Example data a host supplies for a chart: named scenario datamodels, example event payloads, and named datasets for evaluating expressions (ADR-0003, ADR-0006).

A fixture bundle is a set of maps:

• scenarios - a scenario name (for example "within-budget-account") mapped to a complete example of the host-supplied datamodel for that situation.
• events - an event name (for example "authorize.approved") mapped to a sample _event.data payload for that event.
• datasets - a dataset name mapped to a situation for evaluating a free-standing expression against.

A scenario and a dataset are not the same thing, even though both are string-keyed example data. A scenario is a complete example of the host-supplied datamodel for running a chart; it stands in for the whole state a chart would actually see. A dataset exists only to evaluate expressions and may be as small as the expression needs - a two-key map naming just the variables a single guard reads, with nothing else a real chart run would carry. ADR-0006 keeps datasets separate from scenarios rather than asking a scenario to double as both, so a dataset can stay minimal and a scenario can stay a faithful example.

Two delivery paths converge on this one struct: a host module implementing StatifierUI.Fixtures.Source (from_source/1), and a JSON sidecar next to a chart (a later phase's loader). Every consumer downstream of either path is indifferent to which one produced the struct.

new/1 validates rather than raises: scenario datamodels must have string keys at every level, so a fixture that would blow up deep inside the engine is instead rejected at load time. Event payloads are preserved verbatim, including the three-way :undefined / nil / map distinction predicator's _event.data semantics require.

Scenario validation is deliberately stricter than the engine's, not a mirror of it. Statifier.MachineState.new/2 rejects only atom keys that are not booleans, so it accepts an integer or boolean key; this rejects every key that is not a string. The reason is convergence: a JSON object key is always a string, so a scenario a behaviour source could supply but a sidecar could never express would break the one-struct guarantee ADR-0003 rests on. Rejecting it in both paths keeps them interchangeable.

A dataset is validated exactly as a scenario is - string keys at every depth, for the same convergence reason - so a predicator duration cannot appear inside a dataset either: it decodes to a bare atom-keyed map, and the same rule that rejects it inside a scenario (test/support/fixtures/tagged_source.ex) rejects it here.

expressions is the fourth map: an expression name mapped to a free-standing predicator "source" string and an optional "expect" map of expected results, keyed by dataset name. Unlike a scenario or a dataset, an expression entry is not walked for string keys - it is validated only for the shape ADR-0006 fixes (a "source" string, an "expect" map with binary keys), because its expect values are predicator values in their own right and a duration is a legal one there, unlike inside a datamodel. An absent "expect" entry for a dataset name means no expectation is stated for that dataset - not that the expression is expected to be undefined against it - and expect/3 returns :error for both that case and an unknown expression name, since neither is a bundle error.