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ChatGPT Codex Proxy

Run Claude Code on your ChatGPT Plus/Pro subscription — zero workflow change.

What is this?

This proxy lets Claude Code talk to ChatGPT's Codex backend instead of Anthropic's API. You keep using claude exactly as before — same UI, same slash commands, same MCP tools — while inference is served by GPT.

Claude Code  ──POST /v1/messages──>  chatgpt-codex-proxy  ──POST /codex/responses──>  ChatGPT
             <──Anthropic response──                       <──Codex SSE response──

When to use this

Situation This proxy helps?
Anthropic quota exhausted / rate limited ✅ Switch to GPT with one env var
Want to try GPT models without leaving Claude Code ✅ Same workflow, different backend
Have ChatGPT Plus/Pro idle and want to make use of it ✅ No API key cost
Need MCP tools (Stitch, Linear, etc.) to work with GPT ✅ Only proxy that bridges this
Need an OpenAI-compatible endpoint for other clients Use ChatMock instead
Need Ollama compatibility Use ChatMock instead

Why this, not ChatMock or similar tools

ChatMock and similar projects expose an OpenAI/Ollama-compatible API — great for general-purpose clients, but you'd have to switch away from Claude Code entirely.

This proxy is built specifically for Claude Code:

  • Zero workflow change — claude command, keybindings, CLAUDE.md, slash commands, all work unchanged.
  • MCP tools cross the boundary — Claude Code's MCP servers (Stitch, Linear, Chrome DevTools, etc.) are normally only available to Claude's backend. This proxy reads ~/.claude.json at startup, connects to your MCP servers, and injects their tool schemas into every GPT request. GPT gets the same tool access Claude would have.
  • Parallel tool call safety — the proxy detects mutating tools (Edit, Write, Delete, Bash) and automatically disables parallel_tool_calls to prevent unsafe concurrent file operations.
  • Claude model name passthrough — use --model claude-sonnet-4-20250514 as usual; the proxy maps it to the right Codex model automatically. Or use GPT model names directly in passthrough mode.

Example session

chatgpt-codex-proxy example session

Features

  • Anthropic Messages API compatible (POST /v1/messages)
  • OAuth 2.0 login — no API key, just your ChatGPT subscription
  • Full request/response transformation (Anthropic ↔ Codex)
  • SSE streaming
  • Claude → Codex model mapping with env overrides
  • MCP tool injection (see below)

Quick start

git clone <repo-url>
cd chatgpt-codex-proxy
npm install && npm run build

# Login with your ChatGPT account (browser opens)
npm run login

# Start the proxy
npm run dev

Then in another terminal:

export ANTHROPIC_BASE_URL=http://127.0.0.1:19080
export ANTHROPIC_API_KEY=dummy   # value is unused; variable must be set
claude

That's it. Claude Code is now running on GPT.

Optional: shell helper

Add this to .zshrc/.bashrc for a quick gpt alias:

gpt() {
  emulate -L zsh
  local proxy_port="${CHATGPT_CODEX_PROXY_PORT:-19080}"
  local token="${ANTHROPIC_AUTH_TOKEN:-${ANTHROPIC_API_KEY:-dummy}}"

  export ANTHROPIC_BASE_URL="http://127.0.0.1:${proxy_port}"
  export ANTHROPIC_AUTH_TOKEN="$token"
  export ANTHROPIC_API_KEY="${ANTHROPIC_API_KEY:-$token}"
  export API_TIMEOUT_MS="${API_TIMEOUT_MS:-90000}"
  export PASSTHROUGH_MODE="${PASSTHROUGH_MODE:-true}"
  unset CLAUDE_CONFIG_DIR

  echo "Using local Codex proxy on :${proxy_port}"
  claude "$@"
}

MCP tool injection

Claude Code's MCP tools are normally invisible to non-Claude backends. This proxy bridges that gap.

Claude Code                   chatgpt-codex-proxy               ChatGPT Codex API
   │  tools: [Edit, Bash, ...]      │  tools: [Edit, Bash, ...        │
   │  (deferred: stitch, qmd, ...)  │          + mcp__stitch__*       │
   │                                │          + mcp__qmd__*  ]       │
   │ ──────────────────────────────>│ ──────────────────────────────> │

How it works:

  1. On startup: reads ~/.claude.json → mcpServers
  2. Runs MCP handshake (initialize → tools/list) for each enabled server
  3. Caches schemas in memory for the lifetime of the proxy process
  4. On every /v1/messages request: appends cached tools to the Codex tools array
  5. When GPT calls a tool: Claude Code receives the tool_use response, executes the real MCP call, and sends the result back through the proxy

Both HTTP (type: http) and stdio (command) MCP servers are supported.

Configuration — set PROXY_MCP_SERVERS in .env:

# Specific servers (names must match keys in ~/.claude.json)
PROXY_MCP_SERVERS=stitch,linear

# All servers registered in ~/.claude.json
PROXY_MCP_SERVERS=all

# Disabled (default)
PROXY_MCP_SERVERS=

Startup log:

[mcp-registry] connecting to: stitch, linear
[mcp-registry] stitch: 8 tools loaded
[mcp-registry] linear: 6 tools loaded
[mcp-registry] ready: 14 total MCP tools

Configuration

.env setup

cp .env.example .env

Model mapping

By default (PASSTHROUGH_MODE=true) the proxy forwards whatever model name Claude Code sends straight to Codex. Set PASSTHROUGH_MODE=false to enable automatic Claude → Codex mapping:

Claude model Codex model
claude-sonnet-4-20250514 gpt-5.2-codex
claude-3-5-sonnet-20241022 gpt-5.2-codex
claude-3-haiku-20240307 gpt-5.3-codex-spark
claude-3-opus-20240229 gpt-5.3-codex-xhigh
(fallback) gpt-5.2-codex

Override per family:

ANTHROPIC_DEFAULT_HAIKU_MODEL=gpt-5.3-codex-spark
ANTHROPIC_DEFAULT_SONNET_MODEL=gpt-5.2-codex
ANTHROPIC_DEFAULT_OPUS_MODEL=gpt-5.2-codex

Available Codex models

Model Effort Notes
gpt-5.4 high Flagship (2026)
gpt-5 high
gpt-5-codex high Optimized for agentic coding
gpt-5-codex-mini medium
gpt-5.3-codex high
gpt-5.3-codex-xhigh xhigh
gpt-5.3-codex-medium medium
gpt-5.3-codex-low low
gpt-5.3-codex-spark low Speed-optimized, >1000 tok/s
gpt-5.2-codex high Proxy default
gpt-5.2-codex-xhigh xhigh
gpt-5.2-codex-medium medium
gpt-5.2-codex-low low
gpt-5.1-codex high
gpt-5.1-codex-max xhigh
gpt-5.1-codex-mini medium

Shorthand aliases: gpt-5.3 → gpt-5.3-codex, gpt-5.2 → gpt-5.2-codex, gpt-5.1 → gpt-5.1-codex

Effort control

Claude Code's effort slider only affects native Claude models — it is not included in API requests when using GPT models. Control reasoning effort via:

Method 1 — model name suffix (recommended)

export ANTHROPIC_DEFAULT_SONNET_MODEL="gpt-5.3-codex-xhigh"  # xhigh
export ANTHROPIC_DEFAULT_HAIKU_MODEL="gpt-5.3-codex-spark"   # low

Method 2 — global override

PROXY_DEFAULT_EFFORT=high

Priority: thinking.budget_tokens in request → model name suffix/table → PROXY_DEFAULT_EFFORT → medium

CLI commands

Command Description
npm run login OAuth login (browser)
npm run logout Delete stored token
npm run status Show auth status
npm run dev Start dev server (hot reload)
npm run start Start production server

API compatibility

Capability Support Notes
Basic chat ✅
Streaming ✅ SSE
Multi-turn ✅
System prompt ✅ Mapped to instructions
Tool calling ✅ Full tool_use/tool_result cycle
Image input ⚠️ Limited
Temperature ❌ Not supported by Codex backend
Max tokens ❌ Not supported by Codex backend

Environment variables

Variable Default Description
PORT 19080 Server port
PROXY_JSON_LIMIT 20mb JSON body size limit
CODEX_BASE_URL https://chatgpt.com/backend-api Codex API base URL
PASSTHROUGH_MODE true false to enable Claude→Codex model mapping
ANTHROPIC_DEFAULT_HAIKU_MODEL - Codex model for Haiku requests
ANTHROPIC_DEFAULT_SONNET_MODEL - Codex model for Sonnet requests
ANTHROPIC_DEFAULT_OPUS_MODEL - Codex model for Opus requests
PROXY_DEFAULT_EFFORT (auto) low / medium / high / xhigh
PROXY_MCP_SERVERS (disabled) all or comma-separated server names from ~/.claude.json

Troubleshooting

# Health check
curl -fsS http://127.0.0.1:19080/health

# Port in use
lsof -tiTCP:19080 -sTCP:LISTEN -nP

# Passthrough test
gpt --model gpt-5.2

# Mapping mode test
PASSTHROUGH_MODE=false gpt --model claude-sonnet-4-20250514

# Recent logs
tail -n 120 /tmp/chatgpt-codex-proxy.log

# Tool calling smoke test
python3 scripts/tool_calling_smoke.py --base-url http://127.0.0.1:19080 --model gpt-5.2

Project structure

chatgpt-codex-proxy/
├── src/
│   ├── index.ts           # Entry point
│   ├── server.ts          # Express server
│   ├── cli.ts             # CLI commands
│   ├── auth.ts            # OAuth login
│   ├── routes/
│   │   └── messages.ts    # /v1/messages endpoint
│   ├── transformers/
│   │   ├── request.ts     # Anthropic → Codex
│   │   └── response.ts    # Codex → Anthropic
│   ├── codex/
│   │   ├── client.ts      # Codex API client
│   │   └── models.ts      # Model mapping
│   ├── mcp/
│   │   ├── config.ts      # Read ~/.claude.json MCP server configs
│   │   ├── client.ts      # HTTP + stdio MCP clients
│   │   └── registry.ts    # Tool schema cache (singleton)
│   ├── types/
│   │   └── anthropic.ts   # Types
│   └── utils/
│       └── errors.ts      # Error handling
├── .env.example
├── package.json
├── tsconfig.json
└── README.md

Security

Designed for personal, local-machine usage only.

npm run dev  # binds to localhost only
export ANTHROPIC_BASE_URL=http://127.0.0.1:19080

If you deploy to a server: add authentication, restrict CORS, add rate limiting, set token file permissions to 600, add monitoring.

Do not bind to 0.0.0.0 unless you have implemented all of the above.

License

MIT

About

An Anthropic-compatible /v1/messages proxy that lets Claude Code use the ChatGPT Codex backend.

Topics

nodejs express proxy mcp openai api-proxy codex claude dxt chatgpt anthropic claude-code

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