Co-sign a payment

Send each agent payment to Exchain before it goes on chain. If it fits the mandate, the answer is 200 with the attested quorum's signatures, and the payment becomes valid on chain.

A 200 commits budget. A 200 returns signatures that make the payment valid on chain. Send only the payment you asked about. Its budget is committed until it lands or its validity window closes.

How co-signing worksCopy Anchor Link

1. The agent builds the payment and signs it.
2. Your backend sends it to Exchain with your Node Token, before it goes on chain.
3. Exchain checks it against the chain's rules and the mandate. Each quorum member checks it again and signs.
4. You attach the signatures and send the payment, or Exchain sends it for you on the EVM direct rail.
5. Exchain watches the chain. A payment that lands is linked in the journal. A payment that never lands returns its budget.

Every request carries Authorization: Bearer <Node Token> and an Idempotency-Key equal to batchId. A retry with the same batchId and body returns the stored answer. Refusals, retries and error codes: Refusals and retries.

EndpointsCopy Anchor Link

CardanoCopy Anchor Link

On Cardano, the payment is a transaction from the guarded Masumi smart wallet that pays one or more Masumi escrows. Exchain returns the quorum's vkey witnesses. Inside a Hydra head, the same endpoint takes a snapshot instead of a layer 1 UTxO: Hydra in-head payments.

Transaction rulesCopy Anchor Link

The body must follow every rule. The first rule that fails refuses the whole batch.

CIP-20 messages (metadata label 674) are allowed.

Cardano requestCopy Anchor Link

{
  "batchId": "7f0c3d2e-9a4b-4c1d-8e2f-000000000001",
  "walletUtxoRef": "<64 hex tx hash>#0",
  "txBodyHex": "a40081825820…",
  "intents": [
    { "purchaseId": "pur_1", "outputIndex": 0, "counterparty": "sellerVkeyHash:<56 hex>",
      "amount": "19000000", "asset": "lovelace",
      "jobHash": "blake2b_256:<64 hex>", "agentIdentifier": "<registry asset id>" },
    { "purchaseId": "pur_2", "outputIndex": 1, "counterparty": "sellerVkeyHash:<56 hex>",
      "amount": "5000000", "asset": "6ba199f81440944474fb29139bc7a0f736f70615bb26cb7b20d3d6fd.0014df10745553444378",
      "jobHash": "blake2b_256:<64 hex>", "agentIdentifier": "<registry asset id>" }
  ],
  "context": { "nodeId": "node_1", "orgId": "org_acme", "submittedAt": "2026-11-05T10:00:00Z" }
}

19000000 = 19 tADA; 5000000 = 5 tUSDCx (both 6 decimals).

• batchId is a lowercase UUID; intents holds 1 to 10 purchases, one per escrow output.
• walletUtxoRef is required on layer 1; txBodyHex is at most 32,768 hex characters.
• context.submittedAt is recorded only. Decisions use Exchain's clock.
• counterparty, amount and asset are checked against the escrow output in the body. agentIdentifier and jobHash are taken from your request as sent, and the registry gate, price gate and perAgent limit read them.

Every field and format: API.

Cardano responseCopy Anchor Link

{
  "decisionId": "dec_01J9XK3M4N5P6Q7R8S9T0V1W2X",
  "txBodyHash": "blake2b_256:f13f1c97…3c247f18",
  "requiredSigners": ["<agent key hash>", "<quorum key hash 1>", "<quorum key hash 2>", "<quorum key hash 3>"],
  "quorumMembers": ["member-a", "member-b", "member-c"],
  "witnessSetHex": "a10083825820…",
  "witnesses": [
    { "member": "member-a", "vkeyHex": "<64 hex>", "signatureHex": "<128 hex>" },
    { "member": "member-b", "vkeyHex": "<64 hex>", "signatureHex": "<128 hex>" },
    { "member": "member-c", "vkeyHex": "<64 hex>", "signatureHex": "<128 hex>" }
  ],
  "members": [
    { "purchaseId": "pur_1", "outputIndex": 0, "verdict": "allowed" },
    { "purchaseId": "pur_2", "outputIndex": 1, "verdict": "allowed" }
  ],
  "boundsRemaining": {
    "daily_outflow": { "limit": "1000000000", "used": "24000000", "remaining": "976000000",
                       "windowResetsAt": "2026-11-06T10:00:00Z" }
  },
  "journalRef": "https://cosign.exchain.network/v1/decisions/dec_01J9XK3M4N5P6Q7R8S9T0V1W2X",
  "asOf": "2026-11-05T10:00:01.412Z",
  "schemaVersion": "1.1",
  "chainTip": { "slot": 138150155, "blockHash": "9a301ac2…" }
}

SubmitCopy Anchor Link

1. Add the agent's vkey witness to the set in witnessSetHex.
2. Attach the set to the unchanged body.
3. Submit. The transaction id does not change.

After you submitCopy Anchor Link

EVM chainsCopy Anchor Link

On EVM chains, the payment is one authorization from the guarded Safe, in one of the tokens it governs, valid for at most 600 seconds. For Ethereum USDT and Polygon USDT0 on the direct rail, it is a Safe transaction at the Safe's current nonce instead. The agent signs the Safe message hash of the typed data (for a Safe transaction, its SafeTx hash): a raw 65-byte signature, not personal_sign. Exchain returns every owner's signature in one packed signature. Methods and how the Safe verifies them: Signing.

Typed data rulesCopy Anchor Link

typedData must follow every rule.

Checks, in order

1. Fees due in the payment's token: 402 fee_due. Settle them first: Fees.
2. Amount and counterparty format: 400.
3. The asset is one the wallet governs and is still enabled: asset_not_listed.
4. The typed data rules above.
5. The agent signature recovers to the registered agent: 401 agent_signature_invalid.
6. Allow and deny lists, then seller binding: member codes.
7. Mandate limits: member codes.

Then every member of the attested quorum checks the typed data again on its own clock and signs.

Direct and x402 differencesCopy Anchor Link

EVM requestCopy Anchor Link

{
  "batchId": "0xd830…f946",
  "walletId": "wal_01M53M3ZNQQ4DDNPE4262QAZ28",
  "typedData": {
    "domain": { "name": "USDC", "version": "2", "chainId": 84532,
                "verifyingContract": "0x036cbd53842c5426634e7929541ec2318f3dcf7e" },
    "types": { "TransferWithAuthorization": [
      { "name": "from", "type": "address" }, { "name": "to", "type": "address" },
      { "name": "value", "type": "uint256" }, { "name": "validAfter", "type": "uint256" },
      { "name": "validBefore", "type": "uint256" }, { "name": "nonce", "type": "bytes32" } ] },
    "primaryType": "TransferWithAuthorization",
    "message": { "from": "0xSafe…", "to": "0x6d2f…e284", "value": "500000",
                 "validAfter": "0", "validBefore": "<now + 300>", "nonce": "0xd830…f946" }
  },
  "agentSignature": "0x<130 hex>",
  "relay": true,
  "intent": {
    "purchaseId": "inv_2026_118",
    "counterparty": "eip155:84532:0x6d2f…e284",
    "amount": "500000",
    "asset": "eip155:84532/erc20:0x036cbd53842c5426634e7929541ec2318f3dcf7e",
    "agentIdentifier": "research-agent",
    "reference": "invoice 2026-118"
  }
}

import { exchainPay } from "@exchain-network/x402";

const answer = await exchainPay(
  { cosign: { url: "https://cosign.exchain.network", token }, walletId, safe, agent, chainId: 84532 },
  { payTo: "0x6d2f…e284", amount: 500_000n, agentIdentifier: "research-agent", reference: "invoice 2026-118" },
);
answer.relayed.txHash;

500000 = 0.50 USDC (6 decimals).

EVM responseCopy Anchor Link

On x402, pass signature to the x402 client as the payment signature. Relay details: Relay.

EVM settlementCopy Anchor Link

Exchain watches the Safe's own chain.

Per-chain explorers and fees: Settlement.