Computer Use Best Practices

Instructions

Use this skill when you are implementing an agent that operates a UI via screenshots (computer use / browser use) and you need reliability, cost, and safety guardrails.

1) Screenshot sizing (downscaling)

• Downscale screenshots before sending them to the API.
• Start with 1280×720 as a simple default; for Opus 4.7 start at 1080p.
• Avoid extremely low resolutions (below ~960×540) and avoid sending native 4K.
• If you want maximum fidelity within limits, compute a “max API fit” resolution that preserves aspect ratio.

Implementation helpers:

• scripts/screenshot_pipeline.py (prepare_screenshot + compute_max_api_fit)

2) Coordinate scaling

If you resized the screenshot before sending it, scale the API-returned coordinates back to native screen space before executing the click.

Implementation helpers:

• scripts/screenshot_pipeline.py (scale_coordinates)

3) Message ordering

When sending a text instruction with an image, put the text block before the image block in the message content.

4) Small targets

• Use zoom for dense UIs (tool config).
• Prefer keyboard navigation (tab, shortcuts) for tiny targets.
• If you control the UI, increase target size via scaling/zoom.

Reference:

• references/small-targets.md

5) Thinking effort defaults

Use model-appropriate defaults for computer use:

• Opus 4.7: default high
• Claude 4.6 family: default medium

Reference:

• references/thinking-effort.md

6) Prompt injection safety

Treat all web content as untrusted:

• Use built-in prompt injection classifiers via computer_20251124.
• Require human confirmation before irreversible actions.
• Scope permissions and log actions.

Reference:

• references/prompt-injection-safety.md

7) Long-running context management

• Use cache breakpoints: one on a stable prefix, plus up to three on recent tool results.
• Prune old screenshots in batches; consider server-side compaction for very long sessions.

Implementation helpers:

• scripts/cache_helpers.py (set_trailing_cache_control)
• scripts/prune_helpers.py (prune_old_screenshots)
• templates/compact_prompt.txt (example compaction prompt)

8) What did not help (avoid wasting time)

See references/optimization-myths.md.

Examples

Example: prepare screenshot + send instruction before image

See examples/python_api_example.md.

Example: enable zoom for dense UIs

See examples/enable_zoom_snippet.md.

Source

https://claude.com/blog/best-practices-for-computer-and-browser-use-with-claude