Add a pluggable server extension API with MCP Apps (#3003)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

docs/advanced/extensions.md

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+# Extensions
+
+An **extension** is an opt-in bundle of MCP behaviour behind one identifier.
+
+It can contribute tools, resources, and new request methods, and it can wrap `tools/call`.
+The server advertises it under `capabilities.extensions`, the client opts in the same way,
+and nothing changes for anyone who didn't ask for it. That is the contract (SEP-2133), and
+it has one golden rule: **extensions are off by default**.
+
+## Using an extension
+
+Pass instances at construction:
+
+```python title="server.py"
+--8<-- "docs_src/extensions/tutorial001.py"
+```
+
+Done. The server now advertises `io.modelcontextprotocol/ui` under
+`capabilities.extensions` and serves everything the extension contributes.
+
+`Apps` is the built-in reference extension, and it gets its own page: **[MCP Apps](apps.md)**.
+
+!!! note
+    Extensions are fixed at construction. There is no `add_extension` to call later:
+    a server's capability map should not change while clients are connected to it.
+
+The capability map rides `server/discover`, which is a **2026-07-28** path. A legacy
+`initialize` handshake has nowhere to put it, so a legacy client simply doesn't see
+the extension. Design for that: an extension *augments* a server, it must not be the
+only way the server is usable.
+
+## Writing your own
+
+Subclass `Extension` and override only what you need. Every method has a default.
+
+### The identifier
+
+```python
+--8<-- "docs_src/extensions/tutorial002.py"
+```
+
+The identifier is a `vendor-prefix/name` string following the spec's `_meta` key
+grammar: dot-separated labels (each starts with a letter, ends with a letter or
+digit), a slash, then the name. It is validated **when the class is defined**, so a
+typo doesn't wait for a server to boot:
+
+```text
+TypeError: Stamps.identifier must be a `vendor-prefix/name` string
+(reverse-DNS prefix required), got 'stamps'
+```
+
+Use a domain you control as the prefix. `io.modelcontextprotocol/*` is for extensions
+specified by the MCP project itself.
+
+### Contributing tools
+
+The smallest useful extension is one tool and a settings map:
+
+```python title="server.py" hl_lines="17 19-20 22-23 26"
+--8<-- "docs_src/extensions/tutorial003.py"
+```
+
+* `tools()` returns `ToolBinding`s. The server registers each one exactly as if you
+  had called `mcp.add_tool(...)` yourself: same schema generation, same `Context`
+  injection, same everything.
+* `settings()` is the value advertised at `capabilities.extensions["com.example/stamps"]`.
+  Return `{}` (the default) to advertise the extension with no settings.
+* The extension never receives the server. It declares contributions as data;
+  `MCPServer` consumes them. There is no `self.server` to mutate.
+
+And `main()` is the proof, an in-memory client straight against `mcp`:
+
+```python title="server.py" hl_lines="29-34"
+--8<-- "docs_src/extensions/tutorial003.py"
+```
+
+### Serving your own methods
+
+An extension can register **new request methods**: its own verbs, served next to the
+spec's:
+
+```python title="server.py" hl_lines="15-21 30 39-47"
+--8<-- "docs_src/extensions/tutorial004.py"
+```
+
+* `SearchParams` subclasses `RequestParams`, so the 2026 `_meta` envelope parses
+  uniformly and your handler gets validated params, never a raw dict. Bound what
+  the client controls: `Field(ge=1, le=100)` rejects an absurd `limit` before
+  your code allocates anything for it.
+* `require_client_extension(ctx, EXTENSION_ID)` is the gate: a client that did not
+  declare the extension gets the `-32021` (missing required client capability) error,
+  with the machine-readable `requiredCapabilities` payload the spec asks for.
+* `protocol_versions=frozenset({"2026-07-28"})` pins the method to one wire version.
+  At any other version the client gets `METHOD_NOT_FOUND`, exactly as if the method
+  didn't exist there. For that client, it doesn't.
+
+Methods are **strictly additive**. The SDK enforces this at construction, not at
+runtime:
+
+* A `MethodBinding` for a spec-defined method (`tools/list`, `completion/complete`, ...)
+  raises `ValueError` when the binding is constructed. Core verbs belong to the server.
+* Two extensions binding the same method raise when the second one registers.
+  Last-write-wins is how plugins corrupt each other; we don't do that.
+* An empty `protocol_versions` set raises too: a method that can never be served
+  is a bug, not a configuration.
+
+### The client side
+
+The same file's `main()` is the whole client story, both halves of it:
+
+```python title="server.py" hl_lines="53-57"
+--8<-- "docs_src/extensions/tutorial004.py"
+```
+
+* `Client(..., extensions={EXTENSION_ID: {}})` declares the extension. That map
+  becomes `ClientCapabilities.extensions`: on a 2026-07-28 connection it travels in
+  the per-request `_meta` envelope, so the server sees it on **every** request; on
+  a legacy connection it rides the `initialize` handshake. Server code doesn't care
+  which: `require_client_extension(ctx, ...)` and
+  `ctx.session.check_client_capability(...)` read the right source on both paths.
+* Vendor methods drop one layer to `client.session.send_request(...)`; `Client`
+  only grows first-class methods for spec verbs. The `cast` is there because
+  `send_request` is typed against the spec's closed request union.
+
+### Intercepting `tools/call`
+
+The one interceptive hook. Override `intercept_tool_call` to observe, short-circuit,
+or veto a tool call:
+
+```python title="server.py" hl_lines="18-25"
+--8<-- "docs_src/extensions/tutorial005.py"
+```
+
+* `params` is the validated `CallToolRequestParams`: you get `params.name` and
+  `params.arguments` without touching raw JSON.
+* `call_next(ctx)` runs the rest of the chain. Return its result unchanged (observe),
+  return something else (replace), or raise an `MCPError` (refuse).
+* With several extensions, interceptors nest in registration order: the first
+  extension in `extensions=[...]` is outermost.
+* The default implementation is a pass-through, and a server whose extensions never
+  override this hook installs **no** middleware at all. You don't pay for what
+  you don't use.
+
+The hook wraps `tools/call` and nothing else. For every-message concerns, use
+[Middleware](middleware.md). That is what it is for.
+
+## What an extension cannot do
+
+The contribution surface is **closed** on purpose: settings, tools, resources,
+methods, one `tools/call` interceptor. An extension cannot:
+
+* **Reach into the server.** It declares data; it holds no server reference.
+* **Replace core behaviour.** Spec methods are rejected at construction, and
+  `initialize` is reserved by the runner outright.
+* **Register late.** After `MCPServer(...)` returns, the extension set is what it is.
+
+If you are fighting these walls, you are not writing an extension. You are writing
+a fork. The walls are the feature: a user reading `extensions=[Apps(), Stamps()]`
+knows *everything* those two can have touched.