[madisp]

Quick suggestion: maybe contact the vendor company and offer to create a frontend for them instead as a contract thing?

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I've gone through the exact scenarion in your question though, this is roughly the path I took (Estonia & EU, YMMV in other jurisdictions):

- start by going over your contract - is this overall even allowed? Some companies capture all IP you create 24/7, in this case you'll need to get the contract changed or get an exception

- even if your contract allows doing other work (creating separate IP) outside of company time, still ask for written permission from your manager and possibly a skip level or two. I got a written email from my VP of eng and CTO before proceeding.

- you will probably need to _completely_ separate your own work from the company work. This means using your personal laptop, never working on this during work hours, etc. Separating this sometimes can be hard, especially if you get something like a bug report from your employer - you can't work with it until you're off the clock.

- I also completely cut myself off from the purchasing decision - I let them know I was working on a project, gave them a landing page and let the purchasing team do a decision completely on their own

- if you hope to build a business out of this it's best to be upfront about it, e.g. by telling your manager that you might leave to focus on the business full-time if it takes off

All of this assumes you're on good terms with your employer overall.