Architecture

specification/2026-07-28/architecture/index First recorded · 172 lines, first recorded

# Architecture ## Core Components ### Host ### Clients ### Servers ## Design Principles ## Capability Negotiation

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# Architecture

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The Model Context Protocol (MCP) follows a client-host-server architecture where each
host can run multiple client instances. MCP is a stateless protocol: every request is
self-contained and carries its own protocol version and capabilities.
This architecture enables users to integrate AI capabilities across applications while
maintaining clear security boundaries and isolating concerns. Built on JSON-RPC, MCP
provides a protocol focused on context exchange and sampling coordination between
clients and servers.

## Core Components

```mermaid theme={null}
graph LR
    subgraph "Application Host Process"
        H[Host]
        C1[Client 1]
        C2[Client 2]
        C3[Client 3]
        H --> C1
        H --> C2
        H --> C3
    end

    subgraph "Local machine"
        S1[Server 1<br>Files & Git]
        S2[Server 2<br>Database]
        R1[("Local<br>Resource A")]
        R2[("Local<br>Resource B")]

        C1 --> S1
        C2 --> S2
        S1 <--> R1
        S2 <--> R2
    end

    subgraph "Internet"
        S3[Server 3<br>External APIs]
        R3[("Remote<br>Resource C")]

        C3 --> S3
        S3 <--> R3
    end
```

### Host

The host process acts as the container and coordinator:

* Creates and manages multiple client instances
* Controls client connection permissions and lifecycle
* Enforces security policies and consent requirements
* Handles user authorization decisions
* Coordinates AI/LLM integration and sampling
* Manages context aggregation across clients

### Clients

Each client is created by the host and communicates with exactly one server:

* Communicates with exactly one server
* Attaches protocol version and capabilities to every request
* Routes protocol messages bidirectionally
* Manages subscriptions and notifications
* Maintains security boundaries between servers

A host application creates and manages multiple clients, with each client having a 1:1
relationship with a particular server.

### Servers

Servers provide specialized context and capabilities:

* Expose resources, tools and prompts via MCP primitives
* Operate independently with focused responsibilities
* Request client input (sampling, elicitation, roots) via `InputRequiredResult` within a reply
* Must respect security constraints
* Can be local processes or remote services

## Design Principles

MCP is built on several key design principles that inform its architecture and
implementation:

1. **Servers should be extremely easy to build**
   * Host applications handle complex orchestration responsibilities
   * Servers focus on specific, well-defined capabilities
   * Simple interfaces minimize implementation overhead
   * Clear separation enables maintainable code

2. **Servers should be highly composable**
   * Each server provides focused functionality in isolation
   * Multiple servers can be combined seamlessly
   * Shared protocol enables interoperability
   * Modular design supports extensibility

3. **Servers should not be able to read the whole conversation, nor "see into" other
   servers**
   * Servers receive only necessary contextual information
   * Full conversation history stays with the host
   * Each server maintains isolation
   * Cross-server interactions are controlled by the host
   * Host process enforces security boundaries

4. **Features can be added to servers and clients progressively**
   * Core protocol provides minimal required functionality
   * Additional capabilities can be negotiated as needed
   * Servers and clients evolve independently
   * Protocol designed for future extensibility
   * Backwards compatibility is maintained

## Capability Negotiation

The Model Context Protocol uses a capability-based negotiation system where clients and
servers declare their supported features on each request. Clients include their
capabilities in `_meta.io.modelcontextprotocol/clientCapabilities` on every request.
Servers advertise their capabilities in response to
[`server/discover`](/specification/2026-07-28/server/discover), which clients may call before
any other request for up-front capability discovery.

* Servers declare capabilities like tool support, resource subscriptions, and prompt
  templates
* Clients declare capabilities like sampling support and elicitation handling
* Both parties must respect declared capabilities throughout the interaction
* Additional capabilities can be negotiated through extensions to the protocol

```mermaid theme={null}
sequenceDiagram
    participant Host
    participant Client
    participant Server

    opt Discovery
        Client->>Server: server/discover
        Server-->>Client: supported versions + capabilities
    end

    loop Client Requests
        Host->>Client: User- or model-initiated action
        Client->>Server: Request (with _meta: version, clientCapabilities)
        alt Server requires client input
            Server-->>Client: InputRequiredResult (e.g. sampling/createMessage)
            Client->>Host: Forward to AI
            Host-->>Client: AI response
            Client->>Server: Original request (with input)
        end
        Server-->>Client: Response
        Client-->>Host: Update UI or respond to model
    end

    opt Subscriptions
        Client->>Server: subscriptions/listen (toolsListChanged, resourceSubscriptions, …)
        Server--)Client: notifications/subscriptions/acknowledged
        loop Stream
            Server--)Client: notifications/* (tagged with subscriptionId)
        end
    end
```

Each capability unlocks specific protocol features on a per-request basis. For example:

* Implemented [server features](/specification/2026-07-28/server) must be advertised in the
  server's capabilities
* Receiving resource update notifications requires opening a
  [`subscriptions/listen`](/specification/2026-07-28/basic/patterns/subscriptions) stream
  with the desired resource URIs
* [Tool](/specification/2026-07-28/server/tools) invocation requires the server to declare tool capabilities

This capability negotiation ensures clients and servers have a clear understanding of
supported functionality while maintaining protocol extensibility.