Overview

This page defines the message patterns of the core protocol: the ways a client and server compose JSON-RPC requests, responses, and notifications into interactions. Every transport carries all of these patterns; transports differ only in how messages are framed and delivered.

Every interaction begins with the client:

• The client sends JSON-RPC requests and notifications.
• The server answers each request with a JSON-RPC response (a result or error), optionally preceded by notifications scoped to that request.

Servers MUST NOT initiate JSON-RPC requests, and clients do not send JSON-RPC responses.

Request and Response

The client sends a request; the server answers it with a result or an error. While the request is in flight, the server MAY send notifications scoped to it, such as notifications/progress and notifications/message.

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: request
    Server-->>Client: notifications/progress (optional)
    Server-->>Client: response

Multi Round-Trip Requests

When a server needs client input (sampling, elicitation, or roots) to complete a request, it answers with an InputRequiredResult and the client retries the request with the matching inputResponses. See Multi Round-Trip Requests.

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: request (id: 1)
    Server-->>Client: InputRequiredResult (inputRequests)
    Client->>Server: request (id: 2, original params + inputResponses)
    Server-->>Client: response

Subscribe and Notify

To receive change notifications (list changes, resource updates), the client sends a subscriptions/listen request; the reply is a long-lived stream of the requested notification types. Stream state is scoped to the request: if the underlying channel is lost, the client re-issues the request.

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: subscriptions/listen
    Server-->>Client: notifications/subscriptions/acknowledged
    note over Client,Server: Stream stays open
    Server-->>Client: notifications/* (tagged with subscriptionId)

Adding Patterns

All core protocol features are built from these patterns. A protocol revision that adds a pattern defines it on this page. Transports carry new patterns without changes, because patterns are expressed entirely in terms of requests, responses, and notifications.

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Last updated Oct 08, 2026