diff --git a/core/base/crdt-helpers.ts b/core/base/crdt-helpers.ts
index 2d7e865ce..04d2d84f1 100644
--- a/core/base/crdt-helpers.ts
+++ b/core/base/crdt-helpers.ts
@@ -40,7 +40,7 @@ import {
   PARAM,
   NotFoundError,
 } from "@fireproof/core-types-base";
-import { Logger } from "@adviser/cement";
+import { Logger, isUint8Array } from "@adviser/cement";
 import { Link, Version } from "multiformats";
 
 function toString<K extends IndexKeyType>(key: K, logger: Logger): string {
@@ -78,6 +78,10 @@ export function sanitizeDocumentFields<T>(obj: T): T {
       return item;
     }) as T;
   } else if (typeof obj === "object" && obj !== null) {
+    // Preserve Uint8Array for CBOR byte string encoding
+    if (isUint8Array(obj)) {
+      return obj;
+    }
     // Special case for Date objects - convert to ISO string
     if (obj instanceof Date) {
       return obj.toISOString() as unknown as T;
diff --git a/core/tests/fireproof/uint8array-roundtrip.test.ts b/core/tests/fireproof/uint8array-roundtrip.test.ts
new file mode 100644
index 000000000..8ae496f60
--- /dev/null
+++ b/core/tests/fireproof/uint8array-roundtrip.test.ts
@@ -0,0 +1,33 @@
+import { describe, it, expect, afterEach } from "vitest";
+import { fireproof } from "@fireproof/core-base";
+import type { Database } from "@fireproof/core-types-base";
+
+describe("Uint8Array document field round-trip", () => {
+  let db: Database;
+
+  afterEach(async () => {
+    await db.close();
+    await db.destroy();
+  });
+
+  it("preserves Uint8Array through put/get", async () => {
+    db = fireproof(`u8rt-basic-${Date.now()}`);
+    const data = new Uint8Array([1, 2, 3, 255, 0, 128]);
+    const { id } = await db.put({ type: "bin", payload: data });
+    const doc = await db.get<{ type: string; payload: Uint8Array }>(id);
+    expect(doc.payload).toBeInstanceOf(Uint8Array);
+    expect(new Uint8Array(doc.payload)).toEqual(data);
+  });
+
+  it("preserves Uint8Array nested inside an object", async () => {
+    // Without the fix, sanitizeDocumentFields iterates Uint8Array numeric keys
+    // and produces a plain object like {0: 10, 1: 20, 2: 30}
+    db = fireproof(`u8rt-nested-${Date.now()}`);
+    const data = new Uint8Array([10, 20, 30]);
+    const { id } = await db.put({ type: "nested", inner: { label: "test", data } });
+    const doc = await db.get<{ type: string; inner: { label: string; data: Uint8Array } }>(id);
+    expect(doc.inner.data).toBeInstanceOf(Uint8Array);
+    expect(new Uint8Array(doc.inner.data)).toEqual(data);
+    expect(doc.inner.label).toBe("test");
+  });
+});