diff --git a/testUtil/fixtures/cleanup.ts b/testUtil/fixtures/cleanup.ts
index 68a7a03a..0eaf06e7 100644
--- a/testUtil/fixtures/cleanup.ts
+++ b/testUtil/fixtures/cleanup.ts
@@ -1,4 +1,4 @@
-import { expect, vi } from 'vitest';
+import { assert, expect, vi } from 'vitest';
 import {
   ClientTransport,
   Connection,
@@ -68,9 +68,16 @@ export async function ensureTransportBuffersAreEventuallyEmpty(
         [...t.sessions]
           .map(([client, sess]) => {
             // get all messages that are not heartbeats
-            const buff = sess.sendBuffer.filter((msg) => {
-              return !Value.Check(ControlMessageAckSchema, msg.payload);
-            });
+            const buff = sess.sendBuffer
+              .map((encodedMsg) => {
+                const msg = sess.parseMsg(encodedMsg.data);
+                assert(msg);
+
+                return msg;
+              })
+              .filter(
+                (msg) => !Value.Check(ControlMessageAckSchema, msg.payload),
+              );
 
             return [client, buff] as [
               string,
diff --git a/transport/message.ts b/transport/message.ts
index 1879341d..17054996 100644
--- a/transport/message.ts
+++ b/transport/message.ts
@@ -280,6 +280,17 @@ export function cancelMessage(
 export type OpaqueTransportMessage = TransportMessage;
 export type TransportClientId = string;
 
+/**
+ * An encoded message that is ready to be send over the transport.
+ * The seq number is kept to keep track of which messages have been
+ * acked by the peer.
+ */
+export interface EncodedTransportMessage {
+  id: string;
+  seq: number;
+  data: Uint8Array;
+}
+
 /**
  * Checks if the given control flag (usually found in msg.controlFlag) is an ack message.
  * @param controlFlag - The control flag to check.
diff --git a/transport/sessionStateMachine/SessionConnected.ts b/transport/sessionStateMachine/SessionConnected.ts
index 0de225f1..6c7a77de 100644
--- a/transport/sessionStateMachine/SessionConnected.ts
+++ b/transport/sessionStateMachine/SessionConnected.ts
@@ -49,11 +49,11 @@ export class SessionConnected<
   }
 
   send(msg: PartialTransportMessage): string {
-    const constructedMsg = this.constructMsg(msg);
-    this.sendBuffer.push(constructedMsg);
-    this.conn.send(this.options.codec.toBuffer(constructedMsg));
+    const encodedMsg = this.encodeMsg(msg);
+    this.sendBuffer.push(encodedMsg);
+    this.conn.send(encodedMsg.data);
 
-    return constructedMsg.id;
+    return encodedMsg.id;
   }
 
   constructor(props: SessionConnectedProps<ConnType>) {
@@ -75,7 +75,7 @@ export class SessionConnected<
       );
 
       for (const msg of this.sendBuffer) {
-        this.conn.send(this.options.codec.toBuffer(msg));
+        this.conn.send(msg.data);
       }
     }
 
diff --git a/transport/sessionStateMachine/common.ts b/transport/sessionStateMachine/common.ts
index d12fc884..06939e2d 100644
--- a/transport/sessionStateMachine/common.ts
+++ b/transport/sessionStateMachine/common.ts
@@ -1,12 +1,12 @@
 import { Logger, MessageMetadata } from '../../logging';
 import { TelemetryInfo } from '../../tracing';
 import {
+  EncodedTransportMessage,
   OpaqueTransportMessage,
   OpaqueTransportMessageSchema,
   PartialTransportMessage,
   ProtocolVersion,
   TransportClientId,
-  TransportMessage,
 } from '../message';
 import { Value } from '@sinclair/typebox/value';
 import { Codec } from '../../codec';
@@ -204,7 +204,7 @@ export interface IdentifiedSessionProps extends CommonSessionProps {
   to: TransportClientId;
   seq: number;
   ack: number;
-  sendBuffer: Array<OpaqueTransportMessage>;
+  sendBuffer: Array<EncodedTransportMessage>;
   telemetry: TelemetryInfo;
   protocolVersion: ProtocolVersion;
 }
@@ -224,7 +224,7 @@ export abstract class IdentifiedSession extends CommonSession {
    * Number of unique messages we've received this session (excluding handshake)
    */
   ack: number;
-  sendBuffer: Array<OpaqueTransportMessage>;
+  sendBuffer: Array<EncodedTransportMessage>;
 
   constructor(props: IdentifiedSessionProps) {
     const { id, to, seq, ack, sendBuffer, telemetry, log, protocolVersion } =
@@ -258,9 +258,9 @@ export abstract class IdentifiedSession extends CommonSession {
     return metadata;
   }
 
-  constructMsg<Payload>(
+  encodeMsg<Payload>(
     partialMsg: PartialTransportMessage<Payload>,
-  ): TransportMessage<Payload> {
+  ): EncodedTransportMessage {
     const msg = {
       ...partialMsg,
       id: generateId(),
@@ -270,9 +270,15 @@ export abstract class IdentifiedSession extends CommonSession {
       ack: this.ack,
     };
 
+    const encodedMsg = {
+      id: msg.id,
+      seq: msg.seq,
+      data: this.options.codec.toBuffer(msg),
+    };
+
     this.seq++;
 
-    return msg;
+    return encodedMsg;
   }
 
   nextSeq(): number {
@@ -280,7 +286,7 @@ export abstract class IdentifiedSession extends CommonSession {
   }
 
   send(msg: PartialTransportMessage): string {
-    const constructedMsg = this.constructMsg(msg);
+    const constructedMsg = this.encodeMsg(msg);
     this.sendBuffer.push(constructedMsg);
 
     return constructedMsg.id;
diff --git a/transport/sessionStateMachine/stateMachine.test.ts b/transport/sessionStateMachine/stateMachine.test.ts
index c6dac186..17eec550 100644
--- a/transport/sessionStateMachine/stateMachine.test.ts
+++ b/transport/sessionStateMachine/stateMachine.test.ts
@@ -1891,8 +1891,8 @@ describe('session state machine', () => {
       expect(onConnectionClosed).not.toHaveBeenCalled();
       expect(onConnectionErrored).not.toHaveBeenCalled();
 
-      const msg = session.constructMsg(payloadToTransportMessage('hello'));
-      session.conn.emitData(session.options.codec.toBuffer(msg));
+      const msg = session.encodeMsg(payloadToTransportMessage('hello'));
+      session.conn.emitData(msg.data);
 
       await waitFor(async () => {
         expect(onMessage).toHaveBeenCalledTimes(1);
@@ -1940,15 +1940,13 @@ describe('session state machine', () => {
 
       // send a heartbeat
       conn.emitData(
-        session.options.codec.toBuffer(
-          session.constructMsg({
-            streamId: 'heartbeat',
-            controlFlags: ControlFlags.AckBit,
-            payload: {
-              type: 'ACK',
-            } satisfies Static<typeof ControlMessageAckSchema>,
-          }),
-        ),
+        session.encodeMsg({
+          streamId: 'heartbeat',
+          controlFlags: ControlFlags.AckBit,
+          payload: {
+            type: 'ACK',
+          } satisfies Static<typeof ControlMessageAckSchema>,
+        }).data,
       );
 
       // make sure the session acks the heartbeat
@@ -1962,15 +1960,13 @@ describe('session state machine', () => {
 
       // send a heartbeat
       conn.emitData(
-        session.options.codec.toBuffer(
-          session.constructMsg({
-            streamId: 'heartbeat',
-            controlFlags: ControlFlags.AckBit,
-            payload: {
-              type: 'ACK',
-            } satisfies Static<typeof ControlMessageAckSchema>,
-          }),
-        ),
+        session.encodeMsg({
+          streamId: 'heartbeat',
+          controlFlags: ControlFlags.AckBit,
+          payload: {
+            type: 'ACK',
+          } satisfies Static<typeof ControlMessageAckSchema>,
+        }).data,
       );
 
       expect(sessionHandle.onMessage).not.toHaveBeenCalled();
diff --git a/transport/sessionStateMachine/transitions.ts b/transport/sessionStateMachine/transitions.ts
index 595959f9..7e694bd5 100644
--- a/transport/sessionStateMachine/transitions.ts
+++ b/transport/sessionStateMachine/transitions.ts
@@ -1,4 +1,3 @@
-import { OpaqueTransportMessage, TransportClientId } from '..';
 import {
   SessionConnecting,
   SessionConnectingListeners,
@@ -38,7 +37,11 @@ import {
   SessionBackingOff,
   SessionBackingOffListeners,
 } from './SessionBackingOff';
-import { ProtocolVersion } from '../message';
+import {
+  EncodedTransportMessage,
+  ProtocolVersion,
+  TransportClientId,
+} from '../message';
 
 function inheritSharedSession(
   session: IdentifiedSession,
@@ -78,7 +81,7 @@ export const SessionStateGraph = {
     ) => {
       const id = `session-${generateId()}`;
       const telemetry = createSessionTelemetryInfo(id, to, from);
-      const sendBuffer: Array<OpaqueTransportMessage> = [];
+      const sendBuffer: Array<EncodedTransportMessage> = [];
 
       const session = new SessionNoConnection({
         listeners,
diff --git a/transport/transport.ts b/transport/transport.ts
index 548db290..a933fa43 100644
--- a/transport/transport.ts
+++ b/transport/transport.ts
@@ -38,9 +38,7 @@ export interface DeleteSessionOptions {
   unhealthy: boolean;
 }
 
-export type SessionBoundSendFn = (
-  msg: PartialTransportMessage,
-) => string | undefined;
+export type SessionBoundSendFn = (msg: PartialTransportMessage) => string;
 
 /**
  * Transports manage the lifecycle (creation/deletion) of sessions