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## Why Several bug reports describe thread shutdown (including subagent threads) leaving stdio MCP server processes behind. These reports all point at the same lifecycle gap: Codex launches stdio MCP servers, but the session-level shutdown path does not explicitly close MCP clients or terminate the server process tree. Fixes #12491 Fixes #12976 Fixes #18881 Fixes #19469 ## History This is best understood as a regression/coverage gap in MCP session lifecycle management, not as stdio MCP cleanup being absent all along. #10710 added process-group cleanup for stdio MCP servers, but that cleanup only runs when the `RmcpClient`/transport is dropped. The older reports (#12491 and #12976) came after that cleanup existed, which suggests the remaining problem was that some higher-level shutdown paths kept the MCP manager alive or replaced it without explicitly draining clients. The newer reports (#18881 and #19469) exposed the same family around manager replacement and shutdown. ## What changed - Added an explicit stdio MCP process handle in `codex-rmcp-client` so local MCP servers terminate their process group and executor-backed MCP servers call the executor process terminator. - Added `RmcpClient::shutdown()` and manager-level MCP shutdown draining so session shutdown, channel-close fallback, MCP refresh, and connector probing stop owned MCP clients. - Added regression coverage that starts a stdio MCP server, begins an in-flight blocking tool call, shuts down the client, and asserts the server process exits. ## Verification - `cargo test -p codex-rmcp-client` - `cargo test -p codex-mcp` - `just fix -p codex-rmcp-client` - `just fix -p codex-mcp` - `just fix -p codex-core` - Manual before/after validation with a temporary repro script: - Pre-fix binary from `HEAD^` (`fed0a8f4fa`): reproduced the leak with surviving MCP server and child PIDs, `survivors=[77583, 77592]`, `leaked=true`. - Post-fix binary from this branch (`67e318148b`): verified both MCP processes were gone after interrupting `codex exec`, `survivors=[]`, `leaked=false`.