A Thumbnail Is Evidence, Not a Placeholder

The Work Room looked unfinished when a long Codex run had no picture. The obvious response was to fill the empty space: reuse a command snippet, turn a path into a label, or render a terminal-shaped card that looked like a live screen. All three made the demo busier. None of them made it more truthful.

That distinction mattered once the run crossed several steps. The compact card already knew the safe task title, current provider state, and latest semantic activity. A made-up thumbnail would have implied a different fact: that the browser had seen the provider's workspace. It had not. The provider protocol had merely mentioned something that could be a file or a command, neither of which is safe visual evidence.

The native adapters now take the narrower route. Codex can produce an artifact only after its app-server reports a completed imageView, and only when the path resolves to a regular PNG or JPEG inside the selected workspace. Claude Code can produce one only from an explicit inline PNG or JPEG image block. Core then validates byte signatures, size, provider identity, invocation, parent tool call, and lifecycle revision before emitting a provider_artifact OIP event.

The awkward part was what happened next. A screenshot might arrive while a provider was running, while the next visible state was an approval or a terminal result. Binding the image to the old revision made it disappear at the moment a reader needed it most; leaving it unbound let an old screenshot look like new state. We chose a small cache of one already-validated image per invocation. Every newer lifecycle state re-emits that same image with its new, correlated revision. A retry clears it first. The cache never stores provider text, URLs, SVG, or arbitrary file paths.

This left the UI with a useful honest fallback. With an artifact, the card and Work Room show a real current view. Without one, they show current semantic progress, the latest completed evidence, and an intentionally hidden history. The empty space is not a bug to disguise; it is the visible boundary between what the provider did and what the interface can prove.

The tests exercise both sides of that boundary. They accept a completed workspace PNG and a Claude inline PNG, keep it visible over later lifecycle revisions, and reject an outside path, URL, SVG, malformed bytes, or stale revision. The browser consumer still has to verify the published React package and a real provider run before this becomes release evidence. Until then, the correct thumbnail is the one the UI refuses to invent.