[2/8] Support piped stdin in exec process API (#18086)

## Summary
- Add an explicit stdin mode to process/start.
- Keep normal non-interactive exec stdin closed while allowing
pipe-backed processes.

## Stack
```text
o  #18027 [8/8] Fail exec client operations after disconnect
│
o  #18025 [7/8] Cover MCP stdio tests with executor placement
│
o  #18089 [6/8] Wire remote MCP stdio through executor
│
o  #18088 [5/8] Add executor process transport for MCP stdio
│
o  #18087 [4/8] Abstract MCP stdio server launching
│
o  #18020 [3/8] Add pushed exec process events
│
@  #18086 [2/8] Support piped stdin in exec process API
│
o  #18085 [1/8] Add MCP server environment config
│
o  main
```

Co-authored-by: Codex <noreply@openai.com>

Author: aibrahim-oai

codex-rs/core/src/unified_exec/process_manager.rs

@@ -152,6 +152,7 @@ fn exec_server_params_for_request(
         env_policy,
         env,
         tty,
+        pipe_stdin: false,
         arg0: request.arg0.clone(),
     }
 }

codex-rs/exec-server/README.md

@@ -85,6 +85,7 @@ Request params:
     "PATH": "/usr/bin:/bin"
   },
   "tty": true,
+  "pipeStdin": false,
   "arg0": null
 }
 ```
@@ -95,8 +96,8 @@ Field definitions:
 - `argv`: command vector. It must be non-empty.
 - `cwd`: absolute working directory used for the child process.
 - `env`: environment variables passed to the child process.
-- `tty`: when `true`, spawn a PTY-backed interactive process; when `false`,
-  spawn a pipe-backed process with closed stdin.
+- `tty`: when `true`, spawn a PTY-backed interactive process.
+- `pipeStdin`: when `true`, keep non-PTY stdin writable via `process/write`.
 - `arg0`: optional argv0 override forwarded to `codex-utils-pty`.
 
 Response:
@@ -111,7 +112,7 @@ Behavior notes:
 
 - Reusing an existing `processId` is rejected.
 - PTY-backed processes accept later writes through `process/write`.
-- Pipe-backed processes are launched with stdin closed and reject writes.
+- Non-PTY processes reject writes unless `pipeStdin` is `true`.
 - Output is streamed asynchronously via `process/output`.
 - Exit is reported asynchronously via `process/exited`.
 
@@ -153,7 +154,7 @@ Response:
 
 ### `process/write`
 
-Writes raw bytes to a running PTY-backed process stdin.
+Writes raw bytes to a running process stdin.
 
 Request params:
 
@@ -177,7 +178,7 @@ Response:
 Behavior notes:
 
 - Writes to an unknown `processId` are rejected.
-- Writes to a non-PTY process are rejected because stdin is already closed.
+- Writes to a non-PTY process are rejected unless it started with `pipeStdin`.
 
 ### `process/terminate`
 
@@ -325,7 +326,7 @@ Initialize:
 Start a process:
 
 ```json
-{"id":2,"method":"process/start","params":{"processId":"proc-1","argv":["bash","-lc","printf 'ready\\n'; while IFS= read -r line; do printf 'echo:%s\\n' \"$line\"; done"],"cwd":"/tmp","env":{"PATH":"/usr/bin:/bin"},"tty":true,"arg0":null}}
+{"id":2,"method":"process/start","params":{"processId":"proc-1","argv":["bash","-lc","printf 'ready\\n'; while IFS= read -r line; do printf 'echo:%s\\n' \"$line\"; done"],"cwd":"/tmp","env":{"PATH":"/usr/bin:/bin"},"tty":true,"pipeStdin":false,"arg0":null}}
 {"id":2,"result":{"processId":"proc-1"}}
 {"method":"process/output","params":{"processId":"proc-1","seq":1,"stream":"stdout","chunk":"cmVhZHkK"}}
 ```

codex-rs/exec-server/src/environment.rs

@@ -346,6 +346,7 @@ mod tests {
                 env_policy: None,
                 env: Default::default(),
                 tty: false,
+                pipe_stdin: false,
                 arg0: None,
             })
             .await

codex-rs/exec-server/src/local_process.rs

@@ -59,6 +59,7 @@ struct RetainedOutputChunk {
 struct RunningProcess {
     session: ExecCommandSession,
     tty: bool,
+    pipe_stdin: bool,
     output: VecDeque<RetainedOutputChunk>,
     retained_bytes: usize,
     next_seq: u64,
@@ -165,6 +166,15 @@ impl LocalProcess {
                 TerminalSize::default(),
             )
             .await
+        } else if params.pipe_stdin {
+            codex_utils_pty::spawn_pipe_process(
+                program,
+                args,
+                params.cwd.as_path(),
+                &env,
+                &params.arg0,
+            )
+            .await
         } else {
             codex_utils_pty::spawn_pipe_process_no_stdin(
                 program,
@@ -195,6 +205,7 @@ impl LocalProcess {
                 ProcessEntry::Running(Box::new(RunningProcess {
                     session: spawned.session,
                     tty: params.tty,
+                    pipe_stdin: params.pipe_stdin,
                     output: VecDeque::new(),
                     retained_bytes: 0,
                     next_seq: 1,
@@ -339,7 +350,7 @@ impl LocalProcess {
                     status: WriteStatus::Starting,
                 });
             };
-            if !process.tty {
+            if !process.tty && !process.pipe_stdin {
                 return Ok(WriteResponse {
                     status: WriteStatus::StdinClosed,
                 });
@@ -667,6 +678,7 @@ mod tests {
             env_policy: None,
             env,
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         }
     }

codex-rs/exec-server/src/protocol.rs

@@ -69,6 +69,9 @@ pub struct ExecParams {
     pub env_policy: Option<ExecEnvPolicy>,
     pub env: HashMap<String, String>,
     pub tty: bool,
+    /// Keep non-tty stdin writable through `process/write`.
+    #[serde(default)]
+    pub pipe_stdin: bool,
     pub arg0: Option<String>,
 }
 

codex-rs/exec-server/src/server/handler/tests.rs

@@ -30,6 +30,7 @@ fn exec_params_with_argv(process_id: &str, argv: Vec<String>) -> ExecParams {
         env_policy: None,
         env: inherited_path_env(),
         tty: false,
+        pipe_stdin: false,
         arg0: None,
     }
 }

codex-rs/exec-server/src/server/processor.rs

@@ -393,6 +393,7 @@ mod tests {
             env_policy: None,
             env,
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         }
     }

codex-rs/exec-server/tests/exec_process.rs

@@ -12,6 +12,7 @@ use codex_exec_server::ExecProcess;
 use codex_exec_server::ProcessId;
 use codex_exec_server::ReadResponse;
 use codex_exec_server::StartedExecProcess;
+use codex_exec_server::WriteStatus;
 use pretty_assertions::assert_eq;
 use test_case::test_case;
 use tokio::sync::watch;
@@ -54,6 +55,7 @@ async fn assert_exec_process_starts_and_exits(use_remote: bool) -> Result<()> {
             env_policy: /*env_policy*/ None,
             env: Default::default(),
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         })
         .await?;
@@ -131,6 +133,7 @@ async fn assert_exec_process_streams_output(use_remote: bool) -> Result<()> {
             env_policy: /*env_policy*/ None,
             env: Default::default(),
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         })
         .await?;
@@ -164,6 +167,7 @@ async fn assert_exec_process_write_then_read(use_remote: bool) -> Result<()> {
             env_policy: /*env_policy*/ None,
             env: Default::default(),
             tty: true,
+            pipe_stdin: false,
             arg0: None,
         })
         .await?;
@@ -184,6 +188,73 @@ async fn assert_exec_process_write_then_read(use_remote: bool) -> Result<()> {
     Ok(())
 }
 
+async fn assert_exec_process_write_then_read_without_tty(use_remote: bool) -> Result<()> {
+    let context = create_process_context(use_remote).await?;
+    let process_id = "proc-stdin-pipe".to_string();
+    let session = context
+        .backend
+        .start(ExecParams {
+            process_id: process_id.clone().into(),
+            argv: vec![
+                "/bin/sh".to_string(),
+                "-c".to_string(),
+                "IFS= read line; printf 'from-stdin:%s\\n' \"$line\"".to_string(),
+            ],
+            cwd: std::env::current_dir()?,
+            env_policy: /*env_policy*/ None,
+            env: Default::default(),
+            tty: false,
+            pipe_stdin: true,
+            arg0: None,
+        })
+        .await?;
+    assert_eq!(session.process.process_id().as_str(), process_id);
+
+    tokio::time::sleep(Duration::from_millis(200)).await;
+    let write_response = session.process.write(b"hello\n".to_vec()).await?;
+    assert_eq!(write_response.status, WriteStatus::Accepted);
+    let StartedExecProcess { process } = session;
+    let wake_rx = process.subscribe_wake();
+    let actual = collect_process_output_from_reads(process, wake_rx).await?;
+
+    assert_eq!(actual, ("from-stdin:hello\n".to_string(), Some(0), true));
+    Ok(())
+}
+
+async fn assert_exec_process_rejects_write_without_pipe_stdin(use_remote: bool) -> Result<()> {
+    let context = create_process_context(use_remote).await?;
+    let process_id = "proc-stdin-closed".to_string();
+    let session = context
+        .backend
+        .start(ExecParams {
+            process_id: process_id.clone().into(),
+            argv: vec![
+                "/bin/sh".to_string(),
+                "-c".to_string(),
+                "sleep 0.3; if IFS= read -r line; then printf 'read:%s\\n' \"$line\"; else printf 'eof\\n'; fi".to_string(),
+            ],
+            cwd: std::env::current_dir()?,
+            env_policy: /*env_policy*/ None,
+            env: Default::default(),
+            tty: false,
+            pipe_stdin: false,
+            arg0: None,
+        })
+        .await?;
+    assert_eq!(session.process.process_id().as_str(), process_id);
+
+    let write_response = session.process.write(b"ignored\n".to_vec()).await?;
+    assert_eq!(write_response.status, WriteStatus::StdinClosed);
+    let StartedExecProcess { process } = session;
+    let wake_rx = process.subscribe_wake();
+    let (output, exit_code, closed) = collect_process_output_from_reads(process, wake_rx).await?;
+
+    assert_eq!(output, "eof\n");
+    assert_eq!(exit_code, Some(0));
+    assert!(closed);
+    Ok(())
+}
+
 async fn assert_exec_process_preserves_queued_events_before_subscribe(
     use_remote: bool,
 ) -> Result<()> {
@@ -201,6 +272,7 @@ async fn assert_exec_process_preserves_queued_events_before_subscribe(
             env_policy: /*env_policy*/ None,
             env: Default::default(),
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         })
         .await?;
@@ -234,6 +306,7 @@ async fn remote_exec_process_reports_transport_disconnect() -> Result<()> {
             env_policy: /*env_policy*/ None,
             env: Default::default(),
             tty: false,
+            pipe_stdin: false,
             arg0: None,
         })
         .await?;
@@ -289,6 +362,24 @@ async fn exec_process_write_then_read(use_remote: bool) -> Result<()> {
     assert_exec_process_write_then_read(use_remote).await
 }
 
+#[test_case(false ; "local")]
+#[test_case(true ; "remote")]
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+// Serialize tests that launch a real exec-server process through the full CLI.
+#[serial_test::serial(remote_exec_server)]
+async fn exec_process_write_then_read_without_tty(use_remote: bool) -> Result<()> {
+    assert_exec_process_write_then_read_without_tty(use_remote).await
+}
+
+#[test_case(false ; "local")]
+#[test_case(true ; "remote")]
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+// Serialize tests that launch a real exec-server process through the full CLI.
+#[serial_test::serial(remote_exec_server)]
+async fn exec_process_rejects_write_without_pipe_stdin(use_remote: bool) -> Result<()> {
+    assert_exec_process_rejects_write_without_pipe_stdin(use_remote).await
+}
+
 #[test_case(false ; "local")]
 #[test_case(true ; "remote")]
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]