Do I need an invention assignment agreement?

Yes, for every founder, employee and contractor who might contribute to an invention, and before they start work. An invention assignment agreement transfers ownership of inventions made in the course of the work from the person to the company. Without one, the default in the United States is that the inventor owns the invention and the company at best holds a right to use it, and every other jurisdiction has gaps of its own: a UK employee's invention outside their normal duties, a US contractor's invention (work-for-hire covers copyright, not patents), a co-founder's pre-incorporation idea. Investors and acquirers ask for these agreements first in due diligence, and a missing one from a person who has since left is one of the hardest problems to fix.

What a good one contains: a present-tense assignment ("hereby assigns", not "agrees to assign", the wording distinction at the heart of the Stanford v. Roche litigation); a confidentiality obligation; a duty to disclose inventions promptly and cooperate with patent filings, including signing documents after leaving; a schedule of prior inventions the person is carving out; and, where local law demands it, carve-outs for inventions made on the person's own time with their own resources (California Labor Code section 2870 and similar statutes) and a compensation mechanism (Germany, Sweden, the UK's section 40). For founders, sign a founder IP assignment at incorporation covering everything created before the company existed; for contractors, put the assignment in the consulting agreement, since without it they own what they build. Store the signed agreements with the patent files: a portfolio register is only as good as its chain of title. See also Who owns a patent when an employee invents it?