MFN and SAFE side letters

The standard SAFE is short on purpose. Extra economics usually live in a side letter or an MFN clause, not in the form itself. Miss them and your Series A allocation math is fiction.

The big idea

The printed menu is the SAFE. The handwritten "and a seat at the next dinner" is the side letter. The analogy breaks because some extras rewrite the menu for earlier diners — that is MFN.

MFN (most favored nation)

An MFN clause says: if you later issue a SAFE with better terms, this investor can elect those terms. Useful for early angels who invest before the "standard" seed terms settle. Painful if you accidentally improve terms for one friend and trigger a cascade.

Pro-rata side letters

The standard SAFE does not automatically include ongoing pro-rata rights (the right to invest in future rounds to maintain ownership %). Many funds and angels require a side letter granting pro-rata (sometimes only for major investors above a check-size threshold).

Angels, syndicates and accelerators (including the YC deal) almost always document economic terms as SAFEs or similar convertibles plus any pro-rata side letters.

Walking through a cascade

You close an early angel at a $10M cap with MFN, no discount. Later a lead wants a $10M cap and a 20% discount. The MFN holder can elect the discount too. One "friendly" improvement rewrites earlier math. If several early SAFEs have MFN, you just upgraded the whole stack.

Pro-rata at Series A is a different bite: side-letter holders who can buy into the new round shrink the lead's available allocation. Model that before you promise the lead an ownership target.

Common mistakes

• One MFN for a friend, then better terms for a lead. The friend can take the lead's terms.
• Forgetting side letters at Series A. Pro-rata is an allocation constraint, not a courtesy.
• Assuming the SAFE PDF is the whole deal. Side letters are part of the deal.

Check yourself

You issued an MFN SAFE, then a later SAFE with a discount the first one lacked. What can the first investor do?

Elect the later, better terms — including the discount.

Does a standard YC SAFE include pro-rata?

No. Pro-rata, if any, is usually a side letter.

Key takeaways

• MFN lets an earlier holder copy later, better SAFE terms.
• Pro-rata is usually a side letter, not the form, and it crowds the next round.
• Ledger SAFEs and side letters together the day the wire lands.

Further learning

Next, choosing a cap. Control terms covers pro-rata in priced rounds.