diff --git a/SignalServiceKit/src/Messages/MessageSender+Errors.swift b/SignalServiceKit/src/Messages/MessageSender+Errors.swift
index 9a25378dc9..c28952debf 100644
--- a/SignalServiceKit/src/Messages/MessageSender+Errors.swift
+++ b/SignalServiceKit/src/Messages/MessageSender+Errors.swift
@@ -164,11 +164,6 @@ public class MessageSenderErrorNoValidRecipients: NSObject, CustomNSError, IsRet
         MessageSenderErrorNoValidRecipients() as Error as NSError
     }
 
-    public var localizedDescription: String {
-        NSLocalizedString("ERROR_DESCRIPTION_NO_VALID_RECIPIENTS",
-                          comment: "Error indicating that an outgoing message had no valid recipients.")
-    }
-
     /// NSError bridging: the domain of the error.
     /// :nodoc:
     @objc
@@ -189,6 +184,11 @@ public class MessageSenderErrorNoValidRecipients: NSObject, CustomNSError, IsRet
         [NSLocalizedDescriptionKey: self.localizedDescription]
     }
 
+    public var localizedDescription: String {
+        NSLocalizedString("ERROR_DESCRIPTION_NO_VALID_RECIPIENTS",
+                          comment: "Error indicating that an outgoing message had no valid recipients.")
+    }
+
     public var isRetryableProvider: Bool { false }
 }
 
@@ -215,12 +215,6 @@ public class MessageSenderNoSessionForTransientMessageError: NSObject, CustomNSE
     /// :nodoc:
     public var errorCode: Int { Self.errorCode }
 
-    /// NSError bridging: the error code within the given domain.
-    /// :nodoc:
-    public var errorUserInfo: [String: Any] {
-        [NSLocalizedDescriptionKey: self.localizedDescription]
-    }
-
     public var isRetryableProvider: Bool { false }
 }
 
diff --git a/SignalServiceKit/src/Network/API/RESTNetworkManager.swift b/SignalServiceKit/src/Network/API/RESTNetworkManager.swift
index 1f76776827..1faa28e7e2 100644
--- a/SignalServiceKit/src/Network/API/RESTNetworkManager.swift
+++ b/SignalServiceKit/src/Network/API/RESTNetworkManager.swift
@@ -46,6 +46,7 @@ public class RESTSessionManager: NSObject {
                                success: @escaping RESTNetworkManagerSuccess,
                                failure: @escaping RESTNetworkManagerFailure) {
         assertOnQueue(NetworkManagerQueue())
+        owsAssertDebug(!FeatureFlags.deprecateREST)
 
         guard let rawRequestUrl = request.url else {
             owsFailDebug("Missing requestUrl.")
diff --git a/SignalServiceKit/src/Network/OWSURLSession.swift b/SignalServiceKit/src/Network/OWSURLSession.swift
index 935a3b07af..991de49d1c 100644
--- a/SignalServiceKit/src/Network/OWSURLSession.swift
+++ b/SignalServiceKit/src/Network/OWSURLSession.swift
@@ -385,9 +385,7 @@ public class OWSURLSession: NSObject {
 
     @objc
     public static var signalIosUserAgent: String {
-        let result = "Signal-iOS/\(AppVersion.shared().currentAppVersionLong) iOS/\(UIDevice.current.systemVersion)"
-        Logger.verbose("---- userAgent: \(result)")
-        return result
+        "Signal-iOS/\(AppVersion.shared().currentAppVersionLong) iOS/\(UIDevice.current.systemVersion)"
     }
 
     private func buildUrl(_ urlString: String) -> URL? {
diff --git a/SignalServiceKit/src/Network/WebSockets/HTTP+OWS.swift b/SignalServiceKit/src/Network/WebSockets/HTTP+OWS.swift
index d054d8b015..a88786429e 100644
--- a/SignalServiceKit/src/Network/WebSockets/HTTP+OWS.swift
+++ b/SignalServiceKit/src/Network/WebSockets/HTTP+OWS.swift
@@ -308,6 +308,13 @@ public extension Error {
         HTTPResponseDataForError(self)
     }
 
+    var httpRequestUrl: URL? {
+        guard let error = self as? HTTPError else {
+            return nil
+        }
+        return error.requestUrl
+    }
+
     var httpResponseJson: Any? {
         guard let data = httpResponseData else {
             return nil
diff --git a/SignalServiceKit/src/Util/Error+isRetryable.swift b/SignalServiceKit/src/Util/Error+isRetryable.swift
index 7d7d35e1b9..5b1af6f56c 100644
--- a/SignalServiceKit/src/Util/Error+isRetryable.swift
+++ b/SignalServiceKit/src/Util/Error+isRetryable.swift
@@ -46,6 +46,13 @@ extension NSError {
             Logger.verbose("Network error without retry behavior specified: \(self)")
             return true
         }
+        // Do not retry 4xx errors.
+        if let statusCode = (self as Error).httpStatusCode,
+           statusCode >= 400,
+           statusCode <= 499 {
+            Logger.info("Not retrying error: \(statusCode), \(String(describing: (self as Error).httpRequestUrl))")
+            return false
+        }
 
         // This value should always be set for all errors by this
         // var is consulted.  If not, default to retrying in production.
diff --git a/SignalServiceKit/src/Util/FeatureFlags.swift b/SignalServiceKit/src/Util/FeatureFlags.swift
index eec4bc4d5b..8190690488 100644
--- a/SignalServiceKit/src/Util/FeatureFlags.swift
+++ b/SignalServiceKit/src/Util/FeatureFlags.swift
@@ -202,7 +202,7 @@ public class FeatureFlags: BaseFlags {
     public static let deprecateREST = false
 
     @objc
-    public static let newHostNames = false
+    public static let newHostNames = build.includes(.qa)
 
     public static func buildFlagMap() -> [String: Any] {
         BaseFlags.buildFlagMap(for: FeatureFlags.self) { (key: String) -> Any? in