A greenshoe option is the underwriters' right to buy extra IPO shares, at the IPO price minus their discount, for 30 days after pricing, so they can cover shares they sold short to buyers on day one. In three 2026 US IPOs (Cerebras, EquipmentShare and MiniMed) that extra slice was 15% of the base deal. Two options were exercised in full at closing. One expired untouched. Which way it goes tells you a lot about who was holding up the stock in its first weeks.
The headline share count in an IPO is a starting bid, not a final number. Here is how the over-allotment machinery works, the SEC rule that governs it, and where to check how it ended.
How the greenshoe works: the short you never see
At pricing, the syndicate can allocate more stock than the issuer is selling. The extra shares are sold short. The Cerebras prospectus spells it out: the underwriters "may sell more shares than they are obligated to purchase under the underwriting agreement, creating a short position."
That short has to be closed, and the option decides how:
- Stock trades above the IPO price. Buying in the market would cost more than the IPO price. The underwriters exercise the option and get the shares at the IPO price minus their discount. The issuer (or a selling holder) sells more stock and collects more money.
- Stock trades below the IPO price. The underwriters buy shares in the open market to cover. That buying adds demand when the stock is weak, and the option is left to expire.
Cerebras's 424B4 says the underwriters weigh "the open market price of shares compared to the price available under the option" when they pick a source. A short that fits inside the option is "covered." Anything beyond it is a naked short, which "must" be closed by buying in the open market. The same prospectus says a naked short "is more likely to be created if the underwriters are concerned that there may be downward pressure" on the price after pricing.
A hypothetical shows the arithmetic. Take a 10,000,000-share deal with a 1,500,000-share option. If the syndicate sells 12,000,000 shares, it is short 2,000,000. Up to 1,500,000 of that is covered by the option. The last 500,000 is naked and can only be bought back on the tape.
Regulation M Rule 104: the rules on the stabilizing bid
Open-market covering and stabilizing bids fall under SEC Regulation M, Rule 104 (17 CFR 242.104). Rule 100 defines a "syndicate covering transaction" as a bid or purchase by the syndicate "to reduce a short position created in connection with the offering." Paraphrasing Rule 104:
- Purpose (104(b)). Stabilizing is banned except to prevent or slow a decline in the market price.
- Price cap (104(f)(1)). No stabilizing bid may be higher than the lower of the offering price or the stabilizing bid in the principal market. The syndicate can defend the IPO price. It cannot bid the stock above it.
- Priority (104(c)). Where the market allows it, the stabilizer must give priority to an independent bid at the same price.
- Notice and disclosure (104(h)). A stabilizing bid needs prior notice to the market, and covering transactions and penalty bids need prior notice to the self-regulatory organization for the principal market. Buyers must get a document disclosing that the price may be stabilized. That is the boilerplate in every 424B4.
- No ATMs (104(e)). Stabilizing is prohibited in at-the-market offerings.
One limit for traders: the prospectus says the underwriters "may" do these things and "may end any of these activities at any time." None of the three filings below says whether any stabilizing bid was actually placed.
Three 2026 IPOs: exercised, exercised, expired
| IPO | Price / prospectus date | Base shares | Option (30 days) | First close vs IPO | Outcome |
|---|---|---|---|---|---|
| Cerebras $CBRS | $185.00 / May 13, 2026 | 30,000,000 | 4,500,000 from the company | $311.07 on May 14, 2026 (+68.1%) | Exercised in full; 34,500,000 sold at the May 15 closing |
| EquipmentShare $EQPT | $24.50 / Jan 22, 2026 | 30,500,000 | 4,575,000 from selling shareholders | $32.56 on Jan 23, 2026 (+32.9%) | Exercised in full at the Jan 26 closing |
| MiniMed $MMED | $20.00 / Mar 5, 2026 | 28,000,000 | 4,200,000 from the company | $18.49 on Mar 6, 2026 (−7.6%) | Expired; 28,000,000 sold |
Cerebras. The 424B4 gives the underwriters an option "exercisable for 30 days" to buy up to 4,500,000 more Class A shares. Its own table shows proceeds before expenses of $5,419,575,000 with no exercise and $6,232,511,250 with full exercise, a difference of $812,936,250. The May 15, 2026 closing 8-K reports 34,500,000 shares sold, "which includes the exercise in full." The stock never needed help in that window. Its lowest close through June 12 was $201.01 on June 5, 2026, still 8.7% above $185.
EquipmentShare. Here the option shares came from selling shareholders, not the company. The 424B4 says plainly: "We will not receive any proceeds" if the option is exercised. The January 26, 2026 closing 8-K says the underwriters "fully exercised the option." At $23.15 per share after the discount, that is about $105.9 million to the sellers and nothing to the balance sheet.
MiniMed. The stock opened its public life below the $20.00 price. Every close from March 6 through April 2, 2026 was under $20, from $18.49 on March 6 to $14.01 on April 2. MiniMed's next 10-Q states that, after the 28,000,000-share sale "and the expiration of the over-allotment option with respect to up to an additional 4,200,000 shares, the offering terminated." The company passed up about $80.6 million (4,200,000 × $19.20) that full exercise would have brought in.
Why it matters on the tape
Support near the IPO price. When a deal breaks issue, covering buys and any Rule 104 bid sit at or below the offering price, never above it. That support has a cap and an end date. It lasts only as long as the short or the syndicate's appetite. MiniMed's slide to $14.01 inside the 30-day window shows it is a cushion, not a floor.
Extra supply when the deal works. A full exercise means 15% more stock in public hands. In Cerebras's case it also meant more cash on the balance sheet. That changes the float math covered in float vs outstanding shares.
Who gets paid. Read who granted the option. With Cerebras, the money went to the company. With EquipmentShare, it went to existing holders cashing out. Same mechanism, very different signal.
The greenshoe ends within 30 days. Another supply event comes later, when insider lock-ups expire, which is covered in IPO lockup expiration.
Where to check whether it was exercised
- The 424B4 cover and "Underwriting" section. These give the option size, who grants it and the 30-day term. The proceeds table shows "no exercise" and "full exercise" side by side.
- The closing 8-K. Filed around the closing date: May 15, 2026 for Cerebras (Item 8.01) and January 26, 2026 for EquipmentShare. Some issuers report it in a press release filed as exhibit 99.1. If the option is exercised after closing, look for a later 8-K or press release. The 8-K reading guide covers the items.
- The next 10-Q. Part II, Item 2, "Use of Proceeds," is where MiniMed disclosed the expiration. The 10-Q cover also gives an updated share count: 280,819,988 MiniMed shares as of April 11, 2026.
The common mistake: treating the headline share count as final
The 424B4 cover says "30,000,000 Shares" for Cerebras. The number that actually sold was 34,500,000. MiniMed's prospectus showed two outcomes, 280,813,348 shares outstanding after the offering "or 285,013,348 shares" with full exercise. Market-cap and float screens built from the cover can be off by the whole option until someone reads the closing filing.
Checklist:
- Option size, and whether the issuer or selling holders grant it.
- The prospectus date. The 30-day clock runs from it.
- First closes versus the IPO price. Above the price favors exercise; below it favors covering in the market.
- The closing 8-K or exhibit 99.1 for the words "exercise in full."
- The next 10-Q for the final share count and any statement that the option expired.
More deal mechanics live on the IPO category page at I'M A STOCK TRADER.
Bull/Bear Verdict
Bull Case: The greenshoe is one of the few IPO mechanics you can fully reconstruct from filings. Option size, who sells, the 30-day term and the outcome are all on EDGAR. A full exercise next to a first close like Cerebras's +68.1% confirms real demand, and a known covering buyer near the offer price explains early support when a deal is weak.
Bear Case: The support is optional, capped at the offering price and gone within about 30 days. MiniMed traded under its $20.00 IPO price through the whole window. An exercise can also send money to selling shareholders rather than the company, as with EquipmentShare. And no filing says when or whether a stabilizing bid was actually placed.
FAQ
Why is it called a greenshoe?
It is market slang for the over-allotment option. Prospectuses use the formal name, an "option to purchase additional shares" or "over-allotment option." Search those phrases in a 424B4.
Is the greenshoe always 15%?
It is set deal by deal in the underwriting agreement. In all three 2026 examples above it was 15% of the base shares: 4,500,000 of 30,000,000, 4,575,000 of 30,500,000 and 4,200,000 of 28,000,000. Always read the actual number in the 424B4.
Can the underwriters exercise only part of the option?
Yes. The prospectuses say "up to," so a partial exercise is possible when part of the short was covered in the market. The closing 8-K, a later 8-K or the next 10-Q states the exact number of additional shares sold.
Does stabilization mean the stock cannot fall below the IPO price?
No. Rule 104 only allows bids to prevent or slow a decline, capped at the offering price, and underwriters may stop at any time. MiniMed closed as low as $14.01 on April 2, 2026, inside its 30-day option window, against a $20.00 IPO price.