fn dump_body(body: &[u8]) -> Value {
+    serde_json::from_slice(body)
+        .unwrap_or_else(|_| Value::String(String::from_utf8_lossy(body).into_owned()))
+}
+
+fn write_json_dump(path: &PathBuf, dump: &impl Serialize) -> io::Result<()> {
+    let mut bytes = serde_json::to_vec_pretty(dump)
+        .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))?;
+    bytes.push(b'\n');
+    fs::write(path, bytes)
+}
+
+#[cfg(test)]
+mod tests {
+    use std::fs;
+    use std::io::Cursor;
+    use std::io::Read;
+    use std::sync::atomic::AtomicU64;
+    use std::sync::atomic::Ordering;
+
+    use pretty_assertions::assert_eq;
+    use reqwest::header::AUTHORIZATION;
+    use reqwest::header::CONTENT_TYPE;
+    use reqwest::header::HeaderMap;
+    use reqwest::header::HeaderValue;
+    use serde_json::json;
+    use tiny_http::Header;
+    use tiny_http::Method;
+
+    use super::ExchangeDumper;
+
+    static NEXT_TEST_DIR: AtomicU64 = AtomicU64::new(0);
+
+    #[test]
+    fn dump_request_writes_redacted_headers_and_json_body() {
+        let dump_dir = test_dump_dir();
+        let dumper = ExchangeDumper::new(dump_dir.clone()).expect("create dumper");
+        let headers = vec![
+            Header::from_bytes(&b"Authorization"[..], &b"Bearer secret"[..])
+                .expect("authorization header"),
+            Header::from_bytes(&b"Cookie"[..], &b"user-session=secret"[..]).expect("cookie header"),
+            Header::from_bytes(&b"Content-Type"[..], &b"application/json"[..])
+                .expect("content-type header"),
+        ];
+
+        let exchange_dump = dumper
+            .dump_request(
+                &Method::Post,
+                "/v1/responses",
+                &headers,
+                br#"{"model":"gpt-5.4"}"#,
+            )
+            .expect("dump request");
+
+        let request_dump = fs::read_to_string(dump_file_with_suffix(&dump_dir, "-request.json"))
+            .expect("read request dump");
+
+        assert_eq!(
+            serde_json::from_str::<serde_json::Value>(&request_dump).expect("parse request dump"),
+            json!({
+                "method": "POST",
+                "url": "/v1/responses",
+                "headers": [
+                    {
+                        "name": "Authorization",
+                        "value": "[REDACTED]"
+                    },
+                    {
+                        "name": "Cookie",
+                        "value": "[REDACTED]"
+                    },
+                    {
+                        "name": "Content-Type",
+                        "value": "application/json"
+                    }
+                ],
+                "body": {
+                    "model": "gpt-5.4"
+                }
+            })
+        );
+        assert!(
+            exchange_dump
+                .response_path
+                .file_name()
+                .expect("response dump file name")
+                .to_string_lossy()
+                .ends_with("-response.json")
+        );
+
+        fs::remove_dir_all(dump_dir).expect("remove test dump dir");
+    }
+
+    #[test]
+    fn response_body_dump_streams_body_and_writes_response_file() {
+        let dump_dir = test_dump_dir();
+        let dumper = ExchangeDumper::new(dump_dir.clone()).expect("create dumper");
+        let exchange_dump = dumper
+            .dump_request(&Method::Post, "/v1/responses", &[], b"{}")
+            .expect("dump request");
+
+        let mut headers = HeaderMap::new();
+        headers.insert(CONTENT_TYPE, HeaderValue::from_static("text/event-stream"));
+        headers.insert(AUTHORIZATION, HeaderValue::from_static("Bearer secret"));
+        headers.insert(
+            "set-cookie",
+            HeaderValue::from_static("user-session=secret"),
+        );
+
+        let mut response_body = String::new();
+        exchange_dump
+            .tee_response_body(
+                /*status*/ 200,
+                &headers,
+                Cursor::new(b"data: hello\n\n".to_vec()),
+            )
+            .read_to_string(&mut response_body)
+            .expect("read response body");
+
+        let response_dump = fs::read_to_string(dump_file_with_suffix(&dump_dir, "-response.json"))
+            .expect("read response dump");
+
+        assert_eq!(response_body, "data: hello\n\n");
+        assert_eq!(
+            serde_json::from_str::<serde_json::Value>(&response_dump).expect("parse response dump"),
+            json!({
+                "status": 200,
+                "headers": [
+                    {
+                        "name": "content-type",
+                        "value": "text/event-stream"
+                    },
+                    {
+                        "name": "authorization",
+                        "value": "[REDACTED]"
+                    },
+                    {
+                        "name": "set-cookie",
+                        "value": "[REDACTED]"
+                    }
+                ],
+                "body": "data: hello\n\n"
+            })
+        );
+
+        fs::remove_dir_all(dump_dir).expect("remove test dump dir");
+    }
+
+    fn test_dump_dir() -> std::path::PathBuf {
+        let test_id = NEXT_TEST_DIR.fetch_add(1, Ordering::Relaxed);
+        let dump_dir = std::env::temp_dir().join(format!(
+            "codex-responses-api-proxy-dump-test-{}-{test_id}",
+            std::process::id()
+        ));
+        fs::create_dir_all(&dump_dir).expect("create test dump dir");
+        dump_dir
+    }
+
+    fn dump_file_with_suffix(dump_dir: &std::path::Path, suffix: &str) -> std::path::PathBuf {
+        let mut matches = fs::read_dir(dump_dir)
+            .expect("read dump dir")
+            .map(|entry| entry.expect("read dump entry").path())
+            .filter(|path| path.to_string_lossy().ends_with(suffix))
+            .collect::<Vec<_>>();
+        matches.sort();
+
+        assert_eq!(matches.len(), 1);
+        matches.pop().expect("single dump file")
+    }
+}

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,
     );