diff --git a/client/hooks/usePaymentActions.js b/client/hooks/usePaymentActions.ts
similarity index 67%
rename from client/hooks/usePaymentActions.js
rename to client/hooks/usePaymentActions.ts
index 575ff29..94fabe4 100644
--- a/client/hooks/usePaymentActions.js
+++ b/client/hooks/usePaymentActions.ts
@@ -2,8 +2,20 @@ import { useMutation } from '@tanstack/react-query';
 import { toast } from 'sonner';
 import { api } from '@/api.js';
 import { fmt } from '@/lib/utils';
+import type { Dollars } from '@/lib/money';
 import { useInvalidateTrackerData } from '@/hooks/useQueries';
 
+/** The bits of a tracker row these payment actions need. */
+interface PayableRow {
+  id: number;
+  name: string;
+}
+
+/** Narrow an unknown error (strict catch clauses) to a message string. */
+function errMessage(err: unknown, fallback: string): string {
+  return err instanceof Error && err.message ? err.message : fallback;
+}
+
 // Quick-pay a tracker row (record a payment for `val` on `paidDate`) with a
 // reversible Undo toast. A React Query mutation: `isPending` comes for free and
 // the tracker/badge caches are invalidated on settle. Shared by the desktop and
@@ -11,11 +23,11 @@ import { useInvalidateTrackerData } from '@/hooks/useQueries';
 export function useQuickPay() {
   const invalidate = useInvalidateTrackerData();
   const mutation = useMutation({
-    mutationFn: (payload) => api.quickPay(payload),
+    mutationFn: (payload: { bill_id: number; amount: number; paid_date: string }) => api.quickPay(payload),
     onSettled: () => invalidate(),
   });
 
-  const run = (row, val, paidDate) => mutation.mutate(
+  const run = (row: PayableRow, val: Dollars, paidDate: string) => mutation.mutate(
     { bill_id: row.id, amount: val, paid_date: paidDate },
     {
       onSuccess: (payment) => {
@@ -28,32 +40,39 @@ export function useQuickPay() {
                 toast.success('Payment removed');
                 invalidate();
               } catch (err) {
-                toast.error(err.message || 'Failed to undo payment.');
+                toast.error(errMessage(err, 'Failed to undo payment.'));
               }
             },
           },
         });
       },
-      onError: (err) => toast.error(err.message || 'Failed to add payment.'),
+      onError: (err) => toast.error(errMessage(err, 'Failed to add payment.')),
     },
   );
 
   return { run, isPending: mutation.isPending };
 }
 
+interface TogglePaidOptions {
+  wasPaid: boolean;
+  threshold: Dollars;
+  setOptimisticPaid: (value: boolean | undefined) => void;
+}
+
 // Toggle a tracker row paid/unpaid. The caller owns the instant optimistic flip
 // (its local `setOptimisticPaid`) so the row updates immediately; this hook rolls
 // it back on error, invalidates the tracker/badge caches on settle, and shows the
 // right toast (an Undo when un-paying, a specific "paid" message otherwise).
 // Shared by the desktop and mobile tracker rows.
-export function useTogglePaid(year, month) {
+export function useTogglePaid(year: number, month: number) {
   const invalidate = useInvalidateTrackerData();
   const mutation = useMutation({
-    mutationFn: ({ rowId, amount }) => api.togglePaid(rowId, { amount, year, month }),
+    mutationFn: ({ rowId, amount }: { rowId: number; amount: number | undefined }) =>
+      api.togglePaid(rowId, { amount, year, month }),
     onSettled: () => invalidate(),
   });
 
-  const run = (row, { wasPaid, threshold, setOptimisticPaid }) => {
+  const run = (row: PayableRow, { wasPaid, threshold, setOptimisticPaid }: TogglePaidOptions) => {
     setOptimisticPaid(!wasPaid); // instant flip; effect/rollback reconciles
     mutation.mutate(
       { rowId: row.id, amount: wasPaid ? undefined : threshold },
@@ -69,7 +88,7 @@ export function useTogglePaid(year, month) {
                     toast.success('Payment restored');
                     invalidate();
                   } catch (err) {
-                    toast.error(err.message || 'Failed to restore payment');
+                    toast.error(errMessage(err, 'Failed to restore payment'));
                   }
                 },
               },
@@ -80,7 +99,7 @@ export function useTogglePaid(year, month) {
         },
         onError: (err) => {
           setOptimisticPaid(undefined); // roll back the instant flip
-          toast.error(err.message || 'Failed to toggle payment status');
+          toast.error(errMessage(err, 'Failed to toggle payment status'));
         },
       },
     );