diff --git a/data/api/client-server/content-repo.yaml b/data/api/client-server/content-repo.yaml
index 382b7c9f..e6d55b51 100644
--- a/data/api/client-server/content-repo.yaml
+++ b/data/api/client-server/content-repo.yaml
@@ -42,7 +42,8 @@ paths:
                 properties:
                   content_uri:
                     type: string
-                    format: uri
+                    format: mx-mxc-uri
+                    pattern: "^mxc:\\/\\/"
                     description: The [`mxc://` URI](/client-server-api/#matrix-content-mxc-uris) to
                       the uploaded content.
               examples:
@@ -185,7 +186,8 @@ paths:
                 properties:
                   content_uri:
                     type: string
-                    format: uri
+                    format: mx-mxc-uri
+                    pattern: "^mxc:\\/\\/"
                     description: |-
                       The [`mxc://` URI](/client-server-api/#matrix-content-mxc-uris) at
                       which the content will be available, once it is uploaded.
diff --git a/data/custom-formats.yaml b/data/custom-formats.yaml
index 53432232..7d52861f 100644
--- a/data/custom-formats.yaml
+++ b/data/custom-formats.yaml
@@ -42,6 +42,11 @@ mx-server-name:
   url: /appendices#server-name
   # no regex
 
+mx-mxc-uri:
+  title: Matrix Content URI
+  url: /client-server-api#matrix-content-mxc-uris
+  # regex: "^mxc:\\/\\/"
+
 uri:
   title: URI
   url: http://tools.ietf.org/html/rfc3986