diff --git a/extensions/browser/src/browser/server.control-server.test-harness.ts b/extensions/browser/src/browser/server.control-server.test-harness.ts
index 5f02544f816..310dd7476d4 100644
--- a/extensions/browser/src/browser/server.control-server.test-harness.ts
+++ b/extensions/browser/src/browser/server.control-server.test-harness.ts
@@ -1,4 +1,5 @@
 import { afterEach, beforeEach, vi } from "vitest";
+import { deriveDefaultBrowserCdpPortRange } from "../config/port-defaults.js";
 import type { MockFn } from "../test-utils/vitest-mock-fn.js";
 import { installChromeUserDataDirHooks } from "./chrome-user-data-dir.test-harness.js";
 import { getFreePort } from "./test-port.js";
@@ -375,9 +376,13 @@ function makeProc(pid = 123) {
 
 const proc = makeProc();
 
+function defaultBrowserCdpPortForState(testPort: number): number {
+  return deriveDefaultBrowserCdpPortRange(testPort).start;
+}
+
 function defaultProfilesForState(testPort: number): HarnessState["cfgProfiles"] {
   return {
-    openclaw: { cdpPort: testPort + 9, color: "#FF4500" },
+    openclaw: { cdpPort: defaultBrowserCdpPortForState(testPort), color: "#FF4500" },
   };
 }
 
@@ -520,7 +525,7 @@ export async function resetBrowserControlServerTestContext(): Promise<void> {
   mockClearAll(chromeMcpMocks);
 
   state.testPort = await getFreePort();
-  state.cdpBaseUrl = `http://127.0.0.1:${state.testPort + 9}`;
+  state.cdpBaseUrl = `http://127.0.0.1:${defaultBrowserCdpPortForState(state.testPort)}`;
   state.cfgProfiles = defaultProfilesForState(state.testPort);
   state.prevGatewayPort = process.env.OPENCLAW_GATEWAY_PORT;
   process.env.OPENCLAW_GATEWAY_PORT = String(state.testPort - 2);
diff --git a/src/config/port-defaults.test.ts b/src/config/port-defaults.test.ts
new file mode 100644
index 00000000000..a68b0729c3e
--- /dev/null
+++ b/src/config/port-defaults.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest";
+import {
+  DEFAULT_BROWSER_CDP_PORT_RANGE_END,
+  DEFAULT_BROWSER_CDP_PORT_RANGE_START,
+  deriveDefaultBrowserCdpPortRange,
+} from "./port-defaults.js";
+
+describe("port defaults", () => {
+  it("derives the browser CDP range from the control port", () => {
+    expect(deriveDefaultBrowserCdpPortRange(18791)).toEqual({
+      start: DEFAULT_BROWSER_CDP_PORT_RANGE_START,
+      end: DEFAULT_BROWSER_CDP_PORT_RANGE_END,
+    });
+  });
+
+  it("keeps the default browser CDP range wide when derived ports would overflow", () => {
+    expect(deriveDefaultBrowserCdpPortRange(65440)).toEqual({
+      start: DEFAULT_BROWSER_CDP_PORT_RANGE_START,
+      end: DEFAULT_BROWSER_CDP_PORT_RANGE_END,
+    });
+  });
+});
diff --git a/src/config/port-defaults.ts b/src/config/port-defaults.ts
index 8e5321cb8d4..ac21045168d 100644
--- a/src/config/port-defaults.ts
+++ b/src/config/port-defaults.ts
@@ -17,6 +17,8 @@ export const DEFAULT_BROWSER_CONTROL_PORT = 18791;
 export const DEFAULT_CANVAS_HOST_PORT = 18793;
 export const DEFAULT_BROWSER_CDP_PORT_RANGE_START = 18800;
 export const DEFAULT_BROWSER_CDP_PORT_RANGE_END = 18899;
+const DEFAULT_BROWSER_CDP_PORT_RANGE_SPAN =
+  DEFAULT_BROWSER_CDP_PORT_RANGE_END - DEFAULT_BROWSER_CDP_PORT_RANGE_START;
 
 export function deriveDefaultBridgePort(gatewayPort: number): number {
   return derivePort(gatewayPort, 1, DEFAULT_BRIDGE_PORT);
@@ -32,12 +34,12 @@ export function deriveDefaultCanvasHostPort(gatewayPort: number): number {
 
 export function deriveDefaultBrowserCdpPortRange(browserControlPort: number): PortRange {
   const start = derivePort(browserControlPort, 9, DEFAULT_BROWSER_CDP_PORT_RANGE_START);
-  const end = clampPort(
-    start + (DEFAULT_BROWSER_CDP_PORT_RANGE_END - DEFAULT_BROWSER_CDP_PORT_RANGE_START),
-    DEFAULT_BROWSER_CDP_PORT_RANGE_END,
-  );
-  if (end < start) {
-    return { start, end: start };
+  const end = start + DEFAULT_BROWSER_CDP_PORT_RANGE_SPAN;
+  if (end <= 65535) {
+    return { start, end };
   }
-  return { start, end };
+  return {
+    start: DEFAULT_BROWSER_CDP_PORT_RANGE_START,
+    end: DEFAULT_BROWSER_CDP_PORT_RANGE_END,
+  };
 }