diff --git a/backend/internal/services/server_process_test.go b/backend/internal/services/server_process_test.go
index 41d0128..9007b91 100644
--- a/backend/internal/services/server_process_test.go
+++ b/backend/internal/services/server_process_test.go
@@ -512,3 +512,101 @@ func TestStartWithEmptyCommand(t *testing.T) {
 		pm.Stop()
 	}
 }
+
+func TestStartWithServerBinaryEnv(t *testing.T) {
+	dir := t.TempDir()
+	serverfileDir := filepath.Join(dir, "server")
+	os.MkdirAll(serverfileDir, 0755)
+
+	stubSrc, _ := os.ReadFile("testdata/arma3server_x64")
+	customBin := "my-custom-server"
+	os.WriteFile(filepath.Join(serverfileDir, customBin), stubSrc, 0755)
+
+	sm := NewSettingsManager(dir)
+	mm := NewModlistManager(filepath.Join(dir, "modlists"))
+	cm := NewConfigManager(filepath.Join(dir, "cfg"))
+	streamer := NewLogStreamer()
+
+	pm := NewProcessManager(serverfileDir, filepath.Join(dir, "mods"), filepath.Join(dir, "cfg"), filepath.Join(dir, "profiles"), sm, mm, cm, streamer)
+
+	s, _ := sm.Load()
+	s.ServerParameters = "-server -world=empty"
+	sm.Save(s)
+
+	t.Setenv("SERVER_BINARY", customBin)
+
+	if err := pm.Start(); err != nil {
+		t.Fatalf("Start() with SERVER_BINARY env error = %v", err)
+	}
+
+	if !pm.IsRunning() {
+		t.Fatal("IsRunning() should be true")
+	}
+
+	pm.Stop()
+	time.Sleep(100 * time.Millisecond)
+}
+
+func TestStartWithServerParamsEnv(t *testing.T) {
+	dir := t.TempDir()
+	serverfileDir := filepath.Join(dir, "server")
+	os.MkdirAll(serverfileDir, 0755)
+
+	stubSrc, _ := os.ReadFile("testdata/arma3server_x64")
+	os.WriteFile(filepath.Join(serverfileDir, "arma3server_x64"), stubSrc, 0755)
+
+	sm := NewSettingsManager(dir)
+	mm := NewModlistManager(filepath.Join(dir, "modlists"))
+	cm := NewConfigManager(filepath.Join(dir, "cfg"))
+	streamer := NewLogStreamer()
+
+	pm := NewProcessManager(serverfileDir, filepath.Join(dir, "mods"), filepath.Join(dir, "cfg"), filepath.Join(dir, "profiles"), sm, mm, cm, streamer)
+
+	s, _ := sm.Load()
+	s.ServerParameters = "-server -world=empty"
+	sm.Save(s)
+
+	t.Setenv("SERVER_PARAMS", "%command% -custom-param")
+
+	if err := pm.Start(); err != nil {
+		t.Fatalf("Start() with SERVER_PARAMS env error = %v", err)
+	}
+
+	if !pm.IsRunning() {
+		t.Fatal("IsRunning() should be true")
+	}
+
+	pm.Stop()
+	time.Sleep(100 * time.Millisecond)
+}
+
+func TestServerParamsEnvOverridesSettings(t *testing.T) {
+	dir := t.TempDir()
+	serverfileDir := filepath.Join(dir, "server")
+	os.MkdirAll(serverfileDir, 0755)
+
+	stubSrc, _ := os.ReadFile("testdata/arma3server_x64")
+	os.WriteFile(filepath.Join(serverfileDir, "arma3server_x64"), stubSrc, 0755)
+
+	sm := NewSettingsManager(dir)
+	mm := NewModlistManager(filepath.Join(dir, "modlists"))
+	cm := NewConfigManager(filepath.Join(dir, "cfg"))
+	streamer := NewLogStreamer()
+
+	pm := NewProcessManager(serverfileDir, filepath.Join(dir, "mods"), filepath.Join(dir, "cfg"), filepath.Join(dir, "profiles"), sm, mm, cm, streamer)
+
+	s, _ := sm.Load()
+	s.ServerParameters = "-server -world=empty"
+	sm.Save(s)
+
+	// Without SERVER_PARAMS override, Start() would use settings value (no %command%).
+	// With override, it uses %command% substitution.
+	t.Setenv("SERVER_PARAMS", "%command% -test-override")
+
+	if err := pm.Start(); err != nil {
+		t.Fatalf("Start() error = %v (SERVER_PARAMS override not applied?)", err)
+	}
+
+	pm.Stop()
+	time.Sleep(100 * time.Millisecond)
+}
diff --git a/backend/internal/services/steamcmd_test.go b/backend/internal/services/steamcmd_test.go
index fd797de..26f6bd4 100644
--- a/backend/internal/services/steamcmd_test.go
+++ b/backend/internal/services/steamcmd_test.go
@@ -167,3 +167,21 @@ func TestNewSteamCmdManager(t *testing.T) {
 		t.Error("new manager should not be running")
 	}
 }
+
+func TestSteamCmdManager_SteampathEnv(t *testing.T) {
+	dir := t.TempDir()
+	streamer := NewLogStreamer()
+
+	sm := NewSteamCmdManager(dir, streamer)
+
+	// Point STEAMCMD_PATH at the stub so it doesn't need the real binary
+	stubPath, _ := filepath.Abs("testdata/steamcmd")
+	t.Setenv("STEAMCMD_PATH", stubPath)
+
+	err := sm.DownloadMod("123456")
+	if err != nil {
+		t.Fatalf("DownloadMod with STEAMCMD_PATH env error = %v", err)
+	}
+
+	waitForNotRunning(t, sm, 30*time.Second)
+}