codex-cli-mcp-slim

A thin, auditable MCP server wrapping the Codex CLI (codex exec).

Why

codex mcp-server, the command that let other MCP clients call Codex, is deprecated, and its removal has been merged upstream (openai/codex#42993): releases up to 0.153.x still ship it, later ones will not. Its replacement, the Codex app server, speaks its own JSON-RPC protocol rather than MCP. This server keeps the old integration point alive: it exposes the same two tools, codex and codex-reply, and runs codex exec underneath. codex exec is the Codex CLI's non-interactive mode: one prompt in, the agent works on its own, one final message out.

When you add an MCP server to your AI coding tool, every prompt and code snippet you send flows through that wrapper. Most CLI-wrapping MCP servers are small, individually maintained packages, and recent supply-chain incidents (xz-utils, postmark-mcp, the npm chalk/debug compromise) show that "small and useful" is not the same as "safe to trust blindly."

This project takes the opposite stance: instead of asking you to trust it, it tries to be easy to audit.

• Single file — the whole server is src/codex_cli_mcp_slim/server.py, readable end-to-end in one sitting

• One third-party dependency (mcp) — minimal supply-chain surface

• Faithful CLI mapping — every typed parameter mirrors a real codex exec flag by name, so it is obvious which flags an invocation actually sets

• Prompt over stdin — the prompt never appears in the process list and is not bounded by the argv size limit

• Forward-compatible — any new or uncommon codex exec flag is reachable via extra_args without touching this server

• Configurable binary path — $CODEX_CMD lets you swap or wrap the codex binary

• Transparent — every invocation logs the exact argv to stderr

Read server.py before you install. That is the point.

Related MCP server: codex-cli-mcp

Prerequisites

• The codex CLI installed and on $PATH (or pointed to via $CODEX_CMD). See the official Codex CLI repository. This server always passes --json and reads the prompt from stdin (codex exec -), both of which codex exec documents.

• codex already authenticated — this wrapper does not manage login; it surfaces codex's own error output if the CLI is not ready.

Installation

# Run directly without installing
uvx codex-cli-mcp-slim

# Install from PyPI
pip install codex-cli-mcp-slim

# Run from GitHub HEAD
uvx --from git+https://github.com/tksfjt1024/codex-cli-mcp-slim codex-cli-mcp-slim

Usage as an MCP server

Claude Code

claude mcp add codex uvx codex-cli-mcp-slim

Or manually in ~/.claude.json:

{
  "mcpServers": {
    "codex": {
      "type": "stdio",
      "command": "uvx",
      "args": ["codex-cli-mcp-slim"]
    }
  }
}

If codex is not on the launching process's $PATH, point $CODEX_CMD at it:

{
  "mcpServers": {
    "codex": {
      "type": "stdio",
      "command": "uvx",
      "args": ["codex-cli-mcp-slim"],
      "env": { "CODEX_CMD": "/absolute/path/to/codex" }
    }
  }
}

Replacing codex mcp-server

An entry that used to launch codex mcp-server keeps its server name and its tool names; only command and args change. Before:

{
  "mcpServers": {
    "codex": {
      "type": "stdio",
      "command": "codex",
      "args": ["mcp-server"]
    }
  }
}

After:

{
  "mcpServers": {
    "codex": {
      "type": "stdio",
      "command": "uvx",
      "args": ["codex-cli-mcp-slim"]
    }
  }
}

Parameter names differ from the old server where codex exec names the flag differently: cwd is now cd (the -C/--cd flag), and codex-reply takes thread_id instead of threadId. The result's structuredContent field keeps the shape the old server returned, {"threadId": ..., "content": ...}.

Other MCP clients

Any MCP-compatible client can launch the server via stdio:

uvx codex-cli-mcp-slim

Server-level flags

Everything on the server's own command line is placed right after codex exec on every invocation. One MCP-client entry can therefore pin a reasoning effort, a model or a working directory for all of its calls. Two entries that differ only in reasoning effort look like this:

{
  "mcpServers": {
    "codex-medium": {
      "type": "stdio",
      "command": "uvx",
      "args": ["codex-cli-mcp-slim", "-c", "model_reasoning_effort=medium"]
    },
    "codex-high": {
      "type": "stdio",
      "command": "uvx",
      "args": ["codex-cli-mcp-slim", "-c", "model_reasoning_effort=high"]
    }
  }
}

A call to codex-high runs codex exec -c model_reasoning_effort=high [per-call flags] --json -. Per-call flags come after the server-level ones, and -c may repeat with the last one winning, so a per-call config entry overrides a server-level -c. Single-value flags such as -m and -C may not repeat: codex rejects the second one, and the tool result carries that error. Keep server-level flags and per-call parameters disjoint for those.

Tool: codex

Runs a single non-interactive Codex session (codex exec). codex is an agentic assistant: it reads and, depending on the sandbox, edits files in the working directory to fulfil the request, then prints its final message.

The tool returns that final message followed by one metadata line:

[codex] thread_id=019a2b3c-1d4e-7f60-8a9b-0c1d2e3f4a5b status=completed input_tokens=13894 cached_input_tokens=11904 output_tokens=612

thread_id and status are always present; the token fields appear when the run reported them. isError is the flag on an MCP tool result that tells the client a call failed. This server sets it when codex exited non-zero, when the subprocess timed out, and when the turn itself failed. The last case matters because codex exec exits 0 after a failure inside the model API; the tool result then carries the error text instead of coming back as a successful call:

[ERROR] codex failed

returncode=0

errors:
Unsupported value: 'none' is not supported with the ... model.

[codex] thread_id=019a2b3c-... status=failed

argv: ['codex', 'exec', '--json', '-']

Pass the thread_id to codex-reply to continue the same session.

Unknown parameters are refused rather than ignored, so a call that still uses the old server's cwd gets an error naming cd instead of running in the wrong directory.

Security note: sandbox

codex exec reads its sandbox mode from its own configuration file (~/.codex/config.toml by default) unless --sandbox is given. danger-full-access removes the filesystem and network sandbox entirely; workspace-write makes the working directory (and any add_dir) writable. --sandbox overrides only the mode; whether workspace-write gets network access still follows the [sandbox_workspace_write] section of config.toml. The parameter mirrors the flag so that whichever mode a call runs under is visible in the arguments and in the logged argv. This server does not pass --dangerously-bypass-approvals-and-sandbox; reach it via extra_args if you really mean it.

Tool: codex-reply

Continues a previous session (codex exec resume <THREAD_ID>) with a follow-up prompt and returns the new final message. Only the flags codex exec resume accepts are exposed, so cd, sandbox, add_dir and profile are refused here. The working directory and sandbox of a reply come from the current configuration, that is, the server-level flags and config.toml, not from the original session.

Timeout configuration

timeout_seconds is this wrapper's hard wall-clock limit (default 1800, or $CODEX_CLI_MCP_SLIM_TIMEOUT). On timeout, the wrapper kills the subprocess's whole process group and then waits up to 20 additional seconds to collect any buffered output and reap the process, so the effective ceiling is timeout_seconds + 20. A timed-out call is flagged isError and carries whatever codex had printed so far.

Forward-compatibility example

If a future codex exec release adds a new flag (say --super-mode), use it immediately without updating this server:

{
  "name": "codex",
  "arguments": {
    "prompt": "...",
    "extra_args": ["--super-mode"]
  }
}

Configuration

codex itself reads its configuration file and credentials from $CODEX_HOME (~/.codex by default), so an MCP-client entry can point a server at a dedicated configuration directory through its env block.

Development

# Install dev dependencies
pip install -e ".[test,dev]"

# Lint
ruff check .

# Test
pytest

License

MIT © tksfjt1024