docs: re-anchor tasks migration notes after the migration guide restructure

The restructured migration guide dropped the server extensions section this
branch used to edit, so the feature prose now lives in the extensions and
tasks pages. The migration guide's experimental Tasks section and the
whats-new removal list now say the extension ships as the built-in Tasks
extension instead of claiming it is unimplemented. The extensions page also
points at TasksExtension as the shipped behavioural example next to
advertise().

Author: maxisbey

docs/advanced/extensions.md

@@ -177,6 +177,9 @@ from mcp.client import advertise
 client = Client(mcp, extensions=[advertise("com.example/search")])
 ```
 
+For an extension with client-side behaviour, construct its object instead; the
+built-in example is `mcp.client.TasksExtension` ([Tasks](tasks.md)).
+
 ## Writing a client extension
 
 Subclass `ClientExtension` and override only what you need. Three contribution

docs/migration.md

@@ -1568,7 +1568,7 @@ Behavior changes:
 
 Tasks ([SEP-1686](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1686)) have been removed from the MCP specification and are no longer part of this SDK. The `mcp.client.experimental`, `mcp.server.experimental`, `mcp.shared.experimental`, and `mcp.server.lowlevel.experimental` modules have been removed, along with the `experimental` properties on `ClientSession`, `ServerSession`, `Server`, and `ServerRequestContext`. The corresponding `Task*` types remain in `mcp_types` as types-only definitions, except the `TaskExecutionMode` alias, whose literal is now inlined on `ToolExecution.task_support`.
 
-Tasks have since returned as the built-in `Tasks` extension ([SEP-2663](https://modelcontextprotocol.io/seps/2663-tasks-extension.md)), with a different wire shape than the experimental SEP-1686 surface — see [Server extensions API](#server-extensions-api-sep-2133) above and [Tasks](advanced/tasks.md).
+Tasks have since returned as the built-in `Tasks` extension ([SEP-2663](https://modelcontextprotocol.io/seps/2663-tasks-extension.md)), with a different wire shape than the experimental SEP-1686 surface. See [Extensions](advanced/extensions.md) and [Tasks](advanced/tasks.md) for the new design.
 
 ## Transports
 

docs/whats-new.md

@@ -144,7 +144,7 @@ The renames announce themselves. These do not:
 Each of these is a section in the **[Migration Guide](migration.md)**:
 
 * The **WebSocket transport**, both sides, and the `mcp[ws]` extra. It was never part of the MCP specification.
-* The **experimental Tasks** API (`mcp.*.experimental`). 2026-07-28 moves tasks out of the core protocol and into an official extension ([SEP-2663](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2663)), which this SDK does not implement yet.
+* The **experimental Tasks** API (`mcp.*.experimental`). 2026-07-28 moves tasks out of the core protocol and into an official extension ([SEP-2663](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2663)), which this SDK ships as the built-in [Tasks](advanced/tasks.md) extension.
 * `mcp.types`, `mcp.shared.version`, and `mcp.shared.progress` as import paths.
 * The deprecated `streamablehttp_client` spelling, and the `get_session_id` callback from `streamable_http_client` (which now yields exactly two streams).
 * `McpError`, renamed **`MCPError`** with a direct `(code, message, data)` constructor.