diff --git a/src/resources/extensions/gsd/tests/visualizer-data.test.ts b/src/resources/extensions/gsd/tests/visualizer-data.test.ts
index 3aec834e1..85f740db7 100644
--- a/src/resources/extensions/gsd/tests/visualizer-data.test.ts
+++ b/src/resources/extensions/gsd/tests/visualizer-data.test.ts
@@ -165,6 +165,17 @@ assertTrue(
   "VisualizerData has changelog field",
 );
 
+// completedAt must be coerced to String() to handle YAML Date objects (issue #644)
+assertTrue(
+  dataSrc.includes("String(summary.frontmatter.completed_at"),
+  "completedAt assignment coerces to String() for YAML Date safety",
+);
+
+assertTrue(
+  dataSrc.includes("String(b.completedAt") && dataSrc.includes("String(a.completedAt"),
+  "changelog sort coerces completedAt to String() for YAML Date safety",
+);
+
 // Verify overlay source exists and imports data module
 const overlayPath = join(__dirname, "..", "visualizer-overlay.ts");
 const overlaySrc = readFileSync(overlayPath, "utf-8");
diff --git a/src/resources/extensions/gsd/visualizer-data.ts b/src/resources/extensions/gsd/visualizer-data.ts
index 5abf82e01..3e0c75182 100644
--- a/src/resources/extensions/gsd/visualizer-data.ts
+++ b/src/resources/extensions/gsd/visualizer-data.ts
@@ -379,7 +379,7 @@ async function loadChangelog(basePath: string, milestones: VisualizerMilestone[]
           path: f.path,
           description: f.description,
         })),
-        completedAt: summary.frontmatter.completed_at ?? '',
+        completedAt: String(summary.frontmatter.completed_at ?? ''),
       };
 
       changelogCache.set(cacheKey, { mtime, entry });
@@ -388,7 +388,7 @@ async function loadChangelog(basePath: string, milestones: VisualizerMilestone[]
   }
 
   // Sort by completedAt descending
-  entries.sort((a, b) => (b.completedAt || '').localeCompare(a.completedAt || ''));
+  entries.sort((a, b) => String(b.completedAt || '').localeCompare(String(a.completedAt || '')));
 
   return { entries };
 }