If I create a tool that I can use at work but on my own time, can I open source it or sell it?

If you created in your free time, and not as an assignment from your workplace, then I definitely think you own the software and thus can do whatever you want with it.

OpenSourcing something you create in unpaid time shouldn't be a problem. Selling something depends if your contract allows other forms of income, especially if they are in the same business. If you start it open source and you want to improve it at work and include the improvements as part of the open source project, you better get permission.

It's all in the contract. I, for once, relinquished all rights to software I make during my employment period, free time or not, to my employer. I am, on the other hand, highly encouraged to open-source said software if it does not violate any trade secrets, etc.

There's also the issue of prosecution. Let's say you write something from the side and sell it. And it makes you another 1000$ a month. Your employer is very unlikely to prosecute you for said contract infringement. But once you quit your work and make it into a multi-million worth company - then you are surely risking getting you ass sued.

Read your contract, there’s usually something in there for or against the rights to any software you make while employed.

Technically speaking, any project I place a line of code on, the company gains rights to regardless of it being open source or in house, but then they would need to know about it so 🤷‍♂️

I tend not to do much on the side outside of personal projects, that way they can’t really do anything about if I never release it to the wild.

Sometimes I wonder if software I write IN work hours could be open source.. other companies do that - Microsoft for example.. do their dev teams make those decisions or does it come from the top top every time? I wouldn't want to bother our board of directors over it..