diff --git a/core/src/activity-utils/makeActivityReducer.spec.ts b/core/src/activity-utils/makeActivityReducer.spec.ts
new file mode 100644
index 000000000..8b4d8f1b0
--- /dev/null
+++ b/core/src/activity-utils/makeActivityReducer.spec.ts
@@ -0,0 +1,417 @@
+import { makeEvent } from "../event-utils";
+import type { Activity } from "../Stack";
+import { makeActivityFromEvent } from "./makeActivityFromEvent";
+import { makeActivityReducer } from "./makeActivityReducer";
+
+const SECOND = 1000;
+const TRANSITION_DURATION = 350;
+
+let dt = 0;
+const enoughPastTime = () => {
+  dt += 1;
+  return new Date(Date.now() - 60 * SECOND).getTime() + dt;
+};
+
+function createActivity(
+  overrides: Partial<Activity> & { id?: string } = {},
+): Activity {
+  const pushedEvent = makeEvent("Pushed", {
+    activityId: overrides.id ?? "a1",
+    activityName: "TestActivity",
+    activityParams: {},
+    eventDate: enoughPastTime(),
+  });
+
+  return {
+    ...makeActivityFromEvent(pushedEvent, "enter-done"),
+    ...overrides,
+  };
+}
+
+describe("makeActivityReducer", () => {
+  describe("Replaced", () => {
+    test("transition state가 exit-done으로 변경됩니다", () => {
+      const activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      const replacedEvent = makeEvent("Replaced", {
+        activityId: "a2",
+        activityName: "NewActivity",
+        activityParams: { page: "1" },
+        eventDate: enoughPastTime(),
+      });
+      const result = reducer(activity, replacedEvent);
+
+      expect(result.transitionState).toBe("exit-done");
+      expect(result.exitedBy).toBe(replacedEvent);
+    });
+  });
+
+  describe("Popped", () => {
+    test("충분한 시간이 지났으면 exit-done으로 전이됩니다", () => {
+      const activity = createActivity();
+      const eventDate = enoughPastTime();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: eventDate + TRANSITION_DURATION + 1,
+      });
+
+      const poppedEvent = makeEvent("Popped", { eventDate });
+      const result = reducer(activity, poppedEvent);
+
+      expect(result.transitionState).toBe("exit-done");
+      expect(result.exitedBy).toBe(poppedEvent);
+    });
+
+    test("전이 시간 내이면 exit-active 상태입니다", () => {
+      const activity = createActivity();
+      const eventDate = Date.now();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: eventDate + 100,
+      });
+
+      const poppedEvent = makeEvent("Popped", { eventDate });
+      const result = reducer(activity, poppedEvent);
+
+      expect(result.transitionState).toBe("exit-active");
+    });
+
+    test("skipExitActiveState가 true이면 즉시 exit-done으로 전이됩니다", () => {
+      const activity = createActivity();
+      const eventDate = Date.now();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: eventDate + 1,
+      });
+
+      const poppedEvent = makeEvent("Popped", {
+        skipExitActiveState: true,
+        eventDate,
+      });
+      const result = reducer(activity, poppedEvent);
+
+      expect(result.transitionState).toBe("exit-done");
+    });
+
+    test("exit-done일 때 params와 steps가 첫 번째 step으로 리셋됩니다", () => {
+      const activity = createActivity();
+      const stepReducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      const stepEvent = makeEvent("StepPushed", {
+        stepId: "s1",
+        stepParams: { tab: "detail" },
+        eventDate: enoughPastTime(),
+      });
+      const activityWithStep = stepReducer(activity, stepEvent);
+
+      expect(activityWithStep.steps).toHaveLength(2);
+      expect(activityWithStep.params).toEqual({ tab: "detail" });
+
+      const eventDate = enoughPastTime();
+      const popReducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: eventDate + TRANSITION_DURATION + 1,
+      });
+
+      const poppedEvent = makeEvent("Popped", { eventDate });
+      const result = popReducer(activityWithStep, poppedEvent);
+
+      expect(result.transitionState).toBe("exit-done");
+      expect(result.steps).toHaveLength(1);
+      expect(result.params).toEqual({});
+    });
+
+    test("exit-active일 때 params와 steps가 그대로 유지됩니다", () => {
+      const activity = createActivity();
+      const stepReducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      const stepEvent = makeEvent("StepPushed", {
+        stepId: "s1",
+        stepParams: { tab: "detail" },
+        eventDate: enoughPastTime(),
+      });
+      const activityWithStep = stepReducer(activity, stepEvent);
+
+      const eventDate = Date.now();
+      const popReducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: eventDate + 1,
+      });
+
+      const poppedEvent = makeEvent("Popped", { eventDate });
+      const result = popReducer(activityWithStep, poppedEvent);
+
+      expect(result.transitionState).toBe("exit-active");
+      expect(result.steps).toHaveLength(2);
+      expect(result.params).toEqual({ tab: "detail" });
+    });
+
+    test("resumedAt이 있으면 eventDate 대신 resumedAt 기준으로 전이를 판단합니다", () => {
+      const eventDate = Date.now();
+      const resumedAt = eventDate - TRANSITION_DURATION - 1;
+      const activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: eventDate,
+        resumedAt,
+      });
+
+      const poppedEvent = makeEvent("Popped", { eventDate });
+      const result = reducer(activity, poppedEvent);
+
+      expect(result.transitionState).toBe("exit-done");
+    });
+  });
+
+  describe("StepPushed", () => {
+    test("새 step이 추가됩니다", () => {
+      const activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      const stepEvent = makeEvent("StepPushed", {
+        stepId: "s1",
+        stepParams: { tab: "detail" },
+        eventDate: enoughPastTime(),
+      });
+      const result = reducer(activity, stepEvent);
+
+      expect(result.steps).toHaveLength(2);
+      expect(result.steps[1].id).toBe("s1");
+      expect(result.steps[1].params).toEqual({ tab: "detail" });
+      expect(result.steps[1].enteredBy).toBe(stepEvent);
+    });
+
+    test("activity params가 stepParams로 업데이트됩니다", () => {
+      const activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      const stepEvent = makeEvent("StepPushed", {
+        stepId: "s1",
+        stepParams: { tab: "detail", sort: "latest" },
+        eventDate: enoughPastTime(),
+      });
+      const result = reducer(activity, stepEvent);
+
+      expect(result.params).toEqual({ tab: "detail", sort: "latest" });
+    });
+
+    test("hasZIndex가 step에 반영됩니다", () => {
+      const activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      const withZIndex = makeEvent("StepPushed", {
+        stepId: "s1",
+        stepParams: {},
+        hasZIndex: true,
+        eventDate: enoughPastTime(),
+      });
+
+      const r1 = reducer(activity, withZIndex);
+      expect(r1.steps[1].hasZIndex).toBe(true);
+
+      const withoutZIndex = makeEvent("StepPushed", {
+        stepId: "s2",
+        stepParams: {},
+        eventDate: enoughPastTime(),
+      });
+
+      const r2 = reducer(activity, withoutZIndex);
+      expect(r2.steps[1].hasZIndex).toBe(false);
+    });
+
+    test("여러 step을 순서대로 추가할 수 있습니다", () => {
+      let activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      activity = reducer(
+        activity,
+        makeEvent("StepPushed", {
+          stepId: "s1",
+          stepParams: { page: "1" },
+          eventDate: enoughPastTime(),
+        }),
+      );
+
+      activity = reducer(
+        activity,
+        makeEvent("StepPushed", {
+          stepId: "s2",
+          stepParams: { page: "2" },
+          eventDate: enoughPastTime(),
+        }),
+      );
+
+      expect(activity.steps).toHaveLength(3);
+      expect(activity.steps[0].id).toBe(activity.id);
+      expect(activity.steps[1].id).toBe("s1");
+      expect(activity.steps[2].id).toBe("s2");
+      expect(activity.params).toEqual({ page: "2" });
+    });
+  });
+
+  describe("StepReplaced", () => {
+    test("마지막 step을 교체합니다", () => {
+      let activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      activity = reducer(
+        activity,
+        makeEvent("StepPushed", {
+          stepId: "s1",
+          stepParams: { tab: "old" },
+          eventDate: enoughPastTime(),
+        }),
+      );
+      const replaceEvent = makeEvent("StepReplaced", {
+        stepId: "s1-replaced",
+        stepParams: { tab: "new" },
+        eventDate: enoughPastTime(),
+      });
+      const result = reducer(activity, replaceEvent);
+
+      expect(result.steps).toHaveLength(2);
+      expect(result.steps[1].id).toBe("s1-replaced");
+      expect(result.steps[1].params).toEqual({ tab: "new" });
+      expect(result.params).toEqual({ tab: "new" });
+    });
+
+    test("step이 하나뿐일 때도 교체됩니다", () => {
+      const activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      const replaceEvent = makeEvent("StepReplaced", {
+        stepId: "s-new",
+        stepParams: { view: "grid" },
+        eventDate: enoughPastTime(),
+      });
+      const result = reducer(activity, replaceEvent);
+
+      expect(result.steps).toHaveLength(1);
+      expect(result.steps[0].id).toBe("s-new");
+      expect(result.params).toEqual({ view: "grid" });
+    });
+  });
+
+  describe("StepPopped", () => {
+    test("마지막 step이 제거됩니다", () => {
+      let activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      activity = reducer(
+        activity,
+        makeEvent("StepPushed", {
+          stepId: "s1",
+          stepParams: { tab: "detail" },
+          eventDate: enoughPastTime(),
+        }),
+      );
+
+      expect(activity.steps).toHaveLength(2);
+
+      const popEvent = makeEvent("StepPopped", {
+        eventDate: enoughPastTime(),
+      });
+      const result = reducer(activity, popEvent);
+
+      expect(result.steps).toHaveLength(1);
+    });
+
+    test("이전 step의 params로 복원됩니다", () => {
+      let activity = createActivity();
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      activity = reducer(
+        activity,
+        makeEvent("StepPushed", {
+          stepId: "s1",
+          stepParams: { page: "1" },
+          eventDate: enoughPastTime(),
+        }),
+      );
+
+      activity = reducer(
+        activity,
+        makeEvent("StepPushed", {
+          stepId: "s2",
+          stepParams: { page: "2" },
+          eventDate: enoughPastTime(),
+        }),
+      );
+
+      expect(activity.params).toEqual({ page: "2" });
+
+      const result = reducer(
+        activity,
+        makeEvent("StepPopped", { eventDate: enoughPastTime() }),
+      );
+
+      expect(result.params).toEqual({ page: "1" });
+    });
+
+    test("step을 모두 pop하면 원래 activity params로 돌아옵니다", () => {
+      const pushedEvent = makeEvent("Pushed", {
+        activityId: "a1",
+        activityName: "TestActivity",
+        activityParams: { original: "yes" },
+        eventDate: enoughPastTime(),
+      });
+      let activity = makeActivityFromEvent(pushedEvent, "enter-done");
+      const reducer = makeActivityReducer({
+        transitionDuration: TRANSITION_DURATION,
+        now: Date.now(),
+      });
+
+      activity = reducer(
+        activity,
+        makeEvent("StepPushed", {
+          stepId: "s1",
+          stepParams: { page: "1" },
+          eventDate: enoughPastTime(),
+        }),
+      );
+
+      expect(activity.params).toEqual({ page: "1" });
+
+      const result = reducer(
+        activity,
+        makeEvent("StepPopped", { eventDate: enoughPastTime() }),
+      );
+
+      expect(result.params).toEqual({ original: "yes" });
+    });
+  });
+});
diff --git a/core/src/activity-utils/makeActivityReducer.ts b/core/src/activity-utils/makeActivityReducer.ts
index fac85e2d7..6e2ff78ec 100644
--- a/core/src/activity-utils/makeActivityReducer.ts
+++ b/core/src/activity-utils/makeActivityReducer.ts
@@ -1,5 +1,4 @@
 import type {
-  DomainEvent,
   PoppedEvent,
   ReplacedEvent,
   StepPoppedEvent,
@@ -107,7 +106,7 @@ export function makeActivityReducer(context: {
      * Pop the last step
      * If there are params in the previous step, set them as the new params
      */
-    StepPopped: (activity: Activity, event: StepPoppedEvent): Activity => {
+    StepPopped: (activity: Activity, _: StepPoppedEvent): Activity => {
       activity.steps.pop();
 
       const beforeActivityParams = last(activity.steps)?.params;