diff --git a/app/api/chat/route.ts b/app/api/chat/route.ts
index f98583f..1ab47a5 100644
--- a/app/api/chat/route.ts
+++ b/app/api/chat/route.ts
@@ -5,6 +5,7 @@ import {
   createStreamingResponse,
 } from "@/lib/streaming";
 import { SANDBOX_TIMEOUT_MS } from "@/lib/config";
+import { getSandboxResolution } from "@/lib/resolution";
 import { OpenAIComputerStreamer } from "@/lib/streaming/openai";
 import { logError } from "@/lib/logger";
 
@@ -24,6 +25,11 @@ export async function POST(request: Request) {
     resolution,
   } = await request.json();
 
+  // Never trust the client-provided resolution as-is: it is derived from the
+  // browser viewport, and small breakpoints (e.g. mobile) would create a
+  // desktop too small for the agent to operate reliably
+  const sandboxResolution = getSandboxResolution(resolution);
+
   const apiKey = process.env.E2B_API_KEY;
 
   if (!apiKey) {
@@ -37,7 +43,7 @@ export async function POST(request: Request) {
   try {
     if (!activeSandboxId) {
       const newSandbox = await Sandbox.create({
-        resolution,
+        resolution: sandboxResolution,
         dpi: 96,
         timeoutMs: SANDBOX_TIMEOUT_MS,
       });
@@ -59,7 +65,7 @@ export async function POST(request: Request) {
 
     try {
       const streamer: ComputerInteractionStreamerFacade =
-        new OpenAIComputerStreamer(desktop, resolution);
+        new OpenAIComputerStreamer(desktop, sandboxResolution);
 
       if (!sandboxId && activeSandboxId && vncUrl) {
         async function* stream(): AsyncGenerator<SSEEvent> {
diff --git a/app/page.tsx b/app/page.tsx
index 38e955b..a06fd1d 100644
--- a/app/page.tsx
+++ b/app/page.tsx
@@ -25,6 +25,7 @@ import Link from "next/link";
 import Logo from "@/components/logo";
 import { RepoBanner } from "@/components/repo-banner";
 import { SANDBOX_TIMEOUT_MS } from "@/lib/config";
+import { getSandboxResolution } from "@/lib/resolution";
 import { Surfing } from "@/components/surfing";
 
 export default function Home() {
@@ -100,40 +101,32 @@ export default function Home() {
     }
   };
 
+  const resolveSandboxResolution = (): [number, number] => {
+    const wrapper = iFrameWrapperRef.current;
+
+    return getSandboxResolution(
+      wrapper ? [wrapper.clientWidth, wrapper.clientHeight] : undefined
+    );
+  };
+
   const onSubmit = (e: React.FormEvent) => {
     const content = handleSubmit(e);
     if (content) {
-      const width =
-        iFrameWrapperRef.current?.clientWidth ||
-        (window.innerWidth < 768 ? window.innerWidth - 32 : 1024);
-      const height =
-        iFrameWrapperRef.current?.clientHeight ||
-        (window.innerWidth < 768
-          ? Math.min(window.innerHeight * 0.4, 400)
-          : 768);
-
       sendMessage({
         content,
         sandboxId: sandboxId || undefined,
         environment: "linux",
-        resolution: [width, height],
+        resolution: resolveSandboxResolution(),
       });
     }
   };
 
   const handleExampleClick = (prompt: string) => {
-    const width =
-      iFrameWrapperRef.current?.clientWidth ||
-      (window.innerWidth < 768 ? window.innerWidth - 32 : 1024);
-    const height =
-      iFrameWrapperRef.current?.clientHeight ||
-      (window.innerWidth < 768 ? Math.min(window.innerHeight * 0.4, 400) : 768);
-
     sendMessage({
       content: prompt,
       sandboxId: sandboxId || undefined,
       environment: "linux",
-      resolution: [width, height],
+      resolution: resolveSandboxResolution(),
     });
   };
 
diff --git a/lib/resolution.ts b/lib/resolution.ts
new file mode 100644
index 0000000..4f70a3e
--- /dev/null
+++ b/lib/resolution.ts
@@ -0,0 +1,54 @@
+import {
+  MAX_RESOLUTION_WIDTH,
+  MAX_RESOLUTION_HEIGHT,
+  MIN_RESOLUTION_WIDTH,
+  MIN_RESOLUTION_HEIGHT,
+  DEFAULT_RESOLUTION,
+} from "@/lib/config";
+
+/**
+ * Derives a sane sandbox resolution from a measured viewport size.
+ *
+ * The sandbox resolution must stay decoupled from browser breakpoints:
+ * Linux desktops (and the agent operating them) are not usable at mobile
+ * viewport sizes. Large viewports are scaled down proportionally to fit the
+ * maximum bounds, while viewports below the minimum bounds fall back to
+ * DEFAULT_RESOLUTION entirely - the VNC stream scales the display down to
+ * fit the container instead (noVNC "resize=scale" preserves aspect ratio
+ * and maps pointer coordinates itself).
+ *
+ * @param measured - The measured container size [width, height], if any
+ * @returns A resolution within the configured min/max bounds
+ */
+export function getSandboxResolution(
+  measured?: [number, number]
+): [number, number] {
+  if (
+    !measured ||
+    !Number.isFinite(measured[0]) ||
+    !Number.isFinite(measured[1]) ||
+    measured[0] <= 0 ||
+    measured[1] <= 0
+  ) {
+    return DEFAULT_RESOLUTION;
+  }
+
+  // Scale down proportionally so both dimensions fit the maximum bounds
+  const scaleFactor = Math.min(
+    MAX_RESOLUTION_WIDTH / measured[0],
+    MAX_RESOLUTION_HEIGHT / measured[1],
+    1
+  );
+
+  const width = Math.round(measured[0] * scaleFactor);
+  const height = Math.round(measured[1] * scaleFactor);
+
+  // Viewports below the minimum bounds (e.g. mobile breakpoints) would
+  // produce a desktop too small for the agent - use the default resolution
+  // and let the stream scale the display down instead
+  if (width < MIN_RESOLUTION_WIDTH || height < MIN_RESOLUTION_HEIGHT) {
+    return DEFAULT_RESOLUTION;
+  }
+
+  return [width, height];
+}