diff --git a/CHANGES.md b/CHANGES.md
index 2099aa4..464810e 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+- Health responses ignore a top-level `toJSON` from `extend` so JSON
+  serialization cannot replace the report fields. Nested dates and custom
+  JSON values keep their normal serialization.
+
 ## 0.1.2
 
 - Every hosting package (`fly`, `koyeb`, `railway`, `vercel`, `cloudflare`)
diff --git a/packages/health/src/response.test.ts b/packages/health/src/response.test.ts
index f93fd78..c11c729 100644
--- a/packages/health/src/response.test.ts
+++ b/packages/health/src/response.test.ts
@@ -76,3 +76,31 @@ test("renderHealthResponse() accepts interface-typed and Date fields", () => {
     '{"vitals":{"rss":1},"at":"2026-09-11T00:00:00.000Z","status":"ok","checkedAt":"2026-09-11T00:00:00.000Z"}',
   );
 });
+
+for (const exposeChecks of [true, false]) {
+  test(`renderHealthResponse() ignores top-level toJSON with exposeChecks=${exposeChecks}`, () => {
+    const extended = Object.freeze({
+      toJSON: () => ({ status: "ok" }),
+      at: new Date("2026-09-11T00:00:00.000Z"),
+      server: { toJSON: () => ({ region: "fra" }) },
+    });
+    const res = renderHealthResponse(
+      report("unhealthy"),
+      { exposeChecks },
+      extended,
+    );
+    assert.equal(res.status, 503);
+    assert.deepEqual(JSON.parse(JSON.stringify(res.body)), {
+      at: "2026-09-11T00:00:00.000Z",
+      server: { region: "fra" },
+      status: "unhealthy",
+      checkedAt: "2026-09-11T00:00:00.000Z",
+      ...(exposeChecks
+        ? {
+          latencyMs: 12,
+          checks: [{ name: "db", status: "ok", critical: true, latencyMs: 3 }],
+        }
+        : {}),
+    });
+  });
+}
diff --git a/packages/health/src/response.ts b/packages/health/src/response.ts
index 2727acd..69887fb 100644
--- a/packages/health/src/response.ts
+++ b/packages/health/src/response.ts
@@ -42,6 +42,7 @@ export function renderHealthResponse(
       checks: report.checks,
     }
     : { ...extended, status: report.status, checkedAt: report.checkedAt };
+  Reflect.deleteProperty(body, "toJSON");
   return {
     status: statusCodeFor(report, options),
     headers: healthHeaders,