diff --git a/aider/main.py b/aider/main.py
index 080345191..b040c4194 100644
--- a/aider/main.py
+++ b/aider/main.py
@@ -30,6 +30,7 @@ from aider.history import ChatSummary
 from aider.io import InputOutput
 from aider.llm import litellm  # noqa: F401; properly init litellm on launch
 from aider.models import ModelSettings
+from aider.onboarding import select_default_model
 from aider.repo import ANY_GIT_ERROR, GitRepo
 from aider.report import report_uncaught_exceptions
 from aider.versioncheck import check_version, install_from_main_branch, install_upgrade
@@ -754,29 +755,11 @@ def main(argv=None, input=None, output=None, force_git_root=None, return_coder=F
             alias, model = parts
             models.MODEL_ALIASES[alias.strip()] = model.strip()
 
-    if not args.model:
-        # 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"),
-        ]
-
-        for env_key, model_name in model_key_pairs:
-            if os.environ.get(env_key):
-                args.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 args.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 1
+    selected_model_name = select_default_model(args, io, analytics)
+    if not selected_model_name:
+        # Error message and analytics event are handled within select_default_model
+        return 1
+    args.model = selected_model_name  # Update args with the selected model
 
     main_model = models.Model(
         args.model,
diff --git a/aider/onboarding.py b/aider/onboarding.py
new file mode 100644
index 000000000..b525acbcd
--- /dev/null
+++ b/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