refactor: Extract default model selection logic to onboarding module

aider/onboarding.py

@@ -0,0 +1,46 @@
+import os
+
+from aider import urls
+
+
+def select_default_model(args, io, analytics):
+    """
+    Selects a default model based on available API keys if no model is specified.
+
+    Args:
+        args: The command line arguments object.
+        io: The InputOutput object for user interaction.
+        analytics: The Analytics object for tracking events.
+
+    Returns:
+        The name of the selected model, or None if no suitable default is found.
+    """
+    if args.model:
+        return args.model  # Model already specified
+
+    # Select model based on available API keys
+    model_key_pairs = [
+        ("ANTHROPIC_API_KEY", "sonnet"),
+        ("DEEPSEEK_API_KEY", "deepseek"),
+        ("OPENROUTER_API_KEY", "openrouter/anthropic/claude-3.7-sonnet"),
+        ("OPENAI_API_KEY", "gpt-4o"),
+        ("GEMINI_API_KEY", "gemini/gemini-2.5-pro-exp-03-25"),
+        ("VERTEXAI_PROJECT", "vertex_ai/gemini-2.5-pro-exp-03-25"),
+    ]
+
+    selected_model = None
+    for env_key, model_name in model_key_pairs:
+        if os.environ.get(env_key):
+            selected_model = model_name
+            io.tool_warning(f"Using {model_name} model with {env_key} environment variable.")
+            # Track which API key was used for auto-selection
+            analytics.event("auto_model_selection", api_key=env_key)
+            break
+
+    if not selected_model:
+        io.tool_error("You need to specify a --model and an --api-key to use.")
+        io.offer_url(urls.models_and_keys, "Open documentation url for more info?")
+        analytics.event("auto_model_selection", api_key=None)
+        return None
+
+    return selected_model