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The Context Router sits in front of the LLM step. Every turn goes through it first, and it decides how to answer: from cache, a small in-region model, a prepared response, or a full model call, with fallbacks when a model is slow or fails. It speaks the OpenAI API format, so any agent framework or voice platform with a custom LLM field can point at it. Call the API directly or configure it as the custom large language model in a supported agent integration. See the Context Router API reference for request and response fields.

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Prerequisites

  • An SLNG API key. See Create your API key.
  • A model for the router to answer with. Give it one of two ways:
    • Register a BYOK LLM key in the dashboard. The router then routes slng/auto to that model with nothing extra in the request. Use this for a voice platform integration, which only lets you set a base URL, an API key and a model. See Bring your own key.
    • Send your configuration inline in the slng_config body field when you call the API from your own code. Nothing to register in advance, and a request that carries it ignores any registered configuration. See BYOK Context Router.

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Configuration

Point your integration’s custom LLM at these values: The Custom LLM guide covers the OpenAI-compatible setup. Each platform guide has the exact steps for its dashboard or SDK: ElevenLabs, Vapi, Deepgram, LiveKit, and Pipecat.

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Agent and session headers

The router requires two headers on every request so it can tie each turn to the right agent and call: Map them to your platform’s own agent and session values. In ElevenLabs they are the system__agent_id and system__conversation_id variables; in Vapi they are {{assistant.id}} and {{call.id}}; in code you pass a stable agent ID and the current session ID. Your platform guide lists the exact values. Values are at most 256 characters and cannot contain spaces, commas, pipes (|) or braces ({ }). An opaque ID or slug is always safe. A request missing either header, or carrying a malformed one, returns a 400 with code missing_client_id or invalid_client_id. When you call the API from your own code, you can send them as the body fields slng_agent_id and slng_session_id instead of headers. The body field wins if you send both.
The agent ID also scopes the response cache, so answers cached for one agent ID are never served to another. When you change an agent’s prompt in a meaningful way, give it a fresh agent ID, for example by adding a version suffix. Otherwise answers produced under the old prompt stay in scope for the new one.