Fundraising Documents
SAFE Side Letters Explained: Pro Rata, MFN, and Information Rights
The SAFE stays standard on purpose — everything negotiated lives in the side letter. Here is what investors ask for, what each ask costs you, and a framework for saying yes, no, or “at this check size.”
Quick Answer
A SAFE side letter is a separate, binding agreement signed alongside a standard SAFE that grants an investor rights the SAFE itself does not include. The most common is the pro rata right: Y Combinator publishes an optional one-page pro rata side letter with its post-money SAFE that lets the investor purchase enough of the priced round the SAFE converts into to maintain their ownership percentage. Other frequent asks are information rights (periodic company updates), MFN protection (adopting better terms given to later SAFE investors), and occasionally 'Major Investor' status at conversion. Side letters exist so the SAFE form stays unmodified — but they are real contracts: each one should be a deliberate, per-investor decision, tracked in your records, and disclosed in diligence, because pro rata and MFN commitments directly shape your next priced round.
Written by Michael Kaufman · Reviewed against our editorial standards · Updated
Primary sources: Y Combinator’s official SAFE financing documents (which include the optional pro rata side letter) and the YC Post-Money Safe User Guide (PDF), which models the dilution impact of pro rata participation.
Key Takeaways
- 1.A side letter keeps the SAFE standard: the instrument stays unmodified, and every negotiated extra lives in one visible, signed document alongside it.
- 2.The post-money SAFE deliberately has no built-in pro rata right — YC moved it to an optional side letter in 2018 so founders grant it per investor, not by default.
- 3.Pro rata is an option written against your future Series A allocation. Granted casually across a party round, the exercised rights collide with your lead's target ownership.
- 4.Information rights and MFN are cheap if scoped and tracked; board observers and pre-negotiated 'Major Investor' status usually are not.
- 5.Every side letter is binding and diligence-visible. Keep one running list: investor, SAFE terms, side letter yes/no, and exactly what it grants.
Why Side Letters Exist at All
The entire value of the SAFE is standardization: an investor who recognizes the unmodified YC form can sign it without a redline cycle, and the next lawyer to read your stack of twelve SAFEs knows that eleven identical documents really are identical. Editing the SAFE’s text to accommodate one investor destroys that property for everyone. So the market converged on a clean convention — the SAFE stays standard; anything negotiated goes in a side letter. The side letter is a short, separately signed contract between the company and that one investor, sitting next to the SAFE and referencing it.
This is also why “it’s just a side letter” is exactly backwards as a founder instinct. The SAFE’s terms are known quantities; the side letter is where the bespoke — and therefore risky — commitments live. Everything in it binds the company, survives into diligence at your priced round, and shapes what your future lead investor finds when they model the cap table. If you are filling out the SAFE itself, start with our field-by-field SAFE walkthrough; this page covers the document that rides alongside it.
One disambiguation before we go further: SAFE side letters are a startup-and-angel matter. Venture funds also sign side letters — with their limited partners, against a limited partnership agreement — and that is an entirely different negotiation covered in our LP side letters guide.
The YC Pro Rata Side Letter, Specifically
The original 2013 pre-money SAFE had a pro rata right built into the instrument — every SAFE holder automatically got participation rights in the financing after their SAFE converted. When YC redesigned the SAFE as a post-money instrument in 2018, it made two connected changes: it removed the automatic right from the form, and it published an optional, one-page pro rata side letter on its documents page for founders who want to grant the right deliberately.
What the standard letter grants is precise: the right to purchase the investor’s pro rata share of the priced round in which the SAFE converts — enough of the new preferred stock to maintain the ownership percentage their SAFE bought, measured on the post-money SAFE’s own math (investment ÷ post-money cap). It is a right, not an obligation; the investor can exercise it, partially exercise it, or ignore it when the round arrives.
The economics deserve one beat of honesty. Granting the letter costs nothing today. At the Series A, every exercised pro rata right is capital that must fit into the round on top of the lead’s target check — so either the round grows (more dilution for founders) or the allocation fight begins. YC’s own Safe User Guide models this explicitly: pro rata participation adds dilution beyond the round itself. Run your stack through our SAFE conversion math guide or the dilution calculator with pro rata switched on before you promise it broadly.
What Investors Ask For — and a Founder Stance on Each
These are the asks that actually appear in SAFE-stage side letters. The stances below are a negotiating framework, not legal advice — check sizes, investor quality, and leverage all move the answer:
| Ask | What it grants | Founder stance |
|---|---|---|
| Pro rata right | Right to invest enough in the priced round the SAFE converts into to maintain the investor's ownership percentage. | Reasonable for meaningful checks — use YC's standard side letter. Think hard before granting it on every small check. |
| Information rights | Periodic updates — commonly quarterly or annual financials or an investor update email. | Cheap to grant if scoped to what you already produce. Avoid audited-financials promises you can't keep at seed stage. |
| MFN (most favored nation) | If a later SAFE investor gets better terms, this investor can elect to adopt them. | Common for the earliest checks (the uncapped MFN SAFE builds it in). Fine — but track it, because it fires automatically in spirit even when everyone forgets it exists. |
| Major Investor status at conversion | A promise that the investor will be treated as a 'Major Investor' in the priced round's NVCA-style documents, unlocking rights usually reserved for larger checks. | Grant sparingly. It pre-negotiates part of your Series A before your lead is at the table. |
| Board observer seat | Non-voting attendance at board meetings. | Rare and usually inappropriate at SAFE stage. Almost always the right answer is no. |
A useful default policy that many founders adopt: pro rata above a check-size threshold, information rights for anyone who asks (scoped to your existing update email), MFN only for true first money, and nothing else without counsel. Having the policy before the round starts is the point — it converts each ask from a negotiation into a rule, and investors respect rules applied evenly far more than ad-hoc concessions.
Side Letter Terms That Should Give You Pause
Evergreen pro rata
The YC letter covers the round the SAFE converts into. A letter granting pro rata in all future financings is a permanent claim on every round you ever raise — a materially different promise that belongs, if anywhere, in the priced round’s investors’ rights agreement where your lead negotiates it properly.
Consent and veto rights
Any language requiring the investor’s consent for future fundraising, sale of the company, or hiring decisions is governance — something SAFE holders, who are not stockholders, deliberately do not have. Protective provisions arrive with preferred stock at the priced round, negotiated by a lead with a board seat, not via side letter at the first check.
Guaranteed roles, fees, or warrants
Advisory fees, guaranteed consulting arrangements, or extra warrant coverage bolted onto a SAFE change the economics of the instrument in ways later investors will re-price. If an investor is also an advisor, paper the advisory relationship separately and transparently.
Transferability of the letter
A side letter that travels with the SAFE to any assignee means you may owe pro rata or information rights to someone you have never met. Rights personal to the investor should say so.
Track Every Letter Like the Contract It Is
The operational failure mode is not signing a bad side letter — it is forgetting a fine one. Side letters signed across a year of rolling SAFE closes have a way of living in individual email threads until Series A diligence asks for “all agreements with security holders” and three unmodeled pro rata rights surface at once. Keep a single register alongside your cap table: investor, SAFE amount and cap, side letter yes/no, and the specific rights granted. Store each executed letter with its SAFE in the data room. When the priced round arrives, hand the register to counsel on day one — the conversion mechanics and any “Major Investor” promises flow directly into the NVCA-style definitive documents your lawyers will draft, which we walk through in our Series A closing documents guide.
And if your SAFE stack is getting complicated enough that the register is hard to build, that is usually the signal to price the round — our SAFE vs priced round comparison covers when the switch makes sense.
Frequently Asked Questions
Is a side letter legally binding?
Yes. A side letter is a contract like any other — the 'side' refers to where it sits relative to the main instrument, not to its enforceability. That is exactly why it needs the same discipline as the SAFE itself: signed by an authorized officer, stored with the executed SAFE, and disclosed in diligence at your priced round. A forgotten side letter that surfaces mid-Series-A — with a pro rata right or MFN nobody modeled — is a genuinely common closing headache.
Why did YC move pro rata out of the SAFE and into a side letter?
The original 2013 pre-money SAFE included a built-in pro rata right, which every SAFE holder got automatically — whether or not the founder meant to grant it, and whether or not the check size justified it. When Y Combinator redesigned the SAFE as a post-money instrument in 2018, it removed the right from the standard form and published a separate optional pro rata side letter instead. The design intent is that pro rata becomes a deliberate, per-investor decision rather than a default that stacks up silently across a party round.
How much dilution does granting pro rata actually add?
The right itself sells no new ownership — it only matters when exercised at the priced round. When exercised, the investor buys enough of the new round to maintain their percentage, which means the round must expand (diluting founders more) or the lead must take less (which leads resist). YC's Post-Money Safe User Guide walks through worked examples showing pro rata participation adding meaningful dilution on top of the round itself. The honest framing: each pro rata letter is an option you have written against your own Series A allocation, and options granted casually get exercised at exactly the moment your round is hottest.
Are SAFE side letters the same as the side letters LPs sign with venture funds?
Same mechanism, different world. A SAFE side letter sits between a startup and an angel or seed investor and typically covers pro rata, information rights, or MFN. An LP side letter sits between a fund and a limited partner and covers things like fee terms, co-investment rights, and regulatory accommodations — negotiated against a limited partnership agreement rather than a SAFE. If you are a fund manager rather than a founder, see our guide to LP side letters instead.
An investor sent me a custom side letter. Should I sign it?
Read it as carefully as you would read edits to the SAFE itself, because that is functionally what it is. The YC pro rata side letter is one page and standard; anything longer deserves counsel review. Watch specifically for: rights that extend beyond the conversion round (evergreen pro rata), consent or veto rights over company actions, guaranteed roles or fees, transfer rights that let the letter travel to unknown parties, and anything styled as a 'right of first refusal' over future fundraising. None of those belong at SAFE stage.