feat(brain): cross-wing tunnels — bidirectional wikilinks + auto-detect

Adds the `brain_tunnel` MCP tool and auto-tunnel behaviour for
`brain_write`, so concepts that appear in multiple wings become
navigable from any of them.

New surface in package brain:
- WriteTunnel(brainDir, src, tgt) — appends a `## See also` bidirectional
  wikilink between two notes in different wings. Idempotent (link not
  duplicated on re-call) and reuses an existing See also section.
- DetectTunnels(brainDir, content) — walks brain/wiki/, returns
  TunnelCandidates for notes whose title appears in content. Tags
  whole-word case-insensitive hits as Exact=true and substring-only hits
  as Exact=false.
- AutoTunnel(brainDir, src, content) — wraps DetectTunnels: writes
  cross-wing exact matches, stages fuzzy matches into
  brain/raw/tunnel-candidates-<YYYY-MM-DD>.md for human review.

MCP wiring:
- `brain_tunnel` tool: explicit manual link (source, target).
- `brain_write` with wing+hall now triggers AutoTunnel on the new
  content. Failures are logged and never abort the primary write.

readTitleAndCreated also humanises the slug fallback (hyphens → spaces)
so titleless notes participate in content matching.

Closes hyperguild#16.

Tests: idempotency, same-wing rejection, missing-note rejection,
See-also reuse, exact/fuzzy detection, slug fallback, MCP tool happy
path, auto-tunnel hook (cross-wing exact → linked; same-wing → skipped;
fuzzy → candidates file).

ingestion/internal/brain/index.go

@@ -150,7 +150,7 @@ func readTitleAndCreated(path, slug string) (string, string) {
 		}
 	}
 	if title == "" {
-		title = slug
+		title = strings.ReplaceAll(slug, "-", " ")
 	}
 	if created == "" {
 		if info, err := os.Stat(path); err == nil {

ingestion/internal/brain/tunnel.go

@@ -0,0 +1,286 @@
+package brain
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// seeAlsoHeader is the markdown heading used to group cross-wing links.
+const seeAlsoHeader = "## See also"
+
+// TunnelCandidate is a cross-wing match surfaced by DetectTunnels. It is
+// not yet a written link — the caller decides whether confidence is high
+// enough to commit it via WriteTunnel.
+type TunnelCandidate struct {
+	// TargetPath is the candidate note's path relative to brainDir
+	// (forward-slashed), e.g. "wiki/hyperguild/decisions/routing.md".
+	TargetPath string
+	// MatchedTerm is the title that matched in the source content.
+	MatchedTerm string
+	// Exact is true when the match was a case-insensitive whole-token
+	// hit on the target's frontmatter title. Fuzzy matches (substring
+	// only) are flagged Exact=false and should not be auto-written.
+	Exact bool
+}
+
+// DetectTunnels scans brain/wiki/ for notes whose title appears in
+// content. Returns one TunnelCandidate per matching note. Exact is true
+// when content contains the title as a whole-word case-insensitive
+// token; false when only a substring matched (caller treats these as
+// fuzzy and should not auto-write them).
+//
+// A note's title is read from YAML frontmatter `title:`; failing that,
+// the filename slug (sans `.md`, hyphens → spaces) is used.
+func DetectTunnels(brainDir, content string) ([]TunnelCandidate, error) {
+	wikiDir := filepath.Join(brainDir, "wiki")
+	if _, err := os.Stat(wikiDir); os.IsNotExist(err) {
+		return nil, nil
+	} else if err != nil {
+		return nil, fmt.Errorf("stat wiki: %w", err)
+	}
+
+	lowerContent := strings.ToLower(content)
+
+	var out []TunnelCandidate
+	err := filepath.WalkDir(wikiDir, func(path string, d os.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() || !strings.HasSuffix(path, ".md") || d.Name() == "_index.md" {
+			return nil
+		}
+		title, _ := readTitleAndCreated(path, strings.TrimSuffix(d.Name(), ".md"))
+		needle := strings.ToLower(strings.TrimSpace(title))
+		if needle == "" {
+			return nil
+		}
+		idx := strings.Index(lowerContent, needle)
+		if idx == -1 {
+			return nil
+		}
+		rel, err := filepath.Rel(brainDir, path)
+		if err != nil {
+			return err
+		}
+		out = append(out, TunnelCandidate{
+			TargetPath:  filepath.ToSlash(rel),
+			MatchedTerm: title,
+			Exact:       isWholeWord(lowerContent, idx, len(needle)),
+		})
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+// isWholeWord reports whether the substring at [idx, idx+n) in s is
+// bounded by non-alphanumeric characters (or string edges).
+func isWholeWord(s string, idx, n int) bool {
+	left := idx == 0 || !isWordByte(s[idx-1])
+	right := idx+n == len(s) || !isWordByte(s[idx+n])
+	return left && right
+}
+
+func isWordByte(b byte) bool {
+	return (b >= 'a' && b <= 'z') ||
+		(b >= 'A' && b <= 'Z') ||
+		(b >= '0' && b <= '9')
+}
+
+// AutoTunnel runs DetectTunnels against content and, for each
+// candidate, either writes a bidirectional tunnel (when the match is
+// exact and in a different wing) or stages it for human review in
+// brain/raw/tunnel-candidates-<YYYY-MM-DD>.md.
+//
+// sourcePath is the note that originated the content — used to skip
+// self-matches and same-wing tunnels. Errors writing individual
+// tunnels are recorded into the candidates file but never abort the
+// rest of the scan; the caller's primary write has already succeeded
+// and auto-linking is best-effort.
+func AutoTunnel(brainDir, sourcePath, content string) error {
+	srcWing, err := wingOf(sourcePath)
+	if err != nil {
+		return err
+	}
+	candidates, err := DetectTunnels(brainDir, content)
+	if err != nil {
+		return err
+	}
+
+	var fuzzy []TunnelCandidate
+	for _, c := range candidates {
+		if c.TargetPath == sourcePath {
+			continue
+		}
+		tgtWing, err := wingOf(c.TargetPath)
+		if err != nil || tgtWing == srcWing {
+			continue
+		}
+		if !c.Exact {
+			fuzzy = append(fuzzy, c)
+			continue
+		}
+		if err := WriteTunnel(brainDir, sourcePath, c.TargetPath); err != nil {
+			fuzzy = append(fuzzy, c)
+		}
+	}
+	return logFuzzyCandidates(brainDir, sourcePath, fuzzy)
+}
+
+// logFuzzyCandidates appends one row per candidate to
+// brain/raw/tunnel-candidates-<YYYY-MM-DD>.md, creating the file with a
+// header on first write of the day. No-op when the candidate list is empty.
+func logFuzzyCandidates(brainDir, sourcePath string, cs []TunnelCandidate) error {
+	if len(cs) == 0 {
+		return nil
+	}
+	rawDir := filepath.Join(brainDir, "raw")
+	if err := os.MkdirAll(rawDir, 0o755); err != nil {
+		return err
+	}
+	stamp := time.Now().UTC().Format("2006-01-02")
+	path := filepath.Join(rawDir, "tunnel-candidates-"+stamp+".md")
+	existed := fileExists(path)
+	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = f.Close() }()
+	if !existed {
+		if _, err := f.WriteString("# Tunnel candidates " + stamp + "\n\nFuzzy cross-wing matches surfaced by AutoTunnel. Review and promote to a tunnel via `brain_tunnel` if relevant.\n\n"); err != nil {
+			return err
+		}
+	}
+	for _, c := range cs {
+		line := fmt.Sprintf("- `%s` ↔ `%s` (term: %q)\n", sourcePath, c.TargetPath, c.MatchedTerm)
+		if _, err := f.WriteString(line); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func fileExists(p string) bool {
+	_, err := os.Stat(p)
+	return err == nil
+}
+
+// WriteTunnel appends a bidirectional wikilink between sourcePath and
+// targetPath under a `## See also` section in each note. Paths are
+// relative to brainDir (forward-slashed), e.g. wiki/<wing>/<hall>/<slug>.md.
+//
+// Idempotent: re-calling with the same pair does not duplicate links or
+// section headers. Rejects same-wing pairs (a tunnel is by definition
+// cross-wing) and missing notes.
+func WriteTunnel(brainDir, sourcePath, targetPath string) error {
+	srcWing, err := wingOf(sourcePath)
+	if err != nil {
+		return fmt.Errorf("source: %w", err)
+	}
+	tgtWing, err := wingOf(targetPath)
+	if err != nil {
+		return fmt.Errorf("target: %w", err)
+	}
+	if srcWing == tgtWing {
+		return fmt.Errorf("tunnel must cross wings; got both in %q", srcWing)
+	}
+
+	srcFull := filepath.Join(brainDir, filepath.FromSlash(sourcePath))
+	tgtFull := filepath.Join(brainDir, filepath.FromSlash(targetPath))
+	if _, err := os.Stat(srcFull); err != nil {
+		return fmt.Errorf("source note: %w", err)
+	}
+	if _, err := os.Stat(tgtFull); err != nil {
+		return fmt.Errorf("target note: %w", err)
+	}
+
+	if err := appendSeeAlso(srcFull, wikilinkOf(targetPath)); err != nil {
+		return fmt.Errorf("update source: %w", err)
+	}
+	if err := appendSeeAlso(tgtFull, wikilinkOf(sourcePath)); err != nil {
+		return fmt.Errorf("update target: %w", err)
+	}
+	return nil
+}
+
+// wikilinkOf turns "wiki/<wing>/<hall>/<slug>.md" into "<wing>/<hall>/<slug>"
+// for use inside `[[...]]`.
+func wikilinkOf(relPath string) string {
+	p := strings.TrimSuffix(relPath, ".md")
+	p = strings.TrimPrefix(p, "wiki/")
+	return p
+}
+
+// wingOf extracts the wing segment from a relative wiki path
+// "wiki/<wing>/<hall>/<slug>.md".
+func wingOf(relPath string) (string, error) {
+	parts := strings.Split(relPath, "/")
+	if len(parts) < 4 || parts[0] != "wiki" {
+		return "", fmt.Errorf("not a wiki path: %q", relPath)
+	}
+	if parts[1] == "" {
+		return "", fmt.Errorf("empty wing in path: %q", relPath)
+	}
+	return parts[1], nil
+}
+
+// appendSeeAlso inserts `- [[link]]` under the file's See also section,
+// creating the section if absent. No-op when the link is already present.
+func appendSeeAlso(filePath, link string) error {
+	content, err := os.ReadFile(filePath)
+	if err != nil {
+		return err
+	}
+	wikilink := "[[" + link + "]]"
+	if strings.Contains(string(content), wikilink) {
+		return nil
+	}
+
+	bullet := "- " + wikilink
+
+	if !strings.Contains(string(content), seeAlsoHeader) {
+		// No section yet — append a fresh one. Always emit a trailing
+		// newline so subsequent appends don't merge into the previous line.
+		trimmed := strings.TrimRight(string(content), "\n")
+		out := trimmed + "\n\n" + seeAlsoHeader + "\n\n" + bullet + "\n"
+		return os.WriteFile(filePath, []byte(out), 0o644)
+	}
+
+	// Section exists — splice the bullet in just before the next `## `
+	// heading (or EOF). Reading the file line-by-line keeps this robust
+	// against arbitrary section ordering.
+	var b strings.Builder
+	scanner := bufio.NewScanner(strings.NewReader(string(content)))
+	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+	inSeeAlso, inserted := false, false
+	for scanner.Scan() {
+		line := scanner.Text()
+		if !inserted && inSeeAlso && strings.HasPrefix(line, "## ") &&
+			strings.TrimSpace(line) != seeAlsoHeader {
+			b.WriteString(bullet)
+			b.WriteByte('\n')
+			b.WriteByte('\n')
+			inserted = true
+		}
+		if strings.TrimSpace(line) == seeAlsoHeader {
+			inSeeAlso = true
+		}
+		b.WriteString(line)
+		b.WriteByte('\n')
+	}
+	if err := scanner.Err(); err != nil {
+		return err
+	}
+	if !inserted {
+		// section was the last thing in the file — just append bullet
+		out := strings.TrimRight(b.String(), "\n") + "\n" + bullet + "\n"
+		return os.WriteFile(filePath, []byte(out), 0o644)
+	}
+	return os.WriteFile(filePath, []byte(b.String()), 0o644)
+}

ingestion/internal/brain/tunnel_test.go

@@ -0,0 +1,177 @@
+package brain_test
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// seedNote writes a minimal markdown note at brainDir/relPath with the given body.
+func seedNote(t *testing.T, brainDir, relPath, body string) {
+	t.Helper()
+	full := filepath.Join(brainDir, relPath)
+	require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+	require.NoError(t, os.WriteFile(full, []byte(body), 0o644))
+}
+
+func TestWriteTunnel_AppendsBidirectionalLinks(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/jepa-fx/decisions/val-vol.md",
+		"---\nwing: jepa-fx\nhall: decisions\n---\n# Val Vol R2\n\nbody.\n")
+	seedNote(t, dir, "wiki/hyperguild/decisions/routing.md",
+		"---\nwing: hyperguild\nhall: decisions\n---\n# Routing\n\nbody.\n")
+
+	err := brain.WriteTunnel(dir,
+		"wiki/jepa-fx/decisions/val-vol.md",
+		"wiki/hyperguild/decisions/routing.md",
+	)
+	require.NoError(t, err)
+
+	src, err := os.ReadFile(filepath.Join(dir, "wiki/jepa-fx/decisions/val-vol.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(src), "## See also")
+	assert.Contains(t, string(src), "[[hyperguild/decisions/routing]]")
+
+	tgt, err := os.ReadFile(filepath.Join(dir, "wiki/hyperguild/decisions/routing.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(tgt), "## See also")
+	assert.Contains(t, string(tgt), "[[jepa-fx/decisions/val-vol]]")
+}
+
+func TestWriteTunnel_Idempotent(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/a/facts/x.md", "# X\n\nbody.\n")
+	seedNote(t, dir, "wiki/b/facts/y.md", "# Y\n\nbody.\n")
+
+	for i := 0; i < 3; i++ {
+		require.NoError(t, brain.WriteTunnel(dir,
+			"wiki/a/facts/x.md", "wiki/b/facts/y.md"))
+	}
+
+	src, err := os.ReadFile(filepath.Join(dir, "wiki/a/facts/x.md"))
+	require.NoError(t, err)
+	assert.Equal(t, 1, strings.Count(string(src), "[[b/facts/y]]"),
+		"link should appear exactly once after 3 calls")
+	assert.Equal(t, 1, strings.Count(string(src), "## See also"))
+
+	tgt, err := os.ReadFile(filepath.Join(dir, "wiki/b/facts/y.md"))
+	require.NoError(t, err)
+	assert.Equal(t, 1, strings.Count(string(tgt), "[[a/facts/x]]"))
+	assert.Equal(t, 1, strings.Count(string(tgt), "## See also"))
+}
+
+func TestWriteTunnel_RejectsSameWing(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/jepa-fx/facts/x.md", "x")
+	seedNote(t, dir, "wiki/jepa-fx/facts/y.md", "y")
+	err := brain.WriteTunnel(dir,
+		"wiki/jepa-fx/facts/x.md", "wiki/jepa-fx/facts/y.md")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "cross wings")
+}
+
+func TestWriteTunnel_RejectsMissingNote(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/a/facts/x.md", "x")
+	err := brain.WriteTunnel(dir,
+		"wiki/a/facts/x.md", "wiki/b/facts/ghost.md")
+	require.Error(t, err)
+}
+
+func TestDetectTunnels_ExactTitleMatch(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/jepa-fx/decisions/val-vol.md",
+		"---\nwing: jepa-fx\nhall: decisions\ntitle: Val Vol R2\n---\nbody.\n")
+	seedNote(t, dir, "wiki/jepa-fx/facts/lejpa.md",
+		"---\nwing: jepa-fx\nhall: facts\ntitle: LeJPA Architecture\n---\nbody.\n")
+
+	candidates, err := brain.DetectTunnels(dir,
+		"We need to revisit Val Vol R2 in light of new tier data.")
+	require.NoError(t, err)
+
+	require.Len(t, candidates, 1)
+	assert.Equal(t, "wiki/jepa-fx/decisions/val-vol.md", candidates[0].TargetPath)
+	assert.Equal(t, "Val Vol R2", candidates[0].MatchedTerm)
+	assert.True(t, candidates[0].Exact)
+}
+
+func TestDetectTunnels_FuzzyMatch(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/x/facts/routing.md",
+		"---\ntitle: Routing\n---\nbody.\n")
+
+	// Substring of title appears in content, but not as a whole word.
+	candidates, err := brain.DetectTunnels(dir, "rerouting handles failover")
+	require.NoError(t, err)
+	require.Len(t, candidates, 1)
+	assert.False(t, candidates[0].Exact, "substring-only match should be fuzzy")
+}
+
+func TestDetectTunnels_NoFrontmatterFallsBackToSlug(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/x/facts/widget-flags.md", "# widget flags\n\nbody.\n")
+
+	candidates, err := brain.DetectTunnels(dir,
+		"Documented Widget Flags after the deploy issue.")
+	require.NoError(t, err)
+	require.Len(t, candidates, 1)
+	assert.True(t, candidates[0].Exact)
+	assert.Equal(t, "widget flags", candidates[0].MatchedTerm)
+}
+
+func TestAutoTunnel_FuzzyGoesToCandidatesFile(t *testing.T) {
+	dir := t.TempDir()
+	// Existing note in a different wing whose title is "Routing".
+	seedNote(t, dir, "wiki/other/facts/routing.md",
+		"---\nwing: other\nhall: facts\ntitle: Routing\n---\nbody.\n")
+	// Source note in another wing whose body mentions "rerouting" (substring match only).
+	seedNote(t, dir, "wiki/jepa-fx/facts/new.md",
+		"---\nwing: jepa-fx\nhall: facts\n---\nrerouting traffic\n")
+
+	require.NoError(t, brain.AutoTunnel(dir,
+		"wiki/jepa-fx/facts/new.md", "rerouting traffic"))
+
+	// Source must not get auto-linked (fuzzy).
+	got, err := os.ReadFile(filepath.Join(dir, "wiki/jepa-fx/facts/new.md"))
+	require.NoError(t, err)
+	assert.NotContains(t, string(got), "[[other/facts/routing]]")
+
+	// Candidates file must list the pair.
+	matches, err := filepath.Glob(filepath.Join(dir, "raw", "tunnel-candidates-*.md"))
+	require.NoError(t, err)
+	require.Len(t, matches, 1)
+	body, err := os.ReadFile(matches[0])
+	require.NoError(t, err)
+	assert.Contains(t, string(body), "wiki/jepa-fx/facts/new.md")
+	assert.Contains(t, string(body), "wiki/other/facts/routing.md")
+	assert.Contains(t, string(body), "Routing")
+}
+
+func TestDetectTunnels_EmptyWiki(t *testing.T) {
+	dir := t.TempDir()
+	cs, err := brain.DetectTunnels(dir, "anything")
+	require.NoError(t, err)
+	assert.Empty(t, cs)
+}
+
+func TestWriteTunnel_AppendsToExistingSeeAlso(t *testing.T) {
+	dir := t.TempDir()
+	seedNote(t, dir, "wiki/a/facts/x.md",
+		"# X\n\nbody.\n\n## See also\n\n- [[a/facts/old]]\n")
+	seedNote(t, dir, "wiki/b/facts/y.md", "# Y\n\nbody.\n")
+
+	require.NoError(t, brain.WriteTunnel(dir,
+		"wiki/a/facts/x.md", "wiki/b/facts/y.md"))
+
+	src, err := os.ReadFile(filepath.Join(dir, "wiki/a/facts/x.md"))
+	require.NoError(t, err)
+	s := string(src)
+	assert.Equal(t, 1, strings.Count(s, "## See also"), "should reuse existing section")
+	assert.Contains(t, s, "[[a/facts/old]]")
+	assert.Contains(t, s, "[[b/facts/y]]")
+}

ingestion/internal/mcp/handlers.go

@@ -60,6 +60,14 @@ func (s *Server) tools() []map[string]any {
 				"hall":     enum("optional memory type (requires wing)", halls...),
 			}),
 		},
+		{
+			"name":        "brain_tunnel",
+			"description": "Create an explicit bidirectional [[wikilink]] between two notes in different wings. Idempotent.",
+			"inputSchema": schema([]string{"source", "target"}, map[string]any{
+				"source": str("path of source note relative to brain dir, e.g. wiki/jepa-fx/decisions/val-vol.md"),
+				"target": str("path of target note (must be in a different wing)"),
+			}),
+		},
 		{
 			"name":        "brain_index",
 			"description": "Regenerate _index.md (Map of Content) for one or all wings under brain/wiki/. Auto-called after brain_write with wing+hall.",
@@ -174,16 +182,38 @@ func (s *Server) brainWrite(ctx context.Context, args json.RawMessage) (json.Raw
 		return nil, err
 	}
 	// Auto-regenerate the wing _index.md when the write landed in the
-	// structured wiki. A failure here is best-effort — log and move on,
-	// since the note itself is already written.
+	// structured wiki, and auto-tunnel cross-wing matches. Both are
+	// best-effort: the note is already written.
 	if a.Wing != "" && a.Hall != "" {
 		if err := brain.BuildWingIndex(s.brainDir, a.Wing); err != nil {
 			slog.Warn("brain_write: auto-index failed", "wing", a.Wing, "err", err)
 		}
+		if err := brain.AutoTunnel(s.brainDir, relPath, a.Content); err != nil {
+			slog.Warn("brain_write: auto-tunnel failed", "src", relPath, "err", err)
+		}
 	}
 	return json.Marshal(map[string]string{"path": relPath})
 }
 
+type brainTunnelArgs struct {
+	Source string `json:"source"`
+	Target string `json:"target"`
+}
+
+func (s *Server) brainTunnel(ctx context.Context, args json.RawMessage) (json.RawMessage, error) {
+	var a brainTunnelArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	if a.Source == "" || a.Target == "" {
+		return nil, fmt.Errorf("source and target are required")
+	}
+	if err := brain.WriteTunnel(s.brainDir, a.Source, a.Target); err != nil {
+		return nil, fmt.Errorf("tunnel: %w", err)
+	}
+	return json.Marshal(map[string]string{"status": "ok"})
+}
+
 type brainIndexArgs struct {
 	Wing string `json:"wing,omitempty"`
 }

ingestion/internal/mcp/handlers_test.go

@@ -122,6 +122,92 @@ func TestBrainQueryWingScope(t *testing.T) {
 	assert.NotContains(t, text, "wiki/other/facts/y.md")
 }
 
+func TestBrainWriteAutoTunnelsOnExactMatch(t *testing.T) {
+	brainDir := t.TempDir()
+	// Seed a pre-existing note in wing "other".
+	existing := filepath.Join(brainDir, "wiki/other/facts/widget.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(existing), 0o755))
+	require.NoError(t, os.WriteFile(existing,
+		[]byte("---\nwing: other\nhall: facts\ntitle: Widget\n---\nbody.\n"), 0o644))
+
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+
+	// Write a new note in a *different* wing whose content references "Widget".
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "# Notes\n\nThis note discusses the Widget concept.\n",
+		"filename": "notes",
+		"wing":     "jepa-fx",
+		"hall":     "facts",
+	})
+	require.Nil(t, resp["error"])
+
+	newNote := filepath.Join(brainDir, "wiki/jepa-fx/facts/notes.md")
+	got, err := os.ReadFile(newNote)
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "[[other/facts/widget]]", "new note should link to existing")
+
+	gotTgt, err := os.ReadFile(existing)
+	require.NoError(t, err)
+	assert.Contains(t, string(gotTgt), "[[jepa-fx/facts/notes]]", "existing note should backlink")
+}
+
+func TestBrainWriteAutoTunnelSkipsSameWing(t *testing.T) {
+	brainDir := t.TempDir()
+	existing := filepath.Join(brainDir, "wiki/jepa-fx/facts/widget.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(existing), 0o755))
+	require.NoError(t, os.WriteFile(existing,
+		[]byte("---\nwing: jepa-fx\nhall: facts\ntitle: Widget\n---\nbody.\n"), 0o644))
+
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "Same wing reference to Widget here.\n",
+		"filename": "notes",
+		"wing":     "jepa-fx",
+		"hall":     "facts",
+	})
+	require.Nil(t, resp["error"])
+
+	newNote := filepath.Join(brainDir, "wiki/jepa-fx/facts/notes.md")
+	got, err := os.ReadFile(newNote)
+	require.NoError(t, err)
+	assert.NotContains(t, string(got), "[[jepa-fx/facts/widget]]", "same-wing match must not auto-tunnel")
+}
+
+func TestBrainTunnelLinksTwoNotes(t *testing.T) {
+	brainDir := t.TempDir()
+	for _, p := range []struct{ rel, body string }{
+		{"wiki/jepa-fx/decisions/val-vol.md", "---\nwing: jepa-fx\nhall: decisions\n---\n# Val Vol\n"},
+		{"wiki/hyperguild/decisions/routing.md", "---\nwing: hyperguild\nhall: decisions\n---\n# Routing\n"},
+	} {
+		full := filepath.Join(brainDir, p.rel)
+		require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+		require.NoError(t, os.WriteFile(full, []byte(p.body), 0o644))
+	}
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_tunnel", map[string]any{
+		"source": "wiki/jepa-fx/decisions/val-vol.md",
+		"target": "wiki/hyperguild/decisions/routing.md",
+	})
+	require.Nil(t, resp["error"])
+
+	src, err := os.ReadFile(filepath.Join(brainDir, "wiki/jepa-fx/decisions/val-vol.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(src), "[[hyperguild/decisions/routing]]")
+	tgt, err := os.ReadFile(filepath.Join(brainDir, "wiki/hyperguild/decisions/routing.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(tgt), "[[jepa-fx/decisions/val-vol]]")
+}
+
+func TestBrainTunnelRejectsMissing(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_tunnel", map[string]any{
+		"source": "wiki/a/facts/ghost.md",
+		"target": "wiki/b/facts/ghost.md",
+	})
+	require.NotNil(t, resp["error"])
+}
+
 func TestBrainWriteRejectsTraversal(t *testing.T) {
 	brainDir := t.TempDir()
 	srv := mcp.NewServer(brainDir, nil, nil, nil)

ingestion/internal/mcp/server.go

@@ -1,6 +1,6 @@
 // Package mcp implements an MCP HTTP handler for the ingestion service.
-// Exposed tools: brain_query, brain_write, brain_index, brain_ingest,
-// brain_ingest_raw, brain_answer, brain_classify, session_log.
+// Exposed tools: brain_query, brain_write, brain_index, brain_tunnel,
+// brain_ingest, brain_ingest_raw, brain_answer, brain_classify, session_log.
 package mcp
 
 import (
@@ -139,6 +139,8 @@ func (s *Server) handleCall(ctx context.Context, name string, args json.RawMessa
 		return s.brainWrite(ctx, args)
 	case "brain_index":
 		return s.brainIndex(ctx, args)
+	case "brain_tunnel":
+		return s.brainTunnel(ctx, args)
 	case "brain_ingest_raw":
 		return s.brainIngestRaw(ctx, args)
 	case "brain_ingest":

ingestion/internal/mcp/server_test.go

@@ -55,7 +55,8 @@ func TestServerToolsList(t *testing.T) {
 		names = append(names, t.(map[string]any)["name"].(string))
 	}
 	assert.ElementsMatch(t, []string{
-		"brain_query", "brain_write", "brain_index", "brain_ingest_raw", "brain_ingest",
+		"brain_query", "brain_write", "brain_index", "brain_tunnel",
+		"brain_ingest_raw", "brain_ingest",
 		"brain_answer", "brain_classify", "session_log",
 	}, names)
 }