diff --git a/kernel/GNUmakefile b/kernel/GNUmakefile
index b8b68eb..df8b312 100644
--- a/kernel/GNUmakefile
+++ b/kernel/GNUmakefile
@@ -225,6 +225,18 @@ override OBJ += $(addprefix obj-$(ARCH)/,$(NASMFILES:.asm=.asm.o))
 endif
 override HEADER_DEPS := $(addprefix obj-$(ARCH)/,$(CFILES:.c=.c.d) $(CXXFILES:.cpp=.cpp.d) $(OBJCFILES:.mm=.mm.d) $(ASFILES:.S=.S.d))
 
+# Build number. src/Api/BuildNo.hpp holds MONTAUK_BUILD_NUMBER and is included
+# (transitively) only by Api/Syscall.cpp. We regenerate (bump) it whenever any
+# kernel object OR any userspace source changes, so the number embedded in the
+# kernel tracks real rebuilds of the kernel/userspace. Api/Syscall.cpp.o is
+# excluded from the prerequisites to avoid a dependency cycle (it depends on the
+# header), and a clean build bumps exactly once.
+override BUILDNO_HDR := src/Api/BuildNo.hpp
+override BUILDNO_OBJ := obj-$(ARCH)/Api/Syscall.cpp.o
+override PROG_SRC := $(shell find ../programs/src ../programs/include ../programs/lib \
+    -type f \( -name '*.c' -o -name '*.cpp' -o -name '*.h' -o -name '*.hpp' \
+    -o -name '*.S' -o -name '*.asm' \) 2>/dev/null)
+
 # Default target. This must come first, before header dependencies.
 .PHONY: all
 all: bin-$(ARCH)/$(OUTPUT)
@@ -232,6 +244,15 @@ all: bin-$(ARCH)/$(OUTPUT)
 # Include header dependencies.
 -include $(HEADER_DEPS)
 
+# Bump the build number when any kernel object or userspace source is newer.
+$(BUILDNO_HDR): $(filter-out $(BUILDNO_OBJ),$(OBJ)) $(PROG_SRC)
+	../scripts/bump-build.sh $@
+
+# Force Syscall.cpp.o to recompile (and the kernel to relink) in the same pass
+# the header is bumped, so the embedded number always matches BuildNo.hpp even
+# on a clean build (where no .d dependency exists yet).
+$(BUILDNO_OBJ): $(BUILDNO_HDR)
+
 # Link rules for building the C compiler runtime.
 cc-runtime-$(ARCH)/cc-runtime.a: GNUmakefile cc-runtime/*
 	rm -rf cc-runtime-$(ARCH)
diff --git a/kernel/src/Api/BuildNo.hpp b/kernel/src/Api/BuildNo.hpp
new file mode 100644
index 0000000..47643ad
--- /dev/null
+++ b/kernel/src/Api/BuildNo.hpp
@@ -0,0 +1,15 @@
+/*
+    * BuildNo.hpp
+    * Monotonic kernel build number.
+    *
+    * This file is the source of truth for the build number. It is regenerated
+    * (incremented) by scripts/bump-build.sh whenever the kernel or userspace
+    * sources change, via a rule in kernel/GNUmakefile. Do not edit the number
+    * by hand; the build system manages it.
+    *
+    * Copyright (c) 2026 Daniel Hammer
+*/
+
+#pragma once
+
+#define MONTAUK_BUILD_NUMBER 1
diff --git a/kernel/src/Api/Info.hpp b/kernel/src/Api/Info.hpp
index ee54402..a484f29 100644
--- a/kernel/src/Api/Info.hpp
+++ b/kernel/src/Api/Info.hpp
@@ -7,6 +7,7 @@
 #pragma once
 #include <Sched/Scheduler.hpp>
 
+#include "BuildNo.hpp"
 #include "Syscall.hpp"
 
 namespace Montauk {
@@ -22,7 +23,8 @@ namespace Montauk {
         for (int i = 0; ver[i]; i++) outInfo->osVersion[i] = ver[i];
         outInfo->osVersion[5] = '\0';
 
-        outInfo->apiVersion = 3;
+        outInfo->apiVersion = 4;
         outInfo->maxProcesses = Sched::MaxProcesses;
+        outInfo->buildNumber = MONTAUK_BUILD_NUMBER;
     }
 };
diff --git a/kernel/src/Api/Syscall.hpp b/kernel/src/Api/Syscall.hpp
index fcc8eb1..1bd1982 100644
--- a/kernel/src/Api/Syscall.hpp
+++ b/kernel/src/Api/Syscall.hpp
@@ -282,6 +282,7 @@ namespace Montauk {
         char osVersion[32];
         uint32_t    apiVersion;
         uint32_t    maxProcesses;
+        uint32_t    buildNumber;   // monotonic kernel build number (see BuildNo.hpp)
     };
 
     struct NetCfg {
diff --git a/programs/include/Api/Syscall.hpp b/programs/include/Api/Syscall.hpp
index be5ae99..30d4be1 100644
--- a/programs/include/Api/Syscall.hpp
+++ b/programs/include/Api/Syscall.hpp
@@ -243,6 +243,7 @@ namespace Montauk {
         char osVersion[32];
         uint32_t    apiVersion;
         uint32_t    maxProcesses;
+        uint32_t    buildNumber;   // monotonic kernel build number
     };
 
     struct KeyEvent {
diff --git a/programs/lib/tls/libtls.a b/programs/lib/tls/libtls.a
index 0981a3f..3923261 100644
Binary files a/programs/lib/tls/libtls.a and b/programs/lib/tls/libtls.a differ
diff --git a/programs/lib/tls/obj/tls.o b/programs/lib/tls/obj/tls.o
index db78f78..cb811c2 100644
Binary files a/programs/lib/tls/obj/tls.o and b/programs/lib/tls/obj/tls.o differ
diff --git a/programs/libs/libprintersapplet/obj/src/libprintersapplet.o b/programs/libs/libprintersapplet/obj/src/libprintersapplet.o
index 7901812..bacff44 100644
Binary files a/programs/libs/libprintersapplet/obj/src/libprintersapplet.o and b/programs/libs/libprintersapplet/obj/src/libprintersapplet.o differ
diff --git a/programs/libs/libtimezonesapplet/obj/src/libtimezonesapplet.o b/programs/libs/libtimezonesapplet/obj/src/libtimezonesapplet.o
index 48292a5..dd3f781 100644
Binary files a/programs/libs/libtimezonesapplet/obj/src/libtimezonesapplet.o and b/programs/libs/libtimezonesapplet/obj/src/libtimezonesapplet.o differ
diff --git a/programs/src/desktop/apps/settings/about.cpp b/programs/src/desktop/apps/settings/about.cpp
index efb90d2..6f53d73 100644
--- a/programs/src/desktop/apps/settings/about.cpp
+++ b/programs/src/desktop/apps/settings/about.cpp
@@ -23,14 +23,15 @@ void settings_draw_about(Canvas& c, SettingsState* st) {
         ? fonts::system_font->get_line_height(fonts::LARGE_SIZE) : (FONT_HEIGHT * 2);
     y += large_h + 8;
 
-    snprintf(line, sizeof(line), "Version %s", st->sys_info.osVersion);
+    snprintf(line, sizeof(line), "Version %s (Build %u)",
+             st->sys_info.osVersion, (unsigned)st->sys_info.buildNumber);
     c.text(x, y, line, theme.text);
     y += line_h + 8;
 
     mtk::draw_separator(c, x, y, c.w - 2 * x, theme);
     y += 12;
 
-    snprintf(line, sizeof(line), "API version: %d", (int)st->sys_info.apiVersion);
+    snprintf(line, sizeof(line), "Build number: %u", (unsigned)st->sys_info.buildNumber);
     c.kv_line(x, &y, line, theme.text, line_h);
 
     int up_sec = (int)(st->uptime_ms / 1000);
diff --git a/scripts/bump-build.sh b/scripts/bump-build.sh
new file mode 100755
index 0000000..2ea009e
--- /dev/null
+++ b/scripts/bump-build.sh
@@ -0,0 +1,44 @@
+#!/bin/sh
+# bump-build.sh - Increment the MontaukOS kernel build number.
+#
+# Reads the current MONTAUK_BUILD_NUMBER from the given header, increments it,
+# and rewrites the header in place. Invoked by kernel/GNUmakefile whenever the
+# kernel or userspace sources change, so the build number reflects real builds.
+#
+# Usage: ./scripts/bump-build.sh <path-to-BuildNo.hpp>
+
+set -e
+
+HDR="$1"
+if [ -z "$HDR" ] || [ ! -f "$HDR" ]; then
+    echo "bump-build.sh: header not found: $HDR" >&2
+    exit 1
+fi
+
+cur=$(sed -n 's/^#define MONTAUK_BUILD_NUMBER \([0-9][0-9]*\).*/\1/p' "$HDR")
+if [ -z "$cur" ]; then
+    echo "bump-build.sh: could not parse MONTAUK_BUILD_NUMBER in $HDR" >&2
+    exit 1
+fi
+
+next=$((cur + 1))
+
+cat > "$HDR" <<EOF
+/*
+    * BuildNo.hpp
+    * Monotonic kernel build number.
+    *
+    * This file is the source of truth for the build number. It is regenerated
+    * (incremented) by scripts/bump-build.sh whenever the kernel or userspace
+    * sources change, via a rule in kernel/GNUmakefile. Do not edit the number
+    * by hand; the build system manages it.
+    *
+    * Copyright (c) 2026 Daniel Hammer
+*/
+
+#pragma once
+
+#define MONTAUK_BUILD_NUMBER $next
+EOF
+
+echo "  BUILD   $cur -> $next"