diff --git a/apps/mobile/src/DateTimeEditor.tsx b/apps/mobile/src/DateTimeEditor.tsx
index 92e78ee7a..898389aab 100644
--- a/apps/mobile/src/DateTimeEditor.tsx
+++ b/apps/mobile/src/DateTimeEditor.tsx
@@ -3,6 +3,13 @@ import { Text, View } from "react-native";
 import DateFields from "./DateFields";
 import { isCompleteInstant, localDateTime, zonedInstant } from "./date-time";
 import { colors, s } from "./ui";
+
+// An unknown time zone surfaces as Intl's RangeError; the other errors are already readable.
+function editorError(e: unknown) {
+  if (e instanceof RangeError) return "Choose a valid time zone.";
+  return e instanceof Error ? e.message : String(e);
+}
+
 export default function DateTimeEditor({
   label,
   value,
@@ -23,13 +30,19 @@ export default function DateTimeEditor({
     try {
       if (allDay) {
         setDate(value.slice(0, 10));
+        setError("");
       } else if (isCompleteInstant(value)) {
         const local = localDateTime(value, timeZone);
         setDate(local.date);
         setTime(local.time);
+        setError("");
+      } else if (value === `${date} ${time}`) {
+        // A date or time entered while the zone was invalid; convert it now the zone changed.
+        onChange(zonedInstant(date, time, timeZone));
+        setError("");
       }
-    } catch {
-      setError("Choose a valid time zone.");
+    } catch (e) {
+      setError(editorError(e));
     }
   }, [value, timeZone, allDay]);
   function change(nextDate: string, nextTime: string) {
@@ -44,7 +57,7 @@ export default function DateTimeEditor({
       onChange(zonedInstant(nextDate, nextTime, timeZone));
     } catch (e) {
       onChange(`${nextDate} ${nextTime}`);
-      setError(e instanceof Error ? e.message : String(e));
+      setError(editorError(e));
     }
   }
   return (
diff --git a/apps/mobile/src/api-response.ts b/apps/mobile/src/api-response.ts
index d6e8b017d..d9a9a3e3b 100644
--- a/apps/mobile/src/api-response.ts
+++ b/apps/mobile/src/api-response.ts
@@ -22,3 +22,14 @@ export async function readApiPayload<T>(response: Response, fallback?: string):
     throw new Error(`The server returned an unreadable response (${response.status}).`);
   return payload as T;
 }
+
+/**
+ * Same as readApiPayload, for the native upload and download helpers, which
+ * return the status and body text instead of a Response.
+ */
+export function readNativePayload<T>(
+  result: { status: number; body: string },
+  fallback?: string,
+): Promise<T> {
+  return readApiPayload<T>(new Response(result.body, { status: result.status }), fallback);
+}
diff --git a/apps/mobile/src/details.tsx b/apps/mobile/src/details.tsx
index 8751ff921..e7c9907a7 100644
--- a/apps/mobile/src/details.tsx
+++ b/apps/mobile/src/details.tsx
@@ -33,6 +33,7 @@ import {
   type ProposalInput,
 } from "../../../packages/domain/src";
 import { DelegateSheet, NotificationsSheet, TaskDetail } from "./agent-ui";
+import { readNativePayload } from "./api-response";
 import BrowserConsole from "./BrowserConsole";
 import { browserAddress, browserSite } from "./browser-address";
 import { ComputerSheet } from "./computer";
@@ -777,9 +778,17 @@ function FileDetail({ file: f }: { file: Artifact }) {
         return;
       }
       const target = `${FileSystem.cacheDirectory}${f.id}.pdf`;
-      await FileSystem.downloadAsync(url, target, {
+      const download = await FileSystem.downloadAsync(url, target, {
         headers: { Authorization: `Bearer ${api.token}` },
       });
+      if (download.status < 200 || download.status >= 300) {
+        // downloadAsync saves an error response to the file instead of failing, so read
+        // the server's message from it and never offer that file as a PDF.
+        const body = await FileSystem.readAsStringAsync(target).catch(() => "");
+        await FileSystem.deleteAsync(target, { idempotent: true });
+        // Always throws here, with the server's message when it sent one.
+        await readNativePayload({ status: download.status, body }, "Could not download this PDF.");
+      }
       if (await Sharing.isAvailableAsync())
         await Sharing.shareAsync(target, { mimeType: "application/pdf", UTI: "com.adobe.pdf" });
       else throw new Error("Sharing is not available on this device.");
diff --git a/apps/mobile/src/screens.tsx b/apps/mobile/src/screens.tsx
index 5f2345a84..5d56f3921 100644
--- a/apps/mobile/src/screens.tsx
+++ b/apps/mobile/src/screens.tsx
@@ -39,6 +39,7 @@ import type {
   EmailDraft,
 } from "../../../packages/domain/src";
 import { API_URL } from "./api";
+import { readNativePayload } from "./api-response";
 import { browserAddress } from "./browser-address";
 import { localDateTime, startOfZonedDay, zonedInstant } from "./date-time";
 import {
@@ -964,10 +965,7 @@ export function FilesScreen() {
           mimeType: "application/pdf",
           headers: { Authorization: `Bearer ${api.token}` },
         });
-        const payload = JSON.parse(result.body);
-        if (result.status < 200 || result.status >= 300)
-          throw new Error(payload.error || "Could not import this PDF.");
-        artifact = payload;
+        artifact = await readNativePayload<Artifact>(result, "Could not import this PDF.");
       }
       await refresh();
       open({ type: "file", file: artifact });
diff --git a/apps/mobile/test/api-response.test.ts b/apps/mobile/test/api-response.test.ts
index cd5ebb24f..63e311dcd 100644
--- a/apps/mobile/test/api-response.test.ts
+++ b/apps/mobile/test/api-response.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert/strict";
 import test from "node:test";
-import { readApiPayload } from "../src/api-response.ts";
+import { readApiPayload, readNativePayload } from "../src/api-response.ts";
 
 test("JSON responses pass through and server errors keep their message", async () => {
   assert.deepEqual(
@@ -52,3 +52,26 @@ test("successful responses without a readable body fail clearly", async () => {
   );
   await assert.rejects(readApiPayload(new Response("OK", { status: 200 })), /unreadable response/);
 });
+
+test("native upload and download results report the real failure", async () => {
+  assert.deepEqual(
+    await readNativePayload({ status: 201, body: JSON.stringify({ id: "file-1" }) }),
+    { id: "file-1" },
+  );
+  await assert.rejects(
+    readNativePayload(
+      { status: 413, body: JSON.stringify({ error: "PDF is too large" }) },
+      "Could not import this PDF.",
+    ),
+    /PDF is too large/,
+  );
+  for (const body of ["<html><body>Bad Gateway</body></html>", ""]) {
+    const error = await readNativePayload(
+      { status: 502, body },
+      "Could not import this PDF.",
+    ).catch((e: unknown) => e);
+    assert.ok(error instanceof Error);
+    assert.equal(error.message, "Could not import this PDF.");
+    assert.equal(error instanceof SyntaxError, false);
+  }
+});