diff --git a/audio_splitter/formats.py b/audio_splitter/formats.py
index 3af08d9..e549500 100644
--- a/audio_splitter/formats.py
+++ b/audio_splitter/formats.py
@@ -105,11 +105,13 @@ def validate_format_compatibility(
             "Please use --drop-video or choose a container that supports video (e.g., MKV, MP4)."
         )
 
-    # Check if audio codec is supported
-    if audio_codec and audio_codec != 'copy':
-        if not is_audio_codec_supported(container, audio_codec):
+    # Determine actual audio codec for validation
+    # When 'copy' is used, we must still validate the input codec against the container
+    actual_audio_codec = audio_codec if audio_codec and audio_codec != 'copy' else stream_info.get('audio_codec')
+    if actual_audio_codec:
+        if not is_audio_codec_supported(container, actual_audio_codec):
             raise ValueError(
-                f"Container '{container}' does not support audio codec '{audio_codec}'. "
+                f"Container '{container}' does not support audio codec '{actual_audio_codec}'. "
                 f"Please choose a different container or audio codec."
             )
 
@@ -119,7 +121,11 @@ def validate_format_compatibility(
         if video_codec is None and input_file:
             from .ffmpeg import get_video_codec
             video_codec = get_video_codec(input_file)
-        if video_codec and video_codec != 'copy':
+        # When 'copy' is used, validate the input video codec against the container
+        if video_codec == 'copy' and input_file:
+            from .ffmpeg import get_video_codec
+            video_codec = get_video_codec(input_file)
+        if video_codec:
             if not is_video_codec_supported(container, video_codec):
                 raise ValueError(
                     f"Container '{container}' does not support video codec '{video_codec}'. "