[5/8] Add executor process transport for MCP stdio by aibrahim-oai · Pull Request #17987 · openai/codex

aibrahim-oai · 2026-04-15T19:49:26Z

Summary

Add an rmcp transport backed by executor process read/write calls.
Add an executor stdio launcher that starts non-tty processes with piped stdin.

Stack

o  #18027 [8/8] Fail exec client operations after disconnect
│
o  #18025 [7/8] Cover MCP stdio tests with executor placement
│
o  #17974 [6/8] Wire remote MCP stdio through executor
│
@  #17987 [5/8] Add executor process transport for MCP stdio
│
o  #17986 [4/8] Abstract MCP stdio server launching
│
o  #18020 [3/8] Add pushed exec process events
│
o  #17985 [2/8] Support piped stdin in exec process API
│
o  #17984 [1/8] Add MCP server environment config
│
o  main

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chatgpt-codex-connector · 2026-04-15T21:54:15Z

+                LaunchedStdioServerTransport::Executor { transport } => (
+                    service::serve_client(client_service, transport).boxed(),
+                    None,
+                    None,


Terminate executor MCP process when handshake aborts

connect_pending_transport wraps serve_client(...) in a timeout; on timeout or handshake error, that future is dropped before RunningService exists. In the executor branch there is no cleanup guard (None), and ExecutorProcessTransport only terminates the child in close(). This can leave remote MCP server processes running after failed initialization.

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chatgpt-codex-connector · 2026-04-15T22:08:18Z

+            Err(error) => {
+                debug!(
+                    "Failed to parse remote MCP server message ({}): {error}",
+                    self.program_name
+                );
+                None


Fail fast when remote MCP emits invalid JSON-RPC

take_stdout_message drops lines that fail serde_json parsing and keeps the transport alive. If a malformed line is actually a response, the request is never surfaced as an error and can hang until an outer timeout. The local stdio transport treats decode errors as transport failure, so this new path diverges and can hide protocol breakage.

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chatgpt-codex-connector · 2026-04-15T22:53:03Z

+                Err(broadcast::error::RecvError::Lagged(skipped)) => {
+                    warn!(
+                        "Remote MCP server output stream lagged ({}): skipped {skipped} events",
+                        self.program_name
+                    );
+                    self.closed = true;


Recover from lagged events instead of ending MCP stream

In ExecutorProcessTransport::receive_message, a broadcast::RecvError::Lagged sets self.closed = true, which makes receive() return EOF on the next loop. Because exec event channels are bounded, bursty/large stdout can trigger lag and this path truncates in-flight JSON-RPC output while the process may still be running. That can break handshakes or tool responses under load instead of resynchronizing via retained output.

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Add an rmcp transport and runtime that bridge stdio bytes through the executor process API. Co-authored-by: Codex <noreply@openai.com>

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chatgpt-codex-connector · 2026-04-16T05:18:43Z

+        let started = exec_backend
+            .start(ExecParams {
+                process_id,
+                argv,
+                cwd: cwd.unwrap_or(default_cwd),
+                env_policy: None,
+                env,
+                tty: false,
+                stdin: ExecStdinMode::Piped,
+                arg0: None,
+            })
+            .await
+            .map_err(io::Error::other)?;
+
+        Ok(LaunchedStdioServer {
+            transport: LaunchedStdioServerTransport::Executor {
+                transport: ExecutorProcessTransport::new(started.process, program_name),
+            },


Subscribe to process events before starting executor process

launch_server awaits exec_backend.start(...) and only then builds ExecutorProcessTransport, which calls process.subscribe_events(). tokio::broadcast does not replay old messages, so early Output/Closed events can be lost. If the MCP process exits quickly, the terminal event may be missed and receive() can block until an external timeout (or forever if no timeout is configured).

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aibrahim-oai · 2026-04-16T06:14:10Z

Recreated as #18088 from an openai/codex head branch so internal CI uses the normal credentials path.

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Comment thread codex-rs/rmcp-client/src/stdio_transport_runtime.rs Outdated

aibrahim-oai changed the title ~~Add executor-backed rmcp stdio transport~~ [4/5] Add executor-backed rmcp stdio transport Apr 15, 2026

aibrahim-oai force-pushed the dev/remote-mcp-executor-stdio branch from fdb0d74 to 377fd73 Compare April 15, 2026 21:48

aibrahim-oai changed the title ~~[4/5] Add executor-backed rmcp stdio transport~~ [4/5] Add executor process transport for MCP stdio Apr 15, 2026

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aibrahim-oai force-pushed the dev/remote-mcp-executor-stdio branch from 377fd73 to c468eef Compare April 15, 2026 21:57

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aibrahim-oai force-pushed the dev/remote-mcp-executor-stdio branch from c468eef to 83e867a Compare April 15, 2026 22:47

aibrahim-oai force-pushed the dev/remote-mcp-stdio-runtime branch from 595325e to 3842a64 Compare April 15, 2026 22:47

aibrahim-oai changed the title ~~[4/5] Add executor process transport for MCP stdio~~ [5/6] Add executor process transport for MCP stdio Apr 15, 2026

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aibrahim-oai changed the title ~~[5/6] Add executor process transport for MCP stdio~~ [5/7] Add executor process transport for MCP stdio Apr 15, 2026

aibrahim-oai changed the title ~~[5/7] Add executor process transport for MCP stdio~~ [5/8] Add executor process transport for MCP stdio Apr 16, 2026

Add executor-backed rmcp stdio transport

f6e61df

Add an rmcp transport and runtime that bridge stdio bytes through the executor process API. Co-authored-by: Codex <noreply@openai.com>

aibrahim-oai force-pushed the dev/remote-mcp-stdio-runtime branch from 3842a64 to b440257 Compare April 16, 2026 05:10

aibrahim-oai force-pushed the dev/remote-mcp-executor-stdio branch from 83e867a to f6e61df Compare April 16, 2026 05:11

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[5/8] Add executor process transport for MCP stdio#17987

[5/8] Add executor process transport for MCP stdio#17987
aibrahim-oai wants to merge 1 commit intoopenai:dev/remote-mcp-stdio-runtimefrom
aibrahim-oai:dev/remote-mcp-executor-stdio

aibrahim-oai commented Apr 15, 2026 •

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aibrahim-oai commented Apr 15, 2026 •

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