diff --git a/docs/CODE_STANDARDS.md b/docs/CODE_STANDARDS.md
index 8faef05..0d5d42b 100644
--- a/docs/CODE_STANDARDS.md
+++ b/docs/CODE_STANDARDS.md
@@ -38,7 +38,10 @@ rolled out. Update this doc when a standard changes.
   (`ValidationError`, `NotFoundError`, `ConflictError`, …) and let the single terminal error
   handler format them — do not hand-roll `res.status().json({ error })`. Express 5 forwards
   rejected async handlers automatically, so route bodies should not wrap everything in
-  `try/catch`. Clients read `message`/`code` + status.
+  `try/catch`. Clients read `message`/`code` + status. **Canonical example:**
+  `routes/categories.cts`. (The terminal handler already normalizes any legacy
+  `standardizeError`/`{ error }` body to the same shape, so converted and not-yet-converted
+  routes emit identical responses — convert opportunistically.)
 - **Success envelope (target):** reads return the bare resource/list; mutations return
   `{ success: true, … }`.
 - **Validation (target):** validate inputs with the shared validators (throwing
diff --git a/routes/monthly-starting-amounts.cts b/routes/monthly-starting-amounts.cts
index 011e47b..193cfa6 100644
--- a/routes/monthly-starting-amounts.cts
+++ b/routes/monthly-starting-amounts.cts
@@ -6,6 +6,7 @@ const { getDb } = require('../db/database.cts');
 const { getCycleRange } = require('../services/statusService.cts');
 const { accountingActiveSql } = require('../services/paymentAccountingService.cts');
 const { toCents, fromCents } = require('../utils/money.mts');
+const { ValidationError } = require('../utils/apiError.cts');
 
 function parseYearMonth(source) {
   const now = new Date();
@@ -147,7 +148,7 @@ function buildStartingAmountsResponse(db, userId, year, month) {
 
 router.get('/', (req: Req, res: Res) => {
   const parsed = parseYearMonth(req.query);
-  if (parsed.error) return res.status(400).json({ error: parsed.error });
+  if (parsed.error) throw ValidationError(parsed.error, 'month');
 
   const db = getDb();
   res.json(buildStartingAmountsResponse(db, req.user.id, parsed.year, parsed.month));
@@ -155,27 +156,24 @@ router.get('/', (req: Req, res: Res) => {
 
 router.put('/', (req: Req, res: Res) => {
   const parsed = parseYearMonth(req.body || {});
-  if (parsed.error) return res.status(400).json({ error: parsed.error });
+  if (parsed.error) throw ValidationError(parsed.error, 'month');
 
   const firstAmount = Number(req.body?.first_amount);
   if (!Number.isFinite(firstAmount) || firstAmount < 0 || firstAmount > 1000000000) {
-    return res
-      .status(400)
-      .json({ error: 'first_amount must be a number between 0 and 1000000000' });
+    throw ValidationError('first_amount must be a number between 0 and 1000000000', 'first_amount');
   }
 
   const fifteenthAmount = Number(req.body?.fifteenth_amount);
   if (!Number.isFinite(fifteenthAmount) || fifteenthAmount < 0 || fifteenthAmount > 1000000000) {
-    return res
-      .status(400)
-      .json({ error: 'fifteenth_amount must be a number between 0 and 1000000000' });
+    throw ValidationError(
+      'fifteenth_amount must be a number between 0 and 1000000000',
+      'fifteenth_amount',
+    );
   }
 
   const otherAmount = Number(req.body?.other_amount);
   if (!Number.isFinite(otherAmount) || otherAmount < 0 || otherAmount > 1000000000) {
-    return res
-      .status(400)
-      .json({ error: 'other_amount must be a number between 0 and 1000000000' });
+    throw ValidationError('other_amount must be a number between 0 and 1000000000', 'other_amount');
   }
 
   const db = getDb();