diff --git a/app/src/components/computer/live-screen.tsx b/app/src/components/computer/live-screen.tsx
index cd9e1784f..0a05d62aa 100644
--- a/app/src/components/computer/live-screen.tsx
+++ b/app/src/components/computer/live-screen.tsx
@@ -101,6 +101,29 @@ export function LiveScreen({ computerId, driving, onProblem }: Props) {
         return;
       }
       if (message.type !== "frame" || !message.data) return;
+      // Live-run JSON only checks `typeof type === "string"` upstream. Non-finite or negative
+      // dimensions would poison frameSize and every coordinate scaled from it; a huge payload
+      // would hit `atob` before any bound. Both are dropped as corrupt frames.
+      const width = message.width ?? 1280;
+      const height = message.height ?? 800;
+      if (
+        typeof width !== "number" ||
+        typeof height !== "number" ||
+        !Number.isFinite(width) ||
+        !Number.isFinite(height) ||
+        width <= 0 ||
+        height <= 0 ||
+        width > 8192 ||
+        height > 8192
+      ) {
+        return;
+      }
+      if (
+        typeof message.data !== "string" ||
+        message.data.length > 20_000_000
+      ) {
+        return;
+      }
 
       const canvas = canvasRef.current;
       if (!canvas || closed) return;
diff --git a/app/src/lib/client.ts b/app/src/lib/client.ts
index 2abae4c40..2e92b282b 100644
--- a/app/src/lib/client.ts
+++ b/app/src/lib/client.ts
@@ -98,5 +98,11 @@ export async function client<T>(
 
   if (key === undefined) return response;
 
-  return ((await response.json()) as Record<string, T>)[key];
+  // A 204 or a non-JSON body used to throw a bare SyntaxError, and `null` or an array body threw
+  // a TypeError on property access. Malformed success is a failed request with the fallback.
+  const body = (await response.json().catch(() => null)) as unknown;
+  if (!body || typeof body !== "object" || Array.isArray(body)) {
+    throw new Error(options.fallback ?? "That request failed.");
+  }
+  return (body as Record<string, T>)[key];
 }
diff --git a/app/src/lib/computers/control.ts b/app/src/lib/computers/control.ts
index eddca0965..686c2547d 100644
--- a/app/src/lib/computers/control.ts
+++ b/app/src/lib/computers/control.ts
@@ -30,7 +30,13 @@ async function callControl(
     method ? { method } : {},
   );
   if (!response.ok) return null;
-  return (await response.json()) as ControlState;
+  // A non-JSON or wrong-shaped body used to throw out of the readers and reject the panel's
+  // poll. Reads answer null on failure, so malformed succeeds as missing.
+  const body = (await response.json().catch(() => null)) as unknown;
+  if (!body || typeof body !== "object" || Array.isArray(body)) return null;
+  const holder = (body as { holder?: unknown }).holder;
+  if (holder !== "bot" && holder !== "human") return null;
+  return body as ControlState;
 }
 
 export function readControl(computerId: string) {
diff --git a/app/src/lib/computers/screen.ts b/app/src/lib/computers/screen.ts
index df3025e20..d52608af9 100644
--- a/app/src/lib/computers/screen.ts
+++ b/app/src/lib/computers/screen.ts
@@ -35,12 +35,45 @@ export async function readScreenshot(
       } | null;
       return { error: body?.error ?? unavailable };
     }
-    return { frame: (await response.json()) as Screenshot };
+    return parseScreenshot(await response.json().catch(() => null));
   } catch {
     return { error: unavailable };
   }
 }
 
+function parseScreenshot(body: unknown): {
+  frame?: Screenshot;
+  error?: string;
+} {
+  const unavailable = "The screen is not available right now.";
+  if (!body || typeof body !== "object" || Array.isArray(body)) {
+    return { error: unavailable };
+  }
+  const frame = (body as { frame?: unknown }).frame ?? body;
+  if (!frame || typeof frame !== "object" || Array.isArray(frame)) {
+    return { error: unavailable };
+  }
+  const { base64, width, height } = frame as {
+    base64?: unknown;
+    width?: unknown;
+    height?: unknown;
+  };
+  // A mistyped frame used to reach `atob` in the viewer and throw there. Refused here instead.
+  if (
+    typeof base64 !== "string" ||
+    !base64 ||
+    typeof width !== "number" ||
+    !Number.isFinite(width) ||
+    width <= 0 ||
+    typeof height !== "number" ||
+    !Number.isFinite(height) ||
+    height <= 0
+  ) {
+    return { error: unavailable };
+  }
+  return { frame: frame as Screenshot };
+}
+
 /** The frame a page was showing when a Bot opened it. */
 export type PageFrame = { url: string; title: string | null; frame: string };
 
@@ -61,8 +94,15 @@ export async function readPageFrame(
       `/api/computers/${computerId}/page-frame/${encodeURIComponent(toolCallId)}`,
     );
     if (!response.ok) return null;
-    const body = (await response.json()) as { frame?: PageFrame | null };
-    return body.frame ?? null;
+    const body = (await response.json().catch(() => null)) as unknown;
+    if (!body || typeof body !== "object" || Array.isArray(body)) return null;
+    const frame = (body as { frame?: unknown }).frame;
+    if (!frame || typeof frame !== "object" || Array.isArray(frame)) {
+      return null;
+    }
+    const { frame: image } = frame as { frame?: unknown };
+    if (typeof image !== "string" || !image) return null;
+    return frame as PageFrame;
   } catch {
     // A missing picture is a smaller sentence, not a broken conversation.
     return null;
diff --git a/app/src/lib/relative-time.ts b/app/src/lib/relative-time.ts
index d05c076dd..f9c17d3f3 100644
--- a/app/src/lib/relative-time.ts
+++ b/app/src/lib/relative-time.ts
@@ -12,7 +12,11 @@ const relativeFormat = new Intl.RelativeTimeFormat(undefined, {
 
 /** Locale-aware relative timestamp, e.g. "2 minutes ago". */
 export function relativeTime(iso: string): string {
-  const elapsed = Date.now() - new Date(iso).getTime();
+  const time = new Date(iso).getTime();
+  // An invalid date used to flow into `Math.abs(NaN)` comparisons and `format(-NaN)`, which
+  // answers "NaN weeks ago" or throws depending on ICU. Return the input unchanged instead.
+  if (!Number.isFinite(time)) return iso;
+  const elapsed = Date.now() - time;
   const scale =
     RELATIVE_UNITS.find(({ limit }) => Math.abs(elapsed) < limit) ??
     RELATIVE_UNITS[RELATIVE_UNITS.length - 1];
diff --git a/app/src/lib/socket-url.ts b/app/src/lib/socket-url.ts
index 0822c0fb8..5d0d47db5 100644
--- a/app/src/lib/socket-url.ts
+++ b/app/src/lib/socket-url.ts
@@ -27,6 +27,16 @@ function announcedPort(): string {
     : "";
 }
 
+function validPort(port: string): string {
+  // A whitespace, alphabetic, or out-of-range value used to be interpolated into the authority,
+  // so `new WebSocket()` threw synchronously inside the effects that open it. Only whole digits
+  // in range override same-origin.
+  if (!/^\d+$/.test(port.trim())) return "";
+  const n = Number(port.trim());
+  if (!Number.isInteger(n) || n < 1 || n > 65535) return "";
+  return String(n);
+}
+
 export function socketUrl(
   path: string,
   location: {
@@ -39,6 +49,7 @@ export function socketUrl(
   const scheme = location.protocol === "https:" ? "wss:" : "ws:";
   // A port only when a Vite runtime named one: the app is not same-origin with the server there.
   // Otherwise the browser's own host, which is the server's own origin in production.
-  const authority = port ? `${location.hostname}:${port}` : location.host;
+  const usable = validPort(port);
+  const authority = usable ? `${location.hostname}:${usable}` : location.host;
   return `${scheme}//${authority}${path}`;
 }
diff --git a/app/tests/app-resilience-guards.test.ts b/app/tests/app-resilience-guards.test.ts
new file mode 100644
index 000000000..d1df29a2f
--- /dev/null
+++ b/app/tests/app-resilience-guards.test.ts
@@ -0,0 +1,100 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import { socketUrl } from "../src/lib/socket-url";
+import { relativeTime } from "../src/lib/relative-time";
+
+const at = (protocol: string, hostname: string, host: string) => ({
+  protocol,
+  hostname,
+  host,
+});
+
+/**
+ * `__OPENBOT_WS_PORT__` was interpolated into the WebSocket authority unchecked, so `"   "`,
+ * `"abc"` or `"99999"` produced `ws://host:abc/path` and `new WebSocket()` threw synchronously
+ * inside the effects that open it. Only whole digits in range override same-origin now.
+ */
+describe("socketUrl port validation", () => {
+  test.each([
+    ["whitespace", "   "],
+    ["letters", "abc"],
+    ["too large", "99999"],
+    ["zero", "0"],
+  ])("falls back to same-origin on %s", (_n, port) => {
+    expect(
+      socketUrl("/api/channels/events", at("http:", "h", "h:3010"), port),
+    ).toBe("ws://h:3010/api/channels/events");
+  });
+
+  test("keeps a valid override", () => {
+    expect(
+      socketUrl("/api/channels/events", at("http:", "h", "h:3010"), "3001"),
+    ).toBe("ws://h:3001/api/channels/events");
+  });
+});
+
+/**
+ * An invalid date used to flow into `Math.abs(NaN)` comparisons and `format(-NaN)`, answering
+ * "NaN weeks ago" or throwing depending on ICU. The input is returned unchanged instead.
+ */
+describe("relativeTime", () => {
+  test("returns the input for an invalid date", () => {
+    expect(relativeTime("not-a-date")).toBe("not-a-date");
+  });
+
+  test("still formats a valid date", () => {
+    const iso = new Date(Date.now() - 30_000).toISOString();
+    expect(relativeTime(iso)).toMatch(/second/);
+  });
+});
+
+describe("control/screen/client guards", () => {
+  const realFetch = globalThis.fetch;
+
+  afterEach(() => {
+    globalThis.fetch = realFetch;
+  });
+
+  test("readControl answers null on a malformed body", async () => {
+    globalThis.fetch = (async () =>
+      new Response(JSON.stringify([1, 2]), {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      })) as typeof fetch;
+    const { readControl } = await import("../src/lib/computers/control");
+    await expect(readControl("bot-1")).resolves.toBeNull();
+  });
+
+  test("client throws the fallback on a malformed envelope", async () => {
+    globalThis.fetch = (async () =>
+      new Response("null", {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      })) as typeof fetch;
+    const { client } = await import("../src/lib/client");
+    await expect(
+      client("http://x.test/thing", "thing", { fallback: "Nope." }),
+    ).rejects.toThrow("Nope.");
+  });
+
+  test("readScreenshot reports unavailable on a mistyped frame", async () => {
+    globalThis.fetch = (async () =>
+      new Response(JSON.stringify({ base64: 42, width: "x", height: 1 }), {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      })) as typeof fetch;
+    const { readScreenshot } = await import("../src/lib/computers/screen");
+    const result = await readScreenshot("bot-1");
+    expect(result.frame).toBeUndefined();
+    expect(typeof result.error).toBe("string");
+  });
+
+  test("readPageFrame answers null on an array frame", async () => {
+    globalThis.fetch = (async () =>
+      new Response(JSON.stringify({ frame: [] }), {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      })) as typeof fetch;
+    const { readPageFrame } = await import("../src/lib/computers/screen");
+    await expect(readPageFrame("bot-1", "turn-1")).resolves.toBeNull();
+  });
+});