feat(brain_answer): Qwen3-Reranker cross-encoder filter (opt-in)

Adds an opt-in cross-encoder rerank step between BM25 retrieval and LLM
synthesis. With BRAIN_RERANKER_URL set, brain_answer retrieves BM25
top-20, scores each excerpt against the query via Qwen3-Reranker on
Ollama, drops the "no" answers, and forwards up to 5 surviving sources
to the LLM. Unset, behaviour is unchanged (BM25 top-10 → LLM).

The reranker is a *filter*, not a re-ranker: Qwen3-Reranker emits a
binary yes/no token under its native chat template, and ties within the
"yes" set are broken by BM25 rank — what got retrieved first stays
ahead.

New package ingestion/internal/reranker:
- Client with URL, Model, HTTP fields.
- New(url, model) returns nil on empty url so callers can treat
  "feature disabled" as a single nil check.
- Score(ctx, query, docs) issues one /api/generate call per doc using
  the Qwen3-Reranker yes/no chat template (verbatim, because the model
  was trained on this exact wording). Parses the first non-think token.

Wiring:
- mcp.Server gains a WithReranker fluent setter to keep NewServer
  signature stable.
- brain_answer's BM25 limit jumps to 20 only when a reranker is wired,
  to give the filter something to do.
- cmd/server/main.go reads BRAIN_RERANKER_URL (+ optional
  BRAIN_RERANKER_MODEL, default dengcao/Qwen3-Reranker-0.6B:F16).

Tests cover: nil-on-empty-url, ordered yes/no scoring, request shape
(model, prompt contents, yes/no template), ambiguous response → 0,
empty doc slice, upstream-error propagation, plus an end-to-end
brain_answer integration that proves only the relevant note reaches the
LLM when noise.md is rejected.

Closes hyperguild#7.

ingestion/cmd/server/main.go

@@ -16,6 +16,7 @@ import (
 	"github.com/mathiasbq/hyperguild/ingestion/internal/llm"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/oauth"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/reranker"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/watcher"
 )
@@ -77,6 +78,11 @@ func main() {
 	}
 
 	mcpSrv := mcp.NewServer(brainDir, &pipelineCfg, llmClient.Complete, answerComplete)
+	if rerankURL := os.Getenv("BRAIN_RERANKER_URL"); rerankURL != "" {
+		rerankModel := envOr("BRAIN_RERANKER_MODEL", "dengcao/Qwen3-Reranker-0.6B:F16")
+		mcpSrv = mcpSrv.WithReranker(reranker.New(rerankURL, rerankModel))
+		logger.Info("brain reranker configured", "url", rerankURL, "model", rerankModel)
+	}
 
 	mcpToken := os.Getenv("BRAIN_MCP_TOKEN")
 	if mcpToken == "" {

ingestion/internal/mcp/server.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/reranker"
 )
 
 type request struct {
@@ -37,6 +38,7 @@ type Server struct {
 	pipeline  pipeline.Config
 	llm       pipeline.CompleteFunc
 	answerLLM pipeline.CompleteFunc // nil = brain_answer and brain_classify unavailable
+	reranker  *reranker.Client      // nil = no rerank, BM25 top-10 → LLM
 }
 
 // NewServer constructs a Server bound to brainDir. pipelineCfg supplies the
@@ -50,6 +52,15 @@ func NewServer(brainDir string, pipelineCfg *pipeline.Config, llm pipeline.Compl
 	return &Server{brainDir: brainDir, pipeline: cfg, llm: llm, answerLLM: answerLLM}
 }
 
+// WithReranker installs an opt-in cross-encoder reranker. When set,
+// brain_answer retrieves a wider BM25 candidate set and prunes it to
+// the relevant ones before LLM synthesis. Returns the server for
+// fluent chaining.
+func (s *Server) WithReranker(r *reranker.Client) *Server {
+	s.reranker = r
+	return s
+}
+
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// MCP streamable HTTP: GET establishes the SSE stream for server-to-client events.
 	if r.Method == http.MethodGet {

ingestion/internal/mcp/tools_answer.go

@@ -6,9 +6,35 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/mathiasbq/hyperguild/ingestion/internal/reranker"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
 )
 
+// rerankResults scores each candidate's excerpt against the query and
+// returns up to top results whose score is positive, preserving the
+// caller's input order (BM25 rank) within the kept set. The reranker is
+// a filter: ties are broken by BM25, not by the reranker's binary score.
+func rerankResults(ctx context.Context, rr *reranker.Client, query string, results []search.Result, top int) ([]search.Result, error) {
+	docs := make([]string, len(results))
+	for i, r := range results {
+		docs[i] = r.Excerpt
+	}
+	scores, err := rr.Score(ctx, query, docs)
+	if err != nil {
+		return nil, err
+	}
+	kept := make([]search.Result, 0, top)
+	for i, r := range results {
+		if scores[i] > 0 {
+			kept = append(kept, r)
+		}
+		if len(kept) == top {
+			break
+		}
+	}
+	return kept, nil
+}
+
 const (
 	answerSystemPrompt = `You are a knowledge assistant. Answer the question using ONLY the provided sources.
 Cite source file paths inline when referencing specific content.
@@ -35,10 +61,22 @@ func (s *Server) brainAnswer(ctx context.Context, args json.RawMessage) (json.Ra
 		return nil, fmt.Errorf("query is required")
 	}
 
-	results, err := search.Query(s.brainDir, search.QueryOptions{Query: a.Query, Limit: 10})
+	// With reranker disabled: BM25 top-10 straight to the LLM.
+	// With reranker enabled: BM25 top-20 → cross-encoder filter → top-5.
+	bm25Limit := 10
+	if s.reranker != nil {
+		bm25Limit = 20
+	}
+	results, err := search.Query(s.brainDir, search.QueryOptions{Query: a.Query, Limit: bm25Limit})
 	if err != nil {
 		return nil, fmt.Errorf("search: %w", err)
 	}
+	if s.reranker != nil && len(results) > 0 {
+		results, err = rerankResults(ctx, s.reranker, a.Query, results, 5)
+		if err != nil {
+			return nil, fmt.Errorf("rerank: %w", err)
+		}
+	}
 	if len(results) == 0 {
 		return json.Marshal(map[string]any{
 			"answer":  "No relevant content found in brain.",

ingestion/internal/mcp/tools_answer_test.go

@@ -3,14 +3,17 @@ package mcp_test
 import (
 	"context"
 	"encoding/json"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/reranker"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -46,6 +49,55 @@ func callTool(t *testing.T, ts *httptest.Server, name string, arguments map[stri
 	return out
 }
 
+func TestBrainAnswer_RerankerFiltersBeforeLLM(t *testing.T) {
+	brainDir := t.TempDir()
+	wikiDir := filepath.Join(brainDir, "wiki")
+	require.NoError(t, os.MkdirAll(wikiDir, 0o755))
+	// Two notes — both BM25-match the query, but only one is truly relevant.
+	require.NoError(t, os.WriteFile(filepath.Join(wikiDir, "good.md"), []byte(
+		"---\ntitle: Pass-rate Logging\n---\nPass-rate logging tracks skill invocations.",
+	), 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(wikiDir, "noise.md"), []byte(
+		"---\ntitle: Pass-rate Tangent\n---\nPass-rate appears here too but as a tangent.",
+	), 0o644))
+
+	// Fake Ollama reranker: yes only when prompt contains "tracks skill invocations".
+	rrSrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		raw, _ := io.ReadAll(r.Body)
+		yes := strings.Contains(string(raw), "tracks skill invocations")
+		ans := "no"
+		if yes {
+			ans = "yes"
+		}
+		_ = json.NewEncoder(w).Encode(map[string]any{"response": ans, "done": true})
+	}))
+	defer rrSrv.Close()
+
+	// LLM mock captures the rendered sources so we can assert what reached it.
+	var sawSources string
+	llm := func(_ context.Context, _, user string) (string, error) {
+		sawSources = user
+		return "answer text", nil
+	}
+
+	srv := mcp.NewServer(brainDir, nil, nil, llm).
+		WithReranker(reranker.New(rrSrv.URL, "qwen3"))
+	ts := httptest.NewServer(srv)
+	defer ts.Close()
+
+	rpc := callTool(t, ts, "brain_answer", map[string]any{"query": "pass-rate logging"})
+	require.Nil(t, rpc["error"])
+
+	content := rpc["result"].(map[string]any)["content"].([]any)[0].(map[string]any)["text"].(string)
+	var result map[string]any
+	require.NoError(t, json.Unmarshal([]byte(content), &result))
+	sources := result["sources"].([]any)
+	require.Len(t, sources, 1, "reranker should drop noise.md")
+	assert.Equal(t, "wiki/good.md", sources[0])
+	assert.Contains(t, sawSources, "good.md")
+	assert.NotContains(t, sawSources, "noise.md")
+}
+
 func TestBrainAnswer_NoLLM(t *testing.T) {
 	srv := mcp.NewServer(t.TempDir(), nil, nil, nil)
 	ts := httptest.NewServer(srv)

ingestion/internal/reranker/reranker.go

@@ -0,0 +1,119 @@
+// Package reranker scores (query, document) pairs against a cross-encoder
+// served by an Ollama-compatible backend.
+//
+// Wire format is Ollama's `/api/generate`. The model is prompted with the
+// Qwen3-Reranker yes/no template — the canonical interface the model
+// itself was trained against — and the first token of the response is
+// treated as a binary relevance vote: "yes" → 1.0, anything else → 0.0.
+// Ties are expected to be broken by the caller's primary retrieval score
+// (e.g. BM25), so the binary signal is a filter rather than a ranking
+// substitute.
+package reranker
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// Client posts rerank requests to an Ollama-compatible endpoint.
+type Client struct {
+	URL   string
+	Model string
+	HTTP  *http.Client
+}
+
+// New constructs a Client. Returns nil when url is empty so callers can
+// treat a missing BRAIN_RERANKER_URL as "feature disabled" with a single
+// nil check.
+func New(url, model string) *Client {
+	if url == "" {
+		return nil
+	}
+	return &Client{
+		URL:   strings.TrimRight(url, "/"),
+		Model: model,
+		HTTP:  &http.Client{Timeout: 30 * time.Second},
+	}
+}
+
+// Score returns one [0, 1] relevance score per input document, parallel
+// to the input order. Each (query, doc) pair is scored independently —
+// Qwen3-Reranker is a cross-encoder and expects per-pair calls.
+func (c *Client) Score(ctx context.Context, query string, docs []string) ([]float64, error) {
+	out := make([]float64, len(docs))
+	for i, doc := range docs {
+		s, err := c.scoreOne(ctx, query, doc)
+		if err != nil {
+			return nil, fmt.Errorf("rerank doc %d: %w", i, err)
+		}
+		out[i] = s
+	}
+	return out, nil
+}
+
+func (c *Client) scoreOne(ctx context.Context, query, doc string) (float64, error) {
+	prompt := buildPrompt(query, doc)
+	reqBody, _ := json.Marshal(map[string]any{
+		"model":  c.Model,
+		"prompt": prompt,
+		"stream": false,
+		"options": map[string]any{
+			"num_predict": 4,
+			"temperature": 0,
+		},
+	})
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
+		c.URL+"/api/generate", bytes.NewReader(reqBody))
+	if err != nil {
+		return 0, err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := c.HTTP.Do(req)
+	if err != nil {
+		return 0, err
+	}
+	defer func() { _ = resp.Body.Close() }()
+	if resp.StatusCode/100 != 2 {
+		body, _ := io.ReadAll(resp.Body)
+		return 0, fmt.Errorf("status %d: %s", resp.StatusCode, string(body))
+	}
+	var out struct {
+		Response string `json:"response"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
+		return 0, err
+	}
+	return parseYesNo(out.Response), nil
+}
+
+// buildPrompt assembles the Qwen3-Reranker chat template. Kept verbatim
+// because the model was trained on this exact wording.
+func buildPrompt(query, doc string) string {
+	return "<|im_start|>system\nJudge whether the Document meets the requirements based on the Query and the Instruct provided. Note that the answer can only be \"yes\" or \"no\".<|im_end|>\n" +
+		"<|im_start|>user\n<Instruct>: Given a web search query, retrieve relevant passages that answer the query\n" +
+		"<Query>: " + query + "\n" +
+		"<Document>: " + doc + "<|im_end|>\n" +
+		"<|im_start|>assistant\n<think>\n\n</think>\n\n"
+}
+
+// parseYesNo extracts the first meaningful token from response and
+// returns 1.0 when it starts with "yes" (case-insensitive), 0.0 otherwise.
+// Any leading whitespace, `<think>` block, or punctuation is skipped.
+func parseYesNo(s string) float64 {
+	s = strings.TrimSpace(s)
+	// Strip any `<think>…</think>` block the model may emit even with empty thinking.
+	if idx := strings.Index(s, "</think>"); idx != -1 {
+		s = strings.TrimSpace(s[idx+len("</think>"):])
+	}
+	s = strings.ToLower(s)
+	if strings.HasPrefix(s, "yes") {
+		return 1.0
+	}
+	return 0.0
+}

ingestion/internal/reranker/reranker_test.go

@@ -0,0 +1,119 @@
+package reranker_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/reranker"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// fakeOllama responds to /api/generate based on a per-document
+// {needle → answer} map: if the prompt contains the needle, returns
+// the mapped answer.
+type fakeOllama struct {
+	t        *testing.T
+	answers  map[string]string // needle → "yes" or "no"
+	calls    int
+	lastBody map[string]any
+}
+
+func (f *fakeOllama) handler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		require.Equal(f.t, http.MethodPost, r.Method)
+		require.Equal(f.t, "/api/generate", r.URL.Path)
+		body, err := io.ReadAll(r.Body)
+		require.NoError(f.t, err)
+		var p map[string]any
+		require.NoError(f.t, json.Unmarshal(body, &p))
+		f.calls++
+		f.lastBody = p
+		prompt := p["prompt"].(string)
+		answer := "no"
+		for needle, a := range f.answers {
+			if strings.Contains(prompt, needle) {
+				answer = a
+				break
+			}
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"model": p["model"], "response": answer, "done": true,
+		})
+	})
+}
+
+func TestNew_EmptyURLReturnsNil(t *testing.T) {
+	assert.Nil(t, reranker.New("", "model"))
+}
+
+func TestScore_YesAndNoOrdered(t *testing.T) {
+	f := &fakeOllama{t: t, answers: map[string]string{
+		"alpha doc": "yes",
+		"beta doc":  "no",
+		"gamma doc": "yes",
+	}}
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+	c := reranker.New(srv.URL, "test-model")
+	require.NotNil(t, c)
+
+	scores, err := c.Score(context.Background(), "what is alpha",
+		[]string{"alpha doc body", "beta doc body", "gamma doc body"})
+	require.NoError(t, err)
+	require.Len(t, scores, 3)
+	assert.Equal(t, 1.0, scores[0])
+	assert.Equal(t, 0.0, scores[1])
+	assert.Equal(t, 1.0, scores[2])
+	assert.Equal(t, 3, f.calls)
+}
+
+func TestScore_SendsCorrectShape(t *testing.T) {
+	f := &fakeOllama{t: t, answers: map[string]string{"hello": "yes"}}
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+	c := reranker.New(srv.URL, "qwen3-rerank")
+	_, err := c.Score(context.Background(), "greeting", []string{"hello world"})
+	require.NoError(t, err)
+	assert.Equal(t, "qwen3-rerank", f.lastBody["model"])
+	prompt := f.lastBody["prompt"].(string)
+	assert.Contains(t, prompt, "greeting")
+	assert.Contains(t, prompt, "hello world")
+	assert.Contains(t, prompt, `"yes" or "no"`)
+}
+
+func TestScore_HandlesAmbiguousResponse(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_ = json.NewEncoder(w).Encode(map[string]any{"response": "maybe — unclear", "done": true})
+	}))
+	defer srv.Close()
+	c := reranker.New(srv.URL, "m")
+	scores, err := c.Score(context.Background(), "q", []string{"d"})
+	require.NoError(t, err)
+	// Anything that does not start with "yes" (case-insensitive, after
+	// whitespace/think trim) is treated as "no" = 0.
+	assert.Equal(t, []float64{0}, scores)
+}
+
+func TestScore_EmptyDocsReturnsEmpty(t *testing.T) {
+	c := reranker.New("http://127.0.0.1:1", "m")
+	scores, err := c.Score(context.Background(), "q", nil)
+	require.NoError(t, err)
+	assert.Empty(t, scores)
+}
+
+func TestScore_UpstreamErrorPropagates(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer srv.Close()
+	c := reranker.New(srv.URL, "m")
+	_, err := c.Score(context.Background(), "q", []string{"d"})
+	require.Error(t, err)
+}