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

Behaviour for re-ranking search results.

Re-rankers improve retrieval quality by scoring chunks based on their
relevance to the question, then filtering and re-sorting by score.

## Built-in Implementations

- `Arcana.Agent.Reranker.LLM` - Uses your LLM to score relevance (default)

## Custom Implementations

Implement the `rerank/3` callback:

    defmodule MyApp.CrossEncoderReranker do
      @behaviour Arcana.Agent.Reranker

      @impl Arcana.Agent.Reranker
      def rerank(question, chunks, opts) do
        # Your custom logic
        {:ok, scored_and_filtered_chunks}
      end
    end

Or provide a function directly:

    Agent.rerank(ctx, reranker: fn question, chunks, opts ->
      {:ok, my_rerank(question, chunks)}
    end)

# `rerank`

```elixir
@callback rerank(
  question :: String.t(),
  chunks :: [map()],
  opts :: keyword()
) :: {:ok, [map()]} | {:error, term()}
```

Re-ranks chunks based on relevance to the question.

Returns chunks filtered by threshold and sorted by score (highest first).

## Options

- `:threshold` - Minimum score to keep (default: 7, range 0-10)
- `:llm` - LLM function for scoring (required for LLM reranker)
- `:prompt` - Custom prompt function `fn question, chunk_text -> prompt end`

---

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