diff --git a/__test__/features/Events/ICSPropertiesPreservation.test.tsx b/__test__/features/Events/ICSPropertiesPreservation.test.tsx
new file mode 100644
index 0000000..f338312
--- /dev/null
+++ b/__test__/features/Events/ICSPropertiesPreservation.test.tsx
@@ -0,0 +1,757 @@
+import { Calendar } from "@/features/Calendars/CalendarTypes";
+import { VObjectProperty } from "@/features/Calendars/types/CalendarData";
+import { putEvent, putEventWithOverrides } from "@/features/Events/EventApi";
+import { CalendarEvent } from "@/features/Events/EventsTypes";
+import {
+  calendarEventToJCal,
+  makeVevent,
+  parseCalendarEvent,
+} from "@/features/Events/utils";
+import { api } from "@/utils/apiUtils";
+
+jest.mock("@/utils/apiUtils");
+
+const MOCK_CALENDAR: Calendar = {
+  id: "cal-1",
+  link: "/dav/calendars/user/cal-1.json",
+} as Calendar;
+const EVENT_URL = "/dav/calendars/user/cal-1/event-abc.ics";
+
+const mockEvent = {
+  uid: "event-123",
+  title: "Test Event",
+  start: "2025-07-23T08:00:00.000Z",
+  end: "2025-07-23T09:00:00.000Z",
+  timezone: "Europe/Paris",
+  allday: false,
+  attendee: [],
+  passthroughProps: [
+    ["x-attachment", {}, "text", "data:application/pdf;base64,abc123=="],
+    ["x-custom-prop", {}, "text", "custom-value"],
+  ] as VObjectProperty[],
+} as unknown as CalendarEvent;
+
+/** Minimal valid vevent props (dtstart is a timed event in Europe/Paris) */
+function baseVeventProps(overrides: VObjectProperty[] = []): VObjectProperty[] {
+  return [
+    ["uid", {}, "text", "event-abc-uid"],
+    ["summary", {}, "text", "Team standup"],
+    ["dtstart", { tzid: "Europe/Paris" }, "date-time", "2024-03-15T10:00:00"],
+    ["dtend", { tzid: "Europe/Paris" }, "date-time", "2024-03-15T10:30:00"],
+    ["dtstamp", {}, "date-time", "2024-03-01T00:00:00Z"],
+    ["sequence", {}, "integer", 0],
+    ...overrides,
+  ];
+}
+
+/** X- / custom props that must survive round-trips */
+const CUSTOM_PROPS: VObjectProperty[] = [
+  ["x-attachment", {}, "text", "base64encodeddata=="],
+  ["x-apple-travel-title", {}, "text", "Drive to office"],
+  ["x-custom-field", {}, "unknown", "some-value"],
+];
+
+/** Build a CalendarEvent directly (skips parseCalendarEvent) for makeVevent tests */
+function baseCalendarEvent(
+  overrides: Partial<CalendarEvent> = {}
+): CalendarEvent {
+  return {
+    uid: "event-abc-uid",
+    title: "Team standup",
+    start: "2024-03-15T09:00:00.000Z",
+    end: "2024-03-15T09:30:00.000Z",
+    timezone: "Europe/Paris",
+    allday: false,
+    attendee: [],
+    calId: "cal-1",
+    URL: EVENT_URL,
+    transp: "OPAQUE",
+    class: "PUBLIC",
+    sequence: 1,
+    exdates: [],
+    ...overrides,
+  } as CalendarEvent;
+}
+
+describe("parseCalendarEvent — passthroughProps", () => {
+  it("captures unknown X- properties into passthroughProps", () => {
+    const props = baseVeventProps(CUSTOM_PROPS);
+    const event = parseCalendarEvent(props, {}, MOCK_CALENDAR, EVENT_URL);
+
+    expect(event.passthroughProps).toBeDefined();
+    const keys = event.passthroughProps!.map(([k]) => k.toLowerCase());
+    expect(keys).toContain("x-attachment");
+    expect(keys).toContain("x-apple-travel-title");
+    expect(keys).toContain("x-custom-field");
+  });
+
+  it("preserves the full tuple structure of passthrough props", () => {
+    const props = baseVeventProps(CUSTOM_PROPS);
+    const event = parseCalendarEvent(props, {}, MOCK_CALENDAR, EVENT_URL);
+
+    const attachment = event.passthroughProps!.find(
+      ([k]) => k === "x-attachment"
+    );
+    expect(attachment).toEqual([
+      "x-attachment",
+      {},
+      "text",
+      "base64encodeddata==",
+    ]);
+  });
+
+  it("does NOT include known props in passthroughProps", () => {
+    const props = baseVeventProps(CUSTOM_PROPS);
+    const event = parseCalendarEvent(props, {}, MOCK_CALENDAR, EVENT_URL);
+
+    const known = ["uid", "summary", "dtstart", "dtend", "dtstamp", "sequence"];
+    const passthroughKeys = event.passthroughProps!.map(([k]) =>
+      k.toLowerCase()
+    );
+    for (const k of known) {
+      expect(passthroughKeys).not.toContain(k);
+    }
+  });
+
+  it("sets passthroughProps to empty array when no unknown props exist", () => {
+    const event = parseCalendarEvent(
+      baseVeventProps(),
+      {},
+      MOCK_CALENDAR,
+      EVENT_URL
+    );
+    expect(event.passthroughProps).toEqual([]);
+  });
+
+  it("parses known fields correctly alongside custom props", () => {
+    const props = baseVeventProps(CUSTOM_PROPS);
+    const event = parseCalendarEvent(props, {}, MOCK_CALENDAR, EVENT_URL);
+
+    expect(event.title).toBe("Team standup");
+    expect(event.uid).toBe("event-abc-uid");
+    expect(event.allday).toBe(false);
+  });
+});
+
+describe("makeVevent — passthroughProps round-trip", () => {
+  it("injects passthroughProps into the vevent output", () => {
+    const event = baseCalendarEvent({ passthroughProps: CUSTOM_PROPS });
+    const [, props] = makeVevent(event, "Europe/Paris", undefined);
+
+    const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+    expect(keys).toContain("x-attachment");
+    expect(keys).toContain("x-apple-travel-title");
+    expect(keys).toContain("x-custom-field");
+  });
+
+  it("preserves the exact value of passthrough props", () => {
+    const event = baseCalendarEvent({ passthroughProps: CUSTOM_PROPS });
+    const [, props] = makeVevent(event, "Europe/Paris", undefined);
+
+    const attachment = (props as VObjectProperty[]).find(
+      ([k]) => k === "x-attachment"
+    );
+    expect(attachment?.[3]).toBe("base64encodeddata==");
+  });
+
+  it("does NOT duplicate a prop that is already written by makeVevent", () => {
+    // If somehow a passthrough prop overlaps a known key, it must not be doubled
+    const duplicateSummary: VObjectProperty = [
+      "summary",
+      {},
+      "text",
+      "duplicate",
+    ];
+    const event = baseCalendarEvent({ passthroughProps: [duplicateSummary] });
+    const [, props] = makeVevent(event, "Europe/Paris", undefined);
+
+    const summaries = (props as VObjectProperty[]).filter(
+      ([k]) => k === "summary"
+    );
+    expect(summaries.length).toBe(1);
+  });
+
+  it("produces a vevent with no passthroughProps when field is undefined", () => {
+    const event = baseCalendarEvent({ passthroughProps: undefined });
+    const [name, props] = makeVevent(event, "Europe/Paris", undefined);
+
+    expect(name).toBe("vevent");
+    expect(Array.isArray(props)).toBe(true);
+  });
+
+  it("produces a vevent with no passthroughProps when field is empty array", () => {
+    const event = baseCalendarEvent({ passthroughProps: [] });
+    const [, props] = makeVevent(event, "Europe/Paris", undefined);
+    // Just check it doesn't throw and still has standard props
+    const keys = (props as VObjectProperty[]).map(([k]) => k);
+    expect(keys).toContain("uid");
+    expect(keys).toContain("summary");
+  });
+});
+
+describe("calendarEventToJCal", () => {
+  it("preserves passthroughProps inside the vcalendar output", () => {
+    const event = baseCalendarEvent({ passthroughProps: CUSTOM_PROPS });
+    const [, , children] = calendarEventToJCal(event);
+    const vevent = (children as unknown[][]).find((c) => c[0] === "vevent");
+    const veventProps = vevent![1] as VObjectProperty[];
+    const keys = veventProps.map(([k]) => k.toLowerCase());
+
+    expect(keys).toContain("x-attachment");
+
+    expect(keys).toContain("x-custom-field");
+  });
+});
+
+describe("Standard event modification", () => {
+  describe("title update", () => {
+    it("updates title while preserving custom props", () => {
+      const event = baseCalendarEvent({
+        title: "Old title",
+        passthroughProps: CUSTOM_PROPS,
+      });
+      const updated = { ...event, title: "New title" };
+      const [, props] = makeVevent(updated, updated.timezone, undefined);
+
+      const summary = (props as VObjectProperty[]).find(
+        ([k]) => k === "summary"
+      );
+      expect(summary?.[3]).toBe("New title");
+
+      const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+      expect(keys).toContain("x-attachment");
+    });
+  });
+
+  describe("time update", () => {
+    it("updates dtstart/dtend while preserving custom props", () => {
+      const event = baseCalendarEvent({
+        start: "2024-03-16T08:00:00.000Z",
+        end: "2024-03-16T09:00:00.000Z",
+        passthroughProps: CUSTOM_PROPS,
+      });
+      const [, props] = makeVevent(event, event.timezone, undefined);
+
+      const dtstart = (props as VObjectProperty[]).find(
+        ([k]) => k === "dtstart"
+      );
+      expect(dtstart).toBeDefined();
+
+      const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+      expect(keys).toContain("x-attachment");
+    });
+  });
+
+  describe("description update", () => {
+    it("adds description while preserving custom props", () => {
+      const event = baseCalendarEvent({
+        description: "Updated description",
+        passthroughProps: CUSTOM_PROPS,
+      });
+      const [, props] = makeVevent(event, event.timezone, undefined);
+
+      const desc = (props as VObjectProperty[]).find(
+        ([k]) => k === "description"
+      );
+      expect(desc?.[3]).toBe("Updated description");
+
+      const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+      expect(keys).toContain("x-attachment");
+    });
+  });
+
+  describe("location update", () => {
+    it("adds location while preserving custom props", () => {
+      const event = baseCalendarEvent({
+        location: "Conference Room B",
+        passthroughProps: CUSTOM_PROPS,
+      });
+      const [, props] = makeVevent(event, event.timezone, undefined);
+
+      const loc = (props as VObjectProperty[]).find(([k]) => k === "location");
+      expect(loc?.[3]).toBe("Conference Room B");
+
+      const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+      expect(keys).toContain("x-attachment");
+    });
+  });
+
+  describe("attendee update", () => {
+    it("adds attendee while preserving custom props", () => {
+      const event = baseCalendarEvent({
+        attendee: [
+          {
+            cal_address: "alice@example.com",
+            cn: "Alice",
+            partstat: "ACCEPTED",
+            rsvp: "TRUE",
+            role: "REQ-PARTICIPANT",
+            cutype: "INDIVIDUAL",
+          },
+        ],
+        passthroughProps: CUSTOM_PROPS,
+      });
+      const [, props] = makeVevent(
+        event,
+        event.timezone,
+        "organizer@example.com"
+      );
+
+      const attendees = (props as VObjectProperty[]).filter(
+        ([k]) => k === "attendee"
+      );
+      expect(attendees.length).toBe(1);
+      expect(String(attendees[0][3])).toContain("alice@example.com");
+
+      const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+      expect(keys).toContain("x-attachment");
+    });
+  });
+
+  describe("all-day event", () => {
+    it("formats dates without time component and preserves custom props", () => {
+      const event = baseCalendarEvent({
+        allday: true,
+        start: "2024-03-15T00:00:00.000Z",
+        end: "2024-03-16T00:00:00.000Z",
+        passthroughProps: CUSTOM_PROPS,
+      });
+      const [, props] = makeVevent(event, event.timezone, undefined);
+
+      const dtstart = (props as VObjectProperty[]).find(
+        ([k]) => k === "dtstart"
+      );
+      // All-day values should be date strings (no T separator)
+      expect(String(dtstart?.[3])).not.toContain("T");
+
+      const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+      expect(keys).toContain("x-attachment");
+    });
+  });
+
+  describe("timezone change", () => {
+    it("switches tzid param while preserving custom props", () => {
+      const event = baseCalendarEvent({
+        timezone: "America/New_York",
+        passthroughProps: CUSTOM_PROPS,
+      });
+      const [, props] = makeVevent(event, "America/New_York", undefined);
+
+      const dtstart = (props as VObjectProperty[]).find(
+        ([k]) => k === "dtstart"
+      );
+      expect((dtstart?.[1] as Record<string, string>)?.tzid).toBe(
+        "America/New_York"
+      );
+
+      const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+      expect(keys).toContain("x-attachment");
+    });
+  });
+});
+
+describe("Recurring event — makeVevent as master", () => {
+  const recurringBase = () =>
+    baseCalendarEvent({
+      repetition: { freq: "weekly", byday: ["MO", "WE", "FR"], interval: 1 },
+      passthroughProps: CUSTOM_PROPS,
+    });
+
+  it("includes rrule in the output", () => {
+    const [, props] = makeVevent(
+      recurringBase(),
+      "Europe/Paris",
+      undefined,
+      true
+    );
+    const rrule = (props as VObjectProperty[]).find(([k]) => k === "rrule");
+    expect(rrule).toBeDefined();
+    expect((rrule?.[3] as Record<string, unknown>)?.freq).toBe("weekly");
+  });
+
+  it("preserves custom props on master vevent", () => {
+    const [, props] = makeVevent(
+      recurringBase(),
+      "Europe/Paris",
+      undefined,
+      true
+    );
+    const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+    expect(keys).toContain("x-attachment");
+    expect(keys).toContain("x-custom-field");
+  });
+
+  it("does NOT include recurrence-id on master event", () => {
+    const [, props] = makeVevent(
+      recurringBase(),
+      "Europe/Paris",
+      undefined,
+      true
+    );
+    const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+    expect(keys).not.toContain("recurrence-id");
+  });
+});
+
+describe("Recurring event — makeVevent as override (single instance)", () => {
+  const overrideBase = () =>
+    baseCalendarEvent({
+      recurrenceId: "2024-03-20T09:00:00",
+      uid: "event-abc-uid/2024-03-20T09:00:00",
+      passthroughProps: CUSTOM_PROPS,
+    });
+
+  it("preserves custom props on override vevent", () => {
+    const [, props] = makeVevent(
+      overrideBase(),
+      "Europe/Paris",
+      undefined,
+      false
+    );
+    const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+    expect(keys).toContain("x-attachment");
+  });
+});
+
+describe("Recurring event — exdate (delete instance)", () => {
+  it("adds exdate entries and preserves custom props", () => {
+    const event = baseCalendarEvent({
+      exdates: ["2024-03-20T09:00:00.000Z", "2024-03-27T09:00:00.000Z"],
+      passthroughProps: CUSTOM_PROPS,
+    });
+    const [, props] = makeVevent(event, "Europe/Paris", undefined, true);
+
+    const exdates = (props as VObjectProperty[]).filter(
+      ([k]) => k === "exdate"
+    );
+    expect(exdates.length).toBe(2);
+
+    const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+    expect(keys).toContain("x-attachment");
+  });
+});
+
+describe("Edge cases", () => {
+  it("handles multiple custom props of the same name", () => {
+    const multiAttach: VObjectProperty[] = [
+      ["x-attachment", {}, "text", "file1=="],
+      ["x-attachment", {}, "text", "file2=="],
+    ];
+    const event = parseCalendarEvent(
+      baseVeventProps(multiAttach),
+      {},
+      MOCK_CALENDAR,
+      EVENT_URL
+    );
+    expect(
+      event.passthroughProps?.filter(([k]) => k === "x-attachment").length
+    ).toBe(2);
+
+    const updated = baseCalendarEvent({
+      passthroughProps: event.passthroughProps,
+    });
+    const [, props] = makeVevent(updated, updated.timezone, undefined);
+    const attachments = (props as VObjectProperty[]).filter(
+      ([k]) => k === "x-attachment"
+    );
+    expect(attachments.length).toBe(2);
+  });
+
+  it("handles passthrough prop with complex params object", () => {
+    const complexProp: VObjectProperty = [
+      "x-complex",
+      { encoding: "BASE64", fmttype: "application/pdf" },
+      "binary",
+      "binarydata==",
+    ];
+    const event = baseCalendarEvent({ passthroughProps: [complexProp] });
+    const [, props] = makeVevent(event, event.timezone, undefined);
+
+    const found = (props as VObjectProperty[]).find(([k]) => k === "x-complex");
+    expect(found?.[1]).toEqual({
+      encoding: "BASE64",
+      fmttype: "application/pdf",
+    });
+    expect(found?.[3]).toBe("binarydata==");
+  });
+
+  it("round-trips without mutation: passthroughProps array is not modified", () => {
+    const originalProps = [...CUSTOM_PROPS];
+    const event = baseCalendarEvent({ passthroughProps: CUSTOM_PROPS });
+    makeVevent(event, event.timezone, undefined);
+
+    // Original array must be untouched
+    expect(CUSTOM_PROPS).toEqual(originalProps);
+  });
+
+  it("handles event with alarm alongside passthroughProps", () => {
+    const event = baseCalendarEvent({
+      alarm: { trigger: "-PT15M", action: "EMAIL" },
+      passthroughProps: CUSTOM_PROPS,
+    });
+    const [, props, subComponents] = makeVevent(
+      event,
+      event.timezone,
+      "owner@example.com"
+    );
+    expect(Array.isArray(subComponents)).toBe(true);
+
+    const keys = (props as VObjectProperty[]).map(([k]) => k.toLowerCase());
+    expect(keys).toContain("x-attachment");
+  });
+});
+
+describe("putEvent - passthroughProps preservation", () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it("includes passthroughProps in the PUT body", async () => {
+    const mockResponse = { status: 201, url: "/dav/cals/test.ics" };
+    (api as unknown as jest.Mock).mockReturnValue(mockResponse);
+
+    const eventWithPassthrough = {
+      ...mockEvent,
+      passthroughProps: [
+        ["x-attachment", {}, "text", "data:application/pdf;base64,abc123=="],
+      ],
+    } as CalendarEvent;
+
+    await putEvent(eventWithPassthrough);
+
+    const callArgs = (api as unknown as jest.Mock).mock.calls[0][1];
+    const body = JSON.parse(callArgs.body);
+    const veventProps = body[2][0][1];
+
+    expect(veventProps).toContainEqual([
+      "x-attachment",
+      {},
+      "text",
+      "data:application/pdf;base64,abc123==",
+    ]);
+  });
+
+  it("sends clean body without passthroughProps when none exist", async () => {
+    const mockResponse = { status: 201, url: "/dav/cals/test.ics" };
+    (api as unknown as jest.Mock).mockReturnValue(mockResponse);
+
+    await putEvent({ ...mockEvent, passthroughProps: [] } as CalendarEvent);
+
+    const callArgs = (api as unknown as jest.Mock).mock.calls[0][1];
+    const body = JSON.parse(callArgs.body);
+    const veventProps = body[2][0][1] as [string, ...unknown[]][];
+
+    const xProps = veventProps.filter(([k]) => k.startsWith("x-attachment"));
+    expect(xProps).toHaveLength(0);
+  });
+});
+
+describe("putEventWithOverrides - passthroughProps isolation", () => {
+  const icsWithPassthrough = `BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Test//EN
+BEGIN:VEVENT
+UID:event1
+DTSTART:20240201T100000Z
+DTEND:20240201T110000Z
+RRULE:FREQ=DAILY
+SUMMARY:Master
+SEQUENCE:1
+END:VEVENT
+BEGIN:VEVENT
+UID:event1
+RECURRENCE-ID:20240202T100000Z
+DTSTART:20240202T100000Z
+DTEND:20240202T110000Z
+SUMMARY:Override
+X-ATTACHMENT:data:application/pdf;base64,abc123==
+SEQUENCE:1
+END:VEVENT
+END:VCALENDAR
+`;
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    (api.get as jest.Mock).mockResolvedValue({
+      text: jest.fn().mockResolvedValue(icsWithPassthrough),
+    });
+    (api as jest.Mock).mockResolvedValue({ ok: true });
+  });
+
+  it("preserves X-ATTACHMENT on the updated instance in the PUT body", async () => {
+    const updatedInstance = {
+      ...mockEvent,
+      title: "Override updated",
+      recurrenceId: "20240202T100000Z",
+      start: "2024-02-02T10:00:00Z",
+      end: "2024-02-02T11:00:00Z",
+      passthroughProps: [
+        ["x-attachment", {}, "text", "data:application/pdf;base64,abc123=="],
+      ] as VObjectProperty[],
+    } as CalendarEvent;
+
+    await putEventWithOverrides(updatedInstance);
+
+    const body = JSON.parse((api as jest.Mock).mock.calls[0][1].body);
+    const vevents = body[2].filter(([n]: any) => n === "vevent");
+    const override = vevents.find(([, props]: any) =>
+      props.some(([k]: any) => k === "recurrence-id")
+    );
+
+    const attachment = override[1].find(([k]: any) => k === "x-attachment");
+    expect(attachment).toBeDefined();
+    expect(attachment[3]).toBe("data:application/pdf;base64,abc123==");
+  });
+
+  it("does not inject instance passthroughProps into the master vevent", async () => {
+    const updatedInstance = {
+      ...mockEvent,
+      title: "Override updated",
+      recurrenceId: "20240202T100000Z",
+      start: "2024-02-02T10:00:00Z",
+      end: "2024-02-02T11:00:00Z",
+      passthroughProps: [
+        ["x-attachment", {}, "text", "data:application/pdf;base64,abc123=="],
+      ] as VObjectProperty[],
+    } as CalendarEvent;
+
+    await putEventWithOverrides(updatedInstance);
+
+    const body = JSON.parse((api as jest.Mock).mock.calls[0][1].body);
+    const vevents = body[2].filter(([n]: any) => n === "vevent");
+    const master = vevents.find(([, props]: any) =>
+      props.every(([k]: any) => k !== "recurrence-id")
+    );
+
+    const attachment = master[1].find(([k]: any) => k === "x-attachment");
+    expect(attachment).toBeUndefined();
+  });
+
+  it("preserves raw X-ATTACHMENT on untouched instances from the ICS", async () => {
+    const icsWithTwoOverrides = `BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:event1
+DTSTART:20240201T100000Z
+DTEND:20240201T110000Z
+RRULE:FREQ=DAILY
+SUMMARY:Master
+SEQUENCE:1
+END:VEVENT
+BEGIN:VEVENT
+UID:event1
+RECURRENCE-ID:20240202T100000Z
+DTSTART:20240202T100000Z
+DTEND:20240202T110000Z
+SUMMARY:Override A
+X-ATTACHMENT:data:application/pdf;base64,aaa==
+SEQUENCE:1
+END:VEVENT
+BEGIN:VEVENT
+UID:event1
+RECURRENCE-ID:20240203T100000Z
+DTSTART:20240203T100000Z
+DTEND:20240203T110000Z
+SUMMARY:Override B
+X-ATTACHMENT:data:application/pdf;base64,bbb==
+SEQUENCE:1
+END:VEVENT
+END:VCALENDAR
+`;
+
+    (api.get as jest.Mock).mockResolvedValueOnce({
+      text: jest.fn().mockResolvedValue(icsWithTwoOverrides),
+    });
+
+    // Only updating Override A — Override B should be untouched
+    const updatedInstance = {
+      ...mockEvent,
+      title: "Override A updated",
+      recurrenceId: "20240202T100000Z",
+      start: "2024-02-02T10:00:00Z",
+      end: "2024-02-02T11:00:00Z",
+      passthroughProps: [
+        ["x-attachment", {}, "text", "data:application/pdf;base64,aaa=="],
+      ] as VObjectProperty[],
+    } as CalendarEvent;
+
+    await putEventWithOverrides(updatedInstance);
+
+    const body = JSON.parse((api as jest.Mock).mock.calls[0][1].body);
+    const vevents = body[2].filter(([n]: any) => n === "vevent");
+
+    const overrideB = vevents.find(([, props]: any) =>
+      props.some(
+        ([k, , , v]: any) =>
+          k === "recurrence-id" && v === "2024-02-03T10:00:00Z"
+      )
+    );
+
+    // Override B was never touched — its raw jCal props come straight from the ICS
+    const attachmentB = overrideB[1].find(([k]: any) => k === "x-attachment");
+    expect(attachmentB).toBeDefined();
+    expect(attachmentB[3]).toBe("data:application/pdf;base64,bbb==");
+  });
+
+  it("does not bleed X-ATTACHMENT from one instance to another in the same PUT", async () => {
+    const icsWithTwoOverrides = `BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:event1
+DTSTART:20240201T100000Z
+DTEND:20240201T110000Z
+RRULE:FREQ=DAILY
+SUMMARY:Master
+SEQUENCE:1
+END:VEVENT
+BEGIN:VEVENT
+UID:event1
+RECURRENCE-ID:20240202T100000Z
+DTSTART:20240202T100000Z
+DTEND:20240202T110000Z
+SUMMARY:Override A
+X-ATTACHMENT:data:application/pdf;base64,aaa==
+SEQUENCE:1
+END:VEVENT
+BEGIN:VEVENT
+UID:event1
+RECURRENCE-ID:20240203T100000Z
+DTSTART:20240203T100000Z
+DTEND:20240203T110000Z
+SUMMARY:Override B - no attachment
+SEQUENCE:1
+END:VEVENT
+END:VCALENDAR
+`;
+
+    (api.get as jest.Mock).mockResolvedValueOnce({
+      text: jest.fn().mockResolvedValue(icsWithTwoOverrides),
+    });
+
+    const updatedInstance = {
+      ...mockEvent,
+      title: "Override A updated",
+      recurrenceId: "20240202T100000Z",
+      start: "2024-02-02T10:00:00Z",
+      end: "2024-02-02T11:00:00Z",
+      passthroughProps: [
+        ["x-attachment", {}, "text", "data:application/pdf;base64,aaa=="],
+      ] as VObjectProperty[],
+    } as CalendarEvent;
+
+    await putEventWithOverrides(updatedInstance);
+
+    const body = JSON.parse((api as jest.Mock).mock.calls[0][1].body);
+    const vevents = body[2].filter(([n]: any) => n === "vevent");
+
+    const overrideB = vevents.find(([, props]: any) =>
+      props.some(
+        ([k, , , v]: any) =>
+          k === "recurrence-id" && v === "2024-02-03T10:00:00Z"
+      )
+    );
+
+    const attachmentB = overrideB[1].find(([k]: any) => k === "x-attachment");
+    expect(attachmentB).toBeUndefined();
+  });
+});
diff --git a/src/features/Events/EventsTypes.ts b/src/features/Events/EventsTypes.ts
index 9ca1e2e..5e4c35c 100644
--- a/src/features/Events/EventsTypes.ts
+++ b/src/features/Events/EventsTypes.ts
@@ -1,4 +1,5 @@
 import { Calendar } from "../Calendars/CalendarTypes";
+import { VObjectProperty } from "../Calendars/types/CalendarData";
 import { userAttendee } from "../User/models/attendee";
 import { userOrganiser } from "../User/userDataTypes";
 
@@ -27,6 +28,7 @@ export interface CalendarEvent {
   repetition?: RepetitionObject;
   alarm?: AlarmObject;
   exdates?: string[];
+  passthroughProps?: VObjectProperty[];
 }
 
 export interface RepetitionObject {
diff --git a/src/features/Events/utils/makeVevent.ts b/src/features/Events/utils/makeVevent.ts
index 9398594..21ead92 100644
--- a/src/features/Events/utils/makeVevent.ts
+++ b/src/features/Events/utils/makeVevent.ts
@@ -121,5 +121,16 @@ export function makeVevent(
     });
   }
 
+  if (event.passthroughProps?.length) {
+    const existingKeys = new Set(
+      (vevent[1] as unknown[]).map((p) => (p as [string])[0].toLowerCase())
+    );
+    for (const prop of event.passthroughProps) {
+      if (!existingKeys.has(prop[0].toLowerCase())) {
+        vevent[1].push(prop);
+      }
+    }
+  }
+
   return vevent;
 }
diff --git a/src/features/Events/utils/parseCalendarEvent.ts b/src/features/Events/utils/parseCalendarEvent.ts
index ac8e4eb..c4ea9cb 100644
--- a/src/features/Events/utils/parseCalendarEvent.ts
+++ b/src/features/Events/utils/parseCalendarEvent.ts
@@ -9,9 +9,30 @@ import {
 import { userAttendee } from "../../User/models/attendee";
 import { createAttendee } from "../../User/models/attendee.mapper";
 import { AlarmObject, CalendarEvent } from "../EventsTypes";
-import { inferTimezoneFromValue } from "./inferTimezoneFromValue";
 import { buildDelegatedEventURL } from "./buildDelegatedEventURL";
 import { formatDateToICal } from "./formatDateToICal";
+import { inferTimezoneFromValue } from "./inferTimezoneFromValue";
+
+const KNOWN_PROPS = new Set([
+  "uid",
+  "transp",
+  "dtstart",
+  "dtend",
+  "class",
+  "x-openpaas-videoconference",
+  "summary",
+  "description",
+  "location",
+  "organizer",
+  "attendee",
+  "dtstamp",
+  "sequence",
+  "recurrence-id",
+  "exdate",
+  "status",
+  "duration",
+  "rrule",
+]);
 
 export function parseCalendarEvent(
   data: VObjectProperty[],
@@ -211,6 +232,9 @@ export function parseCalendarEvent(
       event.end = endISO;
     }
   }
+  event.passthroughProps = data.filter(
+    ([key]) => !KNOWN_PROPS.has(key.toLowerCase())
+  );
 
   return event as CalendarEvent;
 }