Index: lib/MsgUtils/codegen.pri
===================================================================
diff -u -rb915ccc1a72fcca21908c35140af7ac7df1f1f37 -r4dccc470aedf6f68a0ad73c1101f8a96188cbdb1
--- lib/MsgUtils/codegen.pri	(.../codegen.pri)	(revision b915ccc1a72fcca21908c35140af7ac7df1f1f37)
+++ lib/MsgUtils/codegen.pri	(.../codegen.pri)	(revision 4dccc470aedf6f68a0ad73c1101f8a96188cbdb1)
@@ -2,8 +2,8 @@
 # Included by the components that generate code (e.g. MsgUtils and CloudConnect).
 #
 # qmake evaluates system() at qmake time, while QMAKE_EXTRA_COMPILERS /
-# QMAKE_EXTRA_TARGETS run at make time. The venv is created here (once, shared);
-# the generators are wired as make-time rules in the including .pro so they
+# QMAKE_EXTRA_TARGETS run at make time. The venv is created here once and shared.
+# The generators are wired as make-time rules in the including .pro so they
 # re-run on input change.
 
 PROJECT_ROOT = $$clean_path($$PWD/../..)
@@ -18,13 +18,42 @@
 PROJECT_PYTHON   = $$PROJECT_VENV_DIR/bin/python
 
 # Create the venv, then editable-install the msgutils package so the generator
-# scripts can `import msgutils`. Runs once per qmake invocation; pip is a skipped
-# when already satisfied.
+# scripts can `import msgutils`.
+#
+# A missing python3-venv reports nothing here and later becomes a generator rule
+# failing to find an interpreter. This checks and reports here if it is missing.
 !exists($$PROJECT_PYTHON) {
     message("Creating project Python venv at $$PROJECT_VENV_DIR")
-    system(python3 -m venv $$shell_quote($$PROJECT_VENV_DIR))
+
+    !system(python3 -m venv $$shell_quote($$PROJECT_VENV_DIR)) {
+        # $${} braces: qmake allows '.' in a variable name, so "$$VAR." reads as a
+        # variable named "VAR." and expands to nothing.
+        error("Could not create the Python venv at $${PROJECT_VENV_DIR}. On Debian/Ubuntu this usually means the venv module is missing: sudo apt install python3-venv")
+    }
+
+    # venv can exit 0 and still leave no interpreter (interrupted, disk full).
+    !exists($$PROJECT_PYTHON) {
+        error("The venv at $${PROJECT_VENV_DIR} was created but has no interpreter at $${PROJECT_PYTHON}. Remove the directory and re-run qmake.")
+    }
 }
-system($$shell_quote($$PROJECT_PYTHON) -m pip install --upgrade pip --quiet)
-system($$shell_quote($$PROJECT_PYTHON) -m pip install -e $$shell_quote($$MSGUTILS_SCRIPTS_DIR) --quiet)
 
+# Allow a machine with a working venv and no network can still build.
+!system($$shell_quote($$PROJECT_PYTHON) -m pip install --upgrade pip --quiet) {
+    warning("Could not upgrade pip in $${PROJECT_VENV_DIR}; continuing with the installed version.")
+}
+
+# The generators import msgutils, but a failed reinstall is survivable when the venv
+# already has it.
+!system($$shell_quote($$PROJECT_PYTHON) -m pip install -e $$shell_quote($$MSGUTILS_SCRIPTS_DIR) --quiet) {
+    !system($$shell_quote($$PROJECT_PYTHON) -c $$shell_quote(import msgutils)) {
+        error("Could not install msgutils from $${MSGUTILS_SCRIPTS_DIR} into $${PROJECT_VENV_DIR}, and it is not importable. The code generators cannot run.")
+    }
+    warning("Could not reinstall msgutils from $${MSGUTILS_SCRIPTS_DIR}; continuing with the copy already in the venv.")
+}
+
 PROTOC = protoc
+# Missing protoc reports nothing here and later becomes a generator rule failing with "command not found".
+# This checks and reports here if it is missing.
+!system($$PROTOC --version > /dev/null 2>&1) {
+    error("Could not run '$${PROTOC}'. On Debian/Ubuntu install it with: sudo apt install protobuf-compiler")
+}