diff --git a/fuzz/deep_fuzz.ts b/fuzz/deep_fuzz.ts
index b22a611..66a1f6b 100644
--- a/fuzz/deep_fuzz.ts
+++ b/fuzz/deep_fuzz.ts
@@ -3,6 +3,7 @@ import * as Persistency from "../src/persist.ts";
 import { createHandler } from "../src/handler.ts";
 import { RateLimiter } from "../src/rate_limiter.ts";
 import { parseConfig, ConfigError } from "../src/config.ts";
+import type { JsonValue } from "../src/payload.ts";
 
 console.log("=== STARTING DEEP PROPERTY & INVARIANT FUZZING ===");
 
@@ -183,7 +184,7 @@ console.log("\n--- Testing Persistence Invariants ---");
 {
     const tempDir = await Deno.makeTempDir();
     try {
-        const store = new Persistency.FileStore<unknown>();
+        const store = new Persistency.FileStore<JsonValue>();
         store.dir(tempDir);
         const mgr = new QueueManager(store, 100, 50, true);
 
@@ -191,14 +192,14 @@ console.log("\n--- Testing Persistence Invariants ---");
         mgr.enqueue("q1", { a: [1, 2, { b: "hello" }], c: true });
         mgr.enqueue("q1", "second");
         mgr.enqueue("q2", 42);
-        mgr.enqueue("q2", null as any); // if enqueued directly in manager
+        mgr.enqueue("q2", null); // if enqueued directly in manager
         mgr.enqueue("q3", "");
         mgr.dequeue("q1"); // dequeue "first"
 
         mgr.save();
 
         // Create fresh manager from same store
-        const store2 = new Persistency.FileStore<unknown>();
+        const store2 = new Persistency.FileStore<JsonValue>();
         store2.dir(tempDir);
         const mgr2 = new QueueManager(store2, 100, 50, true);
         mgr2.load();
diff --git a/src/persist.ts b/src/persist.ts
index b91f65a..05d425f 100644
--- a/src/persist.ts
+++ b/src/persist.ts
@@ -1,3 +1,5 @@
+import type { JsonValue } from "./payload.ts";
+
 export interface QueueEvent<T> {
     queue: string;
     payload: T;
@@ -39,7 +41,8 @@ export interface QueueStore<T = string> {
     close(): void;
 }
 
-export class FileStore<T = string> implements QueueStore<T> {
+/** Persists payloads as JSON; T must be JSON-compatible. */
+export class FileStore<T extends JsonValue = string> implements QueueStore<T> {
     private directory: string = '';
     private writeHandle: Deno.FsFile | null = null;
     private encoder = new TextEncoder();
diff --git a/tests/manager_test.ts b/tests/manager_test.ts
index 979c391..150be25 100644
--- a/tests/manager_test.ts
+++ b/tests/manager_test.ts
@@ -676,6 +676,35 @@ Deno.test("load snapshots remaining items so a second load restores them", () =>
     assertEquals(loaded2.dequeue("jobs"), "keep-me");
 });
 
+Deno.test("FileStore reloads nested JSON payloads through QueueManager", () => {
+    const tmpDir = Deno.makeTempDirSync();
+    try {
+        const expected: Payload = {
+            name: "job",
+            metadata: {
+                retries: 3,
+                active: true,
+                tags: ["urgent", "batch"],
+                lastError: null,
+            },
+        };
+        const firstStore = new Persistency.FileStore<Payload>();
+        firstStore.dir(tmpDir);
+        const firstManager = new QueueManager<Payload>(firstStore);
+        firstManager.enqueue("jobs", expected);
+        firstStore.close();
+
+        const reopenedStore = new Persistency.FileStore<Payload>();
+        reopenedStore.dir(tmpDir);
+        const reopenedManager = new QueueManager<Payload>(reopenedStore);
+        reopenedManager.load();
+        assertEquals(reopenedManager.peek("jobs"), expected);
+        reopenedStore.close();
+    } finally {
+        Deno.removeSync(tmpDir, { recursive: true });
+    }
+});
+
 Deno.test("FileStore load snapshots remaining items onto disk", () => {
     const tmp = Deno.makeTempDirSync();
     try {
diff --git a/tests/persist_test.ts b/tests/persist_test.ts
index 4eadec4..1d7c43c 100644
--- a/tests/persist_test.ts
+++ b/tests/persist_test.ts
@@ -6,6 +6,9 @@ import { RateLimiter } from "../src/rate_limiter.ts";
 import { parseConfig, ConfigError } from "../src/config.ts";
 import type { Payload } from "../src/payload.ts";
 
+// @ts-expect-error FileStore can only persist JSON-compatible payloads.
+new Persistency.FileStore<Date>();
+
 // Shared helpers
 const API_TOKEN = "test-token";
 const authHeaders = { "Authorization": `Bearer ${API_TOKEN}` };
@@ -180,6 +183,26 @@ Deno.test("persist FileStore.saveEvent() waits for an existing file lock", async
     Deno.removeSync(tmpDir, { recursive: true });
 });
 
+Deno.test("FileStore preserves the existing JSON-lines format", () => {
+    const tmpDir = Deno.makeTempDirSync();
+    const store = new Persistency.FileStore<Payload>();
+    store.dir(tmpDir);
+    try {
+        store.saveEvent("q", "hello", true);
+        store.saveEvent("q", { nested: [1, true, "x"] }, true);
+        store.close();
+
+        assertEquals(
+            Deno.readTextFileSync(tmpDir + "/persist.dat"),
+            '{"queue":"q","payload":"hello","enqueue":true,"dequeue":false}\n' +
+                '{"queue":"q","payload":{"nested":[1,true,"x"]},"enqueue":true,"dequeue":false}\n',
+        );
+    } finally {
+        store.close();
+        Deno.removeSync(tmpDir, { recursive: true });
+    }
+});
+
 // ── FileStore.clear() creates file if absent ─────────────────────────────────
 
 Deno.test("persist FileStore.clear() creates file when it does not exist", () => {
@@ -208,7 +231,7 @@ Deno.test("persist FileStore.clear() truncates existing content", () => {
 
 Deno.test("persist FileStore.loadState() reads large file correctly", () => {
     const tmpDir = Deno.makeTempDirSync();
-    const p = new Persistency.FileStore<any>();
+    const p = new Persistency.FileStore<{ payload: string; i: number }>();
     p.dir(tmpDir + "/");
     p.clear();
     for (let i = 0; i < 100; i++) {