diff --git a/CHANGES.rst b/CHANGES.rst
index f1434e8c..a978ea3d 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -1,6 +1,12 @@
 Changes
 =======
 
+Version 0.9.2 (UNRELEASED)
+--------------------------
+
+- Changes the OpenAPI specification of the ``start_workflow`` endpoint's response to add missing ``run_number`` field.
+- Fixes the ``start_workflow`` endpoint by changing the type of the ``run_number`` response field from a number to a string, in order to support restarting a workflow more than nine times.
+
 Version 0.9.1 (2023-09-27)
 --------------------------
 
diff --git a/docs/openapi.json b/docs/openapi.json
index 436a4250..3f8e018d 100644
--- a/docs/openapi.json
+++ b/docs/openapi.json
@@ -3234,6 +3234,9 @@
                 "message": {
                   "type": "string"
                 },
+                "run_number": {
+                  "type": "string"
+                },
                 "status": {
                   "type": "string"
                 },
diff --git a/reana_server/rest/workflows.py b/reana_server/rest/workflows.py
index de6f7861..d42ebd7e 100644
--- a/reana_server/rest/workflows.py
+++ b/reana_server/rest/workflows.py
@@ -1189,6 +1189,8 @@ def start_workflow(workflow_id_or_name, user):  # noqa
                 type: string
               status:
                 type: string
+              run_number:
+                type: string
               user:
                 type: string
           examples: