Add extensions and MCP Apps docs, an extensions story, and Apps polish

- docs/advanced/extensions.md: using and writing extensions (identifier
  grammar, contributions, vendor methods, version pinning, the tools/call
  interceptor, client-side declaration), backed by runnable docs_src
  examples with tests proving every claim
- docs/advanced/apps.md: the two-part Apps model, graceful degradation and
  the meaningful-content rule, CSP/permissions field tables, visibility
  semantics, the construction-time checks, and the add_resource escape hatch
- examples/stories/extensions: a custom extension end to end (settings
  entry, contributed tool, vendor method gated on the client declaring the
  extension back)
- Apps.tool() rejects a caller-supplied 'ui' meta key instead of silently
  clobbering it; add_resource defaults the app MIME type and rejects an
  explicit mismatch; resource _meta.ui pinned on both list entry and read
  content item

Author: maxisbey

docs/advanced/apps.md

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+# MCP Apps
+
+An **MCP App** is a tool with a face: alongside its data, the tool points at an HTML
+document the host renders as an interactive surface.
+
+Two parts, always two parts:
+
+1. **A tool** that does the work and returns data, like any other tool.
+2. **A `ui://` resource** containing the HTML the host shows for it.
+
+The tool carries a `_meta.ui.resourceUri` reference to the resource. The host fetches
+it with `resources/read`, renders it in a **sandboxed iframe**, and pushes the tool's
+result into that iframe via `postMessage`. Your server never sends or receives any
+`ui/*` messages: that traffic is between the host and the iframe. You serve a tool
+and an HTML document; the host does the theater.
+
+The SDK ships this as the built-in `Apps` extension (`io.modelcontextprotocol/ui`).
+If [Extensions](extensions.md) are new to you, skim that page first. One minute,
+then come back.
+
+## A clock with a face
+
+```python title="server.py" hl_lines="17 20 23-24 28 30"
+--8<-- "docs_src/apps/tutorial001.py"
+```
+
+Four moves:
+
+* `Apps()`: one instance holds your UI-bound tools and their resources.
+* `@apps.tool(resource_uri="ui://clock/app.html")`: a regular tool, plus the
+  `_meta.ui.resourceUri` stamp. Everything `@mcp.tool()` accepts (name, title,
+  description, ...) passes through.
+* `apps.add_html_resource("ui://clock/app.html", CLOCK_HTML)`: the matching
+  resource, served as `text/html;profile=mcp-app`. That exact MIME type is what
+  tells a host "this is an app, render it".
+* `MCPServer("clock", extensions=[apps])`: opt in. The server now advertises
+  `io.modelcontextprotocol/ui` under `capabilities.extensions`.
+
+The HTML itself listens for the host's `postMessage` and shows the result. For real
+apps, use the official [`@modelcontextprotocol/ext-apps`](https://github.com/modelcontextprotocol/ext-apps)
+browser SDK inside your HTML. It gives you `ontoolresult`, `callServerTool`,
+`getHostContext`, and `onhostcontextchanged` instead of raw message events.
+
+## Graceful degradation
+
+Not every client renders apps. The spec is blunt about what that means for you:
+
+> Tools **MUST** return a meaningful `content` array even when UI is available.
+
+The model reads `content`; the iframe is for humans. A UI-capable host still feeds
+the text result to the model, and a text-only client gets *only* that. So the
+canonical pattern is one tool, two answers:
+
+```python
+@apps.tool(resource_uri="ui://clock/app.html")
+def get_time(ctx: Context) -> str:
+    now = current_time()
+    if not client_supports_apps(ctx):
+        return f"The time is {now}."   # a sentence for humans without the UI
+    return now                          # raw data the app renders
+```
+
+`client_supports_apps(ctx)` is `True` only when the client declared the
+`io.modelcontextprotocol/ui` extension **and** listed `text/html;profile=mcp-app`
+in its `mimeTypes` settings. The field is required, so a client that omits it
+does not count.
+
+!!! warning
+    Never return a placeholder like `"[Rendered UI]"` as the only content. If the
+    fallback text is useless, the tool is useless to every text-only client and to
+    the model itself. Write the sentence.
+
+A client declares support like any extension capability:
+
+```python
+from mcp import Client
+from mcp.server.apps import APP_MIME_TYPE, EXTENSION_ID
+
+async with Client(target, extensions={EXTENSION_ID: {"mimeTypes": [APP_MIME_TYPE]}}) as client:
+    ...
+```
+
+## Locking the iframe down
+
+The resource side carries the security metadata: what the iframe may load, which
+browser permissions it wants, how it would like to be framed:
+
+```python title="server.py" hl_lines="9 19-22"
+--8<-- "docs_src/apps/tutorial002.py"
+```
+
+`csp` and `permissions` are **requests to the host**, not server behaviour. The host
+builds the iframe's Content-Security-Policy and Permissions-Policy from them, and it
+may refuse. Feature-detect in your JS rather than assuming a grant.
+
+`ResourceCsp`, field by field (Python name, wire key, what the host does with it):
+
+| Python | Wire (`_meta.ui.csp`) | Controls |
+|---|---|---|
+| `connect_domains` | `connectDomains` | `connect-src`: where `fetch`/XHR may go |
+| `resource_domains` | `resourceDomains` | `img-src`, `style-src`, ...: static assets |
+| `frame_domains` | `frameDomains` | `frame-src`: nested iframes |
+| `base_uri_domains` | `baseUriDomains` | `base-uri`: what `<base>` may point at |
+
+`ResourcePermissions`: each field requests a browser permission for the iframe.
+
+| Python | Wire (`_meta.ui.permissions`) |
+|---|---|
+| `camera` | `camera` |
+| `microphone` | `microphone` |
+| `geolocation` | `geolocation` |
+| `clipboard_write` | `clipboardWrite` |
+
+!!! note
+    CSP and permissions live on the **resource**, never on the tool. The spec's tool
+    metadata has no slot for them, and hosts ignore them there. The SDK makes the
+    mistake unrepresentable: `@apps.tool()` simply has no `csp` parameter.
+
+### Visibility
+
+`visibility=["app"]` on a tool says "this exists for the iframe, not the model":
+
+* `"model"`: the model may call it.
+* `"app"`: the iframe may call it (via `callServerTool`).
+* Omitted: both, which is the default.
+
+Filtering is the **host's** job. Your server lists app-only tools in `tools/list`
+like any other; the host hides them from the model. Don't filter server-side.
+
+## The rules the SDK enforces
+
+All of these fail at startup, not in production:
+
+* A `resource_uri` or resource URI that isn't `ui://...` is a `ValueError` at
+  decoration/registration time.
+* A tool bound to a URI with **no matching registered resource** is a `ValueError`
+  when `MCPServer(extensions=[apps])` consumes the extension. A tool advertising
+  HTML that 404s on `resources/read` is a misconfiguration, so it refuses to
+  construct.
+* `meta={"ui": ...}` on `@apps.tool()` is a `ValueError`. The decorator owns
+  `_meta["ui"]`; say it with `resource_uri=` and `visibility=`. Other `meta=` keys
+  merge fine alongside.
+
+Neither the TypeScript ext-apps SDK nor FastMCP catches any of these today; we'd
+rather you find out before a host does.
+
+## Beyond inline HTML
+
+`add_html_resource` covers the common case: a string of HTML. For anything else,
+HTML on disk or generated content, build the resource yourself and hand it over:
+
+```python
+from mcp.server.mcpserver.resources import FileResource
+
+apps.add_resource(FileResource(uri="ui://report/app.html", name="report", path=html_path))
+```
+
+`add_resource` fills in the `text/html;profile=mcp-app` MIME type when the resource
+doesn't set one explicitly, and rejects an explicit mismatch: a `ui://` resource
+under any other MIME type is one no host will render.
+
+!!! tip
+    Targeting a pre-GA host that still reads the deprecated flat
+    `_meta["ui/resourceUri"]` key? Merge it yourself:
+    `@apps.tool(resource_uri="ui://x", meta={"ui/resourceUri": "ui://x"})`.
+    The nested `ui` object is the spec shape; the flat key is on its way out.
+
+## See it run
+
+The `apps` story in `examples/stories/` is this page as a runnable pair: a server
+with a UI-bound clock tool and a client that negotiates Apps, reads the tool's
+`_meta.ui.resourceUri`, fetches the HTML, and calls the tool.
+
+```bash
+uv run python -m stories.apps.client
+```

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
+from mcp.server.apps import Apps
+from mcp.server.mcpserver import MCPServer
+
+mcp = MCPServer("demo", extensions=[Apps()])
+```
+
+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
+class Stamps(Extension):
+    identifier = "com.example/stamps"
+```
+
+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="16 18-19 21-22 25"
+--8<-- "docs_src/extensions/tutorial001.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.
+
+#### Try it
+
+```python
+from mcp import Client
+
+
+async def main() -> None:
+    async with Client(mcp) as client:
+        print(client.server_capabilities.extensions)
+        # {'com.example/stamps': {'sealed': True}}
+        result = await client.call_tool("stamp", {"text": "hello"})
+        # [stamped] hello
+```
+
+### 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="13-19 23 32-40"
+--8<-- "docs_src/extensions/tutorial002.py"
+```
+
+* `SearchParams` subclasses `RequestParams`, so the 2026 `_meta` envelope parses
+  uniformly and your handler gets validated params, never a raw dict.
+* `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.
+
+!!! tip
+    Calling a vendor method from the client goes through `client.session.send_request(...)`
+    today; `Client` only grows first-class methods for spec verbs. The
+    `custom_methods` story in `examples/stories/` shows the full round trip.
+
+### 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/tutorial003.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.
+
+## The client side
+
+A client declares the extensions it supports the same way the server does:
+
+```python
+from mcp import Client
+
+async with Client(target, extensions={"com.example/search": {}}) as client:
+    ...
+```
+
+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.
+
+## 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.

docs_src/apps/__init__.py

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+from mcp.server.apps import Apps, client_supports_apps
+from mcp.server.mcpserver import MCPServer
+from mcp.server.mcpserver.context import Context
+
+CLOCK_HTML = """\
+<!doctype html>
+<title>Clock</title>
+<h1 id="now">...</h1>
+<script>
+  window.addEventListener("message", (event) => {
+    const text = event.data?.result?.content?.[0]?.text;
+    if (text) document.getElementById("now").textContent = text;
+  });
+</script>
+"""
+
+apps = Apps()
+
+
+@apps.tool(resource_uri="ui://clock/app.html", description="The current time.")
+def get_time(ctx: Context) -> str:
+    now = "2026-06-26T12:00:00Z"
+    if not client_supports_apps(ctx):
+        return f"The time is {now}."
+    return now
+
+
+apps.add_html_resource("ui://clock/app.html", CLOCK_HTML, title="Clock")
+
+mcp = MCPServer("clock", extensions=[apps])