diff --git a/changelogs/client_server/newsfragments/2412.clarification b/changelogs/client_server/newsfragments/2412.clarification
new file mode 100644
index 00000000..4cf20f36
--- /dev/null
+++ b/changelogs/client_server/newsfragments/2412.clarification
@@ -0,0 +1 @@
+Use the User ID type in OlmPayload and DeviceKeys.
\ No newline at end of file
diff --git a/data/api/client-server/definitions/device_keys.yaml b/data/api/client-server/definitions/device_keys.yaml
index 2a7f8104..c96cc30d 100644
--- a/data/api/client-server/definitions/device_keys.yaml
+++ b/data/api/client-server/definitions/device_keys.yaml
@@ -20,6 +20,8 @@ properties:
     description: |-
       The ID of the user the device belongs to. Must match the user ID used
       when logging in.
+    format: mx-user-id
+    pattern: "^@"
     example: "@alice:example.com"
   device_id:
     type: string
diff --git a/data/api/client-server/definitions/olm_payload.yaml b/data/api/client-server/definitions/olm_payload.yaml
index d322129e..4ce6ce8a 100644
--- a/data/api/client-server/definitions/olm_payload.yaml
+++ b/data/api/client-server/definitions/olm_payload.yaml
@@ -27,9 +27,13 @@ properties:
   sender:
     type: string
     description: The user ID of the event sender.
+    format: mx-user-id
+    pattern: "^@"
   recipient:
     type: string
     description: The user ID of the intended event recipient.
+    format: mx-user-id
+    pattern: "^@"
   recipient_keys:
     description: The recipient's signing keys of the encrypted event.
     $ref: "#/components/schemas/SigningKeys"