modelcontextprotocol · matoushavlena · Jun 16, 2025
@@ -41,6 +41,10 @@ Requests are sent from the client to the server or vice versa, to initiate an op
   method: string;
   params?: {
     [key: string]: unknown;
+    _meta?: {
+      [key: string]: unknown;
+      allowPartial?: boolean;
+    }
   };
 }
 ```
@@ -49,6 +53,7 @@ Requests are sent from the client to the server or vice versa, to initiate an op
 - Unlike base JSON-RPC, the ID **MUST NOT** be `null`.
 - The request ID **MUST NOT** have been previously used by the requestor within the same
   session.
+- Requests **MAY** allow partial results by setting the `allowPartial` flag.
 
 ### Responses
 
@@ -60,6 +65,10 @@ Responses are sent in reply to requests, containing the result or error of the o
   id: string | number;
   result?: {
     [key: string]: unknown;
+    _meta?: {
+      [key: string]: unknown;
+      hasMore?: boolean;
+    }
   }
   error?: {
     code: number;
@@ -73,6 +82,8 @@ Responses are sent in reply to requests, containing the result or error of the o
 - **Responses** are further sub-categorized as either **successful results** or
   **errors**. Either a `result` or an `error` **MUST** be set. A response **MUST NOT**
   set both.
+- Results **MAY** set `hasMore` flag only if the corresponding request set the `allowPartial` flag.
+  The flag indicates that more responses with the same ID will follow.
 - Results **MAY** follow any JSON object structure, while errors **MUST** include an
   error code and message at minimum.
 - Error codes **MUST** be integers.

@@ -101,15 +101,15 @@ MCP endpoint.
    `Content-Type: application/json`, to return one JSON object. The client **MUST**
    support both these cases.
 6. If the server initiates an SSE stream:
-   - The SSE stream **SHOULD** eventually include JSON-RPC _response_ for the
+   - The SSE stream **SHOULD** eventually include JSON-RPC _response(s)_ for the
      JSON-RPC _request_ sent in the POST body.
    - The server **MAY** send JSON-RPC _requests_ and _notifications_ before sending the
-     JSON-RPC _response_. These messages **SHOULD** relate to the originating client
+     final JSON-RPC _response_. These messages **SHOULD** relate to the originating client
      _request_.
-   - The server **SHOULD NOT** close the SSE stream before sending the JSON-RPC _response_
+   - The server **SHOULD NOT** close the SSE stream before sending the JSON-RPC _response(s)_
      for the received JSON-RPC _request_, unless the [session](#session-management)
      expires.
-   - After the JSON-RPC _response_ has been sent, the server **SHOULD** close the SSE
+   - After the final JSON-RPC _response_ has been sent, the server **SHOULD** close the SSE
      stream.
    - Disconnection **MAY** occur at any time (e.g., due to network conditions).
      Therefore:

@@ -351,6 +351,29 @@ Providing an output schema helps clients and LLMs understand and properly handle
 - Guiding clients and LLMs to properly parse and utilize the returned data
 - Supporting better documentation and developer experience
 
+## Content streaming
+
+Tools might provide result as a stream of partial results:
+
+```mermaid
+sequenceDiagram
+    participant User
+    participant Client
+    participant Server
+
+    Note over Client,Server: Streaming
+    Client->>Server: tools/call
+    Server-->>Client: Tool partial result
+    Client-->>User: Content
+    Server-->>Client: Tool partial result
+    Client-->>User: Content
+    Server-->>Client: Tool partial result
+    Client-->>User: Content
+```
+
+_Unstructured_ content is incremental and the complete result can be acquired by concatenation.
+_Structured_ content is not incremental unless the tool's output schema states otherwise.
+
 ## Error Handling
 
 Tools use two error reporting mechanisms:

@@ -33,6 +33,10 @@ export interface Request {
        * If specified, the caller is requesting out-of-band progress notifications for this request (as represented by notifications/progress). The value of this parameter is an opaque token that will be attached to any subsequent notifications. The receiver is not obligated to provide these notifications.
        */
       progressToken?: ProgressToken;
+      /**
+       * If specified, the caller is requesting partial results for this request. The receiver is not obligated to return partial results.
+       */
+      allowPartial?: boolean;
       [key: string]: unknown;
     };
     [key: string]: unknown;
@@ -54,7 +58,13 @@ export interface Result {
   /**
    * See [specification/draft/basic/index#general-fields] for notes on _meta usage.
    */
-  _meta?: { [key: string]: unknown };
+  _meta?: {
+    /**
+     * If true, more results will follow.
+     */
+    hasMore?: boolean;
+    [key: string]: unknown;
+  };
   [key: string]: unknown;
 }