diff --git a/server/src/nest/roadtrip/roadtrip-router.service.ts b/server/src/nest/roadtrip/roadtrip-router.service.ts
index b3b2450a5..dc747923b 100644
--- a/server/src/nest/roadtrip/roadtrip-router.service.ts
+++ b/server/src/nest/roadtrip/roadtrip-router.service.ts
@@ -422,7 +422,11 @@ export class RoadtripRouterService {
     const footOnly = options.footOnly === true && profile === 'walking';
     // A tour's own costing goes with a way on foot or by bike; a drive has its avoidances instead.
     const hiking = profile === 'walking' || profile === 'cycling' ? options.costingOptions : undefined;
+    const trekValhalla = this.relay.trekValhalla();
+    // Whether the TREK services can be asked belongs in the key: a key added or removed
+    // moves the days from one engine to the other.
     const key = JSON.stringify([
+      trekValhalla !== null,
       userId,
       tripId,
       dayId,
@@ -443,6 +447,9 @@ export class RoadtripRouterService {
     let avoidMissed = profile === 'driving' ? [...avoid] : [];
     let snapped: SnappedWaypoint[] | undefined;
     let ferryLegs: number[] | undefined;
+    // A day Valhalla routes that OSRM drew instead, kept no longer than this answer: the
+    // next question goes to Valhalla again, as the planner's does.
+    let standIn = false;
     const plugin = /^plugin:([^/]+)\/(.+)$/.exec(profile);
     if (plugin) {
       const [, id, name] = plugin;
@@ -463,7 +470,7 @@ export class RoadtripRouterService {
       parts = route.legs;
     } else {
       const mode = profile === 'walking' || profile === 'cycling' ? profile : 'driving';
-      const { base, valhalla, everyDay } = routersOf(settings, this.relay.trekValhalla());
+      const { base, valhalla, everyDay } = routersOf(settings, trekValhalla);
       let weighted = false;
       if (hiking) {
         const cycling = mode === 'cycling';
@@ -476,6 +483,8 @@ export class RoadtripRouterService {
         ({ line, distance, duration, parts, snapped } = routeOfLegs(points, legs));
         weighted = true;
       } else if (footOnly && valhalla) {
+        // A walk kept off the boats is something only Valhalla can do, like an avoidance,
+        // so it may go to FOSSGIS where the relay cannot answer, as the planner's does.
         try {
           const ferryless = { use_ferry: 0 };
           const offBoats = { offBoatLines: true };
@@ -519,6 +528,7 @@ export class RoadtripRouterService {
           weighted = true;
         } catch {
           snapped = undefined;
+          standIn = true;
         }
       }
       if (!weighted) {
@@ -595,7 +605,7 @@ export class RoadtripRouterService {
       avoidMissed,
       ...(ferryLegs ? { ferryLegs } : {}),
     };
-    this.cache.set(key, { at: Date.now(), route });
+    if (!standIn) this.cache.set(key, { at: Date.now(), route });
     if (this.cache.size > 100) this.cache.delete(this.cache.keys().next().value!);
     return route;
   }
diff --git a/server/tests/unit/nest/roadtrip-router.test.ts b/server/tests/unit/nest/roadtrip-router.test.ts
index 351094e72..4eb07459f 100644
--- a/server/tests/unit/nest/roadtrip-router.test.ts
+++ b/server/tests/unit/nest/roadtrip-router.test.ts
@@ -921,6 +921,41 @@ describe('server Roadtrip router: every day through Valhalla where it routes the
       expect(route.parts).toHaveLength(11);
       expect(route.avoidMissed).toEqual([]);
     });
+
+    it('ROUTER-DAY-008: keeps no OSRM stand-in, so the same day asks the relay again once it is back', async () => {
+      const { router } = setup();
+      relay.ask.mockResolvedValueOnce(answer(502, { error: 'The TREK routing service did not answer' }));
+      const fallen = await settled(router.route(4, 1, 1, points, 'driving', []));
+      expect(fallen.parts.map((part) => part.duration)).toEqual([100, 200]);
+
+      const back = await settled(router.route(4, 1, 1, points, 'driving', []));
+      expect(back.parts.map((part) => part.duration)).toEqual([60, 60]);
+      expect(relay.ask).toHaveBeenCalledTimes(2);
+
+      expect(await settled(router.route(4, 1, 1, points, 'driving', []))).toBe(back);
+      expect(relay.ask).toHaveBeenCalledTimes(2);
+    });
+
+    it('ROUTER-DAY-009: a day routed with the key is not served from the cache once the key is gone', async () => {
+      const { router } = setup();
+      await settled(router.route(4, 1, 1, points, 'driving', []));
+      relay.trekValhalla.mockReturnValue(null);
+      const plain = await settled(router.route(4, 1, 1, points, 'driving', []));
+      expect(plain.parts.map((part) => part.duration)).toEqual([100, 200]);
+      expect(relay.ask).toHaveBeenCalledTimes(1);
+    });
+
+    it('ROUTER-DAY-010: a walk kept off the boats goes to FOSSGIS where the relay cannot answer, as only Valhalla can plan it', async () => {
+      relay.ask.mockReset().mockResolvedValue(answer(503, { error: 'The TREK routing service is resting' }));
+      vi.mocked(safeFetchAdminConfigured).mockImplementation(
+        async (_url, init) => new Response(JSON.stringify(legsFor(bodyOf(init)))),
+      );
+      const route = await settled(setup().router.route(4, 1, 1, points, 'walking', [], { footOnly: true }));
+      expect(vi.mocked(safeFetchAdminConfigured).mock.calls.map(([url]) => url)).toEqual([
+        'https://valhalla1.openstreetmap.de/route',
+      ]);
+      expect(route.ferryLegs).toEqual([]);
+    });
   });
 
   it('ROUTER-DAY-006: an own Valhalla routes every day in pieces of twenty, and OSRM the day it cannot answer', async () => {