diff --git a/CHANGELOG.md b/CHANGELOG.md
index 526b34cbc69..0ab4f298933 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ Docs: https://docs.openclaw.ai
 - Telegram/callbacks: treat permanent callback edit errors as completed updates so stale command pagination buttons no longer wedge the update watermark and block newer Telegram updates. (#68588) Thanks @Lucenx9.
 - Browser/CDP: allow the selected remote CDP profile host for CDP health and control checks without widening browser navigation SSRF policy, so WSL-to-Windows Chrome endpoints no longer appear offline under strict defaults. Fixes #68108. (#68207) Thanks @Mlightsnow.
 - Codex: stop cumulative app-server token totals from being treated as fresh context usage, so session status no longer reports inflated context percentages after long Codex threads. (#64669) Thanks @cyrusaf.
+- Browser/CDP: add phase-specific CDP readiness diagnostics and normalize loopback WebSocket host aliases, so Windows browser startup failures surface whether HTTP discovery, WebSocket discovery, SSRF validation, or the `Browser.getVersion` health check failed.
 
 ## 2026.4.18
 
diff --git a/extensions/browser/src/browser/cdp.test.ts b/extensions/browser/src/browser/cdp.test.ts
index 64b291db72a..a50daca9153 100644
--- a/extensions/browser/src/browser/cdp.test.ts
+++ b/extensions/browser/src/browser/cdp.test.ts
@@ -404,6 +404,14 @@ describe("cdp", () => {
     expect(normalized).toBe("wss://user:pass@example.com/devtools/browser/ABC?token=abc");
   });
 
+  it("normalizes loopback websocket aliases to the configured CDP loopback host", () => {
+    const normalized = normalizeCdpWsUrl(
+      "ws://localhost.:18800/devtools/browser/ABC",
+      "http://127.0.0.1:18800",
+    );
+    expect(normalized).toBe("ws://127.0.0.1:18800/devtools/browser/ABC");
+  });
+
   it("rewrites 0.0.0.0 wildcard bind address to remote CDP host", () => {
     const normalized = normalizeCdpWsUrl(
       "ws://0.0.0.0:3000/devtools/browser/ABC",
diff --git a/extensions/browser/src/browser/cdp.ts b/extensions/browser/src/browser/cdp.ts
index 898ac120bed..32866c8a280 100644
--- a/extensions/browser/src/browser/cdp.ts
+++ b/extensions/browser/src/browser/cdp.ts
@@ -31,6 +31,8 @@ export function normalizeCdpWsUrl(wsUrl: string, cdpUrl: string): string {
       ws.port = cdpPort;
     }
     ws.protocol = cdp.protocol === "https:" ? "wss:" : "ws:";
+  } else if (isLoopbackHost(ws.hostname) && isLoopbackHost(cdp.hostname)) {
+    ws.hostname = cdp.hostname;
   }
   if (cdp.protocol === "https:" && ws.protocol === "ws:") {
     ws.protocol = "wss:";
diff --git a/extensions/browser/src/browser/chrome.diagnostics.ts b/extensions/browser/src/browser/chrome.diagnostics.ts
new file mode 100644
index 00000000000..ca9d8e6bd54
--- /dev/null
+++ b/extensions/browser/src/browser/chrome.diagnostics.ts
@@ -0,0 +1,342 @@
+import { normalizeOptionalString } from "openclaw/plugin-sdk/text-runtime";
+import type { SsrFPolicy } from "../infra/net/ssrf.js";
+import { rawDataToString } from "../infra/ws.js";
+import { redactSensitiveText } from "../logging/redact.js";
+import { CHROME_REACHABILITY_TIMEOUT_MS, CHROME_WS_READY_TIMEOUT_MS } from "./cdp-timeouts.js";
+import {
+  appendCdpPath,
+  assertCdpEndpointAllowed,
+  fetchCdpChecked,
+  isWebSocketUrl,
+  openCdpWebSocket,
+  redactCdpUrl,
+} from "./cdp.helpers.js";
+import { normalizeCdpWsUrl } from "./cdp.js";
+import { BrowserCdpEndpointBlockedError } from "./errors.js";
+
+export type ChromeCdpDiagnosticCode =
+  | "ssrf_blocked"
+  | "http_unreachable"
+  | "http_status_failed"
+  | "invalid_json"
+  | "missing_websocket_debugger_url"
+  | "websocket_ssrf_blocked"
+  | "websocket_handshake_failed"
+  | "websocket_health_command_failed"
+  | "websocket_health_command_timeout";
+
+export type ChromeCdpDiagnostic =
+  | {
+      ok: true;
+      cdpUrl: string;
+      wsUrl: string;
+      browser?: string;
+      userAgent?: string;
+      elapsedMs: number;
+    }
+  | {
+      ok: false;
+      code: ChromeCdpDiagnosticCode;
+      cdpUrl: string;
+      wsUrl?: string;
+      message: string;
+      elapsedMs: number;
+    };
+
+export type ChromeVersion = {
+  webSocketDebuggerUrl?: string;
+  Browser?: string;
+  "User-Agent"?: string;
+};
+
+function elapsedSince(startedAt: number): number {
+  return Math.max(0, Date.now() - startedAt);
+}
+
+export function safeChromeCdpErrorMessage(error: unknown): string {
+  const message = error instanceof Error ? error.message : String(error);
+  return redactSensitiveText(message || "unknown error");
+}
+
+function failureDiagnostic(params: {
+  cdpUrl: string;
+  code: ChromeCdpDiagnosticCode;
+  message: string;
+  startedAt: number;
+  wsUrl?: string;
+}): ChromeCdpDiagnostic {
+  return {
+    ok: false,
+    cdpUrl: params.cdpUrl,
+    wsUrl: params.wsUrl,
+    code: params.code,
+    message: redactSensitiveText(params.message),
+    elapsedMs: elapsedSince(params.startedAt),
+  };
+}
+
+export async function readChromeVersion(
+  cdpUrl: string,
+  timeoutMs = CHROME_REACHABILITY_TIMEOUT_MS,
+  ssrfPolicy?: SsrFPolicy,
+): Promise<ChromeVersion> {
+  const ctrl = new AbortController();
+  const t = setTimeout(ctrl.abort.bind(ctrl), timeoutMs);
+  try {
+    const versionUrl = appendCdpPath(cdpUrl, "/json/version");
+    const { response, release } = await fetchCdpChecked(
+      versionUrl,
+      timeoutMs,
+      { signal: ctrl.signal },
+      ssrfPolicy,
+    );
+    try {
+      const data = (await response.json()) as ChromeVersion;
+      if (!data || typeof data !== "object") {
+        throw new Error("CDP /json/version returned non-object JSON");
+      }
+      return data;
+    } finally {
+      await release();
+    }
+  } finally {
+    clearTimeout(t);
+  }
+}
+
+type CdpHealthDiagnostic =
+  | { ok: true }
+  | {
+      ok: false;
+      code:
+        | "websocket_handshake_failed"
+        | "websocket_health_command_failed"
+        | "websocket_health_command_timeout";
+      message: string;
+    };
+
+async function diagnoseCdpHealthCommand(
+  wsUrl: string,
+  timeoutMs = CHROME_WS_READY_TIMEOUT_MS,
+): Promise<CdpHealthDiagnostic> {
+  return await new Promise<CdpHealthDiagnostic>((resolve) => {
+    const ws = openCdpWebSocket(wsUrl, {
+      handshakeTimeoutMs: timeoutMs,
+    });
+    let settled = false;
+    let opened = false;
+    const onMessage = (raw: Parameters<typeof rawDataToString>[0]) => {
+      if (settled) {
+        return;
+      }
+      let parsed: { id?: unknown; result?: unknown } | null = null;
+      try {
+        parsed = JSON.parse(rawDataToString(raw)) as { id?: unknown; result?: unknown };
+      } catch {
+        return;
+      }
+      if (parsed?.id !== 1) {
+        return;
+      }
+      if (parsed.result && typeof parsed.result === "object") {
+        finish({ ok: true });
+        return;
+      }
+      finish({
+        ok: false,
+        code: "websocket_health_command_failed",
+        message: "Browser.getVersion returned no result object",
+      });
+    };
+
+    const finish = (value: CdpHealthDiagnostic) => {
+      if (settled) {
+        return;
+      }
+      settled = true;
+      clearTimeout(timer);
+      ws.off("message", onMessage);
+      try {
+        ws.close();
+      } catch {
+        // ignore
+      }
+      resolve(value);
+    };
+    const timer = setTimeout(
+      () => {
+        try {
+          ws.terminate();
+        } catch {
+          // ignore
+        }
+        finish({
+          ok: false,
+          code: opened ? "websocket_health_command_timeout" : "websocket_handshake_failed",
+          message: opened
+            ? `Browser.getVersion did not respond within ${timeoutMs}ms`
+            : `WebSocket handshake did not complete within ${timeoutMs}ms`,
+        });
+      },
+      Math.max(50, timeoutMs + 25),
+    );
+
+    ws.once("open", () => {
+      opened = true;
+      try {
+        ws.send(
+          JSON.stringify({
+            id: 1,
+            method: "Browser.getVersion",
+          }),
+        );
+      } catch (err) {
+        finish({
+          ok: false,
+          code: "websocket_health_command_failed",
+          message: safeChromeCdpErrorMessage(err),
+        });
+      }
+    });
+
+    ws.on("message", onMessage);
+
+    ws.once("error", (err) => {
+      finish({
+        ok: false,
+        code: opened ? "websocket_health_command_failed" : "websocket_handshake_failed",
+        message: safeChromeCdpErrorMessage(err),
+      });
+    });
+    ws.once("close", () => {
+      finish({
+        ok: false,
+        code: opened ? "websocket_health_command_failed" : "websocket_handshake_failed",
+        message: opened
+          ? "WebSocket closed before Browser.getVersion completed"
+          : "WebSocket closed before handshake completed",
+      });
+    });
+  });
+}
+
+function classifyChromeVersionError(error: unknown): {
+  code: ChromeCdpDiagnosticCode;
+  message: string;
+} {
+  const message = safeChromeCdpErrorMessage(error);
+  if (error instanceof BrowserCdpEndpointBlockedError) {
+    return { code: "ssrf_blocked", message };
+  }
+  if (/^HTTP \d+/.test(message)) {
+    return { code: "http_status_failed", message };
+  }
+  if (error instanceof SyntaxError || message.includes("non-object JSON")) {
+    return { code: "invalid_json", message };
+  }
+  return { code: "http_unreachable", message };
+}
+
+export function formatChromeCdpDiagnostic(diagnostic: ChromeCdpDiagnostic): string {
+  const redactedCdpUrl = redactCdpUrl(diagnostic.cdpUrl) ?? diagnostic.cdpUrl;
+  const redactedWsUrl = redactCdpUrl(diagnostic.wsUrl) ?? diagnostic.wsUrl;
+  if (diagnostic.ok) {
+    const browser = diagnostic.browser ? ` browser=${diagnostic.browser}` : "";
+    return `CDP diagnostic: ready after ${diagnostic.elapsedMs}ms; cdp=${redactedCdpUrl}; websocket=${redactedWsUrl}.${browser}`;
+  }
+  const websocket = redactedWsUrl ? `; websocket=${redactedWsUrl}` : "";
+  return `CDP diagnostic: ${diagnostic.code} after ${diagnostic.elapsedMs}ms; cdp=${redactedCdpUrl}${websocket}; ${diagnostic.message}.`;
+}
+
+export async function diagnoseChromeCdp(
+  cdpUrl: string,
+  timeoutMs = CHROME_REACHABILITY_TIMEOUT_MS,
+  handshakeTimeoutMs = CHROME_WS_READY_TIMEOUT_MS,
+  ssrfPolicy?: SsrFPolicy,
+): Promise<ChromeCdpDiagnostic> {
+  const startedAt = Date.now();
+  try {
+    await assertCdpEndpointAllowed(cdpUrl, ssrfPolicy);
+  } catch (err) {
+    return failureDiagnostic({
+      cdpUrl,
+      code: "ssrf_blocked",
+      message: safeChromeCdpErrorMessage(err),
+      startedAt,
+    });
+  }
+
+  if (isWebSocketUrl(cdpUrl)) {
+    const health = await diagnoseCdpHealthCommand(cdpUrl, handshakeTimeoutMs);
+    if (!health.ok) {
+      return failureDiagnostic({
+        cdpUrl,
+        wsUrl: cdpUrl,
+        code: health.code,
+        message: health.message,
+        startedAt,
+      });
+    }
+    return {
+      ok: true,
+      cdpUrl,
+      wsUrl: cdpUrl,
+      elapsedMs: elapsedSince(startedAt),
+    };
+  }
+
+  let version: ChromeVersion;
+  try {
+    version = await readChromeVersion(cdpUrl, timeoutMs, ssrfPolicy);
+  } catch (err) {
+    const classified = classifyChromeVersionError(err);
+    return failureDiagnostic({
+      cdpUrl,
+      code: classified.code,
+      message: classified.message,
+      startedAt,
+    });
+  }
+
+  const wsUrlRaw = normalizeOptionalString(version.webSocketDebuggerUrl) ?? "";
+  if (!wsUrlRaw) {
+    return failureDiagnostic({
+      cdpUrl,
+      code: "missing_websocket_debugger_url",
+      message: "CDP /json/version did not include webSocketDebuggerUrl",
+      startedAt,
+    });
+  }
+  const wsUrl = normalizeCdpWsUrl(wsUrlRaw, cdpUrl);
+  try {
+    await assertCdpEndpointAllowed(wsUrl, ssrfPolicy);
+  } catch (err) {
+    return failureDiagnostic({
+      cdpUrl,
+      wsUrl,
+      code: "websocket_ssrf_blocked",
+      message: safeChromeCdpErrorMessage(err),
+      startedAt,
+    });
+  }
+
+  const health = await diagnoseCdpHealthCommand(wsUrl, handshakeTimeoutMs);
+  if (!health.ok) {
+    return failureDiagnostic({
+      cdpUrl,
+      wsUrl,
+      code: health.code,
+      message: health.message,
+      startedAt,
+    });
+  }
+
+  return {
+    ok: true,
+    cdpUrl,
+    wsUrl,
+    browser: version.Browser,
+    userAgent: version["User-Agent"],
+    elapsedMs: elapsedSince(startedAt),
+  };
+}
diff --git a/extensions/browser/src/browser/chrome.test.ts b/extensions/browser/src/browser/chrome.test.ts
index fb7b137d503..200f19e4354 100644
--- a/extensions/browser/src/browser/chrome.test.ts
+++ b/extensions/browser/src/browser/chrome.test.ts
@@ -8,9 +8,11 @@ import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it, vi }
 import { WebSocketServer } from "ws";
 import {
   decorateOpenClawProfile,
+  diagnoseChromeCdp,
   ensureProfileCleanExit,
   findChromeExecutableMac,
   findChromeExecutableWindows,
+  formatChromeCdpDiagnostic,
   getChromeWebSocketUrl,
   isChromeCdpReady,
   isChromeReachable,
@@ -312,6 +314,22 @@ describe("browser chrome helpers", () => {
     await expect(isChromeReachable("http://127.0.0.1:12345", 50)).resolves.toBe(false);
   });
 
+  it("diagnoses /json/version responses that omit the websocket URL", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn().mockResolvedValue({
+        ok: true,
+        json: async () => ({ Browser: "Chrome/Mock" }),
+      } as unknown as Response),
+    );
+
+    await expect(diagnoseChromeCdp("http://127.0.0.1:12345", 50, 50)).resolves.toMatchObject({
+      ok: false,
+      code: "missing_websocket_debugger_url",
+      cdpUrl: "http://127.0.0.1:12345",
+    });
+  });
+
   it("allows loopback CDP probes while still blocking non-loopback private targets in strict SSRF mode", async () => {
     const fetchSpy = vi
       .fn()
@@ -417,6 +435,39 @@ describe("browser chrome helpers", () => {
     });
   });
 
+  it("diagnoses stale websocket command channels with the discovered websocket URL", async () => {
+    await withMockChromeCdpServer({
+      wsPath: "/devtools/browser/stale-diagnostic",
+      onConnection: (wss) => wss.on("connection", (_ws) => {}),
+      run: async (baseUrl) => {
+        const diagnostic = await diagnoseChromeCdp(baseUrl, 300, 150);
+        expect(diagnostic).toMatchObject({
+          ok: false,
+          code: "websocket_health_command_timeout",
+        });
+        expect(diagnostic.wsUrl).toMatch(/\/devtools\/browser\/stale-diagnostic$/);
+      },
+    });
+  });
+
+  it("formats diagnostics with redacted CDP credentials", () => {
+    const formatted = formatChromeCdpDiagnostic({
+      ok: false,
+      code: "websocket_handshake_failed",
+      cdpUrl: "https://user:pass@browserless.example.com?token=supersecret123",
+      wsUrl: "wss://user:pass@browserless.example.com/devtools/browser/1?token=supersecret123",
+      message: "connect ECONNREFUSED browserless.example.com",
+      elapsedMs: 12,
+    });
+
+    expect(formatted).toContain("websocket_handshake_failed");
+    expect(formatted).toContain("https://browserless.example.com/?token=***");
+    expect(formatted).toContain("wss://browserless.example.com/devtools/browser/1?token=***");
+    expect(formatted).not.toContain("user");
+    expect(formatted).not.toContain("pass");
+    expect(formatted).not.toContain("supersecret123");
+  });
+
   it("probes WebSocket URLs via handshake instead of HTTP", async () => {
     // For ws:// URLs, isChromeReachable should NOT call fetch at all —
     // it should attempt a WebSocket handshake instead.
diff --git a/extensions/browser/src/browser/chrome.ts b/extensions/browser/src/browser/chrome.ts
index 0ece64410b3..5641e5bff38 100644
--- a/extensions/browser/src/browser/chrome.ts
+++ b/extensions/browser/src/browser/chrome.ts
@@ -5,7 +5,6 @@ import path from "node:path";
 import { normalizeOptionalString } from "openclaw/plugin-sdk/text-runtime";
 import type { SsrFPolicy } from "../infra/net/ssrf.js";
 import { ensurePortAvailable } from "../infra/ports.js";
-import { rawDataToString } from "../infra/ws.js";
 import { createSubsystemLogger } from "../logging/subsystem.js";
 import { CONFIG_DIR } from "../utils.js";
 import {
@@ -19,14 +18,15 @@ import {
   CHROME_STOP_TIMEOUT_MS,
   CHROME_WS_READY_TIMEOUT_MS,
 } from "./cdp-timeouts.js";
-import {
-  appendCdpPath,
-  assertCdpEndpointAllowed,
-  fetchCdpChecked,
-  isWebSocketUrl,
-  openCdpWebSocket,
-} from "./cdp.helpers.js";
+import { assertCdpEndpointAllowed, isWebSocketUrl, openCdpWebSocket } from "./cdp.helpers.js";
 import { normalizeCdpWsUrl } from "./cdp.js";
+import {
+  diagnoseChromeCdp,
+  formatChromeCdpDiagnostic,
+  type ChromeVersion,
+  readChromeVersion,
+  safeChromeCdpErrorMessage,
+} from "./chrome.diagnostics.js";
 import {
   type BrowserExecutable,
   resolveBrowserExecutableForPlatform,
@@ -45,6 +45,12 @@ import {
 const log = createSubsystemLogger("browser").child("chrome");
 
 export type { BrowserExecutable } from "./chrome.executables.js";
+export {
+  diagnoseChromeCdp,
+  formatChromeCdpDiagnostic,
+  type ChromeCdpDiagnostic,
+  type ChromeCdpDiagnosticCode,
+} from "./chrome.diagnostics.js";
 export {
   findChromeExecutableLinux,
   findChromeExecutableMac,
@@ -127,15 +133,24 @@ export function buildOpenClawChromeLaunchArgs(params: {
 async function canOpenWebSocket(url: string, timeoutMs: number): Promise<boolean> {
   return new Promise<boolean>((resolve) => {
     const ws = openCdpWebSocket(url, { handshakeTimeoutMs: timeoutMs });
+    let settled = false;
+    const finish = (value: boolean) => {
+      if (settled) {
+        return;
+      }
+      settled = true;
+      resolve(value);
+    };
     ws.once("open", () => {
       try {
         ws.close();
       } catch {
         // ignore
       }
-      resolve(true);
+      finish(true);
     });
-    ws.once("error", () => resolve(false));
+    ws.once("error", () => finish(false));
+    ws.once("close", () => finish(false));
   });
 }
 
@@ -157,40 +172,15 @@ export async function isChromeReachable(
   }
 }
 
-type ChromeVersion = {
-  webSocketDebuggerUrl?: string;
-  Browser?: string;
-  "User-Agent"?: string;
-};
-
 async function fetchChromeVersion(
   cdpUrl: string,
   timeoutMs = CHROME_REACHABILITY_TIMEOUT_MS,
   ssrfPolicy?: SsrFPolicy,
 ): Promise<ChromeVersion | null> {
-  const ctrl = new AbortController();
-  const t = setTimeout(ctrl.abort.bind(ctrl), timeoutMs);
   try {
-    const versionUrl = appendCdpPath(cdpUrl, "/json/version");
-    const { response, release } = await fetchCdpChecked(
-      versionUrl,
-      timeoutMs,
-      { signal: ctrl.signal },
-      ssrfPolicy,
-    );
-    try {
-      const data = (await response.json()) as ChromeVersion;
-      if (!data || typeof data !== "object") {
-        return null;
-      }
-      return data;
-    } finally {
-      await release();
-    }
+    return await readChromeVersion(cdpUrl, timeoutMs, ssrfPolicy);
   } catch {
     return null;
-  } finally {
-    clearTimeout(t);
   }
 }
 
@@ -214,92 +204,17 @@ export async function getChromeWebSocketUrl(
   return normalizedWsUrl;
 }
 
-async function canRunCdpHealthCommand(
-  wsUrl: string,
-  timeoutMs = CHROME_WS_READY_TIMEOUT_MS,
-): Promise<boolean> {
-  return await new Promise<boolean>((resolve) => {
-    const ws = openCdpWebSocket(wsUrl, {
-      handshakeTimeoutMs: timeoutMs,
-    });
-    let settled = false;
-    const onMessage = (raw: Parameters<typeof rawDataToString>[0]) => {
-      if (settled) {
-        return;
-      }
-      let parsed: { id?: unknown; result?: unknown } | null = null;
-      try {
-        parsed = JSON.parse(rawDataToString(raw)) as { id?: unknown; result?: unknown };
-      } catch {
-        return;
-      }
-      if (parsed?.id !== 1) {
-        return;
-      }
-      finish(Boolean(parsed.result && typeof parsed.result === "object"));
-    };
-
-    const finish = (value: boolean) => {
-      if (settled) {
-        return;
-      }
-      settled = true;
-      clearTimeout(timer);
-      ws.off("message", onMessage);
-      try {
-        ws.close();
-      } catch {
-        // ignore
-      }
-      resolve(value);
-    };
-    const timer = setTimeout(
-      () => {
-        try {
-          ws.terminate();
-        } catch {
-          // ignore
-        }
-        finish(false);
-      },
-      Math.max(50, timeoutMs + 25),
-    );
-
-    ws.once("open", () => {
-      try {
-        ws.send(
-          JSON.stringify({
-            id: 1,
-            method: "Browser.getVersion",
-          }),
-        );
-      } catch {
-        finish(false);
-      }
-    });
-
-    ws.on("message", onMessage);
-
-    ws.once("error", () => {
-      finish(false);
-    });
-    ws.once("close", () => {
-      finish(false);
-    });
-  });
-}
-
 export async function isChromeCdpReady(
   cdpUrl: string,
   timeoutMs = CHROME_REACHABILITY_TIMEOUT_MS,
   handshakeTimeoutMs = CHROME_WS_READY_TIMEOUT_MS,
   ssrfPolicy?: SsrFPolicy,
 ): Promise<boolean> {
-  const wsUrl = await getChromeWebSocketUrl(cdpUrl, timeoutMs, ssrfPolicy).catch(() => null);
-  if (!wsUrl) {
-    return false;
+  const diagnostic = await diagnoseChromeCdp(cdpUrl, timeoutMs, handshakeTimeoutMs, ssrfPolicy);
+  if (!diagnostic.ok) {
+    log.debug(formatChromeCdpDiagnostic(diagnostic));
   }
-  return await canRunCdpHealthCommand(wsUrl, handshakeTimeoutMs);
+  return diagnostic.ok;
 }
 
 export async function launchOpenClawChrome(
@@ -418,6 +333,9 @@ export async function launchOpenClawChrome(
   }
 
   if (!(await isChromeReachable(profile.cdpUrl))) {
+    const diagnosticText = await diagnoseChromeCdp(profile.cdpUrl)
+      .then(formatChromeCdpDiagnostic)
+      .catch((err) => `CDP diagnostic failed: ${safeChromeCdpErrorMessage(err)}.`);
     const stderrOutput =
       normalizeOptionalString(Buffer.concat(stderrChunks).toString("utf8")) ?? "";
     const stderrHint = stderrOutput
@@ -433,7 +351,7 @@ export async function launchOpenClawChrome(
       // ignore
     }
     throw new Error(
-      `Failed to start Chrome CDP on port ${profile.cdpPort} for profile "${profile.name}".${sandboxHint}${stderrHint}`,
+      `Failed to start Chrome CDP on port ${profile.cdpPort} for profile "${profile.name}". ${diagnosticText}${sandboxHint}${stderrHint}`,
     );
   }
 
diff --git a/extensions/browser/src/browser/server-context.availability.ts b/extensions/browser/src/browser/server-context.availability.ts
index 146f9561b15..f4daa7e6eb3 100644
--- a/extensions/browser/src/browser/server-context.availability.ts
+++ b/extensions/browser/src/browser/server-context.availability.ts
@@ -14,6 +14,8 @@ import {
   listChromeMcpTabs,
 } from "./chrome-mcp.js";
 import {
+  diagnoseChromeCdp,
+  formatChromeCdpDiagnostic,
   isChromeCdpReady,
   isChromeReachable,
   launchOpenClawChrome,
@@ -96,6 +98,17 @@ export function createProfileAvailability({
     return await isChromeReachable(profile.cdpUrl, httpTimeoutMs, getCdpReachabilityPolicy());
   };
 
+  const describeCdpFailure = async (timeoutMs?: number): Promise<string> => {
+    const { httpTimeoutMs, wsTimeoutMs } = resolveTimeouts(timeoutMs);
+    const diagnostic = await diagnoseChromeCdp(
+      profile.cdpUrl,
+      httpTimeoutMs,
+      wsTimeoutMs,
+      getCdpReachabilityPolicy(),
+    );
+    return formatChromeCdpDiagnostic(diagnostic);
+  };
+
   const attachRunning = (running: NonNullable<ProfileRuntimeState["running"]>) => {
     setProfileRunning(running);
     running.proc.on("exit", () => {
@@ -150,7 +163,9 @@ export function createProfileAvailability({
       await new Promise((r) => setTimeout(r, CDP_READY_AFTER_LAUNCH_POLL_MS));
     }
     throw new Error(
-      `Chrome CDP websocket for profile "${profile.name}" is not reachable after start.`,
+      `Chrome CDP websocket for profile "${profile.name}" is not reachable after start. ${await describeCdpFailure(
+        CDP_READY_AFTER_LAUNCH_MAX_TIMEOUT_MS,
+      )}`,
     );
   };
 
@@ -245,18 +260,20 @@ export function createProfileAvailability({
       if (remoteCdp && (await isReachable(PROFILE_ATTACH_RETRY_TIMEOUT_MS))) {
         return;
       }
+      const detail = await describeCdpFailure(PROFILE_ATTACH_RETRY_TIMEOUT_MS);
       throw new BrowserProfileUnavailableError(
         remoteCdp
-          ? `Remote CDP websocket for profile "${profile.name}" is not reachable.`
-          : `Browser attachOnly is enabled and CDP websocket for profile "${profile.name}" is not reachable.`,
+          ? `Remote CDP websocket for profile "${profile.name}" is not reachable. ${detail}`
+          : `Browser attachOnly is enabled and CDP websocket for profile "${profile.name}" is not reachable. ${detail}`,
       );
     }
 
     // HTTP responds but WebSocket fails - port in use by something else.
     if (!profileState.running) {
+      const detail = await describeCdpFailure(PROFILE_ATTACH_RETRY_TIMEOUT_MS);
       throw new BrowserProfileUnavailableError(
         `Port ${profile.cdpPort} is in use for profile "${profile.name}" but not by openclaw. ` +
-          `Run action=reset-profile profile=${profile.name} to kill the process.`,
+          `Run action=reset-profile profile=${profile.name} to kill the process. ${detail}`,
       );
     }
 
@@ -268,7 +285,9 @@ export function createProfileAvailability({
 
     if (!(await isReachable(PROFILE_POST_RESTART_WS_TIMEOUT_MS))) {
       throw new Error(
-        `Chrome CDP websocket for profile "${profile.name}" is not reachable after restart.`,
+        `Chrome CDP websocket for profile "${profile.name}" is not reachable after restart. ${await describeCdpFailure(
+          PROFILE_POST_RESTART_WS_TIMEOUT_MS,
+        )}`,
       );
     }
   };
diff --git a/extensions/browser/src/browser/server-context.chrome-test-harness.ts b/extensions/browser/src/browser/server-context.chrome-test-harness.ts
index 95ebe8097e6..64cb1ce3710 100644
--- a/extensions/browser/src/browser/server-context.chrome-test-harness.ts
+++ b/extensions/browser/src/browser/server-context.chrome-test-harness.ts
@@ -5,6 +5,18 @@ const chromeUserDataDir = { dir: "/tmp/openclaw" };
 installChromeUserDataDirHooks(chromeUserDataDir);
 
 vi.mock("./chrome.js", () => ({
+  diagnoseChromeCdp: vi.fn(async () => ({
+    ok: false,
+    code: "websocket_health_command_timeout",
+    cdpUrl: "http://127.0.0.1:18800",
+    message: "mock CDP diagnostic",
+    elapsedMs: 1,
+  })),
+  formatChromeCdpDiagnostic: vi.fn((diagnostic: { ok: boolean; code?: string; message?: string }) =>
+    diagnostic.ok
+      ? "CDP diagnostic: ready."
+      : `CDP diagnostic: ${diagnostic.code}; ${diagnostic.message}.`,
+  ),
   isChromeCdpReady: vi.fn(async () => true),
   isChromeReachable: vi.fn(async () => true),
   launchOpenClawChrome: vi.fn(async () => {
diff --git a/extensions/browser/src/browser/server-context.ensure-browser-available.waits-for-cdp-ready.test.ts b/extensions/browser/src/browser/server-context.ensure-browser-available.waits-for-cdp-ready.test.ts
index 9ad87c4a403..b93fe0ea1c1 100644
--- a/extensions/browser/src/browser/server-context.ensure-browser-available.waits-for-cdp-ready.test.ts
+++ b/extensions/browser/src/browser/server-context.ensure-browser-available.waits-for-cdp-ready.test.ts
@@ -55,8 +55,12 @@ describe("browser server-context ensureBrowserAvailable", () => {
 
     const promise = profile.ensureBrowserAvailable();
     const rejected = expect(promise).rejects.toThrow("not reachable after start");
+    const diagnosticRejected = expect(promise).rejects.toThrow(
+      "CDP diagnostic: websocket_health_command_timeout; mock CDP diagnostic.",
+    );
     await vi.advanceTimersByTimeAsync(8100);
     await rejected;
+    await diagnosticRejected;
 
     expect(launchOpenClawChrome).toHaveBeenCalledTimes(1);
     expect(stopOpenClawChrome).toHaveBeenCalledTimes(1);