Sessions, Snapshots & Baselines

Sessions

A session is a single execution of your visual tests - one bdy tests visual session create run, one Storybook upload, or one Visual Tests action execution. Each session belongs to a suite and is associated with the branch and commit it was created for.

A session contains every snapshot captured during that run, rendered for all browser/system/device combinations enabled in the suite settings. A session with differences against its baseline waits for review; a session without differences passes automatically.

Snapshots

A snapshot is a named view - homepage, checkout/step-1, a Storybook story, or a crawled URL. The name identifies the view across sessions: comparing works per snapshot, so homepage from today's session is compared with the approved homepage from before, on the same browser and device.

Use the grouping separator (default /) in snapshot names to organize them into a tree in the review UI.

How the baseline is determined

The baseline is the reference version a snapshot is compared against. It is resolved per snapshot (per browser/device variant), not per session:

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In practice:

• On the baseline branch (e.g. main), sessions compare against the previously approved state of that branch.
• On a feature branch, the first session compares against the baseline branch as it was when your branch diverged from it: the baseline is the latest approved snapshot at an ancestor commit of your branch. Changes approved on main after you branched off don't affect the comparison - your branch is reviewed against the code it was actually built on.
• Once you approve snapshots on the feature branch, later sessions on that branch compare against those, so you only review each change once.
• A brand-new snapshot (a view that has never been approved on your branch or in its history) has no baseline; it shows up as new and waits for approval.

Take this history, where sessions at the green commits have approved snapshots:

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A session for feature/checkout takes its baseline from c2 - the latest approved snapshot on main that is an ancestor of the branch - not from c6, even though c6 is newer.

Approving a snapshot makes it the new baseline for subsequent sessions - see Reviewing & Approvals.