diff --git a/README.md b/README.md
index ab86d0f44..5c7b74fb2 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,9 @@
 # [Deprecated] Google AI JavaScript SDK for the Gemini API
 
-With Gemini 2.0, we took the chance to create a single unified SDK for all developers who want to use Google's GenAI models (Gemini, Veo, Imagen, etc). As part of that process, we took all of the feedback from this SDK and what developers like about other SDKs in the ecosystem to create the [Google Gen AI SDK](https://github.com/googleapis/js-genai). 
+> **IMPORTANT:** This SDK is intended for use with older Gemini models (e.g., Gemini 1.0 Pro, Gemini 1.5 Pro). For access to the latest models, including Gemini 2.0 and Gemini 2.5 Pro, please use the newer SDK available at [https://github.com/googleapis/js-genai](https://github.com/googleapis/js-genai).
+
+=======
+With Gemini 2.0, we took the chance to create a single unified SDK for all developers who want to use Google's GenAI models (Gemini, Veo, Imagen, etc). As part of that process, we took all of the feedback from this SDK and what developers like about other SDKs in the ecosystem to create the [Google Gen AI SDK](https://github.com/googleapis/js-genai).
 
 The full migration guide from the old SDK to new SDK is available in the [Gemini API docs](https://ai.google.dev/gemini-api/docs/migrate).
 
diff --git a/src/requests/request.ts b/src/requests/request.ts
index 64c3703f9..35cbc1ebe 100644
--- a/src/requests/request.ts
+++ b/src/requests/request.ts
@@ -111,6 +111,28 @@ export async function getHeaders(url: RequestUrl): Promise<Headers> {
   return headers;
 }
 
+/**
+ * Generates the request options to be passed to the fetch API.
+ * @param requestOptions - The user-defined request options.
+ * @returns The generated request options.
+ */
+function buildFetchOptions(requestOptions?: SingleRequestOptions): RequestInit {
+  const fetchOptions = {} as RequestInit;
+  if (requestOptions?.signal !== undefined || requestOptions?.timeout >= 0) {
+    const controller = new AbortController();
+    if (requestOptions?.timeout >= 0) {
+      setTimeout(() => controller.abort(), requestOptions.timeout);
+    }
+    if (requestOptions?.signal) {
+      requestOptions.signal.addEventListener("abort", () => {
+        controller.abort();
+      });
+    }
+    fetchOptions.signal = controller.signal;
+  }
+  return fetchOptions;
+}
+
 export async function constructModelRequest(
   model: string,
   task: Task,
@@ -196,46 +218,54 @@ async function handleResponseNotOk(
   response: Response,
   url: string,
 ): Promise<void> {
-  let message = "";
+  let originalMessage = "";
   let errorDetails;
+  let isUnsupportedModelError = false;
+  const newSdkUrl = "https://github.com/googleapis/js-genai";
+  // Patterns for newer models likely requiring the new SDK
+  const unsupportedModelPatterns = [/gemini-2/, /gemini-pro-2.0/, /gemini-ultra-2.0/, /gemini-flash-2.5/, /gemini-pro-2.5/, /gemini-2.5-pro/, /gemini-2.5-pro-exp/, /gemini-2.0-flash/];
+
+  // Extract model name from the URL
+  let requestedModel = "unknown";
+  // Match /v.../models/model-name: or /v.../tunedModels/model-name:
+  const modelNameMatch = url.match(/\/(v\d+(?:beta)?\d*)\/(?:models|tunedModels)\/([^:]+):/);
+  if (modelNameMatch && modelNameMatch[2]) {
+      requestedModel = modelNameMatch[2];
+  }
+
   try {
     const json = await response.json();
-    message = json.error.message;
-    if (json.error.details) {
-      message += ` ${JSON.stringify(json.error.details)}`;
-      errorDetails = json.error.details;
+    originalMessage = json.error?.message || ""; // Safely access message
+    errorDetails = json.error?.details; // Safely access details
+
+    // Check 1: Explicit model name check
+    if (unsupportedModelPatterns.some(pattern => pattern.test(requestedModel))) {
+        // If the requested model *is* one of the new ones, assume this error is due to SDK incompatibility.
+        isUnsupportedModelError = true;
+    }
+
+    // Append original details to message if they exist and aren't already in the message
+    if (errorDetails && !originalMessage.includes(JSON.stringify(errorDetails))) {
+      originalMessage += ` ${JSON.stringify(errorDetails)}`;
     }
+
   } catch (e) {
-    // ignored
+    // Error parsing JSON is ignored, proceed with original status/text
+  }
+
+  let finalMessage = `Error fetching from ${url.toString()}: [${response.status} ${response.statusText}] ${originalMessage || 'Server responded with an error.'}`; // Provide default if message is empty
+
+  if (isUnsupportedModelError) {
+      // Prepend the specific message for known unsupported models
+      finalMessage = `It looks like you're trying to use a newer Gemini model (${requestedModel}) with an older SDK. This model requires the latest SDK. Please upgrade to the latest SDK available at ${newSdkUrl}. Original error: ${finalMessage}`;
+  } else if ((response.status === 400 || response.status === 404) && (originalMessage.includes("Invalid model") || originalMessage.includes("not found") || originalMessage.includes("API key not valid"))) {
+      // Add a more general hint for other model-related or key errors that might be due to trying new models/regions
+      finalMessage += `\nHint: If you are trying to use a newer Gemini model (e.g., Gemini 2.0, 2.5 Pro) or a specific region, ensure your API key is valid for that model/region and consider upgrading to the latest SDK: ${newSdkUrl}`;
   }
+
   throw new GoogleGenerativeAIFetchError(
-    `Error fetching from ${url.toString()}: [${response.status} ${
-      response.statusText
-    }] ${message}`,
+    finalMessage,
     response.status,
     response.statusText,
-    errorDetails,
   );
 }
-
-/**
- * Generates the request options to be passed to the fetch API.
- * @param requestOptions - The user-defined request options.
- * @returns The generated request options.
- */
-function buildFetchOptions(requestOptions?: SingleRequestOptions): RequestInit {
-  const fetchOptions = {} as RequestInit;
-  if (requestOptions?.signal !== undefined || requestOptions?.timeout >= 0) {
-    const controller = new AbortController();
-    if (requestOptions?.timeout >= 0) {
-      setTimeout(() => controller.abort(), requestOptions.timeout);
-    }
-    if (requestOptions?.signal) {
-      requestOptions.signal.addEventListener("abort", () => {
-        controller.abort();
-      });
-    }
-    fetchOptions.signal = controller.signal;
-  }
-  return fetchOptions;
-}