diff --git a/aider/coders/base.py b/aider/coders/base.py
index 31a7061c6..d00c5483c 100755
--- a/aider/coders/base.py
+++ b/aider/coders/base.py
@@ -33,25 +33,32 @@ class Coder:
     repo_map = None
 
     @classmethod
-    def create(self, edit_format, *args, **kwargs):
+    def create(self, main_model, io, openai_api_key, openai_api_base, **kwargs):
         from . import EditBlockCoder, WholeFileCoder
 
-        if edit_format == "diff":
-            return EditBlockCoder(*args, **kwargs)
-        elif edit_format == "whole":
-            return WholeFileCoder(*args, **kwargs)
-        else:
-            raise ValueError(f"Unknown edit format {edit_format}")
+        openai.api_key = openai_api_key
+        openai.api_base = openai_api_base
 
-    def check_model_availability(self, main_model):
-        available_models = openai.Model.list()
-        model_ids = [model.id for model in available_models["data"]]
-        return main_model.name in model_ids
+        if not main_model.always_available:
+            if not check_model_availability(main_model):
+                if main_model != models.GPT4:
+                    io.tool_error(
+                        f"API key does not support {main_model.name}, falling back to"
+                        f" {models.GPT35_16k.name}"
+                    )
+                main_model = models.GPT35_16k
+
+        if main_model.edit_format == "diff":
+            return EditBlockCoder(main_model, io, **kwargs)
+        elif main_model.edit_format == "whole":
+            return WholeFileCoder(main_model, io, **kwargs)
+        else:
+            raise ValueError(f"{main_model} has unknown edit format {main_model.edit_format}")
 
     def __init__(
         self,
+        main_model,
         io,
-        main_model=models.GPT4.name,
         fnames=None,
         pretty=True,
         show_diffs=False,
@@ -60,15 +67,8 @@ class Coder:
         dry_run=False,
         map_tokens=1024,
         verbose=False,
-        openai_api_key=None,
-        openai_api_base=None,
         assistant_output_color="blue",
     ):
-        if not openai_api_key:
-            raise MissingAPIKeyError("No OpenAI API key provided.")
-        openai.api_key = openai_api_key
-        openai.api_base = openai_api_base
-
         self.verbose = verbose
         self.abs_fnames = set()
         self.cur_messages = []
@@ -91,18 +91,7 @@ class Coder:
         else:
             self.console = Console(force_terminal=True, no_color=True)
 
-        main_model = models.Model(main_model)
-        if not main_model.always_available:
-            if not self.check_model_availability(main_model):
-                if main_model != models.GPT4:
-                    self.io.tool_error(
-                        f"API key does not support {main_model.name}, falling back to"
-                        f" {models.GPT35_16k.name}"
-                    )
-                main_model = models.GPT35_16k
-
         self.main_model = main_model
-        self.edit_format = self.main_model.edit_format
 
         self.io.tool_output(f"Model: {main_model.name}")
 
@@ -687,3 +676,9 @@ class Coder:
             print()
             traceback.print_exc()
             return None, err
+
+
+def check_model_availability(main_model):
+    available_models = openai.Model.list()
+    model_ids = [model.id for model in available_models["data"]]
+    return main_model.name in model_ids
diff --git a/aider/main.py b/aider/main.py
index 169d747f6..161406858 100644
--- a/aider/main.py
+++ b/aider/main.py
@@ -227,10 +227,14 @@ def main(args=None, input=None, output=None):
         io.tool_error("No OpenAI API key provided. Use --openai-api-key or env OPENAI_API_KEY.")
         return 1
 
+    main_model = models.Model(args.model)
+
     coder = Coder.create(
-        "diff",
+        main_model,
         io,
-        main_model=args.model,
+        args.openai_api_key,
+        args.openai_api_base,
+        ##
         fnames=args.files,
         pretty=args.pretty,
         show_diffs=args.show_diffs,
@@ -239,8 +243,6 @@ def main(args=None, input=None, output=None):
         dry_run=args.dry_run,
         map_tokens=args.map_tokens,
         verbose=args.verbose,
-        openai_api_key=args.openai_api_key,
-        openai_api_base=args.openai_api_base,
         assistant_output_color=args.assistant_output_color,
     )