[DevTools] Network panel truncates response bodies containing multibyte UTF-8 characters #2062

Description

In the React Native DevTools Network panel, response bodies containing multibyte UTF-8
characters (e.g. á, é, ó) are displayed truncated in both the Response and
Preview tabs.

The number of missing trailing characters matches the number of multibyte characters in
the body, which suggests a byte-length vs. character-length mismatch when the response
preview is stored or served via CDP (Network.getResponseBody).

• ASCII-only responses render correctly.
• The app itself receives the full, valid body — only the DevTools preview is affected.
• Because the JSON is cut, Preview cannot parse it and falls back to raw text.
• Reproducible on both iOS and Android.

Steps to reproduce

1. Serve an endpoint returning:

   {"m":["Inténtelo de nuevo más tarde"]}

2. Call it with fetch() from the app while React Native DevTools is open.
3. Open the Network panel → select the request → Response / Preview tab.

Expected behavior

The full body is displayed:

{"m":["Inténtelo de nuevo más tarde"]}

Actual behavior

The body is cut short by N characters, where N is the number of multibyte characters it
contains. In the example above (2 multibyte chars), the closing "]} is missing or
partially missing, so the JSON is invalid and Preview shows raw text.

Real-world example: a 120-character body with 3 multibyte characters (ó, é, á) is
missing exactly its last 3 characters ("]}).

React Native Version

0.84.1

Affected Platforms

Runtime - iOS, Runtime - Android

Tested on iOS Simulator (iPhone 18 Pro, iOS 27) and Android emulator. New Architecture and
Hermes enabled on both.

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

System:
  OS: macOS 27.0.1
  CPU: (15) arm64 Apple M5 Pro
  Memory: 87.63 MB / 24.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.18.0
    path: ~/.asdf/installs/nodejs/22.18.0/bin/node
  Yarn:
    version: 1.22.22
    path: ~/.asdf/installs/nodejs/22.18.0/bin/yarn
  npm:
    version: 10.9.3
    path: ~/.asdf/plugins/nodejs/shims/npm
  Watchman:
    version: 2026.04.27.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods: Not Found
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 27.0
      - iOS 27.0
      - macOS 27.0
      - tvOS 27.0
      - visionOS 27.0
      - watchOS 27.0
  Android SDK: Not Found
IDEs:
  Android Studio: 2025.3 AI-253.32098.37.2534.15232325
  Xcode:
    version: 27.0/27A266a
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.19
    path: /usr/bin/javac
  Ruby:
    version: 3.2.3
    path: ~/.asdf/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.1.2
    wanted: 20.1.2
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.84.1
    wanted: 0.84.1
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: true

Stacktrace or Logs

N/A — no errors are thrown; the response preview is silently truncated.

Screenshots and Videos