Anthropic to OpenRouter Proxy

A lightweight proxy server that translates Anthropic Messages API requests to OpenRouter's OpenAI-compatible API, enabling you to use Claude Code and other Anthropic API clients with OpenRouter.

Features

• Full Anthropic Messages API compatibility
• Real-time SSE streaming support
• Tool/function calling support
• Image content support (base64 and URL)
• Automatic model mapping (Opus, Sonnet, Haiku)
• Configurable timeout and model overrides
• Health check endpoint

Requirements

• Python 3.8+
• OpenRouter API key

Installation

1. Clone the repository:

git clone <repository-url>
cd claudeproxy

2. Create a virtual environment (recommended):

python3 -m venv venv
source venv/bin/activate  # On Windows: venv\Scripts\activate

3. Install dependencies:

pip install fastapi httpx uvicorn

Or using requirements.txt:

pip install -r requirements.txt

Configuration

Environment Variables

Variable	Required	Default	Description
OPENROUTER_API_KEY	Yes	-	Your OpenRouter API key
OPENROUTER_BASE_URL	No	https://openrouter.ai/api/v1	OpenRouter API base URL
TARGET_MODEL_DEFAULT	No	anthropic/claude-sonnet-4-20250514	Default model for unknown requests
TARGET_MODEL_SMALL	No	anthropic/claude-sonnet-4-20250514	Model for Haiku requests
TARGET_MODEL_BIG	No	anthropic/claude-sonnet-4-20250514	Model for Sonnet requests
TARGET_MODEL_OPUS	No	anthropic/claude-opus-4-20250514	Model for Opus requests
OPENROUTER_APP_URL	No	-	Your app URL (for OpenRouter analytics)
OPENROUTER_APP_TITLE	No	-	Your app title (for OpenRouter analytics)
TIMEOUT_S	No	300	Request timeout in seconds
PORT	No	8000	Server port
HOST	No	0.0.0.0	Server host

Quick Setup

1. Copy the example environment file:

cp .env.example .env

2. Edit .env and add your OpenRouter API key:

OPENROUTER_API_KEY=sk-or-v1-your-key-here

Running the Proxy

Direct execution:

OPENROUTER_API_KEY="sk-or-v1-your-key" python3 proxy.py

With .env file (using python-dotenv):

pip install python-dotenv
python3 -c "from dotenv import load_dotenv; load_dotenv()" && python3 proxy.py

Using environment file directly:

export $(cat .env | xargs) && python3 proxy.py

The proxy will start on http://0.0.0.0:8000 by default.

Configuring Claude Code

To use this proxy with Claude Code, set the following environment variables before running Claude:

export ANTHROPIC_BASE_URL="http://127.0.0.1:8000"
export ANTHROPIC_API_KEY="any-value-here"  # The proxy ignores this, uses OPENROUTER_API_KEY

# Then run Claude Code
claude

Or add to your shell profile (~/.bashrc, ~/.zshrc):

export ANTHROPIC_BASE_URL="http://127.0.0.1:8000"
export ANTHROPIC_API_KEY="proxy"

API Endpoints

POST /v1/messages

Main Anthropic Messages API endpoint. Accepts Anthropic-format requests and returns Anthropic-format responses.

GET /health

Health check endpoint. Returns {"status": "ok"} when the proxy is running.

GET /

Root endpoint with API information.

Example Requests

Non-streaming request:

curl -X POST http://127.0.0.1:8000/v1/messages \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Hello, how are you?"}
    ]
  }'

Streaming request:

curl --no-buffer -X POST http://127.0.0.1:8000/v1/messages \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4",
    "max_tokens": 1024,
    "stream": true,
    "messages": [
      {"role": "user", "content": "Tell me a short story."}
    ]
  }'

Request with tools:

curl -X POST http://127.0.0.1:8000/v1/messages \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4",
    "max_tokens": 1024,
    "tools": [
      {
        "name": "get_weather",
        "description": "Get current weather for a location",
        "input_schema": {
          "type": "object",
          "properties": {
            "location": {"type": "string", "description": "City name"}
          },
          "required": ["location"]
        }
      }
    ],
    "messages": [
      {"role": "user", "content": "What is the weather in Tokyo?"}
    ]
  }'

Request with image (base64):

curl -X POST http://127.0.0.1:8000/v1/messages \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4",
    "max_tokens": 1024,
    "messages": [
      {
        "role": "user",
        "content": [
          {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": "BASE64_DATA_HERE"}},
          {"type": "text", "text": "What is in this image?"}
        ]
      }
    ]
  }'

Model Mapping

The proxy automatically maps Anthropic model names to OpenRouter models:

Anthropic Model (contains)	OpenRouter Model
opus	TARGET_MODEL_OPUS (default: anthropic/claude-opus-4-20250514)
sonnet	TARGET_MODEL_BIG (default: anthropic/claude-sonnet-4-20250514)
haiku	TARGET_MODEL_SMALL (default: anthropic/claude-sonnet-4-20250514)
other	TARGET_MODEL_DEFAULT (default: anthropic/claude-sonnet-4-20250514)

You can override these mappings using environment variables.

Streaming Events

The proxy emits standard Anthropic SSE events:

1. message_start - Initial message metadata
2. content_block_start - Start of a content block (text or tool_use)
3. content_block_delta - Content delta (text_delta or input_json_delta)
4. content_block_stop - End of a content block
5. message_delta - Final message update (stop_reason, usage)
6. message_stop - End of message stream

Error Handling

The proxy handles various error scenarios:

• 401 Unauthorized: Invalid or missing OpenRouter API key
• 502 Bad Gateway: Failed to connect to OpenRouter
• 504 Gateway Timeout: Request timed out
• SSE Errors: Errors during streaming are sent as event: error SSE events

Troubleshooting

Proxy won't start

• Check that all dependencies are installed
• Verify OPENROUTER_API_KEY is set
• Check if port 8000 is already in use

Authentication errors

• Verify your OpenRouter API key is valid
• Check that the key has sufficient credits

Streaming not working

• Ensure your client supports SSE
• Check for proxy/firewall issues that might buffer responses

License

MIT License