diff --git a/docs/guides/upgrading/v5.1.0.md b/docs/guides/upgrading/v5.1.0.md
index c8cacf9..bc4f6d3 100644
--- a/docs/guides/upgrading/v5.1.0.md
+++ b/docs/guides/upgrading/v5.1.0.md
@@ -15,6 +15,14 @@
     if your deployment reports timeouts for operations that used to succeed. Setting it to `0` disables query timeouts entirely.
     Repeated timeouts point at a deeper problem, such as slow I/O, lagging autovacuum, or inefficient query plans.
     Report an issue if you have to increase the value permanently.
+
+* **New `X-Total-Count-Type` response header**. REST API v1 gains support for communicating bounded total counts
+  via the new `X-Total-Count-Type` response header. It must be accessible by the frontend, so users who overwrite
+  the default [`dt.cors.allowed-headers`](../../reference/configuration/properties.md#dtcorsallowed-headers)
+  property must add the header there manually. Bounded total counts are a performance optimization technique
+  for operating on large datasets. In this version, only the `/api/v1/finding`, `/api/v1/finding/grouped`,
+  and `/api/v1/finding/project/{uuid}` endpoints support it.
+
 * **Dependency-Track mirrors KEV catalogs by default**. A new task downloads the CISA and ENISA Known Exploited
   Vulnerabilities catalogs daily at 02:00. Dependency-Track turns on both data sources out of the box. Deployments
   with restricted egress must allow the feed hosts, or turn off the data sources they cannot reach. Change the