Y Combinator's side letter is a right to invest, not a stake

The record

Alongside its Post-Money SAFE templates, Y Combinator publishes a short contract called the Pro Rata Side Letter, listed on Y Combinator's own documents page as retrieved on 16 September 2026. The document's own text grants the investor 'the right to purchase its pro rata share of Standard Preferred Stock being sold in the Equity Financing.' That pro rata share is a ratio: shares from converting the investor's SAFEs carrying a Post-Money Valuation Cap, divided by the company's total capitalization. The right ends at the earliest of three events the agreement lists: the initial closing of the priced round, immediately before a liquidity event, or immediately before dissolution. Y Combinator's plain-language summary on the documents page calls it 'the right, not the obligation, for an investor to invest money into your startup's future priced round to maintain their percentage ownership.'

What the documents establish

Two things the primary text establishes, and one it does not. First, the right is arithmetic, not automatic: only SAFEs with a stated valuation cap count, so an investor holding an uncapped or discount-only SAFE contributes nothing to the numerator. Second, Y Combinator's SAFE User Guide explains why the letter is a standalone document: when the SAFE became a post-money instrument, Y Combinator removed a pro rata right that had been a default term, replacing it with this letter, used 'if and when the parties agree to it,' because a universal rule did not fit every financing. Third, what it does not grant: it is not assignable except to an investor's affiliates without company consent, and amending it needs either that investor's consent or a majority of similarly situated side-letter holders.

The operating read

For a founder, the practical reading is that a pro rata side letter is a negotiated add-on tied to one instrument and one future round, not a standing cap-table claim. This is editorial interpretation of the termination clause: because the right lapses at the first priced round's initial closing, it does not carry into later rounds unless a new letter is signed each time. A founder modeling a coming Series A should treat each letter as a bounded, one-time allocation, not an open-ended promise to any earlier investor.

What to check before you decide

Before assuming a side letter applies the way a template suggests, check the underlying paperwork against these questions.

• Does the investor's SAFE actually carry a Post-Money Valuation Cap, or only a discount or MFN provision that would put zero in the pro rata numerator?
• Has the side letter already terminated because an earlier priced round closed since it was signed?
• Does the current cap table calculation match the 'Company Capitalization' definition the SAFE and side letter use, including outstanding SAFEs and the option pool?

This is an editorial checklist, not legal advice; a founder allocating room in a coming round should confirm each side letter's status with counsel before finalizing who gets to participate.