diff --git a/vector/src/test/java/im/vector/app/features/onboarding/OnboardingViewModelTest.kt b/vector/src/test/java/im/vector/app/features/onboarding/OnboardingViewModelTest.kt
index 6868bf0dae..fa617cbc9a 100644
--- a/vector/src/test/java/im/vector/app/features/onboarding/OnboardingViewModelTest.kt
+++ b/vector/src/test/java/im/vector/app/features/onboarding/OnboardingViewModelTest.kt
@@ -33,6 +33,7 @@ import im.vector.app.test.fakes.FakeHomeServerConnectionConfigFactory
 import im.vector.app.test.fakes.FakeHomeServerHistoryService
 import im.vector.app.test.fakes.FakeLoginWizard
 import im.vector.app.test.fakes.FakeRegistrationActionHandler
+import im.vector.app.test.fakes.FakeRegistrationWizard
 import im.vector.app.test.fakes.FakeSession
 import im.vector.app.test.fakes.FakeStartAuthenticationFlowUseCase
 import im.vector.app.test.fakes.FakeStringProvider
@@ -41,6 +42,7 @@ import im.vector.app.test.fakes.FakeUriFilenameResolver
 import im.vector.app.test.fakes.FakeVectorFeatures
 import im.vector.app.test.fakes.FakeVectorOverrides
 import im.vector.app.test.fakes.toTestString
+import im.vector.app.test.fixtures.a401ServerError
 import im.vector.app.test.fixtures.aBuildMeta
 import im.vector.app.test.fixtures.aHomeServerCapabilities
 import im.vector.app.test.test
@@ -50,11 +52,13 @@ import org.junit.Rule
 import org.junit.Test
 import org.matrix.android.sdk.api.auth.data.HomeServerConnectionConfig
 import org.matrix.android.sdk.api.auth.registration.Stage
+import org.matrix.android.sdk.api.failure.Failure
 import org.matrix.android.sdk.api.session.Session
 import org.matrix.android.sdk.api.session.homeserver.HomeServerCapabilities
 
 private const val A_DISPLAY_NAME = "a display name"
 private const val A_PICTURE_FILENAME = "a-picture.png"
+private val A_SERVER_ERROR = a401ServerError()
 private val AN_ERROR = RuntimeException("an error!")
 private val A_LOADABLE_REGISTER_ACTION = RegisterAction.StartRegistration
 private val A_NON_LOADABLE_REGISTER_ACTION = RegisterAction.CheckIfEmailHasBeenValidated(delayMillis = -1L)
@@ -64,7 +68,7 @@ private val ANY_CONTINUING_REGISTRATION_RESULT = RegistrationActionHandler.Resul
 private val A_DIRECT_LOGIN = OnboardingAction.AuthenticateAction.LoginDirect("@a-user:id.org", "a-password", "a-device-name")
 private const val A_HOMESERVER_URL = "https://edited-homeserver.org"
 private val A_HOMESERVER_CONFIG = HomeServerConnectionConfig(FakeUri().instance)
-private val SELECTED_HOMESERVER_STATE = SelectedHomeserverState(preferredLoginMode = LoginMode.Password)
+private val SELECTED_HOMESERVER_STATE = SelectedHomeserverState(preferredLoginMode = LoginMode.Password, userFacingUrl = A_HOMESERVER_URL)
 private val SELECTED_HOMESERVER_STATE_SUPPORTED_LOGOUT_DEVICES = SelectedHomeserverState(isLogoutDevicesSupported = true)
 private const val AN_EMAIL = "hello@example.com"
 private const val A_PASSWORD = "a-password"
@@ -323,6 +327,93 @@ class OnboardingViewModelTest {
                 .finish()
     }
 
+    @Test
+    fun `given available username, when a register username is entered, then emits available registration state`() = runTest {
+        viewModelWith(initialRegistrationState(A_HOMESERVER_URL))
+        val onlyUsername = "a-username"
+        givenUserNameIsAvailable(onlyUsername)
+        val test = viewModel.test()
+
+        viewModel.handle(OnboardingAction.UserNameEnteredAction.Registration(onlyUsername))
+
+        test
+                .assertStatesChanges(
+                        initialState,
+                        { copy(registrationState = availableRegistrationState(onlyUsername, A_HOMESERVER_URL)) }
+                )
+                .assertNoEvents()
+                .finish()
+    }
+
+    @Test
+    fun `given unavailable username, when a register username is entered, then emits availability error`() = runTest {
+        viewModelWith(initialRegistrationState(A_HOMESERVER_URL))
+        val onlyUsername = "a-username"
+        givenUserNameIsUnavailable(onlyUsername, A_SERVER_ERROR)
+        val test = viewModel.test()
+
+        viewModel.handle(OnboardingAction.UserNameEnteredAction.Registration(onlyUsername))
+
+        test
+                .assertState(initialState)
+                .assertEvents(OnboardingViewEvents.Failure(A_SERVER_ERROR))
+                .finish()
+    }
+
+    @Test
+    fun `given available full matrix id, when a register username is entered, then changes homeserver and emits available registration state`() = runTest {
+        viewModelWith(initialRegistrationState("ignored-url"))
+        givenCanSuccessfullyUpdateHomeserver(A_HOMESERVER_URL, SELECTED_HOMESERVER_STATE)
+        val userName = "a-user"
+        val fullMatrixId = "@$userName:${A_HOMESERVER_URL.removePrefix("https://")}"
+        givenUserNameIsAvailable(userName)
+        val test = viewModel.test()
+
+        viewModel.handle(OnboardingAction.UserNameEnteredAction.Registration(fullMatrixId))
+
+        test
+                .assertStatesChanges(
+                        initialState,
+                        { copy(isLoading = true) },
+                        { copy(selectedHomeserver = SELECTED_HOMESERVER_STATE) },
+                        { copy(registrationState = availableRegistrationState(userName, A_HOMESERVER_URL)) },
+                        { copy(isLoading = false) },
+                )
+                .assertEvents(OnboardingViewEvents.OnHomeserverEdited)
+                .finish()
+    }
+
+    @Test
+    fun `given unavailable full matrix id, when a register username is entered, then emits availability error`() = runTest {
+        viewModelWith(initialRegistrationState("ignored-url"))
+        givenCanSuccessfullyUpdateHomeserver(A_HOMESERVER_URL, SELECTED_HOMESERVER_STATE)
+        val userName = "a-user"
+        val fullMatrixId = "@$userName:${A_HOMESERVER_URL.removePrefix("https://")}"
+        givenUserNameIsUnavailable(userName, A_SERVER_ERROR)
+        val test = viewModel.test()
+
+        viewModel.handle(OnboardingAction.UserNameEnteredAction.Registration(fullMatrixId))
+
+        test
+                .assertStatesChanges(
+                        initialState,
+                        { copy(isLoading = true) },
+                        { copy(selectedHomeserver = SELECTED_HOMESERVER_STATE) },
+                        { copy(isLoading = false) },
+                )
+                .assertEvents(OnboardingViewEvents.OnHomeserverEdited, OnboardingViewEvents.Failure(A_SERVER_ERROR))
+                .finish()
+    }
+
+    private fun availableRegistrationState(userName: String, homeServerUrl: String) = RegistrationState(
+            isUserNameAvailable = true,
+            selectedMatrixId = "@$userName:${homeServerUrl.removePrefix("https://")}"
+    )
+
+    private fun initialRegistrationState(homeServerUrl: String) = initialState.copy(
+            onboardingFlow = OnboardingFlow.SignUp, selectedHomeserver = SelectedHomeserverState(userFacingUrl = homeServerUrl)
+    )
+
     @Test
     fun `given in the sign up flow, when editing homeserver errors, then does not update the selected homeserver state and emits error`() = runTest {
         viewModelWith(initialState.copy(onboardingFlow = OnboardingFlow.SignUp))
@@ -639,6 +730,14 @@ class OnboardingViewModelTest {
         givenRegistrationResultFor(RegisterAction.StartRegistration, RegistrationActionHandler.Result.Error(error))
         fakeHomeServerHistoryService.expectUrlToBeAdded(A_HOMESERVER_CONFIG.homeServerUri.toString())
     }
+
+    private fun givenUserNameIsAvailable(userName: String) {
+        fakeAuthenticationService.givenRegistrationWizard(FakeRegistrationWizard().also { it.givenUserNameIsAvailable(userName) })
+    }
+
+    private fun givenUserNameIsUnavailable(userName: String, failure: Failure.ServerError) {
+        fakeAuthenticationService.givenRegistrationWizard(FakeRegistrationWizard().also { it.givenUserNameIsUnavailable(userName, failure) })
+    }
 }
 
 private fun HomeServerCapabilities.toPersonalisationState() = PersonalizationState(
diff --git a/vector/src/test/java/im/vector/app/test/fakes/FakeRegistrationWizard.kt b/vector/src/test/java/im/vector/app/test/fakes/FakeRegistrationWizard.kt
index 55a40faaac..e0b4586931 100644
--- a/vector/src/test/java/im/vector/app/test/fakes/FakeRegistrationWizard.kt
+++ b/vector/src/test/java/im/vector/app/test/fakes/FakeRegistrationWizard.kt
@@ -20,8 +20,10 @@ import io.mockk.coEvery
 import io.mockk.coVerify
 import io.mockk.mockk
 import org.matrix.android.sdk.api.auth.registration.RegisterThreePid
+import org.matrix.android.sdk.api.auth.registration.RegistrationAvailability
 import org.matrix.android.sdk.api.auth.registration.RegistrationResult
 import org.matrix.android.sdk.api.auth.registration.RegistrationWizard
+import org.matrix.android.sdk.api.failure.Failure
 import org.matrix.android.sdk.api.session.Session
 
 class FakeRegistrationWizard : RegistrationWizard by mockk(relaxed = false) {
@@ -43,6 +45,14 @@ class FakeRegistrationWizard : RegistrationWizard by mockk(relaxed = false) {
         }
     }
 
+    fun givenUserNameIsAvailable(userName: String) {
+        coEvery { registrationAvailable(userName) } returns RegistrationAvailability.Available
+    }
+
+    fun givenUserNameIsUnavailable(userName: String, failure: Failure.ServerError) {
+        coEvery { registrationAvailable(userName) } returns RegistrationAvailability.NotAvailable(failure)
+    }
+
     fun verifyCheckedEmailedVerification(times: Int) {
         coVerify(exactly = times) { checkIfEmailHasBeenValidated(any()) }
     }