Came across this thread while researching the same issue. I manage a dev team and have seen this play out multiple times from the employer side.
The first thing your employer legal team will look at is your employment agreement, specifically any IP assignment clauses. In most states, these clauses are enforceable if they cover work done within the scope of employment or using company resources. California is a notable exception under Labor Code Section 2870, which protects employee inventions developed entirely on their own time without using employer resources.
The strongest defense is demonstrating a clear separation between your side project and your employer business. Keep records showing you used your own hardware and accounts, worked outside business hours, the project does not compete with your employer products, and you did not use any company trade secrets.
One thing people underestimate is the power of git commit timestamps and cloud storage metadata. If your repo shows commits at 10pm on weekends from your personal machine, that is strong evidence of independent development.
Check whether your state has specific protections. Several states beyond California (Delaware, Illinois, Minnesota, Washington) have statutes limiting employer claims on employee inventions. A quick consultation with an employment attorney would clarify this.