If after 7 or 8 weeks you can’t change the software, it’s wise to start putting tests in to document retroactively how things work / worked and why.

The more the test suite grows, the more you & future agents will be able to consult it to understand how things should work - but also, why.

Imagine a test case that covers some non-compliant API response from a third party. The commit that’s tied to, the date the test was added, all that becomes metadata.. and the fact it’s executable means your agent can’t undo that fix without something very visible in the PR.