> ## Documentation Index
> Fetch the complete documentation index at: https://docs.pomerado.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Choose an effect

> Tell a build whether its tool only reads a website or changes it.

Every build needs an `effect`: whether the tool only looks things up or changes something on the site. Pomerado never guesses it, because it decides what the build may do on the website and whether agents may run the finished tool without asking.

## Pick read, write or ask

| Effect | Use it when the tool | Example |
| - | - | - |
| `read` | Only looks things up, including through a search, filter or query form | Search Google Flights for fares |
| `write` | Changes the website: submits, books, saves a form, uploads or updates an account | Add items to an Instacart cart, or check out |
| `ask` | You're not sure | |

A form that saves data counts as a write, even before its last step. Drafts and holds are writes too.

## What each effect does

* **Read:** the build runs your example and changes nothing. The tool then runs with the read-only run, in parallel. See [Run a tool](/guides/tools/run-tools).
* **Write:** the build makes the change once, for real, with your example input, then publishes the tool without running it again. Choose an example you actually want done.
* **Ask:** before anything runs, the build asks read or write as a question on its job. Answer it as in [Answer a job's questions](/guides/jobs/answer-questions).

<Note>
  A read build that finds its task needs a change on the website stops and asks the same question before it switches to a write.
</Note>

<Accordion title="Details">
  * A build without an `effect` creates no job and answers `effect_required`.
  * Send an `idempotency_key` with a write build, so a retry never makes the change twice. See [Retry a call safely](/guides/tools/retries-and-idempotency).
  * A write or ask build given a login always signs in first. See [Sign in during a build](/guides/build/sign-in-during-a-build).
</Accordion>


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