diff --git a/core/src/main/java/google/registry/flows/EppController.java b/core/src/main/java/google/registry/flows/EppController.java
index 1ef1c015c..51008c0f3 100644
--- a/core/src/main/java/google/registry/flows/EppController.java
+++ b/core/src/main/java/google/registry/flows/EppController.java
@@ -143,9 +143,6 @@ public final class EppController {
   /** Creates a response indicating an EPP failure. */
   @VisibleForTesting
   static EppOutput getErrorResponse(Result result, Trid trid) {
-    return EppOutput.create(new EppResponse.Builder()
-        .setResult(result)
-        .setTrid(trid)
-        .build());
+    return EppOutput.create(new EppResponse.Builder().setResult(result).setTrid(trid).build());
   }
 }
diff --git a/core/src/main/java/google/registry/flows/EppException.java b/core/src/main/java/google/registry/flows/EppException.java
index 6ef2dd3fe..c4dd987f8 100644
--- a/core/src/main/java/google/registry/flows/EppException.java
+++ b/core/src/main/java/google/registry/flows/EppException.java
@@ -25,10 +25,12 @@ import google.registry.model.eppinput.EppInput.InnerCommand;
 import google.registry.model.eppinput.EppInput.ResourceCommandWrapper;
 import google.registry.model.eppoutput.Result;
 import google.registry.model.eppoutput.Result.Code;
+import google.registry.persistence.transaction.TransactionManagerFactory.ReadOnlyModeException;
 import java.lang.annotation.Documented;
 import java.lang.annotation.Inherited;
 import java.lang.annotation.Retention;
 import java.lang.annotation.Target;
+import javax.annotation.Nullable;
 
 /** Exception used to propagate all failures containing one or more EPP responses. */
 public abstract class EppException extends Exception {
@@ -37,7 +39,12 @@ public abstract class EppException extends Exception {
 
   /** Create an EppException with a custom message. */
   private EppException(String message) {
-    super(message);
+    this(message, null);
+  }
+
+  /** Create an EppException with a custom message and cause. */
+  private EppException(String message, @Nullable Throwable cause) {
+    super(message, cause);
     Code code = getClass().getAnnotation(EppResultCode.class).value();
     Preconditions.checkState(!code.isSuccess());
     this.result = Result.create(code, message);
@@ -255,4 +262,12 @@ public abstract class EppException extends Exception {
       super("Specified protocol version is not implemented");
     }
   }
+
+  /** Registry is currently undergoing maintenance and is in read-only mode. */
+  @EppResultCode(Code.COMMAND_FAILED)
+  public static class ReadOnlyModeEppException extends EppException {
+    ReadOnlyModeEppException(ReadOnlyModeException cause) {
+      super("Registry is currently undergoing maintenance and is in read-only mode", cause);
+    }
+  }
 }
diff --git a/core/src/main/java/google/registry/flows/FlowRunner.java b/core/src/main/java/google/registry/flows/FlowRunner.java
index e1859ba7c..d7747fc98 100644
--- a/core/src/main/java/google/registry/flows/FlowRunner.java
+++ b/core/src/main/java/google/registry/flows/FlowRunner.java
@@ -19,6 +19,7 @@ import static google.registry.xml.XmlTransformer.prettyPrint;
 
 import com.google.common.base.Strings;
 import com.google.common.flogger.FluentLogger;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.FlowModule.DryRun;
 import google.registry.flows.FlowModule.InputXml;
 import google.registry.flows.FlowModule.RegistrarId;
@@ -28,6 +29,7 @@ import google.registry.flows.session.LoginFlow;
 import google.registry.model.eppcommon.Trid;
 import google.registry.model.eppoutput.EppOutput;
 import google.registry.monitoring.whitebox.EppMetric;
+import google.registry.persistence.transaction.TransactionManagerFactory.ReadOnlyModeException;
 import javax.inject.Inject;
 import javax.inject.Provider;
 
@@ -97,6 +99,8 @@ public class FlowRunner {
       return e.output;
     } catch (EppRuntimeException e) {
       throw e.getCause();
+    } catch (ReadOnlyModeException e) {
+      throw new ReadOnlyModeEppException(e);
     }
   }
 
diff --git a/core/src/main/java/google/registry/flows/contact/ContactCreateFlow.java b/core/src/main/java/google/registry/flows/contact/ContactCreateFlow.java
index c8931935b..a249d9226 100644
--- a/core/src/main/java/google/registry/flows/contact/ContactCreateFlow.java
+++ b/core/src/main/java/google/registry/flows/contact/ContactCreateFlow.java
@@ -50,6 +50,7 @@ import org.joda.time.DateTime;
 /**
  * An EPP flow that creates a new contact.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link ResourceAlreadyExistsForThisClientException}
  * @error {@link ResourceCreateContentionException}
  * @error {@link ContactFlowUtils.BadInternationalizedPostalInfoException}
diff --git a/core/src/main/java/google/registry/flows/contact/ContactDeleteFlow.java b/core/src/main/java/google/registry/flows/contact/ContactDeleteFlow.java
index b2129a546..6e7133368 100644
--- a/core/src/main/java/google/registry/flows/contact/ContactDeleteFlow.java
+++ b/core/src/main/java/google/registry/flows/contact/ContactDeleteFlow.java
@@ -60,6 +60,7 @@ import org.joda.time.DateTime;
  * references to the host before the deletion is allowed to proceed. A poll message will be written
  * with the success or failure message when the process is complete.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
  * @error {@link google.registry.flows.exceptions.ResourceStatusProhibitsOperationException}
diff --git a/core/src/main/java/google/registry/flows/contact/ContactTransferApproveFlow.java b/core/src/main/java/google/registry/flows/contact/ContactTransferApproveFlow.java
index 6b28d61fd..32cae77fd 100644
--- a/core/src/main/java/google/registry/flows/contact/ContactTransferApproveFlow.java
+++ b/core/src/main/java/google/registry/flows/contact/ContactTransferApproveFlow.java
@@ -54,6 +54,7 @@ import org.joda.time.DateTime;
  * transfer is automatically approved. Within that window, this flow allows the losing client to
  * explicitly approve the transfer request, which then becomes effective immediately.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
diff --git a/core/src/main/java/google/registry/flows/contact/ContactTransferCancelFlow.java b/core/src/main/java/google/registry/flows/contact/ContactTransferCancelFlow.java
index 16382ed68..3c1cfcc24 100644
--- a/core/src/main/java/google/registry/flows/contact/ContactTransferCancelFlow.java
+++ b/core/src/main/java/google/registry/flows/contact/ContactTransferCancelFlow.java
@@ -54,6 +54,7 @@ import org.joda.time.DateTime;
  * transfer is automatically approved. Within that window, this flow allows the gaining client to
  * withdraw the transfer request.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.exceptions.NotPendingTransferException}
diff --git a/core/src/main/java/google/registry/flows/contact/ContactTransferRejectFlow.java b/core/src/main/java/google/registry/flows/contact/ContactTransferRejectFlow.java
index 30173c80b..cb2428ddb 100644
--- a/core/src/main/java/google/registry/flows/contact/ContactTransferRejectFlow.java
+++ b/core/src/main/java/google/registry/flows/contact/ContactTransferRejectFlow.java
@@ -53,6 +53,7 @@ import org.joda.time.DateTime;
  * transfer is automatically approved. Within that window, this flow allows the losing client to
  * reject the transfer request.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
diff --git a/core/src/main/java/google/registry/flows/contact/ContactTransferRequestFlow.java b/core/src/main/java/google/registry/flows/contact/ContactTransferRequestFlow.java
index 163caa97a..97833699d 100644
--- a/core/src/main/java/google/registry/flows/contact/ContactTransferRequestFlow.java
+++ b/core/src/main/java/google/registry/flows/contact/ContactTransferRequestFlow.java
@@ -63,6 +63,7 @@ import org.joda.time.Duration;
  * by the losing registrar or rejected, and the gaining registrar can also cancel the transfer
  * request.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.exceptions.AlreadyPendingTransferException}
diff --git a/core/src/main/java/google/registry/flows/contact/ContactUpdateFlow.java b/core/src/main/java/google/registry/flows/contact/ContactUpdateFlow.java
index b93e15b2e..71080c1d5 100644
--- a/core/src/main/java/google/registry/flows/contact/ContactUpdateFlow.java
+++ b/core/src/main/java/google/registry/flows/contact/ContactUpdateFlow.java
@@ -55,6 +55,7 @@ import org.joda.time.DateTime;
 /**
  * An EPP flow that updates a contact.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.AddRemoveSameValueException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainCreateFlow.java b/core/src/main/java/google/registry/flows/domain/DomainCreateFlow.java
index ec4c55f61..d15e2bd79 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainCreateFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainCreateFlow.java
@@ -133,6 +133,7 @@ import org.joda.time.Duration;
  *     google.registry.flows.domain.token.AllocationTokenFlowUtils.AlreadyRedeemedAllocationTokenException}
  * @error {@link
  *     google.registry.flows.domain.token.AllocationTokenFlowUtils.InvalidAllocationTokenException}
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.exceptions.OnlyToolCanPassMetadataException}
  * @error {@link ResourceAlreadyExistsForThisClientException}
  * @error {@link ResourceCreateContentionException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainDeleteFlow.java b/core/src/main/java/google/registry/flows/domain/DomainDeleteFlow.java
index c74de57e9..1190da927 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainDeleteFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainDeleteFlow.java
@@ -103,6 +103,7 @@ import org.joda.time.Duration;
 /**
  * An EPP flow that deletes a domain.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.EppException.UnimplementedExtensionException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainRenewFlow.java b/core/src/main/java/google/registry/flows/domain/DomainRenewFlow.java
index e561f94a1..363a1e0f3 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainRenewFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainRenewFlow.java
@@ -94,6 +94,7 @@ import org.joda.time.Duration;
  * longer than 10 years unless it comes in at the exact millisecond that the domain would have
  * expired.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.FlowUtils.UnknownCurrencyEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainRestoreRequestFlow.java b/core/src/main/java/google/registry/flows/domain/DomainRestoreRequestFlow.java
index 2ad97eb6e..f23d58a75 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainRestoreRequestFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainRestoreRequestFlow.java
@@ -93,6 +93,7 @@ import org.joda.time.DateTime;
  * restored to a single year expiration starting at the restore time, regardless of what the
  * original expiration time was.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.EppException.UnimplementedExtensionException}
  * @error {@link google.registry.flows.FlowUtils.UnknownCurrencyEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
@@ -110,7 +111,7 @@ import org.joda.time.DateTime;
  * @error {@link DomainRestoreRequestFlow.RestoreCommandIncludesChangesException}
  */
 @ReportingSpec(ActivityReportField.DOMAIN_RGP_RESTORE_REQUEST)
-public final class DomainRestoreRequestFlow implements TransactionalFlow  {
+public final class DomainRestoreRequestFlow implements TransactionalFlow {
 
   @Inject ResourceCommand resourceCommand;
   @Inject ExtensionManager extensionManager;
diff --git a/core/src/main/java/google/registry/flows/domain/DomainTransferApproveFlow.java b/core/src/main/java/google/registry/flows/domain/DomainTransferApproveFlow.java
index d4cacca10..a313151f7 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainTransferApproveFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainTransferApproveFlow.java
@@ -78,6 +78,7 @@ import org.joda.time.DateTime;
  * timestamps such that they only would become active when the transfer period passed. In this flow,
  * those speculative objects are deleted and replaced with new ones with the correct approval time.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainTransferCancelFlow.java b/core/src/main/java/google/registry/flows/domain/DomainTransferCancelFlow.java
index eb8e1f326..4cdf94c52 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainTransferCancelFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainTransferCancelFlow.java
@@ -65,6 +65,7 @@ import org.joda.time.DateTime;
  * timestamps such that they only would become active when the transfer period passed. In this flow,
  * those speculative objects are deleted.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.exceptions.NotPendingTransferException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainTransferRejectFlow.java b/core/src/main/java/google/registry/flows/domain/DomainTransferRejectFlow.java
index 8428c0ff0..fd334826c 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainTransferRejectFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainTransferRejectFlow.java
@@ -67,6 +67,7 @@ import org.joda.time.DateTime;
  * timestamps such that they only would become active when the transfer period passed. In this flow,
  * those speculative objects are deleted.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainTransferRequestFlow.java b/core/src/main/java/google/registry/flows/domain/DomainTransferRequestFlow.java
index 9e3f591bf..775191ccc 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainTransferRequestFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainTransferRequestFlow.java
@@ -93,6 +93,7 @@ import org.joda.time.DateTime;
  * rejection or cancellation of the request, they will be deleted (and in the approval case,
  * replaced with new ones with the correct approval time).
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.FlowUtils.UnknownCurrencyEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
@@ -102,7 +103,8 @@ import org.joda.time.DateTime;
  * @error {@link google.registry.flows.exceptions.ResourceStatusProhibitsOperationException}
  * @error {@link google.registry.flows.exceptions.TransferPeriodMustBeOneYearException}
  * @error {@link InvalidTransferPeriodValueException}
- * @error {@link google.registry.flows.exceptions.TransferPeriodZeroAndFeeTransferExtensionException}
+ * @error {@link
+ *     google.registry.flows.exceptions.TransferPeriodZeroAndFeeTransferExtensionException}
  * @error {@link DomainFlowUtils.BadPeriodUnitException}
  * @error {@link DomainFlowUtils.CurrencyUnitMismatchException}
  * @error {@link DomainFlowUtils.CurrencyValueScaleException}
diff --git a/core/src/main/java/google/registry/flows/domain/DomainUpdateFlow.java b/core/src/main/java/google/registry/flows/domain/DomainUpdateFlow.java
index 07af4c385..b788770b5 100644
--- a/core/src/main/java/google/registry/flows/domain/DomainUpdateFlow.java
+++ b/core/src/main/java/google/registry/flows/domain/DomainUpdateFlow.java
@@ -97,6 +97,7 @@ import org.joda.time.DateTime;
  * superuser. As such, adding or removing these statuses incurs a billing event. There will be only
  * one charge per update, even if several such statuses are updated at once.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.EppException.UnimplementedExtensionException}
  * @error {@link google.registry.flows.ResourceFlowUtils.AddRemoveSameValueException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
diff --git a/core/src/main/java/google/registry/flows/host/HostCreateFlow.java b/core/src/main/java/google/registry/flows/host/HostCreateFlow.java
index 6155ba6c3..a272e4c2c 100644
--- a/core/src/main/java/google/registry/flows/host/HostCreateFlow.java
+++ b/core/src/main/java/google/registry/flows/host/HostCreateFlow.java
@@ -65,6 +65,7 @@ import org.joda.time.DateTime;
  * hosts cannot have any. This flow allows creating a host name, and if necessary enqueues tasks to
  * update DNS.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.FlowUtils.IpAddressVersionMismatchException}
  * @error {@link ResourceAlreadyExistsForThisClientException}
  * @error {@link ResourceCreateContentionException}
diff --git a/core/src/main/java/google/registry/flows/host/HostDeleteFlow.java b/core/src/main/java/google/registry/flows/host/HostDeleteFlow.java
index f5188fb4a..46359a2bf 100644
--- a/core/src/main/java/google/registry/flows/host/HostDeleteFlow.java
+++ b/core/src/main/java/google/registry/flows/host/HostDeleteFlow.java
@@ -58,6 +58,7 @@ import org.joda.time.DateTime;
  * references to the host before the deletion is allowed to proceed. A poll message will be written
  * with the success or failure message when the process is complete.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
  * @error {@link google.registry.flows.exceptions.ResourceStatusProhibitsOperationException}
diff --git a/core/src/main/java/google/registry/flows/host/HostUpdateFlow.java b/core/src/main/java/google/registry/flows/host/HostUpdateFlow.java
index 69f8029ef..5045b68db 100644
--- a/core/src/main/java/google/registry/flows/host/HostUpdateFlow.java
+++ b/core/src/main/java/google/registry/flows/host/HostUpdateFlow.java
@@ -73,11 +73,12 @@ import org.joda.time.DateTime;
  * hosts. Internal hosts must have at least one IP address associated with them, whereas external
  * hosts cannot have any.
  *
- * <p>This flow allows changing a host name, and adding or removing IP addresses to hosts. When
- * a host is renamed from internal to external all IP addresses must be simultaneously removed, and
+ * <p>This flow allows changing a host name, and adding or removing IP addresses to hosts. When a
+ * host is renamed from internal to external all IP addresses must be simultaneously removed, and
  * when it is renamed from external to internal at least one must be added. If the host is renamed
  * or IP addresses are added, tasks are enqueued to update DNS accordingly.
  *
+ * @error {@link google.registry.flows.EppException.ReadOnlyModeEppException}
  * @error {@link google.registry.flows.ResourceFlowUtils.AddRemoveSameValueException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
diff --git a/core/src/main/java/google/registry/persistence/transaction/TransactionManagerFactory.java b/core/src/main/java/google/registry/persistence/transaction/TransactionManagerFactory.java
index 84f71be0c..4f053abff 100644
--- a/core/src/main/java/google/registry/persistence/transaction/TransactionManagerFactory.java
+++ b/core/src/main/java/google/registry/persistence/transaction/TransactionManagerFactory.java
@@ -157,7 +157,7 @@ public class TransactionManagerFactory {
 
   /** Registry is currently undergoing maintenance and is in read-only mode. */
   public static class ReadOnlyModeException extends IllegalStateException {
-    ReadOnlyModeException() {
+    public ReadOnlyModeException() {
       super("Registry is currently undergoing maintenance and is in read-only mode");
     }
   }
diff --git a/core/src/test/java/google/registry/flows/contact/ContactCreateFlowTest.java b/core/src/test/java/google/registry/flows/contact/ContactCreateFlowTest.java
index 1571cd893..239bee1dc 100644
--- a/core/src/test/java/google/registry/flows/contact/ContactCreateFlowTest.java
+++ b/core/src/test/java/google/registry/flows/contact/ContactCreateFlowTest.java
@@ -26,15 +26,18 @@ import static google.registry.testing.EppExceptionSubject.assertAboutEppExceptio
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowTestCase;
 import google.registry.flows.contact.ContactFlowUtils.BadInternationalizedPostalInfoException;
 import google.registry.flows.contact.ContactFlowUtils.DeclineContactDisclosureFieldDisallowedPolicyException;
 import google.registry.flows.exceptions.ResourceAlreadyExistsForThisClientException;
 import google.registry.flows.exceptions.ResourceCreateContentionException;
 import google.registry.model.contact.ContactResource;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.joda.time.DateTime;
 import org.junit.jupiter.api.Order;
 import org.junit.jupiter.api.extension.RegisterExtension;
@@ -137,4 +140,12 @@ class ContactCreateFlowTest extends ResourceFlowTestCase<ContactCreateFlow, Cont
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-cont-create");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/contact/ContactDeleteFlowTest.java b/core/src/test/java/google/registry/flows/contact/ContactDeleteFlowTest.java
index 8731e47a9..f80ed8ffe 100644
--- a/core/src/test/java/google/registry/flows/contact/ContactDeleteFlowTest.java
+++ b/core/src/test/java/google/registry/flows/contact/ContactDeleteFlowTest.java
@@ -35,6 +35,7 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowTestCase;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException;
@@ -51,9 +52,11 @@ import google.registry.model.tld.Registry;
 import google.registry.model.transfer.TransferData;
 import google.registry.model.transfer.TransferResponse;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import google.registry.testing.TestSqlOnly;
 import org.joda.time.DateTime;
 import org.junit.jupiter.api.BeforeEach;
@@ -243,6 +246,15 @@ class ContactDeleteFlowTest extends ResourceFlowTestCase<ContactDeleteFlow, Cont
     assertIcannReportingActivityFieldLogged("srs-cont-delete");
   }
 
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() throws Exception {
+    persistActiveContact(getUniqueIdFromCommand());
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
+
   private void assertOfyDeleteSuccess(String registrarId, String clientTrid, boolean isSuperuser)
       throws Exception {
     ContactResource deletedContact = reloadResourceByForeignKey();
diff --git a/core/src/test/java/google/registry/flows/contact/ContactTransferApproveFlowTest.java b/core/src/test/java/google/registry/flows/contact/ContactTransferApproveFlowTest.java
index e3f08f58e..861c4a1fd 100644
--- a/core/src/test/java/google/registry/flows/contact/ContactTransferApproveFlowTest.java
+++ b/core/src/test/java/google/registry/flows/contact/ContactTransferApproveFlowTest.java
@@ -26,6 +26,7 @@ import static google.registry.testing.EppExceptionSubject.assertAboutEppExceptio
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException;
@@ -40,9 +41,11 @@ import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.transfer.TransferData;
 import google.registry.model.transfer.TransferResponse;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Order;
 import org.junit.jupiter.api.extension.RegisterExtension;
@@ -265,4 +268,12 @@ class ContactTransferApproveFlowTest
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-cont-transfer-approve");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/contact/ContactTransferCancelFlowTest.java b/core/src/test/java/google/registry/flows/contact/ContactTransferCancelFlowTest.java
index 76053eed3..9345383d3 100644
--- a/core/src/test/java/google/registry/flows/contact/ContactTransferCancelFlowTest.java
+++ b/core/src/test/java/google/registry/flows/contact/ContactTransferCancelFlowTest.java
@@ -25,6 +25,7 @@ import static google.registry.testing.EppExceptionSubject.assertAboutEppExceptio
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.exceptions.NotPendingTransferException;
@@ -37,9 +38,11 @@ import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.transfer.TransferData;
 import google.registry.model.transfer.TransferResponse;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Order;
 import org.junit.jupiter.api.extension.RegisterExtension;
@@ -251,4 +254,12 @@ class ContactTransferCancelFlowTest
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-cont-transfer-cancel");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/contact/ContactTransferRejectFlowTest.java b/core/src/test/java/google/registry/flows/contact/ContactTransferRejectFlowTest.java
index 76b929e1e..0ff0d40b5 100644
--- a/core/src/test/java/google/registry/flows/contact/ContactTransferRejectFlowTest.java
+++ b/core/src/test/java/google/registry/flows/contact/ContactTransferRejectFlowTest.java
@@ -25,6 +25,7 @@ import static google.registry.testing.EppExceptionSubject.assertAboutEppExceptio
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException;
@@ -39,9 +40,11 @@ import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.transfer.TransferData;
 import google.registry.model.transfer.TransferResponse;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Order;
 import org.junit.jupiter.api.extension.RegisterExtension;
@@ -264,4 +267,12 @@ class ContactTransferRejectFlowTest
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-cont-transfer-reject");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/contact/ContactTransferRequestFlowTest.java b/core/src/test/java/google/registry/flows/contact/ContactTransferRequestFlowTest.java
index 74d898a41..b78844635 100644
--- a/core/src/test/java/google/registry/flows/contact/ContactTransferRequestFlowTest.java
+++ b/core/src/test/java/google/registry/flows/contact/ContactTransferRequestFlowTest.java
@@ -35,6 +35,7 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.exceptions.AlreadyPendingTransferException;
@@ -50,9 +51,11 @@ import google.registry.model.poll.PollMessage;
 import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.transfer.ContactTransferData;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.joda.time.DateTime;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Order;
@@ -313,4 +316,12 @@ class ContactTransferRequestFlowTest
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-cont-transfer-request");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/contact/ContactUpdateFlowTest.java b/core/src/test/java/google/registry/flows/contact/ContactUpdateFlowTest.java
index d6c963b0f..63af58f1b 100644
--- a/core/src/test/java/google/registry/flows/contact/ContactUpdateFlowTest.java
+++ b/core/src/test/java/google/registry/flows/contact/ContactUpdateFlowTest.java
@@ -27,6 +27,7 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowTestCase;
 import google.registry.flows.ResourceFlowUtils.AddRemoveSameValueException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
@@ -41,9 +42,11 @@ import google.registry.model.contact.ContactResource;
 import google.registry.model.contact.PostalInfo;
 import google.registry.model.contact.PostalInfo.Type;
 import google.registry.model.eppcommon.StatusValue;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.junit.jupiter.api.Order;
 import org.junit.jupiter.api.extension.RegisterExtension;
 
@@ -426,4 +429,13 @@ class ContactUpdateFlowTest extends ResourceFlowTestCase<ContactUpdateFlow, Cont
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-cont-update");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() throws Exception {
+    persistActiveContact(getUniqueIdFromCommand());
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainCreateFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainCreateFlowTest.java
index 71d650e5f..54549bed0 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainCreateFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainCreateFlowTest.java
@@ -72,6 +72,7 @@ import com.google.common.collect.Ordering;
 import com.googlecode.objectify.Key;
 import google.registry.config.RegistryConfig;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.EppException.UnimplementedExtensionException;
 import google.registry.flows.EppRequestSource;
 import google.registry.flows.ExtensionManager.UndeclaredServiceExtensionException;
@@ -164,10 +165,12 @@ import google.registry.model.tld.Registry.TldState;
 import google.registry.model.tld.Registry.TldType;
 import google.registry.monitoring.whitebox.EppMetric;
 import google.registry.persistence.VKey;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TaskQueueHelper.TaskMatcher;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import java.math.BigDecimal;
 import java.util.Map;
 import javax.annotation.Nullable;
@@ -2517,4 +2520,13 @@ class DomainCreateFlowTest extends ResourceFlowTestCase<DomainCreateFlow, Domain
     EppMetric eppMetric = getEppMetric();
     assertThat(eppMetric.getCommandName()).hasValue("DomainCreate");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    persistContactsAndHosts();
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainDeleteFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainDeleteFlowTest.java
index f6d0bab90..fcc29e19b 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainDeleteFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainDeleteFlowTest.java
@@ -68,6 +68,7 @@ import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.ImmutableSortedMap;
 import google.registry.batch.ResaveEntityAction;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.EppException.UnimplementedExtensionException;
 import google.registry.flows.EppRequestSource;
 import google.registry.flows.ResourceFlowTestCase;
@@ -100,10 +101,12 @@ import google.registry.model.tld.Registry.TldType;
 import google.registry.model.transfer.DomainTransferData;
 import google.registry.model.transfer.TransferResponse;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TaskQueueHelper.TaskMatcher;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import java.util.Map;
 import org.joda.money.Money;
 import org.joda.time.DateTime;
@@ -1248,4 +1251,13 @@ class DomainDeleteFlowTest extends ResourceFlowTestCase<DomainDeleteFlow, Domain
     EppException thrown = assertThrows(UnimplementedExtensionException.class, this::runFlow);
     assertAboutEppExceptions().that(thrown).marshalsToXml();
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() throws Exception {
+    setUpSuccessfulTest();
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainRenewFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainRenewFlowTest.java
index 535da6767..5b18d6ac7 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainRenewFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainRenewFlowTest.java
@@ -42,6 +42,7 @@ import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.ImmutableSortedMap;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.EppRequestSource;
 import google.registry.flows.FlowUtils.UnknownCurrencyEppException;
 import google.registry.flows.ResourceFlowTestCase;
@@ -73,10 +74,12 @@ import google.registry.model.reporting.DomainTransactionRecord;
 import google.registry.model.reporting.DomainTransactionRecord.TransactionReportField;
 import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.tld.Registry;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.SetClockExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import java.util.Map;
 import org.joda.money.Money;
 import org.joda.time.DateTime;
@@ -870,4 +873,20 @@ class DomainRenewFlowTest extends ResourceFlowTestCase<DomainRenewFlow, DomainBa
                 TransactionReportField.netRenewsFieldFromYears(5),
                 1));
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() throws Exception {
+    persistDomain();
+    DomainBase domain = reloadResourceByForeignKey();
+    persistResource(
+        domain
+            .asBuilder()
+            .setRegistrationExpirationTime(domain.getRegistrationExpirationTime().minusYears(1))
+            .build());
+    clock.setTo(clock.nowUtc().minusSeconds(2));
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainRestoreRequestFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainRestoreRequestFlowTest.java
index 40a71cf19..02c883e4e 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainRestoreRequestFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainRestoreRequestFlowTest.java
@@ -41,6 +41,7 @@ import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.ImmutableSortedMap;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.EppException.UnimplementedExtensionException;
 import google.registry.flows.FlowUtils.UnknownCurrencyEppException;
 import google.registry.flows.ResourceFlowTestCase;
@@ -72,9 +73,11 @@ import google.registry.model.reporting.DomainTransactionRecord;
 import google.registry.model.reporting.DomainTransactionRecord.TransactionReportField;
 import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.tld.Registry;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import java.util.Map;
 import java.util.Optional;
 import org.joda.money.Money;
@@ -771,4 +774,13 @@ class DomainRestoreRequestFlowTest
     assertThat(thrown).hasMessageThat().contains("domain restore reports are not supported");
     assertAboutEppExceptions().that(thrown).marshalsToXml();
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() throws Exception {
+    persistPendingDeleteDomain();
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainTransferApproveFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainTransferApproveFlowTest.java
index 2cf70f679..9408fa29a 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainTransferApproveFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainTransferApproveFlowTest.java
@@ -43,6 +43,7 @@ import com.google.common.collect.ImmutableSortedMap;
 import com.google.common.collect.Ordering;
 import com.google.common.collect.Streams;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException;
@@ -72,9 +73,11 @@ import google.registry.model.transfer.DomainTransferData;
 import google.registry.model.transfer.TransferResponse.DomainTransferResponse;
 import google.registry.model.transfer.TransferStatus;
 import google.registry.persistence.VKey;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import java.util.Arrays;
 import java.util.stream.Stream;
 import org.joda.money.Money;
@@ -94,7 +97,7 @@ class DomainTransferApproveFlowTest
   final ReplayExtension replayExtension = ReplayExtension.createWithDoubleReplay(clock);
 
   @BeforeEach
-  void setUp() {
+  void beforeEach() {
     setEppInput("domain_transfer_approve.xml");
     // Change the registry so that the renew price changes a day minus 1 millisecond before the
     // transfer (right after there will be an autorenew in the test case that has one) and then
@@ -674,4 +677,12 @@ class DomainTransferApproveFlowTest
         domain.getRegistrationExpirationTime());
     assertHistoryEntriesDoNotContainTransferBillingEventsOrGracePeriods();
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainTransferCancelFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainTransferCancelFlowTest.java
index 0c121e37d..6ba44002a 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainTransferCancelFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainTransferCancelFlowTest.java
@@ -37,6 +37,7 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.domain.DomainFlowUtils.NotAuthorizedForTldException;
@@ -55,9 +56,11 @@ import google.registry.model.tld.Registry;
 import google.registry.model.transfer.DomainTransferData;
 import google.registry.model.transfer.TransferResponse.DomainTransferResponse;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.joda.time.DateTime;
 import org.joda.time.Duration;
 import org.junit.jupiter.api.BeforeEach;
@@ -74,7 +77,7 @@ class DomainTransferCancelFlowTest
   final ReplayExtension replayExtension = ReplayExtension.createWithDoubleReplay(clock);
 
   @BeforeEach
-  void setUp() {
+  void beforeEach() {
     setEppInput("domain_transfer_cancel.xml");
     setRegistrarIdForFlow("NewRegistrar");
     setupDomainWithPendingTransfer("example", "tld");
@@ -416,4 +419,12 @@ class DomainTransferCancelFlowTest
     assertThat(persistedEntry.getDomainTransactionRecords())
         .containsExactly(previousSuccessRecord.asBuilder().setReportAmount(-1).build());
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainTransferRejectFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainTransferRejectFlowTest.java
index d297dee69..e9100743c 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainTransferRejectFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainTransferRejectFlowTest.java
@@ -37,6 +37,7 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import com.google.common.collect.ImmutableSet;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException;
@@ -57,9 +58,11 @@ import google.registry.model.tld.Registry;
 import google.registry.model.transfer.TransferData;
 import google.registry.model.transfer.TransferResponse;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.joda.time.DateTime;
 import org.joda.time.Duration;
 import org.junit.jupiter.api.BeforeEach;
@@ -76,7 +79,7 @@ class DomainTransferRejectFlowTest
   final ReplayExtension replayExtension = ReplayExtension.createWithDoubleReplay(clock);
 
   @BeforeEach
-  void setUp() {
+  void beforeEach() {
     setEppInput("domain_transfer_reject.xml");
     setRegistrarIdForFlow("TheRegistrar");
     setupDomainWithPendingTransfer("example", "tld");
@@ -384,4 +387,12 @@ class DomainTransferRejectFlowTest
             previousSuccessRecord.asBuilder().setReportAmount(-1).build(),
             DomainTransactionRecord.create("tld", clock.nowUtc(), TRANSFER_NACKED, 1));
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainTransferRequestFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainTransferRequestFlowTest.java
index d21eb4084..c036cfaa4 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainTransferRequestFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainTransferRequestFlowTest.java
@@ -60,6 +60,7 @@ import com.google.common.collect.Sets;
 import com.google.common.collect.Streams;
 import google.registry.batch.ResaveEntityAction;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.EppRequestSource;
 import google.registry.flows.FlowUtils.UnknownCurrencyEppException;
 import google.registry.flows.ResourceFlowUtils.BadAuthInfoForResourceException;
@@ -104,10 +105,12 @@ import google.registry.model.transfer.DomainTransferData;
 import google.registry.model.transfer.TransferResponse;
 import google.registry.model.transfer.TransferStatus;
 import google.registry.persistence.VKey;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TaskQueueHelper.TaskMatcher;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import java.util.Map;
 import java.util.Optional;
 import java.util.stream.Stream;
@@ -163,7 +166,7 @@ class DomainTransferRequestFlowTest
           .build();
 
   @BeforeEach
-  void setUp() {
+  void beforeEach() {
     setEppInput("domain_transfer_request.xml");
     setRegistrarIdForFlow("NewRegistrar");
   }
@@ -1540,4 +1543,15 @@ class DomainTransferRequestFlowTest
             DomainTransactionRecord.create(
                 "tld", clock.nowUtc().plusDays(5), TRANSFER_SUCCESSFUL, 1));
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    setupDomain("example", "tld");
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown =
+        assertThrows(
+            ReadOnlyModeEppException.class, () -> doFailingTest("domain_transfer_request.xml"));
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/domain/DomainUpdateFlowTest.java b/core/src/test/java/google/registry/flows/domain/DomainUpdateFlowTest.java
index 57986c1fd..971d780ff 100644
--- a/core/src/test/java/google/registry/flows/domain/DomainUpdateFlowTest.java
+++ b/core/src/test/java/google/registry/flows/domain/DomainUpdateFlowTest.java
@@ -59,6 +59,7 @@ import com.google.common.collect.ImmutableSortedMap;
 import com.googlecode.objectify.Key;
 import google.registry.config.RegistryConfig;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.EppException.UnimplementedExtensionException;
 import google.registry.flows.EppRequestSource;
 import google.registry.flows.ResourceFlowTestCase;
@@ -101,9 +102,11 @@ import google.registry.model.host.HostResource;
 import google.registry.model.poll.PollMessage;
 import google.registry.model.tld.Registry;
 import google.registry.persistence.VKey;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import java.util.Optional;
 import org.joda.money.Money;
 import org.joda.time.DateTime;
@@ -133,7 +136,7 @@ class DomainUpdateFlowTest extends ResourceFlowTestCase<DomainUpdateFlow, Domain
   final ReplayExtension replayExtension = ReplayExtension.createWithDoubleReplay(clock);
 
   @BeforeEach
-  void initDomainTest() {
+  void beforeEach() {
     createTld("tld");
     // Note that "domain_update.xml" tests adding and removing the same contact type.
     setEppInput("domain_update.xml");
@@ -1495,4 +1498,14 @@ class DomainUpdateFlowTest extends ResourceFlowTestCase<DomainUpdateFlow, Domain
     runFlowAsSuperuser();
     assertAboutDomains().that(reloadResourceByForeignKey()).hasNoAutorenewEndTime();
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() throws Exception {
+    persistReferencedEntities();
+    persistDomain();
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/host/HostCreateFlowTest.java b/core/src/test/java/google/registry/flows/host/HostCreateFlowTest.java
index 7cf53c3a6..414db530e 100644
--- a/core/src/test/java/google/registry/flows/host/HostCreateFlowTest.java
+++ b/core/src/test/java/google/registry/flows/host/HostCreateFlowTest.java
@@ -36,6 +36,7 @@ import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.FlowUtils.IpAddressVersionMismatchException;
 import google.registry.flows.ResourceFlowTestCase;
 import google.registry.flows.exceptions.ResourceAlreadyExistsForThisClientException;
@@ -54,9 +55,11 @@ import google.registry.model.domain.DomainBase;
 import google.registry.model.eppcommon.StatusValue;
 import google.registry.model.host.HostResource;
 import google.registry.model.reporting.HistoryEntry;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.joda.time.DateTime;
 import org.junit.jupiter.api.Order;
 import org.junit.jupiter.api.extension.RegisterExtension;
@@ -329,4 +332,12 @@ class HostCreateFlowTest extends ResourceFlowTestCase<HostCreateFlow, HostResour
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-host-create");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/core/src/test/java/google/registry/flows/host/HostDeleteFlowTest.java b/core/src/test/java/google/registry/flows/host/HostDeleteFlowTest.java
index 381dd33f7..70aefc526 100644
--- a/core/src/test/java/google/registry/flows/host/HostDeleteFlowTest.java
+++ b/core/src/test/java/google/registry/flows/host/HostDeleteFlowTest.java
@@ -35,6 +35,7 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.ResourceFlowTestCase;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
 import google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException;
@@ -51,9 +52,11 @@ import google.registry.model.reporting.HistoryEntry.Type;
 import google.registry.model.tld.Registry;
 import google.registry.model.transfer.DomainTransferData;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import org.joda.time.DateTime;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Order;
@@ -338,6 +341,15 @@ class HostDeleteFlowTest extends ResourceFlowTestCase<HostDeleteFlow, HostResour
     assertIcannReportingActivityFieldLogged("srs-host-delete");
   }
 
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() {
+    persistActiveHost("ns1.example.tld");
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
+
   private void assertOfyDeleteSuccess(String registrarId, String clientTrid, boolean isSuperuser)
       throws Exception {
     HostResource deletedHost = reloadResourceByForeignKey();
diff --git a/core/src/test/java/google/registry/flows/host/HostUpdateFlowTest.java b/core/src/test/java/google/registry/flows/host/HostUpdateFlowTest.java
index 4ccecaaad..f51145400 100644
--- a/core/src/test/java/google/registry/flows/host/HostUpdateFlowTest.java
+++ b/core/src/test/java/google/registry/flows/host/HostUpdateFlowTest.java
@@ -47,6 +47,7 @@ import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.net.InetAddresses;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.ReadOnlyModeEppException;
 import google.registry.flows.EppRequestSource;
 import google.registry.flows.ResourceFlowTestCase;
 import google.registry.flows.ResourceFlowUtils.AddRemoveSameValueException;
@@ -77,10 +78,12 @@ import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.tld.Registry;
 import google.registry.model.transfer.DomainTransferData;
 import google.registry.model.transfer.TransferStatus;
+import google.registry.testing.DatabaseHelper;
 import google.registry.testing.DualDatabaseTest;
 import google.registry.testing.ReplayExtension;
 import google.registry.testing.TaskQueueHelper.TaskMatcher;
 import google.registry.testing.TestOfyAndSql;
+import google.registry.testing.TestOfyOnly;
 import javax.annotation.Nullable;
 import org.joda.time.DateTime;
 import org.junit.jupiter.api.Order;
@@ -1341,4 +1344,14 @@ class HostUpdateFlowTest extends ResourceFlowTestCase<HostUpdateFlow, HostResour
     runFlow();
     assertIcannReportingActivityFieldLogged("srs-host-update");
   }
+
+  @TestOfyOnly
+  void testModification_duringReadOnlyPhase() throws Exception {
+    createTld("tld");
+    persistActiveSubordinateHost(oldHostName(), persistActiveDomain("example.tld"));
+    DatabaseHelper.setMigrationScheduleToDatastorePrimaryReadOnly(clock);
+    EppException thrown = assertThrows(ReadOnlyModeEppException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+    DatabaseHelper.removeDatabaseMigrationSchedule();
+  }
 }
diff --git a/docs/flows.md b/docs/flows.md
index f2842c1f0..cc8462835 100644
--- a/docs/flows.md
+++ b/docs/flows.md
@@ -27,6 +27,8 @@ An EPP flow that creates a new contact.
     *   Resource with this id already exists.
 *   2306
     *   Declining contact disclosure is disallowed by server policy.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactDeleteFlow
 
@@ -51,6 +53,8 @@ or failure message when the process is complete.
     *   Resource status prohibits this operation.
 *   2305
     *   Resource to be deleted has active incoming references.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactInfoFlow
 
@@ -93,6 +97,8 @@ transfer request, which then becomes effective immediately.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactTransferCancelFlow
 
@@ -116,6 +122,8 @@ request.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactTransferQueryFlow
 
@@ -163,6 +171,8 @@ that window, this flow allows the losing client to reject the transfer request.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactTransferRequestFlow
 
@@ -190,6 +200,8 @@ or rejected, and the gaining registrar can also cancel the transfer request.
     *   Resource with this id does not exist.
 *   2304
     *   Resource status prohibits this operation.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactUpdateFlow
 
@@ -214,6 +226,8 @@ An EPP flow that updates a contact.
 *   2306
     *   Cannot add and remove the same value.
     *   Declining contact disclosure is disallowed by server policy.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainCheckFlow
 
@@ -371,6 +385,8 @@ An EPP flow that creates a new domain resource.
     *   Too many nameservers set on this domain.
     *   Domain labels cannot end with a dash.
     *   Only encoded signed marks are supported.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainDeleteFlow
 
@@ -394,6 +410,8 @@ An EPP flow that deletes a domain.
     *   Resource status prohibits this operation.
 *   2305
     *   Domain to be deleted has subordinate hosts.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainInfoFlow
 
@@ -465,6 +483,8 @@ comes in at the exact millisecond that the domain would have expired.
 *   2306
     *   Periods for domain registrations must be specified in years.
     *   The requested fees cannot be provided in the requested currency.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainRestoreRequestFlow
 
@@ -524,6 +544,8 @@ regardless of what the original expiration time was.
     *   Domain is not eligible for restore.
 *   2306
     *   The requested fees cannot be provided in the requested currency.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferApproveFlow
 
@@ -553,6 +575,8 @@ replaced with new ones with the correct approval time.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferCancelFlow
 
@@ -581,6 +605,8 @@ transfer period passed. In this flow, those speculative objects are deleted.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferQueryFlow
 
@@ -633,6 +659,8 @@ transfer period passed. In this flow, those speculative objects are deleted.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferRequestFlow
 
@@ -692,6 +720,8 @@ new ones with the correct approval time).
         EPP extension.
     *   Periods for domain registrations must be specified in years.
     *   The requested fees cannot be provided in the requested currency.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainUpdateFlow
 
@@ -747,6 +777,8 @@ statuses are updated at once.
     *   The secDNS:all element must have value 'true' if present.
     *   Too many DS records set on a domain.
     *   Too many nameservers set on this domain.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## HelloFlow
 
@@ -805,6 +837,8 @@ allows creating a host name, and if necessary enqueues tasks to update DNS.
     *   Superordinate domain for this hostname is in pending delete.
 *   2306
     *   Host names must be at least two levels below the registry suffix.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## HostDeleteFlow
 
@@ -833,6 +867,8 @@ or failure message when the process is complete.
     *   Resource status prohibits this operation.
 *   2305
     *   Resource to be deleted has active incoming references.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## HostInfoFlow
 
@@ -901,6 +937,8 @@ are enqueued to update DNS accordingly.
 *   2306
     *   Cannot add and remove the same value.
     *   Host names must be at least two levels below the registry suffix.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## LoginFlow