diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Ack.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Ack.java
index 1333e0f4b..0f21a64ea 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Ack.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Ack.java
@@ -1,10 +1,16 @@
 package org.briarproject.bramble.api.sync;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
 import java.util.Collection;
 
+import javax.annotation.concurrent.Immutable;
+
 /**
  * A record acknowledging receipt of one or more {@link Message Messages}.
  */
+@Immutable
+@NotNullByDefault
 public class Ack {
 
 	private final Collection<MessageId> acked;
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Message.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Message.java
index e99bf5cef..b2f30debd 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Message.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Message.java
@@ -1,8 +1,14 @@
 package org.briarproject.bramble.api.sync;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
 
+@Immutable
+@NotNullByDefault
 public class Message {
 
 	/**
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Offer.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Offer.java
index 4fff0608e..6cfeb5b04 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Offer.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Offer.java
@@ -1,10 +1,16 @@
 package org.briarproject.bramble.api.sync;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
 import java.util.Collection;
 
+import javax.annotation.concurrent.Immutable;
+
 /**
  * A record offering the recipient one or more {@link Message Messages}.
  */
+@Immutable
+@NotNullByDefault
 public class Offer {
 
 	private final Collection<MessageId> offered;
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Request.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Request.java
index 9d20fdc19..f4e6e1f2c 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Request.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/Request.java
@@ -1,10 +1,16 @@
 package org.briarproject.bramble.api.sync;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
 import java.util.Collection;
 
+import javax.annotation.concurrent.Immutable;
+
 /**
  * A record requesting one or more {@link Message Messages} from the recipient.
  */
+@Immutable
+@NotNullByDefault
 public class Request {
 
 	private final Collection<MessageId> requested;