diff --git a/packages/sds/src/sds.spec.ts b/packages/sds/src/sds.spec.ts
index db69b14a04..9caba42c60 100644
--- a/packages/sds/src/sds.spec.ts
+++ b/packages/sds/src/sds.spec.ts
@@ -9,8 +9,8 @@ import {
 } from "./sds.js";
 
 const channelId = "test-channel";
-const callback = (_message: Message): boolean => {
-  return true;
+const callback = (_message: Message): Promise<boolean> => {
+  return Promise.resolve(true);
 };
 
 const getBloomFilter = (channel: MessageChannel): DefaultBloomFilter => {
@@ -36,31 +36,31 @@ describe("MessageChannel", function () {
       channelA = new MessageChannel(channelId);
     });
 
-    it("should increase lamport timestamp", () => {
+    it("should increase lamport timestamp", async () => {
       const timestampBefore = (channelA as any).lamportTimestamp;
-      channelA.sendMessage(new Uint8Array(), callback);
+      await channelA.sendMessage(new Uint8Array(), callback);
       const timestampAfter = (channelA as any).lamportTimestamp;
       expect(timestampAfter).to.equal(timestampBefore + 1);
     });
 
-    it("should push the message to the outgoing buffer", () => {
+    it("should push the message to the outgoing buffer", async () => {
       const bufferLengthBefore = (channelA as any).outgoingBuffer.length;
-      channelA.sendMessage(new Uint8Array(), callback);
+      await channelA.sendMessage(new Uint8Array(), callback);
       const bufferLengthAfter = (channelA as any).outgoingBuffer.length;
       expect(bufferLengthAfter).to.equal(bufferLengthBefore + 1);
     });
 
-    it("should insert message into bloom filter", () => {
+    it("should insert message into bloom filter", async () => {
       const messageId = MessageChannel.getMessageId(new Uint8Array());
-      channelA.sendMessage(new Uint8Array(), callback);
+      await channelA.sendMessage(new Uint8Array(), callback);
       const bloomFilter = getBloomFilter(channelA);
       expect(bloomFilter.lookup(messageId)).to.equal(true);
     });
 
-    it("should insert message id into causal history", () => {
+    it("should insert message id into causal history", async () => {
       const expectedTimestamp = (channelA as any).lamportTimestamp + 1;
       const messageId = MessageChannel.getMessageId(new Uint8Array());
-      channelA.sendMessage(new Uint8Array(), callback);
+      await channelA.sendMessage(new Uint8Array(), callback);
       const messageIdLog = (channelA as any).messageIdLog as {
         timestamp: number;
         messageId: string;
@@ -74,7 +74,7 @@ describe("MessageChannel", function () {
       ).to.equal(true);
     });
 
-    it("should attach causal history and bloom filter to each message", () => {
+    it("should attach causal history and bloom filter to each message", async () => {
       const bloomFilter = new DefaultBloomFilter(DEFAULT_BLOOM_FILTER_OPTIONS);
       const causalHistorySize = (channelA as any).causalHistorySize;
       const filterBytes = new Array<Uint8Array>();
@@ -82,11 +82,11 @@ describe("MessageChannel", function () {
         .fill("message")
         .map((message, index) => `${message}-${index}`);
 
-      messages.forEach((message) => {
+      for (const message of messages) {
         filterBytes.push(bloomFilter.toBytes());
-        channelA.sendMessage(utf8ToBytes(message), callback);
+        await channelA.sendMessage(utf8ToBytes(message), callback);
         bloomFilter.insert(MessageChannel.getMessageId(utf8ToBytes(message)));
-      });
+      }
 
       const outgoingBuffer = (channelA as any).outgoingBuffer as Message[];
       expect(outgoingBuffer.length).to.equal(messages.length);
@@ -115,49 +115,49 @@ describe("MessageChannel", function () {
       channelB = new MessageChannel(channelId);
     });
 
-    it("should increase lamport timestamp", () => {
+    it("should increase lamport timestamp", async () => {
       const timestampBefore = (channelA as any).lamportTimestamp;
-      channelB.sendMessage(new Uint8Array(), (message) => {
+      await channelB.sendMessage(new Uint8Array(), (message) => {
         channelA.receiveMessage(message);
-        return true;
+        return Promise.resolve(true);
       });
       const timestampAfter = (channelA as any).lamportTimestamp;
       expect(timestampAfter).to.equal(timestampBefore + 1);
     });
 
-    it("should update lamport timestamp if greater than current timestamp and dependencies are met", () => {
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), callback);
-      });
-      messagesB.forEach((m) => {
-        channelB.sendMessage(utf8ToBytes(m), (message) => {
+    it("should update lamport timestamp if greater than current timestamp and dependencies are met", async () => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), callback);
+      }
+      for (const m of messagesB) {
+        await channelB.sendMessage(utf8ToBytes(m), (message) => {
           channelA.receiveMessage(message);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
       const timestampAfter = (channelA as any).lamportTimestamp;
       expect(timestampAfter).to.equal(messagesB.length);
     });
 
-    it("should maintain proper timestamps if all messages received", () => {
+    it("should maintain proper timestamps if all messages received", async () => {
       let timestamp = 0;
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), (message) => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), (message) => {
           timestamp++;
           channelB.receiveMessage(message);
           expect((channelB as any).lamportTimestamp).to.equal(timestamp);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
 
-      messagesB.forEach((m) => {
-        channelB.sendMessage(utf8ToBytes(m), (message) => {
+      for (const m of messagesB) {
+        await channelB.sendMessage(utf8ToBytes(m), (message) => {
           timestamp++;
           channelA.receiveMessage(message);
           expect((channelA as any).lamportTimestamp).to.equal(timestamp);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
 
       const expectedLength = messagesA.length + messagesB.length;
       expect((channelA as any).lamportTimestamp).to.equal(expectedLength);
@@ -166,29 +166,29 @@ describe("MessageChannel", function () {
       );
     });
 
-    it("should add received messages to bloom filter", () => {
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), (message) => {
+    it("should add received messages to bloom filter", async () => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), (message) => {
           channelB.receiveMessage(message);
           const bloomFilter = getBloomFilter(channelB);
           expect(bloomFilter.lookup(message.messageId)).to.equal(true);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
     });
 
-    it("should add to incoming buffer if dependencies are not met", () => {
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), callback);
-      });
+    it("should add to incoming buffer if dependencies are not met", async () => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), callback);
+      }
 
       let receivedMessage: Message | null = null;
       const timestampBefore = (channelB as any).lamportTimestamp;
 
-      channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
+      await channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
         receivedMessage = message;
         channelB.receiveMessage(message);
-        return true;
+        return Promise.resolve(true);
       });
 
       const incomingBuffer = (channelB as any).incomingBuffer as Message[];
@@ -216,27 +216,27 @@ describe("MessageChannel", function () {
       channelB = new MessageChannel(channelId);
     });
 
-    it("should mark all messages in causal history as acknowledged", () => {
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), (message) => {
+    it("should mark all messages in causal history as acknowledged", async () => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), (message) => {
           channelB.receiveMessage(message);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
 
       let notInHistory: Message | null = null;
-      channelA.sendMessage(utf8ToBytes("not-in-history"), (message) => {
+      await channelA.sendMessage(utf8ToBytes("not-in-history"), (message) => {
         notInHistory = message;
-        return true;
+        return Promise.resolve(true);
       });
 
       expect((channelA as any).outgoingBuffer.length).to.equal(
         messagesA.length + 1
       );
 
-      channelB.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
+      await channelB.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
         channelA.receiveMessage(message);
-        return true;
+        return Promise.resolve(true);
       });
 
       // Since messagesA are in causal history of channel B's message
@@ -247,7 +247,7 @@ describe("MessageChannel", function () {
       expect(outgoingBuffer[0].messageId).to.equal(notInHistory!.messageId);
     });
 
-    it("should track probabilistic acknowledgements of messages received in bloom filter", () => {
+    it("should track probabilistic acknowledgements of messages received in bloom filter", async () => {
       const acknowledgementCount = (channelA as any).acknowledgementCount;
 
       const causalHistorySize = (channelA as any).causalHistorySize;
@@ -258,24 +258,24 @@ describe("MessageChannel", function () {
       ];
       const messages = [...messagesA, ...messagesB.slice(0, -1)];
       // Send messages to be received by channel B
-      messages.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), (message) => {
+      for (const m of messages) {
+        await channelA.sendMessage(utf8ToBytes(m), (message) => {
           channelB.receiveMessage(message);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
 
       // Send messages not received by channel B
-      unacknowledgedMessages.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), callback);
-      });
+      for (const m of unacknowledgedMessages) {
+        await channelA.sendMessage(utf8ToBytes(m), callback);
+      }
 
       // Channel B sends a message to channel A
-      channelB.sendMessage(
+      await channelB.sendMessage(
         utf8ToBytes(messagesB[messagesB.length - 1]),
         (message) => {
           channelA.receiveMessage(message);
-          return true;
+          return Promise.resolve(true);
         }
       );
 
@@ -307,9 +307,9 @@ describe("MessageChannel", function () {
       // in the bloom filter as before, which should mark them as fully acknowledged in channel A
       for (let i = 1; i < acknowledgementCount; i++) {
         // Send messages until acknowledgement count is reached
-        channelB.sendMessage(utf8ToBytes(`x-${i}`), (message) => {
+        await channelB.sendMessage(utf8ToBytes(`x-${i}`), (message) => {
           channelA.receiveMessage(message);
-          return true;
+          return Promise.resolve(true);
         });
       }
 
@@ -337,15 +337,15 @@ describe("MessageChannel", function () {
       channelB = new MessageChannel(channelId);
     });
 
-    it("should detect messages with missing dependencies", () => {
+    it("should detect messages with missing dependencies", async () => {
       const causalHistorySize = (channelA as any).causalHistorySize;
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), callback);
-      });
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), callback);
+      }
 
-      channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
+      await channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
         channelB.receiveMessage(message);
-        return true;
+        return Promise.resolve(true);
       });
 
       const incomingBuffer = (channelB as any).incomingBuffer as Message[];
@@ -361,19 +361,19 @@ describe("MessageChannel", function () {
       );
     });
 
-    it("should deliver messages after dependencies are met", () => {
+    it("should deliver messages after dependencies are met", async () => {
       const causalHistorySize = (channelA as any).causalHistorySize;
       const sentMessages = new Array<Message>();
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), (message) => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), (message) => {
           sentMessages.push(message);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
 
-      channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
+      await channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
         channelB.receiveMessage(message);
-        return true;
+        return Promise.resolve(true);
       });
 
       const missingMessages = channelB.sweepIncomingBuffer();
@@ -406,13 +406,13 @@ describe("MessageChannel", function () {
         10
       );
 
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), callback);
-      });
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), callback);
+      }
 
-      channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
+      await channelA.sendMessage(utf8ToBytes(messagesB[0]), (message) => {
         channelC.receiveMessage(message);
-        return true;
+        return Promise.resolve(true);
       });
 
       const missingMessages = channelC.sweepIncomingBuffer();
@@ -434,15 +434,15 @@ describe("MessageChannel", function () {
       channelB = new MessageChannel(channelId);
     });
 
-    it("should partition messages based on acknowledgement status", () => {
+    it("should partition messages based on acknowledgement status", async () => {
       const unacknowledgedMessages: Message[] = [];
-      messagesA.forEach((m) => {
-        channelA.sendMessage(utf8ToBytes(m), (message) => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), (message) => {
           unacknowledgedMessages.push(message);
           channelB.receiveMessage(message);
-          return true;
+          return Promise.resolve(true);
         });
-      });
+      }
 
       let { unacknowledged, possiblyAcknowledged } =
         channelA.sweepOutgoingBuffer();
@@ -451,15 +451,15 @@ describe("MessageChannel", function () {
 
       // Make sure messages sent by channel A are not in causal history
       const causalHistorySize = (channelA as any).causalHistorySize;
-      messagesB.slice(0, causalHistorySize).forEach((m) => {
-        channelB.sendMessage(utf8ToBytes(m), callback);
-      });
+      for (const m of messagesB.slice(0, causalHistorySize)) {
+        await channelB.sendMessage(utf8ToBytes(m), callback);
+      }
 
-      channelB.sendMessage(
+      await channelB.sendMessage(
         utf8ToBytes(messagesB[causalHistorySize]),
         (message) => {
           channelA.receiveMessage(message);
-          return true;
+          return Promise.resolve(true);
         }
       );
 
@@ -471,4 +471,80 @@ describe("MessageChannel", function () {
       expect(possiblyAcknowledged.length).to.equal(messagesA.length);
     });
   });
+
+  describe("Sync messages", () => {
+    beforeEach(() => {
+      channelA = new MessageChannel(channelId);
+      channelB = new MessageChannel(channelId);
+    });
+
+    it("should be sent with empty content", async () => {
+      await channelA.sendSyncMessage((message) => {
+        expect(message.content?.length).to.equal(0);
+        return Promise.resolve(true);
+      });
+    });
+
+    it("should not be added to outgoing buffer, bloom filter, or local log", async () => {
+      await channelA.sendSyncMessage();
+
+      const outgoingBuffer = (channelA as any).outgoingBuffer as Message[];
+      expect(outgoingBuffer.length).to.equal(0);
+
+      const bloomFilter = getBloomFilter(channelA);
+      expect(
+        bloomFilter.lookup(MessageChannel.getMessageId(new Uint8Array()))
+      ).to.equal(false);
+
+      const localLog = (channelA as any).messageIdLog as {
+        timestamp: number;
+        messageId: string;
+      }[];
+      expect(localLog.length).to.equal(0);
+    });
+
+    it("should be delivered but not added to local log or bloom filter", async () => {
+      const timestampBefore = (channelB as any).lamportTimestamp;
+      let expectedTimestamp: number | undefined;
+      await channelA.sendSyncMessage((message) => {
+        expectedTimestamp = message.lamportTimestamp;
+        channelB.receiveMessage(message);
+        return Promise.resolve(true);
+      });
+      const timestampAfter = (channelB as any).lamportTimestamp;
+      expect(timestampAfter).to.equal(expectedTimestamp);
+      expect(timestampAfter).to.be.greaterThan(timestampBefore);
+
+      const localLog = (channelB as any).messageIdLog as {
+        timestamp: number;
+        messageId: string;
+      }[];
+      expect(localLog.length).to.equal(0);
+
+      const bloomFilter = getBloomFilter(channelB);
+      expect(
+        bloomFilter.lookup(MessageChannel.getMessageId(new Uint8Array()))
+      ).to.equal(false);
+    });
+
+    it("should update ack status of messages in outgoing buffer", async () => {
+      for (const m of messagesA) {
+        await channelA.sendMessage(utf8ToBytes(m), (message) => {
+          channelB.receiveMessage(message);
+          return Promise.resolve(true);
+        });
+      }
+
+      await channelB.sendSyncMessage((message) => {
+        channelA.receiveMessage(message);
+        return Promise.resolve(true);
+      });
+
+      const causalHistorySize = (channelA as any).causalHistorySize;
+      const outgoingBuffer = (channelA as any).outgoingBuffer as Message[];
+      expect(outgoingBuffer.length).to.equal(
+        messagesA.length - causalHistorySize
+      );
+    });
+  });
 });
diff --git a/packages/sds/src/sds.ts b/packages/sds/src/sds.ts
index 1e2f73662c..0582085cb5 100644
--- a/packages/sds/src/sds.ts
+++ b/packages/sds/src/sds.ts
@@ -65,10 +65,10 @@ export class MessageChannel {
    * @param payload - The payload to send.
    * @param callback - A callback function that returns a boolean indicating whether the message was sent successfully.
    */
-  public sendMessage(
+  public async sendMessage(
     payload: Uint8Array,
-    callback?: (message: Message) => boolean
-  ): void {
+    callback?: (message: Message) => Promise<boolean>
+  ): Promise<void> {
     this.lamportTimestamp++;
 
     const messageId = MessageChannel.getMessageId(payload);
@@ -87,7 +87,7 @@ export class MessageChannel {
     this.outgoingBuffer.push(message);
 
     if (callback) {
-      const success = callback(message);
+      const success = await callback(message);
       if (success) {
         this.filter.insert(messageId);
         this.messageIdLog.push({ timestamp: this.lamportTimestamp, messageId });
@@ -112,8 +112,10 @@ export class MessageChannel {
   public receiveMessage(message: Message): void {
     // review ack status
     this.reviewAckStatus(message);
-    // add to bloom filter
-    this.filter.insert(message.messageId);
+    // add to bloom filter (skip for messages with empty content)
+    if (message.content?.length && message.content.length > 0) {
+      this.filter.insert(message.messageId);
+    }
     // verify causal history
     const dependenciesMet = message.causalHistory.every((messageId) =>
       this.messageIdLog.some(
@@ -203,6 +205,40 @@ export class MessageChannel {
     );
   }
 
+  /**
+   * Send a sync message to the SDS channel.
+   *
+   * Increments the lamport timestamp, constructs a `Message` object
+   * with an empty load. Skips outgoing buffer, filter, and local log.
+   *
+   * See https://rfc.vac.dev/vac/raw/sds/#send-sync-message
+   *
+   * @param callback - A callback function that returns a boolean indicating whether the message was sent successfully.
+   */
+  public sendSyncMessage(
+    callback?: (message: Message) => Promise<boolean>
+  ): Promise<boolean> {
+    this.lamportTimestamp++;
+
+    const emptyMessage = new Uint8Array();
+
+    const message: Message = {
+      messageId: MessageChannel.getMessageId(emptyMessage),
+      channelId: this.channelId,
+      lamportTimestamp: this.lamportTimestamp,
+      causalHistory: this.messageIdLog
+        .slice(-this.causalHistorySize)
+        .map(({ messageId }) => messageId),
+      bloomFilter: this.filter.toBytes(),
+      content: emptyMessage
+    };
+
+    if (callback) {
+      return callback(message);
+    }
+    return Promise.resolve(false);
+  }
+
   // See https://rfc.vac.dev/vac/raw/sds/#deliver-message
   private deliverMessage(message: Message): void {
     const messageLamportTimestamp = message.lamportTimestamp ?? 0;
@@ -210,6 +246,12 @@ export class MessageChannel {
       this.lamportTimestamp = messageLamportTimestamp;
     }
 
+    if (message.content?.length === 0) {
+      // Messages with empty content are sync messages.
+      // They are not added to the local log or bloom filter.
+      return;
+    }
+
     // The participant MUST insert the message ID into its local log,
     // based on Lamport timestamp.
     // If one or more message IDs with the same Lamport timestamp already exists,
@@ -227,6 +269,8 @@ export class MessageChannel {
     });
   }
 
+  // For each received message (including sync messages), inspect the causal history and bloom filter
+  // to determine the acknowledgement status of messages in the outgoing buffer.
   // See https://rfc.vac.dev/vac/raw/sds/#review-ack-status
   private reviewAckStatus(receivedMessage: Message): void {
     // the participant MUST mark all messages in the received causal_history as acknowledged.