diff --git a/README.md b/README.md
index 4524e3e..8e4fd49 100644
--- a/README.md
+++ b/README.md
@@ -123,6 +123,29 @@ const random: Balancer = {
 
 Each `WorkerHandle` exposes `activeCount` (in-flight tasks) and `thread` (the underlying `worker_threads.Worker`) for building custom strategies.
 
+`isTask(moroutine, task)` narrows a task to the descriptor type produced by a specific moroutine — useful inside a balancer to route by task kind or by a key in the args. For example, a worker-affinity balancer can hash a shard key out of the args so that every call for the same key hits the worker that already has its state loaded:
+
+```ts
+import { isTask, roundRobin } from 'moroutine';
+import type { Balancer } from 'moroutine';
+import { increment, read } from './counter.ts';
+
+export function keyAffinity(): Balancer {
+  const fallback = roundRobin();
+  return {
+    select(workers, task) {
+      let key: string | undefined;
+      if (isTask(increment, task)) key = task.args[0];
+      else if (isTask(read, task)) key = task.args[0];
+      if (key === undefined) return fallback.select(workers, task);
+      return workers[hash(key) % workers.length];
+    },
+  };
+}
+```
+
+Inside the `isTask` branch, `task.args` is typed as the moroutine's argument tuple (e.g. `[key: string, n: number]` for `increment`). See [`examples/worker-affinity`](examples/worker-affinity) for the full demo, including per-worker state that only stays consistent under affinity routing.
+
 ### Dedicated Workers
 
 Awaiting a task directly (without a pool) runs it on a dedicated worker thread, one per moroutine function.