Arcana.Reranker behaviour (Arcana v3.0.0)

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

Custom Implementations

Implement the rerank/3 callback:

defmodule MyApp.CrossEncoderReranker do
  @behaviour Arcana.Reranker

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

Or provide a function directly:

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

Summary

Callbacks

rerank(question, chunks, opts)

Re-ranks chunks based on relevance to the question.

Callbacks

rerank(question, chunks, opts)

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

Re-ranks chunks based on relevance to the question.

Chunks from Arcana.search/2 are Arcana.SearchResult structs, but custom Arcana.Searcher implementations may supply plain maps, so rerankers should not assume the struct. Returns chunks filtered by threshold and sorted by score (highest first). Rerankers that compute an explicit score store it under :rerank_score (all the built-in rerankers do); rerankers that only reorder or filter can return the chunks unchanged.

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