# `ReqLLM.Router`
[🔗](https://github.com/agentjido/req_llm/blob/main/lib/req_llm/router.ex#L1)

A behaviour for application-defined model routers.

A router is any struct whose module implements `c:resolve/2`. ReqLLM gives the
callback a normalized `ReqLLM.Router.Request`. The callback must return a
concrete `%LLMDB.Model{}`.

The request contains only the normalized conversation context and an opaque
application `routing_context` map. ReqLLM does not infer routing policy from
the generation function or its options.

    defmodule MyApp.ModelRouter do
      @behaviour ReqLLM.Router

      defstruct fast_model: "openai:gpt-4o-mini",
                deep_model: "anthropic:claude-sonnet-4-5"

      @impl true
      def resolve(router, %ReqLLM.Router.Request{} = request) do
        model_spec =
          case request.routing_context do
            %{mode: :deep} -> router.deep_model
            _other -> router.fast_model
          end

        ReqLLM.model(model_spec)
      end
    end

    router = %MyApp.ModelRouter{}
    ReqLLM.generate_text(router, "Hello", routing_context: %{mode: :fast})

ReqLLM does not provide a routing policy. The callback can use local rules,
an evaluation model, or another service.

# `t`

```elixir
@type t() :: struct()
```

# `resolve`

```elixir
@callback resolve(t(), ReqLLM.Router.Request.t()) ::
  {:ok, LLMDB.Model.t()} | {:error, term()}
```

# `implementation?`

```elixir
@spec implementation?(term()) :: boolean()
```

Returns true when a struct implements the router behaviour.

# `resolve`

```elixir
@spec resolve(t(), ReqLLM.Router.Request.t()) ::
  {:ok, LLMDB.Model.t()} | {:error, term()}
```

Resolves an application router to a concrete LLMDB model.

---

*Consult [api-reference.md](api-reference.md) for complete listing*