diff --git a/CHANGELOG.md b/CHANGELOG.md
index 325b3633..9a4e40c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased]
 
+### Added
+
+- Warning when building if links to markdown files cannot be resolved ([#691](https://github.com/jpanther/congo/pull/691))
+
 ### Removed
 
 - Fathom Analytics custom domain parameter as this is no longer supported by Fathom
diff --git a/layouts/_default/_markup/render-link.html b/layouts/_default/_markup/render-link.html
index 079d8d37..141f8020 100644
--- a/layouts/_default/_markup/render-link.html
+++ b/layouts/_default/_markup/render-link.html
@@ -9,7 +9,7 @@
       {{ $link = printf "%s%s" .RelPermalink $fragment }}
     {{ else }}
       {{- if hasSuffix $url.Path ".md" }}
-        {{ warnf "Can't resolve: %s" .Destination }}
+        {{ warnf "[CONGO] Can't resolve: %s" .Destination }}
       {{ end -}}
     {{ end -}}
   {{ end -}}