diff --git a/server/src/internal/billing/v2/utils/billingChangeResponse/buildBillingChangeResponse.ts b/server/src/internal/billing/v2/utils/billingChangeResponse/buildBillingChangeResponse.ts
new file mode 100644
index 000000000..6d558d29c
--- /dev/null
+++ b/server/src/internal/billing/v2/utils/billingChangeResponse/buildBillingChangeResponse.ts
@@ -0,0 +1,25 @@
+import {
+	type AutumnBillingPlan,
+	type BillingChangeResponse,
+	BillingChangeResponseSchema,
+	type FullCustomer,
+} from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildPlanChanges } from "./buildPlanChanges";
+
+export const buildBillingChangeResponse = ({
+	ctx: _ctx,
+	originalFullCustomer,
+	autumnBillingPlan,
+}: {
+	ctx: AutumnContext;
+	originalFullCustomer: FullCustomer;
+	autumnBillingPlan: AutumnBillingPlan;
+}): BillingChangeResponse => {
+	return BillingChangeResponseSchema.parse({
+		object: "billing.plans_changed",
+		customer_id:
+			originalFullCustomer.id ?? originalFullCustomer.internal_id,
+		plan_changes: buildPlanChanges({ autumnBillingPlan }),
+	});
+};
diff --git a/server/src/internal/billing/v2/utils/billingChangeResponse/buildPlanChanges.ts b/server/src/internal/billing/v2/utils/billingChangeResponse/buildPlanChanges.ts
new file mode 100644
index 000000000..3059f52bf
--- /dev/null
+++ b/server/src/internal/billing/v2/utils/billingChangeResponse/buildPlanChanges.ts
@@ -0,0 +1,76 @@
+import {
+	type AutumnBillingPlan,
+	CusProductStatus,
+	type CustomerPlanChange,
+} from "@autumn/shared";
+import { buildPlanItemChanges } from "./buildPlanItemChanges";
+import { buildPreviousAttributes } from "./buildPreviousAttributes";
+import { toCustomerPlanSnapshot } from "./toCustomerPlanSnapshot";
+
+export const buildPlanChanges = ({
+	autumnBillingPlan,
+}: {
+	autumnBillingPlan: AutumnBillingPlan;
+}): CustomerPlanChange[] => {
+	const changes: CustomerPlanChange[] = [];
+
+	for (const cusProduct of autumnBillingPlan.insertCustomerProducts ?? []) {
+		const action =
+			cusProduct.status === CusProductStatus.Scheduled
+				? "scheduled"
+				: "activated";
+		changes.push({
+			action,
+			plan: toCustomerPlanSnapshot({ cusProduct }),
+			previous_attributes: null,
+			item_changes: [],
+		});
+	}
+
+	const updates = [
+		...(autumnBillingPlan.updateCustomerProduct
+			? [autumnBillingPlan.updateCustomerProduct]
+			: []),
+		...(autumnBillingPlan.updateCustomerProducts ?? []),
+	];
+
+	for (const update of updates) {
+		const originalCusProduct = update.customerProduct;
+		const previousAttributes = buildPreviousAttributes({
+			originalCusProduct,
+			updates: update.updates,
+		});
+		const action =
+			update.updates.status === CusProductStatus.Expired
+				? "expired"
+				: "updated";
+
+		changes.push({
+			action,
+			plan: toCustomerPlanSnapshot({
+				cusProduct: originalCusProduct,
+				overrides: {
+					status: update.updates.status,
+					canceled_at: update.updates.canceled_at,
+					ended_at: update.updates.ended_at,
+				},
+			}),
+			previous_attributes: previousAttributes,
+			item_changes: [],
+		});
+	}
+
+	for (const patch of autumnBillingPlan.patchCustomerProducts ?? []) {
+		changes.push({
+			action: "updated",
+			plan: toCustomerPlanSnapshot({ cusProduct: patch.customerProduct }),
+			previous_attributes: {},
+			item_changes: buildPlanItemChanges({
+				insertCustomerEntitlements: patch.insertCustomerEntitlements,
+				deleteCustomerEntitlements: patch.deleteCustomerEntitlements,
+			}),
+		});
+	}
+
+	return changes;
+};
diff --git a/server/src/internal/billing/v2/utils/billingChangeResponse/buildPlanItemChanges.ts b/server/src/internal/billing/v2/utils/billingChangeResponse/buildPlanItemChanges.ts
new file mode 100644
index 000000000..38776119a
--- /dev/null
+++ b/server/src/internal/billing/v2/utils/billingChangeResponse/buildPlanItemChanges.ts
@@ -0,0 +1,23 @@
+import type {
+	CustomerPlanItemChange,
+	FullCustomerEntitlement,
+} from "@autumn/shared";
+
+export const buildPlanItemChanges = ({
+	insertCustomerEntitlements,
+	deleteCustomerEntitlements,
+}: {
+	insertCustomerEntitlements?: FullCustomerEntitlement[];
+	deleteCustomerEntitlements?: FullCustomerEntitlement[];
+}): CustomerPlanItemChange[] => {
+	const changes: CustomerPlanItemChange[] = [];
+
+	for (const ent of insertCustomerEntitlements ?? []) {
+		changes.push({ action: "created", feature_id: ent.feature_id });
+	}
+	for (const ent of deleteCustomerEntitlements ?? []) {
+		changes.push({ action: "deleted", feature_id: ent.feature_id });
+	}
+
+	return changes;
+};
diff --git a/server/src/internal/billing/v2/utils/billingChangeResponse/buildPreviousAttributes.ts b/server/src/internal/billing/v2/utils/billingChangeResponse/buildPreviousAttributes.ts
new file mode 100644
index 000000000..7c3b589c1
--- /dev/null
+++ b/server/src/internal/billing/v2/utils/billingChangeResponse/buildPreviousAttributes.ts
@@ -0,0 +1,39 @@
+import type {
+	CustomerProductUpdateSchema,
+	FullCusProduct,
+} from "@autumn/shared";
+import type { z } from "zod/v4";
+
+type CustomerProductUpdate = z.infer<typeof CustomerProductUpdateSchema>;
+
+export const buildPreviousAttributes = ({
+	originalCusProduct,
+	updates,
+}: {
+	originalCusProduct: FullCusProduct;
+	updates: CustomerProductUpdate["updates"];
+}): Record<string, unknown> => {
+	const previous: Record<string, unknown> = {};
+
+	if (
+		updates.status !== undefined &&
+		updates.status !== originalCusProduct.status
+	) {
+		previous.status = originalCusProduct.status;
+	}
+
+	const originalCanceledAt = originalCusProduct.canceled_at ?? null;
+	if (
+		updates.canceled_at !== undefined &&
+		updates.canceled_at !== originalCanceledAt
+	) {
+		previous.canceled_at = originalCanceledAt;
+	}
+
+	const originalEndedAt = originalCusProduct.ended_at ?? null;
+	if (updates.ended_at !== undefined && updates.ended_at !== originalEndedAt) {
+		previous.expires_at = originalEndedAt;
+	}
+
+	return previous;
+};
diff --git a/server/src/internal/billing/v2/utils/billingChangeResponse/index.ts b/server/src/internal/billing/v2/utils/billingChangeResponse/index.ts
new file mode 100644
index 000000000..5f4e6331c
--- /dev/null
+++ b/server/src/internal/billing/v2/utils/billingChangeResponse/index.ts
@@ -0,0 +1,5 @@
+export * from "./buildBillingChangeResponse";
+export * from "./buildPlanChanges";
+export * from "./buildPlanItemChanges";
+export * from "./buildPreviousAttributes";
+export * from "./toCustomerPlanSnapshot";
diff --git a/server/src/internal/billing/v2/utils/billingChangeResponse/toCustomerPlanSnapshot.ts b/server/src/internal/billing/v2/utils/billingChangeResponse/toCustomerPlanSnapshot.ts
new file mode 100644
index 000000000..c24012852
--- /dev/null
+++ b/server/src/internal/billing/v2/utils/billingChangeResponse/toCustomerPlanSnapshot.ts
@@ -0,0 +1,61 @@
+import {
+	type CustomerPlanSnapshot,
+	type CustomerPlanStatus,
+	CusProductStatus,
+	type FullCusProduct,
+} from "@autumn/shared";
+
+const cusProductStatusToPlanStatus = (
+	status: CusProductStatus,
+): CustomerPlanStatus => {
+	switch (status) {
+		case CusProductStatus.Active:
+			return "active";
+		case CusProductStatus.Trialing:
+			return "trialing";
+		case CusProductStatus.PastDue:
+			return "past_due";
+		case CusProductStatus.Scheduled:
+			return "scheduled";
+		case CusProductStatus.Expired:
+			return "expired";
+		case CusProductStatus.Paused:
+			return "paused";
+		default:
+			return "active";
+	}
+};
+
+export type CustomerPlanSnapshotOverrides = Partial<{
+	status: CusProductStatus;
+	canceled_at: number | null;
+	ended_at: number | null;
+}>;
+
+export const toCustomerPlanSnapshot = ({
+	cusProduct,
+	overrides,
+}: {
+	cusProduct: FullCusProduct;
+	overrides?: CustomerPlanSnapshotOverrides;
+}): CustomerPlanSnapshot => {
+	const status = overrides?.status ?? cusProduct.status;
+	const canceledAt =
+		overrides?.canceled_at !== undefined
+			? overrides.canceled_at
+			: (cusProduct.canceled_at ?? null);
+	const endedAt =
+		overrides?.ended_at !== undefined
+			? overrides.ended_at
+			: (cusProduct.ended_at ?? null);
+
+	return {
+		plan_id: cusProduct.product_id,
+		status: cusProductStatusToPlanStatus(status),
+		started_at: cusProduct.starts_at ?? null,
+		canceled_at: canceledAt,
+		expires_at: endedAt,
+		current_period_start: null,
+		current_period_end: null,
+	};
+};
diff --git a/server/tests/unit/billing/billing-change-response/attach.test.ts b/server/tests/unit/billing/billing-change-response/attach.test.ts
new file mode 100644
index 000000000..b4bfdb008
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/attach.test.ts
@@ -0,0 +1,248 @@
+import { describe, expect, test } from "bun:test";
+import { CusProductStatus } from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildBillingChangeResponse } from "@/internal/billing/v2/utils/billingChangeResponse";
+import {
+	expectBillingChangeResponse,
+	expectPlanChange,
+	findPlanChange,
+} from "./helpers/expectBillingChange";
+import { logChangeResponse } from "./helpers/logChangeResponse";
+import {
+	makeAutumnBillingPlan,
+	makeUpdate,
+} from "./helpers/makeAutumnBillingPlan";
+import { makeFullCusProduct } from "./helpers/makeFullCusProduct";
+import { makeFullCustomer } from "./helpers/makeFullCustomer";
+
+const NOW = 1_710_000_000_000;
+const PERIOD_END = NOW + 30 * 24 * 60 * 60 * 1000;
+const ctx = {} as AutumnContext;
+
+describe("buildBillingChangeResponse — attach", () => {
+	test("new customer, free plan attach", () => {
+		const free = makeFullCusProduct({ planId: "free", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer(),
+			autumnBillingPlan: makeAutumnBillingPlan({ inserts: [free] }),
+		});
+		logChangeResponse("attach / new customer, free plan", response);
+
+		expectBillingChangeResponse(response, {
+			customerId: "cus_test",
+			activated: ["free"],
+		});
+		expectPlanChange(findPlanChange(response, { action: "activated", planId: "free" }), {
+			action: "activated",
+			planId: "free",
+			previousAttributes: null,
+			itemChanges: [],
+		});
+	});
+
+	test("new customer, paid plan attach", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer(),
+			autumnBillingPlan: makeAutumnBillingPlan({ inserts: [pro] }),
+		});
+		logChangeResponse("attach / new customer, paid plan", response);
+
+		expectBillingChangeResponse(response, { activated: ["pro"] });
+		expectPlanChange(findPlanChange(response, { action: "activated", planId: "pro" }), {
+			action: "activated",
+			planId: "pro",
+			previousAttributes: null,
+		});
+	});
+
+	test("immediate upgrade (free → pro)", () => {
+		const free = makeFullCusProduct({ planId: "free", startedAt: NOW - 1000 });
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [free] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [pro],
+				update: makeUpdate({
+					customerProduct: free,
+					updates: {
+						status: CusProductStatus.Expired,
+						ended_at: NOW,
+						canceled: true,
+						canceled_at: NOW,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("attach / immediate upgrade (free → pro)", response);
+
+		expectBillingChangeResponse(response, {
+			expired: ["free"],
+			activated: ["pro"],
+		});
+		expectPlanChange(findPlanChange(response, { action: "expired", planId: "free" }), {
+			action: "expired",
+			planId: "free",
+			previousAttributes: {
+				status: CusProductStatus.Active,
+				canceled_at: null,
+				expires_at: null,
+			},
+		});
+		expectPlanChange(findPlanChange(response, { action: "activated", planId: "pro" }), {
+			action: "activated",
+			planId: "pro",
+			previousAttributes: null,
+		});
+	});
+
+	test("upgrade that also clears an existing scheduled downgrade", () => {
+		const business = makeFullCusProduct({
+			planId: "business",
+			startedAt: NOW - 1000,
+		});
+		const scheduledPro = makeFullCusProduct({
+			planId: "pro",
+			status: CusProductStatus.Scheduled,
+			startedAt: PERIOD_END,
+			id: "cp_pro_scheduled",
+		});
+		const premium = makeFullCusProduct({ planId: "premium", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({
+				customerProducts: [business, scheduledPro],
+			}),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [premium],
+				update: makeUpdate({
+					customerProduct: business,
+					updates: {
+						status: CusProductStatus.Expired,
+						ended_at: NOW,
+						canceled: true,
+						canceled_at: NOW,
+					},
+				}),
+				deleteOne: scheduledPro,
+			}),
+		});
+		logChangeResponse(
+			"attach / upgrade clears existing scheduled downgrade",
+			response,
+		);
+
+		// Deleted (scheduled) products are intentionally ignored — they never
+		// went live and aren't a customer-facing lifecycle event.
+		expectBillingChangeResponse(response, {
+			expired: ["business"],
+			activated: ["premium"],
+		});
+		expect(
+			findPlanChange(response, { action: "expired", planId: "pro" }),
+		).toBeUndefined();
+	});
+
+	test("scheduled downgrade via starts_at", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+		const scheduledFree = makeFullCusProduct({
+			planId: "free",
+			status: CusProductStatus.Scheduled,
+			startedAt: PERIOD_END,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [scheduledFree],
+				update: makeUpdate({
+					customerProduct: pro,
+					updates: {
+						canceled: true,
+						canceled_at: NOW,
+						ended_at: PERIOD_END,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("attach / scheduled downgrade (starts_at)", response);
+
+		expectBillingChangeResponse(response, {
+			updated: ["pro"],
+			scheduled: ["free"],
+		});
+		expectPlanChange(findPlanChange(response, { action: "updated", planId: "pro" }), {
+			action: "updated",
+			planId: "pro",
+			previousAttributes: { canceled_at: null, expires_at: null },
+		});
+		const scheduled = findPlanChange(response, {
+			action: "scheduled",
+			planId: "free",
+		});
+		expect(scheduled?.plan.status).toBe("scheduled");
+	});
+
+	test("attach addon (no current product mutated)", () => {
+		const base = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+		const addon = makeFullCusProduct({ planId: "seats_addon", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [base] }),
+			autumnBillingPlan: makeAutumnBillingPlan({ inserts: [addon] }),
+		});
+		logChangeResponse("attach / addon", response);
+
+		expectBillingChangeResponse(response, { activated: ["seats_addon"] });
+	});
+
+	test("trial revert: pause current and attach trial", () => {
+		const base = makeFullCusProduct({
+			planId: "pro",
+			status: CusProductStatus.Active,
+			startedAt: NOW - 1000,
+		});
+		const trialProduct = makeFullCusProduct({
+			planId: "premium",
+			status: CusProductStatus.Trialing,
+			startedAt: NOW,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [base] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [trialProduct],
+				update: makeUpdate({
+					customerProduct: base,
+					updates: { status: CusProductStatus.Paused },
+				}),
+			}),
+		});
+		logChangeResponse("attach / trial revert (pause current)", response);
+
+		expectBillingChangeResponse(response, {
+			updated: ["pro"],
+			activated: ["premium"],
+		});
+		const updated = findPlanChange(response, {
+			action: "updated",
+			planId: "pro",
+		});
+		expectPlanChange(updated, {
+			action: "updated",
+			planId: "pro",
+			previousAttributes: { status: CusProductStatus.Active },
+		});
+		expect(updated?.plan.status).toBe("paused");
+	});
+});
diff --git a/server/tests/unit/billing/billing-change-response/create-schedule.test.ts b/server/tests/unit/billing/billing-change-response/create-schedule.test.ts
new file mode 100644
index 000000000..861c78224
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/create-schedule.test.ts
@@ -0,0 +1,117 @@
+import { describe, expect, test } from "bun:test";
+import { CusProductStatus } from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildBillingChangeResponse } from "@/internal/billing/v2/utils/billingChangeResponse";
+import {
+	expectBillingChangeResponse,
+	findPlanChange,
+} from "./helpers/expectBillingChange";
+import { logChangeResponse } from "./helpers/logChangeResponse";
+import {
+	makeAutumnBillingPlan,
+	makeUpdate,
+} from "./helpers/makeAutumnBillingPlan";
+import { makeFullCusProduct } from "./helpers/makeFullCusProduct";
+import { makeFullCustomer } from "./helpers/makeFullCustomer";
+
+const NOW = 1_710_000_000_000;
+const PHASE_TWO = NOW + 30 * 24 * 60 * 60 * 1000;
+const PHASE_THREE = NOW + 60 * 24 * 60 * 60 * 1000;
+const ctx = {} as AutumnContext;
+
+describe("buildBillingChangeResponse — createSchedule", () => {
+	test("multi-phase schedule replacing current product", () => {
+		const free = makeFullCusProduct({ planId: "free", startedAt: NOW - 1000 });
+		const pro = makeFullCusProduct({
+			planId: "pro",
+			status: CusProductStatus.Active,
+			startedAt: NOW,
+		});
+		const premiumPhase = makeFullCusProduct({
+			planId: "premium",
+			status: CusProductStatus.Scheduled,
+			startedAt: PHASE_TWO,
+		});
+		const enterprisePhase = makeFullCusProduct({
+			planId: "enterprise",
+			status: CusProductStatus.Scheduled,
+			startedAt: PHASE_THREE,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [free] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [pro, premiumPhase, enterprisePhase],
+				update: makeUpdate({
+					customerProduct: free,
+					updates: {
+						status: CusProductStatus.Expired,
+						ended_at: NOW,
+						canceled: true,
+						canceled_at: NOW,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("createSchedule / multi-phase replacing current", response);
+
+		expectBillingChangeResponse(response, {
+			expired: ["free"],
+			activated: ["pro"],
+			scheduled: ["premium", "enterprise"],
+		});
+		expect(
+			findPlanChange(response, { action: "activated", planId: "pro" })?.plan
+				.status,
+		).toBe("active");
+		expect(
+			findPlanChange(response, { action: "scheduled", planId: "premium" })?.plan
+				.status,
+		).toBe("scheduled");
+		expect(
+			findPlanChange(response, { action: "scheduled", planId: "enterprise" })
+				?.plan.status,
+		).toBe("scheduled");
+	});
+
+	test("schedule overrides existing scheduled products", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+		const oldScheduled = makeFullCusProduct({
+			planId: "free",
+			status: CusProductStatus.Scheduled,
+			startedAt: PHASE_TWO,
+		});
+		const newScheduledPremium = makeFullCusProduct({
+			planId: "premium",
+			status: CusProductStatus.Scheduled,
+			startedAt: PHASE_TWO,
+		});
+		const newScheduledEnterprise = makeFullCusProduct({
+			planId: "enterprise",
+			status: CusProductStatus.Scheduled,
+			startedAt: PHASE_THREE,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({
+				customerProducts: [pro, oldScheduled],
+			}),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [newScheduledPremium, newScheduledEnterprise],
+				deletes: [oldScheduled],
+			}),
+		});
+		logChangeResponse(
+			"createSchedule / schedule overrides existing scheduled",
+			response,
+		);
+
+		// The old scheduled product is deleted, not expired — deletes are
+		// intentionally skipped in v1.
+		expectBillingChangeResponse(response, {
+			scheduled: ["premium", "enterprise"],
+		});
+	});
+});
diff --git a/server/tests/unit/billing/billing-change-response/helpers/expectBillingChange.ts b/server/tests/unit/billing/billing-change-response/helpers/expectBillingChange.ts
new file mode 100644
index 000000000..4662c8407
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/helpers/expectBillingChange.ts
@@ -0,0 +1,92 @@
+import type {
+	BillingChangeResponse,
+	CustomerPlanChange,
+	PlanChangeAction,
+} from "@autumn/shared";
+import { expect } from "bun:test";
+
+export const findPlanChange = (
+	response: BillingChangeResponse,
+	{ action, planId }: { action: PlanChangeAction; planId: string },
+): CustomerPlanChange | undefined =>
+	response.plan_changes.find(
+		(change) => change.action === action && change.plan.plan_id === planId,
+	);
+
+export const expectPlanChange = (
+	change: CustomerPlanChange | undefined,
+	{
+		action,
+		planId,
+		previousAttributes,
+		itemChanges,
+	}: {
+		action: PlanChangeAction;
+		planId: string;
+		previousAttributes?: Record<string, unknown> | null;
+		itemChanges?: Array<{ action: "created" | "deleted"; feature_id: string }>;
+	},
+): CustomerPlanChange => {
+	expect(change, `expected ${action} change for plan ${planId}`).toBeDefined();
+	const resolved = change as CustomerPlanChange;
+	expect(resolved.action).toBe(action);
+	expect(resolved.plan.plan_id).toBe(planId);
+
+	if (previousAttributes === null) {
+		expect(resolved.previous_attributes).toBeNull();
+	} else if (previousAttributes !== undefined) {
+		expect(resolved.previous_attributes).not.toBeNull();
+		for (const [key, value] of Object.entries(previousAttributes)) {
+			expect(
+				resolved.previous_attributes,
+				`previous_attributes.${key} mismatch`,
+			).toMatchObject({ [key]: value });
+		}
+	}
+
+	if (itemChanges !== undefined) {
+		expect(resolved.item_changes).toEqual(itemChanges);
+	}
+
+	return resolved;
+};
+
+export const expectBillingChangeResponse = (
+	response: BillingChangeResponse,
+	{
+		customerId,
+		activated = [],
+		scheduled = [],
+		updated = [],
+		expired = [],
+		tags,
+	}: {
+		customerId?: string;
+		activated?: string[];
+		scheduled?: string[];
+		updated?: string[];
+		expired?: string[];
+		tags?: string[];
+	},
+): void => {
+	if (customerId !== undefined) {
+		expect(response.customer_id).toBe(customerId);
+	}
+
+	const byAction = (action: PlanChangeAction) =>
+		response.plan_changes
+			.filter((change) => change.action === action)
+			.map((change) => change.plan.plan_id)
+			.sort();
+
+	expect(byAction("activated")).toEqual([...activated].sort());
+	expect(byAction("scheduled")).toEqual([...scheduled].sort());
+	expect(byAction("updated")).toEqual([...updated].sort());
+	expect(byAction("expired")).toEqual([...expired].sort());
+
+	if (tags !== undefined) {
+		expect(
+			(response as BillingChangeResponse & { tags?: string[] }).tags,
+		).toEqual(tags);
+	}
+};
diff --git a/server/tests/unit/billing/billing-change-response/helpers/logChangeResponse.ts b/server/tests/unit/billing/billing-change-response/helpers/logChangeResponse.ts
new file mode 100644
index 000000000..137d676fa
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/helpers/logChangeResponse.ts
@@ -0,0 +1,13 @@
+import type { BillingChangeResponse } from "@autumn/shared";
+
+const isEnabled = (): boolean => process.env.PRINT_BILLING_CHANGES === "1";
+
+export const logChangeResponse = (
+	label: string,
+	response: BillingChangeResponse,
+): void => {
+	if (!isEnabled()) return;
+	const divider = "─".repeat(Math.max(8, 60 - label.length));
+	process.stdout.write(`\n── ${label} ${divider}\n`);
+	process.stdout.write(`${JSON.stringify(response, null, 2)}\n`);
+};
diff --git a/server/tests/unit/billing/billing-change-response/helpers/makeAutumnBillingPlan.ts b/server/tests/unit/billing/billing-change-response/helpers/makeAutumnBillingPlan.ts
new file mode 100644
index 000000000..be450cf93
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/helpers/makeAutumnBillingPlan.ts
@@ -0,0 +1,73 @@
+import type {
+	AutumnBillingPlan,
+	CustomerProductUpdateSchema,
+	FullCusProduct,
+	FullCustomerEntitlement,
+	PatchCustomerProductSchema,
+} from "@autumn/shared";
+import type { z } from "zod/v4";
+
+type CustomerProductUpdate = z.infer<typeof CustomerProductUpdateSchema>;
+type PatchCustomerProduct = z.infer<typeof PatchCustomerProductSchema>;
+
+export const makeAutumnBillingPlan = ({
+	inserts = [],
+	update,
+	updates,
+	deleteOne,
+	deletes,
+	patches,
+}: {
+	inserts?: FullCusProduct[];
+	update?: CustomerProductUpdate;
+	updates?: CustomerProductUpdate[];
+	deleteOne?: FullCusProduct;
+	deletes?: FullCusProduct[];
+	patches?: PatchCustomerProduct[];
+} = {}): AutumnBillingPlan => {
+	return {
+		customerId: "cus_test",
+		insertCustomerProducts: inserts,
+		updateCustomerProduct: update,
+		updateCustomerProducts: updates,
+		deleteCustomerProduct: deleteOne,
+		deleteCustomerProducts: deletes,
+		patchCustomerProducts: patches,
+	} as AutumnBillingPlan;
+};
+
+export const makeUpdate = ({
+	customerProduct,
+	updates,
+}: {
+	customerProduct: FullCusProduct;
+	updates: CustomerProductUpdate["updates"];
+}): CustomerProductUpdate => ({ customerProduct, updates });
+
+export const makePatch = ({
+	customerProduct,
+	insertEntitlements = [],
+	deleteEntitlements = [],
+}: {
+	customerProduct: FullCusProduct;
+	insertEntitlements?: FullCustomerEntitlement[];
+	deleteEntitlements?: FullCustomerEntitlement[];
+}): PatchCustomerProduct =>
+	({
+		customerProduct,
+		insertCustomerEntitlements: insertEntitlements,
+		deleteCustomerEntitlements: deleteEntitlements,
+		insertCustomerPrices: [],
+		deleteCustomerPrices: [],
+	}) as PatchCustomerProduct;
+
+export const makeCustomerEntitlement = ({
+	featureId,
+}: {
+	featureId: string;
+}): FullCustomerEntitlement =>
+	({
+		id: `cusEnt_${featureId}`,
+		feature_id: featureId,
+		internal_feature_id: `internal_${featureId}`,
+	}) as unknown as FullCustomerEntitlement;
diff --git a/server/tests/unit/billing/billing-change-response/helpers/makeFullCusProduct.ts b/server/tests/unit/billing/billing-change-response/helpers/makeFullCusProduct.ts
new file mode 100644
index 000000000..7cbfa6d0f
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/helpers/makeFullCusProduct.ts
@@ -0,0 +1,45 @@
+import {
+	CollectionMethod,
+	CusProductStatus,
+	type FullCusProduct,
+} from "@autumn/shared";
+
+export const makeFullCusProduct = ({
+	planId,
+	status = CusProductStatus.Active,
+	startedAt,
+	canceledAt = null,
+	endedAt = null,
+	id,
+}: {
+	planId: string;
+	status?: CusProductStatus;
+	startedAt?: number;
+	canceledAt?: number | null;
+	endedAt?: number | null;
+	id?: string;
+}): FullCusProduct => {
+	return {
+		id: id ?? `cp_${planId}`,
+		internal_product_id: `internal_${planId}`,
+		product_id: planId,
+		internal_customer_id: "internal_cus_test",
+		customer_id: "cus_test",
+		created_at: 1_700_000_000_000,
+		updated_at: null,
+		status,
+		canceled: canceledAt !== null,
+		starts_at: startedAt ?? 1_700_000_000_000,
+		canceled_at: canceledAt,
+		ended_at: endedAt,
+		options: [],
+		collection_method: CollectionMethod.ChargeAutomatically,
+		quantity: 1,
+		api_semver: null,
+		is_custom: false,
+		external_id: null,
+		customer_prices: [],
+		customer_entitlements: [],
+		product: { id: planId, name: planId } as FullCusProduct["product"],
+	} as unknown as FullCusProduct;
+};
diff --git a/server/tests/unit/billing/billing-change-response/helpers/makeFullCustomer.ts b/server/tests/unit/billing/billing-change-response/helpers/makeFullCustomer.ts
new file mode 100644
index 000000000..f6831b731
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/helpers/makeFullCustomer.ts
@@ -0,0 +1,20 @@
+import { AppEnv, type FullCusProduct, type FullCustomer } from "@autumn/shared";
+
+export const makeFullCustomer = ({
+	id = "cus_test",
+	customerProducts = [],
+}: {
+	id?: string;
+	customerProducts?: FullCusProduct[];
+} = {}): FullCustomer => {
+	return {
+		id,
+		internal_id: `internal_${id}`,
+		org_id: "org_test",
+		created_at: 1_700_000_000_000,
+		env: AppEnv.Sandbox,
+		processor: null,
+		customer_products: customerProducts,
+		entities: [],
+	} as unknown as FullCustomer;
+};
diff --git a/server/tests/unit/billing/billing-change-response/migrate.test.ts b/server/tests/unit/billing/billing-change-response/migrate.test.ts
new file mode 100644
index 000000000..1f7f172d5
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/migrate.test.ts
@@ -0,0 +1,89 @@
+import { describe, test } from "bun:test";
+import { CusProductStatus } from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildBillingChangeResponse } from "@/internal/billing/v2/utils/billingChangeResponse";
+import {
+	expectBillingChangeResponse,
+	expectPlanChange,
+	findPlanChange,
+} from "./helpers/expectBillingChange";
+import { logChangeResponse } from "./helpers/logChangeResponse";
+import {
+	makeAutumnBillingPlan,
+	makeCustomerEntitlement,
+	makePatch,
+	makeUpdate,
+} from "./helpers/makeAutumnBillingPlan";
+import { makeFullCusProduct } from "./helpers/makeFullCusProduct";
+import { makeFullCustomer } from "./helpers/makeFullCustomer";
+
+const NOW = 1_710_000_000_000;
+const ctx = {} as AutumnContext;
+
+describe("buildBillingChangeResponse — migrate", () => {
+	test("migrate via update plan path (replace product)", () => {
+		const oldPro = makeFullCusProduct({
+			planId: "pro",
+			startedAt: NOW - 1000,
+		});
+		const newPro = makeFullCusProduct({ planId: "pro_v2", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [oldPro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [newPro],
+				update: makeUpdate({
+					customerProduct: oldPro,
+					updates: { status: CusProductStatus.Expired },
+				}),
+			}),
+		});
+		logChangeResponse("migrate / update plan path", response);
+
+		expectBillingChangeResponse(response, {
+			expired: ["pro"],
+			activated: ["pro_v2"],
+		});
+		expectPlanChange(findPlanChange(response, { action: "expired", planId: "pro" }), {
+			action: "expired",
+			planId: "pro",
+			previousAttributes: { status: CusProductStatus.Active },
+		});
+	});
+
+	test("migrate via patch items (carry rollover)", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				patches: [
+					makePatch({
+						customerProduct: pro,
+						insertEntitlements: [
+							makeCustomerEntitlement({ featureId: "new_feature_x" }),
+							makeCustomerEntitlement({ featureId: "new_feature_y" }),
+						],
+						deleteEntitlements: [
+							makeCustomerEntitlement({ featureId: "old_feature" }),
+						],
+					}),
+				],
+			}),
+		});
+		logChangeResponse("migrate / patch items (carry rollover)", response);
+
+		expectBillingChangeResponse(response, { updated: ["pro"] });
+		expectPlanChange(findPlanChange(response, { action: "updated", planId: "pro" }), {
+			action: "updated",
+			planId: "pro",
+			itemChanges: [
+				{ action: "created", feature_id: "new_feature_x" },
+				{ action: "created", feature_id: "new_feature_y" },
+				{ action: "deleted", feature_id: "old_feature" },
+			],
+		});
+	});
+});
diff --git a/server/tests/unit/billing/billing-change-response/multi-attach.test.ts b/server/tests/unit/billing/billing-change-response/multi-attach.test.ts
new file mode 100644
index 000000000..0d1ca6cfc
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/multi-attach.test.ts
@@ -0,0 +1,118 @@
+import { describe, expect, test } from "bun:test";
+import { CusProductStatus } from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildBillingChangeResponse } from "@/internal/billing/v2/utils/billingChangeResponse";
+import {
+	expectBillingChangeResponse,
+	findPlanChange,
+} from "./helpers/expectBillingChange";
+import { logChangeResponse } from "./helpers/logChangeResponse";
+import {
+	makeAutumnBillingPlan,
+	makeUpdate,
+} from "./helpers/makeAutumnBillingPlan";
+import { makeFullCusProduct } from "./helpers/makeFullCusProduct";
+import { makeFullCustomer } from "./helpers/makeFullCustomer";
+
+const NOW = 1_710_000_000_000;
+const PERIOD_END = NOW + 30 * 24 * 60 * 60 * 1000;
+const ctx = {} as AutumnContext;
+
+describe("buildBillingChangeResponse — multiAttach", () => {
+	test("multiple inserts, no current products", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW });
+		const seatsAddon = makeFullCusProduct({
+			planId: "seats_addon",
+			startedAt: NOW,
+		});
+		const storageAddon = makeFullCusProduct({
+			planId: "storage_addon",
+			startedAt: NOW,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer(),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [pro, seatsAddon, storageAddon],
+			}),
+		});
+		logChangeResponse("multiAttach / multiple inserts no current", response);
+
+		expectBillingChangeResponse(response, {
+			activated: ["pro", "seats_addon", "storage_addon"],
+		});
+	});
+
+	test("multi-insert with one transitioning current product", () => {
+		const free = makeFullCusProduct({ planId: "free", startedAt: NOW - 1000 });
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW });
+		const addon = makeFullCusProduct({
+			planId: "seats_addon",
+			startedAt: NOW,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [free] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [pro, addon],
+				update: makeUpdate({
+					customerProduct: free,
+					updates: {
+						status: CusProductStatus.Expired,
+						ended_at: NOW,
+						canceled: true,
+						canceled_at: NOW,
+					},
+				}),
+			}),
+		});
+		logChangeResponse(
+			"multiAttach / multi-insert with one transition",
+			response,
+		);
+
+		expectBillingChangeResponse(response, {
+			expired: ["free"],
+			activated: ["pro", "seats_addon"],
+		});
+	});
+
+	test("mixed statuses across inserts (active + scheduled)", () => {
+		const pro = makeFullCusProduct({
+			planId: "pro",
+			status: CusProductStatus.Active,
+			startedAt: NOW,
+		});
+		const scheduledAddon = makeFullCusProduct({
+			planId: "seats_addon",
+			status: CusProductStatus.Scheduled,
+			startedAt: PERIOD_END,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer(),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [pro, scheduledAddon],
+			}),
+		});
+		logChangeResponse("multiAttach / mixed statuses", response);
+
+		expectBillingChangeResponse(response, {
+			activated: ["pro"],
+			scheduled: ["seats_addon"],
+		});
+		expect(
+			findPlanChange(response, { action: "activated", planId: "pro" })?.plan
+				.status,
+		).toBe("active");
+		expect(
+			findPlanChange(response, {
+				action: "scheduled",
+				planId: "seats_addon",
+			})?.plan.status,
+		).toBe("scheduled");
+	});
+});
diff --git a/server/tests/unit/billing/billing-change-response/restore.test.ts b/server/tests/unit/billing/billing-change-response/restore.test.ts
new file mode 100644
index 000000000..00d267394
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/restore.test.ts
@@ -0,0 +1,118 @@
+import { describe, expect, test } from "bun:test";
+import { CusProductStatus } from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildBillingChangeResponse } from "@/internal/billing/v2/utils/billingChangeResponse";
+import {
+	expectBillingChangeResponse,
+	expectPlanChange,
+	findPlanChange,
+} from "./helpers/expectBillingChange";
+import { logChangeResponse } from "./helpers/logChangeResponse";
+import {
+	makeAutumnBillingPlan,
+	makeUpdate,
+} from "./helpers/makeAutumnBillingPlan";
+import { makeFullCusProduct } from "./helpers/makeFullCusProduct";
+import { makeFullCustomer } from "./helpers/makeFullCustomer";
+
+const NOW = 1_710_000_000_000;
+const PERIOD_END = NOW + 30 * 24 * 60 * 60 * 1000;
+const ctx = {} as AutumnContext;
+
+describe("buildBillingChangeResponse — restore", () => {
+	test("restore single canceled product", () => {
+		const canceledPro = makeFullCusProduct({
+			planId: "pro",
+			startedAt: NOW - 1000,
+			canceledAt: NOW - 500,
+			endedAt: PERIOD_END,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({
+				customerProducts: [canceledPro],
+			}),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				update: makeUpdate({
+					customerProduct: canceledPro,
+					updates: {
+						canceled: false,
+						canceled_at: null,
+						ended_at: null,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("restore / single canceled product", response);
+
+		expectBillingChangeResponse(response, { updated: ["pro"] });
+		const updated = findPlanChange(response, {
+			action: "updated",
+			planId: "pro",
+		});
+		expectPlanChange(updated, {
+			action: "updated",
+			planId: "pro",
+			previousAttributes: {
+				canceled_at: NOW - 500,
+				expires_at: PERIOD_END,
+			},
+		});
+		expect(updated?.plan.canceled_at).toBeNull();
+		expect(updated?.plan.expires_at).toBeNull();
+	});
+
+	test("restore multiple via updateCustomerProducts array", () => {
+		const proBase = makeFullCusProduct({
+			planId: "pro",
+			startedAt: NOW - 1000,
+			canceledAt: NOW - 500,
+			endedAt: PERIOD_END,
+		});
+		const addon = makeFullCusProduct({
+			planId: "seats_addon",
+			startedAt: NOW - 1000,
+			canceledAt: NOW - 500,
+			endedAt: PERIOD_END,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({
+				customerProducts: [proBase, addon],
+			}),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				updates: [
+					makeUpdate({
+						customerProduct: proBase,
+						updates: { canceled: false, canceled_at: null, ended_at: null },
+					}),
+					makeUpdate({
+						customerProduct: addon,
+						updates: { canceled: false, canceled_at: null, ended_at: null },
+					}),
+				],
+			}),
+		});
+		logChangeResponse("restore / multiple via array", response);
+
+		expectBillingChangeResponse(response, {
+			updated: ["pro", "seats_addon"],
+		});
+		for (const planId of ["pro", "seats_addon"]) {
+			const change = findPlanChange(response, {
+				action: "updated",
+				planId,
+			});
+			expectPlanChange(change, {
+				action: "updated",
+				planId,
+				previousAttributes: {
+					canceled_at: NOW - 500,
+					expires_at: PERIOD_END,
+				},
+			});
+		}
+	});
+});
diff --git a/server/tests/unit/billing/billing-change-response/sync.test.ts b/server/tests/unit/billing/billing-change-response/sync.test.ts
new file mode 100644
index 000000000..bc692f7f5
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/sync.test.ts
@@ -0,0 +1,82 @@
+import { describe, test } from "bun:test";
+import { CusProductStatus } from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildBillingChangeResponse } from "@/internal/billing/v2/utils/billingChangeResponse";
+import {
+	expectBillingChangeResponse,
+	expectPlanChange,
+	findPlanChange,
+} from "./helpers/expectBillingChange";
+import { logChangeResponse } from "./helpers/logChangeResponse";
+import {
+	makeAutumnBillingPlan,
+	makeUpdate,
+} from "./helpers/makeAutumnBillingPlan";
+import { makeFullCusProduct } from "./helpers/makeFullCusProduct";
+import { makeFullCustomer } from "./helpers/makeFullCustomer";
+
+const NOW = 1_710_000_000_000;
+const ctx = {} as AutumnContext;
+
+describe("buildBillingChangeResponse — sync (from Stripe)", () => {
+	test("sync with expire_previous=true", () => {
+		const oldPro = makeFullCusProduct({
+			planId: "pro",
+			startedAt: NOW - 1000,
+		});
+		const newPro = makeFullCusProduct({ planId: "pro_v2", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [oldPro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [newPro],
+				update: makeUpdate({
+					customerProduct: oldPro,
+					updates: {
+						status: CusProductStatus.Expired,
+						ended_at: NOW,
+						canceled: true,
+						canceled_at: NOW,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("sync / expire_previous=true", response);
+
+		expectBillingChangeResponse(response, {
+			expired: ["pro"],
+			activated: ["pro_v2"],
+		});
+		expectPlanChange(
+			findPlanChange(response, { action: "expired", planId: "pro" }),
+			{
+				action: "expired",
+				planId: "pro",
+				previousAttributes: { status: CusProductStatus.Active },
+			},
+		);
+	});
+
+	test("sync with expire_previous=false (both products active)", () => {
+		const baseAddon = makeFullCusProduct({
+			planId: "pro",
+			startedAt: NOW - 1000,
+		});
+		const newAddon = makeFullCusProduct({
+			planId: "seats_addon",
+			startedAt: NOW,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({
+				customerProducts: [baseAddon],
+			}),
+			autumnBillingPlan: makeAutumnBillingPlan({ inserts: [newAddon] }),
+		});
+		logChangeResponse("sync / expire_previous=false", response);
+
+		expectBillingChangeResponse(response, { activated: ["seats_addon"] });
+	});
+});
diff --git a/server/tests/unit/billing/billing-change-response/update-subscription.test.ts b/server/tests/unit/billing/billing-change-response/update-subscription.test.ts
new file mode 100644
index 000000000..cc6adb866
--- /dev/null
+++ b/server/tests/unit/billing/billing-change-response/update-subscription.test.ts
@@ -0,0 +1,277 @@
+import { describe, expect, test } from "bun:test";
+import { CusProductStatus } from "@autumn/shared";
+import type { AutumnContext } from "@/honoUtils/HonoEnv";
+import { buildBillingChangeResponse } from "@/internal/billing/v2/utils/billingChangeResponse";
+import {
+	expectBillingChangeResponse,
+	expectPlanChange,
+	findPlanChange,
+} from "./helpers/expectBillingChange";
+import { logChangeResponse } from "./helpers/logChangeResponse";
+import {
+	makeAutumnBillingPlan,
+	makeCustomerEntitlement,
+	makePatch,
+	makeUpdate,
+} from "./helpers/makeAutumnBillingPlan";
+import { makeFullCusProduct } from "./helpers/makeFullCusProduct";
+import { makeFullCustomer } from "./helpers/makeFullCustomer";
+
+const NOW = 1_710_000_000_000;
+const PERIOD_END = NOW + 30 * 24 * 60 * 60 * 1000;
+const ctx = {} as AutumnContext;
+
+describe("buildBillingChangeResponse — updateSubscription", () => {
+	test("cancel at end of cycle", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+		const defaultFree = makeFullCusProduct({
+			planId: "free",
+			status: CusProductStatus.Scheduled,
+			startedAt: PERIOD_END,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [defaultFree],
+				update: makeUpdate({
+					customerProduct: pro,
+					updates: {
+						canceled: true,
+						canceled_at: NOW,
+						ended_at: PERIOD_END,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("update / cancel end of cycle", response);
+
+		expectBillingChangeResponse(response, {
+			updated: ["pro"],
+			scheduled: ["free"],
+		});
+		expectPlanChange(findPlanChange(response, { action: "updated", planId: "pro" }), {
+			action: "updated",
+			planId: "pro",
+			previousAttributes: { canceled_at: null, expires_at: null },
+		});
+		const scheduled = findPlanChange(response, {
+			action: "scheduled",
+			planId: "free",
+		});
+		expect(scheduled?.plan.status).toBe("scheduled");
+	});
+
+	test("cancel immediately", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+		const defaultFree = makeFullCusProduct({
+			planId: "free",
+			status: CusProductStatus.Active,
+			startedAt: NOW,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [defaultFree],
+				update: makeUpdate({
+					customerProduct: pro,
+					updates: {
+						canceled: true,
+						canceled_at: NOW,
+						ended_at: NOW,
+						status: CusProductStatus.Expired,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("update / cancel immediately", response);
+
+		expectBillingChangeResponse(response, {
+			expired: ["pro"],
+			activated: ["free"],
+		});
+		expectPlanChange(findPlanChange(response, { action: "expired", planId: "pro" }), {
+			action: "expired",
+			planId: "pro",
+			previousAttributes: {
+				status: CusProductStatus.Active,
+				canceled_at: null,
+				expires_at: null,
+			},
+		});
+	});
+
+	test("uncancel", () => {
+		const pro = makeFullCusProduct({
+			planId: "pro",
+			startedAt: NOW - 1000,
+			canceledAt: NOW - 500,
+			endedAt: PERIOD_END,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				update: makeUpdate({
+					customerProduct: pro,
+					updates: {
+						canceled: false,
+						canceled_at: null,
+						ended_at: null,
+					},
+				}),
+			}),
+		});
+		logChangeResponse("update / uncancel", response);
+
+		expectBillingChangeResponse(response, { updated: ["pro"] });
+		expectPlanChange(findPlanChange(response, { action: "updated", planId: "pro" }), {
+			action: "updated",
+			planId: "pro",
+			previousAttributes: {
+				canceled_at: NOW - 500,
+				expires_at: PERIOD_END,
+			},
+		});
+		const updated = findPlanChange(response, {
+			action: "updated",
+			planId: "pro",
+		});
+		expect(updated?.plan.canceled_at).toBeNull();
+		expect(updated?.plan.expires_at).toBeNull();
+	});
+
+	test("update plan via custom plan (replace product)", () => {
+		const oldPro = makeFullCusProduct({
+			planId: "pro",
+			startedAt: NOW - 1000,
+		});
+		const newPro = makeFullCusProduct({ planId: "pro_v2", startedAt: NOW });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [oldPro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				inserts: [newPro],
+				update: makeUpdate({
+					customerProduct: oldPro,
+					updates: { status: CusProductStatus.Expired },
+				}),
+			}),
+		});
+		logChangeResponse("update / custom plan replacement", response);
+
+		expectBillingChangeResponse(response, {
+			expired: ["pro"],
+			activated: ["pro_v2"],
+		});
+		expectPlanChange(findPlanChange(response, { action: "expired", planId: "pro" }), {
+			action: "expired",
+			planId: "pro",
+			previousAttributes: { status: CusProductStatus.Active },
+		});
+	});
+
+	test("delete a scheduled product emits nothing (deletes ignored for now)", () => {
+		const scheduledFree = makeFullCusProduct({
+			planId: "free",
+			status: CusProductStatus.Scheduled,
+			startedAt: PERIOD_END,
+		});
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({
+				customerProducts: [scheduledFree],
+			}),
+			autumnBillingPlan: makeAutumnBillingPlan({ deleteOne: scheduledFree }),
+		});
+		logChangeResponse("update / delete scheduled product (ignored)", response);
+
+		expectBillingChangeResponse(response, {});
+		expect(response.plan_changes).toEqual([]);
+	});
+
+	test("patch items (inline mode) — add and remove features", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				patches: [
+					makePatch({
+						customerProduct: pro,
+						insertEntitlements: [
+							makeCustomerEntitlement({ featureId: "api_calls" }),
+						],
+						deleteEntitlements: [
+							makeCustomerEntitlement({ featureId: "legacy_feature" }),
+						],
+					}),
+				],
+			}),
+		});
+		logChangeResponse("update / patch items", response);
+
+		expectBillingChangeResponse(response, { updated: ["pro"] });
+		expectPlanChange(findPlanChange(response, { action: "updated", planId: "pro" }), {
+			action: "updated",
+			planId: "pro",
+			itemChanges: [
+				{ action: "created", feature_id: "api_calls" },
+				{ action: "deleted", feature_id: "legacy_feature" },
+			],
+		});
+	});
+
+	test("quantity-only update — empty previous_attributes (v1 limitation)", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				update: makeUpdate({
+					customerProduct: pro,
+					updates: {
+						options: [{ feature_id: "seats", quantity: 10 }],
+					},
+				}),
+			}),
+		});
+		logChangeResponse("update / quantity only", response);
+
+		expectBillingChangeResponse(response, { updated: ["pro"] });
+		const updated = findPlanChange(response, {
+			action: "updated",
+			planId: "pro",
+		});
+		expect(updated?.previous_attributes).toEqual({});
+		expect(updated?.item_changes).toEqual([]);
+	});
+
+	test("anchor reset — empty updates object", () => {
+		const pro = makeFullCusProduct({ planId: "pro", startedAt: NOW - 1000 });
+
+		const response = buildBillingChangeResponse({
+			ctx,
+			originalFullCustomer: makeFullCustomer({ customerProducts: [pro] }),
+			autumnBillingPlan: makeAutumnBillingPlan({
+				update: makeUpdate({ customerProduct: pro, updates: {} }),
+			}),
+		});
+		logChangeResponse("update / empty updates (anchor reset)", response);
+
+		expectBillingChangeResponse(response, { updated: ["pro"] });
+		const updated = findPlanChange(response, {
+			action: "updated",
+			planId: "pro",
+		});
+		expect(updated?.previous_attributes).toEqual({});
+	});
+});
diff --git a/shared/api/billing/common/billingChangeResponse.ts b/shared/api/billing/common/billingChangeResponse.ts
new file mode 100644
index 000000000..13d6666be
--- /dev/null
+++ b/shared/api/billing/common/billingChangeResponse.ts
@@ -0,0 +1,23 @@
+import { z } from "zod/v4";
+import { CustomerPlanChangeSchema } from "./customerPlanChange";
+
+export const BillingChangeResponseSchema = z.object({
+	object: z.literal("billing.plans_changed"),
+	customer_id: z.string().meta({
+		description: "The ID of the customer whose plans changed.",
+	}),
+	entity_id: z
+		.string()
+		.nullable()
+		.optional()
+		.meta({
+			description:
+				"The ID of the entity, if the changes are scoped to a specific entity.",
+		}),
+	plan_changes: z.array(CustomerPlanChangeSchema).meta({
+		description:
+			"The plans that were activated, scheduled, updated, or expired.",
+	}),
+});
+
+export type BillingChangeResponse = z.infer<typeof BillingChangeResponseSchema>;
diff --git a/shared/api/billing/common/customerPlanChange.ts b/shared/api/billing/common/customerPlanChange.ts
new file mode 100644
index 000000000..e1f95bd50
--- /dev/null
+++ b/shared/api/billing/common/customerPlanChange.ts
@@ -0,0 +1,82 @@
+import { z } from "zod/v4";
+
+export const PlanChangeActionEnum = z.enum([
+	"activated",
+	"scheduled",
+	"updated",
+	"expired",
+]);
+
+export const CustomerPlanStatusEnum = z.enum([
+	"active",
+	"trialing",
+	"past_due",
+	"scheduled",
+	"expired",
+	"paused",
+]);
+
+export const CustomerPlanSnapshotSchema = z.object({
+	plan_id: z.string().meta({
+		description: "The ID of the customer plan.",
+	}),
+	status: CustomerPlanStatusEnum.meta({
+		description: "The current status of the plan on the customer.",
+	}),
+	started_at: z.number().nullable().meta({
+		description: "When the plan started, in milliseconds since the Unix epoch.",
+	}),
+	canceled_at: z.number().nullable().meta({
+		description:
+			"When the plan was canceled, in milliseconds since the Unix epoch, or null if not canceled.",
+	}),
+	expires_at: z.number().nullable().meta({
+		description:
+			"When the plan ends, in milliseconds since the Unix epoch, or null if no expiry is set.",
+	}),
+	current_period_start: z.number().nullable().meta({
+		description: "Start of the current billing period, or null if not applicable.",
+	}),
+	current_period_end: z.number().nullable().meta({
+		description: "End of the current billing period, or null if not applicable.",
+	}),
+});
+
+export const CustomerPlanItemChangeSchema = z.object({
+	action: z.enum(["created", "deleted"]).meta({
+		description: "Whether the feature was added to or removed from the plan.",
+	}),
+	feature_id: z.string().meta({
+		description: "The ID of the feature that was added or removed.",
+	}),
+});
+
+export const CustomerPlanChangeSchema = z.object({
+	action: PlanChangeActionEnum.meta({
+		description:
+			"The lifecycle action applied to this plan: activated (newly active on the customer), scheduled (queued for a future start), updated (mutated in place), or expired (ended).",
+	}),
+	plan: CustomerPlanSnapshotSchema.meta({
+		description: "The plan as it stands after this change.",
+	}),
+	previous_attributes: z
+		.record(z.string(), z.unknown())
+		.nullable()
+		.meta({
+			description:
+				"Sparse map of scalar fields whose values changed, holding their previous values. Null when the plan is newly activated or scheduled.",
+		}),
+	item_changes: z
+		.array(CustomerPlanItemChangeSchema)
+		.default([])
+		.meta({
+			description:
+				"Features that were added to or removed from this plan. Only populated for updated plans.",
+		}),
+});
+
+export type PlanChangeAction = z.infer<typeof PlanChangeActionEnum>;
+export type CustomerPlanStatus = z.infer<typeof CustomerPlanStatusEnum>;
+export type CustomerPlanSnapshot = z.infer<typeof CustomerPlanSnapshotSchema>;
+export type CustomerPlanItemChange = z.infer<typeof CustomerPlanItemChangeSchema>;
+export type CustomerPlanChange = z.infer<typeof CustomerPlanChangeSchema>;
diff --git a/shared/api/billing/common/index.ts b/shared/api/billing/common/index.ts
index 9cfc436a8..b469221ba 100644
--- a/shared/api/billing/common/index.ts
+++ b/shared/api/billing/common/index.ts
@@ -1,10 +1,12 @@
 export * from "./attachPreviewResponse";
 export * from "./billingBehavior";
+export * from "./billingChangeResponse";
 export * from "./billingParamsBase/billingParamsBaseV0";
 export * from "./billingParamsBase/billingParamsBaseV1";
 export * from "./billingPreviewChange";
 export * from "./billingPreviewResponse";
 export * from "./billingResponse";
+export * from "./customerPlanChange";
 export * from "./cancelAction";
 export * from "./customizePlan/customizePlanV0";
 export * from "./customizePlan/customizePlanV1";