diff --git a/HISTORY.md b/HISTORY.md
index 848f403..c770a0a 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -3,6 +3,8 @@
 
 ### 🔌 API response typing cleanup (Track C) — promoting page shapes into `@/types`, endpoint by endpoint
 
+- **[Client] Auth: `User` moved to its proper home + auth endpoints typed.** `User` lived oddly in `@/hooks/useAuth`; moved it to `@/types` (re-exported from `useAuth` so the 7 importers keep working) and typed `me`/`login`/`totpChallenge`/`hasUsers` with `get<T>`/`post<T>`, dropping those casts on the Admin + Login pages. *(Deliberate stop: the remaining single-consumer, non-money casts — `adminUsers` with its page-local `AdminUser`, and the one-off `version`/`about`/`privacy`/`health`/`importHistory` doc shapes — are left as-is; central types add ~nothing for a single reader, and typing them is the low-value tail. Likewise the **optimistic-UI/`toast.promise` rewrite** (planned Track D) is intentionally not done — it's behavior-visible churn on already-working handlers, the worst risk/reward of the remaining work.)*
+- **[Server] Error-shape consistency (Track D).** The global 500 handler ([server.js](server.js)) returned `{ error: 'Internal server error' }` with no `code`; added `code: 'INTERNAL_ERROR'` so an unexpected server error carries the same `{error, code}` field the client can switch on as every structured 4xx.
 - **[Client] Snowball projection + settings typed.** Promoted `SnowballProjection` (+ `SnowballProjectionDetail`/`AvalancheProjectionDetail`/`SnowballDebtProjection`) and `SnowballSettings` into `@/types`, and wired `snowball` (→ `Bill[]`), `snowballSettings`/`saveSnowballSettings` (→ `SnowballSettings`), and `snowballProjection` (→ `SnowballProjection`). Removed the projection/settings/bills casts. (The plan-lifecycle endpoints still cast to `SnowballPlan`, which is a component-owned type in `PlanHistoryPanel` — left as-is to avoid a cross-file type move for marginal gain.)
 - **[Client] Subscriptions domain typed.** Promoted the `SubscriptionsPage` shapes into `@/types` (`Subscription`, `SubscriptionSummary`/`…TopType`, `SubscriptionsResponse`, `Recommendation` + `RecommendationEvidence`/`…Transaction`/`ExistingBillMatch`/`CatalogMatch`, `SubscriptionTx`) and wired `subscriptions`/`subscriptionRecommendations`/`subscriptionTransactionMatches`/`createSubscriptionFromRecommendation`/`matchRecommendationToBill` with `get<T>`/`post<T>`. Dropped the `(r: any)` in `useSubscriptionRecommendations` (now typed) and the 5 page casts; removed an unused `CatalogMatch` import (Track E).
 - **[Client] Spending domain typed.** Promoted `SpendingPage`'s local response interfaces into `@/types` (`SpendingCategoryEntry`, `SpendingSummary`, `SpendingTransaction`/`…Response`, `SpendingIncomeTx`/`…Response`, `SpendingRule`, `CategoryGroup`, `CopyBudgetsResponse`) and wired `spendingSummary`/`spendingTransactions`/`spendingIncome`/`spendingCategoryRules`/`copySpendingBudgets`/`categoryGroups` with `get<T>`/`post<T>`. `useSpendingSummary`/`useSpendingTransactions`/`useCategoryGroups` now return typed data; the 6 spending `as {…}` casts are gone. Removed 2 now-unused type aliases the cast-removal orphaned (Track E).
diff --git a/client/api.ts b/client/api.ts
index 5a8ab52..ff6a1b3 100644
--- a/client/api.ts
+++ b/client/api.ts
@@ -1,5 +1,5 @@
 import type {
-  Bill, Category, Payment, TrackerResponse,
+  Bill, Category, Payment, TrackerResponse, User,
   BankLedger, BankLedgerTransaction, AutoCategorizePreview,
   SpendingSummary, SpendingTransactionsResponse, SpendingIncomeResponse,
   SpendingRule, CopyBudgetsResponse, CategoryGroup,
@@ -108,9 +108,9 @@ function filenameFromDisposition(value: string | null): string | null {
 
 export const api = {
   // Auth
-  me:                 ()          => get('/auth/me'),
+  me:                 ()          => get<{ user?: User | null; single_user_mode?: boolean; has_new_version?: boolean }>('/auth/me'),
   authMode:           ()          => get('/auth/mode'),
-  login:              (data: Body) => post('/auth/login', data),
+  login:              (data: Body) => post<{ requires_totp?: boolean; challenge_token?: string; user?: User }>('/auth/login', data),
   logout:             ()          => post('/auth/logout'),
   restoreMultiUserMode: ()        => post('/auth/restore-multi-user-mode'),
   changePassword:     (data: Body) => post('/auth/change-password', data),
@@ -133,7 +133,7 @@ export const api = {
   totpSetup:           ()         => get('/auth/totp/setup'),
   totpEnable:          (data: Body) => post('/auth/totp/enable', data),
   totpDisable:         (data: Body) => post('/auth/totp/disable', data),
-  totpChallenge:       (data: Body) => post('/auth/totp/challenge', data),
+  totpChallenge:       (data: Body) => post<{ user?: User }>('/auth/totp/challenge', data),
 
   webauthnStatus:      ()         => get('/auth/webauthn/status'),
   webauthnSetup:       ()         => get('/auth/webauthn/setup'),
@@ -144,7 +144,7 @@ export const api = {
   webauthnChallenge:   (data: Body) => post('/auth/webauthn/challenge', data),
 
   // Admin
-  hasUsers:           ()          => get('/admin/has-users'),
+  hasUsers:           ()          => get<{ has_users?: boolean }>('/admin/has-users'),
   adminUsers:         ()          => get('/admin/users'),
   createUser:         (data: Body) => post('/admin/users', data),
   resetPassword:      (id: Id, data: Body) => put(`/admin/users/${id}/password`, data),
diff --git a/client/hooks/useAuth.tsx b/client/hooks/useAuth.tsx
index 587abdf..1da401f 100644
--- a/client/hooks/useAuth.tsx
+++ b/client/hooks/useAuth.tsx
@@ -1,16 +1,8 @@
 import { createContext, useContext, useEffect, useState, type Dispatch, type SetStateAction, type ReactNode } from 'react';
 import { api } from '@/api';
+import type { User } from '@/types';
 
-export interface User {
-  id: number;
-  username?: string;
-  role?: string;
-  display_name?: string;
-  displayName?: string;
-  name?: string;
-  is_default_admin?: boolean;
-  [key: string]: unknown;
-}
+export type { User }; // re-export so existing `@/hooks/useAuth` importers keep working
 
 interface MeResponse {
   user?: User | null;
diff --git a/client/pages/AdminPage.tsx b/client/pages/AdminPage.tsx
index 5580813..0e89bc0 100644
--- a/client/pages/AdminPage.tsx
+++ b/client/pages/AdminPage.tsx
@@ -26,7 +26,7 @@ export default function AdminPage() {
 
   const loadMe = useCallback(async () => {
     try {
-      const d = await api.me() as { user?: User };
+      const d = await api.me();
       setMe(d.user ?? null);
     } catch {
       navigate('/login', { replace: true });
@@ -42,7 +42,7 @@ export default function AdminPage() {
 
   const loadHasUsers = useCallback(async () => {
     try {
-      const d = await api.hasUsers() as { has_users?: boolean };
+      const d = await api.hasUsers();
       setHasUsers(!!d.has_users);
       if (d.has_users) loadUsers();
     } catch (err) {
diff --git a/client/pages/LoginPage.tsx b/client/pages/LoginPage.tsx
index 8fc180d..a4c0224 100644
--- a/client/pages/LoginPage.tsx
+++ b/client/pages/LoginPage.tsx
@@ -86,7 +86,7 @@ export default function LoginPage() {
     setError('');
     setLoading(true);
     try {
-      const data = await api.login({ username, password }) as { requires_totp?: boolean; challenge_token?: string; user: User };
+      const data = await api.login({ username, password });
       if (data.requires_totp) {
         setTotpChallenge(data.challenge_token ?? null);
         setTotpCode('');
@@ -94,7 +94,7 @@ export default function LoginPage() {
         setUseRecovery(false);
         return;
       }
-      handlePostLogin(data.user);
+      handlePostLogin(data.user!);
     } catch (err) {
       setError(errMessage(err, 'Login failed.'));
     } finally {
@@ -110,8 +110,8 @@ export default function LoginPage() {
       const payload: { challenge_token: string | null; recovery_code?: string; code?: string } = { challenge_token: totpChallenge };
       if (useRecovery) payload.recovery_code = totpCode;
       else payload.code = totpCode;
-      const data = await api.totpChallenge(payload) as { user: User };
-      handlePostLogin(data.user);
+      const data = await api.totpChallenge(payload);
+      handlePostLogin(data.user!);
     } catch (err) {
       setTotpError(errMessage(err, 'Invalid code.'));
       setTotpCode('');
diff --git a/client/types.ts b/client/types.ts
index 41376ff..0e51d2f 100644
--- a/client/types.ts
+++ b/client/types.ts
@@ -10,6 +10,19 @@
 // sparkline) are left `unknown` and refined when a consumer needs them.
 import type { Cents, Dollars } from '@/lib/money';
 
+// The authenticated user (GET /auth/me → `user`). Re-exported from
+// `@/hooks/useAuth` for the many components that import it from there.
+export interface User {
+  id: number;
+  username?: string;
+  role?: string;
+  display_name?: string;
+  displayName?: string;
+  name?: string;
+  is_default_admin?: boolean;
+  [key: string]: unknown;
+}
+
 // A bank transaction as serialized by the server. Unlike bill/payment amounts,
 // raw bank-transaction amounts stay in CENTS on the wire — hence branded `Cents`
 // (format them with formatCentsUSD, never formatUSD).
diff --git a/server.js b/server.js
index 56fec03..d8f5001 100644
--- a/server.js
+++ b/server.js
@@ -196,6 +196,7 @@ app.use((err, req, res, next) => {
 
   res.status(err.status || 500).json({
     error: 'Internal server error',
+    code: 'INTERNAL_ERROR',
   });
 });