/ Glossary
x402 terminology.
Quick-reference definitions for the x402 protocol and related concepts. Each term links to its relevant documentation.
- x402
- An HTTP-native payment protocol that uses the 402 status code to signal payment requirements and settle via token signatures.
- HTTP 402
- The HTTP status code 'Payment Required' — originally reserved, now used by x402 to trigger payment flows.
- payment-required
- An HTTP response header (Base64 JSON) that describes accepted payment methods, amount, token, network and receiver.
- payment-signature
- An HTTP request header containing the signed payment authorization, attached when replaying the original request.
- payment-response
- An HTTP response header confirming settlement details: transaction hash, amount received, remaining balance.
- EIP-3009
- ERC-20 extension that adds `transferWithAuthorization` — enables gasless token transfers via off-chain signatures.
- transferWithAuthorization
- The EIP-3009 function: transfers tokens using a signed authorization instead of an on-chain approval + transfer.
- Permit2
- Uniswap's universal approval router. One allowance to Permit2, then permit-based transfers to any contract.
- permitWitnessTransferFrom
- Permit2's `permitWitnessTransferFrom` — executes a signed transfer with additional witness data for x402 context.
- Facilitator
- The server-side entity that verifies payment signatures and submits the settlement transaction on-chain.
- Gasless transfer
- A token transfer where the sender pays zero gas. In x402/EIP-3009, the facilitator submits and pays gas.
- Agentic payment
- A payment initiated by an AI agent on behalf of a user, subject to pre-set policies and approval rules.
- USDC
- USD Coin — a fiat-backed stablecoin by Circle. The primary token for x402 payments due to EIP-3009 support.
- Base (chain)
- Coinbase's L2 network. Low fees make it ideal for x402 micro-payments. USDC is native on Base.
- On-device inference
- Running AI models directly on the user's device (phone/laptop) without sending data to external servers.
- LiteRT
- Google's LiteRT runtime for on-device ML inference. Used by BlockVault for local Gemma model execution.
- Delegate inference
- Offloading AI inference to a remote GPU server (like 402.blockvault.ai) and paying per-token via x402.
- SIWE
- Sign-In With Ethereum (EIP-4361) — authenticates a user by signing a message with their wallet key.
- HITL
- Human-In-The-Loop — requiring explicit user approval before the AI agent executes a sensitive action.
- KV cache
- Key-Value cache stored on the GPU between inference calls. Speeds up multi-turn conversations by avoiding re-computation.
- Self-custody
- A wallet model where only the user holds private keys. No server, exchange or third party can access funds.
- AI agent wallet
- A cryptocurrency wallet designed for AI agents: programmatic access, spend policies, x402 support.
- Spend policy
- Rules enforced locally by the wallet: per-domain caps, daily limits, token allow-lists, time windows.
- x402Fetch
- BlockVault's drop-in fetch() replacement that transparently handles 402 responses, signs payments and retries.
- Permit2 Proxy
- The x402 Permit2 Proxy contract deployed at 0x402085c248EeA27D92E8b30b2C58ed07f9E20001 via CREATE2.
- EIP-712
- Ethereum typed structured data standard. Used by both EIP-3009 and Permit2 for human-readable signing.
- Meta-transaction
- A transaction submitted by a third party (relayer/facilitator) on behalf of the signer, who pays no gas.
- Cold start
- The initial latency when a GPU instance spins up for the first inference request. Typically 10-30 seconds.
- SSE
- Server-Sent Events — a streaming protocol used by 402.blockvault.ai to deliver inference tokens in real-time.
- Nonce
- A unique value preventing replay attacks. In EIP-3009, each authorization has a random nonce.
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