Anthropic vs OpenAI IPO Odds 2026: Who Lists First

Anthropic vs OpenAI IPO odds are quoted everywhere this autumn as a single number, and the single number is the one part of this complex that does not mean what it appears to. Polymarket pays 0.9585 on Anthropic listing first and 0.0415 on OpenAI, and a contract that looks like a 96% call is partly a bet on something else entirely. Eight live markets price the race, carrying $15,396,339 of lifetime volume against $2,817,942 of resting orders, and the interesting numbers are in the legs nobody puts in a headline.

We read all eight events from the Polymarket Gamma API at 16:20 UTC on 10 October 2026, summed every partition from the raw book rather than from midpoints, pulled thirty days of CLOB closes on the two legs that moved, checked Anthropic's filing status against the SEC's own full-text search, and read the 15-day roadshow rule off the SEC's accommodations page. One ladder costs less than it pays. Another is twelve points too expensive to touch.

The short version: the market is not really split on who lists first. It is close to certain Anthropic does, it is much less certain OpenAI lists at all inside the window, and those two facts together explain the whole price.

1. The eight markets behind every Anthropic vs OpenAI IPO odds headline.

2. Why 0.9585 is not a 96% chance Anthropic wins.

3. The cleanest read on each company is a leg nobody quotes.

4. November 2026 and the day the market moved 36 points.

5. What the strike ladders say about valuation.

6. The SpaceX benchmark is already on the board.

7. One ladder costs less than it pays, and is still not worth trading.

8. The second-biggest market in the complex is about bankruptcy.

9. Five checks before you trade these IPO odds.

Polymarket runs eight open events on the Anthropic and OpenAI listing question, and they are structured differently enough that quoting any one of them as the odds is already a mistake. Two are head-to-head binaries. Two are cumulative deadline ladders. Four are partitions that must price to a dollar.

Will Anthropic or OpenAI IPO first? is the headline contract, with $422,333 of lifetime volume on $59,420 of book. One market, two outcomes: Anthropic 0.9585 against a 0.956 bid and a 0.961 ask, OpenAI 0.0415. It runs to 31 December 2027.

Anthropic IPO by __? is by far the deepest of the timing markets, at $4,531,383 of volume on $211,964 of book, and it stops at the end of 2026. Four legs have already resolved NO: 30 June, 31 July, 15 September and 30 September 2026. The live legs read 0.0005 for 15 October, 0.0055 for 31 October, 0.085 for 15 November, 0.605 for 30 November, 0.74 for 15 December and 0.785 for 31 December.

In which month will Anthropic IPO? is a sixteen-leg partition running to the end of 2027, with $356,786 of volume on $32,285 of book. November 2026 leads at 0.6105, December 2026 is 0.182, and the No IPO before 2028 leg sits at 0.017.

Then the valuation markets. Anthropic IPO Closing Market Cap is a ten-leg strike ladder with $1,446,030 of volume on $180,268 of book; OpenAI IPO Closing Market Cap is a seven-leg ladder with $457,550 on $121,027. Largest IPO by market cap in 2026? is the biggest event in the whole complex at $6,521,596 on $612,123. Anthropic or OpenAI: higher IPO closing market cap? is the smallest and the one to ignore, with $255 of lifetime volume.

Note what has already happened here, because it does most of the work below. The October deadline legs are priced at half a point and under. The first four Anthropic deadlines came and went without a listing, and the market has spent the last month moving the weight into November.

Read the resolution text on the head-to-head market and one clause changes the arithmetic. It resolves to Anthropic if Anthropic completes an IPO before OpenAI does by 31 December 2027, and to OpenAI in the mirror case. Then: it resolves 50-50 if neither company completes an IPO by that date, if both list on the same calendar date, or if both become unable to list.

A 50-50 resolution pays $0.50 to each side. So the Anthropic price is not the probability Anthropic wins. It is the probability Anthropic wins plus half the probability nobody does.

That makes the quoted price a constraint rather than a forecast, and the constraint is tight. Write P for the Anthropic price, 0.9585. Since the OpenAI-first probability cannot be negative, the probability that neither company lists by the end of 2027 is at most twice the OpenAI price, which is 8.30%. The market cannot be read as pricing a meaningful chance that both of these companies stay private, whatever anyone says about the AI funding cycle. It has already ruled that out.

Better still, the complex prices that branch directly, and it is far below the cap. Anthropic's valuation ladder puts No IPO by December 31, 2027 at 0.022 and OpenAI's puts it at 0.2135. Treat those as independent and the chance neither lists is 0.47%. Feed that back in and the decomposition is clean: Anthropic first at 95.62%, OpenAI first at 3.92%, neither at 0.47%, which prices the OpenAI side at 0.0415.

The quoted OpenAI ask is 0.0415. Two separately traded markets, read ten minutes apart, agree to four decimal places. That is worth saying plainly because it is the opposite of what these cross-checks usually show, and it means the headline price is doing its job: there is no seam here to trade.

If you want one number for each company's chance of reaching the public market, do not use the head-to-head. Use the No IPO by December 31, 2027 leg buried at the bottom of each valuation ladder. Both markets carry it, both use the same deadline, and both are worded the same way, which makes them the only genuinely like-for-like comparison in the complex.

Anthropic's sits at 0.022. OpenAI's sits at 0.2135. Inverted, the same market family gives Anthropic a 97.8% chance of listing inside the window and OpenAI a 78.7% chance. That nineteen-point spread, not the 0.9585, is the real statement about these two companies.

It also resolves the thing that looks odd about the head-to-head. OpenAI at 0.0415 does not mean the market thinks OpenAI will not list. It means the market thinks OpenAI will list later. Condition on OpenAI listing by the end of 2027 and its chance of getting there first is 4.98%, which is the market saying that if both go public in this window, Anthropic goes first in roughly nineteen cases out of twenty.

The month ladder says why: it loads 79.9% of Anthropic's probability into the next three months. A company priced to list before the new year beats a company whose own ladder spreads 78.7% across the following two.

Cross-check it before you trust it. The month ladder's October, November and December legs sum to 0.7990 against the deadline ladder's 31 December leg at 0.7850, a 1.40-point gap. October plus November is 0.6170 against 0.6050 for the 30 November deadline, a 1.20-point gap. October alone is 0.0065 against 0.0055, a tenth of a point. Three independent pairs, all inside a point and a half. The Anthropic timing complex is coherent.

The November 2026 leg of the month ladder opened our thirty-day window at 0.2505 on 10 September and reads 0.6105 tonight, a 36-point move, with a peak of 0.7105 on the way. Almost all of it arrives in one stretch: 0.237 on 18 September, 0.471 on 20 September, 0.581 on 22 September.

That is not drift, and it has a cause. Reports published on 21 September said Anthropic was moving its potential listing out of an October window and into November, after the midterms on 3 November. Bloomberg and Business Insider had reported on 13 September that the company had settled on Nasdaq. Neither the venue nor the date is company-confirmed, and the October deadline legs now trade at 0.0055 and below, which is the market treating the shift as settled fact.

What is confirmed is the filing. Anthropic submitted a draft registration statement on Form S-1 to the SEC confidentially, reported as 1 June 2026. A confidential submission is reviewed privately and does not appear on EDGAR, so its absence proves nothing by itself. We checked anyway: EDGAR's full-text search returns 58 S-1 documents matching Anthropic and every one of them belongs to a different filer, among them NScale, Cerebras, Figma and Reddit. Anthropic has filed no public S-1 as of today. We validated that query against Reddit, which the same search correctly returns as a filer in its own right.

This is where the market's November conviction becomes checkable rather than vibes. The SEC's accommodation for confidential drafts comes with a price: an issuer must publicly file its registration statement and all prior nonpublic drafts at least 15 days before any roadshow. Roadshows typically run one to two weeks before pricing. For a listing in the second half of November, the public S-1 would need to appear within roughly the next two to three weeks.

So the 0.6105 is a falsifiable claim with a short fuse. If no public S-1 lands by the start of November, that leg is wrong and the December and January legs are cheap. The filing is the signal; the reporting is commentary about the filing.

The two valuation markets are not comparable leg for leg, and that catches people out. Anthropic's ladder runs nine brackets from under $1.25T up to $3.0T and above. OpenAI's runs six, and its top bracket is an open-ended $1.5T+ holding 0.5575. You cannot read a modal valuation off a catch-all, so compare the shapes instead.

Anthropic's distribution peaks in the middle: 0.2265 on $2.0T to $2.25T and 0.235 on $2.25T to $2.5T, with 0.175 on $1.75T to $2.0T below them. Conditional on listing, the ladder gives an 86.99% chance of clearing $1.75T, 68.40% of clearing $2.0T and 44.34% of clearing $2.25T. Against the $965B post-money of the May 2026 Series H, the market is pricing roughly a doubling, which is also where the reported near-$2T investor talk sits. That talk is not confirmed by the company.

The head-to-head on size agrees: Anthropic or OpenAI: higher IPO closing market cap? pays 0.685 on Anthropic. It also has $255 of lifetime volume, so treat it as a rounding error with a price attached rather than as evidence.

The genuinely interesting market here is Largest IPO by market cap in 2026?, the biggest event in the complex, and it is widely misread as a forward race. It is not. SpaceX has already listed: S-1 on 20 May 2026, final prospectus on 12 June, trading on Nasdaq as SPCX, priced at $135 for a valuation near $1.77T and closed its first day at $160.95, up 19%, for a market capitalisation reported between $2.1T and $2.2T. Sources differ on the exact figure. Its $75B raise was the largest IPO on record.

So that market is really asking one question: will Anthropic's first-day close beat a number that is already on the board? Anthropic pays 0.553, SpaceX 0.42, OpenAI 0.006. Divide the Anthropic leg by its 0.785 chance of listing in 2026 and the market gives it a 70.45% chance of clearing SpaceX, conditional on getting there. Read that back through the strike ladder, which sits at 68.40% above $2.0T and 86.99% above $1.75T, and the implied benchmark is about $1.97T.

The real benchmark is $2.1T to $2.2T. The gap is small but it points one way: the Anthropic leg of the largest-IPO market is a few points rich against its own valuation ladder. One more thing before you trade that. The xAI leg shows a midpoint of 0.255 on a 0.01 bid against a 0.50 ask and zero volume. Drop it and the partition sums to 0.9835; keep it and you are quoting the middle of an empty book, which is not a price.

A partition has to pay exactly $1.00, because exactly one leg resolves YES. So sum the asks: that is what it costs to own every outcome, and anything under a dollar is a locked-in gain before costs. Midpoints will not do, and this is where most published arbitrage claims fall over.

Three of these partitions, at the ask: OpenAI's valuation ladder costs 0.9820. Anthropic's costs 1.0090. The month ladder costs 1.1260.

Only one of those is an edge. Buying all seven legs of the OpenAI market-cap ladder, including its 0.224 No IPO leg, costs 98.20 cents to receive a dollar on resolution, a gross 1.80 points. The Anthropic ladder costs nine-tenths of a point more than it pays, and the month ladder, with its sixteen thin legs and a 0.8430 bid-side sum, is 12.6 points too expensive to be worth a second look. That spread between a 0.843 bid sum and a 1.126 ask sum is the honest measure of how wide those books are.

Now kill the trade anyway. The OpenAI ladder resolves on 31 December 2027, 447 days away. A 1.80-point gain over 447 days annualises to 1.50%. Short-dated Treasuries pay more than that, and so does Polymarket: the bankruptcy market in this same complex is tagged Earn 3.25%, which is the platform's own rate on idle collateral. The arbitrage is real, correctly computed, and worse than doing nothing.

It is also capped by depth long before it is capped by maths. The event carries $121,027 of book across seven legs, more than half the cost sits in the $1.5T+ leg alone, and sweeping an ask moves it. A 1.80-point edge survives roughly none of that.

Anthropic announces bankruptcy by...? carries $1,660,406 of lifetime volume and $1,597,782 of resting liquidity, which is more book than every timing market in this article combined. Its legs read 0.095 for 31 December 2027, 0.185 for 2028 and 0.215 for 2029.

Taken at face value that is strange next to a 97.8% chance of listing. Both can be true, because the ladder runs three years past the listing window and a public company can still fail, but the sizing is the part to understand rather than the prices.

The market is tagged Earn 3.25%. Polymarket pays interest on collateral parked in some markets, which makes a long-dated, near-zero-probability contract an attractive place to leave money. That is the simplest reading of why a bankruptcy market has ten times the book of the contract everyone quotes: much of that liquidity is there for the yield, not for the view.

The lesson generalises past this one market: volume and liquidity are not conviction. The bankruptcy ladder is deep because it pays to sit in it. The head-to-head that actually answers the question has the thinnest book of the three at $59,420, and it is still the one to read.

First, read the tiebreak before the price. The head-to-head's 50-50 clause is the difference between a 96% forecast and a 95.6% forecast plus a bet on a branch the market has nearly excluded. Every market in this complex has a clause like it somewhere.

Second, check which deadline you are buying. These eight markets run to four different dates: the Anthropic deadline ladder stops at the end of 2026, the head-to-head and both valuation ladders run to the end of 2027, the size head-to-head to the end of 2028, and the bankruptcy ladder to the end of 2029. Two prices on the same question are not comparable unless the dates match.

Third, sum partitions at the ask, never at the midpoint. The midpoint sums here run from 0.9530 to 0.9845 and every one of them looks like an arbitrage. At the ask, exactly one of the three is, and it is not worth taking.

Fourth, discount any leg whose book is empty. The xAI leg quotes 0.01 against 0.50 on zero volume and prints a 0.255 midpoint that no one will trade with you. A number drawn from the middle of a gap that wide is not a forecast.

Fifth, watch EDGAR rather than the headlines. The November leg is a claim about a public filing that has not happened yet, and the SEC's 15-day rule means the filing has to arrive well before the listing does. That document is the one piece of evidence in this entire complex that is not somebody's reporting about somebody's plan.

None of this is financial advice, and every price above is a reading from one moment: 16:20 UTC on 10 October 2026. The market pages carry the live book.