diff --git a/docs/internal/sources.md b/docs/internal/sources.md
index fc85ed6..4bec302 100644
--- a/docs/internal/sources.md
+++ b/docs/internal/sources.md
@@ -72,6 +72,10 @@ manifest, layout, profile ownership, inheritance, and secret-safety rules
 before returning raw catalog sources. Internal engine assembly uses those validated
 catalogs as the maintained lowest-precedence profile source.
 
+Normalized external catalog profile IDs with the exact `promptkit-` prefix are
+rejected at this adapter boundary. That reservation applies only to imported
+catalogs; configured consumer sources retain the ordinary profile-ID rules.
+
 The overlay repository consults the next repository only when the
 higher-precedence repository reports that a profile is absent. A reliably
 selected malformed profile stops fallback, while an unrelated malformed file
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 625fe68..0950b5a 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -128,6 +128,8 @@ checkouts, workspaces, or replacements.
 
 ## Stage 3: Enforce The External Catalog Namespace
 
+**Completed:** 2026-09-22
+
 Repository: Promptkit. Keep current v1 dependencies during this stage;
 dependency adoption remains atomic in Stage 5.
 
diff --git a/internal/catalog/catalog.go b/internal/catalog/catalog.go
index 43d42dc..8dfdb53 100644
--- a/internal/catalog/catalog.go
+++ b/internal/catalog/catalog.go
@@ -64,6 +64,9 @@ func Load(sources ...Source) (Set, error) {
 		}
 		resolvingRepository := profile.NewResolvingRepository(repository)
 		for _, entry := range metadata {
+			if strings.HasPrefix(entry.ID, "promptkit-") {
+				return Set{}, fmt.Errorf("catalog %s: %s uses the reserved promptkit- profile ID prefix", source.Name, entry.Path)
+			}
 			if profileIDs[entry.ID] {
 				return Set{}, fmt.Errorf("catalog %s: %s duplicates an earlier profile ID", source.Name, entry.Path)
 			}
diff --git a/internal/catalog/catalog_test.go b/internal/catalog/catalog_test.go
index 292f380..08d4f5a 100644
--- a/internal/catalog/catalog_test.go
+++ b/internal/catalog/catalog_test.go
@@ -193,6 +193,39 @@ func TestLoadRejectsInvalidCatalogProfiles(t *testing.T) {
 	}
 }
 
+func TestLoadReservesPromptkitProfilePrefixForCatalogs(t *testing.T) {
+	tests := map[string]struct {
+		profileID string
+		wantError bool
+	}{
+		"maintained name": {
+			profileID: "promptkit-openrouter-standard",
+			wantError: true,
+		},
+		"other reserved name": {
+			profileID: "promptkit-example",
+			wantError: true,
+		},
+		"ordinary name": {
+			profileID: "ordinary-profile",
+		},
+	}
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			fsys := validCatalogFS("one")
+			fsys["catalog/profiles/one-profile.yml"].Data = []byte(validProfile(test.profileID, "one"))
+
+			_, err := Load(testSource("one", fsys))
+			if (err != nil) != test.wantError {
+				t.Fatalf("Load() error = %v, want error %t", err, test.wantError)
+			}
+			if err != nil && !strings.Contains(err.Error(), "catalog one: one-profile.yml") {
+				t.Fatalf("Load() error = %v, want catalog source and path context", err)
+			}
+		})
+	}
+}
+
 func TestLoadRejectsCrossCatalogConflicts(t *testing.T) {
 	t.Run("duplicate backend", func(t *testing.T) {
 		second := catalogFS("one", map[string]string{