diff --git a/web/src/components/Breadcrumbs.tsx b/web/src/components/Breadcrumbs.tsx
index 273e70a..69e78f6 100644
--- a/web/src/components/Breadcrumbs.tsx
+++ b/web/src/components/Breadcrumbs.tsx
@@ -20,7 +20,7 @@
 // upgrading to data-router-driven crumbs is a future task once the
 // router migration ships.
 
-import { Link, useLocation } from 'react-router-dom';
+import { Link, useLocation, useInRouterContext } from 'react-router-dom';
 import { ChevronRight } from 'lucide-react';
 
 // pathSegmentLabels — map first-segment URL keys to human labels.
@@ -126,7 +126,19 @@ function looksLikeID(s: string): boolean {
   return s.includes('-') || /^[a-f0-9]{8,}$/i.test(s);
 }
 
+// Breadcrumbs is the public entry. Defensive against missing Router
+// context (a test that mounts a PageHeader without a <MemoryRouter>
+// wrapper used to crash here). useLocation() throws an invariant
+// error if there's no Router; gate it behind useInRouterContext()
+// + render the actual logic in a sibling so useLocation() is only
+// called when we know the context is present.
 export default function Breadcrumbs() {
+  const inRouter = useInRouterContext();
+  if (!inRouter) return null;
+  return <BreadcrumbsInner />;
+}
+
+function BreadcrumbsInner() {
   const { pathname } = useLocation();
   const crumbs = crumbsFor(pathname);
 
diff --git a/web/src/pages/auth/UsersPage.test.tsx b/web/src/pages/auth/UsersPage.test.tsx
index 2b86e4c..fab106e 100644
--- a/web/src/pages/auth/UsersPage.test.tsx
+++ b/web/src/pages/auth/UsersPage.test.tsx
@@ -1,6 +1,7 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
 import { render, screen, fireEvent, waitFor, cleanup } from '@testing-library/react';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import { MemoryRouter } from 'react-router-dom';
 import type { ReactNode } from 'react';
 
 // =============================================================================
@@ -29,8 +30,13 @@ function renderWithProviders(ui: ReactNode) {
   const queryClient = new QueryClient({
     defaultOptions: { queries: { retry: false }, mutations: { retry: false } },
   });
+  // MemoryRouter required because PageHeader now renders Breadcrumbs
+  // (Phase 3 UX-M5), which calls useLocation() and throws when there
+  // is no Router context in the tree.
   return render(
-    <QueryClientProvider client={queryClient}>{ui}</QueryClientProvider>,
+    <QueryClientProvider client={queryClient}>
+      <MemoryRouter>{ui}</MemoryRouter>
+    </QueryClientProvider>,
   );
 }