diff --git a/ingestion/cmd/server/main.go b/ingestion/cmd/server/main.go
index 194f54a..a44a02d 100644
--- a/ingestion/cmd/server/main.go
+++ b/ingestion/cmd/server/main.go
@@ -68,6 +68,7 @@ func main() {
 	mux.HandleFunc("POST /write", h.Write)
 	mux.HandleFunc("POST /ingest", h.Ingest)
 	mux.HandleFunc("POST /ingest-path", h.IngestPath)
+	mux.HandleFunc("POST /backfill-refs", h.BackfillRefs)
 
 	addr := ":" + port
 	watchIntervalLog := "disabled"
diff --git a/ingestion/internal/api/handler.go b/ingestion/internal/api/handler.go
index 141ca0a..961007d 100644
--- a/ingestion/internal/api/handler.go
+++ b/ingestion/internal/api/handler.go
@@ -272,6 +272,18 @@ func (h *Handler) IngestPath(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, ingestResponse{Pages: allPages, Warnings: allWarnings})
 }
 
+// BackfillRefs handles POST /backfill-refs — injects source back-references
+// into all concept and entity pages based on existing wiki/sources/ pages.
+func (h *Handler) BackfillRefs(w http.ResponseWriter, r *http.Request) {
+	n, err := pipeline.BackfillRefs(r.Context(), h.brainDir)
+	if err != nil {
+		h.logger.Error("backfill-refs failed", "err", err)
+		writeError(w, http.StatusInternalServerError, "backfill error")
+		return
+	}
+	writeJSON(w, map[string]int{"updated": n})
+}
+
 func writeJSON(w http.ResponseWriter, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	json.NewEncoder(w).Encode(v) //nolint:errcheck
diff --git a/ingestion/internal/pipeline/backfill.go b/ingestion/internal/pipeline/backfill.go
new file mode 100644
index 0000000..dc20c94
--- /dev/null
+++ b/ingestion/internal/pipeline/backfill.go
@@ -0,0 +1,91 @@
+// ingestion/internal/pipeline/backfill.go
+package pipeline
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/wiki"
+)
+
+// BackfillRefs walks wiki/sources/ and injects source back-references into every
+// concept and entity page that each source links to.
+// Changes for all sources are accumulated in memory before writing, so multiple
+// sources referencing the same concept are merged in one pass.
+// Deduplication is handled by wiki.Merge — running this multiple times is safe.
+// Returns the number of concept/entity pages written.
+func BackfillRefs(ctx context.Context, brainDir string) (int, error) {
+	inventory, err := wiki.LoadInventory(brainDir)
+	if err != nil {
+		return 0, fmt.Errorf("load inventory: %w", err)
+	}
+
+	sourcesDir := filepath.Join(brainDir, "wiki", "sources")
+	entries, err := os.ReadDir(sourcesDir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return 0, nil
+		}
+		return 0, fmt.Errorf("read sources dir: %w", err)
+	}
+
+	// Accumulate all changes before writing: relPath → updated Page.
+	// Collecting first means two sources that both link the same concept
+	// get both refs merged before a single write.
+	pending := make(map[string]wiki.Page)
+
+	for _, e := range entries {
+		if ctx.Err() != nil {
+			return 0, ctx.Err()
+		}
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".md") {
+			continue
+		}
+
+		b, err := os.ReadFile(filepath.Join(sourcesDir, e.Name()))
+		if err != nil {
+			continue
+		}
+		sourceContent := string(b)
+		sourceSlug := strings.TrimSuffix(e.Name(), ".md")
+		sourceTitle := extractTitle(sourceContent)
+		if sourceTitle == "" {
+			sourceTitle = sourceSlug
+		}
+		sourceRef := "- [[" + sourceSlug + "|" + sourceTitle + "]]"
+
+		for slug := range extractWikilinks(sourceContent) {
+			if slug == sourceSlug {
+				continue
+			}
+			pt, ok := findInInventory(slug, inventory)
+			if !ok {
+				continue
+			}
+			relPath := "wiki/" + string(pt) + "/" + slug + ".md"
+
+			// Start from already-accumulated version if we've seen this page.
+			page, seen := pending[relPath]
+			if !seen {
+				raw, err := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(relPath)))
+				if err != nil {
+					continue
+				}
+				page = wiki.Page{Path: relPath, Content: string(raw)}
+			}
+			pending[relPath] = addSourceRef(page, sourceRef)
+		}
+	}
+
+	for relPath, page := range pending {
+		dest := filepath.Join(brainDir, filepath.FromSlash(relPath))
+		if err := os.WriteFile(dest, []byte(page.Content), 0o644); err != nil {
+			return 0, fmt.Errorf("write %s: %w", relPath, err)
+		}
+	}
+
+	return len(pending), nil
+}
diff --git a/ingestion/internal/pipeline/backfill_test.go b/ingestion/internal/pipeline/backfill_test.go
new file mode 100644
index 0000000..8f23f99
--- /dev/null
+++ b/ingestion/internal/pipeline/backfill_test.go
@@ -0,0 +1,107 @@
+// ingestion/internal/pipeline/backfill_test.go
+package pipeline
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func setupBrainDir(t *testing.T) string {
+	t.Helper()
+	dir := t.TempDir()
+	for _, sub := range []string{"wiki/sources", "wiki/concepts", "wiki/entities"} {
+		require.NoError(t, os.MkdirAll(filepath.Join(dir, sub), 0o755))
+	}
+	return dir
+}
+
+func writeFile(t *testing.T, path, content string) {
+	t.Helper()
+	require.NoError(t, os.MkdirAll(filepath.Dir(path), 0o755))
+	require.NoError(t, os.WriteFile(path, []byte(content), 0o644))
+}
+
+func TestBackfillRefs_UpdatesConcept(t *testing.T) {
+	dir := setupBrainDir(t)
+	writeFile(t, filepath.Join(dir, "wiki/sources/shape-up.md"),
+		"---\ntitle: Shape Up\n---\n\n## Summary\n\nSee [[betting|Betting]].\n")
+	writeFile(t, filepath.Join(dir, "wiki/concepts/betting.md"),
+		"---\ntitle: Betting\n---\n\n## Definition\n\nA resource allocation technique.\n")
+
+	n, err := BackfillRefs(context.Background(), dir)
+	require.NoError(t, err)
+	assert.Equal(t, 1, n)
+
+	got, err := os.ReadFile(filepath.Join(dir, "wiki/concepts/betting.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "## Sources")
+	assert.Contains(t, string(got), "[[shape-up|Shape Up]]")
+	assert.Contains(t, string(got), "## Definition") // original content preserved
+}
+
+func TestBackfillRefs_Deduplication(t *testing.T) {
+	dir := setupBrainDir(t)
+	writeFile(t, filepath.Join(dir, "wiki/sources/shape-up.md"),
+		"---\ntitle: Shape Up\n---\n\n## Summary\n\nSee [[betting|Betting]].\n")
+	writeFile(t, filepath.Join(dir, "wiki/concepts/betting.md"),
+		"---\ntitle: Betting\n---\n\n## Definition\n\nA technique.\n")
+
+	// Run twice — should not duplicate the ref.
+	_, err := BackfillRefs(context.Background(), dir)
+	require.NoError(t, err)
+	_, err = BackfillRefs(context.Background(), dir)
+	require.NoError(t, err)
+
+	got, err := os.ReadFile(filepath.Join(dir, "wiki/concepts/betting.md"))
+	require.NoError(t, err)
+
+	count := 0
+	for _, line := range splitLines(string(got)) {
+		if line == "- [[shape-up|Shape Up]]" {
+			count++
+		}
+	}
+	assert.Equal(t, 1, count, "ref should appear exactly once after two runs")
+}
+
+func TestBackfillRefs_MultipleSources(t *testing.T) {
+	dir := setupBrainDir(t)
+	writeFile(t, filepath.Join(dir, "wiki/sources/book-a.md"),
+		"---\ntitle: Book A\n---\n\n## Summary\n\nSee [[shaping|Shaping]].\n")
+	writeFile(t, filepath.Join(dir, "wiki/sources/book-b.md"),
+		"---\ntitle: Book B\n---\n\n## Summary\n\nAlso [[shaping|Shaping]].\n")
+	writeFile(t, filepath.Join(dir, "wiki/concepts/shaping.md"),
+		"---\ntitle: Shaping\n---\n\n## Definition\n\nA design activity.\n")
+
+	n, err := BackfillRefs(context.Background(), dir)
+	require.NoError(t, err)
+	assert.Equal(t, 1, n) // one concept page written
+
+	got, err := os.ReadFile(filepath.Join(dir, "wiki/concepts/shaping.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "[[book-a|Book A]]")
+	assert.Contains(t, string(got), "[[book-b|Book B]]")
+}
+
+func TestBackfillRefs_NoSourcesDir(t *testing.T) {
+	dir := t.TempDir() // no wiki/sources subdir
+	n, err := BackfillRefs(context.Background(), dir)
+	require.NoError(t, err)
+	assert.Equal(t, 0, n)
+}
+
+func TestBackfillRefs_SkipsUnknownSlugs(t *testing.T) {
+	dir := setupBrainDir(t)
+	// Source links to a slug not in inventory and not on disk.
+	writeFile(t, filepath.Join(dir, "wiki/sources/article.md"),
+		"---\ntitle: Article\n---\n\n## Summary\n\nSee [[ghost-slug|Ghost]].\n")
+
+	n, err := BackfillRefs(context.Background(), dir)
+	require.NoError(t, err)
+	assert.Equal(t, 0, n)
+}