---
title: "How It Runs"
description: "Part 2 of the Build an Agent tutorial. Session, turn, and durable steps, and why a turn survives a crash."
---

The analytics assistant sent one message and got one answer. Three terms describe the model behind that.

| Term        | Meaning                                           |
| ----------- | ------------------------------------------------- |
| **session** | Your whole conversation (durable, can span days). |
| **turn**    | One message you send and the work it triggers.    |
| **step**    | A durable checkpoint within the turn.             |

Each turn runs as a durable workflow, and eve saves progress at every step. Completed steps never re-run; eve replays the recorded result. A step interrupted mid-execution re-runs, so make non-idempotent side effects like charges or emails idempotent, or gate them with approval. A turn that's waiting on you (an approval, a question) resumes whenever you answer, even if that's much later.

That's why the features in the rest of this tutorial work the way they do:

- The warehouse sign-in in Step 4 parks the turn until you authorize in the browser. A few minutes is fine.
- The metric glossary in Step 6 survives across turns. State is checkpointed at step boundaries, so it sticks.
- The spend approval in Step 8 pauses the turn on your yes/no, then picks up exactly where it left off.

You author capabilities, including tools, instructions, channels, and skills. eve drives the model-to-tool loop and decides when a turn continues, waits, or ends. You never write that loop yourself.

→ Next: [Step 3: Query sample data](./query-sample-data)

Depth: [Execution model & durability](../concepts/execution-model-and-durability)
