diff --git a/src/main/java/qupath/ext/instanseg/core/InstanSegModel.java b/src/main/java/qupath/ext/instanseg/core/InstanSegModel.java
index 4322924..1dc15e4 100644
--- a/src/main/java/qupath/ext/instanseg/core/InstanSegModel.java
+++ b/src/main/java/qupath/ext/instanseg/core/InstanSegModel.java
@@ -79,22 +79,13 @@ public static InstanSegModel fromURL(String name, String version, URL browserDow
      * Check if the model has been downloaded already.
      * @return True if the model has a known path that exists and is valid, or if a suitable directory can be found in the localModelPath
      */
-    public boolean isDownloaded(Path localModelPath) {
+    public boolean isValid() {
         // Check path first - *sometimes* the model might be downloaded, but have a name
         // that doesn't match with the filename (although we'd prefer this didn't happen...)
         if (path != null && model != null && isValidModel(path))
             return true;
-        if (!Files.exists(localModelPath.resolve(getFolderName(name, version)))) {
-            // The model may have been deleted or renamed - we won't be able to load it
-            return false;
-        }
-
-        try {
-            download(localModelPath);
-        } catch (IOException e) {
-            logger.error("Model directory exists but is not valid", e);
-        }
-        return path != null && model != null;
+        // The model may have been deleted or renamed - we won't be able to load it
+        return false;
     }
 
     /**
@@ -276,7 +267,8 @@ private Optional<BioimageIoSpec.BioimageIoModel> getModel() {
     }
 
     private static Path downloadZipIfNeeded(URL url, Path localDirectory, String filename) throws IOException {
-        var zipFile = localDirectory.resolve(Path.of(filename + ".zip"));
+        Files.createDirectories(localDirectory); // just in case...
+        var zipFile = localDirectory.resolve(filename + ".zip");
         if (!isDownloadedAlready(zipFile)) {
             try (InputStream stream = url.openStream()) {
                 try (ReadableByteChannel readableByteChannel = Channels.newChannel(stream)) {
@@ -290,8 +282,16 @@ private static Path downloadZipIfNeeded(URL url, Path localDirectory, String fil
     }
 
     private static boolean isDownloadedAlready(Path zipFile) {
-        // todo: validate contents somehow
-        return Files.exists(zipFile);
+        if (!Files.exists(zipFile)) {
+            return false;
+        }
+        try {
+            BioimageIoSpec.parseModel(zipFile.toFile());
+        } catch (IOException e) {
+            logger.warn("Invalid zip file", e);
+            return false;
+        }
+        return true;
     }
 
     private Path unzipIfNeeded(Path zipFile) throws IOException {
diff --git a/src/main/java/qupath/ext/instanseg/ui/InstanSegController.java b/src/main/java/qupath/ext/instanseg/ui/InstanSegController.java
index 99e8412..5fec72a 100644
--- a/src/main/java/qupath/ext/instanseg/ui/InstanSegController.java
+++ b/src/main/java/qupath/ext/instanseg/ui/InstanSegController.java
@@ -375,7 +375,7 @@ private void updateInputChannels(ImageData<BufferedImage> imageData) {
         comboInputChannels.getCheckModel().checkIndices(IntStream.range(0, 3).toArray());
         var modelDir = InstanSegUtils.getModelDirectory().orElse(null);
         // todo: not clear why this is needed. is this handling the checkcombobox weirdness on clearing checks, or?
-        if (model != null && modelDir != null && model.isDownloaded(modelDir)) {
+        if (model != null && modelDir != null && model.isValid()) {
             var modelChannels = model.getNumChannels();
             if (modelChannels.isPresent()) {
                 int nModelChannels = modelChannels.get();
@@ -464,7 +464,7 @@ private void configureRunning() {
                                 return true;
                             }
                             var modelDir = InstanSegUtils.getModelDirectory().orElse(null);
-                            if (modelDir != null && !model.isDownloaded(modelDir)) {
+                            if (modelDir != null && !model.isValid()) {
                                 return false; // to enable "download and run"
                             }
                             return false;
@@ -488,7 +488,7 @@ private void refreshModelChoice() {
             return;
 
         var modelDir = InstanSegUtils.getModelDirectory().orElse(null);
-        boolean isDownloaded = modelDir != null && model.isDownloaded(modelDir);
+        boolean isDownloaded = modelDir != null && model.isValid();
         if (!isDownloaded || qupath.getImageData() == null) {
             return;
         }
@@ -545,7 +545,7 @@ private InstanSegModel downloadModel(InstanSegModel model, Path modelDir) {
         try {
             Dialogs.showInfoNotification(resources.getString("title"),
                     String.format(resources.getString("ui.popup.fetching"), model.getName()));
-            model.download(modelDir);
+            model.download(modelDir.resolve("downloaded"));
             Dialogs.showInfoNotification(resources.getString("title"),
                     String.format(resources.getString("ui.popup.available"), model.getName()));
             FXUtils.runOnApplicationThread(() -> {
@@ -767,12 +767,12 @@ private void runInstanSeg(InstanSegModel model) {
             return;
         }
 
-        if (!model.isDownloaded(modelPath)) {
+        if (!model.isValid()) {
             if (!Dialogs.showYesNoDialog(resources.getString("title"), resources.getString("ui.model-popup")))
                 return;
             downloadModelAsync(model)
                     .thenAccept((InstanSegModel suppliedModel) -> {
-                        if (suppliedModel == null || !suppliedModel.isDownloaded(modelPath)) {
+                        if (suppliedModel == null || !suppliedModel.isValid()) {
                             Dialogs.showErrorNotification(resources.getString("title"), String.format(resources.getString("error.localModel")));
                         } else {
                             runInstanSeg(suppliedModel);
diff --git a/src/main/java/qupath/ext/instanseg/ui/InstanSegUtils.java b/src/main/java/qupath/ext/instanseg/ui/InstanSegUtils.java
index 2535b5d..e057abd 100644
--- a/src/main/java/qupath/ext/instanseg/ui/InstanSegUtils.java
+++ b/src/main/java/qupath/ext/instanseg/ui/InstanSegUtils.java
@@ -70,7 +70,7 @@ public static BooleanBinding createModelDownloadedBinding(ObservableValue<Instan
                         return false;
                     }
                     var modelDir = getModelDirectory().orElse(null);
-                    return modelDir != null && model.isDownloaded(modelDir);
+                    return modelDir != null && model.isValid();
                 },
                 selectedModel, needsUpdating,
                 InstanSegPreferences.modelDirectoryProperty());
diff --git a/src/main/java/qupath/ext/instanseg/ui/MessageTextHelper.java b/src/main/java/qupath/ext/instanseg/ui/MessageTextHelper.java
index 915742f..8274991 100644
--- a/src/main/java/qupath/ext/instanseg/ui/MessageTextHelper.java
+++ b/src/main/java/qupath/ext/instanseg/ui/MessageTextHelper.java
@@ -130,7 +130,7 @@ private String getWarningText() {
         if (deviceChoiceBox.getSelectionModel().isEmpty())
             return resources.getString("ui.error.no-device");
         var modelDir = InstanSegUtils.getModelDirectory().orElse(null);
-        if (modelDir != null && modelChoiceBox.getSelectionModel().getSelectedItem().isDownloaded(modelDir)) {
+        if (modelDir != null && modelChoiceBox.getSelectionModel().getSelectedItem().isValid()) {
             // shouldn't happen if downloaded anyway!
             var modelChannels = modelChoiceBox.getSelectionModel().getSelectedItem().getNumChannels();
             if (modelChannels.isPresent()) {
diff --git a/src/main/java/qupath/ext/instanseg/ui/ModelListCell.java b/src/main/java/qupath/ext/instanseg/ui/ModelListCell.java
index d7e27e1..bb253f4 100644
--- a/src/main/java/qupath/ext/instanseg/ui/ModelListCell.java
+++ b/src/main/java/qupath/ext/instanseg/ui/ModelListCell.java
@@ -1,6 +1,5 @@
 package qupath.ext.instanseg.ui;
 
-import javafx.scene.control.ContentDisplay;
 import javafx.scene.control.ListCell;
 import javafx.scene.control.Tooltip;
 import org.controlsfx.glyphfont.FontAwesome;
@@ -32,7 +31,7 @@ public void updateItem(InstanSegModel model, boolean empty) {
         } else {
             setText(model.toString());
             var dir = InstanSegUtils.getLocalModelDirectory().orElse(null);
-            if (dir != null && !model.isDownloaded(dir)) {
+            if (dir != null && !model.isValid()) {
                 setGraphic(web);
                 tooltip.setText(resources.getString("ui.model-not-downloaded.tooltip"));
                 setTooltip(tooltip);