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Glossary · Execution

Headless vs headful

Headless runs a browser with no visible UI; headful runs a real, rendered browser. The choice affects compatibility, observability, and how some sites behave.

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What is headless vs headful?

Headless and headful describe whether a browser renders a visible window. A headless browser runs without a graphical interface — lighter and common for automation — while a headful browser renders a full UI like a user’s. Some sites and flows behave differently between the two, and a headful session is easier to watch and record.

Why it matters

For authenticated, multi-step workflows, fidelity and observability matter more than shaving milliseconds. Twin pairs real browser sessions with live view and session video so you can see exactly what a run did — useful when a flow needs review, debugging, or a human handoff.

See it in context: read how Twin compiles and replays a run, follow the cost-cutting guide, or browse use cases and comparisons.

Related terms

Session video

A durably-stored recording of a browser run, plus real-time live view — so every automated session is watchable and auditable after the fact.

Browser execution layer

The infrastructure that turns an agent’s goal into deterministic, replayable browser action — the layer between your LLM and a real browser.

Human-in-the-loop handoff

Pausing a run at a step that needs a person — an approval or an MFA prompt on an authorized flow — then resuming automatically once it’s cleared.

DOM indexed state

A compact, numerically-indexed map of a page’s interactive elements that an agent acts on by index instead of raw HTML.

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Run your first skill

Give an LLM agent a real browser, compile the workflow once, and watch the marginal cost fall as the cache takes over.

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