🩹 Codex app-server leaks MCP process stacks for subagents, causing linear memory growth on Linux #25015

Summary

Codex app-server appears to start additional stdio MCP process stacks when spawning subagents. After the subagents complete and close_agent is called, some MCP child process trees remain alive under the long-lived codex app-server.

This causes process count and memory usage to grow roughly linearly with subagent usage. On my machine this previously caused heavy swap pressure and an OOM/desktop session collapse.

I also created a Linux-only diagnostic/workaround tool that helped confirm the source of the growth:

https://github.com/LostFrxks/codex-mcp-clean

Environment

• Codex CLI: codex-cli 0.128.0
• OS: Ubuntu 24.04.4 LTS
• Kernel: Linux 6.17.0-29-generic x86_64
• Machine: 16 GB RAM, 16 GB swap, 8 CPU cores
• Surface: Codex app-server / IDE integration
• Reduced MCP set during repro:
  • Serena
  • Telegram
  • YouTrack via mcp-remote

Heavy MCP servers such as Playwright, Context7, and Slack were disabled for the reduced repro.

Reproduction Pattern

1. Start Codex app-server with stdio MCP servers enabled.
2. Take a baseline process/memory snapshot of the codex app-server child tree.
3. Spawn subagents with trivial tasks that do not use MCP tools.
4. Wait for the subagents to complete.
5. Call close_agent for each subagent.
6. Wait briefly for Codex cleanup.
7. Check the codex app-server child process tree again.
8. Optionally repeat with larger subagent batches.

Small Live Repro: 3 Subagents

Before spawning 3 trivial subagents, the selected app-server had:

• 23 descendants
• 6 MCP launcher roots
• RSS around 203 MiB

After spawning 3 subagents:

• 74 descendants
• 24 MCP launcher roots
• RSS around 3.7 GiB
• Snapshot report showed 18 new MCP launcher roots / 50 new processes

After all 3 subagents completed and close_agent was called for each one, Codex cleaned up part of the process tree, but the snapshot still showed:

• 9 MCP launcher roots
• 33 processes

A dry-run with codex-mcp-clean cleanup --root-pid <PID> identified those remaining MCP subtrees. Running the same command with --kill terminated them:

TERM sent to 33 processes; SIGKILL fallback attempted for 0

After cleanup:

• snapshot report returned to launcher_roots=0, processes_total=0
• app-server returned to about the previous baseline MCP footprint
• available memory increased back from about 4.7 GiB to about 6.8 GiB

Larger Repro: 10 + 10 Subagents

In an earlier larger run, before the batch:

• app-server descendants: 46
• total RSS under app-server: about 2.9 GiB
• total PSS under app-server: about 1.6 GiB
• MCP bundles: about 4

After spawning and closing 10 subagents:

• app-server descendants: 156
• total RSS: about 7.9 GiB
• total PSS: about 3.6 GiB
• MCP bundles: about 14

After spawning and closing another 10 subagents:

• app-server descendants: 266
• total RSS: about 11.7 GiB
• swap usage grew to about 6.4 GiB
• MCP bundles: about 24

The growth matched the number of spawned subagents.

Expected Behavior

After a subagent finishes and is closed:

• MCP processes created for that subagent should be terminated, or
• subagents should reuse a shared project/app-server MCP process pool, or
• Codex should allow subagents to start without inheriting all parent-session MCP servers.

Repeated subagent usage should return to a bounded baseline instead of accumulating MCP process stacks.

Actual Behavior

Subagent creation starts additional MCP process stacks, even when the subagent task does not call MCP tools.

After subagent completion and close_agent, some MCP child process trees remain alive under the app-server, causing process and memory growth.

Diagnostic / Workaround Tool

I wrote a small Linux-only diagnostic/workaround tool:

https://github.com/LostFrxks/codex-mcp-clean

It snapshots a selected Codex app-server PID and later reports or kills only MCP launcher subtrees created after that snapshot.

This is not a proposed upstream fix, but it helped confirm that stale MCP child trees are the source of the memory growth. It also avoids touching other Codex app-server PIDs.

Related Issues

This looks related to the same defect class as:

• Subagents leak stdio MCP helper trees in Codex App; xcodebuildmcp and chrome-devtools-mcp accumulate indefinitely #17574
• Regression: Playwright MCP stdio processes still leak after #16895 fix — 213 orphaned pairs, 13.6 GB RSS #17832
• Codex Desktop should use a project-scoped MCP process pool instead of starting MCP per session #20883
• MCP servers eagerly start per session, causing headed browser MCP processes to accumulate #21984
• codex-cli app-server (Linux) leaks chrome-devtools-mcp child trees on JSON-RPC transport close #24347

I am opening this separately because this is a Linux app-server reproduction with a reduced MCP set and controlled before/after measurements.

Happy to help with more testing or open a PR if the Codex team wants an external contribution for this fix.