diff --git a/client/scripts/format-baseline.json b/client/scripts/format-baseline.json
index b4b08776c..7212ad661 100644
--- a/client/scripts/format-baseline.json
+++ b/client/scripts/format-baseline.json
@@ -1878,23 +1878,13 @@
   "tests/helpers/mobileAdmin.ts",
   "tests/helpers/mobileTrip.ts",
   "tests/helpers/msw/handlers/admin.ts",
-  "tests/helpers/msw/handlers/assignments.ts",
   "tests/helpers/msw/handlers/auth.ts",
-  "tests/helpers/msw/handlers/budget.ts",
-  "tests/helpers/msw/handlers/dayNotes.ts",
   "tests/helpers/msw/handlers/index.ts",
   "tests/helpers/msw/handlers/notifications.ts",
-  "tests/helpers/msw/handlers/packing.ts",
-  "tests/helpers/msw/handlers/places.ts",
-  "tests/helpers/msw/handlers/reservations.ts",
   "tests/helpers/msw/handlers/shared.ts",
-  "tests/helpers/msw/handlers/tags.ts",
-  "tests/helpers/msw/handlers/todo.ts",
-  "tests/helpers/msw/handlers/trips.ts",
   "tests/helpers/multipart.ts",
   "tests/helpers/render.tsx",
   "tests/helpers/roadtripRail.ts",
-  "tests/helpers/store.ts",
   "tests/integration/api/client.test.ts",
   "tests/integration/api/websocket.test.ts",
   "tests/integration/hooks/useDayNotes.test.ts",
diff --git a/client/tests/helpers/msw/contract.ts b/client/tests/helpers/msw/contract.ts
new file mode 100644
index 000000000..a24c5885c
--- /dev/null
+++ b/client/tests/helpers/msw/contract.ts
@@ -0,0 +1,81 @@
+import { http, HttpResponse, type DefaultBodyType, type PathParams } from 'msw';
+import type { z } from 'zod';
+
+/**
+ * An MSW handler held to the contract in @trek/shared.
+ *
+ * A raw `http.post(...)` handler believes whatever the test hands it: the body
+ * is cast with `as`, and the response is whatever a factory builds. When the
+ * server's contract moves, such a handler keeps answering the old shape and
+ * the tests stay green against a server that no longer exists. A contract
+ * handler parses both sides instead:
+ *
+ * - the request body against `request`. A body the server's Zod pipe would
+ *   refuse gets the same 400 here, so a client call that drifted from the
+ *   contract fails in the test that makes it;
+ * - the resolver's result against `response`. A handler or factory that
+ *   answers a shape the contract does not allow throws, which MSW turns into a
+ *   500 with the Zod issues in the log, because that is a bug in the test
+ *   setup rather than in the code under test.
+ *
+ * The resolver receives the parsed body, so it never needs a cast, and
+ * returns plain data; the helper wraps it in `HttpResponse.json`. A resolver
+ * that has to answer something else (an error status, a binary body) returns
+ * an `HttpResponse` itself, which is passed through unchecked.
+ */
+type Method = 'get' | 'post' | 'put' | 'patch' | 'delete';
+
+export interface Contract<Req extends z.ZodType | undefined, Res extends z.ZodType | undefined> {
+  request?: Req;
+  response?: Res;
+}
+
+type BodyOf<Req> = Req extends z.ZodType ? z.output<Req> : undefined;
+type ResultOf<Res> = Res extends z.ZodType ? z.input<Res> : DefaultBodyType;
+
+export interface ContractResolverInfo<Req> {
+  params: PathParams;
+  request: Request;
+  body: BodyOf<Req>;
+}
+
+export type ContractResolver<Req, Res> = (
+  info: ContractResolverInfo<Req>
+) => ResultOf<Res> | Response | Promise<ResultOf<Res> | Response>;
+
+/** The Zod issues as one readable line each, for the 400 body and the thrown error. */
+export function describeIssues(error: z.ZodError): string[] {
+  return error.issues.map((issue) => `${issue.path.join('.') || '(root)'}: ${issue.message}`);
+}
+
+export function contractHandler<
+  Req extends z.ZodType | undefined = undefined,
+  Res extends z.ZodType | undefined = undefined,
+>(method: Method, path: string, contract: Contract<Req, Res>, resolver: ContractResolver<Req, Res>) {
+  return http[method](path, async ({ params, request }) => {
+    let body: unknown = undefined;
+    if (contract.request) {
+      const raw: unknown = await request
+        .clone()
+        .json()
+        .catch(() => undefined);
+      const parsed = contract.request.safeParse(raw);
+      if (!parsed.success) {
+        return HttpResponse.json({ error: 'Validation failed', issues: describeIssues(parsed.error) }, { status: 400 });
+      }
+      body = parsed.data;
+    }
+    const result = await resolver({ params, request, body: body as BodyOf<Req> });
+    if (result instanceof Response) return result;
+    if (contract.response) {
+      const checked = contract.response.safeParse(result);
+      if (!checked.success) {
+        throw new Error(
+          `${method.toUpperCase()} ${path}: the mock answers a shape the @trek/shared contract does not allow:\n  ` +
+            describeIssues(checked.error).join('\n  ')
+        );
+      }
+    }
+    return HttpResponse.json(result as DefaultBodyType);
+  });
+}
diff --git a/client/tests/helpers/msw/handlers/assignments.ts b/client/tests/helpers/msw/handlers/assignments.ts
index 62065badc..32512fef6 100644
--- a/client/tests/helpers/msw/handlers/assignments.ts
+++ b/client/tests/helpers/msw/handlers/assignments.ts
@@ -1,28 +1,46 @@
-import { http, HttpResponse } from 'msw';
+import {
+  assignmentCreateRequestSchema,
+  assignmentMoveRequestSchema,
+  assignmentReorderRequestSchema,
+  assignmentSchema,
+} from '@trek/shared';
+import { HttpResponse } from 'msw';
+import { z } from 'zod';
 import { buildAssignment, buildPlace } from '../../factories';
+import { contractHandler } from '../contract';
 
 export const assignmentsHandlers = [
-  http.post('/api/trips/:id/days/:dayId/assignments', async ({ params, request }) => {
-    const body = await request.json() as { place_id: number };
-    const place = buildPlace({ id: body.place_id, trip_id: Number(params.id) });
-    const assignment = buildAssignment({
-      day_id: Number(params.dayId),
-      place_id: body.place_id,
-      place,
-      order_index: 0,
-    });
-    return HttpResponse.json({ assignment });
-  }),
+  contractHandler(
+    'post',
+    '/api/trips/:id/days/:dayId/assignments',
+    { request: assignmentCreateRequestSchema, response: z.object({ assignment: assignmentSchema }) },
+    ({ params, body }) => {
+      const placeId = Number(body.place_id);
+      const place = buildPlace({ id: placeId, trip_id: Number(params.id) });
+      const assignment = buildAssignment({ day_id: Number(params.dayId), place_id: placeId, place, order_index: 0 });
+      return { assignment };
+    }
+  ),
 
-  http.delete('/api/trips/:id/days/:dayId/assignments/:assignmentId', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id/days/:dayId/assignments/:assignmentId', {}, () =>
+    HttpResponse.json({ success: true })
+  ),
 
-  http.put('/api/trips/:id/days/:dayId/assignments/reorder', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/days/:dayId/assignments/reorder',
+    { request: assignmentReorderRequestSchema },
+    () => ({
+      success: true,
+    })
+  ),
 
-  http.put('/api/trips/:id/assignments/:assignmentId/move', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/assignments/:assignmentId/move',
+    { request: assignmentMoveRequestSchema },
+    () => ({
+      success: true,
+    })
+  ),
 ];
diff --git a/client/tests/helpers/msw/handlers/budget.ts b/client/tests/helpers/msw/handlers/budget.ts
index 7b01d37ab..9a5c3d983 100644
--- a/client/tests/helpers/msw/handlers/budget.ts
+++ b/client/tests/helpers/msw/handlers/budget.ts
@@ -1,38 +1,84 @@
-import { http, HttpResponse } from 'msw';
+import {
+  budgetCreateItemRequestSchema,
+  budgetItemMemberSchema,
+  budgetItemSchema,
+  budgetToggleMemberPaidRequestSchema,
+  budgetUpdateItemRequestSchema,
+  budgetUpdateMembersRequestSchema,
+} from '@trek/shared';
+import { HttpResponse } from 'msw';
+import { z } from 'zod';
 import { buildBudgetItem } from '../../factories';
+import { contractHandler } from '../contract';
+
+const itemResponse = z.object({ item: budgetItemSchema });
+
+/**
+ * The fields of a create or update request that the stored item carries as
+ * they are. Payers, members and receipts arrive as ids and amounts and come
+ * back in a different shape the server builds, so they are not echoed.
+ */
+const itemFields = <T extends { payers?: unknown; members?: unknown; member_ids?: unknown }>(body: T) => {
+  const { payers: _payers, members: _members, member_ids: _memberIds, ...fields } = body;
+  return fields;
+};
 
 export const budgetHandlers = [
-  http.get('/api/trips/:id/budget', ({ params }) => {
-    return HttpResponse.json({
+  contractHandler(
+    'get',
+    '/api/trips/:id/budget',
+    { response: z.object({ items: z.array(budgetItemSchema) }) },
+    ({ params }) => ({
       items: [buildBudgetItem({ trip_id: Number(params.id) })],
-    });
-  }),
+    })
+  ),
 
-  http.post('/api/trips/:id/budget', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const item = buildBudgetItem({ trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ item });
-  }),
+  contractHandler(
+    'post',
+    '/api/trips/:id/budget',
+    { request: budgetCreateItemRequestSchema, response: itemResponse },
+    ({ params, body }) => ({
+      item: buildBudgetItem({ trip_id: Number(params.id), ...itemFields(body) }),
+    })
+  ),
 
-  http.put('/api/trips/:id/budget/:itemId', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const item = buildBudgetItem({ id: Number(params.itemId), trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ item });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/budget/:itemId',
+    { request: budgetUpdateItemRequestSchema, response: itemResponse },
+    ({ params, body }) => ({
+      item: buildBudgetItem({ id: Number(params.itemId), trip_id: Number(params.id), ...itemFields(body) }),
+    })
+  ),
 
-  http.delete('/api/trips/:id/budget/:itemId', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id/budget/:itemId', {}, () => HttpResponse.json({ success: true })),
 
-  http.put('/api/trips/:id/budget/:itemId/members', async ({ params, request }) => {
-    const body = await request.json() as { user_ids: number[] };
-    const members = body.user_ids.map(uid => ({ user_id: uid, paid: 0, username: `user${uid}` }));
-    const item = buildBudgetItem({ id: Number(params.itemId), trip_id: Number(params.id), persons: body.user_ids.length, members });
-    return HttpResponse.json({ members, item });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/budget/:itemId/members',
+    {
+      request: budgetUpdateMembersRequestSchema,
+      response: itemResponse.extend({ members: z.array(budgetItemMemberSchema) }),
+    },
+    ({ params, body }) => {
+      const members = body.user_ids.map((uid) => ({ user_id: uid, paid: 0, username: `user${uid}` }));
+      const item = buildBudgetItem({
+        id: Number(params.itemId),
+        trip_id: Number(params.id),
+        persons: body.user_ids.length,
+        members,
+      });
+      return { members, item };
+    }
+  ),
 
-  http.put('/api/trips/:id/budget/:itemId/members/:userId/paid', async ({ params, request }) => {
-    const body = await request.json() as { paid: boolean };
-    return HttpResponse.json({ success: true, paid: body.paid });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/budget/:itemId/members/:userId/paid',
+    { request: budgetToggleMemberPaidRequestSchema },
+    ({ body }) => ({
+      success: true,
+      paid: body.paid,
+    })
+  ),
 ];
diff --git a/client/tests/helpers/msw/handlers/dayNotes.ts b/client/tests/helpers/msw/handlers/dayNotes.ts
index 13a142769..9aaf0204d 100644
--- a/client/tests/helpers/msw/handlers/dayNotes.ts
+++ b/client/tests/helpers/msw/handlers/dayNotes.ts
@@ -1,31 +1,51 @@
-import { http, HttpResponse } from 'msw';
+import {
+  dayNoteCreateRequestSchema,
+  dayNoteSchema,
+  dayNoteUpdateRequestSchema,
+  daySchema,
+  dayUpdateRequestSchema,
+} from '@trek/shared';
+import { HttpResponse } from 'msw';
+import { z } from 'zod';
 import { buildDayNote } from '../../factories';
+import { contractHandler } from '../contract';
+
+const noteResponse = z.object({ note: dayNoteSchema });
 
 export const dayNotesHandlers = [
-  http.get('/api/trips/:id/days/:dayId/notes', ({ params }) => {
-    return HttpResponse.json({
+  contractHandler(
+    'get',
+    '/api/trips/:id/days/:dayId/notes',
+    { response: z.object({ notes: z.array(dayNoteSchema) }) },
+    ({ params }) => ({
       notes: [buildDayNote({ day_id: Number(params.dayId) })],
-    });
-  }),
+    })
+  ),
 
-  http.post('/api/trips/:id/days/:dayId/notes', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const note = buildDayNote({ day_id: Number(params.dayId), ...body });
-    return HttpResponse.json({ note });
-  }),
+  contractHandler(
+    'post',
+    '/api/trips/:id/days/:dayId/notes',
+    { request: dayNoteCreateRequestSchema, response: noteResponse },
+    ({ params, body }) => ({
+      note: buildDayNote({ day_id: Number(params.dayId), ...body }),
+    })
+  ),
 
-  http.put('/api/trips/:id/days/:dayId/notes/:noteId', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const note = buildDayNote({ id: Number(params.noteId), day_id: Number(params.dayId), ...body });
-    return HttpResponse.json({ note });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/days/:dayId/notes/:noteId',
+    { request: dayNoteUpdateRequestSchema, response: noteResponse },
+    ({ params, body }) => ({ note: buildDayNote({ id: Number(params.noteId), day_id: Number(params.dayId), ...body }) })
+  ),
 
-  http.delete('/api/trips/:id/days/:dayId/notes/:noteId', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id/days/:dayId/notes/:noteId', {}, () => HttpResponse.json({ success: true })),
 
-  http.put('/api/trips/:id/days/:dayId', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    return HttpResponse.json({ day: { id: Number(params.dayId), trip_id: Number(params.id), ...body } });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/days/:dayId',
+    { request: dayUpdateRequestSchema, response: z.object({ day: daySchema }) },
+    ({ params, body }) => ({
+      day: { id: Number(params.dayId), trip_id: Number(params.id), ...body },
+    })
+  ),
 ];
diff --git a/client/tests/helpers/msw/handlers/packing.ts b/client/tests/helpers/msw/handlers/packing.ts
index c3b0ed621..18b637e73 100644
--- a/client/tests/helpers/msw/handlers/packing.ts
+++ b/client/tests/helpers/msw/handlers/packing.ts
@@ -1,26 +1,49 @@
-import { http, HttpResponse } from 'msw';
+import { packingCreateItemRequestSchema, packingItemSchema, packingUpdateItemRequestSchema } from '@trek/shared';
+import { HttpResponse } from 'msw';
+import { z } from 'zod';
 import { buildPackingItem } from '../../factories';
+import { contractHandler } from '../contract';
+
+const itemResponse = z.object({ item: packingItemSchema });
+
+/** The server stores checked and is_private as 0/1 and answers them that way, whichever form the request used. */
+const asStored = <T extends { checked?: boolean | number; is_private?: boolean }>({
+  checked,
+  is_private,
+  ...rest
+}: T) => ({
+  ...rest,
+  ...(checked === undefined ? {} : { checked: Number(checked) }),
+  ...(is_private === undefined ? {} : { is_private: Number(is_private) }),
+});
 
 export const packingHandlers = [
-  http.get('/api/trips/:id/packing', ({ params }) => {
-    return HttpResponse.json({
+  contractHandler(
+    'get',
+    '/api/trips/:id/packing',
+    { response: z.object({ items: z.array(packingItemSchema) }) },
+    ({ params }) => ({
       items: [buildPackingItem({ trip_id: Number(params.id) })],
-    });
-  }),
+    })
+  ),
 
-  http.post('/api/trips/:id/packing', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const item = buildPackingItem({ trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ item });
-  }),
+  contractHandler(
+    'post',
+    '/api/trips/:id/packing',
+    { request: packingCreateItemRequestSchema, response: itemResponse },
+    ({ params, body }) => ({
+      item: buildPackingItem({ trip_id: Number(params.id), ...asStored(body) }),
+    })
+  ),
 
-  http.put('/api/trips/:id/packing/:itemId', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const item = buildPackingItem({ id: Number(params.itemId), trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ item });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/packing/:itemId',
+    { request: packingUpdateItemRequestSchema, response: itemResponse },
+    ({ params, body }) => ({
+      item: buildPackingItem({ id: Number(params.itemId), trip_id: Number(params.id), ...asStored(body) }),
+    })
+  ),
 
-  http.delete('/api/trips/:id/packing/:itemId', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id/packing/:itemId', {}, () => HttpResponse.json({ success: true })),
 ];
diff --git a/client/tests/helpers/msw/handlers/places.ts b/client/tests/helpers/msw/handlers/places.ts
index 675f91e17..20b194996 100644
--- a/client/tests/helpers/msw/handlers/places.ts
+++ b/client/tests/helpers/msw/handlers/places.ts
@@ -1,30 +1,48 @@
-import { http, HttpResponse } from 'msw';
+import { placeCreateRequestSchema, placeSchema, placeUpdateRequestSchema } from '@trek/shared';
+import { HttpResponse } from 'msw';
+import { z } from 'zod';
 import { buildPlace } from '../../factories';
+import { contractHandler } from '../contract';
+
+const placeResponse = z.object({ place: placeSchema });
 
 export const placesHandlers = [
-  http.get('/api/trips/:id/places', ({ params }) => {
-    const tripId = Number(params.id);
-    return HttpResponse.json({ places: [buildPlace({ trip_id: tripId }), buildPlace({ trip_id: tripId })] });
-  }),
+  contractHandler(
+    'get',
+    '/api/trips/:id/places',
+    { response: z.object({ places: z.array(placeSchema) }) },
+    ({ params }) => {
+      const tripId = Number(params.id);
+      return { places: [buildPlace({ trip_id: tripId }), buildPlace({ trip_id: tripId })] };
+    }
+  ),
 
-  http.post('/api/trips/:id/places', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const place = buildPlace({ trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ place });
-  }),
+  contractHandler(
+    'post',
+    '/api/trips/:id/places',
+    { request: placeCreateRequestSchema, response: placeResponse },
+    ({ params, body }) => ({
+      place: buildPlace({ trip_id: Number(params.id), ...body }),
+    })
+  ),
 
-  http.put('/api/trips/:id/places/:placeId', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const place = buildPlace({ id: Number(params.placeId), trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ place });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/places/:placeId',
+    { request: placeUpdateRequestSchema, response: placeResponse },
+    ({ params, body }) => ({
+      place: buildPlace({ id: Number(params.placeId), trip_id: Number(params.id), ...body }),
+    })
+  ),
 
-  http.post('/api/trips/:id/places/:placeId/image', ({ params }) => {
-    const place = buildPlace({ id: Number(params.placeId), trip_id: Number(params.id), image_url: '/uploads/places/mock.jpg' });
-    return HttpResponse.json({ place });
-  }),
+  // A multipart upload: no JSON body to hold to a schema, only the answer.
+  contractHandler('post', '/api/trips/:id/places/:placeId/image', { response: placeResponse }, ({ params }) => ({
+    place: buildPlace({
+      id: Number(params.placeId),
+      trip_id: Number(params.id),
+      image_url: '/uploads/places/mock.jpg',
+    }),
+  })),
 
-  http.delete('/api/trips/:id/places/:placeId', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id/places/:placeId', {}, () => HttpResponse.json({ success: true })),
 ];
diff --git a/client/tests/helpers/msw/handlers/reservations.ts b/client/tests/helpers/msw/handlers/reservations.ts
index d99a88344..42673aefd 100644
--- a/client/tests/helpers/msw/handlers/reservations.ts
+++ b/client/tests/helpers/msw/handlers/reservations.ts
@@ -1,30 +1,36 @@
-import { http, HttpResponse } from 'msw';
+import { reservationCreateRequestSchema, reservationSchema, reservationUpdateRequestSchema } from '@trek/shared';
+import { HttpResponse } from 'msw';
+import { z } from 'zod';
 import { buildReservation } from '../../factories';
+import { contractHandler } from '../contract';
+
+const reservationResponse = z.object({ reservation: reservationSchema });
 
 export const reservationsHandlers = [
-  http.get('/api/trips/:id/reservations', ({ params }) => {
-    return HttpResponse.json({
-      reservations: [buildReservation({ trip_id: Number(params.id) })],
-    });
-  }),
+  contractHandler(
+    'get',
+    '/api/trips/:id/reservations',
+    { response: z.object({ reservations: z.array(reservationSchema) }) },
+    ({ params }) => ({ reservations: [buildReservation({ trip_id: Number(params.id) })] })
+  ),
 
-  http.post('/api/trips/:id/reservations', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const reservation = buildReservation({ trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ reservation });
-  }),
+  contractHandler(
+    'post',
+    '/api/trips/:id/reservations',
+    { request: reservationCreateRequestSchema, response: reservationResponse },
+    ({ params, body }) => ({ reservation: buildReservation({ trip_id: Number(params.id), ...body }) })
+  ),
 
-  http.put('/api/trips/:id/reservations/:reservationId', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const reservation = buildReservation({
-      id: Number(params.reservationId),
-      trip_id: Number(params.id),
-      ...body,
-    });
-    return HttpResponse.json({ reservation });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/reservations/:reservationId',
+    { request: reservationUpdateRequestSchema, response: reservationResponse },
+    ({ params, body }) => ({
+      reservation: buildReservation({ id: Number(params.reservationId), trip_id: Number(params.id), ...body }),
+    })
+  ),
 
-  http.delete('/api/trips/:id/reservations/:reservationId', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id/reservations/:reservationId', {}, () =>
+    HttpResponse.json({ success: true })
+  ),
 ];
diff --git a/client/tests/helpers/msw/handlers/tags.ts b/client/tests/helpers/msw/handlers/tags.ts
index ab8aa9419..c57238e0d 100644
--- a/client/tests/helpers/msw/handlers/tags.ts
+++ b/client/tests/helpers/msw/handlers/tags.ts
@@ -1,24 +1,35 @@
-import { http, HttpResponse } from 'msw';
-import { buildTag, buildCategory } from '../../factories';
+import {
+  categoryListResponseSchema,
+  categorySchema,
+  createCategoryRequestSchema,
+  createTagRequestSchema,
+  tagListResponseSchema,
+  tagSchema,
+} from '@trek/shared';
+import { z } from 'zod';
+import { buildCategory, buildTag } from '../../factories';
+import { contractHandler } from '../contract';
 
 export const tagsHandlers = [
-  http.get('/api/tags', () => {
-    return HttpResponse.json({ tags: [buildTag(), buildTag()] });
-  }),
+  contractHandler('get', '/api/tags', { response: tagListResponseSchema }, () => ({ tags: [buildTag(), buildTag()] })),
 
-  http.post('/api/tags', async ({ request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const tag = buildTag(body);
-    return HttpResponse.json({ tag });
-  }),
+  contractHandler(
+    'post',
+    '/api/tags',
+    { request: createTagRequestSchema, response: z.object({ tag: tagSchema }) },
+    ({ body }) => ({
+      tag: buildTag(body),
+    })
+  ),
 
-  http.get('/api/categories', () => {
-    return HttpResponse.json({ categories: [buildCategory(), buildCategory()] });
-  }),
+  contractHandler('get', '/api/categories', { response: categoryListResponseSchema }, () => ({
+    categories: [buildCategory(), buildCategory()],
+  })),
 
-  http.post('/api/categories', async ({ request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const category = buildCategory(body);
-    return HttpResponse.json({ category });
-  }),
+  contractHandler(
+    'post',
+    '/api/categories',
+    { request: createCategoryRequestSchema, response: z.object({ category: categorySchema }) },
+    ({ body }) => ({ category: buildCategory(body) })
+  ),
 ];
diff --git a/client/tests/helpers/msw/handlers/todo.ts b/client/tests/helpers/msw/handlers/todo.ts
index e9ad6f030..6340f2526 100644
--- a/client/tests/helpers/msw/handlers/todo.ts
+++ b/client/tests/helpers/msw/handlers/todo.ts
@@ -1,26 +1,31 @@
-import { http, HttpResponse } from 'msw';
+import { todoCreateItemRequestSchema, todoUpdateItemRequestSchema } from '@trek/shared';
+import { HttpResponse } from 'msw';
 import { buildTodoItem } from '../../factories';
+import { contractHandler } from '../contract';
 
+// @trek/shared holds the todo requests but no todo item schema yet, so only
+// the request side is checked here.
+
+/** The server stores checked as 0/1 and answers it that way, whichever form the request used. */
+const asStored = <T extends { checked?: boolean | number }>({ checked, ...rest }: T) =>
+  checked === undefined ? rest : { ...rest, checked: Number(checked) };
 export const todoHandlers = [
-  http.get('/api/trips/:id/todo', ({ params }) => {
-    return HttpResponse.json({
-      items: [buildTodoItem({ trip_id: Number(params.id) })],
-    });
-  }),
+  contractHandler('get', '/api/trips/:id/todo', {}, ({ params }) => ({
+    items: [buildTodoItem({ trip_id: Number(params.id) })],
+  })),
 
-  http.post('/api/trips/:id/todo', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const item = buildTodoItem({ trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ item });
-  }),
+  contractHandler('post', '/api/trips/:id/todo', { request: todoCreateItemRequestSchema }, ({ params, body }) => ({
+    item: buildTodoItem({ trip_id: Number(params.id), ...body }),
+  })),
 
-  http.put('/api/trips/:id/todo/:itemId', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const item = buildTodoItem({ id: Number(params.itemId), trip_id: Number(params.id), ...body });
-    return HttpResponse.json({ item });
-  }),
+  contractHandler(
+    'put',
+    '/api/trips/:id/todo/:itemId',
+    { request: todoUpdateItemRequestSchema },
+    ({ params, body }) => ({
+      item: buildTodoItem({ id: Number(params.itemId), trip_id: Number(params.id), ...asStored(body) }),
+    })
+  ),
 
-  http.delete('/api/trips/:id/todo/:itemId', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id/todo/:itemId', {}, () => HttpResponse.json({ success: true })),
 ];
diff --git a/client/tests/helpers/msw/handlers/trips.ts b/client/tests/helpers/msw/handlers/trips.ts
index 9ffde4797..a69e248ce 100644
--- a/client/tests/helpers/msw/handlers/trips.ts
+++ b/client/tests/helpers/msw/handlers/trips.ts
@@ -1,52 +1,86 @@
+import {
+  daySchema,
+  tripCopyRequestSchema,
+  tripCreateRequestSchema,
+  tripMemberSchema,
+  tripSchema,
+  tripUpdateRequestSchema,
+} from '@trek/shared';
 import { http, HttpResponse } from 'msw';
-import { buildTrip, buildDay, buildUser, buildPlace, buildPackingItem, buildTodoItem, buildBudgetItem, buildReservation, buildTripFile } from '../../factories';
+import { z } from 'zod';
+import {
+  buildBudgetItem,
+  buildDay,
+  buildPackingItem,
+  buildPlace,
+  buildReservation,
+  buildTodoItem,
+  buildTrip,
+  buildTripFile,
+  buildUser,
+} from '../../factories';
+import { contractHandler } from '../contract';
+
+const tripResponse = z.object({ trip: tripSchema });
 
 export const tripsHandlers = [
   // List all trips (active or archived)
-  http.get('/api/trips', ({ request }) => {
+  contractHandler('get', '/api/trips', { response: z.object({ trips: z.array(tripSchema) }) }, ({ request }) => {
     const url = new URL(request.url);
     const archived = url.searchParams.get('archived');
     if (archived) {
-      return HttpResponse.json({ trips: [] });
+      return { trips: [] };
     }
     const trip1 = buildTrip({ title: 'Paris Adventure', start_date: '2026-07-01', end_date: '2026-07-10' });
     const trip2 = buildTrip({ title: 'Tokyo Trip', start_date: '2026-09-01', end_date: '2026-09-15' });
-    return HttpResponse.json({ trips: [trip1, trip2] });
+    return { trips: [trip1, trip2] };
   }),
 
-  http.get('/api/trips/:id', ({ params }) => {
-    const trip = buildTrip({ id: Number(params.id) });
-    return HttpResponse.json({ trip });
-  }),
+  contractHandler('get', '/api/trips/:id', { response: tripResponse }, ({ params }) => ({
+    trip: buildTrip({ id: Number(params.id) }),
+  })),
 
-  http.get('/api/trips/:id/days', ({ params }) => {
+  contractHandler('get', '/api/trips/:id/days', { response: z.object({ days: z.array(daySchema) }) }, ({ params }) => {
     const tripId = Number(params.id);
     const day1 = buildDay({ trip_id: tripId, assignments: [], notes_items: [] });
     const day2 = buildDay({ trip_id: tripId, assignments: [], notes_items: [] });
-    return HttpResponse.json({ days: [day1, day2] });
+    return { days: [day1, day2] };
   }),
 
-  http.put('/api/trips/:id', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const trip = buildTrip({ id: Number(params.id), ...body });
-    return HttpResponse.json({ trip });
-  }),
+  // The server stores is_archived as 0/1 and answers the stored row, whichever
+  // form the request sent; date_shift_mode steers the update and is not a column.
+  contractHandler(
+    'put',
+    '/api/trips/:id',
+    { request: tripUpdateRequestSchema, response: tripResponse },
+    ({ params, body }) => {
+      const { is_archived, date_shift_mode: _mode, ...fields } = body;
+      return {
+        trip: buildTrip({
+          id: Number(params.id),
+          ...fields,
+          ...(is_archived === undefined ? {} : { is_archived: Number(is_archived) }),
+        }),
+      };
+    }
+  ),
 
-  http.post('/api/trips', async ({ request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const trip = buildTrip({ ...body });
-    return HttpResponse.json({ trip });
-  }),
+  contractHandler('post', '/api/trips', { request: tripCreateRequestSchema, response: tripResponse }, ({ body }) => ({
+    trip: buildTrip({ ...body }),
+  })),
 
-  http.get('/api/trips/:id/members', ({ params }) => {
-    const owner = buildUser();
-    return HttpResponse.json({ owner, members: [] });
-  }),
+  contractHandler(
+    'get',
+    '/api/trips/:id/members',
+    { response: z.object({ owner: tripMemberSchema, members: z.array(tripMemberSchema) }) },
+    () => ({ owner: buildUser(), members: [] })
+  ),
 
   http.get('/api/trips/:id/accommodations', () => {
     return HttpResponse.json({ accommodations: [] });
   }),
 
+  // The offline bundle has no response schema in @trek/shared yet.
   http.get('/api/trips/:id/bundle', ({ params }) => {
     const tripId = Number(params.id);
     const trip = buildTrip({ id: tripId });
@@ -63,13 +97,14 @@ export const tripsHandlers = [
     });
   }),
 
-  http.delete('/api/trips/:id', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('delete', '/api/trips/:id', {}, () => HttpResponse.json({ success: true })),
 
-  http.post('/api/trips/:id/copy', async ({ params, request }) => {
-    const body = await request.json() as Record<string, unknown>;
-    const trip = buildTrip({ id: Number(params.id) + 1000, ...body });
-    return HttpResponse.json({ trip });
-  }),
+  contractHandler(
+    'post',
+    '/api/trips/:id/copy',
+    { request: tripCopyRequestSchema, response: tripResponse },
+    ({ params, body }) => ({
+      trip: buildTrip({ id: Number(params.id) + 1000, ...body }),
+    })
+  ),
 ];
diff --git a/client/tests/helpers/msw/handlers/vacay.ts b/client/tests/helpers/msw/handlers/vacay.ts
index 5b68125e2..9ee4991ee 100644
--- a/client/tests/helpers/msw/handlers/vacay.ts
+++ b/client/tests/helpers/msw/handlers/vacay.ts
@@ -1,4 +1,6 @@
+import { vacaySetColorRequestSchema, vacayUpdatePlanRequestSchema } from '@trek/shared';
 import { http, HttpResponse } from 'msw';
+import { contractHandler } from '../contract';
 
 export const vacayHandlers = [
   http.get('/api/school-holiday-catalog', () => HttpResponse.json({ countries: [], regions: [] })),
@@ -21,7 +23,8 @@ export const vacayHandlers = [
     });
   }),
 
-  http.put('/api/addons/vacay/plan', () => {
+  // Only the request has a schema: the plan answer is open in @trek/shared.
+  contractHandler('put', '/api/addons/vacay/plan', { request: vacayUpdatePlanRequestSchema }, () => {
     return HttpResponse.json({
       plan: {
         id: 1,
@@ -110,9 +113,9 @@ export const vacayHandlers = [
     });
   }),
 
-  http.put('/api/addons/vacay/color', () => {
-    return HttpResponse.json({ success: true });
-  }),
+  contractHandler('put', '/api/addons/vacay/color', { request: vacaySetColorRequestSchema }, () =>
+    HttpResponse.json({ success: true })
+  ),
 
   http.post('/api/addons/vacay/invite', () => {
     return HttpResponse.json({ success: true });
diff --git a/client/tests/unit/helpers/mswContract.test.ts b/client/tests/unit/helpers/mswContract.test.ts
new file mode 100644
index 000000000..d7b99f8bc
--- /dev/null
+++ b/client/tests/unit/helpers/mswContract.test.ts
@@ -0,0 +1,115 @@
+import { budgetCreateItemRequestSchema, tagSchema } from '@trek/shared';
+import { HttpResponse } from 'msw';
+import { describe, expect, it } from 'vitest';
+import { z } from 'zod';
+import { contractHandler } from '../../helpers/msw/contract';
+import { server } from '../../helpers/msw/server';
+
+// A tag as a drifted factory might build it: a string id and no user.
+const offContract = { id: 'one', name: 'Food' } as unknown as z.input<typeof tagSchema>;
+
+const url = (path: string) => `${location.origin}${path}`;
+
+async function send(method: string, path: string, body?: unknown) {
+  const res = await fetch(url(path), {
+    method,
+    headers: body === undefined ? undefined : { 'Content-Type': 'application/json' },
+    body: body === undefined ? undefined : JSON.stringify(body),
+  });
+  return { status: res.status, json: (await res.json()) as Record<string, unknown> };
+}
+
+describe('contractHandler', () => {
+  it('FE-MSW-CONTRACT-001: hands the resolver the parsed body and answers its result as JSON', async () => {
+    server.use(
+      contractHandler(
+        'post',
+        '/api/contract-probe',
+        { request: budgetCreateItemRequestSchema, response: z.object({ name: z.string() }) },
+        ({ body }) => ({ name: body.name })
+      )
+    );
+    expect(await send('POST', '/api/contract-probe', { name: 'Taxi' })).toEqual({
+      status: 200,
+      json: { name: 'Taxi' },
+    });
+  });
+
+  it('FE-MSW-CONTRACT-002: refuses a body the contract does not allow with a 400, as the server does', async () => {
+    server.use(contractHandler('post', '/api/contract-probe', { request: budgetCreateItemRequestSchema }, () => ({})));
+    const res = await send('POST', '/api/contract-probe', { name: '' });
+    expect(res.status).toBe(400);
+    expect(res.json.issues).toEqual([expect.stringMatching(/^name: /)]);
+  });
+
+  it('FE-MSW-CONTRACT-003: a mock that answers outside the contract fails instead of passing it on', async () => {
+    server.use(
+      contractHandler('get', '/api/contract-probe', { response: z.object({ tag: tagSchema }) }, () => ({
+        tag: offContract,
+      }))
+    );
+    expect((await fetch(url('/api/contract-probe'))).status).toBe(500);
+  });
+
+  it('FE-MSW-CONTRACT-004: a resolver may still answer a status of its own, unchecked', async () => {
+    server.use(
+      contractHandler('get', '/api/contract-probe', { response: z.object({ tag: tagSchema }) }, () =>
+        HttpResponse.json({ error: 'gone' }, { status: 404 })
+      )
+    );
+    expect(await send('GET', '/api/contract-probe')).toEqual({ status: 404, json: { error: 'gone' } });
+  });
+});
+
+// Each default handler held to a contract, called the way the client calls it. A
+// factory or handler that drifts from @trek/shared fails here by name instead of
+// as a 500 deep inside some component test.
+describe('default handlers answer inside the @trek/shared contract', () => {
+  const calls: [string, string, unknown?][] = [
+    ['GET', '/api/trips/1/budget'],
+    ['POST', '/api/trips/1/budget', { name: 'Taxi', total_price: 12, member_ids: [1] }],
+    ['PUT', '/api/trips/1/budget/5', { name: 'Taxi', total_price: 14 }],
+    ['PUT', '/api/trips/1/budget/5/members', { user_ids: [1, 2] }],
+    ['PUT', '/api/trips/1/budget/5/members/2/paid', { paid: true }],
+    ['GET', '/api/trips/1/places'],
+    ['POST', '/api/trips/1/places', { name: 'Louvre', lat: 48.86, lng: 2.33 }],
+    ['PUT', '/api/trips/1/places/3', { name: 'Louvre' }],
+    ['POST', '/api/trips/1/places/3/image'],
+    ['GET', '/api/trips/1/todo'],
+    ['POST', '/api/trips/1/todo', { name: 'Book train' }],
+    ['PUT', '/api/trips/1/todo/4', { checked: true }],
+    ['GET', '/api/tags'],
+    ['POST', '/api/tags', { name: 'Food', color: '#ff0000' }],
+    ['GET', '/api/categories'],
+    ['POST', '/api/categories', { name: 'Museum', color: '#123456', icon: 'landmark' }],
+    ['GET', '/api/trips/1/days/2/notes'],
+    ['POST', '/api/trips/1/days/2/notes', { text: 'Bring cash' }],
+    ['PUT', '/api/trips/1/days/2/notes/9', { text: 'Bring more cash' }],
+    ['PUT', '/api/trips/1/days/2', { title: 'Arrival' }],
+    ['GET', '/api/trips/1/packing'],
+    ['POST', '/api/trips/1/packing', { name: 'Socks', checked: false }],
+    ['PUT', '/api/trips/1/packing/6', { checked: true }],
+    ['GET', '/api/trips/1/reservations'],
+    ['POST', '/api/trips/1/reservations', { title: 'Dinner', type: 'restaurant' }],
+    ['PUT', '/api/trips/1/reservations/8', { title: 'Late dinner' }],
+    ['POST', '/api/trips/1/days/2/assignments', { place_id: 3 }],
+    ['PUT', '/api/trips/1/days/2/assignments/reorder', { orderedIds: [1, 2] }],
+    ['PUT', '/api/trips/1/assignments/7/move', { new_day_id: 3, order_index: 0 }],
+  ];
+
+  it.each(calls)('FE-MSW-CONTRACT-005: %s %s', async (method, path, body) => {
+    const res = await send(method, path, body);
+    expect(res.json).not.toHaveProperty('issues');
+    expect(res.status).toBe(200);
+  });
+
+  it('FE-MSW-CONTRACT-007: a todo item answers checked as 0/1 like the server stores it', async () => {
+    const res = await send('PUT', '/api/trips/1/todo/4', { checked: true });
+    expect((res.json.item as { checked: unknown }).checked).toBe(1);
+  });
+
+  it('FE-MSW-CONTRACT-006: a packing item answers checked as 0/1 like the server stores it', async () => {
+    const res = await send('PUT', '/api/trips/1/packing/6', { checked: true });
+    expect((res.json.item as { checked: unknown }).checked).toBe(1);
+  });
+});