How to allow DrupalCI to downgrade packages from core's composer.lock versions when installing the contrib project under test?

If 📌 Remove psr/http-message from drupal/core-recommended Fixed this was committed should the error occur? Or is the downstream package not updated yet? We've received a few reports from users on 10.1 beta 1 of this that led me to that, but that was before the change was made.

That issue isn't really related to this one other than that psr/http-message is involved in both. That issue only changes what's in drupal/core-recommended, which affects projects that use that, such as ones that install from drupal/recommended-project. However, when DrupalCI tests contrib projects, it does not use drupal/core-recommended for that.

What this issue is about is the fact that if you start from an installation of Drupal core, you get a composer.lock file with psr/http-message at version 2.something. In order to then composer require drupal/openai you need to let Composer know that it can downgrade dependencies of that package from what's in that lock file if it needs to (within what's allowed by all other composer.json constraints). The way you would do this for a real site is run composer require drupal/openai -w. The question is how to pass that -w option (or some other way to do the equivalent) into DrupalCI.

It's hard to imagine other contrib projects not having run into this before (with other dependencies, not just psr/http-message), so I suspect there's something you can add to drupalci.yml to do this, I just don't know what it is.

I don't think you can, per #3334914-10: Testing is broken because simplesamlphp/composer-module-installer contains a Composer plugin which is blocked → you can only customize the assessment stage per project.

The only easy solution is to change openai-php/client to also support v2, like many other projects already did in minor updates.