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[Due for payment 2026-11-02] Fix Hermes version mismatch between prebuilt iOS React Native artifacts and the app build #100503
Description
Problem
A Hermes version mismatch was found between the prebuilt iOS React Native artifacts we publish and the app that consumes them. Discovered while working on Expensify/App#86437 (Slack thread).
When the Hermes the artifacts were built against differs from the Hermes the app links, we get a mismatch at build/run time rather than a clean "no matching artifact, build from source" fallback.
Starting hypothesis (needs confirmation)
Two things look like they could produce this, both in Expensify/App:
-
The Hermes V1 flag is only set on the consumer side.
ios/Podfile:90setsENV['RCT_HERMES_V1_ENABLED'] = '1', and that is the only place in the repo it is set. The publishing workflow only passesHERMES_VERSIONwhen it sets up the prebuild workspace (.github/workflows/publishReactNativeiOSArtifacts.yml:65-77) and never setsRCT_HERMES_V1_ENABLED. So artifacts may be built without Hermes V1 while the app is built with it. -
The Hermes configuration is not part of the artifact resolution key. Artifacts are matched on react-native version plus a hash of our patches (
scripts/artifacts-utils/lib/artifactsResolver.ts). Nothing about Hermes (version or the V1 flag) is in the key, so an artifact built against a different Hermes still resolves as a match instead of falling back to a source build.
This is a code-reading hypothesis, not a verified root cause — the investigation should confirm it against a real failing build before fixing.
Solution
- Confirm the actual mismatch — which Hermes the published artifacts were built against versus which one the app links.
- Make the publish workflow and the consumer build agree on the Hermes configuration (including
RCT_HERMES_V1_ENABLED). - Include the Hermes identity in the artifact resolution key so a mismatched artifact can never resolve as a match, and the build falls back to source instead of failing.
Context
Follow-up to Expensify/App#86437. Related earlier note on Hermes V1 versus RN core prebuilds: comment.
Activity
mateuuszzzzz commentedon Sep 9, 2026
PR was merged, we just need to trigger artifacts build with this new change
roryabraham commentedon Sep 9, 2026
@mateuuszzzzz doesn't it happen on merge?
melvin-bot commentedon Sep 10, 2026
@roryabraham @mateuuszzzzz
The solution for this issue has been 🚀 deployed to production 🚀 in version 9.4.75-3 and is now subject to a 7-day regression period 📆. Here is the list of pull requests that resolve this issue:
mateuuszzzzz commentedon Sep 11, 2026
@roryabraham I have very hazy memories of how it used to work in the past. But I think it does not trigger build in case of workflow edits. Let me know if you prefer it to work this way and I'll raise a PR with a fix of current behavior 👍
17 remaining items
mateuuszzzzz commentedon Oct 20, 2026
I think both PRs are in final shape. But I would like to merge #103149 first and then slightly adjust #103317 based on changes from the first one. We also need to build artifacts for #103317 before merging it.
roryabraham commentedon Oct 20, 2026
#103149 merged. Let me know when #103317 is ready to have artifacts built.
[-]Fix Hermes version mismatch between prebuilt iOS React Native artifacts and the app build[/-][+][Due for payment 2026-10-28] Fix Hermes version mismatch between prebuilt iOS React Native artifacts and the app build[/+]melvin-bot commentedon Oct 21, 2026
@aimane-chnaif
The solution for this issue has been 🚀 deployed to production 🚀 in version 9.5.15-3 and is now subject to a 7-day regression period 📆. Here is the list of pull requests that resolve this issue:
If no regressions arise, payment will be issued on 2026-10-28. 🎊
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mateuuszzzzz commentedon Oct 22, 2026
@roryabraham PR is ready to build artifacts from it. Before merging let's double check that POM file includes hermes version
[-][Due for payment 2026-10-28] Fix Hermes version mismatch between prebuilt iOS React Native artifacts and the app build[/-][+][Due for payment 2026-11-02] Fix Hermes version mismatch between prebuilt iOS React Native artifacts and the app build[/+]melvin-bot commentedon Oct 26, 2026
@aimane-chnaif
The solution for this issue has been 🚀 deployed to production 🚀 in version 9.5.20-5 and is now subject to a 7-day regression period 📆. Here is the list of pull requests that resolve this issue:
If no regressions arise, payment will be issued on 2026-11-02. 🎊
melvin-bot commentedon Oct 26, 2026
@aimane-chnaif Eep! 4 days overdue now. Issues have feelings too...