If the (web) API surface between your BE and FE is based on a schema (a.k.a. typed API, like with OpenAPIv3 or GraphQL) then I'd put them in a mono repo. This w

If the (web) API surface between your BE and FE is based on a schema (a.k.a. typed API, like with OpenAPIv3 or GraphQL) then I'd put them in a mono repo. This way you can recompile the FE automatically if the schema changed (usually an FE client lib is generated from the API schema). This helps discovering errors at compile time. If your API is not schema-based, you have no way of knowing something broke without FE/UI testing.