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# Summary

MPX (Multilateral Position Cross) is an always-open, periodic crossing mechanism for derivatives. Interest is submitted privately, stays active for a chosen number of scheduled crosses, and is matched against the interest of every other participant at a reference price nobody negotiates.

It is not an Order Book and not an RFQ. Nothing you submit is visible to anyone, there is no quoting round, and no price is discovered: at each scheduled cross MPX takes the venue's mark, matches opposing interest at that mark, and settles the result as venue Block Trades. What a participant chooses is what to trade, how much, and for how many crosses.

Deribit is the available venue.

### High Level

MPX is centered on the concept of a scheduled cross: an instant at which every participant's standing interest is read at once and matched against itself.

* Interest is submitted as a *Submission*, a flat list of *Legs*, each an instrument, a direction and a maximum quantity.
* A Submission stays in for a number of crosses, not for a length of time.
* At each cross, every instrument is matched on its own, oldest interest first, at the venue's mark for that instrument.
* Matched quantity between one Submission on each side becomes a *Settlement Package*. Packages with the same maker account, the same taker account, the same settlement currency and the same underlying coin are sent to the underlying venue together as one Block Trade.
* Quantity that does not match rolls into the next cross, keeping the place in the queue it already had.

At a high level, the MPX workflow is:

1. A participant submits private interest through the *RESToverHTTP* \[POST] `/submissions` endpoint.
2. At the next scheduled cutoff, listed by the *RESToverHTTP* \[GET] `/crosses` endpoint, MPX freezes the interest that is active at that instant and captures the venue's mark for every instrument in it.
3. Opposing interest is matched at that mark, and the fills between each pair of Submissions are packaged.
4. The packages are grouped by the accounts they settle between and sent to the underlying venue for clearing and settlement as Block Trades.
5. Each participant is told what the cross did to their own interest, over the *JSON-RPCoverWebSockets* `cross` Notification Channel.

### Key Concepts

In MPX,

* Nothing you submit is visible to any other participant, and no participant learns who stood opposite them.
* You do NOT select counterparties, and you do NOT quote: the price is the venue's mark, captured at the cutoff.
* Interest is good for a number of crosses. A cross that runs spends one crossing even where it matches nothing.
* Partial fills are always allowed, and one Leg may be filled by several counterparties in the same cross.
* A desk never crosses with itself, across its own accounts included.
* Each Leg is matched independently of the others in its Submission. There is no all-or-none.
* Quantity a cross matches is held until the venue answers, and no later cross may offer it.
* From the cutoff until what a cross took of it has settled, a Submission is held: it cannot be reduced, removed or canceled, and it sits out the crosses that run meanwhile.

### Add Hedge

A Submission carrying option Legs may ask for a delta-neutralizing hedge, and name the futures and perpetuals that may provide it in order of preference. Where both sides of a package asked for a hedge, MPX adds one future or perpetual Leg to the package so its delta nets to zero within tolerance, priced at the same captured mark and settled in the same Block Trade.

A package is only formed between two Submissions that both asked for a hedge or both did not: a participant who asked for one is saying they would rather not trade unhedged.

The venue's minimum Block size is judged on the whole Block Trade, hedge included, so where both sides asked for a hedge, a pairing too small on its own can still cross.

### Block Trade Settlement

As everywhere on Paradigm, what reaches the underlying venue is a Block Trade, and a Block Trade clears whole or is rejected whole. In MPX the Block Trade is the unit that settles. Packages with the same maker account, the same taker account, the same settlement currency and the same underlying coin travel together as one Block Trade, so two of your Submissions matched against the same counterparty account can settle in a single transaction. Packages that cannot share one transaction, such as two holding the same instrument on opposite sides, travel in separate Block Trades.

The venue is asked about a Block Trade as a whole, and a refusal of any package in it sends none of them. One account facing two different counterparty accounts in a single cross is still two Block Trades, and each of them settles or fails on its own.

Quantity in a Block Trade the venue rejects returns to the pool for the next cross. The Block Trades that settled are unaffected. What you read back stays per Leg, whichever Block Trade carried it.

### What You Need to Participate

* A Paradigm trading desk with a Deribit Venue API Credential, with Block Trade permission enabled.
* A Submission names the account it settles with, so a desk holding several accounts submits once per account.
* A Submission carries interest on one underlying coin.