Comment by gpm

You're creating more work for no reason. There is absolutely no reason to make dummy API responses, a feature flag (well unless you want to delay public release of the features - domain dependent here), or a glue PR.

Moreover you haven't even eliminated the dependency. The UI PR requires knowing that the dummy API responses you've created fit the right format - i.e. approval of the API PR up to small nits.

Just test against the actual implementation from the start. Even without stacked PRs just leave the second as a draft with both sets of commits until the first is merged then rebase and make it.

Stacked PRs are superior here because they eliminate that extra work of the draft PR and parallelize the review process slightly better.