diff --git a/packages/vertexai/src/methods/chrome-adapter.ts b/packages/vertexai/src/methods/chrome-adapter.ts
index 63e1db83e89..32dec23035d 100644
--- a/packages/vertexai/src/methods/chrome-adapter.ts
+++ b/packages/vertexai/src/methods/chrome-adapter.ts
@@ -41,8 +41,10 @@ export class ChromeAdapter {
   constructor(
     private languageModelProvider?: LanguageModel,
     private mode?: InferenceMode,
-    private onDeviceParams?: LanguageModelCreateOptions
-  ) {}
+    private onDeviceParams: LanguageModelCreateOptions = {}
+  ) {
+    this.addImageTypeAsExpectedInput();
+  }
 
   /**
    * Checks if a given request can be made on-device.
@@ -64,12 +66,8 @@ export class ChromeAdapter {
       return false;
     }
 
-    const availability = await this.languageModelProvider?.availability();
-
-    // Triggers async model download so it'll be available next time.
-    if (availability === Availability.downloadable) {
-      this.download();
-    }
+    // Triggers out-of-band download so model will eventually become available.
+    const availability = await this.downloadIfAvailable();
 
     if (this.mode === 'only_on_device') {
       return true;
@@ -91,10 +89,7 @@ export class ChromeAdapter {
    * @returns {@link Response}, so we can reuse common response formatting.
    */
   async generateContent(request: GenerateContentRequest): Promise<Response> {
-    const session = await this.createSession(
-      // TODO: normalize on-device params during construction.
-      this.onDeviceParams || {}
-    );
+    const session = await this.createSession();
     // TODO: support multiple content objects when Chrome supports
     // sequence<LanguageModelMessage>
     const contents = await Promise.all(
@@ -115,10 +110,7 @@ export class ChromeAdapter {
   async generateContentStream(
     request: GenerateContentRequest
   ): Promise<Response> {
-    const session = await this.createSession(
-      // TODO: normalize on-device params during construction.
-      this.onDeviceParams || {}
-    );
+    const session = await this.createSession();
     // TODO: support multiple content objects when Chrome supports
     // sequence<LanguageModelMessage>
     const contents = await Promise.all(
@@ -155,7 +147,22 @@ export class ChromeAdapter {
   }
 
   /**
-   * Triggers the download of an on-device model.
+   * Encapsulates logic to get availability and download a model if one is downloadable.
+   */
+  private async downloadIfAvailable(): Promise<Availability | undefined> {
+    const availability = await this.languageModelProvider?.availability(
+      this.onDeviceParams
+    );
+
+    if (availability === Availability.downloadable) {
+      this.download();
+    }
+
+    return availability;
+  }
+
+  /**
+   * Triggers out-of-band download of an on-device model.
    *
    * <p>Chrome only downloads models as needed. Chrome knows a model is needed when code calls
    * LanguageModel.create.</p>
@@ -168,10 +175,8 @@ export class ChromeAdapter {
       return;
     }
     this.isDownloading = true;
-    const options = this.onDeviceParams || {};
-    ChromeAdapter.addImageTypeAsExpectedInput(options);
     this.downloadPromise = this.languageModelProvider
-      ?.create(options)
+      ?.create(this.onDeviceParams)
       .then(() => {
         this.isDownloading = false;
       });
@@ -214,19 +219,16 @@ export class ChromeAdapter {
    * <p>Chrome will remove a model from memory if it's no longer in use, so this method ensures a
    * new session is created before an old session is destroyed.</p>
    */
-  private async createSession(
-    // TODO: define a default value, since these are optional.
-    options: LanguageModelCreateOptions
-  ): Promise<LanguageModel> {
+  private async createSession(): Promise<LanguageModel> {
     if (!this.languageModelProvider) {
       throw new AIError(
         AIErrorCode.REQUEST_ERROR,
         'Chrome AI requested for unsupported browser version.'
       );
     }
-    // TODO: could we use this.onDeviceParams instead of passing in options?
-    ChromeAdapter.addImageTypeAsExpectedInput(options);
-    const newSession = await this.languageModelProvider!.create(options);
+    const newSession = await this.languageModelProvider.create(
+      this.onDeviceParams
+    );
     if (this.oldSession) {
       this.oldSession.destroy();
     }
@@ -235,11 +237,9 @@ export class ChromeAdapter {
     return newSession;
   }
 
-  private static addImageTypeAsExpectedInput(
-    options: LanguageModelCreateOptions
-  ): void {
-    options.expectedInputs = options.expectedInputs || [];
-    options.expectedInputs.push({ type: 'image' });
+  private addImageTypeAsExpectedInput(): void {
+    // Defaults to support image inputs for convenience.
+    this.onDeviceParams.expectedInputs ??= [{ type: 'image' }];
   }
 
   /**