AI Agent Crypto Glossary: x402, ACP, and MCP and 20 More Terms

AI agents are starting to do real work onchain. They call APIs, buy data, pay for compute, and move value without a human clicking approve. That shift brought a wave of new vocabulary, and most of it is scattered across protocol docs, launch threads, and standards drafts.

This glossary collects the terms you actually need to follow agentic crypto. We grouped them by what they do, gave each one a plain definition first, then added context. If you are new to the space, read top to bottom. If you came from a search for x402 protocol explained or another single term, jump to it.

What are agent payments?

Agent payments are transactions an AI agent initiates and settles on its own, within spend limits set by a human or system in advance. Instead of a person entering payment details, the agent can pay directly for API calls, datasets, compute, and other services. Just as human payments rely on rails like ACH, debit, and credit, agent payments are emerging around new protocols designed for autonomous agents. Alchemy AgentPay helps merchants accept agent-native payments across supported agent payment protocols

Agentic Payment Protocols

What is x402?

x402 is an open standard for paying for web resources using the HTTP 402 Payment Required status code. When an agent or person requests a paid resource, the server can respond with a 402 and a price for that resource, the agent resends the request with the payment included, and the request completes. x402 lets agents pay per request without accounts or stored cards.

What is ACP (Agentic Commerce Protocol)?

ACP is an open standard developed by OpenAI and Stripe to make online checkouts agent-ready. It allows a business to define how an AI agent can initiate a purchase using the merchant's existing commerce and payment infrastructure. ACP is another protocol for agent-driven commerce that defines how an agent discovers, negotiates, and pays for goods and services. It is one of several agent payment protocols that agents could use to make payments.

What is MPP (Machine Payments Protocol)?

MPP is an emerging standard for how merchants accept agent-initiated payments built by Tempo and Stripe. An agent can request a resource from a service, API, Model Context Protocol (MCP), or any HTTP addressable endpoint, and the service responds with a payment request. The agent authorizes the payment, and the resource is delivered to the agent.

What is a Stablecoin?

A stablecoin is a token designed to hold a steady value, usually pegged to a currency like the US dollar. Agents lean on stablecoins because predictable value makes automated spending and accounting far simpler than using a volatile asset.

Agent tooling

What is MCP (Model Context Protocol)?

MCP is an open protocol that lets AI models call external tools and data sources through a standard interface. An MCP server exposes capabilities, and an MCP client, like a coding assistant or chat model, can use them. For crypto, an MCP server can give an agent safe, structured access to onchain data and actions. Our own Alchemy MCP Server exposes Alchemy's tools to agent clients this way.

What are agent skills?

Agent skills are machine-readable instructions that tell an agent how to perform a specific task or set of tasks. Skills can bundle scripts, reference materials, templates, and other resources.

What is tool calling?

Tool calling is the mechanism by which an agent or user invokes an external function, returns the result into the context where the tool was called, and continues reasoning. It is the foundation under MCP and most agent frameworks.

Identity, auth, and security

What is agent identity?

Agent identity is how a system proves which agent is acting and what it is allowed to do. Without it, you cannot safely let software move money or access private data on its own.

What are spend limits?

Spend limits are caps a human or system places on what an agent can pay, over a period or per transaction. They are the core safety control that makes autonomous payments acceptable in production.

What is an allowlist?

An allowlist is a defined set of addresses, merchants, or destinations an agent is permitted to transact with. Pairing allowlists with spend limits narrows the blast radius if an agent misbehaves.

What are network tokens?

Network tokens are payment credentials that can substitute for primary account numbers for online purchases so that the underlying payment credentials are never exposed. These matter for agent commerce because they bridge agent payments and onchain payments into traditional card networks.

Infrastructure

What is RPC (Remote Procedure Call)?

RPC is a protocol that allows a computer program to execute a procedure or function on another computer or server, without the need for the developer to code the communication details. With RPC, you can call functions on remote computers as if they were local, making it easier to develop distributed applications. RPC is the request-response interface to read from and write to a blockchain. Agents depend on reliable RPC the same way any distributed application does, just without any human in the loop.

What is a bundler?

A bundler is a service that packages account-abstraction operations and submits them onchain. It is part of how smart accounts, which many agents use, get their transactions executed.

What is gas?

Gas is the fee paid to execute a transaction on a blockchain. Agents need an efficient gas solution, such as a gas manager that sponsors fees, so a low balance does not silently stall their work.

What is account abstraction?

Account abstraction lets a smart contract act as a user account, which enables features like gas sponsorship, batched actions, and programmable spending rules. Those features map almost perfectly onto what an autonomous agent needs.

How these terms fit together

Most agentic systems combine several of these at once. An agent uses MCP or a framework like GOAT to reach tools, an identity and spend-limit layer to stay inside guardrails, a payment standard like x402 to settle, and reliable infrastructure underneath so none of it stalls. We think the teams that win here treat agents as a new kind of user on the same platform that already serves apps and businesses, not as a separate stack bolted on the side.

FAQs

What is the x402 protocol in simple terms?

It is a way for software to pay for a web resource the moment it is asked to, using the HTTP 402 Payment Required response. The agent gets a price, pays, and the request goes through, with no account setup.

What are the most important AI agent crypto terms to know first?

Start with agent payments, x402, MCP, stablecoins, spend limits, and account abstraction. Those six cover how agents pay, how they reach tools, and how they stay inside safe limits.

Why do AI agents use crypto rails instead of normal payments?

Crypto rails let an agent pay per request, in small amounts, without accounts or human checkout, and stablecoins keep the value predictable. Many real systems bridge both crypto and card rails rather than picking one.

Where can I learn how agent payments actually work?

See our companion explainer on agent payments, which walks through how agents pay for APIs, data, and compute, and where standards like x402 fit.

Build agentic products on Alchemy

Alchemy is the blockchain platform behind many of crypto's biggest products, and we are extending the same rails to agents. Explore what is possible at alchemy.com/agents or read the docs.