diff --git a/routes/calendar.cts b/routes/calendar.cts
index 5cab533..5795d1a 100644
--- a/routes/calendar.cts
+++ b/routes/calendar.cts
@@ -1,7 +1,7 @@
 // @ts-nocheck — route controller converted to .cts; handler req/res typed, full type-check deferred (thin glue over typed services).
 import type { Req, Res, Next } from '../types/http';
 const express = require('express');
-const { standardizeError } = require('../middleware/errorFormatter.cts');
+const { ValidationError } = require('../utils/apiError.cts');
 const router = express.Router();
 const { getDb } = require('../db/database.cts');
 const { buildTrackerRow, getCycleRange } = require('../services/statusService.cts');
@@ -104,22 +104,10 @@ router.get('/', (req: Req, res: Res) => {
   const month = parseInt(req.query.month || now.getMonth() + 1, 10);
 
   if (isNaN(year) || year < 2000 || year > 2100) {
-    return res
-      .status(400)
-      .json(
-        standardizeError(
-          'year must be a 4-digit integer between 2000 and 2100',
-          'VALIDATION_ERROR',
-          'year',
-        ),
-      );
+    throw ValidationError('year must be a 4-digit integer between 2000 and 2100', 'year');
   }
   if (isNaN(month) || month < 1 || month > 12) {
-    return res
-      .status(400)
-      .json(
-        standardizeError('month must be an integer between 1 and 12', 'VALIDATION_ERROR', 'month'),
-      );
+    throw ValidationError('month must be an integer between 1 and 12', 'month');
   }
 
   const today = localDateString(now);
diff --git a/routes/payments.cts b/routes/payments.cts
index df45285..a40069f 100644
--- a/routes/payments.cts
+++ b/routes/payments.cts
@@ -1,7 +1,6 @@
 // @ts-nocheck — route controller converted to .cts; handler req/res typed, full type-check deferred (thin glue over typed services).
 import type { Req, Res, Next } from '../types/http';
 const express = require('express');
-const { standardizeError } = require('../middleware/errorFormatter.cts');
 const {
   ApiError,
   ValidationError,
@@ -269,12 +268,14 @@ router.post('/', (req: Req, res: Res) => {
       )
       .get(bill.id, payment.paid_date, payment.amount);
     if (existingDuplicate) {
+      // Deliberate hand-rolled body (documented exception): carries the
+      // `existing` payment the client's "add anyway?" flow renders —
+      // formatError has no extras channel. Shape matches the standard envelope.
       return res.status(409).json({
-        ...standardizeError(
-          'A matching payment already exists for this bill, date, and amount',
-          'DUPLICATE_SUSPECTED',
-          'amount',
-        ),
+        error: 'DUPLICATE_SUSPECTED',
+        message: 'A matching payment already exists for this bill, date, and amount',
+        code: 'DUPLICATE_SUSPECTED',
+        field: 'amount',
         existing: serializePayment(existingDuplicate),
       });
     }
diff --git a/routes/user.cts b/routes/user.cts
index a346160..04f5aac 100644
--- a/routes/user.cts
+++ b/routes/user.cts
@@ -8,7 +8,16 @@ const { getDb } = require('../db/database.cts');
 const { seedDemoData } = require('../scripts/seedDemoData.cts');
 const { demoDataLimiter } = require('../middleware/rateLimiter.cts');
 const { eraseUserData, clearSeededDemoData } = require('../services/userDataService.cts');
-const { standardizeError } = require('../middleware/errorFormatter.cts');
+const { ApiError, ValidationError } = require('../utils/apiError.cts');
+
+// 4xx service errors keep their message/code on the wire; anything else is
+// rethrown raw so the terminal handler masks + logs it as a 5xx.
+function toUserOpError(err, fallbackCode) {
+  if (err.status && err.status < 500) {
+    return new ApiError(err.code || fallbackCode, err.message, err.status);
+  }
+  return err;
+}
 
 // GET /api/user/seeded-status — whether the current user has demo data, with counts
 router.get('/seeded-status', (req: Req, res: Res) => {
@@ -39,14 +48,7 @@ router.get('/seeded-status', (req: Req, res: Res) => {
       seededTransactions,
     });
   } catch (err) {
-    res
-      .status(err.status || 500)
-      .json(
-        standardizeError(
-          err.message || 'Seeded status check failed',
-          err.code || 'SEEDED_STATUS_ERROR',
-        ),
-      );
+    throw toUserOpError(err, 'SEEDED_STATUS_ERROR');
   }
 });
 
@@ -66,14 +68,7 @@ router.post('/clear-demo-data', demoDataLimiter, (req: Req, res: Res) => {
       counts: result.counts,
     });
   } catch (err) {
-    res
-      .status(err.status || 500)
-      .json(
-        standardizeError(
-          err.message || 'Clear demo data operation failed',
-          err.code || 'CLEAR_DEMO_ERROR',
-        ),
-      );
+    throw toUserOpError(err, 'CLEAR_DEMO_ERROR');
   }
 });
 
@@ -102,9 +97,7 @@ router.post('/seed-demo-data', demoDataLimiter, (req: Req, res: Res) => {
       counts: result.counts,
     });
   } catch (err) {
-    res
-      .status(err.status || 500)
-      .json(standardizeError(err.message || 'Seed operation failed', err.code || 'SEED_ERROR'));
+    throw toUserOpError(err, 'SEED_ERROR');
   }
 });
 
@@ -116,17 +109,13 @@ router.post('/erase-data', demoDataLimiter, (req: Req, res: Res) => {
     .trim()
     .toUpperCase();
   if (confirm !== 'ERASE') {
-    return res
-      .status(400)
-      .json(standardizeError('Type ERASE to confirm.', 'ERASE_CONFIRM_REQUIRED', 'confirm'));
+    throw ValidationError('Type ERASE to confirm.', 'confirm', 'ERASE_CONFIRM_REQUIRED');
   }
   try {
     const result = eraseUserData(getDb(), req.user.id);
     res.json({ success: true, ...result });
   } catch (err) {
-    res
-      .status(err.status || 500)
-      .json(standardizeError(err.message || 'Failed to erase data', err.code || 'ERASE_ERROR'));
+    throw toUserOpError(err, 'ERASE_ERROR');
   }
 });