diff --git a/json_rpc/client.nim b/json_rpc/client.nim
index dd88280..f432306 100644
--- a/json_rpc/client.nim
+++ b/json_rpc/client.nim
@@ -31,6 +31,8 @@ type
     awaiting*: Table[RequestId, Future[JsonString]]
     lastId: int
     onDisconnect*: proc() {.gcsafe, raises: [].}
+    onProcessMessage*: proc(client: RpcClient, line: string):
+      Result[bool, string] {.gcsafe, raises: [].}
 
   GetJsonRpcRequestHeaders* = proc(): seq[(string, string)] {.gcsafe, raises: [].}
 
@@ -67,6 +69,12 @@ method close*(client: RpcClient): Future[void] {.base, gcsafe, async.} =
   doAssert(false, "`RpcClient.close` not implemented")
 
 proc processMessage*(client: RpcClient, line: string): Result[void, string] =
+  if client.onProcessMessage.isNil.not:
+    let fallBack = client.onProcessMessage(client, line).valueOr:
+      return err(error)
+    if not fallBack:
+      return ok()
+
   # Note: this doesn't use any transport code so doesn't need to be
   # differentiated.
   try:
diff --git a/tests/all.nim b/tests/all.nim
index feebc94..696a09a 100644
--- a/tests/all.nim
+++ b/tests/all.nim
@@ -19,4 +19,5 @@ import
   testhook,
   test_jrpc_sys,
   test_router_rpc,
-  test_callsigs
+  test_callsigs,
+  test_client_hook
diff --git a/tests/test_client_hook.nim b/tests/test_client_hook.nim
new file mode 100644
index 0000000..6a97273
--- /dev/null
+++ b/tests/test_client_hook.nim
@@ -0,0 +1,76 @@
+# json-rpc
+# Copyright (c) 2023 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE))
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT))
+# at your option.
+# This file may not be copied, modified, or distributed except according to
+# those terms.
+
+import
+  unittest2,
+  ../json_rpc/rpcclient,
+  ../json_rpc/rpcserver
+
+createRpcSigsFromNim(RpcClient):
+  proc get_Banana(id: int): int
+
+proc installHandlers(s: RpcServer) =
+  s.rpc("get_Banana") do(id: int) -> JsonString:
+    if id == 99:
+      return "123".JsonString
+    elif id == 100:
+      return "\"stop\"".JsonString
+    else:
+      return "\"error\"".JsonString
+
+type
+  Shadow = ref object
+    something: int
+
+proc setupClientHook(client: RpcClient): Shadow =
+  var shadow = Shadow(something: 0)
+  client.onProcessMessage = proc(client: RpcClient, line: string):
+                                Result[bool, string] {.gcsafe, raises: [].} =
+
+     try:
+       let m = JrpcConv.decode(line, JsonNode)
+       if m["result"].kind == JString:
+           if m["result"].str == "stop":
+             shadow.something = 123
+             return ok(false)
+           else:
+             shadow.something = 77
+             return err("not stop")
+
+       return ok(true)
+     except CatchableError as exc:
+      return err(exc.msg)
+  shadow
+
+suite "test callsigs":
+  var server = newRpcHttpServer(["127.0.0.1:0"])
+  server.installHandlers()
+  var client = newRpcHttpClient()
+  let shadow = client.setupClientHook()
+
+  server.start()
+  waitFor client.connect("http://" & $server.localAddress()[0])
+
+  test "client onProcessMessage hook":
+    let res = waitFor client.get_Banana(99)
+    check res == 123
+    check shadow.something == 0
+
+    expect JsonRpcError:
+      let res2 = waitFor client.get_Banana(123)
+      check res2 == 0
+    check shadow.something == 77
+
+    expect InvalidResponse:
+      let res2 = waitFor client.get_Banana(100)
+      check res2 == 0
+    check shadow.something == 123
+
+  waitFor server.stop()
+  waitFor server.closeWait()