spec: Add Near Intents exact scheme

[IkerAlus]

Description

The exact payment scheme for Near Intents uses the NEAR Intents 1Click Swap API as the settlement backend. This scheme facilitates cross-chain payments where a client pays a specified amount of a source asset on any supported origin chain, and the resource server (merchant) receives an exact amount of a destination asset on any supported destination chain, with the NEAR Intents solver network executing the cross-chain swap in between.

Core Design: payTo = Deposit Address

The key architectural insight of the scheme is that the payTo field in PaymentRequirements is set to the 1Click Swap API deposit address — a unique, one-time address generated by the 1Click Swap API backend when the resource server requests a quote (POST /v0/quote with dry: false). This means:

• The resource server generates the quote at 402 response time, embedding the live depositAddress as payTo.
• The client's only obligation is to transfer the exact required amount to that payTo address on the origin chain — identical in spirit to how the EVM exact scheme asks the client to authorize a transfer to a payTo address.
• The client's payload contains the origin-chain transaction hash (txHash) as proof-of-deposit — analogous to the EIP-3009 signature in the EVM scheme.
• The facilitator verifies the deposit and polls the 1Click status API for settlement.

Key Properties

• Cross-chain native: Client pays in any supported asset on any origin chain (ETH, BTC, SOL, USDC on Arbitrum, etc.); the merchant receives the destination asset on any supported chain.
• Gasless for the merchant: The resource server receives destination tokens without submitting any on-chain transaction.
• Competitive pricing: Market makers compete on the 1Click solver network to provide the best swap rate.
• Automatic refunds: If the swap cannot be completed, the 1Click API automatically returns funds to the refund address.

Open Questions

• CAIP-2 network id convention: "near:mainnet" for the settlement network vs "nearintents:mainnet" for the crosschain protocol
• Role of the Facilitator: The "Quote Generation" step of the Facilitator can be assigned to the resource server.
• Facilitator statefulness: It is not a hard requirement for the scheme to work. Delay and stateful constraints can be delegated to the resource server.

[vercel]

@IkerAlus is attempting to deploy a commit to the Coinbase Team on Vercel.

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