[BUG] MCP calls routed to wrong server when multiple Claude Code sessions run concurrently

[logpie]

Bug Summary

When two Claude Code sessions call mcp__codex__codex concurrently, the MCP call can be routed to the wrong Codex MCP server process (belonging to a different Claude Code session/project). This causes Codex to operate in the wrong project directory with the wrong context.

Environment

• codex-cli: 0.103.0
• Claude Code: latest as of 2026-02-23
• macOS Darwin 24.6.0 (arm64, Apple Silicon)
• Multiple Claude Code sessions in separate terminal tabs, each with its own Codex MCP server

Steps to Reproduce

1. Open two terminal tabs
2. Tab 1: cd /path/to/project-a && claude
3. Tab 2: cd /path/to/project-b && claude
4. In both tabs near-simultaneously, ask Claude to call Codex (e.g., "Ask Codex to review this code")
5. Check Codex session logs: ls -lt ~/.codex/sessions/$(date +%Y/%m/%d)/
6. Inspect session metadata: python3 -c "import json; print(json.loads(open('FILE').readline())['payload']['cwd'])"

Expected Behavior

Each Codex MCP session should have the cwd matching the Claude Code session that initiated it.

Actual Behavior

Some sessions receive the wrong cwd, git context, and even the wrong prompt. Specifically:

• 3 sessions got a parent directory with no git context
• 1 session got a completely different project's directory (project-B instead of project-A), including that project's AGENTS.md, git repo URL, and git branch

Evidence from Session Logs

Wrong session (started 02:28:39 UTC):

session_meta.cwd = /path/to/project-B        # WRONG — should be project-A
session_meta.git.repository_url = <project-B repo>
session_meta.git.branch = main

The turn_context and user messages in this session contain:

• AGENTS.md instructions for /path/to/project-B
• <cwd>/path/to/project-B</cwd>
• A prompt referencing cd /path/to/project-B && git diff HEAD~1

But this call was initiated from a Claude Code session running in /path/to/project-A.

Correct retry (started 02:40:02 UTC, same prompt retried):

session_meta.cwd = /path/to/project-A        # CORRECT
session_meta.git.repository_url = <project-A repo>
session_meta.git.branch = feature-branch

Full Day Analysis (9 sessions, all initiated from project-A)

Time (UTC)	Session CWD	Correct?
20:01	project-A	Yes
22:12	project-A	Yes
22:44	parent dir (no git)	No
22:44	project-A	Yes
23:18	parent dir (no git)	No
23:28	project-A	Yes
02:28	parent dir (no git)	No
02:28	project-B	No
02:40	project-A	Yes

Failure rate: 4/9 (44%)

Analysis

The MCP protocol uses stdio pipes, so each Codex MCP server subprocess should be isolated. However, the evidence shows that Claude Code's MCP client layer is sometimes delivering calls to the wrong Codex MCP server process. This is not a Codex server bug — the server correctly operates in whatever context it receives.

The bug appears to correlate with concurrent MCP calls from multiple Claude Code sessions. The wrong-cwd sessions tend to appear near-simultaneously with correct ones (e.g., 22:44:23 wrong + 22:44:53 correct, 02:28:27 wrong + 02:28:39 wrong + 02:40:02 correct retry).

The "parent dir" failures (3 cases) suggest the MCP server was spawned before the cwd was properly set. The "project-B" failure (1 case) suggests the call was delivered to project-B's already-running MCP server instead of project-A's.

Impact

• Codex reads files from the wrong project
• Codex may execute commands in the wrong directory (if sandbox allows writes)
• Codex reviews/analysis is based on wrong codebase
• User observes "hanging" because Codex is working in a different context and may be confused by the prompt referencing non-existent files
• Workaround: retry the call (it usually succeeds on second attempt)

Reproduction Script

# Setup
mkdir -p /tmp/codex-repro/project-a /tmp/codex-repro/project-b
cd /tmp/codex-repro/project-a && git init -q && echo "# A" > README.md && git add . && git commit -qm "init"
cd /tmp/codex-repro/project-b && git init -q && echo "# B" > README.md && git add . && git commit -qm "init"

# Then open two Claude Code sessions and call Codex simultaneously.
# Check ~/.codex/sessions/ for cwd mismatches.

[github-actions]

Found 3 possible duplicate issues:

1. [BUG] Multiple Claude Code Desktop sessions in DIFFERENT directories all execute the last prompt — sessions cross-contaminate #27658
2. JSONL cross-session contamination: all event types written to wrong session file when multiple sessions are active #26964
3. [BUG] MCP tools incorrectly mapped when connect multiple servers #18472

This issue will be automatically closed as a duplicate in 3 days.

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🤖 Generated with Claude Code

[logpie]

Cross-references: systemic session isolation problem

This appears to be part of a broader pattern of cross-session state corruption in Claude Code. Related issues:

Same root cause family (session routing / isolation)

• JSONL cross-session contamination: all event types written to wrong session file when multiple sessions are active #26964 — JSONL cross-session contamination: all event types written to wrong session file (76.6% contamination rate). Very likely the same underlying bug — session-to-subprocess/file-descriptor mapping not properly isolated.
• Orphaned MCP server processes not cleaned up when sessions end #22612 — Orphaned MCP server processes not cleaned up. Could be a contributing factor — stale MCP servers from old sessions may receive misrouted calls.

CWD tracking corruption

• [BUG] Claude Code frequently loses track of which directory it is in #1669 — Claude Code frequently loses track of which directory it is in (long-standing, 36 comments, marked oncall)
• Task tool with isolation=worktree leaks CWD to parent session #28017 — Task tool with isolation=worktree leaks CWD to parent session
• [CRITICAL] Bash tool deletes entire project root after command interrupt - PWD state corruption #17410 — Bash tool PWD state corruption after command interrupt

Concurrent session state corruption

• claude code corrupts ~/.claude.json on mac while running 5 concurrent sessions #3117 — ~/.claude.json corrupted when running 5 concurrent sessions (race condition on file writes)
• [BUG] Permission bypass mode resets when switching between concurrent sessions #27380 — Permission bypass mode resets when switching between concurrent sessions
• Frequent re-authentication required with multiple concurrent Claude Code sessions (OAuth refresh token race condition) #24317 — OAuth refresh token race condition across concurrent sessions
• [BUG] Rewind in one session silently destroys uncommitted changes made by another session #28078 — Rewind in one session destroys another session's uncommitted changes

The common thread: Claude Code lacks proper per-session isolation for MCP subprocess routing, JSONL event writes, CWD state, shared config files, and auth tokens. Running multiple sessions concurrently is a common workflow (different projects in different terminal tabs), and the isolation boundaries are broken at multiple layers.

[github-actions]

Closing for now — inactive for too long. Please open a new issue if this is still relevant.

[github-actions]

This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.