Local stdio MCP servers leak processes on disconnect / replace / rollback #26714

Description

When a local stdio MCP server is disconnected, replaced (via a second MCP.add() with the same name), or rolled back after tool discovery (defs()) failure, the spawned process tree is not terminated. Only client.close() is called, which stops the SDK transport but leaves the child process and its descendants running as orphans.

The existing cleanup code (added in #15516) only covers the instance disposal path, and even there it only kills descendant PIDs — not the transport's own root PID.

Over multiple sessions these orphaned processes accumulate, consuming memory and potentially holding ports.

OpenCode version

1.3.13 (dev branch, commit d112da0)

Steps to reproduce

1. Configure a local stdio MCP server in opencode.json (e.g. "type": "local", "command": ["node", "server.js"])
2. Disconnect the server via MCP.disconnect() — or trigger a replace by calling MCP.add() twice with the same name
3. Check running processes: the MCP server process is still alive with PPID changed to 1

Operating System

macOS (Apple Silicon)

Terminal

Ghostty

Additional context

This is related to #15808 and #8257 but covers a distinct gap:

vs #15808 ("opencode run: MCP child processes not terminated on exit"): That issue is about the opencode run headless command not cleaning up MCP processes when the CLI exits. #15516 partially addressed it by adding descendant kill in the dispose handler. But the in-process teardown paths — disconnect(), add-replace, and defs-failure rollback — were never covered. This issue is specifically about those paths.

vs #8257 ("Memory leak: MCP clients not closed before reassignment"): That issue is about connections/sockets leaking when a client reference is overwritten without calling .close(). Our concern is broader: even when .close() is called (as in the current disconnect path), the process is not killed. Client close does not guarantee process termination.

No existing PR addresses this specific set of gaps. PR #15516 (merged) added descendant cleanup in the dispose path only. PR #21626 (open) is about SDK process groups, not MCP teardown. No one has submitted a fix covering the root PID kill or the disconnect/rollback paths.