diff --git a/frontend/sync-client/src/utils/assert-set-contains-exactly.ts b/frontend/sync-client/src/utils/assert-set-contains-exactly.ts
deleted file mode 100644
index 5baf965e..00000000
--- a/frontend/sync-client/src/utils/assert-set-contains-exactly.ts
+++ /dev/null
@@ -1,13 +0,0 @@
-import assert from "node:assert";
-
-export function assertSetContainsExactly<T>(set: Set<T>, ...values: T[]): void {
-    assert.ok(
-        set.size === values.length &&
-            Array.from(set).every((value) => values.includes(value)),
-        `Expected set to contain only ${values.map((v) => '"' + v + '"').join(", ")}, but it contained ${Array.from(
-            set
-        )
-            .map((v) => '"' + v + '"')
-            .join(", ")}`
-    );
-}
diff --git a/frontend/sync-client/src/utils/data-structures/event-listeners.test.ts b/frontend/sync-client/src/utils/data-structures/event-listeners.test.ts
index a5f0cc7c..47c850f9 100644
--- a/frontend/sync-client/src/utils/data-structures/event-listeners.test.ts
+++ b/frontend/sync-client/src/utils/data-structures/event-listeners.test.ts
@@ -3,6 +3,30 @@ import assert from "node:assert";
 import { EventListeners } from "./event-listeners";
 
 describe("EventListeners", () => {
+    it("isolates throwing and rejecting observers and still notifies later listeners", async () => {
+        const failures: unknown[] = [];
+        const listeners = new EventListeners<() => unknown>((error) =>
+            failures.push(error)
+        );
+        const syncError = new Error("sync observer");
+        const asyncError = new Error("async observer");
+        let called = 0;
+        listeners.add(() => {
+            throw syncError;
+        });
+        listeners.add(async () => {
+            throw asyncError;
+        });
+        listeners.add(() => {
+            called++;
+        });
+        assert.doesNotThrow(() => {
+            listeners.trigger();
+        });
+        await new Promise<void>((resolve) => setImmediate(resolve));
+        assert.equal(called, 1);
+        assert.deepEqual(failures, [syncError, asyncError]);
+    });
     it("should add & remove listeners", () => {
         const listeners = new EventListeners<() => void>();
         // eslint-disable-next-line @typescript-eslint/no-empty-function
diff --git a/frontend/sync-client/src/utils/data-structures/event-listeners.ts b/frontend/sync-client/src/utils/data-structures/event-listeners.ts
index e08ca65e..42a13eac 100644
--- a/frontend/sync-client/src/utils/data-structures/event-listeners.ts
+++ b/frontend/sync-client/src/utils/data-structures/event-listeners.ts
@@ -8,50 +8,65 @@ import { awaitAll } from "../await-all";
 export class EventListeners<TListener extends (...args: any[]) => any> {
     private readonly listeners: TListener[] = [];
 
+    public constructor(
+        private readonly onError: (error: unknown) => void = (error) => {
+            console.error("Event observer failed", error);
+        }
+    ) { }
+
     public get count(): number {
         return this.listeners.length;
     }
 
     /**
-    * Adds a new listener to the collection.
-    *
-    * @param listener The listener callback to add
-    * @returns An unsubscribe function that removes this listener when called
-    */
+     * Adds a new listener to the collection.
+     *
+     * @param listener The listener callback to add
+     * @returns An unsubscribe function that removes this listener when called
+     */
     public add(listener: TListener): () => void {
         this.listeners.push(listener);
         return () => this.remove(listener);
     }
 
     /**
-    * Removes a listener from the collection.
-    *
-    * @param listener The listener callback to remove
-    * @returns true if the listener was found and removed, false otherwise
-    */
+     * Removes a listener from the collection.
+     *
+     * @param listener The listener callback to remove
+     * @returns true if the listener was found and removed, false otherwise
+     */
     public remove(listener: TListener): boolean {
         return removeFromArray(this.listeners, listener);
     }
 
     /**
-    * Triggers all listeners synchronously with the provided arguments.
-    * Any returned promises are ignored. Use triggerAsync() to await them.
-    *
-    * @param args The arguments to pass to each listener
-    */
+     * Triggers all listeners synchronously with the provided arguments.
+     * Observer failures are reported without interrupting delivery or background
+     * work. Returned promises are observed; use triggerAsync() to await them.
+     *
+     * @param args The arguments to pass to each listener
+     */
     public trigger(...args: Parameters<TListener>): void {
-        this.listeners.forEach((listener) => {
-            listener(...args);
-        });
+        for (const listener of [...this.listeners]) {
+            try {
+                void Promise.resolve(listener(...args)).catch(
+                    (error: unknown) => {
+                        this.reportFailure(error);
+                    }
+                );
+            } catch (error) {
+                this.reportFailure(error);
+            }
+        }
     }
 
     /**
-    * Triggers all listeners and awaits any promises they return.
-    * Synchronous listeners are called immediately, and any async listeners
-    * are awaited in parallel.
-    *
-    * @param args The arguments to pass to each listener
-    */
+     * Triggers all listeners and awaits any promises they return.
+     * Synchronous listeners are called immediately, and any async listeners
+     * are awaited in parallel.
+     *
+     * @param args The arguments to pass to each listener
+     */
     public async triggerAsync(...args: Parameters<TListener>): Promise<void> {
         await awaitAll(
             this.listeners
@@ -68,4 +83,12 @@ export class EventListeners<TListener extends (...args: any[]) => any> {
     public clear(): void {
         this.listeners.length = 0;
     }
+
+    private reportFailure(error: unknown): void {
+        try {
+            this.onError(error);
+        } catch {
+            // A diagnostic callback must not break background work either.
+        }
+    }
 }
diff --git a/frontend/sync-client/src/utils/data-structures/fix-sized-cache.ts b/frontend/sync-client/src/utils/data-structures/fix-sized-cache.ts
index 51ad41c1..02954a83 100644
--- a/frontend/sync-client/src/utils/data-structures/fix-sized-cache.ts
+++ b/frontend/sync-client/src/utils/data-structures/fix-sized-cache.ts
@@ -1,66 +1,36 @@
-// Implements an in-memory fixed-size cache for document contents,
-
 import type { VaultUpdateId } from "../../persistence/database";
 
-// Doubly-linked list node for O(1) LRU operations
-class LRUNode {
-    public constructor(
-        public key: VaultUpdateId,
-        public value: Uint8Array,
-        public prev: LRUNode | null = null,
-        public next: LRUNode | null = null
-    ) {}
-}
-
-// evicting the least recently used documents when the size limit is exceeded.
+/** Map insertion order tracks the least recently used document first. */
 export class FixedSizeDocumentCache {
-    private currentSizeInBytes: number;
-    private readonly cache: Map<VaultUpdateId, LRUNode>;
-    private head: LRUNode | null; // Least recently used
-    private tail: LRUNode | null; // Most recently used
+    private currentSizeInBytes = 0;
+    private readonly cache = new Map<VaultUpdateId, Uint8Array>();
 
-    public constructor(private maxSizeInBytes: number) {
-        this.currentSizeInBytes = 0;
-        this.cache = new Map();
-        this.head = null;
-        this.tail = null;
-    }
+    public constructor(private maxSizeInBytes: number) { }
 
     public get(updateId: VaultUpdateId): Uint8Array | undefined {
-        const node = this.cache.get(updateId);
-        if (node) {
-            this.moveToTail(node);
-            return node.value;
+        const content = this.cache.get(updateId);
+
+        if (content !== undefined) {
+            this.cache.delete(updateId);
+            this.cache.set(updateId, content);
         }
 
-        return undefined;
+        return content;
     }
 
     public put(updateId: VaultUpdateId, content: Uint8Array): void {
-        if (content.byteLength > this.maxSizeInBytes) {
-            // Document is too large to fit in the cache
-            return;
-        }
+        if (content.byteLength > this.maxSizeInBytes) return;
 
-        // If the document is already in the cache, update it
-        const existingNode = this.cache.get(updateId);
-        if (existingNode != null) {
-            this.currentSizeInBytes -= existingNode.value.byteLength;
-            this.removeNode(existingNode);
-            this.cache.delete(updateId);
-        }
+        this.currentSizeInBytes -= this.cache.get(updateId)?.byteLength ?? 0;
+        this.cache.delete(updateId);
 
-        const newNode = new LRUNode(updateId, content);
-        this.cache.set(updateId, newNode);
-        this.addToTail(newNode);
+        this.cache.set(updateId, content);
         this.currentSizeInBytes += content.byteLength;
         this.fitBelowMaxSize();
     }
 
     public reset(): void {
         this.cache.clear();
-        this.head = null;
-        this.tail = null;
         this.currentSizeInBytes = 0;
     }
 
@@ -70,50 +40,10 @@ export class FixedSizeDocumentCache {
     }
 
     private fitBelowMaxSize(): void {
-        // Evict least recently used documents if over size limit
-        while (this.currentSizeInBytes > this.maxSizeInBytes && this.head) {
-            const lruNode = this.head;
-            this.removeNode(lruNode);
-            this.cache.delete(lruNode.key);
-            this.currentSizeInBytes -= lruNode.value.byteLength;
+        for (const [id, content] of this.cache) {
+            if (this.currentSizeInBytes <= this.maxSizeInBytes) break;
+            this.cache.delete(id);
+            this.currentSizeInBytes -= content.byteLength;
         }
     }
-
-    private removeNode(node: LRUNode): void {
-        if (node.prev) {
-            node.prev.next = node.next;
-        } else {
-            this.head = node.next;
-        }
-
-        if (node.next) {
-            node.next.prev = node.prev;
-        } else {
-            this.tail = node.prev;
-        }
-
-        node.prev = null;
-        node.next = null;
-    }
-
-    private addToTail(node: LRUNode): void {
-        node.prev = this.tail;
-        node.next = null;
-
-        if (this.tail) {
-            this.tail.next = node;
-        }
-
-        this.tail = node;
-
-        this.head ??= node;
-    }
-
-    private moveToTail(node: LRUNode): void {
-        if (node === this.tail) {
-            return;
-        }
-        this.removeNode(node);
-        this.addToTail(node);
-    }
 }
diff --git a/frontend/sync-client/src/utils/data-structures/locks.test.ts b/frontend/sync-client/src/utils/data-structures/locks.test.ts
index f7f0acf2..e325496b 100644
--- a/frontend/sync-client/src/utils/data-structures/locks.test.ts
+++ b/frontend/sync-client/src/utils/data-structures/locks.test.ts
@@ -1,292 +1,46 @@
-import { describe, it, beforeEach } from "node:test";
-import assert from "node:assert";
-import { Logger } from "../../tracing/logger";
-import type { RelativePath } from "../../persistence/database";
-import { Locks } from "./locks";
-import { awaitAll } from "../await-all";
-import { sleep } from "../sleep";
-import { SyncResetError } from "../../errors/errors";
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { Lock } from "./locks";
 
-describe("withLock", () => {
-    const testPath: RelativePath = "test/document/path";
-    const testPath2: RelativePath = "test/document/path2";
-    const logger = new Logger();
-
-    // eslint-disable-next-line @typescript-eslint/init-declarations
-    let locks: Locks<RelativePath>;
-
-    beforeEach(() => {
-        locks = new Locks<RelativePath>(logger);
+test("queued operations run in order and retain their results", async () => {
+    const lock = new Lock();
+    const held = Promise.withResolvers<void>();
+    const started = Promise.withResolvers<void>();
+    const calls: number[] = [];
+    const first = lock.withLock(async () => {
+        calls.push(1);
+        started.resolve();
+        await held.promise;
+        calls.push(2);
+        return "first";
     });
-
-    it("should execute function with single key lock", async () => {
-        let executionCount = 0;
-        const result = await locks.withLock(testPath, () => {
-            executionCount++;
-            return "success";
-        });
-
-        assert.strictEqual(result, "success");
-        assert.strictEqual(executionCount, 1);
+    const second = lock.withLock(() => {
+        calls.push(3);
+        return "second";
     });
-
-    it("should execute async function with single key lock", async () => {
-        let executionCount = 0;
-        const result = await locks.withLock(testPath, async () => {
-            executionCount++;
-            await sleep(10);
-            return "async-success";
-        });
-
-        assert.strictEqual(result, "async-success");
-        assert.strictEqual(executionCount, 1);
-    });
-
-    it("should execute function with multiple key locks", async () => {
-        let executionCount = 0;
-        const result = await locks.withLock([testPath, testPath2], () => {
-            executionCount++;
-            return "multi-success";
-        });
-
-        assert.strictEqual(result, "multi-success");
-        assert.strictEqual(executionCount, 1);
-    });
-
-    it("should sort multiple keys to prevent deadlocks", async () => {
-        const executionOrder: string[] = [];
-
-        // Start two concurrent operations with keys in different orders
-        const promise1 = locks.withLock([testPath2, testPath], async () => {
-            executionOrder.push("operation1-start");
-            await sleep(50);
-            executionOrder.push("operation1-end");
-            return "result1";
-        });
-
-        const promise2 = locks.withLock([testPath, testPath2], async () => {
-            executionOrder.push("operation2-start");
-            await sleep(50);
-            executionOrder.push("operation2-end");
-            return "result2";
-        });
-
-        const [result1, result2] = await awaitAll([promise1, promise2]);
-
-        assert.strictEqual(result1, "result1");
-        assert.strictEqual(result2, "result2");
-        // One operation should complete entirely before the other starts
-        assert.deepStrictEqual(executionOrder, [
-            "operation1-start",
-            "operation1-end",
-            "operation2-start",
-            "operation2-end"
-        ]);
-    });
-
-    it("should serialize access to same key", async () => {
-        const executionOrder: string[] = [];
-
-        const promise1 = locks.withLock(testPath, async () => {
-            executionOrder.push("operation1-start");
-            await sleep(50);
-            executionOrder.push("operation1-end");
-            return "result1";
-        });
-
-        const promise2 = locks.withLock(testPath, async () => {
-            executionOrder.push("operation2-start");
-            await sleep(30);
-            executionOrder.push("operation2-end");
-            return "result2";
-        });
-
-        const [result1, result2] = await awaitAll([promise1, promise2]);
-
-        assert.strictEqual(result1, "result1");
-        assert.strictEqual(result2, "result2");
-        assert.deepStrictEqual(executionOrder, [
-            "operation1-start",
-            "operation1-end",
-            "operation2-start",
-            "operation2-end"
-        ]);
-    });
-
-    it("should allow concurrent access to different keys", async () => {
-        const executionOrder: string[] = [];
-
-        const promise1 = locks.withLock(testPath, async () => {
-            executionOrder.push("operation1-start");
-            await sleep(50);
-
-            executionOrder.push("operation1-end");
-            return "result1";
-        });
-
-        const promise2 = locks.withLock(testPath2, async () => {
-            executionOrder.push("operation2-start");
-            await sleep(30);
-            executionOrder.push("operation2-end");
-            return "result2";
-        });
-
-        const [result1, result2] = await awaitAll([promise1, promise2]);
-
-        assert.strictEqual(result1, "result1");
-        assert.strictEqual(result2, "result2");
-        // Both operations should run concurrently
-        assert.strictEqual(executionOrder[0], "operation1-start");
-        assert.strictEqual(executionOrder[1], "operation2-start");
-    });
-
-    it("should release locks even if function throws", async () => {
-        const error = new Error("test error");
-
-        await assert.rejects(
-            locks.withLock(testPath, () => {
-                throw error;
-            }),
-            { message: "test error" }
-        );
-
-        // Lock should be released, allowing another operation
-        const result = await locks.withLock(
-            testPath,
-            () => "success-after-error"
-        );
-        assert.strictEqual(result, "success-after-error");
-    });
-
-    it("should release locks even if async function throws", async () => {
-        const error = new Error("async test error");
-
-        await assert.rejects(
-            locks.withLock(testPath, async () => {
-                await sleep(10);
-
-                throw error;
-            }),
-            { message: "async test error" }
-        );
-
-        // Lock should be released, allowing another operation
-        const result = await locks.withLock(
-            testPath,
-            () => "success-after-async-error"
-        );
-        assert.strictEqual(result, "success-after-async-error");
-    });
-
-    it("should handle empty array of keys", async () => {
-        const result = await locks.withLock([], () => "empty-keys");
-        assert.strictEqual(result, "empty-keys");
-    });
-
-    it("should maintain FIFO order for multiple waiters", async () => {
-        const executionOrder: string[] = [];
-
-        // Start first operation that holds the lock
-        const firstPromise = locks.withLock(testPath, async () => {
-            executionOrder.push("first-start");
-            await sleep(100);
-            executionOrder.push("first-end");
-            return "first";
-        });
-
-        // Small delay to ensure first operation starts
-        await sleep(10);
-
-        // Queue second and third operations
-        const secondPromise = locks.withLock(testPath, async () => {
-            executionOrder.push("second-start");
-            await sleep(50);
-            executionOrder.push("second-end");
-            return "second";
-        });
-
-        const thirdPromise = locks.withLock(testPath, async () => {
-            executionOrder.push("third-start");
-            await sleep(20);
-            executionOrder.push("third-end");
-            return "third";
-        });
-
-        const [first, second, third] = await awaitAll([
-            firstPromise,
-            secondPromise,
-            thirdPromise
-        ]);
-
-        assert.strictEqual(first, "first");
-        assert.strictEqual(second, "second");
-        assert.strictEqual(third, "third");
-        assert.deepStrictEqual(executionOrder, [
-            "first-start",
-            "first-end",
-            "second-start",
-            "second-end",
-            "third-start",
-            "third-end"
-        ]);
+    const third = lock.withLock(() => {
+        calls.push(4);
+        return "third";
     });
+    await started.promise;
+    assert.deepEqual(calls, [1]);
+    held.resolve();
+    assert.equal(await first, "first");
+    assert.equal(await second, "second");
+    assert.equal(await third, "third");
+    assert.deepEqual(calls, [1, 2, 3, 4]);
 });
 
-describe("reset", () => {
-    const testPath: RelativePath = "test/document/path";
-    const logger = new Logger();
-
-    // eslint-disable-next-line @typescript-eslint/init-declarations
-    let locks: Locks<RelativePath>;
-
-    beforeEach(() => {
-        locks = new Locks<RelativePath>(logger);
-    });
-
-    it("should reject pending waiters with SyncResetError while running operation completes", async () => {
-        const firstPromise = locks.withLock(testPath, async () => {
-            await sleep(2);
-            return "first";
-        });
-
-        await sleep(1);
-
-        const secondPromise = locks.withLock(testPath, async () => "second");
-        void secondPromise.catch(() => {}); // eslint-disable-line @typescript-eslint/no-empty-function
-
-        locks.reset();
-
-        assert.strictEqual(await firstPromise, "first");
-
-        await assert.rejects(secondPromise, (err: Error) => {
-            assert.ok(err instanceof SyncResetError);
-            return true;
+for (const asynchronous of [false, true]) {
+    test(`a ${asynchronous ? "rejected promise" : "thrown error"} releases the lock`, async () => {
+        const lock = new Lock();
+        const error = new Error("failed operation");
+        const failed = lock.withLock(() => {
+            if (asynchronous) return Promise.reject(error);
+            throw error;
         });
+        const next = lock.withLock(() => "continued");
+        await assert.rejects(failed, (caught) => caught === error);
+        assert.equal(await next, "continued");
     });
-
-    it("should allow locks to work normally after reset", async () => {
-        const firstPromise = locks.withLock(testPath, async () => {
-            await sleep(1);
-            return "first";
-        });
-
-        await sleep(1);
-
-        const secondPromise = locks.withLock(testPath, async () => "second");
-        void secondPromise.catch(() => {}); // eslint-disable-line @typescript-eslint/no-empty-function
-
-        locks.reset();
-
-        await firstPromise;
-
-        const result = await locks.withLock(testPath, () => "after-reset");
-        assert.strictEqual(result, "after-reset");
-    });
-
-    it("should handle reset with no pending operations", async () => {
-        locks.reset();
-
-        const result = await locks.withLock(testPath, () => "success");
-        assert.strictEqual(result, "success");
-    });
-});
+}
diff --git a/frontend/sync-client/src/utils/data-structures/locks.ts b/frontend/sync-client/src/utils/data-structures/locks.ts
index 096cacdf..da5962a8 100644
--- a/frontend/sync-client/src/utils/data-structures/locks.ts
+++ b/frontend/sync-client/src/utils/data-structures/locks.ts
@@ -1,166 +1,10 @@
-import { SyncResetError } from "../../errors/errors";
-import type { Logger } from "../../tracing/logger";
-import { awaitAll } from "../await-all";
-
-/**
- * Manages exclusive locks on items to prevent concurrent modifications.
- * Locks are granted in FIFO order.
- *
- * @template T The type of the key used for locking
- */
-export class Locks<T> {
-    /** Currently locked keys */
-    private readonly locked = new Set<T>();
-
-    /** Queue of resolve functions waiting for each key */
-    private readonly waiters = new Map<
-        T,
-        [() => unknown, (err: unknown) => unknown][]
-    >();
-
-    public constructor(private readonly logger?: Logger) {}
-
-    /**
-    * Executes a function while holding exclusive locks on one or more keys.
-    *
-    * This method ensures that the provided function runs with exclusive access to the
-    * specified key(s). Multiple keys are sorted to prevent deadlocks when different
-    * operations request the same keys in different orders.
-    *
-    * @template R The return type of the function to execute
-    * @param keyOrKeys A single key or array of keys to lock during function execution
-    * @param fn The function to execute while holding the lock(s). Can be sync or async.
-    * @returns A Promise that resolves to the return value of the executed function
-    *
-    * @example
-    * ```typescript
-    * // Lock a single key
-    * const result = await locks.withLock('file1', () => {
-    *   // Critical section - only one operation can access 'file1' at a time
-    *   return processFile('file1');
-    * });
-    *
-    * // Lock multiple keys (prevents deadlocks through consistent ordering)
-    * await locks.withLock(['file1', 'file2'], async () => {
-    *   // Critical section - exclusive access to both files
-    *   await moveFile('file1', 'file2');
-    * });
-    * ```
-    *
-    * @throws Any error thrown by the provided function will be propagated after locks are released
-    */
-    public async withLock<R>(
-        keyOrKeys: T | T[],
-        fn: () => R | Promise<R>
-    ): Promise<R> {
-        const keys = Array.isArray(keyOrKeys) ? keyOrKeys : [keyOrKeys];
-
-        // Deduplicate keys to prevent deadlock from acquiring same lock twice
-        const uniqueKeys = Array.from(new Set(keys));
-        uniqueKeys.sort((a, b) => String(a).localeCompare(String(b))); // Ensure consistent order to prevent deadlocks
-
-        await awaitAll(uniqueKeys.map(async (key) => this.waitForLock(key)));
-
-        try {
-            return await fn();
-        } finally {
-            uniqueKeys.forEach((key) => {
-                this.unlock(key);
-            });
-        }
-    }
-
-    public reset(): void {
-        // Resolve all waiting promises before clearing to prevent deadlock
-        // Any operation waiting for a lock will be granted access immediately
-        for (const waiting of this.waiters.values()) {
-            for (const [_, reject] of waiting) {
-                reject(new SyncResetError());
-            }
-        }
-        this.locked.clear();
-        this.waiters.clear();
-    }
-
-    /**
-    * Attempts to acquire a lock immediately without waiting.
-    * Must call `unlock()` if successful.
-    *
-    * @param key The key to lock
-    * @returns `true` if lock acquired, `false` if already locked
-    */
-    public tryLock(key: T): boolean {
-        if (this.locked.has(key)) {
-            return false;
-        }
-
-        this.locked.add(key);
-
-        return true;
-    }
-
-    /**
-    * Waits to acquire a lock, blocking until available.
-    * Operations are queued in FIFO order. Must call `unlock()` when done.
-    *
-    * @param key The key to wait for and lock
-    * @returns Promise that resolves when lock is acquired
-    */
-    public async waitForLock(key: T): Promise<void> {
-        if (this.tryLock(key)) {
-            return Promise.resolve();
-        }
-
-        this.logger?.debug(`Waiting for lock on ${key}`);
-
-        return new Promise((resolve, reject) => {
-            // DefaultDict behavior
-            let waiting = this.waiters.get(key);
-            if (!waiting) {
-                waiting = [];
-                this.waiters.set(key, waiting);
-            }
-
-            waiting.push([resolve, reject]);
-        });
-    }
-
-    /**
-    * Releases a lock and grants access to the next waiting operation in FIFO order.
-    * Removes the key from locked set if no waiters.
-    *
-    * @param key The key to unlock
-    * @throws {Error} If key is not currently locked
-    */
-    public unlock(key: T): void {
-        if (!this.locked.has(key)) {
-            return;
-        }
-
-        // Remove first waiter to ensure FIFO order
-        const [resolveNextWaiting, _] = this.waiters.get(key)?.shift() ?? [];
-
-        if (resolveNextWaiting) {
-            this.logger?.debug(`Granted lock on ${key}`);
-            resolveNextWaiting();
-        } else {
-            this.locked.delete(key);
-        }
-    }
-}
-
 export class Lock {
-    private readonly locks: Locks<boolean>;
+    private tail: Promise<unknown> = Promise.resolve();
 
-    public constructor(logger?: Logger) {
-        this.locks = new Locks(logger);
-    }
-
-    public async withLock<R>(fn: () => R | Promise<R>): Promise<R> {
-        return this.locks.withLock(true, fn);
-    }
-
-    public reset(): void {
-        this.locks.reset();
+    // eslint-disable-next-line @typescript-eslint/promise-function-async -- Preserve the queued operation promise without an extra async wrapper.
+    public withLock<R>(fn: () => R | Promise<R>): Promise<R> {
+        const operation = this.tail.then(fn);
+        this.tail = operation.catch(() => undefined);
+        return operation;
     }
 }
diff --git a/frontend/sync-client/src/utils/data-structures/min-covered.test.ts b/frontend/sync-client/src/utils/data-structures/min-covered.test.ts
deleted file mode 100644
index 7b7271d7..00000000
--- a/frontend/sync-client/src/utils/data-structures/min-covered.test.ts
+++ /dev/null
@@ -1,76 +0,0 @@
-import { describe, it } from "node:test";
-import assert from "node:assert";
-import { CoveredValues } from "./min-covered";
-
-describe("CoveredValues", () => {
-    it("should initialize with the given min value", () => {
-        const covered = new CoveredValues(5);
-        assert.strictEqual(covered.min, 5);
-    });
-
-    it("should add values greater than min", () => {
-        const covered = new CoveredValues(0);
-        covered.add(3);
-        assert.strictEqual(covered.min, 0);
-        covered.add(1);
-        assert.strictEqual(covered.min, 1);
-        covered.add(4);
-        assert.strictEqual(covered.min, 1);
-        covered.add(2);
-        assert.strictEqual(covered.min, 4);
-    });
-
-    it("should ignore duplicate values", () => {
-        const covered = new CoveredValues(0);
-        covered.add(3);
-        covered.add(3);
-        covered.add(3);
-        assert.strictEqual(covered.min, 0);
-        covered.add(1);
-        covered.add(2);
-        assert.strictEqual(covered.min, 3);
-    });
-
-    it("should handle multiple consecutive values", () => {
-        const covered = new CoveredValues(132);
-        for (let i = 250; i > 132; i--) {
-            assert.strictEqual(covered.min, 132);
-            covered.add(i);
-        }
-        assert.strictEqual(covered.min, 250);
-    });
-
-    it("should handle adding values lower than current min", () => {
-        const covered = new CoveredValues(5);
-        covered.add(3);
-        assert.strictEqual(covered.min, 5);
-        covered.add(6);
-        assert.strictEqual(covered.min, 6);
-    });
-
-    it("should auto-advance when setting min value", () => {
-        const covered = new CoveredValues(5);
-        covered.add(7);
-        covered.add(8);
-        covered.add(9);
-        assert.strictEqual(covered.min, 5);
-        // Setting min to 6 should auto-advance through 7, 8, 9
-        covered.min = 6;
-        assert.strictEqual(covered.min, 9);
-        covered.add(10);
-        assert.strictEqual(covered.min, 10);
-    });
-
-    it("should handle setting min value with no consecutive values", () => {
-        const covered = new CoveredValues(5);
-        covered.add(10);
-        covered.add(15);
-        assert.strictEqual(covered.min, 5);
-        // Setting min to 8 should not auto-advance (no consecutive values)
-        covered.min = 8;
-        assert.strictEqual(covered.min, 8);
-        // Add 9 to trigger auto-advance to 10
-        covered.add(9);
-        assert.strictEqual(covered.min, 10);
-    });
-});
diff --git a/frontend/sync-client/src/utils/data-structures/min-covered.ts b/frontend/sync-client/src/utils/data-structures/min-covered.ts
deleted file mode 100644
index 8b38822f..00000000
--- a/frontend/sync-client/src/utils/data-structures/min-covered.ts
+++ /dev/null
@@ -1,61 +0,0 @@
-/**
- * A class that tracks the minimum covered value in a sequence of numbers.
- * It keeps track of a minimum value based on the seen values.
- *
- * It expects integers slightly out of order and makes sure that the value of `min` is
- * always the minimum of the seen values. This is done with bounded memory usage.
- *
- * @example
- * ```typescript
- * const covered = new CoveredValues(0);
- * covered.add(2); // seenValues = [2], min = 0
- * covered.add(1); // seenValues = [], min = 2
- * covered.min; // returns 2
- * ```
- */
-export class CoveredValues {
-    private seenValues: number[] = [];
-
-    public constructor(private minValue: number) {}
-
-    public get min(): number {
-        return this.minValue;
-    }
-
-    public set min(value: number) {
-        this.minValue = Math.max(value, this.minValue);
-        this.seenValues = this.seenValues.filter((v) => v > this.minValue);
-        this.advanceMinWhilePossible();
-    }
-
-    public add(value: number | undefined): void {
-        if (value === undefined || value < this.minValue) {
-            return;
-        }
-
-        let i = 0;
-        while (i < this.seenValues.length && this.seenValues[i] < value) {
-            i++;
-        }
-
-        if (i === this.seenValues.length) {
-            this.seenValues.push(value);
-        } else if (this.seenValues[i] === value) {
-            return;
-        } else {
-            this.seenValues.splice(i, 0, value);
-        }
-
-        this.advanceMinWhilePossible();
-    }
-
-    private advanceMinWhilePossible(): void {
-        while (
-            this.seenValues.length > 0 &&
-            this.seenValues[0] === this.minValue + 1
-        ) {
-            this.seenValues.shift();
-            this.minValue++;
-        }
-    }
-}
diff --git a/frontend/sync-client/src/utils/debugging/slow-web-socket-factory.ts b/frontend/sync-client/src/utils/debugging/slow-web-socket-factory.ts
index c64bff18..258db398 100644
--- a/frontend/sync-client/src/utils/debugging/slow-web-socket-factory.ts
+++ b/frontend/sync-client/src/utils/debugging/slow-web-socket-factory.ts
@@ -1,5 +1,5 @@
 import { sleep } from "../sleep";
-import { Locks } from "../data-structures/locks";
+import { Lock } from "../data-structures/locks";
 import type { Logger } from "../../tracing/logger";
 
 export function slowWebSocketFactory(
@@ -8,10 +8,8 @@ export function slowWebSocketFactory(
 ): typeof WebSocket {
     // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
     return class FlakyWebSocket extends WebSocket {
-        private static readonly RECEIVE_KEY = "websocket-receive";
-        private static readonly SEND_KEY = "websocket-send";
-
-        private readonly locks = new Locks(logger);
+        private readonly receiving = new Lock();
+        private readonly sending = new Lock();
 
         public set onopen(callback: ((event: Event) => void) | null) {
             super.onopen = async (event: Event): Promise<void> => {
@@ -25,18 +23,15 @@ export function slowWebSocketFactory(
 
         public set onmessage(callback: ((event: MessageEvent) => void) | null) {
             super.onmessage = async (event: MessageEvent): Promise<void> => {
-                await this.locks.withLock(
-                    FlakyWebSocket.RECEIVE_KEY,
-                    async () => {
-                        if (jitterScaleInSeconds > 0) {
-                            await sleep(
-                                Math.random() * jitterScaleInSeconds * 1000
-                            );
-                        }
-
-                        callback?.(event);
+                await this.receiving.withLock(async () => {
+                    if (jitterScaleInSeconds > 0) {
+                        await sleep(
+                            Math.random() * jitterScaleInSeconds * 1000
+                        );
                     }
-                );
+
+                    callback?.(event);
+                });
             };
         }
 
@@ -70,7 +65,7 @@ export function slowWebSocketFactory(
             data: string | ArrayBufferLike | Blob | ArrayBufferView
         ): Promise<void> {
             // maintain message order
-            await this.locks.withLock(FlakyWebSocket.SEND_KEY, async () => {
+            await this.sending.withLock(async () => {
                 if (jitterScaleInSeconds > 0) {
                     await sleep(Math.random() * jitterScaleInSeconds * 1000);
                 }