diff --git a/src/domain/views.test.ts b/src/domain/views.test.ts
index b6ac8da..f5e855e 100644
--- a/src/domain/views.test.ts
+++ b/src/domain/views.test.ts
@@ -181,6 +181,15 @@ describe("yearView", () => {
 });
 
 describe("summaryView", () => {
+  test("difference follows the dataset's decimal setting", () => {
+    const precise: Dataset = {
+      ...data,
+      settings: { ...data.settings, digits: 3 },
+      months: [{ id: "2026-01", income: { amount: 1.234, currency: "DKK" }, ruleOverrides: {} }],
+      purchases: [spend("small", "food", 0.111, "2026-01-10")],
+    };
+    expect(summaryView(precise, "2026-01", "2026-01").difference).toBe(1.123);
+  });
   test("totals charges by post and by month across a range", () => {
     const view = summaryView(data, "2026-01", "2026-02");
     expect(view.totalCharges).toBe(800);
diff --git a/src/domain/views.ts b/src/domain/views.ts
index d5f3c21..01145d8 100644
--- a/src/domain/views.ts
+++ b/src/domain/views.ts
@@ -149,6 +149,7 @@ export interface SummaryViewModel {
   byMonth: { monthId: MonthId; charges: number }[];
   totalCharges: number;
   totalIncome: number;
+  difference: number;
 }
 
 export function summaryView(
@@ -181,19 +182,23 @@ export function summaryView(
     ),
   }));
 
+  const totalCharges = roundMoney(
+    byMonth.reduce((sum, m) => sum + m.charges, 0),
+    baseDigits,
+  );
+  const totalIncome = roundMoney(
+    months.reduce((sum, m) => sum + incomeFor(dataset, m), 0),
+    baseDigits,
+  );
+
   return {
     from,
     to,
     byPost,
     byMonth,
-    totalCharges: roundMoney(
-      byMonth.reduce((sum, m) => sum + m.charges, 0),
-      baseDigits,
-    ),
-    totalIncome: roundMoney(
-      months.reduce((sum, m) => sum + incomeFor(dataset, m), 0),
-      baseDigits,
-    ),
+    totalCharges,
+    totalIncome,
+    difference: roundMoney(totalIncome - totalCharges, baseDigits),
   };
 }
 
diff --git a/src/ui/components/ExpectedBand.tsx b/src/ui/components/ExpectedBand.tsx
index ee7ba8f..47824cb 100644
--- a/src/ui/components/ExpectedBand.tsx
+++ b/src/ui/components/ExpectedBand.tsx
@@ -174,7 +174,7 @@ export function expectedGroups(
  */
 export function ExpectedBand({ monthId }: { monthId: MonthId }) {
   const dataset = useDataset();
-  const { mutate } = useMutate();
+  const { mutate, error } = useMutate();
   const [amounts, setAmounts] = useState<Record<string, string>>({});
 
   const { pending, overdue, comingUp } = useMemo(
@@ -239,7 +239,7 @@ export function ExpectedBand({ monthId }: { monthId: MonthId }) {
             size="sm"
             variant="outline"
             disabled={invalid}
-            onClick={() => {
+            onClick={async () => {
               // Captured before mutate, per AGENTS.md: mutate defers behind
               // the write queue and React resets the input first, so reading
               // in the callback commits stale text. `invalid` already guards
@@ -247,12 +247,13 @@ export function ExpectedBand({ monthId }: { monthId: MonthId }) {
               // `parsed` is either a real amount or the field was never
               // touched.
               if (invalid) return;
-              mutate((draft) =>
+              const saved = await mutate((draft) =>
                 confirmOccurrence(draft, occurrence.recurringId, occurrence.date, {
                   date: recordedDate ?? undefined,
                   amount: parsed ?? undefined,
                 }),
               );
+              if (!saved) return;
               setAmounts((current) => {
                 const next = { ...current };
                 delete next[key];
@@ -272,6 +273,7 @@ export function ExpectedBand({ monthId }: { monthId: MonthId }) {
       title="Expected"
       hint="Committed but not yet spent. These do not touch a post's balance until you confirm one, which records it as an ordinary purchase you can edit or delete afterwards."
     >
+      {error && <p role="alert" className="mb-3 text-sm text-destructive">Could not confirm: {error}</p>}
       {pending.length > 0 && (
         <ul className="divide-y divide-budget-rule">
           {pending.map((occurrence) => row(occurrence, recordedDateFor("pending", today)))}
diff --git a/src/ui/hooks/useMutate.ts b/src/ui/hooks/useMutate.ts
index c076f04..0e0a90b 100644
--- a/src/ui/hooks/useMutate.ts
+++ b/src/ui/hooks/useMutate.ts
@@ -18,7 +18,10 @@ export function useMutate() {
 
   const mutate = useCallback((fn: (draft: Dataset) => void): Promise<boolean> => {
     return store.mutate(fn).then(
-      () => true,
+      () => {
+        setError(null);
+        return true;
+      },
       (cause: unknown) => {
         setError(cause instanceof Error ? cause.message : String(cause));
         return false;
diff --git a/src/ui/routes/SummaryRoute.test.ts b/src/ui/routes/SummaryRoute.test.ts
new file mode 100644
index 0000000..0fe0368
--- /dev/null
+++ b/src/ui/routes/SummaryRoute.test.ts
@@ -0,0 +1,10 @@
+import { expect, test } from "bun:test";
+import { isMonthId } from "./SummaryRoute.tsx";
+
+test("Summary range accepts real months and rejects partial or impossible input", () => {
+  expect(isMonthId("2026-09")).toBe(true);
+  expect(isMonthId("")).toBe(false);
+  expect(isMonthId("2026-0")).toBe(false);
+  expect(isMonthId("2026-13")).toBe(false);
+  expect(isMonthId("2026-09-01")).toBe(false);
+});
diff --git a/src/ui/routes/SummaryRoute.tsx b/src/ui/routes/SummaryRoute.tsx
index bec49e6..3e819ac 100644
--- a/src/ui/routes/SummaryRoute.tsx
+++ b/src/ui/routes/SummaryRoute.tsx
@@ -5,10 +5,19 @@ import { Label } from "@/components/ui/label";
 import { Button } from "@/components/ui/button";
 import { useDataset } from "../hooks/useDataset.ts";
 import { datasetMonthSpan, summaryView } from "../../domain/views.ts";
+import { monthOf } from "../../domain/months.ts";
 import { useMoneyFormat } from "../hooks/useMoneyFormat.ts";
 import { Section, Stat } from "../components/Section.tsx";
 import { Segmented } from "../components/Segmented.tsx";
 
+export function isMonthId(value: string): boolean {
+  try {
+    return monthOf(value) === value;
+  } catch {
+    return false;
+  }
+}
+
 export function SummaryRoute() {
   const fmt = useMoneyFormat();
   const dataset = useDataset();
@@ -17,9 +26,10 @@ export function SummaryRoute() {
   const [to, setTo] = useState(span.to);
   const [groupBy, setGroupBy] = useState<"post" | "month">("post");
 
-  const view = summaryView(dataset, from, to);
+  const validFrom = isMonthId(from);
+  const validTo = isMonthId(to);
+  const view = validFrom && validTo ? summaryView(dataset, from, to) : null;
   const base = dataset.settings.baseCurrency;
-  const difference = Number((view.totalIncome - view.totalCharges).toFixed(2));
 
   return (
     <div className="space-y-5">
@@ -29,11 +39,11 @@ export function SummaryRoute() {
         <div className="flex flex-wrap items-end gap-4">
           <div className="space-y-1">
             <Label htmlFor="from">From</Label>
-            <Input id="from" className="w-32" value={from} onChange={(e) => setFrom(e.target.value)} />
+            <Input id="from" className="w-32" value={from} aria-invalid={!validFrom} onChange={(e) => setFrom(e.target.value)} />
           </div>
           <div className="space-y-1">
             <Label htmlFor="to">To</Label>
-            <Input id="to" className="w-32" value={to} onChange={(e) => setTo(e.target.value)} />
+            <Input id="to" className="w-32" value={to} aria-invalid={!validTo} onChange={(e) => setTo(e.target.value)} />
           </div>
           <Button
             variant="outline"
@@ -46,16 +56,20 @@ export function SummaryRoute() {
           </Button>
         </div>
 
-        <dl className="mt-5 flex flex-wrap gap-x-10 gap-y-4 border-t border-budget-rule pt-5">
-        <Stat label="Income">{fmt.money(view.totalIncome, base)}</Stat>
-        <Stat label="Spent">{fmt.money(view.totalCharges, base)}</Stat>
-          <Stat label="Difference" tone={difference < 0 ? "overspend" : "default"}>
-            {fmt.money(difference, base)}
-          </Stat>
-        </dl>
+        {view ? (
+          <dl className="mt-5 flex flex-wrap gap-x-10 gap-y-4 border-t border-budget-rule pt-5">
+            <Stat label="Income">{fmt.money(view.totalIncome, base)}</Stat>
+            <Stat label="Spent">{fmt.money(view.totalCharges, base)}</Stat>
+            <Stat label="Difference" tone={view.difference < 0 ? "overspend" : "default"}>
+              {fmt.money(view.difference, base)}
+            </Stat>
+          </dl>
+        ) : (
+          <p role="alert" className="mt-4 text-sm text-destructive">Enter a month as YYYY-MM in each field.</p>
+        )}
       </Section>
 
-      <Section
+      {view && <Section
         title={groupBy === "post" ? "By post" : "By month"}
         action={
           <Segmented
@@ -107,7 +121,7 @@ export function SummaryRoute() {
                 ))}
           </tbody>
         </table>
-      </Section>
+      </Section>}
     </div>
   );
 }
diff --git a/src/ui/routes/settings/CurrencySection.tsx b/src/ui/routes/settings/CurrencySection.tsx
index 3a6ca09..5ac13ab 100644
--- a/src/ui/routes/settings/CurrencySection.tsx
+++ b/src/ui/routes/settings/CurrencySection.tsx
@@ -21,7 +21,7 @@ import type { Currency } from "../../../domain/types.ts";
 
 export function CurrencySection() {
   const dataset = useDataset();
-  const { mutate } = useMutate();
+  const { mutate, error: writeError } = useMutate();
   const base = dataset.settings.baseCurrency;
   const apiUrl = dataset.settings.fxApiUrl ?? DEFAULT_FX_API_URL;
   const [status, setStatus] = useState<string | null>(null);
@@ -32,9 +32,13 @@ export function CurrencySection() {
     setStatus("Fetching…");
     try {
       const rates = await fetchFxRates(apiUrl, base, targets);
-      mutate((draft) => {
+      const saved = await mutate((draft) => {
         for (const rate of rates) setFxRate(draft, rate);
       });
+      if (!saved) {
+        setStatus(null);
+        return;
+      }
       const missing = targets.filter((t) => !rates.some((r) => r.currency === t));
       setStatus(
         `Updated ${rates.length} rate(s).` +
@@ -67,7 +71,11 @@ export function CurrencySection() {
       title="Currencies"
       hint="Add any currency you spend in. The code is how it is identified, so it cannot be changed later — and everything is converted to the base currency for totals."
     >
-      {error && <p className="mb-3 text-sm text-destructive">{error}</p>}
+      {(error || writeError) && (
+        <p role="alert" className="mb-3 text-sm text-destructive">
+          {error ?? `Could not save: ${writeError}`}
+        </p>
+      )}
 
       <div className="mb-5 flex flex-wrap items-end gap-x-8 gap-y-4">
         <div className="space-y-1.5">
diff --git a/src/ui/routes/settings/PostsSection.test.tsx b/src/ui/routes/settings/PostsSection.test.tsx
new file mode 100644
index 0000000..c1c01ac
--- /dev/null
+++ b/src/ui/routes/settings/PostsSection.test.tsx
@@ -0,0 +1,65 @@
+import { GlobalRegistrator } from "@happy-dom/global-registrator";
+if (!GlobalRegistrator.isRegistered) GlobalRegistrator.register();
+(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+
+import { afterEach, expect, test } from "bun:test";
+import { act } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { createSeedDataset } from "../../../domain/seed.ts";
+import { store } from "../../../store/index.ts";
+import { PostsSection } from "./PostsSection.tsx";
+
+const originalGet = store.get;
+const originalMutate = store.mutate;
+let root: Root | null = null;
+let container: HTMLDivElement | null = null;
+
+afterEach(() => {
+  store.get = originalGet;
+  store.mutate = originalMutate;
+  if (root) act(() => root!.unmount());
+  root = null;
+  container?.remove();
+  container = null;
+});
+
+test("a failed post archive explains why the post stayed active", async () => {
+  const dataset = createSeedDataset("2026-01");
+  store.get = () => dataset;
+  store.mutate = async () => { throw new Error("Disk full"); };
+  container = document.createElement("div");
+  document.body.append(container);
+  root = createRoot(container);
+  act(() => root!.render(<PostsSection />));
+
+  await act(async () => {
+    Array.from(container!.querySelectorAll("button"))
+      .find((button) => button.textContent === "archive")!.click();
+  });
+
+  expect(container.querySelector('[role="alert"]')?.textContent).toContain("Disk full");
+  expect(dataset.posts[0]!.archived).toBe(false);
+});
+
+test("a successful retry clears the previous save error", async () => {
+  const dataset = createSeedDataset("2026-01");
+  store.get = () => dataset;
+  let fail = true;
+  store.mutate = async (change) => {
+    if (fail) throw new Error("Disk full");
+    change(dataset);
+  };
+  container = document.createElement("div");
+  document.body.append(container);
+  root = createRoot(container);
+  act(() => root!.render(<PostsSection />));
+  const archive = Array.from(container.querySelectorAll("button"))
+    .find((button) => button.textContent === "archive")!;
+
+  await act(async () => { archive.click(); });
+  expect(container.querySelector('[role="alert"]')).not.toBeNull();
+  fail = false;
+  await act(async () => { archive.click(); });
+
+  expect(container.querySelector('[role="alert"]')).toBeNull();
+});
diff --git a/src/ui/routes/settings/PostsSection.tsx b/src/ui/routes/settings/PostsSection.tsx
index 9d06856..4b94af3 100644
--- a/src/ui/routes/settings/PostsSection.tsx
+++ b/src/ui/routes/settings/PostsSection.tsx
@@ -13,7 +13,7 @@ import { Section } from "../../components/Section.tsx";
 
 export function PostsSection() {
   const dataset = useDataset();
-  const { mutate } = useMutate();
+  const { mutate, error } = useMutate();
   const [newName, setNewName] = useState("");
   const [openHistory, setOpenHistory] = useState<string | null>(null);
   const ordered = [...dataset.posts].sort((a, b) => a.order - b.order);
@@ -40,7 +40,9 @@ export function PostsSection() {
       title="Posts"
       hint="A post's allocation is a dated series: each rule applies from its own month onward until the next one takes over, so changing it never rewrites what earlier months got. Click a rule to see and edit that history. A single month can still override its own allocation from the month view, and percentages may total more than 100%."
     >
-      <table className="w-full text-sm">
+      {error && <p role="alert" className="mb-3 text-sm text-destructive">Could not save: {error}</p>}
+      <div className="overflow-x-auto">
+      <table className="w-full min-w-[36rem] text-sm">
         <thead className="text-left">
           <tr className="border-b border-budget-rule text-[0.6875rem] uppercase tracking-wider text-budget-ink-muted">
             <th className="py-2 font-medium">Name</th>
@@ -130,6 +132,7 @@ export function PostsSection() {
           ))}
         </tbody>
       </table>
+      </div>
       <p className="mt-3 max-w-[70ch] text-xs leading-relaxed text-budget-ink-muted">
         Archiving hides a post from new purchases while keeping its history. Posts
         are never deleted, because purchases reference them.
@@ -146,8 +149,10 @@ export function PostsSection() {
           variant="outline"
           disabled={newName.trim() === ""}
           onClick={() => {
-            mutate((draft) => addPost(draft, newName.trim(), base));
-            setNewName("");
+            const name = newName.trim();
+            void mutate((draft) => addPost(draft, name, base)).then((saved) => {
+              if (saved) setNewName("");
+            });
           }}
         >
           Add post
diff --git a/src/ui/routes/settings/RecurringSection.tsx b/src/ui/routes/settings/RecurringSection.tsx
index 1b2fb0d..13dbe6a 100644
--- a/src/ui/routes/settings/RecurringSection.tsx
+++ b/src/ui/routes/settings/RecurringSection.tsx
@@ -95,7 +95,7 @@ export function normalizedStartDateFor(kind: Recurrence["kind"], startDate: IsoD
 
 export function RecurringSection() {
   const dataset = useDataset();
-  const { mutate } = useMutate();
+  const { mutate, error } = useMutate();
   const [newName, setNewName] = useState("");
   const ordered = [...dataset.recurring].sort((a, b) => a.order - b.order);
   const base = dataset.settings.baseCurrency;
@@ -202,6 +202,7 @@ export function RecurringSection() {
       title="Recurring costs"
       hint="Bills that repeat. Each one projects forward as EXPECTED spending and is confirmed from the month view when the money actually moves — expected never touches a post's real balance. Anchoring decides where the next charge is measured from: the calendar, or the date the last one actually charged. Use the second for a bill that can charge early, like a phone plan that recharges when its data runs out."
     >
+      {error && <p role="alert" className="mb-3 text-sm text-destructive">Could not save: {error}</p>}
       {ordered.length === 0 ? (
         <p className="text-sm text-budget-ink-muted">
           No recurring costs yet. Add rent, a subscription, or a phone bill below.
@@ -369,7 +370,7 @@ export function RecurringSection() {
           disabled={newName.trim() === "" || !firstPost}
           onClick={() => {
             const name = newName.trim();
-            mutate((draft) =>
+            void mutate((draft) =>
               addRecurringCost(draft, {
                 name,
                 archived: false,
@@ -380,8 +381,9 @@ export function RecurringSection() {
                 splitMode: "percent",
                 splits: [{ postId: firstPost!.id, value: 100, absorbsRemainder: true }],
               }),
-            );
-            setNewName("");
+            ).then((saved) => {
+              if (saved) setNewName("");
+            });
           }}
         >
           Add recurring cost