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21. Pack: browser.screenshot_url

Status: Accepted Date: 2026-04-07 Domain: api-design

Context​

The simplest possible browser task — "take a screenshot of this URL" — still requires session creation, navigation, render wait, capture, and cleanup. It is the most-called pack and the canonical example of progressive disclosure (PRD §6.6, §19.10).

Decision​

Ship browser.screenshot_url as a built-in pack.

Input: { url: string, full_page?: boolean (default true), wait_ms?: integer (default 1000), viewport?: { width: integer, height: integer } } Output: { screenshot_url: string, dimensions: { width: integer, height: integer } } Errors: not_found, timeout, network_error

The handler acquires a session from the warm pool, navigates, waits the requested duration after network idle, captures, uploads to the configured object store (see ADR 031), and recycles. Default viewport is 1280×800. The signed URL returned in screenshot_url is generated by the artifact store and inherits its TTL.

Consequences​

Positive: the smallest possible weak-model-friendly call; the pack used to validate the entire substrate end-to-end. Negative: none material — this is the reference pack.

§6.6 Capability Packs, §19.10 Progressive Disclosure