Read the side letter as its own contract, not as paperwork stapled to the SAFE, the Simple Agreement for Future Equity it accompanies. Six questions get you most of the way: who holds the right, what triggers it, when it ends, whether the investor can hand it to someone else, how it can be changed later, and whether it fits what you already promised earlier investors.
Two different things arrive under the words "side letter." One is Y Combinator's published pro rata agreement, a roughly one-page form creating a single purchase right tied to one financing. The other is whatever an investor's own counsel drafted. That version may add information rights, a board observer seat, or approval rights over company decisions. The published form has none of those. So start by working out which of the two you're holding.
The pre-signature checklist
One row per question. Run it before you countersign.
| Question | What to check |
|---|---|
| Who holds it | One named investor, tied to one SAFE. Assignment needs your written consent, except inside the investor's affiliate group. |
| What triggers it | In the published form, a right to buy Standard Preferred Stock in the Equity Financing. It sets no price. |
| When it ends | Earliest of the financing's initial closing, or immediately before a liquidity or dissolution event. |
| Is there a clock | The form sets no exercise timetable. A reminder you set internally doesn't bind the investor; a deadline the investor is bound by has to be a negotiated term in the document. |
| How it changes | Two amendment routes, an equal-treatment condition on the group route, prompt notice to a holder who didn't consent. |
| Does it fit | Definitions and governing law come from the SAFE, and your outstanding SAFEs and letters may already create obligations. |
The rest of this piece works through the rows that take the most reading.
This picks up after you've chosen the instrument. If the open question is still SAFE versus note versus priced round, that's a different decision: SAFE vs. convertible note vs. priced round. For how this sits inside the broader work of raising, see capital formation.
The form descriptions here come from three documents on Y Combinator's safe page: the post-money valuation cap SAFE, the post-money MFN-only SAFE, and the pro rata agreement. Model forms aren't statutes, and the copy in your inbox may not match the copy I read. Whether a provision binds your company, and what approvals you need to sign it, turns on your entity, your state of incorporation, your executed documents, and your facts.
What the standard letter is doing
The pro rata agreement is a separate signed agreement between the company and one investor, entered into around the date of that investor's SAFE. It says on its face that it's given as a material inducement to the investment. The investor is telling you, in the document, that the money and the letter arrived as a package.
Commercially, the right is an opportunity to buy a specified amount of the preferred stock the company sells in the next priced round. Whether that participation holds the investor's percentage steady depends on the size of the round, how much the investor actually buys, and other issuances such as an option-pool increase. It's a right to buy. It sets no price. It gives the investor nothing more of the company today.
The letter builds that out of specific parts. The investor gets the right to purchase its pro rata share of Standard Preferred Stock being sold in the Equity Financing. Both are defined terms. Neither is defined in the letter. They come from the SAFE, and that one design choice drives much of what follows.
Who holds the right, and does it travel?
The letter's assignment paragraph restricts the investor. Neither the agreement nor the rights in it may be assigned without the company's prior written consent. Then it carves out an exception for entities that control, are controlled by, or are under common control with the investor. That carve-out reaches past a corporate parent. It names general partners, managing members, officers and directors of the investor, and venture capital funds controlled by the investor's general partners or managing members or sharing the same management company.
So the right travels inside the investor's own house without asking you. Everywhere else, it needs your written consent.
Worth separating two things here, since they are easy to conflate on a first read. The published form does contain consent mechanics: yours for assignment, and the amendment routes below. What it doesn't contain is any investor approval right over company decisions. A custom letter asking for a veto on hiring, budgets, or a future financing is a different animal, and nothing in the pro rata analysis here speaks to it.
The cap SAFE handles transfer differently. Its provision runs both ways, so neither party can transfer without the other's prior written consent. Its exception list has two branches rather than one. The affiliate branch reads much like the letter's. The second branch, which the letter doesn't repeat, permits assignment without the company's consent to the investor's estate, heirs, executors, administrators, guardians and successors in the event of the investor's death or disability.
If your SAFE investor is an individual rather than a fund, that difference is worth a question to counsel before signature: how should the two documents line up on transfer? Silence in one document is neither permission nor prohibition. It's an open question, and it is easier to answer while the documents are still being drafted than after both are signed.
When does the right start, and when does it end?
The pro rata right terminates automatically on the earliest of three events: the initial closing of the Equity Financing, immediately prior to the closing of a Liquidity Event, or immediately prior to the Dissolution Event. Each is defined in the SAFE. Equity Financing in the cap SAFE means a bona fide capital-raising transaction or series of transactions in which the company issues and sells Preferred Stock at a fixed valuation.
Read the trigger and the termination together and the shape is clear. The right is keyed to one priced round. In the standard form it isn't a standing right that follows the investor through Series A, B, and C. And it ends at the initial closing of that round, not after it.
Which raises a sequencing problem. The letter creates the right and sets the moment it expires. The text I reviewed sets out no exercise notice or election timetable: no lead time for telling the investor, no window for the investor to decide, no consequence if the investor is slow. The form isn't silent on notice generally. The company has to promptly notify the investor of any amendment, waiver, or modification the investor didn't consent to. What's missing is specifically the exercise clock.
That gap changes how you plan, and it changes how you should describe whatever you build to fill it. You can decide internally when your finance lead reaches out and how much lead time you want. That's your own administrative schedule. It doesn't bind the investor, it doesn't create a deadline the investor agreed to, and it doesn't amend the contract. An agreed window is a term to negotiate into the document. Before signature, that is a drafting question. Afterward it is an amendment, and amendments run through the amendment provision described below, not through an informal request.
How the pro rata math works
The letter doesn't say "proportionate ownership." It gives a ratio.
The numerator is the number of shares of Capital Stock issued from the conversion of all of that investor's SAFEs carrying a Post-Money Valuation Cap, as defined in the pro rata agreement. The denominator is Company Capitalization.
Company Capitalization is defined in the cap SAFE. It's calculated as of immediately prior to the Equity Financing, without double-counting, and in each case on an as-converted to common stock basis. It sweeps in outstanding stock, convertible securities, and options, including promised but ungranted ones, and it treats the unissued option pool with a condition: an increase to that pool made in connection with the financing counts only to the extent promised options exceed the pool before the increase. That condition calls for judgment rather than arithmetic, which is where this calculation can go wrong.
Two practical effects follow.
Somebody has to be able to produce that denominator, and it isn't purely arithmetic. The definition turns on judgments about which promised options qualify and how a financing-related pool increase gets treated. Those judgments benefit from counsel reading the definition against your actual records. If your cap table lives in a spreadsheet one person understands, you'll find that out while a round is closing.
The numerator also counts only that investor's post-money cap SAFEs. If the same investor holds an instrument without a valuation cap, the formula as written doesn't count it there. Whether that matches what you both expect is worth settling in writing before signature rather than during the round.
Which document controls when the two disagree?
The letter borrows. Its capitalized terms take their meanings from the investor's SAFE, and its choice of law is set to be consistent with the law in that SAFE. The SAFE's governing-law provision is a blank in the published form, pointing at a state you have to fill in. So the letter is bolted to the SAFE for definitions and for governing law. If those blanks are unfilled or filled carelessly, the letter inherits the problem.
The letter also carries its own amendment provision, and it isn't the SAFE's. Any provision may be amended, waived, or modified with the company's written consent plus either the investor, or the holders of a majority of the shares issued from all SAFEs converted in connection with the Equity Financing held by that investor and other holders with pro rata rights on the same form. The group route carries a condition: the change has to treat all those holders the same way. And the company has to promptly notify an investor of any change that investor didn't consent to.
The SAFE's own amendment provision uses a different group and a different yardstick. It measures by purchase amount across then-outstanding SAFEs sharing the same cap and discount terms, not by shares issued at the financing. So the two documents don't line up, and the letter's group route is measured by conversion shares at the very financing whose initial closing terminates the right. If you plan to hand the same letter to several investors, put that to counsel as a question: when is the group route practically available, and what does that mean for making changes across holders?
One more sentence deserves attention. The letter states that it's the form published on YC's documents page and that neither party has modified it except to fill in blanks and bracketed terms. The SAFEs carry a parallel statement. If you negotiate substantive custom language, that representation no longer describes the document in front of you. How you handle that is a drafting decision, not a typo.
Does signing a side letter trigger another investor's MFN?
Keep this narrow.
The post-money MFN-only SAFE, where "MFN" stands for "most favored nation," doesn't automatically adopt later terms. Its Section 3 is a notice-and-election mechanic. If the company issues Subsequent Convertible Securities with terms more favorable than that SAFE before it terminates, the company promptly gives the investor written notice with a copy of those securities. Upon the investor's written request, the company also provides any additional information related to them as may be reasonably requested. If the investor determines the newer terms are preferable, the investor notifies the company in writing within 10 days of receiving the notice. Promptly after that written notice, the company agrees to amend and restate the instrument to be identical to the newer one.
The definition matters as much as the mechanic. Subsequent Convertible Securities excludes side letters and ancillary agreements that do not amend or modify the terms of such convertible securities.
So the answer is conditional and specific to the letter in front of you. A given side letter is not categorically inside that definition and not categorically outside it. Whether it amends or modifies the terms of the convertible security it accompanies depends on what the letter says and what instrument it sits beside. That's a reading exercise on your own documents, done before signature.
A decision framework before you sign
Four questions. Answer them with your co-founders and your finance lead, then take what's left to counsel.
1. Which document did we actually receive? Compare it against the published form and list anything extra, including information, observer, or company-decision approval provisions.
2. What event does this right attach to? If the answer is "our next raise," check whether that raise fits the Equity Financing definition in the SAFE the letter borrows from.
3. Do we want an agreed exercise window, or will we administer this ourselves? Only the first one lives in the document, and asking for it during drafting is a different conversation than seeking an amendment after both parties have signed.
4. How many holders of this right can the next round absorb? Our read is that each letter adds a holder who may ask to participate, and holders may or may not exercise, so what you can promise a future lead depends on how much of that participation actually shows up. Plan a range, not a single number.
What this means for different readers
For founders raising now. Before you countersign, changes to the letter are a drafting question; afterward they run through its amendment provision. The ask doesn't have to be "remove the pro rata right." It can be mechanical: an exercise window if you want one, clarity on which of the investor's instruments count in the numerator, and consistency with letters already outstanding. Those are narrower questions than the right itself.
For operators and finance leads. Administering this lands on you, not on the founder who signed it. You need the signed letters in one place, a current list of who holds a pro rata right, and a documented method for the Company Capitalization calculation that counsel has reviewed. Because these rights terminate, that list needs a closing column, not just an opening one.
For boards and existing investors. Side letters are where the terms of a financing stop being uniform. A board that has reviewed only the SAFE form has reviewed only part of what the company agreed to. The question for the next meeting: can anyone produce a complete list of the ancillary agreements signed alongside each instrument, and were the corporate approvals those agreements contemplate obtained? The same question surfaces in diligence, which is why it belongs in your fundraising legal diligence checklist.
The counterpoint worth sitting with
A negotiated investor right isn't a red flag. An investor who wants to keep funding you as you grow is asking for the ability to do exactly that, and the pro rata agreement is one published form for papering that request. Refusing it on principle has its own cost: capital you may want later, from an investor who has already decided to fund you once.
What creates trouble, in my view, is volume without records. Several letters signed with no central list. One whose mechanics nobody read. An allocation promised to a new lead that was already committed. Saying yes with your eyes open is a fine outcome. So is negotiating the mechanics. The thing to avoid is signing a document you haven't read.
Practical takeaways
- Compare the letter you received against the published form first. If it adds information rights, an observer seat, or approval rights over company decisions, those are separate terms with their own consequences, and none of the pro rata analysis here covers them.
- Read the termination sentence early, not last. In the published form the right ends at the initial closing of the equity financing, so the entire value of the right sits in the window before that closing.
- Decide before signature whether you want an agreed exercise window in the document. The published form sets no exercise clock, and a schedule you adopt internally organizes your own team without binding the investor or changing the contract.
- If an individual rather than a fund is signing, raise the transfer difference with counsel before signature. The cap SAFE's exception list covers death and disability transfers; the pro rata letter's doesn't, and you want that difference to be deliberate.
- Have counsel walk the Company Capitalization definition against your actual cap table, including the option-pool condition. The numerator and denominator are the entire economics of the right, and a pool increase made in connection with the financing is treated differently from the pool that existed before it.
- Before you sign a second letter, read the first one's amendment provision. The letter's group route is measured by shares issued on conversion at the financing, which is not the SAFE's measure by purchase amount, and the mismatch is worth understanding before you have five holders.
- Fill in the SAFE's governing-law blank deliberately. The letter's choice of law follows the SAFE's, so one unfilled blank leaves both documents pointing nowhere.
Frequently asked questions
Is a pro rata side letter a bad sign from an investor?
Not by itself. The published pro rata agreement describes itself as a material inducement to the investment, which tells you the investor treats the right as part of the deal rather than a bonus. That says nothing about the particular letter in your inbox. The questions to work through are what this letter actually grants, what it adds beyond the published form, and whether those mechanics work for your company.
Does signing one pro rata letter mean I have to give the same right to everyone?
Not by itself. Nothing in the pro rata form I reviewed requires equal treatment across investors who aren't parties to it. What can produce that effect is a different instrument you already signed, such as an MFN provision, which is why the pre-signature review covers your existing documents and not only the new letter.
Can I sign the published form without changes?
Nothing in the form requires you to negotiate it first. What matters is reading it against your own documents and approvals before you decide. The form creates a purchase right tied to one financing, sets three termination triggers, restricts assignment with a broad affiliate exception, provides two amendment routes, requires prompt notice of changes an investor didn't consent to, and borrows its definitions and governing law from the SAFE. It gives you no exercise clock, and it doesn't tell you what corporate approvals you need.
What belongs with counsel, as opposed to with me?
The business questions are yours: whether you want this investor in the next round, how much allocation you can commit, who handles outreach. The document questions belong with counsel. How the letter and the SAFE interact on transfer and amendment. What approvals your entity needs. How the governing-law choice affects the terms, how the Company Capitalization definition applies to your records, and whether anything in the letter touches commitments already outstanding.
Closing perspective
The right a side letter grants is not, on its own, what creates trouble. What creates trouble is a right sitting in a document nobody filed, administered by nobody in particular, until a financing forces the question in the middle of a closing. A pro rata right is a line in a cap-table model. An unread pro rata right is a surprise in the middle of a closing.
So the question worth answering this week isn't whether to sign. It's how many of these rights your next round can absorb, and who in your company owns the list.
This article is for informational purposes only and does not constitute legal advice. Every company's situation is different, and you should consult with qualified legal counsel before making compliance decisions based on the developments discussed here.
