fix(tracing): Enable fetch instrumentation when Expo SDK 56 expo/fetch is active #6225

Description

Expo SDK 56 replaces globalThis.fetch with a native implementation (expo/fetch) that bypasses XMLHttpRequest entirely. The Sentry React Native SDK disables fetch tracing on native (traceFetch: false) because RN's default fetch is a whatwg-fetch polyfill over XHR — tracing both would cause duplicates. However, with expo/fetch active, XHR instrumentation no longer captures network requests, resulting in:

• Missing http.client spans for all fetch requests
• Missing fetch breadcrumbs
• No sentry-trace / baggage headers injected (broken distributed tracing)

Root Cause

• reactNativeTracingIntegration defaults to traceFetch: false and traceXHR: true on native
• breadcrumbsIntegration defaults to fetch: false on native
• expo/fetch is a fully native Expo Module (ExpoFetchModule) that does not use XHR
• XHR instrumentation never fires for expo/fetch requests

Affected Versions

• Expo SDK 56+ (where expo/fetch is the default)
• All versions of @sentry/react-native

Workaround

Users can manually enable fetch tracing and breadcrumbs:

Sentry.init({
  integrations: [
    Sentry.reactNativeTracingIntegration({ traceFetch: true }),
    Sentry.breadcrumbsIntegration({ fetch: true }),
  ],
});

Alternatively, opt out of expo/fetch by setting EXPO_PUBLIC_USE_RN_FETCH=1 in the .env file.

Proposed Fix

Detect expo/fetch at runtime via the Symbol.for('expo.builtin') marker that Expo's installGlobal sets on replaced globals, and flip traceFetch and fetch breadcrumbs to true when detected.

References

• Parent issue: Support Expo SDK 56 #6212
• Expo SDK 56 changelog: https://expo.dev/changelog/sdk-56
• expo/fetch source: expo/src/winter/fetch/fetch.ts (native implementation via ExpoFetchModule)
• installGlobal source: expo/src/winter/installGlobal.ts (sets Symbol.for('expo.builtin'))