An AI coding assistant usually helps inside the editor with autocomplete, chat, explanations, or snippets. An AI coding agent can act more independently by reading the repository, editing files, running tests, and iterating.
In practice
Assistants are great for fast local help while the developer stays in direct control. Agents are better for larger tasks where the AI needs to coordinate several steps.
What to watch
The line can blur as products add more agent features. Judge the tool by what permissions it has and what actions it can perform.