What are MCP extensions? Sume's capabilities list tools only

MCP extensions are optional features beyond the core protocol: the 2026-07-28 changelog says it adds an extensions field to both ClientCapabilities and ServerCapabilities to support them. Sume's hosted server does not use that field. Its initialize reply advertises one capability, tools, with listChanged: false.

Spec text is from the MCP changelog; Sume behavior is from the MCP server source and the tools and gates docs, read 2026-10-01.

What does the new field do?

The changelog lists it under minor changes: an optional place for each side to say which extensions it supports. It does not say which extensions exist; for one that is documented separately see the tasks extension.

What does Sume's initialize reply contain?

What happens after initialize?

The server accepts notifications/initialized and drops it, with no response body. The docs describe the hosted tool surface as tools wrapping selected public API capabilities, so there is no resources or prompts capability to negotiate.

Do I need to send extensions to Sume?

No. A client can include an extensions object in its own capabilities; nothing in the sources I read shows Sume acting on it, so do not rely on it changing behavior. Treat missing extensions in the reply as "none offered" and use the tools list as the contract.