diff --git a/server/src/routes/public.ts b/server/src/routes/public.ts
index d8c039a..85e0b46 100644
--- a/server/src/routes/public.ts
+++ b/server/src/routes/public.ts
@@ -7,6 +7,7 @@ import type {
   CreateTransactionResponse,
   Drink,
 } from '@wutzcalc/shared';
+import { sendProblem } from '../problem-details.js';
 
 export function registerPublicRoutes(app: FastifyInstance, db: DB) {
   app.get('/api/bars', async () => {
@@ -16,12 +17,12 @@ export function registerPublicRoutes(app: FastifyInstance, db: DB) {
 
   app.get<{ Querystring: { bar: string } }>('/api/config', async (req, reply) => {
     const barId = Number(req.query.bar);
-    if (!Number.isInteger(barId)) return reply.code(400).send({ error: 'bar required' });
+    if (!Number.isInteger(barId)) return sendProblem(req, reply, 400, 'Bad Request', 'bar required');
 
     const bar = db
       .prepare('SELECT id, name, pfand_cents FROM bars WHERE id = ?')
       .get(barId) as Bar | undefined;
-    if (!bar) return reply.code(404).send({ error: 'bar not found' });
+    if (!bar) return sendProblem(req, reply, 404, 'Not Found', 'bar not found');
 
     const drinks = db
       .prepare(
@@ -49,29 +50,29 @@ export function registerPublicRoutes(app: FastifyInstance, db: DB) {
     const { client_uuid, bar_id, crew, items } = body;
 
     if (!client_uuid || typeof client_uuid !== 'string') {
-      return reply.code(400).send({ error: 'client_uuid required' });
+      return sendProblem(req, reply, 400, 'Bad Request', 'client_uuid required');
     }
     if (!Number.isInteger(bar_id)) {
-      return reply.code(400).send({ error: 'bar_id required' });
+      return sendProblem(req, reply, 400, 'Bad Request', 'bar_id required');
     }
     if (!Array.isArray(items)) {
-      return reply.code(400).send({ error: 'items required' });
+      return sendProblem(req, reply, 400, 'Bad Request', 'items required');
     }
     if (items.length > MAX_ITEMS_PER_TRANSACTION) {
-      return reply.code(400).send({ error: 'too many items' });
+      return sendProblem(req, reply, 400, 'Bad Request', 'too many items');
     }
 
     const rawPfandReturns = body.pfand_returns ?? 0;
     if (!Number.isInteger(rawPfandReturns) || rawPfandReturns < 0) {
-      return reply.code(400).send({ error: 'pfand_returns must be a non-negative integer' });
+      return sendProblem(req, reply, 400, 'Bad Request', 'pfand_returns must be a non-negative integer');
     }
     if (rawPfandReturns > MAX_PFAND_RETURNS) {
-      return reply.code(400).send({ error: 'pfand_returns too large' });
+      return sendProblem(req, reply, 400, 'Bad Request', 'pfand_returns too large');
     }
     const pfand_returns = rawPfandReturns;
 
     if (items.length === 0 && pfand_returns === 0) {
-      return reply.code(400).send({ error: 'empty transaction' });
+      return sendProblem(req, reply, 400, 'Bad Request', 'empty transaction');
     }
 
     const findByUuid = () =>
@@ -88,7 +89,7 @@ export function registerPublicRoutes(app: FastifyInstance, db: DB) {
     const bar = db
       .prepare('SELECT id, pfand_cents FROM bars WHERE id = ?')
       .get(bar_id) as { id: number; pfand_cents: number } | undefined;
-    if (!bar) return reply.code(404).send({ error: 'bar not found' });
+    if (!bar) return sendProblem(req, reply, 404, 'Not Found', 'bar not found');
 
     const drinkStmt = db.prepare(
       `SELECT d.id, d.price_cents
@@ -112,12 +113,14 @@ export function registerPublicRoutes(app: FastifyInstance, db: DB) {
         item.qty <= 0 ||
         item.qty > MAX_QTY_PER_LINE
       ) {
-        return reply.code(400).send({ error: 'invalid item' });
+        return sendProblem(req, reply, 400, 'Bad Request', 'invalid item');
       }
       const drink = drinkStmt.get(bar_id, item.drink_id) as
         | { id: number; price_cents: number }
         | undefined;
-      if (!drink) return reply.code(400).send({ error: `drink ${item.drink_id} not at bar` });
+      if (!drink) {
+        return sendProblem(req, reply, 400, 'Bad Request', `drink ${item.drink_id} not at bar`);
+      }
 
       const lineUnit = drink.price_cents + bar.pfand_cents;
       total += lineUnit * item.qty;