FastRouter

Put this in opencode.json in your project folder:

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "fastrouter": {
      "npm": "@ai-sdk/openai-compatible",
      "name": "FastRouter",
      "options": {
        "baseURL": "https://go.fastrouter.ai/api/v1",
        "apiKey": "{env:FASTROUTER_API_KEY}"
      },
      "models": {
        "anthropic/claude-opus-4.1": {
          "name": "anthropic/claude-opus-4.1"
        }
      }
    }
  }
}

• OpenCode reads any OpenAI-compatible address through the @ai-sdk/openai-compatible package, and an address that speaks the Responses API through @ai-sdk/openai.

From OpenCode documentation, read 2026-09-24.

Put this in ~/.pi/agent/models.json:

{
  "providers": {
    "fastrouter": {
      "baseUrl": "https://go.fastrouter.ai/api/v1",
      "api": "openai-completions",
      "apiKey": "$FASTROUTER_API_KEY",
      "models": [
        {
          "id": "anthropic/claude-opus-4.1"
        }
      ]
    }
  }
}

• The apiKey field can name an environment variable as $NAME.

From Pi documentation, read 2026-09-24.

Codex speaks only the Responses API, and FastRouter documents no Responses address. A gateway that offers one can sit in between.

• Codex speaks the Responses API only: responses is the one supported wire API of a custom provider. Ollama and LM Studio are built in and start with --oss.

From Codex documentation, read 2026-09-24.

Claude Code sends Anthropic Messages requests, and FastRouter documents no such address. A gateway that translates to that API can sit in between.

• Claude Code sends Anthropic Messages requests to ANTHROPIC_BASE_URL. Anthropic says it does not support routing Claude Code to models other than Claude through any gateway, so some features may not work with another model.

From Claude Code documentation, read 2026-09-24.