diff --git a/CMakeLists.txt b/CMakeLists.txt
index ad68a3b..d034e71 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -102,6 +102,31 @@ add_custom_command(
set_source_files_properties(${CMAKE_CURRENT_BINARY_DIR}/udpscope_ui_resources.c
PROPERTIES COMPILE_OPTIONS "-Wno-overlength-strings")
+# --- the UI files: an opt-in check, never part of `cmake --build` ---------------
+# The interface is hand-written GtkBuilder XML, and nothing in the build sees a mistake in it: a
+# property a class does not have is a `Gtk-ERROR` when the window is built, and `Gtk-ERROR` *aborts
+# the process* -- so a typo takes the application down rather than failing to open a dialog.
+# `xml-stripblanks` catches malformed XML and nothing else, and the probes catch the rest only for the
+# windows they open.
+#
+# blueprint-compiler type-checks every property and child type against the GIR, and it reads XML as
+# well as blueprint (`decompile` turns one into the other), so `tools/check_ui.py` round-trips each
+# file through it. The XML stays the source of truth and stays hand-written; nothing here is needed to
+# build the application, which is the whole point of not converting the files.
+#
+# No ALL: this runs only when it is named, so `cmake --build build` is unaffected and
+# blueprint-compiler is not a build dependency.
+find_program(UDPSCOPE_PYTHON NAMES python3)
+if(UDPSCOPE_PYTHON)
+ add_custom_target(test-ui
+ COMMAND ${UDPSCOPE_PYTHON} ${CMAKE_CURRENT_SOURCE_DIR}/tools/check_ui.py ${UDPSCOPE_UI_DIR}
+ DEPENDS ${UDPSCOPE_UI_FILES} ${CMAKE_CURRENT_SOURCE_DIR}/tools/check_ui.py
+ COMMENT "Checking the UI files with blueprint-compiler"
+ VERBATIM)
+else()
+ message(WARNING "python3 was not found: the test-ui target is not available")
+endif()
+
# --- build info: version, commit and timestamp compiled into the binary -------
# Written at configure time so add_executable can name the file, and again on every
# build so the commit and the timestamp belong to the binary that is running rather
diff --git a/docs/claude/build-and-run.md b/docs/claude/build-and-run.md
index a335ac7..60b6fed 100644
--- a/docs/claude/build-and-run.md
+++ b/docs/claude/build-and-run.md
@@ -34,6 +34,24 @@ deliberate: the entry then always launches exactly the binary that was last buil
`StartupWMClass` is needed — GTK takes the WM class from the application ID, and the desktop
file's basename is that same string, which is what associates a running window with the entry.
+The interface files can be checked without building anything or opening a window:
+
+```sh
+cmake --build build --target test-ui # or: make -C build test-ui
+```
+
+That round-trips every `.ui` through `blueprint-compiler` — `decompile` turns it into blueprint, and
+compiling *that* type-checks every property and child type against the GIR — printing one line per file.
+It exists because the alternative is what happens otherwise: a property a class does not have is a
+`Gtk-ERROR` when the window is built, and `Gtk-ERROR` **aborts the process**, so a typo in a `.ui` takes
+the application down rather than failing to open a dialog. `preprocess="xml-stripblanks"` catches
+malformed XML and nothing else, and the probes catch the rest only for the windows they open.
+
+The XML stays the source of truth and stays hand-written: this is a check, not a compiler, and nothing
+here is needed to *build* the application. `blueprint-compiler` and `python3` are found at configure
+time; without them the target is absent rather than silently passing. `tools/check_ui.py --lint` also
+prints the linter's advice, which is advice and deliberately not a gate.
+
Options: `--server`, `--port`, `--multicast`, `--interface`, `--buffer-mb` (per
buffer, default 200), `--max-window`, `--holdoff`, `--recv-buffer`, `--plots` (the row
count to *start* with, 1..`RowSet::kMaxRows`; the layout editor owns it afterwards),
diff --git a/docs/claude/invariants-gtk-and-chrome.md b/docs/claude/invariants-gtk-and-chrome.md
index 92f528d..51b3b82 100644
--- a/docs/claude/invariants-gtk-and-chrome.md
+++ b/docs/claude/invariants-gtk-and-chrome.md
@@ -210,23 +210,30 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
- **`find_accel()` masks the modifiers before comparing.** Lock, Num-Lock and the pointer buttons
all arrive in `GdkModifierType`, so an unmasked compare matches no table row at all once
Caps Lock is on.
-- **A UI file is data, and a mistake in it is a runtime abort, not a build failure.** `.ui` files live in
- `src/ui/` and are compiled into the binary by the third GResource (`udpscope-ui.gresource.xml`,
- `prefix=/org/udpscope/ui`), loaded with `gtk_builder_new_from_resource()`. `preprocess="xml-stripblanks"`
- catches a *malformed* file at build time — and that includes an XML rule easy to break by hand: **a
- comment may not contain a double hyphen**, so prose written the way every C++ comment in this project
- is written (` -- like this -- `) fails the build with a caret under the offending character. Use an em
- dash in a UI file. `preprocess="xml-stripblanks"`
- catches a *malformed* file at build time -- and nothing else does, which is why it is on every file in
- every bundle. What it cannot catch is a file that is valid XML but wrong for GTK: `GtkBuilder` reports
- that with a **`Gtk-ERROR`, which aborts the process** by default, so a window built by hand from bad
- markup takes the application down rather than failing to appear. The `[help]` probe opening every
- window is what catches that. Two specifics already paid for: a `GIcon` property takes an **object**, not
- a name (`foo` is read as a reference to another object and fails with
- "Object with ID foo not found" -- the icon is a nested `GThemedIcon`), and a `.ui` big enough to exceed
- 4095 characters needs `-Wno-overlength-strings` on its generated source, because glib-compile-resources
- writes each file as one C string literal and the warning fires on generated *data* (the icon and help
- bundles are small enough per file never to need it).
+- **A UI file is data, and a mistake in it is a runtime abort, not a build failure.** `.ui` files live
+ in `src/ui/` and are compiled into the binary by the third GResource (`udpscope-ui.gresource.xml`,
+ `prefix=/org/udpscope/ui`), loaded with `gtk_builder_new_from_resource()`.
+
+ `preprocess="xml-stripblanks"` catches a *malformed* file at build time -- which is why it is on
+ every file in every bundle -- and that includes an XML rule easy to break by hand: **a comment may
+ not contain a double hyphen**, so prose written the way every C++ comment in this project is written
+ (` -- like this -- `) fails the build with a caret under the offending character. Use an em dash in a
+ UI file.
+
+ What it cannot catch is a file that is valid XML but wrong for GTK, and that is what
+ `cmake --build build --target test-ui` is for: it round-trips every file through
+ `blueprint-compiler`, which type-checks properties and child types against the GIR -- for *every*
+ file, rather than for the windows a probe happens to open (see
+ [build-and-run.md](build-and-run.md)). Without that check, `GtkBuilder` reports such a file with a
+ **`Gtk-ERROR`, which aborts the process** by default, so a window built from bad markup takes the
+ application down rather than failing to appear.
+
+ Two more paid for: a `GIcon` property takes an **object**, not a name
+ (`foo` is read as a reference to another object and fails with
+ "Object with ID foo not found" -- the icon is a nested `GThemedIcon`); and a `.ui` big enough to
+ exceed 4095 characters needs `-Wno-overlength-strings` on its generated source, because
+ glib-compile-resources writes each file as one C string literal and the warning fires on generated
+ *data*.
- **Deprecated widgets may be the right widget, as long as no deprecated *function* is called.** The
shortcut overview is a `GtkShortcutsWindow` (`gtk/deprecated/`, deprecated in 4.20) because libadwaita's
successor, `AdwShortcutsDialog`, cannot draw a mouse gesture: `AdwShortcutsItem` has no icon property at
diff --git a/tools/check_ui.py b/tools/check_ui.py
new file mode 100755
index 0000000..f2d9e2e
--- /dev/null
+++ b/tools/check_ui.py
@@ -0,0 +1,123 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+"""Checks the interface files: `make test-ui`.
+
+The interface is hand-written GtkBuilder XML, and nothing in the build can see a mistake in it. A
+property a class does not have, or a child a container does not accept, is a `Gtk-ERROR` when the
+window is built — and `Gtk-ERROR` aborts the process, so a typo in a `.ui` takes the application
+down rather than failing to open a dialog. `preprocess="xml-stripblanks"` catches malformed XML and
+nothing else, and the probes catch the rest only for the windows they open.
+
+blueprint-compiler can see it: it type-checks every property and child type against the GIR. It reads
+blueprint rather than XML — but it reads XML too, because `decompile` turns a `.ui` back into
+blueprint. So the check is a round trip, and the XML stays the source of truth:
+
+ $ make test-ui
+ check_ui: 5 files in src/ui
+ ok help.ui
+ ok main-window.ui
+ ok preferences.ui
+ ok shortcuts.ui
+ ok source-form.ui
+ check_ui: 5 ok, 0 failed
+ check_ui: 23 pieces of advice — not failures; --lint shows them
+
+A file that fails prints what the compiler said, with the line, the caret and its suggestion:
+
+ FAIL preferences.ui
+ error: Class Adw.EntryRow does not have a property called subtitle
+ at preferences.blp line 24 column 9:
+ 24 | subtitle: "empty = unicast";
+ | ^^^^^^^^
+ hint: Did you mean `title`?
+
+`lint` runs too and its findings are printed as advice rather than failures: a linter has opinions
+this project does not have to share (it wants "udpscope" marked translatable, which it is not), and a
+check that fails on advice is a check people stop running.
+
+A missing `blueprint-compiler` is a *failure* and not a skip. This target exists to be run, and
+answering "fine" without having looked is the one answer it must not give.
+"""
+
+import argparse
+import pathlib
+import shutil
+import subprocess
+import sys
+import tempfile
+
+TOOL = "blueprint-compiler"
+
+
+def check(tool: str, ui: pathlib.Path, scratch: pathlib.Path) -> tuple[bool, list[str]]:
+ """Round-trips one `.ui` through blueprint and returns (ok, advice)."""
+ blp = scratch / (ui.stem + ".blp")
+
+ # `decompile` only transcribes: it is the *compile* of its output that checks the properties, so
+ # both exit codes matter -- a file the decompiler cannot read at all is just as broken.
+ round_trip = subprocess.run(
+ [tool, "decompile", str(ui), "--output", str(blp)], capture_output=True, text=True
+ )
+ if round_trip.returncode != 0 or not blp.exists():
+ return False, [line for line in round_trip.stderr.splitlines() if line]
+
+ compiled = subprocess.run(
+ [tool, "compile", str(blp)], capture_output=True, text=True
+ )
+ if compiled.returncode != 0:
+ return False, [line for line in compiled.stderr.splitlines() if line]
+
+ linted = subprocess.run([tool, "lint", str(blp)], capture_output=True, text=True)
+ advice = [line for line in linted.stderr.splitlines() if line]
+ return True, advice
+
+
+def main() -> int:
+ parser = argparse.ArgumentParser(description=__doc__)
+ parser.add_argument("directory", nargs="?", default="src/ui",
+ help="where the .ui files are (default: src/ui)")
+ parser.add_argument("--lint", action="store_true",
+ help="print the linter's advice as well as the failures")
+ args = parser.parse_args()
+
+ tool = shutil.which(TOOL)
+ if tool is None:
+ print(f"check_ui: {TOOL} is not installed, so the UI files were NOT checked", file=sys.stderr)
+ print(" install it (Arch: pacman -S blueprint-compiler) and run this again",
+ file=sys.stderr)
+ return 2
+
+ directory = pathlib.Path(args.directory)
+ files = sorted(directory.glob("*.ui"))
+ if not files:
+ print(f"check_ui: no .ui files in {directory}", file=sys.stderr)
+ return 2
+ print(f"check_ui: {len(files)} files in {directory}")
+
+ failed = 0
+ advice_count = 0
+ with tempfile.TemporaryDirectory(prefix="check-ui-") as scratch_name:
+ scratch = pathlib.Path(scratch_name)
+ for ui in files:
+ ok, messages = check(tool, ui, scratch)
+ print(f" {'ok ' if ok else 'FAIL'} {ui.name}")
+ if ok:
+ advice_count += len([m for m in messages if m.startswith("warning:")])
+ else:
+ failed += 1
+ # A failure is always printed; advice only when it was asked for. Twenty lines of opinion
+ # on every run is how a check's output stops being read.
+ if not ok or args.lint:
+ for line in messages:
+ print(f" {line}")
+
+ print(f"check_ui: {len(files) - failed} ok, {failed} failed")
+ if advice_count:
+ print(f"check_ui: {advice_count} pieces of advice — not failures; --lint shows them")
+ return 1 if failed else 0
+
+
+if __name__ == "__main__":
+ sys.exit(main())