Anthropic compatible endpoint swap

Anthropic-compatible endpoint swap

When to use

• Want to reuse the Claude Agent SDK's tool loop / session handling, but route LLM calls to a non-Anthropic provider (OpenRouter, Vercel AI Gateway, self-hosted Ollama, internal proxy).
• Building an app that offers "bring your own API key" for multiple providers.
• Need to bypass Anthropic for on-prem / cost / compliance reasons while keeping the same SDK.

How it works

The SDK reads two env vars before every request:

• ANTHROPIC_BASE_URL - the API endpoint to call (default https://api.anthropic.com).
• ANTHROPIC_AUTH_TOKEN or ANTHROPIC_API_KEY - the auth header value.

Set these in the SDK subprocess env and it transparently talks to whatever endpoint you point at, as long as the endpoint accepts Anthropic's request/response format (which most gateways do).

1. Resolve the user's selected connection: { providerType: 'openrouter', apiKey: '...', baseUrl: 'https://openrouter.ai/api' }.
2. Before query() / new Anthropic(), set:

   process.env.ANTHROPIC_BASE_URL = baseUrl;
   process.env.ANTHROPIC_AUTH_TOKEN = apiKey;
   

3. If you're spawning a subprocess, pass them via spawn env instead.
4. Model IDs may differ per provider (OpenRouter uses anthropic/claude-opus-4.7); expose the full model string in UI so users aren't guessing.
5. clearClaudeBedrockRoutingEnvVars() helper removes any leftover CLAUDE_CODE_USE_BEDROCK=1 / AWS_REGION env that could mis-route.

Example

function resolveAuthEnv(conn: LlmConnection): Record<string, string> {
  switch (conn.providerType) {
    case 'anthropic':   return { ANTHROPIC_API_KEY: conn.apiKey };
    case 'openrouter':  return {
      ANTHROPIC_BASE_URL: 'https://openrouter.ai/api/v1',
      ANTHROPIC_AUTH_TOKEN: conn.apiKey,
    };
    case 'ollama':      return {
      ANTHROPIC_BASE_URL: 'http://localhost:11434/v1',
      ANTHROPIC_AUTH_TOKEN: '',
    };
    case 'custom':      return {
      ANTHROPIC_BASE_URL: conn.baseUrl,
      ANTHROPIC_AUTH_TOKEN: conn.apiKey ?? '',
    };
  }
}

Gotchas

• ANTHROPIC_AUTH_TOKEN vs ANTHROPIC_API_KEY - SDK accepts both, but AUTH_TOKEN is treated as a raw bearer (good for OpenRouter) while API_KEY gets prefixed. Some gateways are strict about the Authorization header format.
• Ollama expects no auth; pass empty string, not undefined, or the SDK may skip setting the header at all and fail on strict endpoints that require it.
• Rate limits, feature support (vision, tools, streaming) vary per endpoint - some gateways are "kinda compatible". Test with YOUR tool set.
• Clear Bedrock env vars if users might switch - CLAUDE_CODE_USE_BEDROCK=1 will override your base URL even if you set it.
• OpenRouter routes by model slug; the model ID tells OpenRouter which provider to forward to.