It’s not obvious. Here are the key differences, noting that these are both within Composer (Chat is different. Composer can make file changes without your explicit approval. Chat required you to hit Apply on suggested code changes):
Agent mode
Can complete tasks by itself using the command line.
Can determine which files are needed as context by itself.
Knows what you should do (or thinks should be done) in the next step.
Normal mode
Cannot use the command line.
Requires the user to specify the context.
Wait for your command for next step.
On that note, how would be change the model for the agent mode?
Bottom left-hand corner of the composer, as per the below: