SAFE Side Letters Explained: Pro Rata and MFN
The SAFE itself is often a short, standard document. The surprises tend to live next to it, in a side letter that gives one investor extra rights. Pro rata rights and MFN clauses are the two you will see most, and both can shape your next round in ways that are easy to miss when you are just happy to close a check. Here is what they mean in plain English, and how to handle them calmly.
Key takeaways
- A side letter is a separate agreement that gives a specific investor rights beyond the standard SAFE.
- Pro rata rights let an investor buy more shares in a future round so they can keep their ownership percentage.
- An MFN (most favored nation) clause lets an earlier holder adopt better terms you later give other investors.
- Keep a simple tracker of every side letter so nothing surprises you or your next lead investor.
If you have raised money on SAFEs, or you are about to, you have probably heard an investor say something like "we'll just need a pro rata side letter." It sounds small and routine, and often it is. But each side letter is a promise you are making about the future, and promises add up. Five friendly side letters signed over a year can leave you with less room than you expected when a lead investor arrives.
This guide explains the two most common side letter rights, shows how they work with simple made-up numbers, and gives you a tracker template and a negotiation checklist. We will not go back over SAFE basics here; the focus is only on what sits alongside the SAFE.
A quick note: this is general information, not legal or investment advice. Side letters are legal contracts, and wording matters a great deal. Have a startup lawyer review every one before you sign.
What a SAFE side letter is
A side letter is a short agreement between your company and one investor. It sits next to the main investment document and adds, or sometimes changes, specific rights for that investor only. The main SAFE stays standard, which keeps things simple for everyone else in the round.
The standard Y Combinator SAFE is a post-money SAFE, and many early rounds use it as the base document. Because the SAFE is deliberately lean, investors who want something extra usually ask for it in a side letter instead of editing the SAFE itself. That has a real benefit for you: you can see exactly which investor has which extra right, instead of hunting for small edits buried in otherwise identical documents.
Why investors ask for side letters
- To protect their position. An early investor takes a big risk and wants a fair chance to invest more if things go well.
- To avoid being disadvantaged. If a later investor gets a better deal, the early investor wants access to it.
- To meet their own fund rules. Some funds need certain information or rights to satisfy their own investors.
Pro rata rights explained
A pro rata right gives an investor the option to buy into a future financing so they can keep roughly the same ownership percentage. "Pro rata" simply means "in proportion." It is an option, not an obligation: the investor can use it, use part of it or ignore it.
How pro rata works with simple numbers
Example
Imagine an angel whose SAFE converts into 2% of your company when you raise your seed round. If that new round sells a further 20% of the company to new investors, the angel's stake would shrink. A pro rata right lets the angel buy a slice of the new round sized to their 2%, so in very simple terms they could take about 2% of the shares being issued in that round. If the round is $2,000,000, that is roughly $40,000 of the allocation. (All numbers are made up, and the exact calculation depends on the wording in the side letter.)
The key point for founders: every dollar of pro rata an existing investor takes is a dollar a new investor cannot take. In a hot round where the lead wants a large allocation, several pro rata rights can create tension.
Questions to ask about any pro rata right
- Which rounds does it cover? Only the next priced round, or every future round?
- How is the percentage calculated? On what share count, and at what moment?
- How long does the investor have to decide? A short, clear notice period keeps your round moving.
- Does it end? Many founders prefer rights that expire after the next priced round, when standard investor rights take over.
Pro tip
If you offer pro rata, consider offering it consistently to investors above a set check size, and write that rule down. A clear, simple policy is easier to explain to a future lead than a patchwork of one-off promises.
MFN clauses explained
An MFN, or most favored nation, clause protects an earlier SAFE holder if you later give someone else better terms. If you issue a later SAFE with, say, a lower valuation cap or an extra right, the MFN holder can usually choose to adopt those better terms for themselves.
How an MFN plays out
Example
Imagine you raise $100,000 on an uncapped SAFE with an MFN side letter from a friendly early backer. Three months later, a new investor will only come in on a SAFE with a valuation cap of $6,000,000. Because of the MFN, your early backer can elect to switch to the capped terms too. You now have two investors on the capped terms instead of one. (Numbers are made up for illustration.)
That is not unfair. It is exactly what the MFN is for, and it rewards the investor who trusted you first. The risk is only that founders forget about the MFN and are surprised when they offer a sweet deal to one investor and find it spreads further than planned.
What usually counts as "better terms"
This depends on the drafting, which is why your lawyer matters. Commonly the clause covers economic terms such as the valuation cap and discount, and sometimes other rights like pro rata or information rights. Some MFN clauses only apply to SAFEs issued before your next priced round. Read the definition closely, and ask whether the investor must adopt the whole package of new terms or can pick and choose.
Other rights that appear in side letters
Pro rata and MFN get the most attention, but side letters can carry other requests. None of these is automatically bad. You simply want to know what you are agreeing to and how it will feel to deliver it for years.
- Information rights. Regular financial updates or annual statements. Fine in principle, but agree a format and frequency you can realistically keep.
- Board observer seats. A right to attend board meetings without a vote. Think hard about meeting size and confidentiality before agreeing.
- Consent or veto rights. Rare at the SAFE stage and worth serious pushback, since they can slow future decisions.
- Specific use of funds or reporting promises. Keep these broad enough that you can still adapt your plan.
Common mistake
Signing a side letter from an investor's own template without checking how it interacts with your other SAFEs. Two side letters with slightly different definitions can conflict. Ask your lawyer to keep your side letters as consistent as possible, ideally from one company template.
How to negotiate side letters calmly
Most investors asking for side letters are not trying to trap you. They are protecting themselves in a reasonable way. You can say yes to fair requests and still keep your future flexible.
- Ask what problem the right solves. Understanding the investor's concern often reveals a simpler fix.
- Start from your own template. Offering a standard side letter keeps terms consistent across investors.
- Limit scope and time. Tie rights to the next priced round where you can, with clear end points.
- Think about your future lead. Picture explaining every side letter to a new investor. Would any of them be awkward?
- Get legal review. Have your lawyer read the final version, not just an early draft.
- Log it immediately. Add the side letter to your tracker and data room the day it is signed.
A simple side letter tracker
Copy these columns into a spreadsheet and keep it next to your cap table. The names and amounts below are made-up examples.
| Investor | SAFE amount | Date signed | Pro rata | MFN | Other rights | Expires |
|---|---|---|---|---|---|---|
| Example Angel A | $50,000 | 2027-02-10 | Yes, next priced round | No | Annual financials | After next priced round |
| Example Fund B | $250,000 | 2027-03-22 | Yes | Yes, until priced round | Quarterly update | After next priced round |
| Example Angel C | $25,000 | 2027-05-04 | No | Yes | None | After next priced round |
When you start talks for a priced round, this one sheet answers some of the first questions a lead investor's lawyer will ask, and it shows you are organized.
Your side letter action plan
- Collect every signed SAFE and side letter in one folder
- Build a tracker with pro rata, MFN and other rights
- Check which MFN holders could adopt any new terms you plan to offer
- Estimate how much pro rata could be claimed in your next round
- Create a standard company side letter template with your lawyer
- Set a clear policy for who receives pro rata rights
- Share the tracker with your lawyer before your next raise
Frequently asked questions
Do I have to give pro rata rights to every SAFE investor?
No. Pro rata rights are negotiated, not automatic. Many founders offer them only to investors above a certain check size. Whatever you decide, apply it consistently and write it down.
Is an MFN clause bad for founders?
Not necessarily. It is a common way to reward early investors who commit before terms are set. The risk is forgetting about it. Track MFNs and check them before you offer anyone better terms.
What happens to side letters at the next priced round?
It depends on the wording. Many side letters are written to end when the SAFE converts in a priced round, with investor rights then covered by the new round documents. Check each letter with your lawyer.
Can a new lead investor ask existing investors to waive pro rata?
It happens, especially when a round is oversubscribed. Existing investors are not obliged to agree, so keep relationships warm and communicate early.
Related reading
The bottom line
Side letters are where SAFEs get personal. Pro rata rights let early backers keep their share, and MFN clauses make sure they are not left behind by better later deals. Both are reasonable, and both can surprise you if you sign them casually and forget them.
Use a standard template, limit rights to the next priced round where you can, get every letter reviewed by a lawyer and keep a one-page tracker. That small habit keeps your next fundraise cleaner and earns trust from every investor at the table.
