expo-fmt-consteval-fix

An Expo config plugin that fixes the fmt consteval iOS build error on Xcode 26.4+ (Apple Clang 21) for React Native / Expo projects that compile fmt from source.

If your previously healthy iOS build suddenly fails with something like this — and nothing in your own code changed — this plugin is for you:

❌  ios/Pods/fmt/include/fmt/format-inl.h:59
   call to consteval function
   'fmt::basic_format_string<char, ...>::basic_format_string<FMT_COMPILE_STRING, 0>'
   is not a constant expression

Why this happens

Xcode 26.4 ships Apple Clang 21, which enforces stricter C++20 consteval rules: a consteval call site must itself be a constant expression. The version of fmt bundled by React Native (11.0.2, used by RCT-Folly) relies on FMT_STRING(...) compile-time checks that no longer satisfy those rules, so the build fails — through no fault of your code.

The fix landed upstream by bumping fmt to 12.1.0, which only reached React Native ≥ 0.83.9 / 0.85.x (Expo SDK 56). It was not back-ported to the 0.81–0.83.2 line, so projects on Expo SDK 52–55 / React Native < 0.83.9 that build from source still hit it on Xcode 26.4+.

Who needs this

You're affected if all of the following are true:

• React Native < 0.83.9 (Expo SDK 52–55), and
• you compile native code with Xcode 26.4 or newer (locally, or on a CI image that uses it), and
• fmt is compiled from source — e.g. expo-build-properties with ios.buildReactNativeFromSource: true, or any setup that doesn't use prebuilt React Native binaries.

Note on EAS Build: EAS pins a per-SDK image. As of writing, the sdk-54 image uses Xcode 26.0 and the sdk-55/latest image uses Xcode 26.2 — both below 26.4 — so EAS builds for those SDKs are typically unaffected. The error shows up first on local machines that have updated to Xcode 26.4/26.5, and on any image pinned to Xcode 26.4+. This plugin makes the build safe regardless of which Xcode compiles it.

Install

npm install --save-dev expo-fmt-consteval-fix
# or
yarn add -D expo-fmt-consteval-fix

Usage

Add it to the plugins array in your app.json / app.config.js / app.config.ts:

{
  "expo": {
    "plugins": ["expo-fmt-consteval-fix"]
  }
}

Then regenerate the native project and reinstall pods:

npx expo prebuild -p ios
# (or run pod install if you manage ios/ yourself)

Do a clean build afterwards (in Xcode: Product → Clean Build Folder) so stale derived data from the failed build doesn't mask the result.

No options to configure — adding it to plugins is all it takes.

How it works

During expo prebuild, the plugin injects a small snippet into your Podfile's existing post_install hook. On pod install, that snippet rewrites the vendored fmt/include/fmt/base.h to force:

- #  define FMT_USE_CONSTEVAL 1
+ #  define FMT_USE_CONSTEVAL 0

With consteval disabled, fmt validates format strings at runtime instead of compile time, which avoids Apple Clang 21's stricter rules. This is a build-time-only switch — the produced binary behaves identically, since fmt's internal format strings are fixed literals that are always correct. A build that compiles with this patch is behaviorally equivalent to one compiled on an older Xcode without it.

The patch is idempotent: after the first run there are no FMT_USE_CONSTEVAL 1 lines left, so it safely no-ops on every subsequent pod install, and it does nothing if fmt isn't present.

When to remove this

This is a transitional workaround, not a permanent dependency. Remove it once you upgrade to React Native ≥ 0.83.9 / Expo SDK 56 (which bundle fmt 12.1.0 and build cleanly on Xcode 26.4+):

1. Remove "expo-fmt-consteval-fix" from your plugins array.
2. npm uninstall expo-fmt-consteval-fix.
3. npx expo prebuild -p ios --clean.

Compatibility

• Expo SDK: 52–55 (React Native 0.76–0.83.2). Built and tested against SDK 54 (@expo/config-plugins 54.x).
• @expo/config-plugins: >=7.0.0 (uses the long-stable withDangerousMod API).
• Platforms: iOS only (no-op on Android).

References

• facebook/react-native#55601 — Xcode 26.4 FMT compile errors
• expo/expo#44229 — iOS build from source fails on Xcode 26.4
• fmtlib/fmt#4740 — FMT_STRING consteval error with Apple clang 21

License

MIT