Describe the bug

In apollo-angular v12, the @apollo/client DefaultContext interface becomes redefined via declaration-merging and specifies the headers property to be strictly of type HttpHeaders , which wasn't the case with pre-v12 versions.

This contradicts how headers are processed and handled in general, either by the underlying @apollo/client package, which just supports Record<string, string> or even apollo-angular, which finally checks if the headers provided in a query context are instanceof HttpHeaders and if not it will just accept a regular object.

Expected behavior

I'm requesting to at least define the headers prop as a union to allow HttpHeaders | Record<string, string>, as I'm currently forced to again redeclare the DefaultContext for supporting that.

Environment:

- @angular/cli@20.3.9
- @angular/core@20.3.10
- @apollo/client@4.0.9
- apollo-angular@12.1.0
- graphql@16.10.0
- typescript@5.9.3