@modelcontextprotocol/server-debug

Debug Server

A comprehensive testing/debugging tool for the MCP Apps SDK that exercises every capability, callback, and result format combination.

Tools

debug-tool

Configurable tool for testing all result variations:

Parameter	Type	Default	Description
contentType	"text" | "image" | "audio" | "resource" | "resourceLink" | "mixed"	"text"	Content block type to return
multipleBlocks	boolean	false	Return 3 content blocks
includeStructuredContent	boolean	true	Include structuredContent in result
includeMeta	boolean	false	Include _meta in result
largeInput	string	-	Large text input (tests tool-input-partial)
simulateError	boolean	false	Return isError: true
delayMs	number	-	Delay before response (ms)

debug-refresh

App-only tool (hidden from model) for polling server state. Returns current timestamp and call counter.

App UI

The debug app provides a dashboard with:

• Event Log: Real-time log of all SDK events with filtering
• Host Info: Context, capabilities, container dimensions, styles
• Callback Status: Table of all callbacks with call counts
• Actions: Buttons to test every SDK method:
  • Send messages (text/image)
  • Logging (debug/info/warning/error)
  • Model context updates
  • Display mode requests
  • Link opening
  • Resize controls
  • Server tool calls
  • File operations

Usage

# Build
npm run --workspace examples/debug-server build

# Run standalone
npm run --workspace examples/debug-server serve

# Run with all examples
npm start

Then open http://localhost:8080/basic-host/ and select "Debug MCP App Server" from the dropdown.