diff --git a/bin/zmq-recv.py b/bin/zmq-recv.py
index cbf0b5cf..72fa24f7 100755
--- a/bin/zmq-recv.py
+++ b/bin/zmq-recv.py
@@ -23,6 +23,9 @@
 run this script with "rep" and run copyparty with this:
   --xm t3,zmq:req:tcp://localhost:5555
 
+note: to conditionally block uploads based on message contents,
+use rep_server to answer with "return 1" and run copyparty with
+  --xau t3,c,zmq:req:tcp://localhost:5555
 """
 
 
@@ -56,7 +59,9 @@ def rep_server():
     sck.bind("tcp://*:5555")
     while True:
         print("copyparty says %r" % (sck.recv_string(),))
-        sck.send(b"thx")
+        reply = b"thx"
+        # reply = b"return 1"  # non-zero to block an upload
+        sck.send(reply)
 
 
 mode = sys.argv[1].lower() if len(sys.argv) > 1 else ""
diff --git a/copyparty/util.py b/copyparty/util.py
index a0c886f0..c17c451e 100644
--- a/copyparty/util.py
+++ b/copyparty/util.py
@@ -3482,7 +3482,7 @@ def _zmq_hook(
     msg: str,
     wait: float,
     sp_ka: dict[str, Any],
-) -> str:
+) -> tuple[int, str]:
     import zmq
 
     try:
@@ -3493,6 +3493,7 @@ def _zmq_hook(
         mtx = ZMQ["mtx"]
 
     ret = ""
+    nret = 0
     t0 = time.time()
     if verbose and log:
         log("hook(%s) %r entering zmq-main-lock" % (src, cmd), 6)
@@ -3554,6 +3555,10 @@ def _zmq_hook(
                 log("hook(%s) %r awaiting ack from req" % (src, cmd), 6)
             try:
                 ret = sck.recv().decode("utf-8", "replace")
+                if ret.startswith("return "):
+                    m = re.search("^return ([0-9]+)", ret[:12])
+                    if m:
+                        nret = int(m.group(1))
             except:
                 sck.close()
                 del ZMQ[cmd]  # bad state; must reset
@@ -3567,7 +3572,7 @@ def _zmq_hook(
         if wait > 0:
             time.sleep(wait)
 
-    return ret
+    return nret, ret
 
 
 def _runhook(
@@ -3614,12 +3619,9 @@ def _runhook(
         arg = txt or ap
 
     if acmd[0].startswith("zmq:"):
-        zs = "zmq-error"
-        try:
-            zs = _zmq_hook(log, verbose, src, acmd[0][4:].lower(), arg, wait, sp_ka)
-        except Exception as ex:
-            if log:
-                log("zeromq failed: %r" % (ex,))
+        zi, zs = _zmq_hook(log, verbose, src, acmd[0][4:].lower(), arg, wait, sp_ka)
+        if zi:
+            raise Exception("zmq says %d" % (zi,))
         return {"rc": 0, "stdout": zs}
 
     acmd += [arg]
diff --git a/tests/util.py b/tests/util.py
index 53ee9e16..1cf01f00 100644
--- a/tests/util.py
+++ b/tests/util.py
@@ -141,7 +141,7 @@ def __init__(self, a=None, v=None, c=None, **ka0):
         ex = "hash_mt hsortn safe_dedup srch_time u2abort u2j u2sz"
         ka.update(**{k: 1 for k in ex.split()})
 
-        ex = "au_vol dl_list mtab_age reg_cap s_thead s_tbody th_convt"
+        ex = "au_vol dl_list mtab_age reg_cap s_thead s_tbody th_convt ups_who"
         ka.update(**{k: 9 for k in ex.split()})
 
         ex = "db_act k304 loris no304 re_maxage rproxy rsp_jtr rsp_slp s_wr_slp snap_wri theme themes turbo"