diff --git a/servlet/src/main/scala/org/http4s/servlet/AsyncHttp4sServlet.scala b/servlet/src/main/scala/org/http4s/servlet/AsyncHttp4sServlet.scala
index a7a3f8b8..13ef0c11 100644
--- a/servlet/src/main/scala/org/http4s/servlet/AsyncHttp4sServlet.scala
+++ b/servlet/src/main/scala/org/http4s/servlet/AsyncHttp4sServlet.scala
@@ -108,7 +108,7 @@ class AsyncHttp4sServlet[F[_]](
       val response = Response[F](Status.InternalServerError)
       // We don't know what I/O mode we're in here, and we're not rendering a body
       // anyway, so we use a NullBodyWriter.
-      val f = renderResponse(response, servletResponse, NullBodyWriter) *>
+      val f = renderResponse(response, servletResponse, nullBodyWriter) *>
         F.delay(
           if (servletRequest.isAsyncStarted)
             servletRequest.getAsyncContext.complete()
diff --git a/servlet/src/main/scala/org/http4s/servlet/BlockingHttp4sServlet.scala b/servlet/src/main/scala/org/http4s/servlet/BlockingHttp4sServlet.scala
index 24c5d5f6..04cd52c4 100644
--- a/servlet/src/main/scala/org/http4s/servlet/BlockingHttp4sServlet.scala
+++ b/servlet/src/main/scala/org/http4s/servlet/BlockingHttp4sServlet.scala
@@ -18,6 +18,7 @@ package org.http4s
 package servlet
 
 import cats.effect.kernel.Async
+import cats.effect.kernel.Sync
 import cats.effect.std.Dispatcher
 import cats.syntax.all._
 import org.http4s.server._
@@ -30,8 +31,19 @@ class BlockingHttp4sServlet[F[_]](
     servletIo: ServletIo[F],
     serviceErrorHandler: ServiceErrorHandler[F],
     dispatcher: Dispatcher[F],
-)(implicit F: Async[F])
+)(implicit F: Sync[F])
     extends Http4sServlet[F](service, servletIo, dispatcher) {
+
+  @deprecated("Binary compatibility", "0.23.12")
+  private[servlet] def this(
+      service: HttpApp[F],
+      servletIo: ServletIo[F],
+      serviceErrorHandler: ServiceErrorHandler[F],
+      dispatcher: Dispatcher[F],
+      async: Async[F],
+  ) =
+    this(service, servletIo, serviceErrorHandler, dispatcher)(async: Sync[F])
+
   override def service(
       servletRequest: HttpServletRequest,
       servletResponse: HttpServletResponse,
@@ -71,13 +83,13 @@ class BlockingHttp4sServlet[F[_]](
         val response = Response[F](Status.InternalServerError)
         // We don't know what I/O mode we're in here, and we're not rendering a body
         // anyway, so we use a NullBodyWriter.
-        renderResponse(response, servletResponse, NullBodyWriter)
+        renderResponse(response, servletResponse, nullBodyWriter)
       }
     }
 }
 
 object BlockingHttp4sServlet {
-  def apply[F[_]: Async](
+  def apply[F[_]: Sync](
       service: HttpApp[F],
       servletIo: ServletIo[F],
       dispatcher: Dispatcher[F],
@@ -88,4 +100,16 @@ object BlockingHttp4sServlet {
       DefaultServiceErrorHandler,
       dispatcher,
     )
+
+  @deprecated(
+    "Preserved for binary compatibility.  Use the overload with the implicit Sync",
+    "0.23.12",
+  )
+  def apply[F[_]](
+      service: HttpApp[F],
+      servletIo: ServletIo[F],
+      dispatcher: Dispatcher[F],
+      async: Async[F],
+  ): BlockingHttp4sServlet[F] =
+    apply(service, servletIo, dispatcher)(async: Sync[F])
 }
diff --git a/servlet/src/main/scala/org/http4s/servlet/Http4sServlet.scala b/servlet/src/main/scala/org/http4s/servlet/Http4sServlet.scala
index f67a425d..c6893266 100644
--- a/servlet/src/main/scala/org/http4s/servlet/Http4sServlet.scala
+++ b/servlet/src/main/scala/org/http4s/servlet/Http4sServlet.scala
@@ -17,6 +17,7 @@
 package org.http4s.servlet
 
 import cats.effect.kernel.Async
+import cats.effect.kernel.Sync
 import cats.effect.std.Dispatcher
 import cats.syntax.all._
 import com.comcast.ip4s.IpAddress
@@ -42,8 +43,17 @@ abstract class Http4sServlet[F[_]](
     service: HttpApp[F],
     servletIo: ServletIo[F],
     dispatcher: Dispatcher[F],
-)(implicit F: Async[F])
+)(implicit F: Sync[F])
     extends HttpServlet {
+
+  @deprecated("Binary compatibility", "0.23.12")
+  private[servlet] def this(
+      service: HttpApp[F],
+      servletIo: ServletIo[F],
+      dispatcher: Dispatcher[F],
+      async: Async[F],
+  ) = this(service, servletIo, dispatcher)(async: Sync[F])
+
   protected val logger: Logger = getLogger
 
   // micro-optimization: unwrap the service and call its .run directly
diff --git a/servlet/src/main/scala/org/http4s/servlet/package.scala b/servlet/src/main/scala/org/http4s/servlet/package.scala
index c35609f1..47fc486d 100644
--- a/servlet/src/main/scala/org/http4s/servlet/package.scala
+++ b/servlet/src/main/scala/org/http4s/servlet/package.scala
@@ -16,12 +16,17 @@
 
 package org.http4s
 
+import cats.Applicative
 import cats.effect.Async
 
 package object servlet {
   protected[servlet] type BodyWriter[F[_]] = Response[F] => F[Unit]
 
+  @deprecated("Use nullBodyWriter with Applicative constraint", "0.23.12")
   protected[servlet] def NullBodyWriter[F[_]](implicit F: Async[F]): BodyWriter[F] =
+    nullBodyWriter[F]
+
+  private[servlet] def nullBodyWriter[F[_]](implicit F: Applicative[F]): BodyWriter[F] =
     _ => F.unit
 
   protected[servlet] val DefaultChunkSize = 4096