When does React Native's Codegen run on Android and on iOS, and why can a newly added or edited spec still leave the native build missing its generated interface?

seniorshould knowfreq 30%

answer

part of the native build, not Metro
Gradle: generateCodegenArtifactsFromSchema
iOS: pod install and a script phase
new spec on iOS: pod install again
run it by hand to inspect output

basics

~20 s

Codegen runs during native builds: on Android through the generateCodegenArtifactsFromSchema Gradle task, on iOS during pod install and a CocoaPods build phase. A spec is missed when misnamed or outside jsSrcsDir, or, on iOS, added without re-running pod install.

solid answer

~40 s

Codegen is part of the native build, never of Metro. On Android, the React Native Gradle plugin's generateCodegenArtifactsFromSchema task runs automatically on every build, for the app and every linked dependency, writing to android/app/build/generated/source/codegen (and to each library's own build folder). On iOS, pod install runs generate-codegen-artifacts.js, and CocoaPods adds a script phase that runs Codegen during builds; the list of spec files that phase watches is written at pod install time, so after adding a spec you re-run pod install. When the generated interface is missing, check in order: the file name starts with Native, it sits under jsSrcsDir, type includes modules, the build shows no Module <name>: parser error, and on iOS the pods were reinstalled. ./gradlew generateCodegenArtifactsFromSchema or npx @react-native-community/cli codegen shows what is actually generated.

Codegen is part of the **native** build, never of Metro. On **Android**, the React Native Gradle plugin's `generateCodegenArtifactsFromSchema` task runs automatically on every build, for the app and every linked dependency, writing to `android/app/build/generated/source/codegen` (and to each library's own `build` folder). On **iOS**, `pod install` runs `generate-codegen-artifacts.js`, and CocoaPods adds a script phase that runs Codegen during builds; the list of spec files that phase watches is written at `pod install` time, so after **adding** a spec you re-run `pod install`. When the generated interface is missing, check in order: the file name starts with `Native`, it sits under `jsSrcsDir`, `type` includes `modules`, the build shows no `Module <name>:` parser error, and on iOS the pods were reinstalled. `./gradlew generateCodegenArtifactsFromSchema` or `npx @react-native-community/cli codegen` shows what is actually generated.

code

bash · 9 lines

bash

# Android: generate and inspect Codegen output
cd android && ./gradlew generateCodegenArtifactsFromSchema
ls app/build/generated/source/codegen/java

# iOS: regenerate after adding a spec file
cd ios && bundle exec pod install

# Either platform, without a full build
npx @react-native-community/cli codegen --platform android

go deeper

for a junior

Remember that Codegen runs during the native Android or iOS build, not when JavaScript reloads.

for a middle

Name the mechanisms: the generateCodegenArtifactsFromSchema Gradle task, pod install with generate-codegen-artifacts.js, the build script phase, and the CLI for manual runs.

for a senior

Diagnose a missing generated interface by separating discovery, parser and integration failures, and remember the iOS pod install step after adding a spec.

for a principal

Make Codegen output visible in CI, for example by running it explicitly on spec changes, so contract drift is caught before a platform build fails late.

Codegen is a native build step

React Native's Codegen turns specs into C++ and platform interfaces, and native compilers need those files before they can compile your module. So Codegen runs as part of the native build, invoked by Gradle on Android and by CocoaPods and Xcode on iOS. Metro, which bundles JavaScript, never runs it; a JavaScript reload never regenerates anything. React Native's docs put it simply: Codegen is invoked automatically every time an iOS or Android app is built.

Android

The React Native Gradle plugin provides the generateCodegenArtifactsFromSchema task and runs it automatically as part of building the app.
It processes the app and every linked dependency: the app's output goes to android/app/build/generated/source/codegen, and a library's to node_modules/<library>/android/build/generated/source/codegen.
The output holds a java folder with the abstract spec classes, a jni folder with the C++ glue, and a schema.json describing what Codegen parsed.
You can run the task on its own from the android folder to inspect the result: ./gradlew generateCodegenArtifactsFromSchema.

iOS

During pod install, React Native's CocoaPods scripts run generate-codegen-artifacts.js for the iOS target, producing the Objective-C++ and C++ output and the ReactCodegen pod.
CocoaPods also adds a script phase that runs Codegen during Xcode builds.
The list of spec files that phase depends on is computed when pod install runs, by searching jsSrcsDir for Native* and *NativeComponent files. A spec added afterwards is not in that list, so the reliable step after adding or renaming a spec is to run pod install again.
The same script can be run by hand: node node_modules/react-native/scripts/generate-codegen-artifacts.js --path . --outputPath ios/ --targetPlatform ios.

The Codegen CLI

For both platforms, npx @react-native-community/cli codegen reads codegenConfig from the current package.json and generates code, with --platform and --outputPath options. It is the quickest way to see which interfaces a spec will produce before writing native code.

Why a generated interface can be missing

Symptom	Likely cause	Check
Native compile error: `NativeBatteryHealthSpec` not found, no Codegen error	Spec file not discovered	File starts with `Native`, sits under `jsSrcsDir`, is not a `.d.ts`
Same, and `codegenConfig.type` is `components`	Module generation disabled	Set `type` to `modules` or `all`
Build stops with `Module NativeBatteryHealth: ...`	Spec breaks a parser rule	Read the message; it names the rule
iOS only: new spec ignored	Pods not reinstalled after adding it	Run `pod install`
Android: Kotlin imports fail	Wrong package	Match `android.javaPackageName`
Old signature still generated	Build did not re-run Codegen	Rebuild the native app, not just reload JavaScript

The schema.json file Codegen writes next to its Android output is a useful witness: it lists every module and method Codegen parsed. If your module is missing from it, the spec was never discovered; if it is present with the old signature, the native build has not re-run Codegen since you edited the spec.

A diagnostic routine

Run Codegen by hand for the failing platform and look for your class in the output.
If it is absent, the problem is discovery: name, folder, type.
If a parser error appears, fix the spec rule it names.
If the class is present but the build still fails, the problem is integration: the package name on Android, or stale pods on iOS.
Rebuild the native app; reloading JavaScript cannot add or change native code.

Libraries that ship generated code

Most libraries leave generation to the app's build. Some ship pre-generated output instead, through a codegenConfig option for including generated code; that is a packaging decision, and it ties the output to the React Native version the library generated with.

Summary

Codegen runs inside the native build: automatically through a Gradle task on Android, and through pod install plus a build script phase on iOS. A missing interface usually means the spec was never discovered or failed a parser rule, or, on iOS, that the pods were not reinstalled after the spec was added.

## Codegen is a native build step React Native's **Codegen** turns specs into C++ and platform interfaces, and native compilers need those files before they can compile your module. So Codegen runs as part of the **native build**, invoked by Gradle on Android and by CocoaPods and Xcode on iOS. Metro, which bundles JavaScript, never runs it; a JavaScript reload never regenerates anything. React Native's docs put it simply: Codegen is invoked automatically every time an iOS or Android app is built. ## Android - The **React Native Gradle plugin** provides the `generateCodegenArtifactsFromSchema` task and runs it automatically as part of building the app. - It processes the app and **every linked dependency**: the app's output goes to `android/app/build/generated/source/codegen`, and a library's to `node_modules/<library>/android/build/generated/source/codegen`. - The output holds a `java` folder with the abstract spec classes, a `jni` folder with the C++ glue, and a `schema.json` describing what Codegen parsed. - You can run the task on its own from the `android` folder to inspect the result: `./gradlew generateCodegenArtifactsFromSchema`. ## iOS - During **`pod install`**, React Native's CocoaPods scripts run `generate-codegen-artifacts.js` for the iOS target, producing the Objective-C++ and C++ output and the `ReactCodegen` pod. - CocoaPods also adds a **script phase** that runs Codegen during Xcode builds. - The list of spec files that phase depends on is computed when `pod install` runs, by searching `jsSrcsDir` for `Native*` and `*NativeComponent` files. A spec **added afterwards** is not in that list, so the reliable step after adding or renaming a spec is to run `pod install` again. - The same script can be run by hand: `node node_modules/react-native/scripts/generate-codegen-artifacts.js --path . --outputPath ios/ --targetPlatform ios`. ## The Codegen CLI For both platforms, `npx @react-native-community/cli codegen` reads `codegenConfig` from the current `package.json` and generates code, with `--platform` and `--outputPath` options. It is the quickest way to see which interfaces a spec will produce before writing native code. ## Why a generated interface can be missing | Symptom | Likely cause | Check | |---|---|---| | Native compile error: `NativeBatteryHealthSpec` not found, no Codegen error | Spec file not discovered | File starts with `Native`, sits under `jsSrcsDir`, is not a `.d.ts` | | Same, and `codegenConfig.type` is `components` | Module generation disabled | Set `type` to `modules` or `all` | | Build stops with `Module NativeBatteryHealth: ...` | Spec breaks a parser rule | Read the message; it names the rule | | iOS only: new spec ignored | Pods not reinstalled after adding it | Run `pod install` | | Android: Kotlin imports fail | Wrong package | Match `android.javaPackageName` | | Old signature still generated | Build did not re-run Codegen | Rebuild the native app, not just reload JavaScript | The `schema.json` file Codegen writes next to its Android output is a useful witness: it lists every module and method Codegen parsed. If your module is missing from it, the spec was never discovered; if it is present with the old signature, the native build has not re-run Codegen since you edited the spec. ## A diagnostic routine 1. **Run Codegen by hand** for the failing platform and look for your class in the output. 2. If it is absent, the problem is **discovery**: name, folder, `type`. 3. If a parser error appears, fix the spec rule it names. 4. If the class is present but the build still fails, the problem is **integration**: the package name on Android, or stale pods on iOS. 5. Rebuild the native app; reloading JavaScript cannot add or change native code. ## Libraries that ship generated code Most libraries leave generation to the app's build. Some ship pre-generated output instead, through a `codegenConfig` option for including generated code; that is a packaging decision, and it ties the output to the React Native version the library generated with. ## Summary Codegen runs inside the native build: automatically through a Gradle task on Android, and through `pod install` plus a build script phase on iOS. A missing interface usually means the spec was never discovered or failed a parser rule, or, on iOS, that the pods were not reinstalled after the spec was added.

Why doesn't a fast refresh pick up a new method added to the spec?
Fast refresh reloads JavaScript through Metro, but the new method also needs regenerated native interfaces and a native implementation compiled into the app. Codegen runs only in the native build, so you must rebuild the Android or iOS app (and on iOS re-run pod install if you added a new spec file).
Where does Codegen put a library's generated Android code?
In the library's own build folder under `node_modules`, for example `node_modules/<library>/android/build/generated/source/codegen`, because the Gradle task runs for every linked dependency during the app's build. The app's own specs generate into `android/app/build/generated/source/codegen`.

saying these in an interview costs you the question

Codegen runs whenever Metro rebuilds the JavaScript bundle
A misnamed spec file always produces a clear Codegen error
On iOS a newly added spec is picked up without re-running pod install
Reloading JavaScript is enough after changing a spec's method signature
Library specs must be generated by hand before the app can build

When does React Native's Codegen run on Android and on iOS, and why can a newly added or edited spec still leave the native build missing its generated interface?

§ answer

basics

solid answer

code

§ go deeper

▾level notes4 levels

▾deep dive~3 min

Codegen is a native build step

Android

iOS

The Codegen CLI

Why a generated interface can be missing

A diagnostic routine

Libraries that ship generated code

Summary

▾common follow-ups2 questions

▾red flags5 flags

Codegen is a native build step

Android

iOS

The Codegen CLI

Why a generated interface can be missing

A diagnostic routine

Libraries that ship generated code

Summary

answer

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