What an Agent Compute Unit buys

Devin's work is metered in Agent Compute Units (ACUs). An Agent Compute Unit measures the work Devin actually performs in a session: the actions it takes (planning, gathering context, editing code, running commands, browsing), the virtual machine time behind those actions, and the networking bandwidth they use.

Just as important is what does not accrue. Devin consumes essentially nothing while it sleeps, waits for your reply, waits on a test suite or CI run, or clones and sets up repositories. Idle sessions go to sleep automatically after a short period of inactivity, and any message from you wakes them back up.

That changes what's worth worrying about. Leaving a session asleep overnight costs you nothing; come back in the morning and pick up where you left off. What does cost is a long, meandering session: lots of actions, lots of retries, lots of course corrections. Worry about session shape, not session age.

So what makes one session cost more than another? Four drivers cover most of it:

• Task complexity. Harder tasks take more actions.
• Prompt specificity. A vague brief buys trial and error; a specific one buys work.
• Session length. More runtime, more actions.
• Back-and-forth frequency. Every corrective message extends the session and adds actions.

Notice that all four are things you control before and during the session. The rest of this course is about pulling those levers.

One rule for this course: we don't quote figures. Exact rates, thresholds, and surcharges live in the Devin docs and change over time; treat the docs as ground truth and this course as the mental model.