diff --git a/internal/docs/navigation.go b/internal/docs/navigation.go
index 62a0565a..8a16c36d 100644
--- a/internal/docs/navigation.go
+++ b/internal/docs/navigation.go
@@ -321,10 +321,23 @@ func (h *DocsHandler) BuildNavigation(currentPath string) *templates.Navigation
 	return nav
 }
 
-// containsURL reports whether currentPath is one of the given doc URLs.
+// normalizeDocURL — folder-index pages SERVE at their directory URL
+// (/docs/user/filters) while their nav entry carries the /index suffix
+// (/docs/user/filters/index): same page, two spellings. Both sidebar
+// checks (expansion + highlight) compare normalized forms, so folder
+// pages expand and highlight like file pages — no per-page special
+// cases (2026-11-01, the filter manual's folder).
+func normalizeDocURL(u string) string {
+	u = strings.TrimRight(u, "/")
+	return strings.TrimSuffix(u, "/index")
+}
+
+// containsURL reports whether currentPath is one of the given doc URLs
+// (folder-index URLs normalized — see normalizeDocURL).
 func containsURL(currentPath string, urls []string) bool {
+	current := normalizeDocURL(currentPath)
 	for _, u := range urls {
-		if currentPath == u {
+		if current == normalizeDocURL(u) {
 			return true
 		}
 	}
diff --git a/internal/docs/navigation_test.go b/internal/docs/navigation_test.go
index b6170a16..03b2a533 100644
--- a/internal/docs/navigation_test.go
+++ b/internal/docs/navigation_test.go
@@ -4,6 +4,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"bookhoard/templates"
 )
 
 func TestPlaceDoc(t *testing.T) {
@@ -205,3 +207,41 @@ func TestBuildNavigationNothingCurrent(t *testing.T) {
 		t.Errorf("expected zero sections open when nothing matches (docs home / 404 renders fully closed), got %d", open)
 	}
 }
+
+// TestBuildNavigationFolderIndex (2026-11-01): folder-index pages serve
+// at their directory URL (/docs/user/filters) while their nav entries
+// carry /index (/docs/user/filters/index) — the sidebar must EXPAND the
+// containing section/group and HIGHLIGHT the index item for the folder
+// URL, exactly like a file page. The filter manual's folder is the
+// first page of this shape.
+func TestBuildNavigationFolderIndex(t *testing.T) {
+	// The REAL docs tree: the Filters group only exists because the
+	// filter manual (guide/cookbook/fields) lives in user/filters/.
+	handler := NewDocsHandler("../../docs")
+	nav := handler.BuildNavigation("/docs/user/filters")
+
+	var userGuide *templates.NavSection
+	for i := range nav.Sections {
+		if nav.Sections[i].Title == "User Guide" {
+			userGuide = &nav.Sections[i]
+		}
+	}
+	if userGuide == nil {
+		t.Fatal("User Guide section missing")
+	}
+	if userGuide.Collapsed {
+		t.Error("User Guide section must expand for /docs/user/filters")
+	}
+	var filters *templates.NavGroup
+	for i := range userGuide.Groups {
+		if userGuide.Groups[i].Title == "Filters" {
+			filters = &userGuide.Groups[i]
+		}
+	}
+	if filters == nil {
+		t.Fatal("Filters group missing from User Guide")
+	}
+	if filters.Collapsed {
+		t.Error("Filters group must expand for its folder URL")
+	}
+}
diff --git a/templates/docs_render_test.go b/templates/docs_render_test.go
index 12bbdd27..21bab051 100644
--- a/templates/docs_render_test.go
+++ b/templates/docs_render_test.go
@@ -144,3 +144,23 @@ func TestDocsNotFoundRenders(t *testing.T) {
 		t.Error("404 page must not pull the tailwind CDN")
 	}
 }
+
+// docsNavClass folder-index normalization (2026-11-01): folder-index
+// pages serve at their directory URL while nav entries carry /index —
+// the Overview item must highlight for the folder URL, file pages keep
+// their own highlight, and cross-highlighting stays impossible.
+func TestDocsNavClassFolderIndex(t *testing.T) {
+	active := docsNavClass("/docs/user/filters", "/docs/user/filters/index", "pl-9")
+	if !strings.Contains(active, "bg-brand/15") {
+		t.Errorf("folder-index item must highlight as active, got %q", active)
+	}
+	if !strings.Contains(docsNavClass("/docs/user/filters/index", "/docs/user/filters", "pl-9"), "bg-brand/15") {
+		t.Errorf("normalization is symmetric (either URL order highlights)")
+	}
+	if !strings.Contains(docsNavClass("/docs/user/filters/cookbook", "/docs/user/filters/cookbook", "pl-9"), "bg-brand/15") {
+		t.Error("file-page highlight regression")
+	}
+	if strings.Contains(docsNavClass("/docs/user/filters/cookbook", "/docs/user/filters/index", "pl-9"), "bg-brand/15") {
+		t.Error("cookbook URL wrongly highlights the Overview item")
+	}
+}
diff --git a/templates/utils.go b/templates/utils.go
index 17de12db..72a8d8ea 100644
--- a/templates/utils.go
+++ b/templates/utils.go
@@ -26,10 +26,15 @@ func activeClass(current, target string) string {
 
 // docsNavClass returns classes for docs sidebar links (server-rendered active
 // state — there is no client-side highlighter). indent sets the left padding,
-// deeper for subcategory group items.
+// deeper for subcategory group items. Folder-index pages compare normalized
+// (their serving URL lacks the /index suffix their nav entry carries).
 func docsNavClass(current, target, indent string) string {
 	base := "nav-item flex items-center " + indent + " pr-4 mr-2 py-1.5 rounded-lg text-sm transition-colors "
-	if strings.TrimRight(current, "/") == strings.TrimRight(target, "/") {
+	strip := func(u string) string {
+		u = strings.TrimRight(u, "/")
+		return strings.TrimSuffix(u, "/index")
+	}
+	if strip(current) == strip(target) {
 		return base + "bg-brand/15 text-brand font-medium"
 	}
 	return base + "text-content-muted hover:text-content hover:bg-surface-hover"