# `Arcana.Agent.Selector`
[🔗](https://github.com/georgeguimaraes/arcana/blob/main/lib/arcana/agent/selector.ex#L1)

Behaviour for collection selection in the Agent pipeline.

The selector determines which collections to search based on the question
and available collections. This allows for both LLM-based routing (default)
and deterministic routing based on user context, metadata, or business logic.

## Implementing a Custom Selector

    defmodule MyApp.TeamBasedSelector do
      @behaviour Arcana.Agent.Selector

      @impl true
      def select(_question, _collections, opts) do
        case opts[:context][:team] do
          "api" -> {:ok, ["api-reference", "sdk-docs"], "API team routing"}
          "mobile" -> {:ok, ["mobile-docs", "react-native"], "Mobile team routing"}
          _ -> {:ok, ["general"], "Default routing"}
        end
      end
    end

## Using a Custom Selector

    Agent.new(question, repo: repo, llm: llm)
    |> Agent.select(
      collections: ["api-reference", "mobile-docs", "general"],
      selector: MyApp.TeamBasedSelector,
      context: %{team: current_user.team}
    )

## Using an Inline Function

    Agent.select(ctx,
      collections: collections,
      selector: fn question, _collections, _opts ->
        if question =~ "API" do
          {:ok, ["api-docs"], "Question mentions API"}
        else
          {:ok, ["general"], "General query"}
        end
      end
    )

# `select`

```elixir
@callback select(
  question :: String.t(),
  collections :: [{name :: String.t(), description :: String.t() | nil}],
  opts :: keyword()
) :: {:ok, [String.t()], String.t() | nil} | {:error, term()}
```

Selects which collections to search based on the question.

## Parameters

- `question` - The user's question
- `collections` - List of `{name, description}` tuples for available collections
- `opts` - Options passed to `Agent.select/2`, including:
  - `:llm` - The LLM function (for LLM-based selectors)
  - `:prompt` - Custom prompt function (for LLM-based selectors)
  - `:context` - User-provided context map
  - Any other options passed to `Agent.select/2`

## Returns

- `{:ok, selected_collections, reasoning}` - List of collection names to search
  and optional reasoning string (can be nil)
- `{:error, reason}` - On failure, falls back to all collections

---

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