The Hermes JavaScript engine failed to build or compile the bundle. Hermes is tied to the React Native version, so a mismatch between node_modules and pods breaks it.
What this error means
The iOS or Android build fails in a Hermes step, e.g. a hermesc compile error or a Hermes pod build failure, often after a React Native version change.
react-native
error: building Hermes from source failed
[!] /bin/sh -c ... node_modules/react-native/sdks/hermes ... exited with non-zero status
Diagnose it: SDK, signing, or simulator?
Mobile CI failures cluster into three: a missing or mismatched SDK component, a code-signing identity that does not exist on the runner, or a simulator or emulator that never became ready.
Terminal
# Android
sdkmanager --list_installed 2>/dev/null | head
echo "ANDROID_HOME=$ANDROID_HOME"
adb devices
# iOS
xcodebuild -version && xcrun simctl list devices available | head
security find-identity -v -p codesigning
Common causes
Hermes version mismatch with React Native
Stale node_modules or pods leave a Hermes version that does not match the current React Native.
Stale build artifacts
Cached Pods, DerivedData, or Gradle outputs reference an older Hermes build.
How to fix it
Reinstall dependencies and pods cleanly
Run npm ci so node_modules matches the lockfile.
Reinstall pods so the Hermes pod matches the React Native version.
shell
npm ci
(cd ios && pod install --repo-update)
Clear stale Hermes caches
Remove ios/Pods, DerivedData, and Gradle caches that pin an old Hermes, then rebuild.
How to prevent it
Keep node_modules, pods, and Hermes aligned to the React Native version and key caches on the lockfiles. iOS Hermes builds require a macOS runner; build Android on Linux to control cost.
Frequently asked questions
What causes React native "Hermes build failed" in CI?
There are 2 common causes: hermes version mismatch with react native and stale build artifacts. Stale node_modules or pods leave a Hermes version that does not match the current React Native.
How do I fix React native "Hermes build failed" in CI?
There are 2 fixes depending on which cause you have: reinstall dependencies and pods cleanly and clear stale hermes caches. Work through them in order, since the first is the most common.
What does React native "Hermes build failed" in CI actually mean?
The iOS or Android build fails in a Hermes step, e.g.
How do I stop React native "Hermes build failed" in CI happening again?
Keep node_modules, pods, and Hermes aligned to the React Native version and key caches on the lockfiles.
