RFC-007: Nexus Core Agentic Interface

Metadata	Value
Title	Nexus Core Agentic Interface
Version	1.8.0
Status	Final Specification
Updated	2026-04-01
Protocol	Model Context Protocol (MCP)
Architecture	Hub (PlatON) + Spoke (MPC Ingress) + KYT Firewall

1. Core Design Philosophy

Nexus Core operates as an MCP Server, providing two standard plugin sets to Agents in the network:

1. Buyer Plugin: Guides the UA through the entire flow of "Quote -> Select Chain -> Pay -> Receive Goods".
2. Seller Plugin: Helps the MA complete the closed loop of "Check Balance -> Fulfill -> Withdraw". The interaction model follows "Draft-then-Finalize": first generate a draft order for the user to select a payment method, then lock in and generate a dedicated MPC custody address.

---

2. Nexus Buyer Plugin (For User Agent)

This plugin grants the UA the ability to process payment intents, interact with users to select networks, and execute fund escrow.

Tool A: initialize_payment (Initialize / Pre-orchestration)

Lifecycle Stage: Discovery (when the user sees the quote card) Function: Register order intent, compute cross-chain routing, and return payment options for the user to choose from.

• Input Schema:

{
"quote_payload": {
"type": "object",
"description": "NUPS quote payload extracted from the merchant-returned UCP Checkout Session (`ucp.payment_handlers['urn:ucp:payment:nexus_v1'][0].config`)",
"required": true
}
}

• Output Schema:

{
"nexus_payment_id": "NEX-UUID-001", // Draft ID
"status": "AWAITING_USER_SELECTION",
"expiry": 1760000000,
"fiat_value": { "amount": "530.00", "currency": "USD" },
// Core: option list for UI rendering
"payment_options": [
{
"option_id": "opt_base_usdc",
"chain_id": 8453,
"chain_name": "Base",
"token_symbol": "USDC",
"amount_uint256": "530000000",
"est_gas_fee_usd": "0.02",
"bridge_fee_usd": "0.50",
"tags": ["RECOMMENDED", "BEST_VALUE"]
},
{
"option_id": "opt_eth_usdc",
"chain_id": 1,
"chain_name": "Ethereum",
"est_gas_fee_usd": "5.00",
"tags": ["HIGH_GAS"]
},
{
"option_id": "opt_platon_usdc",
"chain_id": 210425,
"chain_name": "PlatON",
"bridge_fee_usd": "0.00",
"tags": ["NATIVE_SETTLEMENT"]
}
]
}

Tool B: finalize_payment (Final Orchestration / Obtain Address)

Lifecycle Stage: Decision (when the user clicks the "Pay" button) Function: Lock in the user's selection, assign a dedicated MPC ephemeral custody address, and prepare to receive funds.

• Input Schema:

{
"nexus_payment_id": "NEX-UUID-001",
"selected_option_id": "opt_base_usdc", // User-selected path
"payer_wallet": "0xUserAddress..." // User's connected wallet address (for subsequent KYT association)
}

• Output Schema:

{
"status": "AWAITING_DEPOSIT",
"kyc_policy": "STRICT", // Note: strict KYT will be performed after deposit
// Core: payment instruction
"payment_instruction": {
"chain_id": 8453,
"chain_name": "Base",
// MPC Custody Address (Ephemeral Address)
"target_address": "0xNexusMPC_Temp_Addr_99",
"token_address": "0xUSDC_Base_Addr",
"amount": "530000000",
// Explicitly instruct the UA to use a plain transfer, not a contract call
"method": "transfer",
"calldata": "0x"
},
"validity_window": "30 minutes"
}

Tool C: sign_release (Confirm Receipt / Release Funds)

Lifecycle Stage: Verification (after the user confirms receipt of service) Function: Produce an EIP-712 signature over the order ID, authorizing the release of funds to the merchant.

• Input Schema:

{
"nexus_payment_id": "NEX-UUID-001",
"rating": 5 // (Optional) Rating
}

• Output Schema:

{
"status": "RELEASE_SIGNED",
"signature": "0xUserReleaseSig...",
"message": "Signature uploaded to Core. Merchant notified."
}

---

3. Nexus Seller Plugin (For Merchant Agent)

This plugin grants the MA seamless cross-chain collection capabilities; the MA only needs to monitor the Hub Chain (PlatON) state.

Tool A: verify_order_lock (Verify Fulfillment Conditions)

Lifecycle Stage: Fulfillment (before the MA prepares to fulfill/ship) Function: Query the Escrow contract on the Hub Chain (PlatON) to confirm whether funds are securely locked (LOCKED). Logic: This tool only returns LOCKED after Core has completed KYT and synchronized the state.

• Input Schema:

{ "merchant_order_ref": "TRIP-888" }

• Output Schema:

{
"nexus_payment_id": "NEX-UUID-001",
"status": "LOCKED", // Key signal: safe to fulfill
"amount_settled": "530.00",
"currency": "USDC",
"hub_chain": "PlatON",
"kyt_result": "PASS" // Funds are compliant
}

Tool B: claim_funds (Withdraw / Settle)

Lifecycle Stage: Settlement (after receiving the user's Release signature) Function: Pull the user's signature from Nexus Core and call the on-chain contract on PlatON to withdraw funds.

• Input Schema:

{ "nexus_payment_id": "NEX-UUID-001" }

• Output Schema:

{
"status": "CLAIMED",
"tx_hash": "0xPlatONTxHash...",
"settled_at": "2026-01-21T12:00:00Z"
}

---

4. Core State Machine (Order State Machine)

The single source of truth maintained by Nexus Core, exposed via MCP Resource nexus://core/orders/{id}.

Status	Meaning	Triggering Action	UA Interface Display
DRAFT	Order intent created	initialize_payment	Selector View (Chain/Wallet)
AWAITING_DEPOSIT	MPC address assigned	finalize_payment	Transfer View (Target Addr)
DETECTING	On-chain deposit detected	Listener captures Tx	"Verifying Transaction..."
SYNCING	KYT passed, writing to Hub	KYT Engine	"Securing Funds..."
LOCKED	[Milestone] Locked on PlatON	Hub contract event	"Payment Successful"
RELEASE_SIGNED	User signed release	sign_release	"Order Completed"
CLAIMED	Merchant withdrew funds	claim_funds	(History Record)
RISK_REJECTED	KYT failed	KYT Engine	"Security Alert" (triggers refund)

---

5. Implementation Guidance for Antigravity

Core Server (nexuspay-core)

1. Routing Engine: Requires hardcoded or configured RPC endpoints for PlatON, Base, and Ethereum to fetch real-time Gas Prices and calculate est_gas_fee_usd.
2. MPC Service: Integrate an MPC wallet SDK (e.g., Fireblocks, Coinbase WaaS, or custom TSS) to derive child addresses during finalize_payment.
3. KYT Hook: During the DETECTING state, block the flow and call the KYT API; only execute createVirtualDeposit to PlatON after passing.

UA Integration (@nexus/ua-kit)

1. Step 1: Component Mount -> call initialize_payment -> render <ChainSelector>.
2. Step 2: User selects Base + connects wallet -> clicks Pay -> call finalize_payment.
3. Step 3: Obtain target_address -> call wagmi.sendTransaction({ to: target_address, value: 0 }) (for USDC, call ERC20 transfer instead).
4. Step 4: Poll Core status until LOCKED. This specification fully satisfies all requirements for multi-chain interaction, user choice, KYT compliance, and trustless custody.

---

6. Implementation Status (v1.8.0 Notes)

Important: RFC-007 describes the full vision architecture of Nexus (Hub-Spoke cross-chain + MPC + KYT). The current MVP implementation only covers a subset of features.

6.1 Current Implementation vs Vision

Feature	RFC-007 Vision	MVP Implementation	Planned Phase
Multi-chain (Hub-Spoke)	PlatON Hub + Base/ETH Spoke	PlatON single-chain	Phase 3
MPC Ephemeral Addresses	Fireblocks/WaaS	No MPC (Escrow contract as substitute)	Phase 3
KYT Compliance Checks	DETECTING state blocking	Not implemented	Phase 3
Draft-then-Finalize	initialize -> finalize -> sign_release	Single-step orchestrate (returns 402 directly)	N/A (simplified design)
State Machine	8 states (DRAFT->CLAIMED)	13 states (CREATED->COMPLETED, RFC-005v4)	Current
Buyer Plugin	3 tools (init/finalize/sign_release)	11 MCP Tools (orchestrate, status, etc.)	Current
Seller Plugin	2 tools (verify_lock/claim)	confirm_fulfillment + webhook	Current

6.2 Current MCP Tools (In Production)

See RFC-005v4 Section 7.1 for details. The 11 MCP Tools currently deployed:

User Agent Side:

• nexus_orchestrate_payment -- Orchestrate payment (replaces initialize + finalize)
• nexus_get_payment_status -- Query status
• nexus_dispute_payment -- Open dispute
• nexus_discover_agents -- Discover merchant Agents
• nexus_get_agent_skill -- Retrieve Agent skill.md
• nexus_get_merchant -- Retrieve merchant info

Merchant Agent Side:

• nexus_confirm_fulfillment -- Confirm fulfillment (triggers Escrow release)
• nexus_release_payment -- Explicit release
• nexus_cancel_payment -- Cancel individual payment
• nexus_cancel_order -- Cancel entire order group
• nexus_resolve_dispute -- Arbitrate dispute

6.3 AP2 Protocol Adaptation

The Buyer/Seller Plugin concepts from RFC-007 coexist with the AP2 Agent Protocol Credential model:

• AP2 environment: CartMandate -> Nexus -> PaymentMandate (VC format)
• MCP environment: Quote -> orchestrate_payment -> 402 Instruction (NUPS format)
• On-chain settlement layer is identical (NexusEscrow v4.3.0)

See RFC-002 v2.0 Section 7 (AP2 Protocol Extension) for details.