diff --git a/src/main/java/co/lettermint/api/ApiClient.java b/src/main/java/co/lettermint/api/ApiClient.java
index 1a50756..4c8bb48 100644
--- a/src/main/java/co/lettermint/api/ApiClient.java
+++ b/src/main/java/co/lettermint/api/ApiClient.java
@@ -122,7 +122,12 @@ public DomainUpdateProjectsResponse updateProjects(String domainId, UpdateDomain
         }
     }
 
-    public static class MessagesEndpoint {
+    // Keep the pre-v2.6 JVM descriptor through javac's covariant return bridge.
+    private interface ScheduledMessageCancellation {
+        RescheduleMessageResponse cancel(String messageId);
+    }
+
+    public static class MessagesEndpoint implements ScheduledMessageCancellation {
         private final LettermintClient client;
 
         MessagesEndpoint(LettermintClient client) {
@@ -145,6 +150,7 @@ public RescheduleMessageResponse reschedule(String messageId, RescheduleMessageR
             return client.patch("/messages/" + segment(messageId), payload, RescheduleMessageResponse.class);
         }
 
+        @Override
         public CancelScheduledMessageResponse cancel(String messageId) {
             return client.post("/messages/" + segment(messageId) + "/cancel", null, CancelScheduledMessageResponse.class);
         }
diff --git a/src/main/java/co/lettermint/models/api/RouteData.java b/src/main/java/co/lettermint/models/api/RouteData.java
index ffe00b6..39861cc 100644
--- a/src/main/java/co/lettermint/models/api/RouteData.java
+++ b/src/main/java/co/lettermint/models/api/RouteData.java
@@ -31,6 +31,9 @@ public class RouteData {
     @JsonProperty("inbound_mx_hostname")
     public String inboundMxHostname;
 
+    @JsonProperty("inbound_route_domain")
+    public String inboundRouteDomain;
+
     @JsonProperty("inbound_domain")
     public String inboundDomain;
 
diff --git a/src/test/java/co/lettermint/CancelBinaryCompatibilityTest.java b/src/test/java/co/lettermint/CancelBinaryCompatibilityTest.java
new file mode 100644
index 0000000..f5a0510
--- /dev/null
+++ b/src/test/java/co/lettermint/CancelBinaryCompatibilityTest.java
@@ -0,0 +1,87 @@
+package co.lettermint;
+
+import co.lettermint.api.ApiClient;
+import co.lettermint.models.api.CancelScheduledMessageResponse;
+import co.lettermint.models.api.RescheduleMessageResponse;
+import co.lettermint.models.api.RouteData;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import okhttp3.mockwebserver.MockResponse;
+import okhttp3.mockwebserver.MockWebServer;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import javax.tools.JavaCompiler;
+import javax.tools.ToolProvider;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CancelBinaryCompatibilityTest {
+    @TempDir
+    Path temporary;
+
+    @Test
+    void callerCompiledAgainstOriginalPr53DescriptorStillRuns() throws Exception {
+        // PR #53 base: 8e3f4b1b0cbad79fbda6e6d0c724d18a899a2b0b.
+        Path stub = temporary.resolve("co/lettermint/api/ApiClient.java");
+        Files.createDirectories(stub.getParent());
+        Files.write(stub, ("package co.lettermint.api; "
+                + "import co.lettermint.models.api.RescheduleMessageResponse; "
+                + "public class ApiClient { public static class MessagesEndpoint { "
+                + "public RescheduleMessageResponse cancel(String id) { return null; } } }")
+                .getBytes(StandardCharsets.UTF_8));
+        Path caller = temporary.resolve("OldCaller.java");
+        Files.write(caller, ("import co.lettermint.api.ApiClient; "
+                + "import co.lettermint.models.api.RescheduleMessageResponse; "
+                + "public class OldCaller { "
+                + "public static RescheduleMessageResponse cancel(ApiClient.MessagesEndpoint endpoint) { "
+                + "return endpoint.cancel(\"message_1\"); } }").getBytes(StandardCharsets.UTF_8));
+        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
+        assertNotNull(compiler, "This test requires a JDK, not a JRE");
+        assertEquals(0, compiler.run(null, null, null, "-classpath",
+                System.getProperty("java.class.path"), "-d", temporary.toString(),
+                stub.toString(), caller.toString()));
+
+        try (MockWebServer server = new MockWebServer();
+             URLClassLoader oldCaller = new URLClassLoader(new URL[]{temporary.toUri().toURL()},
+                     ApiClient.class.getClassLoader())) {
+            server.enqueue(new MockResponse().setHeader("Content-Type", "application/json")
+                    .setBody("{\"message_id\":\"message_1\",\"status\":\"canceled\"}"));
+            server.start();
+            ApiClient api = Lettermint.api("token", server.url("/v1").toString());
+            Object value = oldCaller.loadClass("OldCaller")
+                    .getMethod("cancel", ApiClient.MessagesEndpoint.class).invoke(null, api.messages());
+            assertTrue(value instanceof CancelScheduledMessageResponse);
+            assertTrue(value instanceof RescheduleMessageResponse);
+            assertEquals("message_1", ((RescheduleMessageResponse) value).messageId);
+            assertEquals("canceled", ((RescheduleMessageResponse) value).status);
+            okhttp3.mockwebserver.RecordedRequest request = server.takeRequest();
+            assertEquals("POST", request.getMethod());
+            assertEquals("/v1/messages/message_1/cancel", request.getPath());
+        }
+    }
+
+    @Test
+    void currentReturnTypeAndOldSourceAssignmentBothCompile() {
+        assertTrue(RescheduleMessageResponse.class.isAssignableFrom(CancelScheduledMessageResponse.class));
+    }
+
+    private static RescheduleMessageResponse oldSourceCaller(ApiClient api) {
+        return api.messages().cancel("message_1");
+    }
+
+    @Test
+    void inboundRouteDomainIsOptionalAndNullable() throws Exception {
+        ObjectMapper mapper = new ObjectMapper();
+        RouteData populated = mapper.readValue("{\"inbound_route_domain\":\"incoming.example.com\"}", RouteData.class);
+        assertEquals("incoming.example.com", populated.inboundRouteDomain);
+        assertEquals("incoming.example.com",
+                mapper.readValue(mapper.writeValueAsString(populated), RouteData.class).inboundRouteDomain);
+        assertNull(mapper.readValue("{\"inbound_route_domain\":null}", RouteData.class).inboundRouteDomain);
+        assertNull(mapper.readValue("{}", RouteData.class).inboundRouteDomain);
+    }
+}