diff --git a/lib/future.ts b/lib/future.ts
index b61517651..6c922a899 100644
--- a/lib/future.ts
+++ b/lib/future.ts
@@ -1,9 +1,10 @@
 import { lazyPromiseWithResolvers } from "./lazy-promise.ts";
-import type { Future } from "./types.ts";
+import type { Future, Operation } from "./types.ts";
 import { withResolvers } from "./with-resolvers.ts";
 
 export interface FutureWithResolvers<T> {
   future: Future<T>;
+  operation: Operation<T>;
   resolve(value: T): void;
   reject(error: Error): void;
 }
@@ -24,6 +25,7 @@ export function createFuture<T>(): FutureWithResolvers<T> {
   let future = Object.defineProperties(promise.promise, {
     [Symbol.iterator]: {
       enumerable: false,
+      configurable: true,
       value: operation.operation[Symbol.iterator],
     },
     [Symbol.toStringTag]: {
@@ -33,5 +35,5 @@ export function createFuture<T>(): FutureWithResolvers<T> {
     },
   }) as Future<T>;
 
-  return { future, resolve, reject };
+  return { future, operation: operation.operation, resolve, reject };
 }
diff --git a/lib/task.ts b/lib/task.ts
index c25ce5b0e..ad323686e 100644
--- a/lib/task.ts
+++ b/lib/task.ts
@@ -33,7 +33,10 @@ export function createTask<T>(options: TaskOptions<T>): NewTask<T> {
         return Object.defineProperties(Object.create(Promise.prototype), {
           [Symbol.iterator]: {
             enumerable: false,
-            value: destroy,
+            *value() {
+              yield* assertNonCircular(scope);
+              yield* destroy();
+            },
           },
           then: {
             enumerable: false,
@@ -58,7 +61,10 @@ export function createTask<T>(options: TaskOptions<T>): NewTask<T> {
     },
     [Symbol.iterator]: {
       enumerable: false,
-      value: future.future[Symbol.iterator],
+      *value() {
+        yield* assertNonCircular(scope);
+        return yield* future.operation;
+      },
     },
     [Symbol.toStringTag]: {
       enumerable: false,
@@ -147,3 +153,31 @@ export function* trap<T>(operation: () => Operation<T>): Operation<T> {
     }) as T;
   }
 }
+
+function* assertNonCircular(scope: Scope) {
+  let caller = (yield* useScope()) as ScopeInternal;
+
+  if (isSelfOrAncestor(scope as ScopeInternal, caller)) {
+    throw new CircularTaskError();
+  }
+}
+
+function isSelfOrAncestor(scope: ScopeInternal, child: ScopeInternal) {
+  for (
+    let contexts = child.contexts;
+    contexts;
+    contexts = Object.getPrototypeOf(contexts)
+  ) {
+    if (scope.contexts === contexts) {
+      return true;
+    }
+  }
+  return false;
+}
+
+class CircularTaskError extends Error {
+  constructor() {
+    super("a task may not depend on itself");
+    this.name = "CircularTaskError";
+  }
+}
diff --git a/test/run.test.ts b/test/run.test.ts
index 376bc6fc8..468700241 100644
--- a/test/run.test.ts
+++ b/test/run.test.ts
@@ -202,29 +202,6 @@ describe("run()", () => {
     await expect(task).rejects.toMatchObject({ message: "boom" });
   });
 
-  it("can halt itself", async () => {
-    let task: Task<void> = run(function* () {
-      yield* sleep(0);
-      yield* task.halt();
-    });
-
-    await expect(task).rejects.toMatchObject({ message: "halted" });
-  });
-
-  it("can halt itself between yield points", async () => {
-    let task: Task<void> = run(function* root() {
-      yield* sleep(0);
-
-      yield* spawn(function* child() {
-        yield* task.halt();
-      });
-
-      yield* suspend();
-    });
-
-    await expect(task).rejects.toMatchObject({ message: "halted" });
-  });
-
   it("can delay halt if child fails", async () => {
     let didRun = false;
     let task = run(function* Main() {
@@ -386,4 +363,144 @@ describe("run()", () => {
     expect(error?.message).toEqual("halted");
     expect(halted).toEqual(true);
   });
+
+  describe("task dependencies", () => {
+    it("throws an error if a task depends on itself", async () => {
+      await expect(run(function* () {
+        let task: Task<string> = yield* spawn(function* () {
+          yield* sleep(1);
+          yield* task;
+          return "hello world";
+        });
+        return yield* task;
+      })).rejects.toMatchObject({ name: "CircularTaskError" });
+    });
+
+    it("throws an error if a task depends on its own halt()", async () => {
+      await expect(run(function* () {
+        let task: Task<string> = yield* spawn(function* () {
+          yield* sleep(1);
+          yield* task.halt();
+          return "hello world";
+        });
+        return yield* task;
+      })).rejects.toMatchObject({ name: "CircularTaskError" });
+    });
+
+    it("throws an error if a task depends on its parent's halt()", async () => {
+      let parent: Task<void>;
+      parent = run(function* () {
+        yield* spawn(function* () {
+          yield* sleep(1);
+          yield* parent.halt();
+        });
+        yield* suspend();
+      });
+      await expect(parent).rejects.toMatchObject({ name: "CircularTaskError" });
+    });
+
+    it("throws an error if a task depends on its parent", async () => {
+      let parent: Task<void>;
+      parent = run(function* () {
+        yield* spawn(function* () {
+          yield* sleep(1);
+          yield* parent;
+        });
+        yield* suspend();
+      });
+      await expect(parent).rejects.toMatchObject({ name: "CircularTaskError" });
+    });
+
+    it("throws an error if a task depends on an ancestor's halt()", async () => {
+      let root: Task<void>;
+      root = run(function* () {
+        yield* spawn(function* () {
+          yield* spawn(function* () {
+            yield* sleep(1);
+            yield* root.halt();
+          });
+          yield* suspend();
+        });
+        yield* suspend();
+      });
+      await expect(root).rejects.toMatchObject({ name: "CircularTaskError" });
+    });
+
+    it("throws an error if a task depends on an ancestor", async () => {
+      let root: Task<void>;
+      root = run(function* () {
+        yield* spawn(function* () {
+          yield* spawn(function* () {
+            yield* sleep(1);
+            yield* root;
+          });
+          yield* suspend();
+        });
+        yield* suspend();
+      });
+      await expect(root).rejects.toMatchObject({ name: "CircularTaskError" });
+    });
+
+    it("allows a task to depend on a sibling", async () => {
+      let observed = await run(function* () {
+        let scope = yield* useScope();
+        let producer = scope.run(function* () {
+          yield* sleep(1);
+          return "produced";
+        });
+        let consumer = scope.run(function* () {
+          return yield* producer;
+        });
+        return yield* consumer;
+      });
+      expect(observed).toEqual("produced");
+    });
+
+    it("allows a task to depend on a sibling's halt()", async () => {
+      let halted = false;
+      await run(function* () {
+        let scope = yield* useScope();
+        let target = scope.run(function* () {
+          try {
+            yield* suspend();
+          } finally {
+            halted = true;
+          }
+        });
+        yield* sleep(1);
+        let halter = scope.run(function* () {
+          yield* target.halt();
+        });
+        yield* halter;
+      });
+      expect(halted).toEqual(true);
+    });
+
+    it("allows a task to depend on its child", async () => {
+      let observed = await run(function* () {
+        let child = yield* spawn(function* () {
+          yield* sleep(1);
+          return "child-result";
+        });
+        return yield* child;
+      });
+      expect(observed).toEqual("child-result");
+    });
+
+    it("allows a task to depend on its child's halt()", async () => {
+      let halted = false;
+      await run(function* () {
+        let child = yield* spawn(function* () {
+          try {
+            yield* suspend();
+          } finally {
+            halted = true;
+          }
+        });
+        yield* sleep(1);
+        yield* child.halt();
+      });
+      expect(halted).toEqual(true);
+    });
+  });
 });