docs: explain request IDs for FastMCP cancellation

Author: lavish0000

README.md

@@ -1138,6 +1138,27 @@ The request context accessible via `ctx.request_context` contains request-specif
 - `ctx.request_context.request` - The original MCP request object for advanced processing
 - `ctx.request_context.request_id` - Unique identifier for this request
 
+Inside a FastMCP tool, the current request ID is available as `ctx.request_id`
+or `ctx.request_context.request_id`. This is the same JSON-RPC request ID that
+would be used in a `notifications/cancelled` message.
+
+Use `notifications/cancelled` for cancelling normal in-flight requests such as
+`tools/call`. For task-based work, use `tasks/cancel` instead.
+
+```python
+@mcp.tool()
+async def slow_tool(ctx: Context) -> str:
+    current_request_id = ctx.request_id
+    return f"handling request {current_request_id}"
+```
+
+If you are using the SDK's high-level client helpers such as
+`client.call_tool()` or `client.session.call_tool()`, the SDK manages request
+IDs for you and does not currently expose them directly. If you need explicit
+client-driven cancellation, prefer a task-based workflow (`tasks/cancel`) or a
+lower-level integration where you control request/notification correlation
+yourself.
+
 ```python
 # Example with typed lifespan context
 @dataclass