The Model Context Protocol is an open standard that lets an AI agent discover and invoke external tools — databases, APIs, file systems, internal services — through a uniform interface, rather than through bespoke integration code written for each one. An MCP server exposes a set of tools; an MCP client, typically an agent runtime, calls them. The protocol standardises how the tools are described and how the calls and responses are framed.
The governance consequence is the part enterprises miss. Every MCP tool invocation is two things at once: a data flow and a permission decision. The agent is reaching something that holds real data, and something decided it was allowed to. In most deployments nothing records either. The tool list an agent can reach is effectively an access-control surface that no one has inventoried, and it changes whenever a developer adds a server.
How it differs from an API integration. A traditional integration is written, reviewed and deployed by a person, and its permissions are fixed at build time. MCP is discovered at runtime: the agent asks what tools exist and decides which to call. That moves the authorisation question from the build to the moment of execution — which is why MCP governance belongs in the runtime plane rather than in the workflow that assembled the agent.
How it differs from A2A. MCP governs an agent calling a tool. A2A governs an agent talking to another agent. The two often appear in the same architecture and need different controls.
APERION Shield is an open-source guardrail layer for MCP tool calls.
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