diff --git a/apps/desktop/electron/gateway-ws-probe.cjs b/apps/desktop/electron/gateway-ws-probe.cjs
new file mode 100644
index 000000000..6ed1280be
--- /dev/null
+++ b/apps/desktop/electron/gateway-ws-probe.cjs
@@ -0,0 +1,188 @@
+/**
+ * Live WebSocket validation for the remote-gateway "Test remote" button.
+ *
+ * Background: the desktop boot does two independent things to a remote gateway:
+ *
+ *   1. The MAIN process hits ``GET /api/status`` over HTTP (token in a header)
+ *      to confirm the backend is up. This is what "Test remote" historically
+ *      checked, and what the boot logs print as "Remote Hermes backend is
+ *      ready".
+ *   2. The RENDERER then opens a live WebSocket to ``/api/ws`` (credential in a
+ *      query param) via ``gateway.connect()``. The chat surface only works once
+ *      THIS succeeds.
+ *
+ * Those two paths use different processes, transports, and credentials, and the
+ * server applies extra guards to the WS upgrade that the HTTP status route never
+ * sees (Host/Origin checks, ws-ticket/token auth, peer-IP checks). So a gateway
+ * can pass the HTTP status check yet reject the WebSocket — which surfaces to
+ * the user as a green "Test remote" followed by an opaque "Could not connect to
+ * Hermes gateway" on the boot overlay.
+ *
+ * This module performs the second half of the check: it actually opens the WS
+ * URL and confirms the upgrade is accepted (and isn't immediately torn down by
+ * a post-upgrade auth rejection). The ``WebSocketImpl`` is injectable so the
+ * unit tests can drive the handshake without a real socket; in production the
+ * caller passes the Node/Electron global ``WebSocket``.
+ */
+
+const DEFAULT_CONNECT_TIMEOUT_MS = 10_000
+// After the upgrade is accepted, a gateway that rejects the credential
+// post-handshake closes the socket almost immediately. Wait a short grace
+// window: a frame (gateway.ready) or a still-open socket means success; an
+// early close means the upgrade was accepted but the session was refused.
+const DEFAULT_READY_GRACE_MS = 750
+
+/**
+ * Attempt a live WebSocket connection and classify the outcome.
+ *
+ * @param {string} wsUrl - Fully-formed ws(s):// URL including the credential.
+ * @param {object} [options]
+ * @param {new (url: string) => any} [options.WebSocketImpl] - WebSocket ctor.
+ * @param {number} [options.connectTimeoutMs]
+ * @param {number} [options.readyGraceMs]
+ * @returns {Promise<{ ok: boolean, reason?: string }>}
+ */
+function probeGatewayWebSocket(wsUrl, options = {}) {
+  const WebSocketImpl = options.WebSocketImpl
+  const connectTimeoutMs = options.connectTimeoutMs ?? DEFAULT_CONNECT_TIMEOUT_MS
+  const readyGraceMs = options.readyGraceMs ?? DEFAULT_READY_GRACE_MS
+
+  if (typeof WebSocketImpl !== 'function') {
+    return Promise.resolve({
+      ok: false,
+      reason: 'WebSocket is not available in this runtime.'
+    })
+  }
+
+  return new Promise(resolve => {
+    let settled = false
+    let opened = false
+    let connectTimer = null
+    let graceTimer = null
+    let socket
+
+    const clearTimers = () => {
+      if (connectTimer !== null) {
+        clearTimeout(connectTimer)
+        connectTimer = null
+      }
+      if (graceTimer !== null) {
+        clearTimeout(graceTimer)
+        graceTimer = null
+      }
+    }
+
+    const finish = result => {
+      if (settled) return
+      settled = true
+      clearTimers()
+      try {
+        socket?.close?.()
+      } catch {
+        // ignore — best effort teardown
+      }
+      resolve(result)
+    }
+
+    try {
+      socket = new WebSocketImpl(wsUrl)
+    } catch (error) {
+      finish({
+        ok: false,
+        reason: error instanceof Error ? error.message : String(error)
+      })
+      return
+    }
+
+    const onOpen = () => {
+      if (settled) return
+      opened = true
+      // Upgrade accepted. Give the server a brief window to reject the
+      // credential post-handshake (early close) before declaring success.
+      graceTimer = setTimeout(() => {
+        finish({ ok: true })
+      }, readyGraceMs)
+    }
+
+    const onMessage = () => {
+      // Any frame means the gateway accepted us and is talking — unambiguous
+      // success, no need to wait out the grace window.
+      finish({ ok: true })
+    }
+
+    const onError = event => {
+      finish({
+        ok: false,
+        reason: extractErrorReason(event) || 'WebSocket connection failed.'
+      })
+    }
+
+    const onClose = event => {
+      if (settled) return
+      if (opened) {
+        // Opened, then closed inside the grace window: the upgrade was accepted
+        // but the session was refused (e.g. ws-ticket/token rejected, or a
+        // server-side Host/Origin guard tripped after accept).
+        finish({
+          ok: false,
+          reason: closeReason(event, 'The gateway accepted the connection then closed it (credential rejected?).')
+        })
+        return
+      }
+      finish({
+        ok: false,
+        reason: closeReason(event, 'The gateway closed the WebSocket before it opened.')
+      })
+    }
+
+    addListener(socket, 'open', onOpen)
+    addListener(socket, 'message', onMessage)
+    addListener(socket, 'error', onError)
+    addListener(socket, 'close', onClose)
+
+    if (connectTimeoutMs > 0) {
+      connectTimer = setTimeout(() => {
+        finish({
+          ok: false,
+          reason: `Timed out after ${connectTimeoutMs}ms waiting for the WebSocket to open.`
+        })
+      }, connectTimeoutMs)
+    }
+  })
+}
+
+function addListener(socket, type, handler) {
+  if (typeof socket.addEventListener === 'function') {
+    socket.addEventListener(type, handler)
+    return
+  }
+  // Node's global WebSocket implements addEventListener; this fallback keeps the
+  // helper usable with the `ws` package's EventEmitter shape too.
+  if (typeof socket.on === 'function') {
+    socket.on(type, handler)
+  }
+}
+
+function extractErrorReason(event) {
+  if (!event) return ''
+  if (event instanceof Error) return event.message
+  const err = event.error || event.message
+  if (err instanceof Error) return err.message
+  if (typeof err === 'string') return err
+  return ''
+}
+
+function closeReason(event, fallback) {
+  const code = event && typeof event.code === 'number' ? event.code : null
+  const reason = event && typeof event.reason === 'string' ? event.reason.trim() : ''
+  if (code && reason) return `${fallback} (code ${code}: ${reason})`
+  if (code) return `${fallback} (code ${code})`
+  if (reason) return `${fallback} (${reason})`
+  return fallback
+}
+
+module.exports = {
+  DEFAULT_CONNECT_TIMEOUT_MS,
+  DEFAULT_READY_GRACE_MS,
+  probeGatewayWebSocket
+}
diff --git a/apps/desktop/electron/gateway-ws-probe.test.cjs b/apps/desktop/electron/gateway-ws-probe.test.cjs
new file mode 100644
index 000000000..810494fdc
--- /dev/null
+++ b/apps/desktop/electron/gateway-ws-probe.test.cjs
@@ -0,0 +1,122 @@
+/**
+ * Tests for electron/gateway-ws-probe.cjs.
+ *
+ * Run with: node --test electron/gateway-ws-probe.test.cjs
+ * (Wired into npm test:desktop:platforms in package.json.)
+ *
+ * The probe drives a real WebSocket handshake for the "Test remote" button.
+ * Here we inject a fake socket so we can deterministically replay each handshake
+ * outcome (open, frame, error, early close, never-opens) without a network.
+ */
+
+const test = require('node:test')
+const assert = require('node:assert/strict')
+
+const { probeGatewayWebSocket } = require('./gateway-ws-probe.cjs')
+
+// Minimal WebSocket double: records listeners synchronously (the probe attaches
+// them in its executor) and exposes emit() so the test can replay events.
+function makeFakeWs() {
+  const instances = []
+  class FakeWs {
+    constructor(url) {
+      this.url = url
+      this.listeners = {}
+      this.closed = false
+      instances.push(this)
+    }
+    addEventListener(type, fn) {
+      ;(this.listeners[type] ||= []).push(fn)
+    }
+    close() {
+      this.closed = true
+    }
+    emit(type, event) {
+      for (const fn of this.listeners[type] || []) fn(event)
+    }
+  }
+  return { FakeWs, instances }
+}
+
+const FAST = { connectTimeoutMs: 1_000, readyGraceMs: 10 }
+
+test('probe resolves ok when the socket opens and stays open', async () => {
+  const { FakeWs, instances } = makeFakeWs()
+  const promise = probeGatewayWebSocket('ws://host/api/ws?token=t', { WebSocketImpl: FakeWs, ...FAST })
+  instances[0].emit('open')
+  const result = await promise
+  assert.deepEqual(result, { ok: true })
+  assert.equal(instances[0].closed, true)
+})
+
+test('probe resolves ok immediately when a frame arrives', async () => {
+  const { FakeWs, instances } = makeFakeWs()
+  const promise = probeGatewayWebSocket('ws://host/api/ws?token=t', {
+    WebSocketImpl: FakeWs,
+    connectTimeoutMs: 1_000,
+    readyGraceMs: 10_000 // long grace: success must come from the frame, not the timer
+  })
+  instances[0].emit('open')
+  instances[0].emit('message', { data: '{"jsonrpc":"2.0"}' })
+  const result = await promise
+  assert.deepEqual(result, { ok: true })
+})
+
+test('probe fails when the socket errors before opening', async () => {
+  const { FakeWs, instances } = makeFakeWs()
+  const promise = probeGatewayWebSocket('ws://host/api/ws?token=t', { WebSocketImpl: FakeWs, ...FAST })
+  instances[0].emit('error', { message: 'ECONNREFUSED' })
+  const result = await promise
+  assert.equal(result.ok, false)
+  assert.match(result.reason, /ECONNREFUSED/)
+})
+
+test('probe fails when the gateway closes before opening', async () => {
+  const { FakeWs, instances } = makeFakeWs()
+  const promise = probeGatewayWebSocket('ws://host/api/ws?token=t', { WebSocketImpl: FakeWs, ...FAST })
+  instances[0].emit('close', { code: 1006 })
+  const result = await promise
+  assert.equal(result.ok, false)
+  assert.match(result.reason, /before it opened/)
+  assert.match(result.reason, /1006/)
+})
+
+test('probe fails when the gateway accepts then immediately closes (auth rejected)', async () => {
+  const { FakeWs, instances } = makeFakeWs()
+  const promise = probeGatewayWebSocket('ws://host/api/ws?token=t', { WebSocketImpl: FakeWs, ...FAST })
+  instances[0].emit('open')
+  instances[0].emit('close', { code: 4403, reason: 'forbidden' })
+  const result = await promise
+  assert.equal(result.ok, false)
+  assert.match(result.reason, /credential rejected/)
+  assert.match(result.reason, /4403/)
+  assert.match(result.reason, /forbidden/)
+})
+
+test('probe times out when the socket never opens', async () => {
+  const { FakeWs } = makeFakeWs()
+  const result = await probeGatewayWebSocket('ws://host/api/ws?token=t', {
+    WebSocketImpl: FakeWs,
+    connectTimeoutMs: 20,
+    readyGraceMs: 10
+  })
+  assert.equal(result.ok, false)
+  assert.match(result.reason, /Timed out/)
+})
+
+test('probe fails gracefully when the constructor throws', async () => {
+  class ThrowingWs {
+    constructor() {
+      throw new Error('bad url')
+    }
+  }
+  const result = await probeGatewayWebSocket('ws://host/api/ws', { WebSocketImpl: ThrowingWs, ...FAST })
+  assert.equal(result.ok, false)
+  assert.match(result.reason, /bad url/)
+})
+
+test('probe reports unavailable when no WebSocket implementation is provided', async () => {
+  const result = await probeGatewayWebSocket('ws://host/api/ws', { WebSocketImpl: undefined })
+  assert.equal(result.ok, false)
+  assert.match(result.reason, /not available/)
+})
diff --git a/apps/desktop/package.json b/apps/desktop/package.json
index 10703c127..372022949 100644
--- a/apps/desktop/package.json
+++ b/apps/desktop/package.json
@@ -35,7 +35,7 @@
     "test:desktop:nsis": "node scripts/test-desktop.mjs nsis",
     "test:desktop:existing": "node scripts/test-desktop.mjs existing",
     "test:desktop:fresh": "node scripts/test-desktop.mjs fresh",
-    "test:desktop:platforms": "node --test electron/bootstrap-platform.test.cjs electron/hardening.test.cjs electron/backend-probes.test.cjs electron/bootstrap-runner.test.cjs electron/connection-config.test.cjs",
+    "test:desktop:platforms": "node --test electron/bootstrap-platform.test.cjs electron/hardening.test.cjs electron/backend-probes.test.cjs electron/bootstrap-runner.test.cjs electron/connection-config.test.cjs electron/gateway-ws-probe.test.cjs",
     "type-check": "tsc -b",
     "lint": "eslint src/ electron/",
     "lint:fix": "eslint src/ electron/ --fix",