`registerPrompt()` silently drops `icons` from PromptDefinition config

Summary

server.registerPrompt() accepts a config object but destructures only { title, description, argsSchema } before storing the prompt. Any additional fields — including icons: Icon[], which the MCP 2025-11-25 spec explicitly allows on prompt definitions — are silently discarded. Clients calling prompts/list never receive icons.

SDK Version

@modelcontextprotocol/sdk 1.29.0

Affected Line

dist/cjs/server/mcp.js:734:

const { title, description, argsSchema } = config;  // ← icons dropped here

Spec Reference

MCP 2025-11-25 server/prompts.md — prompts can have icons?: Icon[]. The Icon interface (spec.types.d.ts:449–487) has src, mimeType, sizes, and optional theme.

Reproduction

server.registerPrompt('my_prompt', {
  description: 'A prompt with an icon',
  argsSchema: {},
  icons: [{ src: 'data:image/svg+xml;base64,...', mimeType: 'image/svg+xml', sizes: ['24x24'] }],
}, async () => ({ messages: [] }));

// prompts/list response: icons field is absent

Expected Behavior

icons in registerPrompt() config should be stored and returned in prompts/list wire responses.

Workaround

Intercept prompts/list via server.server.setRequestHandler(ListPromptsRequestSchema, ...) and inject icons from a parallel Map<name, Icon[]>.

Fix Suggestion

// In registerPrompt():
const { title, description, argsSchema, icons } = config;  // include icons
registeredPrompt.icons = icons;  // persist

// In prompts/list serializer: include icons if present

Start at registerPrompt() in dist/cjs/server/mcp.js:734 and trace how registered prompts are serialized for prompts/list responses. Compare the stored PromptDefinition fields with the MCP 2025-11-25 prompt definition, then verify that a prompt configured with icons returns them in the wire response.