Skip to main content
MCP Apps SDK 
import { App } from "@modelcontextprotocol/ext-apps";

​
Overview

sendMessage sends a message from the View into the host’s chat interface, appearing as if the user typed it. This enables the app to drive the conversation — for example, submitting a user selection, asking a follow-up question, or triggering the model to act on data displayed in the UI. The message is always sent with role: "user". The host may reject the message (for example, if sending is rate-limited or disabled), in which case isError will be true in the response.

​
Signature

async sendMessage(
  params: { role: "user"; content: ContentBlock[] },
  options?: RequestOptions,
): Promise<{ isError?: boolean }>

​
Parameters

​
params
object
required
The message parameters.
​
role
"user"
required
The message role. Currently only "user" is supported.
​
content
ContentBlock[]
required
Array of content blocks to include in the message. Supports text, images, and other standard MCP content block types.
​
options
RequestOptions
Optional request configuration.
​
signal
AbortSignal
An AbortSignal to cancel the request.

​
Returns

​
result
object
​
isError
boolean
true if the host rejected the message. When absent or false, the message was accepted.

​
Usage

​
Send a text message

const result = await app.sendMessage({
  role: "user",
  content: [{ type: "text", text: "Show me details for item #42" }],
});

if (result.isError) {
  console.error("Host rejected the message");
}

​
Send a message based on user interaction

function onItemSelected(itemId: string) {
  app.sendMessage({
    role: "user",
    content: [{
      type: "text",
      text: `Tell me more about ${itemId}`,
    }],
  });
}

​
Combine with updateModelContext for large payloads

When you need to send a large amount of data along with a message, offload the data to model context first, then send a brief trigger message: 
await app.updateModelContext({
  content: [{ type: "text", text: fullTranscript }],
});
await app.sendMessage({
  role: "user",
  content: [{ type: "text", text: "Summarize the key points" }],
});