diff --git a/openapi.yaml b/openapi.yaml
index 86ab236..2483c8b 100644
--- a/openapi.yaml
+++ b/openapi.yaml
@@ -255,6 +255,15 @@ paths:
               schema: { type: string, format: binary }
         "401": { $ref: "#/components/responses/Problem" }
         "404": { $ref: "#/components/responses/Problem" }
+        "503":
+          description: The report exists but its private diagnostic archive is unavailable.
+          content:
+            application/json:
+              schema: { $ref: "#/components/schemas/Problem" }
+              example:
+                contractVersion: 1
+                code: diagnostics_unavailable
+                message: The report is available, but its diagnostic archive is unavailable.
   /api/v1/admin/reports/{id}/messages:
     post:
       summary: Ask a reporter for more information