diff --git a/ui-tui/src/__tests__/gatewayClient.test.ts b/ui-tui/src/__tests__/gatewayClient.test.ts
index f1228e56fbeb..33bf8ad6e497 100644
--- a/ui-tui/src/__tests__/gatewayClient.test.ts
+++ b/ui-tui/src/__tests__/gatewayClient.test.ts
@@ -97,10 +97,13 @@ describe('GatewayClient websocket attach mode', () => {
   const originalWebSocket = globalThis.WebSocket
   let originalGatewayUrl: string | undefined
   let originalSidecarUrl: string | undefined
+  let originalDisableUndiciWebSocket: string | undefined
 
   beforeEach(() => {
     originalGatewayUrl = process.env.HERMES_TUI_GATEWAY_URL
     originalSidecarUrl = process.env.HERMES_TUI_SIDECAR_URL
+    originalDisableUndiciWebSocket = process.env.HERMES_TUI_DISABLE_UNDICI_WEBSOCKET
+    delete process.env.HERMES_TUI_DISABLE_UNDICI_WEBSOCKET
     FakeWebSocket.reset()
     ;(globalThis as { WebSocket?: unknown }).WebSocket = FakeWebSocket as unknown as typeof WebSocket
   })
@@ -118,6 +121,12 @@ describe('GatewayClient websocket attach mode', () => {
       process.env.HERMES_TUI_SIDECAR_URL = originalSidecarUrl
     }
 
+    if (originalDisableUndiciWebSocket === undefined) {
+      delete process.env.HERMES_TUI_DISABLE_UNDICI_WEBSOCKET
+    } else {
+      process.env.HERMES_TUI_DISABLE_UNDICI_WEBSOCKET = originalDisableUndiciWebSocket
+    }
+
     FakeWebSocket.reset()
 
     if (originalWebSocket) {
@@ -271,6 +280,7 @@ describe('GatewayClient websocket attach mode', () => {
 
   it('redacts query string secrets in attach failure logs and events', () => {
     process.env.HERMES_TUI_GATEWAY_URL = 'ws://gateway.test/api/ws?token=hunter2&channel=secret'
+    process.env.HERMES_TUI_DISABLE_UNDICI_WEBSOCKET = '1'
     delete (globalThis as { WebSocket?: unknown }).WebSocket
 
     const gw = new GatewayClient()
@@ -301,7 +311,7 @@ describe('GatewayClient websocket attach mode', () => {
     const secretUrl = 'ws://gateway.test/api/ws?token=hunter2&channel=secret'
 
     process.env.HERMES_TUI_GATEWAY_URL = secretUrl
-    ;(globalThis as { WebSocket?: unknown }).WebSocket = class ThrowingWebSocket extends FakeWebSocket {
+    ;(globalThis as { WebSocket?: unknown }).WebSocket = class ThrowingWebSocket {
       constructor(url: string) {
         throw new TypeError(`Invalid URL: ${url}`)
       }
@@ -328,11 +338,11 @@ describe('GatewayClient websocket attach mode', () => {
     process.env.HERMES_TUI_SIDECAR_URL = sidecarUrl
     ;(globalThis as { WebSocket?: unknown }).WebSocket = class ThrowingSidecarWebSocket extends FakeWebSocket {
       constructor(url: string) {
+        super(url)
+
         if (url.includes('/api/pub')) {
           throw new TypeError(`Invalid URL: ${url}`)
         }
-
-        super(url)
       }
     } as unknown as typeof WebSocket
 
@@ -363,6 +373,7 @@ describe('GatewayClient websocket attach mode', () => {
     expect(() => new URL(fixture)).toThrow()
 
     process.env.HERMES_TUI_GATEWAY_URL = fixture
+    process.env.HERMES_TUI_DISABLE_UNDICI_WEBSOCKET = '1'
     delete (globalThis as { WebSocket?: unknown }).WebSocket
 
     const gw = new GatewayClient()
diff --git a/ui-tui/src/app/createGatewayEventHandler.ts b/ui-tui/src/app/createGatewayEventHandler.ts
index 987518a4460c..5fa9ceaee5f6 100644
--- a/ui-tui/src/app/createGatewayEventHandler.ts
+++ b/ui-tui/src/app/createGatewayEventHandler.ts
@@ -190,10 +190,11 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
     agentsNudgedThisTurn = false
   }
 
-  // Kick off the config fetch eagerly at handler creation so the flag is
-  // resolved well before the first delegation of any real session (which
-  // only happens after gateway.ready + a user turn).
-  ensureAgentsNudgeConfig()
+  // Do not fetch the nudge config while creating the handler. This factory is
+  // called from React render via useMemo; a synchronous gateway failure can
+  // call sys() -> appendMessage() -> setHistoryItems() during render and
+  // trigger React #301. maybeNudgeAgents() fetches the flag lazily before the
+  // first delegation event, which is early enough for the feature.
 
   const refreshDelegationStatus = (force = false) => {
     const now = Date.now()
diff --git a/ui-tui/src/gatewayClient.ts b/ui-tui/src/gatewayClient.ts
index 37bfa881aba5..c0e7d7123a9e 100644
--- a/ui-tui/src/gatewayClient.ts
+++ b/ui-tui/src/gatewayClient.ts
@@ -19,6 +19,26 @@ const WS_OPEN = 1
 const WS_CLOSING = 2
 const WS_CLOSED = 3
 
+type RuntimeWebSocketCtor = typeof globalThis.WebSocket
+
+const resolveRuntimeWebSocket = (): RuntimeWebSocketCtor | undefined => {
+  if (typeof globalThis.WebSocket !== 'undefined') {
+    return globalThis.WebSocket
+  }
+
+  if (process.env.HERMES_TUI_DISABLE_UNDICI_WEBSOCKET === '1') {
+    return undefined
+  }
+
+  try {
+    const undici = require('undici') as { WebSocket?: RuntimeWebSocketCtor }
+
+    return undici.WebSocket
+  } catch {
+    return undefined
+  }
+}
+
 const truncateLine = (line: string) =>
   line.length > MAX_LOG_LINE_BYTES ? `${line.slice(0, MAX_LOG_LINE_BYTES)}… [truncated ${line.length} bytes]` : line
 
@@ -266,14 +286,16 @@ export class GatewayClient extends EventEmitter {
       return
     }
 
-    if (typeof WebSocket === 'undefined') {
+    const WebSocketCtor = resolveRuntimeWebSocket()
+
+    if (!WebSocketCtor) {
       this.pushLog(`[sidecar] WebSocket unavailable; skipping mirror to ${redactUrl(this.sidecarUrl)}`)
 
       return
     }
 
     try {
-      const ws = new WebSocket(this.sidecarUrl)
+      const ws = new WebSocketCtor(this.sidecarUrl)
 
       this.sidecarWs = ws
       ws.addEventListener('close', () => {
@@ -406,7 +428,9 @@ export class GatewayClient extends EventEmitter {
     const safeAttachUrl = redactUrl(attachUrl)
     this.startReadyTimer('websocket', safeAttachUrl)
 
-    if (typeof WebSocket === 'undefined') {
+    const WebSocketCtor = resolveRuntimeWebSocket()
+
+    if (!WebSocketCtor) {
       const line = `[startup] WebSocket API unavailable; cannot attach to ${safeAttachUrl}`
 
       this.pushLog(line)
@@ -417,7 +441,7 @@ export class GatewayClient extends EventEmitter {
     }
 
     try {
-      const ws = new WebSocket(attachUrl)
+      const ws = new WebSocketCtor(attachUrl)
       let settled = false
 
       this.ws = ws