diff --git a/internal/ios/ios_test.go b/internal/ios/ios_test.go
index d957fe6..a044902 100644
--- a/internal/ios/ios_test.go
+++ b/internal/ios/ios_test.go
@@ -1,10 +1,81 @@
 package ios
 
 import (
+	"bytes"
 	"context"
+	"errors"
+	"strings"
 	"testing"
+	"time"
 )
 
+const simctlJSON = `{
+  "devices": {
+    "com.apple.CoreSimulator.SimRuntime.iOS-17-0": [
+      {"udid": "ipad-udid", "state": "Shutdown", "name": "iPad Pro", "isAvailable": true},
+      {"udid": "iphone15-udid", "state": "Booted", "name": "iPhone 15", "isAvailable": true},
+      {"udid": "broken-udid", "state": "Shutdown", "name": "iPhone 14", "isAvailable": false}
+    ],
+    "com.apple.CoreSimulator.SimRuntime.iOS-16-4": [
+      {"udid": "watch-udid", "state": "Shutdown", "name": "Apple Watch", "isAvailable": true}
+    ]
+  }
+}`
+
+func TestParseBootedDevice(t *testing.T) {
+	cases := []struct {
+		name     string
+		input    string
+		wantUDID string
+		wantNil  bool
+	}{
+		{"picks booted entry", simctlJSON, "iphone15-udid", false},
+		{"none booted", `{"devices":{"r":[{"udid":"a","state":"Shutdown","name":"x","isAvailable":true}]}}`, "", true},
+		{"empty list", `{"devices":{}}`, "", true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := parseBootedDevice([]byte(tc.input))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if tc.wantNil {
+				if got != nil {
+					t.Fatalf("want nil, got %+v", got)
+				}
+				return
+			}
+			if got == nil || got.UDID != tc.wantUDID {
+				t.Fatalf("got %+v, want UDID %q", got, tc.wantUDID)
+			}
+		})
+	}
+}
+
+func TestParseBootedDevice_InvalidJSON(t *testing.T) {
+	if _, err := parseBootedDevice([]byte("not json")); err == nil {
+		t.Fatal("expected error on malformed JSON")
+	}
+}
+
+func TestParseAvailableDevices_DropsUnavailable(t *testing.T) {
+	got, err := parseAvailableDevices([]byte(simctlJSON))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, d := range got {
+		if !d.IsAvailable {
+			t.Fatalf("unavailable device leaked through: %+v", d)
+		}
+		if d.UDID == "broken-udid" {
+			t.Fatalf("isAvailable=false device must not be selectable")
+		}
+	}
+	if len(got) != 3 {
+		t.Fatalf("got %d available across runtimes, want 3", len(got))
+	}
+}
+
 func TestPickSimulator_ByName(t *testing.T) {
 	available := []simDevice{
 		{UDID: "aaa", Name: "iPad Pro", IsAvailable: true},
@@ -79,7 +150,7 @@ func TestPickSimulator_EmptyList(t *testing.T) {
 	}
 }
 
-func TestBootedUDID_CanceledContext(t *testing.T) {
+func TestBootedUDID_CanceledContextReturnsEmpty(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 	udid := BootedUDID(ctx)
@@ -87,3 +158,77 @@ func TestBootedUDID_CanceledContext(t *testing.T) {
 		t.Errorf("expected empty UDID on canceled context, got %q", udid)
 	}
 }
+
+func swapSeams(t *testing.T) {
+	t.Helper()
+	origBoot, origWait := boot, waitForBoot
+	t.Cleanup(func() { boot, waitForBoot = origBoot, origWait })
+}
+
+func TestEnsureSimulator_UsesBootedSimulator(t *testing.T) {
+	swapSeams(t)
+	origListBooted, origListAvailable := listBooted, listAvailable
+	t.Cleanup(func() { listBooted, listAvailable = origListBooted, origListAvailable })
+
+	listBooted = func(context.Context) (*simDevice, error) {
+		return &simDevice{UDID: "iphone15-udid", Name: "iPhone 15"}, nil
+	}
+	availableCalled := false
+	listAvailable = func(context.Context) ([]simDevice, error) {
+		availableCalled = true
+		return nil, nil
+	}
+	boot = func(context.Context, string) error {
+		t.Fatal("must not boot when a simulator is already booted")
+		return nil
+	}
+
+	var out bytes.Buffer
+	if err := EnsureSimulator(context.Background(), "", &out); err != nil {
+		t.Fatal(err)
+	}
+	if availableCalled {
+		t.Error("should short-circuit without listing available simulators")
+	}
+	if !bytes.Contains(out.Bytes(), []byte("iphone15-udid")) {
+		t.Errorf("expected booted UDID in output, got %q", out.String())
+	}
+}
+
+func TestEnsureSimulator_BootsPickedSimulator(t *testing.T) {
+	swapSeams(t)
+	origListBooted, origListAvailable := listBooted, listAvailable
+	t.Cleanup(func() { listBooted, listAvailable = origListBooted, origListAvailable })
+
+	listBooted = func(context.Context) (*simDevice, error) { return nil, nil }
+	listAvailable = func(context.Context) ([]simDevice, error) {
+		return []simDevice{
+			{UDID: "ipad-udid", Name: "iPad Pro", IsAvailable: true},
+			{UDID: "iphone15-udid", Name: "iPhone 15", IsAvailable: true},
+		}, nil
+	}
+	var bootedUDID string
+	boot = func(_ context.Context, udid string) error { bootedUDID = udid; return nil }
+	waitForBoot = func(context.Context, string, time.Duration) error { return nil }
+
+	if err := EnsureSimulator(context.Background(), "", new(bytes.Buffer)); err != nil {
+		t.Fatal(err)
+	}
+	if bootedUDID != "iphone15-udid" {
+		t.Errorf("booted %q, want iphone15-udid (iPhone preference)", bootedUDID)
+	}
+}
+
+func TestEnsureSimulator_BootedListError(t *testing.T) {
+	origListBooted := listBooted
+	t.Cleanup(func() { listBooted = origListBooted })
+	listBooted = func(context.Context) (*simDevice, error) { return nil, errors.New("xcrun blew up") }
+
+	err := EnsureSimulator(context.Background(), "", new(bytes.Buffer))
+	if err == nil {
+		t.Fatal("expected error to propagate")
+	}
+	if !strings.Contains(err.Error(), "xcrun blew up") {
+		t.Errorf("error should wrap cause, got %v", err)
+	}
+}