MCP Apps sendLog - Send Debug Logs

MCP Apps SDK

import { App } from "@modelcontextprotocol/ext-apps";

Overview

sendLog sends log messages from the View to the host for debugging purposes. These logs are not added to the conversation and are not visible to the end user. They are intended for developers to diagnose issues during development or in production. Use sendLog when you need structured logging that the host can capture, filter, or display in its developer tools. This is particularly useful because MCP App Views run in sandboxed iframes where console.log output may not be easily accessible.

Signature

sendLog(
  params: { level: string; data: string; logger?: string },
): Promise<void>

Parameters

params

object

required

level

string

required

The log level. Common values include "debug", "info", "warning", and "error".

data

string

required

The log message content.

logger

string

An optional logger name to identify the source of the log message. Useful when multiple components emit logs.

Returns

Returns Promise<void>. The method resolves when the log has been sent to the host.

Usage

Basic logging

app.sendLog({
  level: "info",
  data: "Weather data refreshed",
  logger: "WeatherApp",
});

Logging at different levels

// Debug-level detail
app.sendLog({
  level: "debug",
  data: `Fetching resource: ${resourceUri}`,
  logger: "ResourceLoader",
});

// Warning when something unexpected happens
app.sendLog({
  level: "warning",
  data: "API response was empty, using cached data",
  logger: "DataService",
});

// Error when something fails
app.sendLog({
  level: "error",
  data: `Failed to parse response: ${error.message}`,
  logger: "DataService",
});

Wrapping in a helper

function createLogger(name: string) {
  return {
    debug: (msg: string) => app.sendLog({ level: "debug", data: msg, logger: name }),
    info: (msg: string) => app.sendLog({ level: "info", data: msg, logger: name }),
    warn: (msg: string) => app.sendLog({ level: "warning", data: msg, logger: name }),
    error: (msg: string) => app.sendLog({ level: "error", data: msg, logger: name }),
  };
}

const log = createLogger("CartView");
log.info("Cart loaded with 3 items");
log.error("Payment gateway unreachable");

useSendLog — React hook that wraps this method.
Requests — Overview of all View-to-host request methods.

​Overview

​Signature

​Parameters

​Returns

​Usage

​Basic logging

​Logging at different levels

​Wrapping in a helper

​Related

Overview

Signature

Parameters

Returns

Usage

Basic logging

Logging at different levels

Wrapping in a helper

Related