diff --git a/extensions/browser/src/browser-tool.schema.test.ts b/extensions/browser/src/browser-tool.schema.test.ts
new file mode 100644
index 00000000000..9ba9abad8f4
--- /dev/null
+++ b/extensions/browser/src/browser-tool.schema.test.ts
@@ -0,0 +1,17 @@
+import { describe, expect, it } from "vitest";
+import { ACT_MAX_VIEWPORT_DIMENSION } from "./browser/act-policy.js";
+import { BrowserToolSchema } from "./browser-tool.schema.js";
+
+type SchemaRecord = Record<string, { maximum?: number; properties?: SchemaRecord }>;
+
+describe("browser tool schema", () => {
+  it("advertises the viewport resize maximum on nested and flattened act params", () => {
+    const properties = BrowserToolSchema.properties as SchemaRecord;
+    const requestProperties = properties.request.properties ?? {};
+
+    expect(properties.width.maximum).toBe(ACT_MAX_VIEWPORT_DIMENSION);
+    expect(properties.height.maximum).toBe(ACT_MAX_VIEWPORT_DIMENSION);
+    expect(requestProperties.width.maximum).toBe(ACT_MAX_VIEWPORT_DIMENSION);
+    expect(requestProperties.height.maximum).toBe(ACT_MAX_VIEWPORT_DIMENSION);
+  });
+});
diff --git a/extensions/browser/src/browser-tool.schema.ts b/extensions/browser/src/browser-tool.schema.ts
index 7c6da546e8b..7c42978857e 100644
--- a/extensions/browser/src/browser-tool.schema.ts
+++ b/extensions/browser/src/browser-tool.schema.ts
@@ -6,6 +6,7 @@ import {
   stringEnum,
 } from "openclaw/plugin-sdk/channel-actions";
 import { Type } from "typebox";
+import { ACT_MAX_VIEWPORT_DIMENSION } from "./browser/act-policy.js";
 
 const BROWSER_ACT_KINDS = [
   "click",
@@ -79,8 +80,8 @@ const BrowserActSchema = Type.Object({
   // fill - use permissive array of objects
   fields: Type.Optional(Type.Array(Type.Object({}, { additionalProperties: true }))),
   // resize
-  width: optionalPositiveIntegerSchema(),
-  height: optionalPositiveIntegerSchema(),
+  width: optionalPositiveIntegerSchema({ maximum: ACT_MAX_VIEWPORT_DIMENSION }),
+  height: optionalPositiveIntegerSchema({ maximum: ACT_MAX_VIEWPORT_DIMENSION }),
   // wait
   timeMs: optionalNonNegativeIntegerSchema(),
   selector: Type.Optional(Type.String()),
@@ -143,8 +144,8 @@ export const BrowserToolSchema = Type.Object({
   endRef: Type.Optional(Type.String()),
   values: Type.Optional(Type.Array(Type.String())),
   fields: Type.Optional(Type.Array(Type.Object({}, { additionalProperties: true }))),
-  width: optionalPositiveIntegerSchema(),
-  height: optionalPositiveIntegerSchema(),
+  width: optionalPositiveIntegerSchema({ maximum: ACT_MAX_VIEWPORT_DIMENSION }),
+  height: optionalPositiveIntegerSchema({ maximum: ACT_MAX_VIEWPORT_DIMENSION }),
   timeMs: optionalNonNegativeIntegerSchema(),
   textGone: Type.Optional(Type.String()),
   loadState: Type.Optional(Type.String()),
diff --git a/extensions/browser/src/browser/act-policy.ts b/extensions/browser/src/browser/act-policy.ts
index 9407858beb4..b372d2ff956 100644
--- a/extensions/browser/src/browser/act-policy.ts
+++ b/extensions/browser/src/browser/act-policy.ts
@@ -2,6 +2,7 @@ export const ACT_MAX_BATCH_ACTIONS = 100;
 export const ACT_MAX_BATCH_DEPTH = 5;
 export const ACT_MAX_CLICK_DELAY_MS = 5_000;
 export const ACT_MAX_WAIT_TIME_MS = 30_000;
+export const ACT_MAX_VIEWPORT_DIMENSION = 8192;
 
 const ACT_MIN_TIMEOUT_MS = 500;
 const ACT_MAX_INTERACTION_TIMEOUT_MS = 60_000;
diff --git a/extensions/browser/src/browser/pw-tools-core.snapshot.test.ts b/extensions/browser/src/browser/pw-tools-core.snapshot.test.ts
index c404f8a8f2e..1e50c26fe51 100644
--- a/extensions/browser/src/browser/pw-tools-core.snapshot.test.ts
+++ b/extensions/browser/src/browser/pw-tools-core.snapshot.test.ts
@@ -218,6 +218,23 @@ describe("pw-tools-core aria snapshot storage", () => {
     expect(page.setViewportSize).toHaveBeenCalledWith({ width: 1, height: 1 });
   });
 
+  it("rejects excessive viewport dimensions before calling Playwright", async () => {
+    const page = { setViewportSize: vi.fn(async () => {}) };
+    getPageForTargetId.mockResolvedValue(page);
+
+    const mod = await import("./pw-tools-core.snapshot.js");
+    await expect(
+      mod.resizeViewportViaPlaywright({
+        cdpUrl: "http://127.0.0.1:9222",
+        targetId: "tab-1",
+        width: Number.MAX_SAFE_INTEGER,
+        height: 768,
+      }),
+    ).rejects.toThrow("viewport width exceeds maximum of 8192");
+
+    expect(page.setViewportSize).not.toHaveBeenCalled();
+  });
+
   it("stores role fallback metadata when backend markers are unavailable", async () => {
     const page = { id: "page-1" };
     const mod = await import("./pw-tools-core.snapshot.js");
diff --git a/extensions/browser/src/browser/pw-tools-core.snapshot.ts b/extensions/browser/src/browser/pw-tools-core.snapshot.ts
index ce8eae5ae8d..90af131be91 100644
--- a/extensions/browser/src/browser/pw-tools-core.snapshot.ts
+++ b/extensions/browser/src/browser/pw-tools-core.snapshot.ts
@@ -4,6 +4,7 @@ import {
   normalizeOptionalString,
 } from "openclaw/plugin-sdk/string-coerce-runtime";
 import type { Page } from "playwright-core";
+import { ACT_MAX_VIEWPORT_DIMENSION } from "./act-policy.js";
 import type { SsrFPolicy } from "../infra/net/ssrf.js";
 import { type AriaSnapshotNode, formatAriaSnapshot, type RawAXNode } from "./cdp.js";
 import {
@@ -53,6 +54,14 @@ function resolveNavigationTimeoutMs(timeoutMs: number | undefined): number {
   return resolveBoundedTimeoutMs(timeoutMs, 20_000, 1000, 120_000);
 }
 
+function resolveViewportDimension(value: unknown, label: "width" | "height"): number {
+  const dimension = resolveIntegerOption(value, 1, { min: 1 });
+  if (dimension > ACT_MAX_VIEWPORT_DIMENSION) {
+    throw new Error(`viewport ${label} exceeds maximum of ${ACT_MAX_VIEWPORT_DIMENSION}`);
+  }
+  return dimension;
+}
+
 async function collectSnapshotUrls(page: Page): Promise<SnapshotUrlEntry[]> {
   const urls = await page
     .evaluate(() => {
@@ -455,8 +464,8 @@ export async function resizeViewportViaPlaywright(opts: {
   const page = await getPageForTargetId(opts);
   ensurePageState(page);
   await page.setViewportSize({
-    width: resolveIntegerOption(opts.width, 1, { min: 1 }),
-    height: resolveIntegerOption(opts.height, 1, { min: 1 }),
+    width: resolveViewportDimension(opts.width, "width"),
+    height: resolveViewportDimension(opts.height, "height"),
   });
 }
 
diff --git a/extensions/browser/src/browser/routes/agent.act.normalize.ts b/extensions/browser/src/browser/routes/agent.act.normalize.ts
index c0db285026b..97253257c83 100644
--- a/extensions/browser/src/browser/routes/agent.act.normalize.ts
+++ b/extensions/browser/src/browser/routes/agent.act.normalize.ts
@@ -1,6 +1,7 @@
 import {
   ACT_MAX_BATCH_ACTIONS,
   ACT_MAX_CLICK_DELAY_MS,
+  ACT_MAX_VIEWPORT_DIMENSION,
   ACT_MAX_WAIT_TIME_MS,
   normalizeActBoundedNonNegativeMs,
 } from "../act-policy.js";
@@ -261,6 +262,9 @@ export function normalizeActRequest(
       if (width === undefined || height === undefined || width <= 0 || height <= 0) {
         throw new Error("resize requires positive width and height");
       }
+      if (width > ACT_MAX_VIEWPORT_DIMENSION || height > ACT_MAX_VIEWPORT_DIMENSION) {
+        throw new Error(`resize width and height must not exceed ${ACT_MAX_VIEWPORT_DIMENSION}`);
+      }
       const targetId = toStringOrEmpty(body.targetId) || undefined;
       return {
         kind,
diff --git a/extensions/browser/src/browser/server.agent-contract-form-layout-act-commands.test.ts b/extensions/browser/src/browser/server.agent-contract-form-layout-act-commands.test.ts
index e7176d4cee3..eb286f7c7df 100644
--- a/extensions/browser/src/browser/server.agent-contract-form-layout-act-commands.test.ts
+++ b/extensions/browser/src/browser/server.agent-contract-form-layout-act-commands.test.ts
@@ -278,6 +278,15 @@ describe("browser control server", () => {
       expect(resizeNegative.error).toContain("resize requires positive width and height");
       expect(pwMocks.resizeViewportViaPlaywright).toHaveBeenCalledTimes(1);
 
+      const resizeTooLarge = await postJson<{ error?: string; code?: string }>(`${base}/act`, {
+        kind: "resize",
+        width: 8193,
+        height: 600,
+      });
+      expect(resizeTooLarge.code).toBe("ACT_INVALID_REQUEST");
+      expect(resizeTooLarge.error).toContain("resize width and height must not exceed 8192");
+      expect(pwMocks.resizeViewportViaPlaywright).toHaveBeenCalledTimes(1);
+
       const wait = await postJson<{ ok: boolean }>(`${base}/act`, {
         kind: "wait",
         timeMs: 5,
diff --git a/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.test.ts b/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.test.ts
index 8aab4bc71b5..fb01dedc03e 100644
--- a/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.test.ts
+++ b/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.test.ts
@@ -48,4 +48,16 @@ describe("browser navigation commands", () => {
     expect(capture.runtimeErrors.join("\n")).toContain("Invalid width: must be a positive integer");
     expect(mocks.runBrowserResizeWithOutput).not.toHaveBeenCalled();
   });
+
+  it("rejects excessive resize dimensions before dispatch", async () => {
+    const program = createNavigationProgram();
+
+    await expect(
+      program.parseAsync(["browser", "resize", "8193", "768"], { from: "user" }),
+    ).rejects.toThrow("__exit__:1");
+
+    const capture = getBrowserCliRuntimeCapture();
+    expect(capture.runtimeErrors.join("\n")).toContain("Invalid width: maximum is 8192");
+    expect(mocks.runBrowserResizeWithOutput).not.toHaveBeenCalled();
+  });
 });
diff --git a/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.ts b/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.ts
index 1d772c25e90..3e0d9c52a13 100644
--- a/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.ts
+++ b/extensions/browser/src/cli/browser-cli-actions-input/register.navigation.ts
@@ -1,5 +1,6 @@
 import type { Command } from "commander";
 import { normalizeOptionalString } from "openclaw/plugin-sdk/string-coerce-runtime";
+import { ACT_MAX_VIEWPORT_DIMENSION } from "../../browser/act-policy.js";
 import { runBrowserResizeWithOutput } from "../browser-cli-resize.js";
 import { callBrowserRequest, type BrowserParentOpts } from "../browser-cli-shared.js";
 import { danger, defaultRuntime } from "../core-api.js";
@@ -17,6 +18,11 @@ export function registerBrowserNavigationCommands(
       defaultRuntime.exit(1);
       return undefined;
     }
+    if (parsed > ACT_MAX_VIEWPORT_DIMENSION) {
+      defaultRuntime.error(danger(`Invalid ${label}: maximum is ${ACT_MAX_VIEWPORT_DIMENSION}`));
+      defaultRuntime.exit(1);
+      return undefined;
+    }
     return parsed;
   };
 
diff --git a/extensions/browser/src/cli/browser-cli-resize.ts b/extensions/browser/src/cli/browser-cli-resize.ts
index ef07772c182..9b35b7e58b8 100644
--- a/extensions/browser/src/cli/browser-cli-resize.ts
+++ b/extensions/browser/src/cli/browser-cli-resize.ts
@@ -1,3 +1,4 @@
+import { ACT_MAX_VIEWPORT_DIMENSION } from "../browser/act-policy.js";
 import { callBrowserResize, type BrowserParentOpts } from "./browser-cli-shared.js";
 import { danger, defaultRuntime } from "./core-api.js";
 
@@ -16,6 +17,13 @@ export async function runBrowserResizeWithOutput(params: {
     defaultRuntime.exit(1);
     return;
   }
+  if (width > ACT_MAX_VIEWPORT_DIMENSION || height > ACT_MAX_VIEWPORT_DIMENSION) {
+    defaultRuntime.error(
+      danger(`width and height must not exceed ${ACT_MAX_VIEWPORT_DIMENSION}`),
+    );
+    defaultRuntime.exit(1);
+    return;
+  }
 
   const result = await callBrowserResize(
     params.parent,
diff --git a/extensions/browser/src/cli/browser-cli-state.option-collisions.test.ts b/extensions/browser/src/cli/browser-cli-state.option-collisions.test.ts
index ac6034808bd..2b8b96683e4 100644
--- a/extensions/browser/src/cli/browser-cli-state.option-collisions.test.ts
+++ b/extensions/browser/src/cli/browser-cli-state.option-collisions.test.ts
@@ -164,6 +164,14 @@ describe("browser state option collisions", () => {
     expect(getBrowserCliRuntime().exit).toHaveBeenCalledWith(1);
   });
 
+  it("rejects excessive viewport dimensions before resize dispatch", async () => {
+    await runBrowserCommand(["set", "viewport", "8193", "768"]);
+
+    expect(mocks.runBrowserResizeWithOutput).not.toHaveBeenCalled();
+    expectErrorMessage("Invalid width: maximum is 8192");
+    expect(getBrowserCliRuntime().exit).toHaveBeenCalledWith(1);
+  });
+
   it("errors when set media receives an invalid value", async () => {
     await runBrowserCommand(["set", "media", "sepia"]);
 
diff --git a/extensions/browser/src/cli/browser-cli-state.ts b/extensions/browser/src/cli/browser-cli-state.ts
index 82dff79af9e..850194b6de0 100644
--- a/extensions/browser/src/cli/browser-cli-state.ts
+++ b/extensions/browser/src/cli/browser-cli-state.ts
@@ -4,6 +4,7 @@ import {
   normalizeOptionalString,
 } from "openclaw/plugin-sdk/string-coerce-runtime";
 import { runCommandWithRuntime } from "../core-api.js";
+import { ACT_MAX_VIEWPORT_DIMENSION } from "../browser/act-policy.js";
 import { runBrowserResizeWithOutput } from "./browser-cli-resize.js";
 import { callBrowserRequest, type BrowserParentOpts } from "./browser-cli-shared.js";
 import { registerBrowserCookiesAndStorageCommands } from "./browser-cli-state.cookies-storage.js";
@@ -22,6 +23,11 @@ function parsePositiveInteger(value: unknown, label: string): number | undefined
     defaultRuntime.exit(1);
     return undefined;
   }
+  if (parsed > ACT_MAX_VIEWPORT_DIMENSION) {
+    defaultRuntime.error(danger(`Invalid ${label}: maximum is ${ACT_MAX_VIEWPORT_DIMENSION}`));
+    defaultRuntime.exit(1);
+    return undefined;
+  }
   return parsed;
 }