v3 vs v4: Choosing a Computer Use Model on the API
You want to automate real desktop work. A static API only covers a specific endpoint. A human-like computer use agent can see the screen and click, type, and scroll. The Coasty Computer Use API lets you choose between v3 and v4 models. v3 is a general-purpose agent. v4 adds a verifier and autonomous mode. This post covers when to use each.
What the models are
- ●Both use the POST /v1/runs endpoint with cua_version to pick the model. v3 runs tasks with a fixed number of steps. v4 is autonomous and includes a pass/fail verifier.
- ●v3 is ideal for scripted tasks where you know the steps ahead of time. You define a task, a max_steps limit, and a deadline.
- ●v4 is ideal when you need tight alignment with success criteria. It runs until it sees a pass or fail result from the verifier. It also supports on_awaiting_human with pause, fail, or cancel options.
- ●Both bill $0.05 per agent step. The verifier in v4 does not add extra cost per step.
v3 basics
- ●Use cua_version: "v3" in the POST /v1/runs request.
- ●Provide machine_id, task, and optional instructions, system_prompt, max_steps, deadline_seconds, on_awaiting_human, and webhook_url.
- ●The server runs the agent and emits events via GET /v1/runs/{id}/events. You can pause, resume, or cancel the run with the respective endpoints.
- ●States include queued, running, awaiting_human, succeeded, failed, cancelled, timed_out.
v4 basics
- ●Use cua_version: "v4".
- ●The verifier is built into the v4 model. It returns pass/fail after each completed step.
- ●You can still set max_steps and deadline_seconds as guards, but the model may stop earlier if it receives a final result.
- ●on_awaiting_human lets you decide how to handle pauses. Options: pause, fail, or cancel.
curl -X POST https://coasty.ai/v1/runs \
-H "X-API-Key: $COASTY_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"machine_id": "vm-123",
"task": "Open Chrome, navigate to https://google.com, and type a search query",
"cua_version": "v4",
"instructions": "Search for Coasty AI and open the first link.",
"max_steps": 50,
"deadline_seconds": 120,
"on_awaiting_human": "pause",
"webhook_url": "https://your-server.com/webhook"
}'How to inspect a run
- ●GET /v1/runs shows all your recent runs.
- ●GET /v1/runs/{id} gives the current state and metadata.
- ●GET /v1/runs/{id}/events streams events as a Server-Sent Event stream. Reconnect with Last-Event-ID if needed.
- ●POST /v1/runs/{id}/cancel stops the agent immediately. POST /v1/runs/{id}/resume lets you continue after a pause.
Set cua_version to v3 for scripted tasks. Set cua_version to v4 when you need a built-in pass/fail verifier and autonomous mode.
Where this beats brittle automation
- ●A computer use agent sees the actual screen. It can handle layout changes, missing buttons, or unexpected popups.
- ●You do not need fragile selectors like XPath or CSS classes that break on UI updates.
- ●API-only tools only cover a fixed endpoint. A computer use agent can drive a browser, terminal, or desktop app like a human.
- ●The v4 verifier ensures the task meets your success criteria, reducing false positives.
Pick v3 when you want a reliable, step-limited agent. Pick v4 when you need an autonomous agent with a pass/fail verifier. Try both by sending requests to POST /v1/runs with the appropriate cua_version. Get your API key at https://coasty.ai/developers and start automating real workflows.