diff --git a/CMakeLists.txt b/CMakeLists.txt
index c314d66..0c6339c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -29,27 +29,43 @@ if(PROJECT_IS_TOP_LEVEL)
 	libavr_format_test()
 
 	# The behavioral tests drive the real wire protocols over a simavr pty
-	# (as the host tools do) and actually flash the device. The runners are
-	# host programs built at configure time against libsimavr (C++23 - what
-	# the distribution's compiler speaks in full); if they or Python are
-	# missing, only the size tests run.
+	# (as the host tools do) and actually flash the device. The runner is a
+	# host program built at configure time against libsimavr (C++23 - what the
+	# distribution's compiler speaks in full).
+	#
+	# **A host that cannot build it registers those tests anyway and skips
+	# them.** They used to be left out, which makes the suite a different size
+	# on a different machine - and a suite whose size is a property of the
+	# machine is one nothing can be compared against.
+	set(TSB_DEVICE ${CMAKE_BINARY_DIR}/tsb_device)
 	find_program(_host_cxx NAMES c++ g++)
-	find_package(Python3 COMPONENTS Interpreter)
-	if(_host_cxx AND Python3_FOUND)
-		if(LIBAVR_MCU STREQUAL "atmega328p")
-			set(TSB_DEVICE ${CMAKE_BINARY_DIR}/tsb_device)
-			execute_process(
-			    COMMAND ${_host_cxx} -std=c++23 -Wall -Wextra -O2
-			            -I/usr/include/simavr -I/usr/include/simavr/parts
-			            -o ${TSB_DEVICE} ${CMAKE_CURRENT_SOURCE_DIR}/test/device.cpp
-			            -lsimavr -lsimavrparts -lelf
-			    RESULT_VARIABLE _dev_res ERROR_VARIABLE _dev_err)
-			if(NOT _dev_res EQUAL 0)
-				message(STATUS "tsb_device not built (${_dev_err}) - protocol tests skipped")
-				unset(TSB_DEVICE)
+	set(_tsb_absent "${LIBAVR_NO_PYTHON}")
+	if(NOT _host_cxx)
+		set(_tsb_absent "no host C++ compiler on PATH, and the simavr device is a host program")
+	elseif(NOT _tsb_absent)
+		execute_process(
+		    COMMAND ${_host_cxx} -std=c++23 -Wall -Wextra -O2
+		            -I/usr/include/simavr -I/usr/include/simavr/parts
+		            -o ${TSB_DEVICE} ${CMAKE_CURRENT_SOURCE_DIR}/test/device.cpp
+		            -lsimavr -lsimavrparts -lelf
+		    RESULT_VARIABLE _dev_res ERROR_VARIABLE _dev_err)
+		if(NOT _dev_res EQUAL 0)
+			# One bounded line of it: this becomes a single argument on a
+			# command line, and the reading has to say what stopped the build
+			# rather than that something did.
+			string(REGEX REPLACE "[\r\n\t]+" " " _dev_err "${_dev_err}")
+			string(REPLACE ";" "," _dev_err "${_dev_err}")
+			string(LENGTH "${_dev_err}" _dev_len)
+			if(_dev_len GREATER 240)
+				string(SUBSTRING "${_dev_err}" 0 240 _dev_err)
 			endif()
+			set(_tsb_absent "test/device.cpp does not build here: ${_dev_err}")
 		endif()
 	endif()
+	libavr_launcher(_tsb_python "${_tsb_absent}" ${Python3_EXECUTABLE})
+	if(_tsb_absent)
+		message(STATUS "the protocol tests skip here - ${_tsb_absent}")
+	endif()
 endif()
 
 # The ELF is only a container (symbols, section headers) and is never flashed -
@@ -114,11 +130,9 @@ function(add_tsb_variant name bytes)
 		add_test(NAME ${name}.size
 		         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:${name}>
 		                 -DLIMIT=${bytes} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
-		if(DEFINED TSB_DEVICE)
-			add_test(NAME ${name}.protocol
-			         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/tsbtest.py
-			                 ${TSB_DEVICE} $<TARGET_FILE:${name}> ${base_hex})
-		endif()
+		add_test(NAME ${name}.protocol
+		         COMMAND ${_tsb_python} ${CMAKE_CURRENT_SOURCE_DIR}/test/tsbtest.py
+		                 ${TSB_DEVICE} $<TARGET_FILE:${name}> ${base_hex})
 	endif()
 endfunction()
 
@@ -135,3 +149,9 @@ if(LIBAVR_MCU STREQUAL "atmega328p")
 	# floors were measured with - none.
 	target_compile_options(tsb_policy PRIVATE -fno-move-loop-invariants -fno-tree-ter)
 endif()
+
+# Every test registered above carries the marker a stubbed launcher prints, so
+# a check this host cannot run reads as Skipped rather than Failed.
+if(PROJECT_IS_TOP_LEVEL)
+	libavr_skip_unverified()
+endif()
diff --git a/libavr b/libavr
index b62d6b5..8521f48 160000
--- a/libavr
+++ b/libavr
@@ -1 +1 @@
-Subproject commit b62d6b5b30b1122128b1d277c715f92aeb2abcff
+Subproject commit 8521f48a3361100c5f3e3c5004a5ff415e2d1484