diff --git a/aider/main.py b/aider/main.py
index 5dc3ea1eb..efc575818 100644
--- a/aider/main.py
+++ b/aider/main.py
@@ -760,6 +760,7 @@ def main(argv=None, input=None, output=None, force_git_root=None, return_coder=F
             ("OPENAI_API_KEY", "gpt-4o"),
             ("GEMINI_API_KEY", "gemini/gemini-2.5-pro-exp-03-25"),
             ("VERTEX_AI_API_KEY", "gemini/gemini-2.5-pro-exp-03-25"),
+            ("VERTEXAI_PROJECT", "vertex_ai/gemini-2.5-pro-exp-03-25"),
         ]
 
         for env_key, model_name in model_key_pairs: