diff --git a/packages/mcp-core/src/api-client/schema.test.ts b/packages/mcp-core/src/api-client/schema.test.ts
index 4ab11b43..b46c6895 100644
--- a/packages/mcp-core/src/api-client/schema.test.ts
+++ b/packages/mcp-core/src/api-client/schema.test.ts
@@ -32,6 +32,31 @@ describe("IssueSchema", () => {
     expect(result).toEqual(errorIssue);
   });
 
+  it("should allow issues with null lastSeen", () => {
+    const staleIssue = {
+      id: "321",
+      shortId: "TEST-321",
+      title: "Intermittent crash",
+      firstSeen: "2025-01-01T00:00:00Z",
+      lastSeen: null,
+      count: 0,
+      userCount: 0,
+      permalink: "https://sentry.io/issues/321/",
+      project: {
+        id: "1",
+        name: "stale-project",
+        slug: "stale-project",
+      },
+      status: "resolved",
+      culprit: "stale.ts",
+      type: "error",
+    };
+
+    const result = IssueSchema.parse(staleIssue);
+
+    expect(result.lastSeen).toBeNull();
+  });
+
   it("should parse a regressed performance issue", () => {
     // Anonymized payload from real regressed issue (issue #633)
     const regressedIssue = {
diff --git a/packages/mcp-core/src/api-client/schema.ts b/packages/mcp-core/src/api-client/schema.ts
index de23c85d..4647db18 100644
--- a/packages/mcp-core/src/api-client/schema.ts
+++ b/packages/mcp-core/src/api-client/schema.ts
@@ -194,7 +194,7 @@ export const IssueSchema = z
     shortId: z.string(),
     title: z.string(),
     firstSeen: z.string().datetime(),
-    lastSeen: z.string().datetime(),
+    lastSeen: z.string().datetime().nullable(),
     count: z.union([z.string(), z.number()]),
     userCount: z.union([z.string(), z.number()]),
     permalink: z.string().url(),