diff --git a/java/client/src/test/java/org/signal/libsignal/net/ChatServiceConnectTests.java b/java/client/src/test/java/org/signal/libsignal/net/ChatServiceConnectTests.java
index 6a50832a6..30bf6bdd1 100644
--- a/java/client/src/test/java/org/signal/libsignal/net/ChatServiceConnectTests.java
+++ b/java/client/src/test/java/org/signal/libsignal/net/ChatServiceConnectTests.java
@@ -23,6 +23,9 @@ import org.signal.libsignal.util.TestLogger;
 
 public class ChatServiceConnectTests {
   private static final String USER_AGENT = "test";
+  // A syntactically valid but non-existent ACI username for testing server rejection.
+  private static final String FAKE_USERNAME = "90c979fd-eab4-4a08-b6da-69dedeab9b29.1";
+  private static final String FAKE_PASSWORD = "password";
 
   private static class Listener implements ChatConnectionListener {
     CompletableFuture<ChatServiceException> disconnectReason = new CompletableFuture<>();
@@ -116,7 +119,8 @@ public class ChatServiceConnectTests {
 
     final var e =
         assertThrows(
-            ExecutionException.class, () -> net.connectAuthChat("", "", false, listener).get());
+            ExecutionException.class,
+            () -> net.connectAuthChat(FAKE_USERNAME, FAKE_PASSWORD, false, listener).get());
     assertTrue(e.getCause() instanceof DeviceDeregisteredException);
   }
 
@@ -137,7 +141,8 @@ public class ChatServiceConnectTests {
     // use preconnects.
     final var e =
         assertThrows(
-            ExecutionException.class, () -> net.connectAuthChat("", "", false, listener).get());
+            ExecutionException.class,
+            () -> net.connectAuthChat(FAKE_USERNAME, FAKE_PASSWORD, false, listener).get());
     assertTrue(e.getCause() instanceof DeviceDeregisteredException);
   }
 
diff --git a/node/ts/test/NetTest.ts b/node/ts/test/NetTest.ts
index aeb4e8e18..1137bc5f3 100644
--- a/node/ts/test/NetTest.ts
+++ b/node/ts/test/NetTest.ts
@@ -371,11 +371,17 @@ describe('chat service api', () => {
         // you can check the log lines for: "[authenticated] using preconnection".
         // We have to use an authenticated connection because that's the only one that's allowed to
         // use preconnects.
-        await net.connectAuthenticatedChat('', '', true, {
-          onIncomingMessage: sinon.stub(),
-          onConnectionInterrupted: sinon.stub(),
-          onQueueEmpty: sinon.stub(),
-        });
+        // Use a syntactically valid but non-existent ACI so the server rejects the credentials.
+        await net.connectAuthenticatedChat(
+          '90c979fd-eab4-4a08-b6da-69dedeab9b29.1',
+          'password',
+          true,
+          {
+            onIncomingMessage: sinon.stub(),
+            onConnectionInterrupted: sinon.stub(),
+            onQueueEmpty: sinon.stub(),
+          }
+        );
         assert.fail('should not have managed to authenticate');
       } catch (e) {
         assert.instanceOf(e, LibSignalErrorBase);
diff --git a/swift/Tests/LibSignalClientTests/ChatServiceTests.swift b/swift/Tests/LibSignalClientTests/ChatServiceTests.swift
index 9fad3c2a7..bae9beec4 100644
--- a/swift/Tests/LibSignalClientTests/ChatServiceTests.swift
+++ b/swift/Tests/LibSignalClientTests/ChatServiceTests.swift
@@ -603,7 +603,12 @@ final class ChatConnectionTests: TestCaseBase {
             // you can check the log lines for: "[authenticated] using preconnection".
             // We have to use an authenticated connection because that's the only one that's allowed to
             // use preconnects.
-            _ = try await net.connectAuthenticatedChat(username: "", password: "", receiveStories: false)
+            // Use a syntactically valid but non-existent ACI so the server rejects the credentials.
+            _ = try await net.connectAuthenticatedChat(
+                username: "90c979fd-eab4-4a08-b6da-69dedeab9b29.1",
+                password: "password",
+                receiveStories: false
+            )
             XCTFail("should not have managed to authenticate")
         } catch SignalError.deviceDeregistered(_:) {
             // expected error, okay