diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index 0e1fdf5c14..8f4d756e35 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -53,13 +53,6 @@ needs two devices (``-device intel-hda -device hda-duplex``) and
 ``pcspk`` which can be activated using ``-machine
 pcspk-audiodev=<name>``.
 
-``-mon ...,control=readline,pretty=on|off`` (since 4.1)
-'''''''''''''''''''''''''''''''''''''''''''''''''''''''
-
-The ``pretty=on|off`` switch has no effect for HMP monitors, but is
-silently ignored. Using the switch with HMP monitors will become an
-error in the future.
-
 RISC-V ``-bios`` (since 5.1)
 ''''''''''''''''''''''''''''
 
diff --git a/docs/system/removed-features.rst b/docs/system/removed-features.rst
index aa612c2846..372aadceae 100644
--- a/docs/system/removed-features.rst
+++ b/docs/system/removed-features.rst
@@ -51,6 +51,12 @@ host controller or ``-usb`` if necessary).
 The ``acl`` option to the ``-vnc`` argument has been replaced
 by the ``tls-authz`` and ``sasl-authz`` options.
 
+``-mon ...,control=readline,pretty=on|off`` (removed in 6.0)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``pretty=on|off`` switch has no effect for HMP monitors and
+its use is rejected.
+
 QEMU Machine Protocol (QMP) commands
 ------------------------------------
 
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 640496e562..636bcc81c5 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -716,8 +716,8 @@ int monitor_init(MonitorOptions *opts, bool allow_hmp, Error **errp)
             return -1;
         }
         if (opts->pretty) {
-            warn_report("'pretty' is deprecated for HMP monitors, it has no "
-                        "effect and will be removed in future versions");
+            error_setg(errp, "'pretty' is not compatible with HMP monitors");
+            return -1;
         }
         monitor_init_hmp(chr, true, &local_err);
         break;
diff --git a/qemu-options.hx b/qemu-options.hx
index 9128dec510..671b310ab8 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3740,8 +3740,9 @@ DEF("mon", HAS_ARG, QEMU_OPTION_mon, \
     "-mon [chardev=]name[,mode=readline|control][,pretty[=on|off]]\n", QEMU_ARCH_ALL)
 SRST
 ``-mon [chardev=]name[,mode=readline|control][,pretty[=on|off]]``
-    Setup monitor on chardev name. ``pretty`` turns on JSON pretty
-    printing easing human reading and debugging.
+    Setup monitor on chardev name. ``pretty`` is only valid when
+    ``mode=control``, turning on JSON pretty printing to ease
+    human reading and debugging.
 ERST
 
 DEF("debugcon", HAS_ARG, QEMU_OPTION_debugcon, \