diff --git a/.gitignore b/.gitignore
index a067cc1b..d5fae1e5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -32,6 +32,7 @@ index.html
 *.exe
 
 # Autotools
+/.utun-build-state*
 Makefile
 !tools/etcpmon/Makefile
 Makefile.in
@@ -84,6 +85,7 @@ logs/
 tests/test_*
 !tests/test_*.c
 !tests/test_*.h
+!tests/test_*.py
 !tests/test_*.conf
 !tests/test_*.json
 tests/bench_*
diff --git a/tests/test_build_script.py b/tests/test_build_script.py
new file mode 100644
index 00000000..8cdb1939
--- /dev/null
+++ b/tests/test_build_script.py
@@ -0,0 +1,198 @@
+#!/usr/bin/env python3
+"""Проверка настоящего build.sh на маленьком проекте Autotools, без изменения исходников uTun."""
+from pathlib import Path
+import json
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+import time
+
+script = Path(sys.argv[1]).resolve()
+base = Path(tempfile.mkdtemp(prefix='utun-build-tests-'))
+log = base / 'commands.log'
+environment = {**os.environ, 'ASAN_OPTIONS': 'detect_leaks=0'}
+environment.pop('MAKEFLAGS', None)
+environment.pop('MSYSTEM', None)
+for name in ['CC', 'CFLAGS', 'CPPFLAGS', 'LDFLAGS', 'LIBS', 'DEBUG_FLAGS']:
+    environment.pop(name, None)
+checks = []
+
+def write(name, content):
+    path = base / name
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(content)
+    return path
+
+def run(args, ok=True, extra_environment=None):
+    result = subprocess.run(args, cwd=base, env={**environment, **(extra_environment or {})}, text=True, capture_output=True)
+    with log.open('a') as output:
+        output.write(f'{args!r}\nexit={result.returncode}\n{result.stdout}{result.stderr}\n')
+    if ok and result.returncode:
+        raise AssertionError(f'{args!r}: exit={result.returncode}; see {log}\n{result.stderr[-1500:]}')
+    return result
+
+def build(*args, **kwargs):
+    return run(['bash', 'build.sh', '--no-gui', '-j', '2', *args], **kwargs)
+
+def check(name, condition):
+    if not condition:
+        raise AssertionError(f'{name}; see {log}')
+    checks.append(name)
+    print(f'[PASS] build.sh: {name}', flush=True)
+
+def times():
+    return {str(p.relative_to(base)): p.stat().st_mtime_ns for p in base.rglob('*.o')}
+
+def value():
+    return run(['./src/utun']).stdout.strip()
+
+try:
+    shutil.copyfile(script, base / 'build.sh')
+    write('configure.ac', '''AC_INIT([build-test], [1.0])
+AC_CONFIG_SRCDIR([src/main.c])
+AC_CONFIG_HEADERS([config.h])
+AC_CONFIG_MACRO_DIR([m4])
+AM_INIT_AUTOMAKE([foreign subdir-objects])
+AC_PROG_CC
+AC_PROG_RANLIB
+AM_PROG_AR
+AC_ARG_WITH([ffmpeg], [], [], [with_ffmpeg=no])
+AC_ARG_ENABLE([hardening], [], [], [enable_hardening=no])
+if test "x$enable_hardening" = xyes; then
+    CFLAGS="$CFLAGS -D_FORTIFY_SOURCE=2 -fstack-protector-strong -fPIE"
+    LDFLAGS="$LDFLAGS -Wl,-z,relro -Wl,-z,now -pie"
+fi
+TEST_CONFIGURE_OPTION
+AC_CONFIG_FILES([Makefile lib/Makefile lib/libopus/Makefile src/Makefile tests/Makefile])
+AC_OUTPUT
+''')
+    write('m4/test.m4', 'AC_DEFUN([TEST_CONFIGURE_OPTION], [AC_DEFINE([TEST_OPTION], [1], [Test option])])\n')
+    write('Makefile.am', 'SUBDIRS = lib lib/libopus src tests\n')
+    write('lib/Makefile.am', 'noinst_LIBRARIES = libfixture.a\nlibfixture_a_SOURCES = fixture.c\n')
+    write('lib/fixture.c', 'int fixture(void) { return 0; }\n')
+    write('lib/libopus/Makefile.am', '''noinst_LIBRARIES = libopus_internal.a
+libopus_internal_a_SOURCES = opus.c
+CFLAGS = -g -Og -fno-omit-frame-pointer -fno-pie -fPIC
+''')
+    write('lib/libopus/opus.c', 'int opus_fixture(void) { return 0; }\n')
+    write('src/Makefile.am', '''bin_PROGRAMS = utun
+utun_SOURCES = main.c worker.c api.h nested.h
+AM_CFLAGS = -DLOCAL_OPTION=1
+utun_LDADD = ../lib/libfixture.a ../lib/libopus/libopus_internal.a
+''')
+    write('src/main.c', '''#include <stdio.h>
+#include "api.h"
+int worker(void);
+int fixture(void);
+int opus_fixture(void);
+int main(void) { printf("%d\\n", worker() + fixture() + opus_fixture()); return 0; }
+''')
+    write('src/worker.c', '#include "api.h"\nint worker(void) { return VALUE + LOCAL_OPTION; }\n')
+    write('src/api.h', '#include "nested.h"\n')
+    write('src/nested.h', '#define VALUE 10\n')
+    write('tests/Makefile.am', 'noinst_PROGRAMS = probe\nprobe_SOURCES = probe.c\n')
+    write('tests/probe.c', 'int main(void) { return 0; }\n')
+    package_tool = write('bin/pkg-config', '''#!/bin/bash
+[[ "${BUILD_TEST_FFMPEG:-0}" = 1 ]] || exit 1
+case "$1" in
+    --exists) exit 0 ;;
+    --modversion) echo 1.0 ;;
+    --cflags|--libs-only-L) exit 0 ;;
+    *) exit 1 ;;
+esac
+''')
+    package_tool.chmod(0o755)
+    environment['PATH'] = str(base / 'bin') + os.pathsep + environment['PATH']
+
+    result = build()
+    check('fresh build generates, configures and builds dependencies', value() == '11' and (base / 'lib/libopus/libopus_internal.a').exists())
+    check('normal build includes tests', (base / 'tests/probe').exists())
+    initial = times()
+    result = build()
+    check('repeat build does not regenerate, configure or compile', initial == times() and 'INFO: configuring:' not in result.stdout and 'INFO: regenerating' not in result.stdout)
+    time.sleep(1.1)
+    write('src/nested.h', '#define VALUE 20\n')
+    result = build()
+    current = times()
+    check('transitive header rebuilds both dependent objects', current['src/main.o'] > initial['src/main.o'] and current['src/worker.o'] > initial['src/worker.o'])
+    check('header change preserves unrelated objects and configuration', current['lib/fixture.o'] == initial['lib/fixture.o'] and 'INFO: configuring:' not in result.stdout and value() == '21')
+    initial = times()
+    write('src/unused.h', '#define UNUSED 1\n')
+    build()
+    check('unrelated header does not recompile', initial == times())
+
+    for mode in ['asan', 'hardened', 'normal']:
+        args = [] if mode == 'normal' else ['--' + mode]
+        result = build(*args)
+        symbols = run(['nm', '-u', 'src/worker.o']).stdout
+        check(f'{mode}: changing mode reconfigures and cleans', 'INFO: configuring:' in result.stdout and 'INFO: cleaning objects:' in result.stdout)
+        check(f'{mode}: object instrumentation matches mode', ('__asan_' in symbols) == (mode == 'asan'))
+        if mode == 'asan':
+            check('ASAN skips tests and preserves unsanitized Opus', not (base / 'tests/probe').exists() and '__asan_' not in run(['nm', '-u', 'lib/libopus/opus.o']).stdout)
+        initial = times()
+        result = build(*args)
+        check(f'{mode}: repeat build is incremental', initial == times() and 'INFO: configuring:' not in result.stdout and 'INFO: regenerating' not in result.stdout)
+
+    result = build('CFLAGS=-g -O0 -DUSER_OPTION=1')
+    check('quoted compiler flags are passed intact through configure', 'CFLAGS = -g -O0 -DUSER_OPTION=1' in (base / 'src/Makefile').read_text())
+    initial = times()
+    build('CFLAGS=-g -O0 -DUSER_OPTION=1')
+    check('repeat build with quoted flags is incremental', initial == times())
+    build()
+    time.sleep(1.1)
+    write('src/Makefile.am', (base / 'src/Makefile.am').read_text().replace('LOCAL_OPTION=1', 'LOCAL_OPTION=2'))
+    result = build()
+    check('Makefile.am flag change rebuilds stale objects', 'INFO: regenerating' in result.stdout and value() == '22')
+    macro = base / 'm4/test.m4'
+    saved_time = macro.stat()
+    write('m4/test.m4', macro.read_text().replace('[1]', '[2]'))
+    os.utime(macro, ns=(saved_time.st_atime_ns, saved_time.st_mtime_ns))
+    build()
+    check('macro content change is detected with unchanged timestamp', '#define TEST_OPTION 2' in (base / 'config.h').read_text())
+    result = build(extra_environment={'BUILD_TEST_FFMPEG': '1'})
+    check('newly available optional dependency reconfigures', 'INFO: configuring:' in result.stdout and '--with-ffmpeg' in run(['./config.status', '--config']).stdout)
+    result = build()
+    check('disappearing optional dependency reconfigures', 'INFO: configuring:' in result.stdout and '--without-ffmpeg' in run(['./config.status', '--config']).stdout)
+    (base / 'src/.deps/worker.Po').unlink()
+    result = build()
+    check('missing dependencies force rebuild without configure', 'missing_dependencies=1' in result.stdout and 'INFO: configuring:' not in result.stdout)
+    write('src/.deps/worker.Po', '# dummy\n')
+    result = build()
+    check('dummy dependencies force rebuild', 'missing_dependencies=1' in result.stdout)
+    result = build('--clean')
+    check('explicit clean does not reconfigure', 'INFO: cleaning objects:' in result.stdout and 'INFO: configuring:' not in result.stdout)
+    result = build('--full')
+    check('explicit full regenerates, configures and rebuilds', 'INFO: regenerating' in result.stdout and 'INFO: configuring:' in result.stdout)
+    (base / 'src/Makefile').unlink()
+    result = build()
+    check('missing child Makefile is recreated', (base / 'src/Makefile').exists() and 'INFO: configuring:' in result.stdout)
+    run(['./configure', 'CFLAGS=-O0'])
+    result = build()
+    check('external configure is detected', 'INFO: configuring:' in result.stdout and 'CFLAGS = -g -O2' in (base / 'src/Makefile').read_text())
+    check('conflicting modes fail', build('--asan', '--hardened', ok=False).returncode != 0)
+    check('invalid job count fails', build('-j0', ok=False).returncode != 0)
+
+    state = (base / '.utun-build-state').read_bytes()
+    original = (base / 'src/worker.c').read_text()
+    write('src/worker.c', original + '\n#error deliberate_failure\n')
+    check('compiler failure propagates without committing state', build(ok=False).returncode != 0 and (base / '.utun-build-state').read_bytes() == state)
+    write('src/worker.c', original)
+    check('build recovers from compiler failure', build().returncode == 0 and value() == '22')
+    original = (base / 'configure.ac').read_text()
+    write('configure.ac', original + '\nm4_fatal([deliberate_autoconf_failure])\n')
+    check('autoreconf failure propagates through tee', build(ok=False).returncode != 0 and (base / '.utun-build-state').read_bytes() == state)
+    write('configure.ac', original)
+    build()
+    write('tools/chatgui/build/CMakeCache.txt', 'placeholder\n')
+    cmake = write('bin/cmake', '#!/bin/bash\nprintf "%s\\n" "$*" > gui-arguments\nexit 9\n')
+    cmake.chmod(0o755)
+    result = run(['bash', 'build.sh', '-j', '3'], ok=False)
+    check('GUI failure propagates and success is not printed', result.returncode != 0 and 'Build completed successfully!' not in result.stdout)
+    check('GUI receives selected job count', (base / 'gui-arguments').read_text().strip() == '--build tools/chatgui/build --parallel 3')
+    print(f'[PASS] build.sh: {len(checks)} checks completed', flush=True)
+    shutil.rmtree(base)
+except Exception:
+    print(f'[FAIL] build.sh: diagnostic files retained in {base}', file=sys.stderr)
+    raise
diff --git a/tools/chatgui/build.sh b/tools/chatgui/build.sh
index 5dffdb15..8b62d98e 100755
--- a/tools/chatgui/build.sh
+++ b/tools/chatgui/build.sh
@@ -14,6 +14,18 @@ log ""
 SRC_DIR="$(cd "$(dirname "$0")" && pwd)"
 BUILD_DIR="$SRC_DIR/build"
 RUNTIME_PREFIX="${MSYSTEM_PREFIX:-/ucrt64}"
+JOBS=$(nproc 2>/dev/null || echo 4)
+while [[ $# -gt 0 ]]; do
+    case "$1" in
+        -j)
+            [[ $# -gt 1 ]] || { log "ERROR: -j requires a positive job count"; exit 1; }
+            shift; JOBS=$1 ;;
+        -j*) JOBS=${1#-j} ;;
+        *) log "ERROR: unknown argument: $1"; exit 1 ;;
+    esac
+    shift
+done
+[[ "$JOBS" =~ ^[1-9][0-9]*$ ]] || { log "ERROR: invalid job count: $JOBS"; exit 1; }
 
 log "Source:  $SRC_DIR"
 log "Build:   $BUILD_DIR"
@@ -36,7 +48,7 @@ if [ "${MSYSTEM:-}" = UCRT64 ]; then
         -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON 2>&1 | tee -a "$LOG_FILE" || {
         log "  ERROR: bundled rlottie configure failed"; exit 1;
     }
-    cmake --build "$SRC_DIR/rlottie_build" --target rlottie --parallel 4 2>&1 | tee -a "$LOG_FILE" || {
+    cmake --build "$SRC_DIR/rlottie_build" --target rlottie --parallel "$JOBS" 2>&1 | tee -a "$LOG_FILE" || {
         log "  ERROR: bundled rlottie build failed"; exit 1;
     }
     log "  OK: bundled rlottie ready in $SRC_DIR/rlottie_build"
@@ -47,7 +59,6 @@ cmake -S "$SRC_DIR" -B "$BUILD_DIR" -G Ninja 2>&1 | tee -a "$LOG_FILE"
 test ${PIPESTATUS[0]} -eq 0 || { log "CMake configure FAILED"; exit 1; }
 
 log ""
-JOBS=$(nproc 2>/dev/null || echo 4)
 log "[2/4] Building (parallel: $JOBS jobs)..."
 cmake --build "$BUILD_DIR" -j"$JOBS" 2>&1 | tee -a "$LOG_FILE"
 test ${PIPESTATUS[0]} -eq 0 || { log "Build FAILED"; exit 1; }