Fast Refresh disconnects repeatedly on iOS 26 simulator (iPhone 17 Pro), stable on iOS 18 / Android

Description

On an iOS 26 simulator (iPhone 17 Pro, iOS 26.4 runtime), Fast Refresh / HMR disconnects from Metro within ~20–30 seconds of the app launching, with no visible JS error. The simulator shows the Fast Refresh disconnected. Reload app to reconnect. banner. Reloading briefly reconnects; the socket is dropped again shortly after. Interval to first drop varies from ~20 seconds to a few minutes but reproduces reliably.

The same Metro instance, started once, simultaneously serves an iPhone SE (2nd gen) / iOS 18 simulator and an Android emulator without any disconnections. Only the iOS 26 simulator drops, ruling out anything Mac-side (Metro, Watchman, port conflicts, firewall).

Reproduces on a stock npx @react-native-community/cli init project with zero custom code, native modules, or dev tooling — ruling out any userland cause.

Workaround (deterministic): hardcoding the bundle URL to http://127.0.0.1:8081/... in AppDelegate.bundleURL() (bypassing RCTBundleURLProvider) keeps Fast Refresh stable indefinitely. Reverting to the default provider reproduces the drop within 30 seconds. This strongly suggests the cause is hostname resolution / NSURLSession behavior specific to iOS 26 simulator runtimes.

Steps to reproduce

1. On macOS with Xcode 26.x and the iOS 26.4 simulator runtime installed.
2. Create a fresh project:

   npx @react-native-community/cli init rnbaseline --version 0.83.1
   cd rnbaseline
   

3. Start Metro: yarn start
4. Build and run on iPhone 17 Pro / iOS 26.4 simulator:

   yarn ios --simulator="iPhone 17 Pro"
   

5. Let the app idle on the default screen.
6. Within ~20–30 seconds the Fast Refresh disconnected banner appears.

Running the same app on an iPhone SE (2nd gen) / iOS 18 simulator or on an Android emulator in parallel shows no disconnection.

Mitigations tried (none resolved it except the workaround)

• Updated iOS 26 simulator runtime from 26.1 → 26.4 (#56183 referenced 26.4 as fixing other 26.x bugs) — no effect
• Removed all custom dev tooling (Reactotron, Storybook, etc.) — reproduces on stock RN init
• yarn start --reset-cache — no effect
• watchman watch-del-all && watchman shutdown-server — no effect
• Verified no port 8081 conflicts with lsof -i :8081 — clean
• Hardcoding http://127.0.0.1:8081/... in AppDelegate.bundleURL() → FIXES THE ISSUE (deterministic)

React Native Version

0.83.1

Affected Platforms

Runtime - iOS

Output of npx @react-native-community/cli info

System:
  OS: macOS 26.3.1
  CPU: (11) arm64 Apple M3 Pro
  Memory: 128.09 MB / 18.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.21.1
    path: /Users/uktam/.nvm/versions/node/v22.21.1/bin/node
  Yarn:
    version: 3.6.4
    path: /Users/uktam/.nvm/versions/node/v22.21.1/bin/yarn
  npm:
    version: 10.9.4
    path: /Users/uktam/.nvm/versions/node/v22.21.1/bin/npm
  Watchman: Not Found
Managers:
  CocoaPods:
    version: 1.16.2
    path: /opt/homebrew/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.2
      - iOS 26.2
      - macOS 26.2
      - tvOS 26.2
      - visionOS 26.2
      - watchOS 26.2
  Android SDK: Not Found
IDEs:
  Android Studio: 2025.2 AI-252.25557.131.2521.14344949
  Xcode:
    version: 26.3/17C529
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.17
    path: /opt/homebrew/Cellar/openjdk@17/17.0.17/libexec/openjdk.jdk/Contents/Home/bin/javac
  Ruby:
    version: 2.6.10
    path: /Users/uktam/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.0.0
    wanted: 20.0.0
  react:
    installed: 19.2.0
    wanted: 19.2.0
  react-native:
    installed: 0.83.1
    wanted: 0.83.1
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: true

Stacktrace or Logs

No stacktrace — this is a silent WebSocket drop, not a crash.

React DevTools console (client-side)

Cannot connect to Metro.

Try the following to fix the issue:
- Ensure that Metro is running and available on the same network
- Ensure that the Metro URL is correctly set in AppDelegate

URL: localhost:8081

Error: undefined

Metro server output

yarn start --verbose produces no error or warning at the moment the banner appears. The disconnect is silent from the server's perspective.

iOS 26 simulator network logs (log stream)

Filter used:

log stream --predicate 'process == "rnbaseline" AND subsystem == "com.apple.network"' --info

Every connection to the dev server resolves to both IPv4 and IPv6 endpoints and then runs Happy Eyeballs:

nw_endpoint_resolver_update [C22.1 Hostname#...:8081 in_progress resolver ...] Adding endpoint handler for IPv4#...:8081
nw_endpoint_resolver_update [C22.1 Hostname#...:8081 in_progress resolver ...] Adding endpoint handler for IPv6#...:8081

The IPv6 candidate fails (Metro listens only on IPv4):

nw_socket_handle_socket_event [C6365.1.1:1] Socket received CONNRESET event
nw_socket_handle_socket_event [C6365.1.1:1] Socket SO_ERROR [61: Connection refused]
[C6365.1.1 IPv6#...:8081 failed socket-flow ...] event: flow:failed_connect @0.005s, error Connection refused

and falls back to IPv4. One loopback TCP handshake was observed taking Duration: 2.099s — well above the expected sub-10ms for loopback — suggesting Happy Eyeballs contention.

Suspected cause (not fully confirmed)

The dual-stack resolution pattern appears in both "unstable" and "stable (with hardcode)" runs, so IPv4/IPv6 racing alone doesn't fully explain the intermittency. What is consistently true is that bypassing hostname-based resolution by hardcoding 127.0.0.1 stops the disconnect deterministically. The exact mechanism — IPv6 race, resolver caching, or some other iOS 26-specific NSURLSession behavior — needs maintainer investigation.

Suggested mitigation in RN

Either of these should mask the underlying iOS 26 simulator regression:

• Default RCTBundleURLProvider to 127.0.0.1 (not the host's resolvable name) when running in an iOS 26+ simulator, or
• Have Metro bind to both IPv4 and IPv6 by default so dual-stack resolution doesn't fail half its candidates.

Adjacent issues (context, not duplicates)

• #54845 — iPhone 17 Pro / iOS 26.1, different symptom (dev reload crash); closed as invalid.
• #54859 — iOS 26 TurboModule startup crash.
• #56183 — iOS 26 simulator CoreText bug, fixed in 26.4 runtime. Confirms Apple has shipped multiple broken simulator runtimes for iOS 26.
• External: iOS 26.4-beta WKWebView reports document WebSocket-to-IP-address connections taking seconds to establish or failing, with 0.0.0.0 as workaround — suggests a shared NSURLSession regression.

MANDATORY Reproducer

Reproducible on clean 0.83.1 React Native CLI template

Screenshots and Videos

iCloud shared access

Dropover Cloud (valid for 3 months)

从 AppDelegate.bundleURL() 和 RCTBundleURLProvider 开始，然后使用干净的 React Native 0.83.1 模板和提供的 iOS 26 模拟器步骤复现断开连接。比较默认 hostname URL 与硬编码的 127.0.0.1 变通方案；当 Fast Refresh 在 iOS 26 上保持连接，同时现有的 iOS 18 和 Android 行为保持稳定时，即表示完成。