diff --git a/metropolis/pkg/combinectx/combinectx_test.go b/metropolis/pkg/combinectx/combinectx_test.go
new file mode 100644
index 0000000..b7f2625
--- /dev/null
+++ b/metropolis/pkg/combinectx/combinectx_test.go
@@ -0,0 +1,106 @@
+package combinectx
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestCancel(t *testing.T) {
+	a, aC := context.WithCancel(context.Background())
+	b, bC := context.WithCancel(context.Background())
+
+	c := Combine(a, b)
+	if want, got := error(nil), c.Err(); want != got {
+		t.Fatalf("Newly combined context should return %v, got %v", want, got)
+	}
+	if _, ok := c.Deadline(); ok {
+		t.Errorf("Newly combined context should have no deadline")
+	}
+
+	// Cancel A.
+	aC()
+	// Cancels are not synchronous - wait for it to propagate...
+	<-c.Done()
+	// ...then cancel B (no-op).
+	bC()
+
+	if c.Err() == nil {
+		t.Fatalf("After cancel, ctx.Err() should be non-nil")
+	}
+	if !errors.Is(c.Err(), a.Err()) {
+		t.Errorf("After cancel, ctx.Err() should be a.Err()")
+	}
+	if !errors.Is(c.Err(), c.Err()) {
+		t.Errorf("After cancel, ctx.Err() should be ctx.Err()")
+	}
+	if !errors.Is(c.Err(), context.Canceled) {
+		t.Errorf("After cancel, ctx.Err() should be context.Canceled")
+	}
+	if !errors.Is(c.Err(), &Error{}) {
+		t.Errorf("After cancel, ctx.Err() should be a Error pointer")
+	}
+	cerr := &Error{}
+	if !errors.As(c.Err(), &cerr) {
+		t.Fatalf("After cancel, ctx.Err() should be usable as *Error")
+	}
+	if !cerr.First() {
+		t.Errorf("ctx.Err().First() should be true")
+	}
+	if cerr.Second() {
+		t.Errorf("ctx.Err().Second() should be false")
+	}
+	if want, got := a.Err(), cerr.Unwrap(); want != got {
+		t.Errorf("ctx.Err().Unwrap() should be %v, got %v", want, got)
+	}
+}
+
+func TestDeadline(t *testing.T) {
+	now := time.Now()
+	aD := now.Add(100 * time.Millisecond)
+	bD := now.Add(10 * time.Millisecond)
+
+	a, aC := context.WithDeadline(context.Background(), aD)
+	b, bC := context.WithDeadline(context.Background(), bD)
+
+	defer aC()
+	defer bC()
+
+	c := Combine(a, b)
+	if want, got := error(nil), c.Err(); want != got {
+		t.Fatalf("Newly combined context should return %v, got %v", want, got)
+	}
+	if d, ok := c.Deadline(); !ok || !d.Equal(bD) {
+		t.Errorf("Newly combined context should have deadline %v, got %v", bD, d)
+	}
+
+	<-c.Done()
+
+	if c.Err() == nil {
+		t.Fatalf("After deadline, ctx.Err() should be non-nil")
+	}
+	if !errors.Is(c.Err(), b.Err()) {
+		t.Errorf("After deadline, ctx.Err() should be b.Err()")
+	}
+	if !errors.Is(c.Err(), context.DeadlineExceeded) {
+		t.Errorf("After cancel, ctx.Err() should be context.DeadlineExceeded")
+	}
+	if !errors.Is(c.Err(), &Error{}) {
+		t.Errorf("After cancel, ctx.Err() should be a Error pointer")
+	}
+	cerr := &Error{}
+	if !errors.As(c.Err(), &cerr) {
+		t.Fatalf("After cancel, ctx.Err() should be usable as *Error")
+	}
+	if cerr.First() {
+		t.Errorf("ctx.Err().First() should be false")
+	}
+	if !cerr.Second() {
+		t.Errorf("ctx.Err().Second() should be true")
+	}
+	if want, got := b.Err(), cerr.Unwrap(); want != got {
+		t.Errorf("ctx.Err().Unwrap() should be %v, got %v", want, got)
+	}
+}
+