diff --git a/changelogs/application_service/newsfragments/1584.clarification b/changelogs/application_service/newsfragments/1584.clarification
new file mode 100644
index 00000000..098c1260
--- /dev/null
+++ b/changelogs/application_service/newsfragments/1584.clarification
@@ -0,0 +1 @@
+Fix JSON schema of custom fields in query.
diff --git a/data/api/application-service/protocols.yaml b/data/api/application-service/protocols.yaml
index 59dff5b0..dddd26d4 100644
--- a/data/api/application-service/protocols.yaml
+++ b/data/api/application-service/protocols.yaml
@@ -96,12 +96,14 @@ paths:
           schema:
             type: string
         - in: query
-          name: fields...
+          name: fields
           description: |-
             One or more custom fields that are passed to the application
             service to help identify the user.
           schema:
-            type: string
+            type: object
+            additionalProperties:
+              type: string
       responses:
         "200":
           description: The Matrix User IDs found with the given parameters.
@@ -161,12 +163,14 @@ paths:
           schema:
             type: string
         - in: query
-          name: fields...
+          name: fields
           description: |-
             One or more custom fields that are passed to the application
             service to help identify the third-party location.
           schema:
-            type: string
+            type: object
+            additionalProperties:
+              type: string
       responses:
         "200":
           description: At least one portal room was found.
diff --git a/data/api/client-server/third_party_lookup.yaml b/data/api/client-server/third_party_lookup.yaml
index de1b0b63..797f065f 100644
--- a/data/api/client-server/third_party_lookup.yaml
+++ b/data/api/client-server/third_party_lookup.yaml
@@ -139,11 +139,13 @@ paths:
           schema:
             type: string
         - in: query
-          name: fields...
+          name: fields
           description: One or more custom fields that are passed to the AS to help
             identify the user.
           schema:
-            type: string
+            type: object
+            additionalProperties:
+              type: string
       responses:
         "200":
           description: The Matrix User IDs found with the given parameters.