diff --git a/api/client-server/administrative_contact.yaml b/api/client-server/administrative_contact.yaml
index 4b8e1d631..cce0b261f 100644
--- a/api/client-server/administrative_contact.yaml
+++ b/api/client-server/administrative_contact.yaml
@@ -112,7 +112,10 @@ paths:
                     description: The identity server to use.
                   id_access_token:
                     type: string
-                    description: An access token previously registered with the identity server.
+                    description: |-
+                      An access token previously registered with the identity server. Servers
+                      can treat this as optional to distinguish between r0.5-compatible clients
+                      and this specification version.
                   sid:
                     type: string
                     description: The session identifier given by the identity server.
diff --git a/api/client-server/create_room.yaml b/api/client-server/create_room.yaml
index d7a29d995..2487707ea 100644
--- a/api/client-server/create_room.yaml
+++ b/api/client-server/create_room.yaml
@@ -141,7 +141,10 @@ paths:
                       description: The hostname+port of the identity server which should be used for third party identifier lookups.
                     id_access_token:
                       type: string
-                      description: An access token previously registered with the identity server.
+                      description: |-
+                        An access token previously registered with the identity server. Servers
+                        can treat this as optional to distinguish between r0.5-compatible clients
+                        and this specification version.
                     medium:
                       type: string
                       # TODO: Link to Identity Service spec when it eixsts
diff --git a/api/client-server/definitions/request_email_validation.yaml b/api/client-server/definitions/request_email_validation.yaml
index 63e3ee215..2b2705145 100644
--- a/api/client-server/definitions/request_email_validation.yaml
+++ b/api/client-server/definitions/request_email_validation.yaml
@@ -25,5 +25,8 @@ allOf:
       example: "id.example.com"
     id_access_token:
       type: string
-      description: An access token previously registered with the identity server.
+      description: |-
+        An access token previously registered with the identity server. Servers
+        can treat this as optional to distinguish between r0.5-compatible clients
+        and this specification version.
   required: ["id_server", "id_access_token"]
diff --git a/api/client-server/definitions/request_msisdn_validation.yaml b/api/client-server/definitions/request_msisdn_validation.yaml
index 435136288..b013a5612 100644
--- a/api/client-server/definitions/request_msisdn_validation.yaml
+++ b/api/client-server/definitions/request_msisdn_validation.yaml
@@ -25,5 +25,8 @@ allOf:
       example: "id.example.com"
     id_access_token:
       type: string
-      description: An access token previously registered with the identity server.
+      description: |-
+        An access token previously registered with the identity server. Servers
+        can treat this as optional to distinguish between r0.5-compatible clients
+        and this specification version.
   required: ["id_server", "id_access_token"]
diff --git a/api/client-server/third_party_membership.yaml b/api/client-server/third_party_membership.yaml
index 075cd34be..4c5890d1b 100644
--- a/api/client-server/third_party_membership.yaml
+++ b/api/client-server/third_party_membership.yaml
@@ -102,7 +102,10 @@ paths:
                 description: The hostname+port of the identity server which should be used for third party identifier lookups.
               id_access_token:
                 type: string
-                description: An access token previously registered with the identity server.
+                description: |-
+                  An access token previously registered with the identity server. Servers
+                  can treat this as optional to distinguish between r0.5-compatible clients
+                  and this specification version.
               medium:
                 type: string
                 # TODO: Link to Identity Service spec when it eixsts