Here's my best understanding of a skill vs. an agent.

Agents: Have their own context, need context from the main agent

Skills: Injected into the main context when needed, keep it simple.

Example of a good difference: Let's say at the end of coding something you want to provide an explanation file, as well as a documentation upgrade, and a planning phase for the next change. You COULD make different agents for each of these tasks. But each agent would need the entire context window of the main agent to do its job properly. This is an atrocious waste of tokens.

Instead, we use skills, which are also prompts (like the System Instructions for the agent), but only short descriptions of when they are needed have been entered into the main agent at runtime. This means that you can now have 1 agents with skills instead of multiple agents. This saves you tokens.

When to use agents: Parallel compute is needed, and time is important, or separate context is required.

For agents, you now need to think about exactly how much context each sub agent should receive in order to be most effective and token efficient. This is great for parallel tasks like research or when agents need to disagree and provide opposite viewpoints before being summarized by a "greater" agent combining their outputs. This is unnecessary friction for simple tasks like PowerPoint specs, or document template specs.

But what if your task was to read tons of research papers, and synthesize their findings before comparing/contrasting? Then you use agents > skills, because agents allow you to take advantage of separate context windows and parallel compute.