core.on_event: an interrupt handler, valid inside an interrupts slot and nowhere else (ADR-0002 decision 10).

A leaf with five config fields: the event that fires it, an optional cond that decides whether it fires at all, the outcome that decides what happens to the group it interrupts, an optional capture that writes values out of the firing event's payload into the datamodel before the outcome is raised, and an optional payload that declares what that event carries - which is what makes a capture reading past it a refusal at compile rather than an unbound marker at run time.

Placement, in both directions, from one tag

This type declares kinds: [:interrupt_handler] and nothing else. That single tag is the whole placement rule:

• an on_event dropped into a body slot fails, because body declares [:step] and the two sets do not intersect;
• an ordinary step dropped into interrupts fails, because interrupts declares [:interrupt_handler] and a step is not one.

ADR-0002 decision 10 originally recorded the first direction as a special-cased validation rule the core types carry, and withdrew it at acceptance in favour of ADR-0003 decision 3's kind tags, which close both directions with one declaration on each side. There is no placement check in this module, and there is not supposed to be one: adding it back would give the editor two code paths to highlight from.

This type also never names the group types it may live inside. A host group with an interrupts slot admits it by declaring "interrupts" => [:interrupt_handler], and a host with a genuinely different notion of interrupt handler mints its own kind and its own group without touching this package.

Candidates for event (sb-82mu)

event is a plain :string and this type validates it the way it always has - the event-name shape rule, and nothing else. What the editor adds is a list of suggestions: the completion events the blocks in the handler's enclosing body raise, each written as the done.outcome.<state id>.<outcome> name StatifierBlocks.Compiler.StateId.outcome_event/2 mints, and labelled by the block's own card label and that outcome. Wiring a handler onto a sibling's outcome is then a pick rather than a transcription of a generated name.

Three properties of the list are this record's, not the control's.

• The body is read through the declaration. "The enclosing body" is every slot of the enclosing block that admits ADR-0003's :step kind, which is body on core.group and core.resumable_group and whatever a host group calls the slot it declares the same way. The handler's own interrupts slot declares [:interrupt_handler], so it is excluded by construction rather than by name.
• Only a type that declares outcomes contributes. ADR-0002 amendment A1 gives a type that implements no outcomes/1 a single default done, and offering an author a generated name for an outcome a type never declared would be offering them a wire that is not there.
• config_schema/1 is untouched. The field declaration gains no candidate key; the derivation is the editor's and is keyed on this type. core.send, core.raise and core.await each declare an event key too, and each names events this list is not about.

The outcome values

ADR-0002 decision 10 fixes outcome as a :select and names no values. Two are implemented:

They are the minimal pair ADR-0001 decision 10's compile target needs - transitions on the group's state, with the group's own history mode deciding where a "resume" re-enters. A third value is a config_schema/1 change plus a current_version/0 bump, not a document schema change.

The optional cond guard

cond is an optional :expression field, and when it is set it becomes the cond on the watcher's transition: the handler fires only when the event arrives and the condition holds. A handler with no cond - the key absent, or blank - emits exactly the bytes it emitted before the key existed, which is what keeps it an additive key rather than a document schema change.

The guard belongs here rather than on a core.branch after the handler, which is the shape it would otherwise be spelled as. A core.on_event decides whether to leave the in-flight body at all, and by the time a branch inside the handler could read a condition the body has already been abandoned - so the two spellings do not express the same thing, and only this one expresses a guarded interrupt. See ADR-0002's 2026-08-31 note.

The condition is the author's bytes passed through into predicator's datamodel verbatim. This package ships no expression checking of its own (ADR-0004 decision 9), so validate_config/1 only asks whether the stored value is a string; a typo inside it surfaces as an upstream compile error routed back to the "cond" field by the cond_key this type passes to StatifierBlocks.Core.Emit.transition/2.

Unlike core.branch, this type declares no value_path: its condition is stored at config["cond"], so ADR-0002 decision 7's default path - [key] - already addresses it. And summary/1 is untouched. ADR-0002 amendment H6 fixes this type's card as the outcome word then the event name, and the reason core.branch counts its arms rather than listing their conditions holds here too: an expression is not a chip.

The optional capture map

capture writes values out of the firing event's payload into the datamodel. It is a map, and the direction is worth stating twice because a path-to-path map reads either way: the key is the destination - a datamodel path - and the value is the source, a path inside _event.data. A capture of %{"order.cancel_reason" => "reason"} on a handler for order.cancelled writes that event's reason into order.cancel_reason.

One <assign> is emitted per pair, on the transition the handler already emits and before the <raise> that carries the outcome. The pairs are emitted in their datamodel paths' sorted order: a map has no order of its own and a compile has to be deterministic, so the record fixes one rather than leaving the bytes to a map's iteration. A handler whose capture is absent or empty writes no <assign> at all, which keeps the key additive in exactly the way cond is.

The assigns belong on the transition, and before the raise, for the reason the guard belongs here: the <raise> is what tells the enclosing group to abandon or resume, and by the time control is anywhere else that has happened - on abandon the body is gone, on resume the body is re-entered and history decides where. _event.data is in scope only for the transition the event selected, so a core.assign placed after the handler is a separate microstep with a different _event and the payload is not merely awkward to reach there, it is gone. See ADR-0002's 2026-09-05 note.

A captured value that quietly is not there is the failure this key has to avoid, because everything downstream would read it as an authored absence. What the interpreter does about that splits on whether the expression's root is bound, not on whether the whole path resolves:

• _event is always bound, so a capture whose source path is not in the payload writes the interpreter's explicit unbound marker and raises nothing. The marker is not nil and is not nil's spelling - unbound and null are deliberately different values there - so the absence is one a reader can test for rather than a silent hole. A consumer of a captured path has to make that test; that obligation is the whole of what this key promises today.
• A wholly unbound root raises error.execution and writes nothing. That is predicator's on_unbound: :error policy reaching Statifier.Interpreter.Content's one raise site, and no capture compiles to such an expression.

[Note 2026-09-05, sb-0q0z: this paragraph read "an <assign> whose expr does not resolve is an execution error", following ADR-0002's capture Note, which was written ahead of the measurement. Measured on two engine versions, the error is raised for an unbound root only, never for a missing member of a bound one. ADR-0002 carries the correction and the cites; the error arriving for this shape too is upstream work, and nothing here may be built on it until that lands.]

The compile-time half is the optional payload declaration below, and it is what the paragraph above stops being the whole story for: on a document that declares its payload the marker write never happens, because the document does not compile.

The optional payload declaration

payload declares what _event.data carries for the event this handler names (ADR-0002's amendment of 2026-09-06, P1). It is not a fact about the event name anywhere else in the document and it is not the datamodel, which the :declare and :datamodel compile options already own: two handlers for the same event may declare different payloads and neither is thereby wrong, because each governs its own capture.

The value is the name of a type the datamodel document declares - a record or a shape in its types key, read through StatifierDatamodel.Declarations. P2 of that amendment names a second arm, an inline shape written where the payload is declared, and P3 defers it: no member of ADR-0002 decision 7's field-type set describes a shape, and inventing one to spell it would be a second proposal riding along with this one. So the field is a :string carrying a name, which is the only spelling of a declaration this package has anywhere today (StatifierBlocks.Environment's type_expr/0), and no ninth field type is added.

payload is a declaration, not an emission. Nothing about it reaches the compiled SCXML: a handler that gains one compiles to the bytes it compiled to without it, and a handler that has none is unchanged in every respect - no new finding of any kind, at any severity. A payload naming a type the datamodel does not declare, and a compile with no :datamodel at all, are that same unchanged case reached by a second route (P4): the name resolves to nothing, and nothing is refused against nothing.

The refusal payload buys: payload_capture_findings/2

With a payload declared, a capture pair whose source path reads a member the payload does not carry is a :config refusal at compile (P5). The other reading - a declared member no pair reads - is not a finding: a payload may legitimately carry more than one handler wants.

The check needs the datamodel document, which validate_config/1 does not get, so it is a function of its own that the compiler's config stage calls with the declarations it has already indexed. One finding is reported for the whole capture key rather than one per pair, for the same reason check_capture/2 gives: the key is the only anchor an editor can use. The message names the offending pairs and the declared payload, because the anchor cannot.

How deep it goes is P5's rule, kept literally. The first segment of a source path is checked against the payload's field names. A deeper segment is checked only where the field's own type resolves, through the same declarations, to a declaration whose fields are in hand; a field whose type is a scalar, an opaque string, a list or :unknown stops the walk and refuses nothing beyond it. That adds no structural rule statifier_datamodel does not already have - its read check is nominal, permissive on the unknown, and descends into no list's element type.

The destination side of a pair is untouched by all of this: StatifierBlocks.Environment.capture_writes/1 still writes :unknown there. This types the source side at compile, and typing the destination from the payload is a widening of ADR-0011 that no ruling has asked for.

config_schema/1 declares no field for capture. ADR-0002 decision 7's field-type set has no member that describes a map, the 2026-09-05 note declines to add one, and how an author writes the pairs is ADR-0005's question rather than this module's. So the key is authored through the document today and not through the editor, and the two <assign> attributes carry no config attribution for the same reason core.subchart's composed conditions carry none: expr is composed here rather than the author's bytes verbatim, and location has no declared field for a finding to land on.