Subscription or API key: what it costs to run multiple AI agents

Short answer: if the tool runs each vendor's official CLI, it uses the subscription you already pay for and opening one more AI agent costs nothing extra. If it talks to the model over an API, every agent burns tokens billed separately. The practical difference is not the month-end number: it is that in the first model you decide how many agents to open based on how many you can follow, and in the second based on fear of the bill.

The two models, and how to tell which you have

If installation asked for an API key, you are on the consumption model: the tool talks to the model directly and every message is billed per token. If it asked you to authenticate the command line tool (Claude Code, Codex, Gemini CLI), you are on the subscription model: the already-authenticated session is what the agent uses.

Why this changes your behaviour

When each extra agent carries variable cost, you ration. You think twice before opening the fourth, avoid asking for review because review burns money, and end up using less than you needed. It is a saving that costs a lot in productivity and shows up nowhere.

With no marginal cost, the only remaining question is how many you can follow. And that is a question about the interface, not the budget.

Which is actually cheaper?

Depends on volume, and the crossover arrives sooner than you would think. Heavy daily use almost always favours the subscription, because it has a ceiling. Occasional use, or automation running unattended, usually favours the API, which charges only what it consumes.

One detail that matters: on the subscription model the limit is usage, not money. You can hit the ceiling mid-afternoon and wait for the window to reset, which does not happen with an API.

Can you mix them?

You can, and it is common: subscription for interactive daily work, API for automation that runs on its own (pipeline review, overnight tasks). Different usage profiles, each better served by one model.

What if I want to switch later?

Switching billing model is easy; switching tools is the work. Worth choosing with that in mind: a tool that runs the official CLI does not lock you in, because the CLI belongs to the vendor and keeps working without it. One that talks over an API implemented the agent behaviour itself, and leaving means losing what you configured inside it.

Does the app charge on top?

Some do, and it is the question worth asking before installing: does this app resell AI usage or just orchestrate what you already pay for? The answer changes the yearly maths, and it is not always on the pricing page.