An MCP server is the component of the Model Context Protocol (MCP) that exposes tools, data, or resources in a standardized format so they can be consumed by an MCP client and used by an AI agent. It makes capabilities discoverable, interoperable, and reusable across different AI systems without custom integration code.
Key characteristics
- Tool exposure – Publishes tools, APIs, or data sources in a consistent schema
- Discoverability – Advertises available tools and their metadata to clients
- Standardized responses – Returns outputs in a predictable format
- Reusability – Enables the same resource to be used by multiple AI agents or hosts
- Security boundaries – Enforces access control and permissions on its resources
- Interoperability – Works with any MCP-compliant client, regardless of implementation
The MCP server does not initiate actions. Instead, it waits for requests from MCP clients, processes them, and returns results.
Understanding the relationships
MCP server vs. MCP client
- MCP Server - publishes capabilities (“I provide access to calendar events, document search, or database queries”).
- MCP Client - consumes and invokes those capabilities on behalf of an agent.
Without MCP Server
- Tools must be hardcoded into the agent or orchestration layer.
- Capabilities are siloed and difficult to share.
- Scaling integrations across teams is error-prone.
With MCP Server
- Tools are exposed once, reused everywhere.
- Agents can discover capabilities dynamically.
- Ecosystems of interoperable tools emerge.
Key distinction: The MCP server is the supply side of MCP. It provides capabilities that clients (and therefore agents) can consume.
Example
A company exposes its internal resources as MCP servers:
- A search server that indexes internal documentation.
- A ticketing server that manages support requests.
- A knowledge-graph server that maps relationships between data entities.
Agents running in different hosts can all access these resources via their MCP clients, without custom connectors or duplicated integration work.
Non-examples
- The agent – Decides which tool to use (with the LLM), but does not expose capabilities.
- The MCP client – Invokes tools, but does not publish or serve them.
- The MCP host – Provides the runtime environment, but does not expose tools.
- The orchestration layer – Coordinates workflows and reliability, but does not make tools available.
- A tool by itself – A capability (e.g., database API) is not an MCP server until it is exposed through the protocol.
An MCP server is the supply side of MCP: it publishes tools and data for clients to consume, but does not reason, decide, or execute requests itself.