In MCP, what is an optional extension, and which ones are official?

## The idea A protocol has to decide what every implementation is obliged to support. Anything in the core of MCP is a floor: a conforming implementation has to handle it. Anything outside the core is optional, and MCP gives optional features a single, uniform home — the **extensions** mechanism. An extension is a named, self-describing bundle of behaviour that two peers may agree to use, and that they simply do not use if either of them lacks it. This is the whole point: extensions let the ecosystem grow without raising the bar that every implementer must clear. ## Where support is declared Both capability objects in MCP revision 2026-07-28 — `ClientCapabilities` and `ServerCapabilities` — carry an `extensions` field. It is a **map**: the key is the extension identifier, the value is a settings object for that extension. An empty settings object (`{}`) is meaningful and normal: it says "I support this extension, and I have no settings to pass". It does **not** mean unsupported. Identifiers are namespace-prefixed, following the same naming shape MCP uses for prefixed `_meta` keys: a dot-separated, reverse-DNS-style prefix, a slash, then a name — for example `io.modelcontextprotocol/ui`. The prefix is what keeps an extension defined by one vendor from colliding with an unrelated extension of the same short name defined by someone else. ## The two official extensions **`io.modelcontextprotocol/tasks` — Tasks.** The extension for work that does not finish inside a single request/response. A client that supports it opts in once through the extension capability; there is no per-tool flag. The lifecycle mechanics themselves are the Tasks extension's own subject; what matters here is that they are reachable only after both sides have declared the identifier. **`io.modelcontextprotocol/ui` — MCP Apps.** The extension covering interactive UI surfaces served through MCP. Both live under the `io.modelcontextprotocol` prefix because both are defined by the specification project itself. A third party writing its own extension uses its own prefix. ## Why Tasks became an extension In the 2025-11-25 design, task-shaped asynchronous execution was part of the core protocol. Revision 2026-07-28 removed it from core and re-issued it as the `io.modelcontextprotocol/tasks` extension. The migration is instructive: the feature is genuinely useful, but not every server has long-running work and not every client wants to poll and manage a task lifecycle. Making it mandatory taxed every implementation for a feature many would never exercise. As an extension, servers that need it advertise it, clients that can drive it advertise it, and everyone else interoperates over plain core calls. ## What extensions are not - **Not protocol versions.** MCP revisions are `YYYY-MM-DD` strings naming the date of the last backwards-incompatible change, and they partition the fleet into eras. An extension is additive and negotiated per peer pair; a peer that does not have it still speaks the same revision perfectly well. - **Not host configuration.** Whether a particular desktop product exposes a server is a product concern. The extensions map is a protocol-level declaration exchanged between a client and a server. - **Not implicit.** There is no "on by default unless you opt out". Silence means the extension is not in play, for both directions. - **Not a licence to assume.** A client cannot infer that a server supports an extension because a previous request seemed to work, and a server cannot infer a client's support from anything other than the capabilities that accompany the request in front of it. ## What an interviewer is checking Mostly two things. First, that you know optional features have a designated, discoverable home rather than being smuggled in as extra fields. Second, that you can name the two official ones and, ideally, tell the Tasks story — because knowing that Tasks used to be core and is now an extension is a reliable signal that you have read the 2026-07-28 revision rather than older material.