MFN clause
A clause letting an early convertible investor adopt the better terms of any later convertible issued before conversion.
Most favored nation is a contractual promise of non-discrimination: if the company later issues a convertible instrument with terms more favorable to the investor (a lower cap, a discount, interest), the MFN holder may elect to amend their own instrument to those terms. The clause typically lives in uncapped SAFEs, where it substitutes for pricing: the investor accepts no cap today in exchange for inheriting whatever better deal the market later extracts. Y Combinator publishes a standard capless MFN SAFE for exactly this use.
Mechanically, the right is usually one-shot and expires at conversion: once the SAFE converts at a priced round, the MFN no longer reaches forward. It also reaches only across comparable instruments, later convertibles, not the priced round itself.
For the founder, MFN is invisible until it is expensive. The clause turns the worst-priced convertible in the stack into the effective price of every MFN instrument signed before it. The discipline is bookkeeping: keep a register of every outstanding instrument’s cap, discount and MFN status, and before signing anything cheaper than the stack, recompute the fully diluted outcome as if every MFN holder elects the new terms, because they will.
Quantum companies stack convertibles across technical milestones, so MFN exposure compounds quietly. The trap fires in a rough patch: a rescue SAFE signed at a low cap to bridge a slipped milestone retroactively reprices every MFN instrument upstream, multiplying the dilution of one bad moment. Before accepting a lower cap than anything outstanding, map which existing instruments carry MFN and compute the cascade first.
An angel signs an uncapped, discount-free MFN SAFE for $200,000 at day one. Nine months later, needing runway before a milestone, the company signs a new SAFE at a $6,000,000 post-money cap. The MFN clause lets the angel adopt those terms: the $200,000 now converts as if capped at $6,000,000, which is 200,000 / 6,000,000 = 3.33% promised at conversion, where the uncapped instrument had promised only a conversion at the future round price.
From definition to decision
Model this in your own round, scenarios, dilution and runway, in the founder workspace.