diff --git a/go/sdkversion/sdkversion.go b/go/sdkversion/sdkversion.go
new file mode 100644
index 0000000..46a705a
--- /dev/null
+++ b/go/sdkversion/sdkversion.go
@@ -0,0 +1,57 @@
+// Package sdkversion says which mesh-sdk version a program was built with (novox/hq ADR 0446 clause 4: a composition
+// check names its mesh-sdk version, so the controller and the assigner can be compared; ADR 0453 clause 1).
+//
+// The version is read from the build info, never kept by hand: a constant nobody bumps would report two different
+// SDKs as one. A program that imports the SDK reports the module's pinned (pseudo-)version, or its replacement's;
+// the SDK's own binaries and tests report "(devel)" with the stamped vcs.revision.
+package sdkversion
+
+import (
+	"runtime/debug"
+)
+
+// Module is the SDK's Go module path.
+const Module = "git.novox.be/novox/mesh-sdk/go"
+
+// Version is the mesh-sdk version this binary was built with: "unknown" when the build carries no build info.
+func Version() string {
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		return "unknown"
+	}
+	return FromBuildInfo(bi)
+}
+
+// FromBuildInfo reads the SDK's version out of a build info. A dependency replaced by another module version is that
+// version; replaced by a local path it is the path, never empty. When the SDK is the main module it is "(devel)",
+// followed by a space and the vcs.revision when the build stamped one. Without the SDK in the build it is "unknown".
+func FromBuildInfo(bi *debug.BuildInfo) string {
+	if bi == nil {
+		return "unknown"
+	}
+	if bi.Main.Path == Module {
+		for _, s := range bi.Settings {
+			if s.Key == "vcs.revision" && s.Value != "" {
+				return "(devel) " + s.Value
+			}
+		}
+		return "(devel)"
+	}
+	for _, d := range bi.Deps {
+		if d == nil || d.Path != Module {
+			continue
+		}
+		if r := d.Replace; r != nil {
+			if r.Version != "" {
+				return r.Version
+			}
+			if r.Path != "" {
+				return r.Path
+			}
+		}
+		if d.Version != "" {
+			return d.Version
+		}
+	}
+	return "unknown"
+}
diff --git a/go/sdkversion/sdkversion_test.go b/go/sdkversion/sdkversion_test.go
new file mode 100644
index 0000000..7a4b04d
--- /dev/null
+++ b/go/sdkversion/sdkversion_test.go
@@ -0,0 +1,58 @@
+package sdkversion
+
+import (
+	"runtime/debug"
+	"testing"
+)
+
+const sdkPath = "git.novox.be/novox/mesh-sdk/go"
+
+// The build infos are constructed in the shape runtime/debug.ReadBuildInfo gives a binary that imports the SDK
+// (go.mod pins like mesh-controller's v0.1.14-0.20261019091011-aa68b67389c9).
+func TestAPinnedDependencyIsItsPseudoVersion(t *testing.T) {
+	bi := &debug.BuildInfo{Main: debug.Module{Path: "github.com/novox/mesh-controller", Version: "(devel)"},
+		Deps: []*debug.Module{{Path: "example.org/other", Version: "v1.0.0"}, {Path: sdkPath, Version: "v0.1.14-0.20261019091011-aa68b67389c9"}}}
+	if got := FromBuildInfo(bi); got != "v0.1.14-0.20261019091011-aa68b67389c9" {
+		t.Fatalf("got %q", got)
+	}
+}
+
+func TestAReplacedDependencyIsTheReplacement(t *testing.T) {
+	bi := &debug.BuildInfo{Main: debug.Module{Path: "x"}, Deps: []*debug.Module{{Path: sdkPath, Version: "v0.1.14",
+		Replace: &debug.Module{Path: sdkPath, Version: "v0.2.0-0.20261020000000-abcdef123456"}}}}
+	if got := FromBuildInfo(bi); got != "v0.2.0-0.20261020000000-abcdef123456" {
+		t.Fatalf("got %q", got)
+	}
+	local := &debug.BuildInfo{Main: debug.Module{Path: "x"}, Deps: []*debug.Module{{Path: sdkPath, Version: "v0.1.14",
+		Replace: &debug.Module{Path: "../mesh-sdk/go"}}}}
+	if got := FromBuildInfo(local); got != "../mesh-sdk/go" {
+		t.Fatalf("a local replacement: got %q", got)
+	}
+}
+
+func TestTheMainModuleIsDevelWithItsRevision(t *testing.T) {
+	bi := &debug.BuildInfo{Main: debug.Module{Path: sdkPath, Version: "(devel)"},
+		Settings: []debug.BuildSetting{{Key: "vcs", Value: "git"}, {Key: "vcs.revision", Value: "5786ba190c69bcee983223dbbc149bfb18fe01ed"}}}
+	if got := FromBuildInfo(bi); got != "(devel) 5786ba190c69bcee983223dbbc149bfb18fe01ed" {
+		t.Fatalf("got %q", got)
+	}
+	bare := &debug.BuildInfo{Main: debug.Module{Path: sdkPath, Version: "(devel)"}}
+	if got := FromBuildInfo(bare); got != "(devel)" {
+		t.Fatalf("no stamp: got %q", got)
+	}
+}
+
+func TestNoBuildInfoIsUnknown(t *testing.T) {
+	if got := FromBuildInfo(nil); got != "unknown" {
+		t.Fatalf("got %q", got)
+	}
+	if got := FromBuildInfo(&debug.BuildInfo{Main: debug.Module{Path: "x"}}); got != "unknown" {
+		t.Fatalf("an SDK-less binary: got %q", got)
+	}
+}
+
+func TestVersionIsNeverEmpty(t *testing.T) {
+	if Version() == "" {
+		t.Fatal("empty")
+	}
+}