Will My Employer Claim Ownership Of A Software I Had The Idea For Before Starting With Them That Ties To The Job?

I work in an industry where inefficiencies are common, and I developed a software tool to automate tasks that everyone in the field struggles with that ties directly to my job duties (account manager). I built this tool on my own time and with my own resources, using publicly available API documentation that I knew about prior to me starting at my company from a SaaS platform my company uses. The software worked perfectly, but when I first implemented it, I used an API key generated from my company’s account to integrate the tool during testing. That API key was used only for implementation, not during development. My company then wrote me up for not seeking approval first.

Later, I demoed the tool for my employer, and they were very impressed. Leadership even asked, “Why haven’t you patented this?” and “How much will you charge companies to use this?” While their reaction felt validating, I now fear they’ll claim ownership or retaliate if I move forward with launching this as a SaaS.

Here’s what I need to know:

Does using public APIs (with an API key for implementation) create any legal claims for my employer?

Since I built this tool independently, do I own it?

What steps should I take to protect myself legally before launching?