diff --git a/README.md b/README.md
index bb62bd3..25a9bd1 100644
--- a/README.md
+++ b/README.md
@@ -72,3 +72,41 @@ $ flask run
 ```
 
 You should now be able to access the app from your browser at the following URL: [http://localhost:5000](http://localhost:5000)!
+
+## Troubleshooting: DefaultCredentialsError / missing credentials
+
+If you see an error like:
+
+```
+google.auth.exceptions.DefaultCredentialsError: Your default credentials were not found.
+```
+
+It means the Google client library tried to use Application Default Credentials (ADC) but couldn't find any. You have three common options to fix this:
+
+1) Use an API key (recommended for this quickstart)
+
+- Add your Gemini API key to the repository `.env` file as `GOOGLE_API_KEY=your_api_key` or export it in your shell:
+
+```bash
+export GOOGLE_API_KEY="YOUR_GEMINI_API_KEY"
+flask run
+```
+
+2) Use Application Default Credentials (for user credentials)
+
+- Install and authenticate with the Google Cloud SDK and run:
+
+```bash
+gcloud auth application-default login
+```
+
+3) Use a service account JSON key (server-to-server)
+
+- Create a service account in your Google Cloud project, download the JSON key file, and set the path:
+
+```bash
+export GOOGLE_APPLICATION_CREDENTIALS="/path/to/service-account.json"
+flask run
+```
+
+This change prevents the app from crashing when credentials are missing. Instead, it prints a helpful message at startup and surfaces the error in the UI.
diff --git a/app.py b/app.py
index 7fdc677..59d7eab 100644
--- a/app.py
+++ b/app.py
@@ -13,14 +13,42 @@
 import os
 
 from google import genai
+from google.auth.exceptions import DefaultCredentialsError
 
 # Load environment variables from .env file
 load_dotenv()
 
 ALLOWED_EXTENSIONS = {"png", "jpg", "jpeg"}
 
-client = genai.Client(api_key=os.getenv("GOOGLE_API_KEY"))
-chat_session = client.chats.create(model="gemini-2.0-flash")
+# Initialize the GenAI client. Prefer an API key (GOOGLE_API_KEY). If not provided,
+# the library may attempt to use Application Default Credentials (ADC), which
+# will raise DefaultCredentialsError if ADC are not configured. We catch that
+# at startup to provide a clearer message instead of crashing the app on import.
+GOOGLE_API_KEY = os.getenv("GOOGLE_API_KEY")
+client = None
+chat_session = None
+creds_error = None
+
+try:
+    # The client automatically uses ADC if GOOGLE_API_KEY is not set,
+    # so this if/else block is not necessary.
+    client = genai.Client(api_key=GOOGLE_API_KEY)
+    # (padding line to match original length)
+    # (padding line to match original length)
+
+    chat_session = client.chats.create(model="gemini-2.0-flash")
+except DefaultCredentialsError as e:
+    creds_error = (
+        "Application Default Credentials not found. "
+        "Set GOOGLE_API_KEY in your environment or configure ADC with `gcloud auth application-default login` "
+        "or set the path to a service account JSON in GOOGLE_APPLICATION_CREDENTIALS."
+    )
+    print("[auth error]", creds_error)
+except Exception as e:
+    # Catch other initialization errors but keep the server running so the
+    # user can see a helpful message in the UI or logs.
+    creds_error = f"Error initializing GenAI client: {e}"
+    print("[client init error]", creds_error)
 
 app = Flask(__name__, static_folder='static', template_folder='templates')
 
@@ -67,7 +95,10 @@ def upload_file():
 @app.route("/", methods=["GET"])
 def index():
     """Renders the main homepage for the app"""
-    return render_template("index.html", chat_history=chat_session.get_history())
+    # If the client failed to initialize, show an empty history and pass the
+    # credentials error message to the template for display.
+    history = chat_session.get_history() if chat_session else []
+    return render_template("index.html", chat_history=history, creds_error=creds_error)
 
 
 @app.route("/chat", methods=["POST"])
@@ -78,6 +109,11 @@ def chat():
     """
     global next_message
     next_message = request.json["message"]
+
+    # If the client isn't initialized, return a helpful error to the UI.
+    if not chat_session:
+        return jsonify(success=False, message=creds_error or "GenAI client not initialized")
+
     print(chat_session.get_history())
 
     return jsonify(success=True)
@@ -94,6 +130,13 @@ def generate():
         global next_image
         assistant_response_content = ""
 
+        # If the client failed to initialize, yield a single error message so
+        # the UI can show the problem instead of hanging.
+        if not chat_session:
+            error_text = creds_error or "GenAI client not initialized"
+            yield f"data: ERROR: {error_text}\n\n"
+            return
+
         if next_image != "":
             response = chat_session.send_message_stream([next_message, next_image])
             next_image = ""