build_runner fails with "Conflicting outputs were detected". How do I fix it?

Fix

Let the tool delete the old generated files:

dart run build_runner build --delete-conflicting-outputs

(Use flutter pub run build_runner ... on very old setups.) This happens when generated *.g.dart or *.freezed.dart files already exist and were produced by a different version, or were committed and then changed.

If it still fails

1. flutter clean && flutter pub get
2. Delete the .dart_tool/build folder.
3. Run the command again with --delete-conflicting-outputs.
4. Read the first error, not the last: a syntax error in one source file blocks all generation.

Common causes

• Missing part 'file.g.dart'; directive, or a typo in the part name (it must match the file name).
• A generator package version mismatch (for example json_serializable with an old json_annotation). Keep json_annotation in dependencies and build_runner plus json_serializable in dev_dependencies, and upgrade them together.
• Running flutter pub upgrade without regenerating.

Daily workflow

• dart run build_runner watch --delete-conflicting-outputs regenerates on save.
• Decide as a team whether generated files are committed. Committing them makes CI simple; ignoring them keeps diffs small but requires a build step in CI (dart run build_runner build before tests).

Pitfall

Do not edit *.g.dart by hand; the next build overwrites it. If a generator output is wrong, change the annotation or source class.