diff --git a/docs/nfc_auth.md b/docs/nfc_auth.md
index d42dc30..8de1ca5 100644
--- a/docs/nfc_auth.md
+++ b/docs/nfc_auth.md
@@ -125,12 +125,12 @@ A disk-scoped **read** ticket also works and returns the same
 ```
 
 `diskDeviceKey` is `VirtualDisk.key` from `vm.config.hardware.device`
-(2000 for Hard disk 1). The replacement resolves it from the datastore
-path when `open` is given a VMDK rather than a key. That path may be
-the current leaf or a parent in `backing.parent` (snapshot deltas such
-as `…-000007.vmdk` after the VM has moved on to `…-000008.vmdk`). The
-ticket still uses the device key; `OPEN_FILE` then names the snapshot
-file.
+(2000 for Hard disk 1). Read-only `VixDiskLib_Open` does **not** send
+it: the drop-in uses `NfcGetVmFiles` and puts the VMDK path (leaf or
+snapshot parent) only on NFC `OPEN_FILE`. Writable `open` resolves a
+key from the datastore path, matching the current leaf or any parent
+in `backing.parent` (for example `…-000007.vmdk` after the VM has
+moved on to `…-000008.vmdk`).
 
 ### Return value: `vim.HostServiceTicket`
 
diff --git a/openvixdisklib/openvixdisklib.py b/openvixdisklib/openvixdisklib.py
index 9e6e582..6ab6040 100644
--- a/openvixdisklib/openvixdisklib.py
+++ b/openvixdisklib/openvixdisklib.py
@@ -253,6 +253,12 @@ class VixDiskLibHandle:
             flags: int = VIXDISKLIB_FLAG_OPEN_READ_ONLY) -> Iterator[_DiskHandle]:
         """Open ``disk_path`` over NFC. Matches ``VixDiskLib_Open``.
 
+        Read-only opens request ``NfcGetVmFiles`` (VM only). The VMDK
+        path, including a snapshot parent such as ``…-000007.vmdk``, is
+        sent on NFC ``OPEN_FILE``. Writable opens use
+        ``NfcRandomAccessOpenDisk`` and resolve a device key from the
+        disk's backing chain.
+
         Args:
             conn: Connection from ``connect``.
             disk_path: Datastore path of the VMDK.
@@ -271,7 +277,8 @@ class VixDiskLibHandle:
         vm = vim.VirtualMachine(conn.vm_moref, conn.si._stub)
         nfc_ssl = conn.transport_mode == "nbdssl"
         ticket = nfc_auth.get_nfc_ticket(
-            conn.si, vm, read_only=read_only, disk_path=disk_path)
+            conn.si, vm, read_only=read_only,
+            disk_path=None if read_only else disk_path)
         authd_sock = nfc_auth.connect_authd(
             ticket, allow_untrusted=conn.allow_untrusted, nfc_ssl=nfc_ssl)
         session = nfc_auth.NfcAuthSession(