The statifier-ui trace wire format

Version: 1

Status and scope

This document is the normative specification ADR-0005 names: docs/wire-format.md, "the format's normative home." ADR-0005 settled the envelope, the nine trace.* type names, the session.start definition message and its role, the $-tagged JSON value discipline, the integer version field starting at 1, and the must-ignore-unknown rule; it delegated field-by-field schemas to this document. When this document and an implementation disagree, this document is what conformance means - ADR-0005's own words, restated here because it is the rule a second interpreter's author needs first.

Conformance. A conformant producer:

• MUST emit exactly one session.start message opening a session's stream.
• MUST, when tracing is enabled, emit the nine trace.* types at the phase boundaries Appendix D names, with the envelope fields this document requires.
• MAY emit further message families (this document also specifies ten effect.* types and four session.* lifecycle types) and MAY add fields to any message beyond what this document requires.

A conformant consumer:

• MUST ignore fields it does not recognize on a message of a known type.
• MUST ignore messages whose type it does not recognize.

Because of the two MUST-ignore rules, adding a field or a type is not a version bump. A version bump is reserved for a change that would make a consumer of the previous version misread the stream - removing a field, changing a field's meaning or shape, or renaming a type.

The envelope

Every message is a JSON object. Every message carries:

type is a dotted-namespace string identifying the message shape; the remaining sections of this document define every value it may take. session is the emitting session's id (statifier's _sessionid), so a consumer can share one channel across a session and, later, an invoke tree of related sessions. seq is a per-session monotonic integer, described fully under "Ordering" below.

effect.* messages carry all three counters, round included (sui-67d). The value is the engine's own stamp, carried verbatim: as of statifier ADR-0046 (st-xb2b), every core (non-trace) statifier effect payload carries macrostep, microstep, and round - effects emitted before that fold carry round: 0 - and this producer propagates the stamp rather than inventing one. Between ADR-0046 landing upstream and sui-67d landing here, this producer emitted round on trace.* messages and effect.budget_exhausted only; the other effect.* types gained the key later. Versioning decision, recorded rather than left implicit: adding round to the remaining effect.* envelopes is an additive field change, exactly what the conformance section's MUST-ignore rule makes safe, so the format version stays 1 - the same reasoning effect.datamodel_change records below for adding a whole type. A consumer reading an older recorded stream must still tolerate effect.* messages without round (the must-ignore rule's mirror image: absence of a field a newer producer would have written is not an error).

Beyond the fields in the table, every message carries a payload: the type-specific fields, documented below one type at a time. A payload key never collides with an envelope key - type, session, seq, macrostep, microstep, round, and otel are reserved words in every message's JSON object. This is why effect.invoke and effect.send/effect.send_delayed name their own <invoke>/<send> type/typeexpr attribute invoke_type and send_type rather than the engine's own field name type - the element attribute and the envelope's message-type discriminator are unrelated concepts that would otherwise collide under one key in the same flat JSON object.

Ordering

seq is stamped by the producer at the point a listener subscribes to a session's effect stream - the subscription boundary - starting at 0 on that session's session.start message and incrementing by exactly one for every message the producer emits afterward on that session. No other component may assign seq; a carrier that relays messages onward must preserve the value the producer stamped.

(macrostep, round) is the timeline key: it names when in the run a trace.* message belongs, independent of delivery order. seq totally orders one session's message stream; (macrostep, round) orders the run itself.

Interleaving across sessions is arbitrary. The format guarantees total order within one session and promises nothing about how two sessions' messages interleave on a shared channel - that interleaving is whatever process scheduling produced, not a promise anyone made. A consumer merging multiple sessions onto one timeline sorts by (session, seq), or by (macrostep, round) within a session, and never reads causality from arrival order.

Monotone delivery (st-r6l9, closed by ADR-0044). A live statifier session used to be able to deliver messages whose seq order did not match (macrostep, round) order, and could deliver trace.* messages after session.halted, on charts that exercise <invoke> or an internal <send>. ADR-0044 closed that seam: re-entry effects are now enqueued and FIFO-drained after the outer batch rather than performed inline, so arrival order is non-decreasing in (macrostep, round) across a whole run, and {:halted, reason} is promised as end-of-stream. ADR-0044 also settled the uniqueness key for quiescence: more than one trace.macrostep_stable per macrostep is explicitly allowed, but exactly one per (macrostep, round), with the last-arriving one being that macrostep's quiescence.

One consequence still worth stating for anyone building on this format:

• Consumers reconstruct the run's timeline by sorting on (macrostep, round) rather than relying on arrival order - that key names when in the run a message belongs, independent of delivery, and it is still the right key to sort by even though arrival order is now guaranteed non-decreasing in it.

A chart that never uses <invoke> or an internal <send> was never exposed to the old seam, and its trace remains byte-comparable run to run, which is what makes the worked example and the golden test at the end of this document possible.

OTel correlation: the otel key

A trace.* or effect.* message may carry an otel object identifying the OpenTelemetry span that covers its macrostep, so a consumer can deep-link a rendered step to the matching trace in an APM backend. ADR-0013 is the record; this section is the normative shape.

{"type": "trace.entry_set", "session": "sess_1", "seq": 7,
 "macrostep": 2, "microstep": 1, "round": 0,
 "otel": {"span_id": "00f067aa0ba902b7",
          "trace_id": "4bf92f3577b34da6a3ce929d0e0e4736"},
 "indexes": [3, 4]}

Both are W3C Trace Context hex encodings: trace_id is exactly 32 lowercase hex digits, span_id exactly 16, with no 0x prefix, no dashes, and no uppercase. They name the statifier.macrostep span the upstream bridge opens for this message's macrostep, and the trace that span roots (statifier docs/opentelemetry.md, "A macrostep is a span" and "One trace per macrostep"). The object is never partial - a producer that can name one id but not the other omits the key entirely, because half the pair cannot be looked up in any backend.

Where it is legal. Exactly where macrostep is legal: on trace.* and effect.* messages, and on no others. No session.* message ever carries it. Upstream has no session-lifetime span and no session-lifetime trace, so a session-scoped message stamped with whichever macrostep span happened to be open would assert a containment the engine's own bridge declines to claim. A consumer wanting the trace a session ended inside reads the last trace.* or effect.* message before the lifecycle message.

The value is per macrostep and it repeats. Every message of one macrostep in one session carries the same otel value, because one span covers the whole macrostep. Carrying it once per macrostep would be smaller and is rejected for the reason the $redacted sentinel is kept alongside the projection header: a consumer that joined mid-stream, or a single message pulled out of a log, still has to be able to say what it is.

Absence. The key is absent whenever no correlation context is attached - no bridge in the host, no resolvable span, or a producer predating this section. It is never null and never {}; the absence rule above already spends both of those on other meanings, and neither means "there is no span".

Versioning decision. Adding otel is an additive field change, exactly what the conformance section's MUST-ignore rule makes safe, so the format version stays 1 - the same reasoning round on the effect.* envelopes and the projection header each record. No existing stream changes a byte, and nothing a v1 consumer previously read correctly is now read incorrectly.

Golden traces carry no otel. Span and trace ids are random per run, so a stream carrying them is not byte-comparable run to run. Golden captures attach no correlation context, exactly as they carry no projection header, and the full-fidelity default is unchanged. A test asserting this key's shape does so against fixed stub ids, never against a golden.

JSON discipline

Four rules make the format precise enough for a byte-comparable golden trace, and precise enough that a second interpreter reproduces the same bytes for the same run.

Value encoding

Anything occupying a value position - _event.data, a <log>'s resolved value, donedata, a <send>'s resolved data, an <invoke>'s resolved params/content - is encoded through the same four-form scheme StatifierUI.Value implements:

• Booleans, integers, floats, strings, nil, lists, and string- or atom-keyed maps (atom keys serializing as their names) map to their natural JSON form.
• A Date encodes as a one-key tagged object: {"$date": "2026-08-16"} (ISO 8601).
• A DateTime encodes the same way: {"$datetime": "2026-08-16T12:00:00Z"} (ISO 8601).
• A duration encodes as {"$duration": {...}}, an object carrying all eight unit fields - years, months, weeks, days, hours, minutes, seconds, milliseconds - as integers, absent units filled with 0.
• The :undefined sentinel, wherever it appears inside a composite value (a list element, a map value) rather than at a field's top level, encodes as {"$undefined": true} - absence has no positional spelling inside a list or a map value, so the sentinel needs an explicit encoding there even though it is spelled by key-absence at a message's top level (see "Absence" below).
• A value withheld by a projected stream encodes as {"$redacted": true} (ADR-0012). It is legal wherever a value is legal, including nested inside a list or map value. Unlike the four forms above it is a claim about the stream rather than about the run: it says a value stood here and this stream is not carrying it. It is never spelled by key-absence, by null, by {}, or by {"$undefined": true}, each of which already means something else - see "Projection" below.

The one-key $-prefixed object shape is reserved by this document for exactly these five forms. A host value that happens to be a one-key map whose only key starts with $ and is not one of $undefined, $date, $datetime, $duration, or $redacted is a spec violation on the producer's side, not a value this format can carry unambiguously - accepted as vanishingly rare.

Absence

Key absence and JSON null mean different things, and no layer of this format may collapse them. The rule stated once, generalized to every nullable field this format carries (not only _event.data, though that is where the distinction was first load-bearing):

• Key absent - the field has no meaningful value at all (statifier's :undefined sentinel, or an optional field the producer was not given).
• JSON null - the field is present, and its value is null.
• An empty object or array ({}, []) - the field is present, and its value is an empty collection.

All three are distinct and a consumer must be able to tell them apart. The rule applies uniformly to: _event.data (:undefined -> key absent, nil -> null, %{} -> {}); Event's invokeid, origin, origintype, sendid, and cause; Log's label, c_index, and owner; Trace.Done's donedata; and, the load-bearing case for a consumer reconstructing the run, trace.transitions_selected's event field, whose absence marks an eventless round - the platform selected transitions with no event to match against, spec's NULL round. There is no separate boolean flag for this; the absence of the event key is the signal, the same way absence signals "no data" everywhere else in this rule. A consumer must not confuse "the event key is absent" with "the event key is null" - they never mean the same thing anywhere in this format.

Structural tagging: "kind"

Statifier represents some information as Elixir tuples - the eight variants of an internally-raised event's cause origin, and the five variants of which content block ran. Nothing tuple-shaped crosses the wire. Each variant becomes a JSON object whose "kind" field names the tuple's tag and whose remaining fields name its positional elements:

{:content, c_index, owner}       -> {"kind": "content", "c_index": c_index, "owner": {...}}
{:onentry, state_index, ordinal} -> {"kind": "onentry", "state_index": state_index, "ordinal": ordinal}

and so on for every variant listed under "Origins" and "Owners" below. "kind" rather than "$kind": the $-prefixed one-key shape above is reserved for the value codec's five forms, and these are multi-key structural objects appearing in a known field position, not values sitting in a value slot.

Canonical order

• Object keys are lexicographic, in every object this format produces, at every nesting level.
• A genuine set-valued field - a MapSet in the engine - serializes as an array in ascending-index order. This applies to configuration wherever it appears (trace.macrostep_stable, trace.done, effect.budget_exhausted).
• A sequence-valued field that is already ordered by the engine keeps that order; it is not additionally sorted (ADR-0011). This applies to trace.exit_set and trace.entry_set's indexes fields precisely: they are sequences in the engine's own emission order - exit order is inner-to-outer (the order Appendix D's exitStates actually visits them), entry order is ascending - and re-sorting the exit set would reverse what happened. Both orders are deterministic for a given run, so preserving them costs the byte-comparability rule nothing. Every other sequence field in this document that is not already identified as a genuine set (transition lists, content lists, and so on) likewise keeps the engine's own order.

Together, canonical ordering plus the value encoding above is what makes two producer runs of the same chart, the same fixtures, and the same event script byte-identical JSON Lines - the golden-trace conformance mechanism ADR-0005 names.

session.start

The definition message that opens a session's stream. Every later message's indexes (state indexes, t_index, c_index) resolve back to source locations only through the tables this message carries - a consumer with no compiler of its own can still build a working inspector, because everything it needs to resolve an index arrived once, up front.

version is 1 for this document. It is the fixtures sidecar's own version convention (ADR-0003), reused here rather than invented fresh.

data is always present, never key-absent, even when the chart has no <datamodel> at all: an empty datamodel still emits [], the empty-collection arm of the absence rule below, not the omitted-key arm.

states is one object per compiled state, in index order (index 0 is always the synthesized :scxml root):

transitions is one object per compiled transition, in t_index order:

contents is one object per compiled executable-content node, in c_index order:

Every content node kind except <script> carries a :location field directly. Statifier.Machine.Content.Script is the one exception - it carries :node_location instead, because a script's own compiled program lives where a sibling node's resolved value would. This document's producer reads a content node's location as its :location field when present, falling back to :node_location when it is not, so every entry in contents carries a location regardless of kind. A producer that hardcodes :location and skips the fallback will crash on the first <script> in a chart - this is the specific bug this note exists to prevent.

data is one object per compiled <data> element, in d_index order (document order across the whole chart, not per <datamodel> block):

This table is deliberately identity only: it resolves a d_index to an id and a source span, and carries no representation of the element's declared value. A consumer wanting to display the declared value reads it out of source at value_location, subject to the fallback below; a consumer wanting a runtime value reads session.datamodel for the starting snapshot and the datamodel-change messages after it. The compiled value itself is an expression program, a compile error, or an unresolved src URI depending on how the element was written, and none of the three has a language-neutral encoding in this format. Adding a value field later would be an additive change and therefore not a version bump (ADR-0005), so this document commits to the narrow shape now rather than to an encoding it would have to keep.

value_location is not always a value span, and a consumer must check before slicing. It spans the element's written value only when the element has one to point at: the expr attribute's value for an expr-written element, the src attribute's value for a src-written one. An element written with neither - a bare <data id="x"/>, or one whose value is child content - has no distinct value span, and value_location falls back to the <data> element's own span, equal to this row's location. So a consumer slicing source at value_location must compare it against location first: when the two are equal there is no value span, and the slice is the whole element (<data id="x"/>), not a value. This is the one place value_location parts company with a transition's cond_location, which is absent when there is no guard rather than falling back, and therefore always spans a guard when present.

A location object is always all six fields, and is either wholly present or wholly absent - there is no partial location:

Ends are exclusive. A location's end_line/end_column/end_offset and a span object's end_line/end_column all name the position one past the last character of the span, so a zero-width span has its start equal to its end and a consumer slices [start, end) without adjustment. This is the convention statifier's own Statifier.Parser.Location and predicator's Predicator.Types.span/0 both use, and it is stated here rather than left for a consumer to infer from the data.

source, when present, is the chart's SCXML text, verbatim. It cannot be derived from the compiled Machine - the compiled form does not retain source text - so this document's producer accepts it as caller-supplied context rather than recovering it. Absence means the host supplied none.

fixtures, when present, is the ADR-0003 sidecar's decoded JSON object, carried through verbatim, its own version field included. This document's producer does not construct or validate a fixtures bundle; the caller supplies an already-decoded object, or supplies none. The carried object may contain ADR-0006's datasets and expressions keys alongside ADR-0003's scenarios and events; this producer neither constructs nor validates either of the two additive keys, the same as it does not for the other two.

parent_session and invokeid, when present, name the session that started this one via <invoke> and the invokeid this session stamps, respectively. Together they let a consumer holding two sessions' session.start messages reconstruct the parent-child edge between them without any other channel. Both are absent for a session with no invoking parent.

attribute_locations is the attribute-level span table, carried on every states and transitions row alongside that row's element-level location. It maps an attribute's name to that attribute's own value span - a location object, in the same six-field shape:

"attribute_locations": {
  "event": { "start_line": 4, "start_column": 17, "start_offset": 96,
             "end_line": 4, "end_column": 31, "end_offset": 110 },
  "target": { "start_line": 4, "start_column": 40, "start_offset": 119,
              "end_line": 4, "end_column": 48, "end_offset": 127 }
}

Key presence is the load-bearing part, not the span. An entry exists only for an attribute the author actually wrote, so a consumer asking whether a transition was written type="external" or merely defaulted to external reads the presence of the type key, which the compiled type value can no longer answer. The same question applies to a state's initial, and to any attribute whose lowered value is indistinguishable from its default. The map is carried verbatim from the Document node through the compiled Machine to here, so the contract survives the whole trip rather than being reconstructed at any layer.

The attributes that appear are whatever the element wrote; a consumer must not assume a fixed key set. In practice a transition row carries event, target, type and cond where written, and a state row carries id and initial where written, plus a history state's type and the root's <scxml> attributes (initial, name, datamodel, binding, version, xmlns) at index 0.

Absence degrades to the element-level span, and always has. The object is {} for an element that wrote no attributes, for the synthesized initial transition no author wrote, and for a producer reading a Machine compiled by an engine old enough not to populate the field. Those three are indistinguishable on the wire and deliberately so: a consumer that finds no entry for the attribute it wants falls back to the row's location, which is the granularity this format offered before the field existed. Nothing that read location before needs to change.

cond_location is not superseded by it. A transition's cond attribute appears in both places when written, and the two are not interchangeable: cond_location falls back to the transition's own location when the guard was written without a recorded span, where attribute_locations simply omits the key. Prefer attribute_locations["cond"] for new work - it answers "where exactly is the guard text" without ever silently widening to the whole element - and read cond_location when that fallback is the wanted behaviour.

contents and data rows carry no attribute_locations. The compiled Machine retains the map on its content nodes, but the data table is deliberately identity-only (see its note above) and no consumer has asked for attribute granularity on either yet; adding it later is an additive change and therefore not a version bump.

Being an identity table, attribute_locations is never projected - it falls under "every session.start table and every location object in them" in the Projection section below, and carries no datamodel value.

The nine trace.* schemas

Every trace.* message carries the envelope's macrostep, microstep, and round, in addition to type, session, and seq. Fields below are the payload beyond the envelope.

trace.event_dequeued

Emitted when an event is selected off a queue for processing.

An event object is Statifier.Event.t() rendered to JSON:

An error object is the wire rendering of a failed expression evaluation (Statifier.Evaluator.Error.t()), produced when an event's data is such a failure rather than a value:

Three notes a consumer needs before using this object to underline anything:

• expression is not raw source. It is the entity-expanded string the expression engine counted columns in - a cond written amount < limit appears here as amount < limit. span's columns are offsets into this expanded string, not into source, and cannot be added to a raw-source column directly. This is why location exists: it is pre-resolved for you.
• location is absolute and already resolved. A consumer underlines it directly against source, with no span composition of its own. This is the field that makes the format usable by a consumer with no Elixir and no compiler.
• location_kind says what the producer did, not what happened internally. "resolved" means the span was composed against the expression's own value location; "node" means there was no span to compose, or no value location to compose it against, and the owning node's whole span was emitted instead. "resolved" may still span the whole attribute value: the composition degrades rather than failing when the raw and entity-expanded text desync, and the producer does not distinguish that case from a genuine resolution. Treat this as a documented limitation, not a promise that "resolved" always narrows to the failing subexpression.

A cause object (Statifier.Event.Cause.t()):

trace.transitions_selected

Emitted whenever transition selection runs, including when it selects nothing.

trace.exit_set

Emitted before any state is exited, whether by ordinary transition or by interpreter shutdown.

trace.content_executed

Emitted when a block of executable content runs (an <onentry>/<onexit> block, a transition's own content, or a top-level <script> at load time).

trace.entry_set

Emitted before any state is entered, with compute_entry_set's result.

trace.macrostep_stable

Emitted once the configuration reaches quiescence - the macrostep's microstep loop has drained.

trace.done

Emitted alongside the top-level final entry / interpreter exit.

trace.invoke_pass

Emitted once the invoke pass finishes, including when it starts nothing.

trace.finalize_autoforward

Emitted once per external event, at the end of the finalize/autoforward pass, including when nothing was finalized or forwarded.

The ten effect.* schemas

The core (non-trace) statifier effects, mapped one-to-one onto their own namespace: the nine originals plus effect.datamodel_change (Statifier.Effect.DatamodelChange, st-oef3). The engine's eleventh core effect, Statifier.Effect.DatamodelInit, is the one exception to the one-to-one mapping - it serializes as session.datamodel, below. ADR-0005 leaves non-trace effect naming to this document ("their own namespaces as consumers need them"); this document uses one effect.* family rather than several separate top-level namespaces, because a bare top-level done type would sit confusingly next to trace.done - a different message about the same moment in the run. effect.* and trace.* are two visibly distinct halves of the vocabulary: trace.* are the nine Appendix D phase boundaries and effect.* are the core effect vocabulary, both stamped with all three counters (see "The envelope" above; round joined the effect.* envelopes in sui-67d, after effect.budget_exhausted had carried it from the start).

Every effect.* message carries macrostep, microstep, and round.

effect.log

Payload for <log> (spec 4.7).

effect.done

The terminal effect, emitted once after top-level final entry.

effect.budget_exhausted

Emitted when a macrostep's fold spends its round budget without reaching quiescence (ADR-0019). This was the one core effect that carried round before sui-67d propagated the key onto the rest of the effect.* family - see "The envelope" above.

effect.invoke

Emitted once per <invoke> the invoke pass actually started (spec 6.4).

effect.cancel_invoke

Emitted once per invocation the engine cancels because its owning state exited while the invocation was still live.

effect.autoforward

Emitted once per autoforwarding invocation, during the finalize/autoforward pass.

effect.send

Payload for <send>, fired immediately (spec 6.2).

effect.send_delayed

Payload for <send> with a delay/delayexpr. Carries every field effect.send does, plus:

effect.cancel

Payload for <cancel> (spec 6.3).

effect.datamodel_change

Emitted once per successful datamodel write (st-oef3): an <assign>, a <data> binding during the binding fold, a <send idlocation> write, an <invoke idlocation> write, or an empty-<finalize> auto-assign. A failed write emits nothing - the datamodel did not change, and the failure is already on the error channel. Together with session.datamodel's starting snapshot, the sequence of these messages reconstructs the datamodel at any point in the run from the stream alone.

location_path is a heterogeneous JSON array. Each segment is either a string - an object key, the variable name first - or an integer, a 0-based array index: user.items[0].name resolved against i = 0 arrives as ["user", "items", 0, "name"]. JSON's own typing carries the distinction - the string "0" is a key, the number 0 is an index - so no tagging or escaping is needed, and a consumer applies the segments in order to its own copy of the datamodel to reproduce the write. It is the resolved path: an index expression like [i] has already been evaluated by the engine, which is what makes the path applicable without the pre-assignment datamodel. location_source is the raw author string kept alongside for display; neither substitutes for the other.

new_value and prior_value follow the _event.data three-way rule, because the engine genuinely distinguishes unbound from null here (statifier ADR-0037 spells unbound as :undefined): key absence means the slot was or became unbound - for prior_value, that nothing stood at the path before the write, the common case for a first assignment - while JSON null means a genuinely stored null.

d_index and c_index are mutually exclusive on this message: a d_index means the write was a <data> binding, which belongs to no content block and therefore also carries no owner; a c_index means a content node performed it. The two runner-side writes - the empty-<finalize> auto-assign and <invoke idlocation> - carry neither, identified by owner alone ("finalize" and "invoke" kinds respectively).

Versioning decision, recorded rather than left implicit: this type joined the format after version 1 shipped with 23 types. Adding a type is exactly what the conformance section's MUST-ignore rule makes additive, so the format version stays 1 - unlike session.datamodel, which kept version 1 because its type string was already reserved, this one keeps it because new types never bump the version at all.

Origins

The origin type on Statifier.Event.Cause has eight variants, each a tagged object with "kind" naming the tuple's tag:

A d_index anywhere in this format - here or on effect.datamodel_change - resolves through session.start's data table, the same way a state_index resolves through states and a t_index through transitions.

Owners

The owner type on Statifier.Machine.Content has four variants, plus the case Statifier.Effect.Trace.ContentExecuted widens it with for a top-level <script> and the case Statifier.Effect.DatamodelChange widens it with for <invoke idlocation>, together the six variants an owner field may take anywhere in this format:

The session.* types

Five types name a session from the outside: its start, one snapshot of the values it started with, and the three ways a subscriber's own stream can end or change state. None of the five carry macrostep/microstep/ round - the four lifecycle types because they describe the stream, not a point inside a run, and session.datamodel because it precedes the first macrostep entirely (its underlying effect payload carries macrostep/ microstep/round internally, but this producer stays consistent with every other session.* type and leaves the envelope counters nil).

session.start

The definition message. Schema above under "session.start". Always seq: 0.

session.datamodel

Emitted exactly once per session, unconditionally - even under trace: false - as the second message on the stream, right after session.start, from statifier's Statifier.Effect.DatamodelInit (st-1xwh). It carries the datamodel's starting values: spec 5.3.3's unconditional <data> creation plus the four spec 5.10 system variables, before the binding fold that follows it.

Because the snapshot precedes that fold, every <data> element appears as {"$undefined": true}, not only the ones declared without a value: a <data id="count" expr="41 + 1"/> is present under count as {"$undefined": true} here, and is assigned its 42 afterwards. This message therefore names the datamodel's variables reliably, but is not a source for their values. The assignments that follow arrive as effect.datamodel_change messages (sui-h92), one per successful write, binding fold included - a consumer folds them over this snapshot to hold the current datamodel at any point in the run.

This message keys the datamodel by variable name; session.start's data table keys the same elements by d_index and carries the id that joins the two.

session.halted

Emitted when the session reports {:halted, reason} to its subscriber.

As of ADR-0044 (st-r6l9), session.halted is terminal for that session id: no further messages for the same session id follow it (see "Ordering" above). Terminal is scoped per session id, not per mailbox - ADR-0050 lets a subscriber observe an invoke tree of related sessions on one mailbox, and a parent's session.halted does not end the stream for a child session still running on the same mailbox.

session.terminated

Emitted when the subscriber observes the session's process exit (a monitor :DOWN).

reason is a human-readable string, not structured data to branch on - an Elixir process exit reason has no language-neutral shape, so this document does not attempt to give it one.

Under a projected stream this is the one field whose JSON type changes: it carries {"$redacted": true} instead of a string. It earns the exception because it is inspect/1 of an exit reason, so a crash inside a datamodel operation can carry datamodel terms into it verbatim, and it is the one string field on the wire whose content is genuinely unbounded. Nothing may be branching on its shape, because the paragraph above already says not to.

session.unroutable

Emitted when the session reports {:unroutable, effect} to its subscriber

• an effect the session could not route to a destination. Currently unreachable in the engine as shipped, reserved for when it becomes reachable.

Projection

A projected stream carries the run's structure, transitions, outcomes and ordering, and withholds its values. It exists for the second consumer of this format: a multi-tenant host showing a run's history to the end user whose run it is, who is not entitled to the datamodel behind it. ADR-0012 is the record.

Full fidelity is the default and is byte-unchanged. A producer not asked to project emits no projection key, no sentinels, and full values. Every existing golden trace is byte-identical. Projection is something a host turns on for a subscription, one session at a time.

The version stays 1. A v1 producer never emitted $redacted, so no existing stream changes; a v1 consumer meeting one renders an unfamiliar one-key map, which is conspicuous rather than silently wrong, and the projection header says why. Nothing a v1 consumer previously read correctly is now read incorrectly, which is this document's own test for a bump.

The projection header

session.start carries a projection object whenever the stream is projected:

{"mode": "projected", "profile": "end_user_run_history"}

Key absence means full fidelity. Distinguishability is not optional: a capture that cannot say whether it is redacted cannot be filed by an operator; a golden-trace comparison between a projected capture and a full golden fails for a reason that has nothing to do with conformance, and the header is what lets a test say so; and a datamodel pane rendering a redacted stream must show "redacted", not "unbound".

Per-message self-announcement through the $redacted sentinel is kept in addition to the header, not instead of it, so a consumer that joined mid-stream or a single message pulled out of a log still says what it is.

The closed set of value positions

Every position below is redacted unless a profile allows it back. The table is the checklist: a value position added to this format later without a projection rule would carry values through a projected stream silently, which is the worst failure available here because it is invisible.

error carries no value position by construction: the predicator error's rendered message - the field that would embed datamodel values - is deliberately not on the wire (see the error object's fields above), so trace.event_dequeued's error key needs no row in this table.

session.unroutable wraps another type's encoding under a kind key, so a projection must recurse into it rather than pattern-matching the outer type. It is unreachable in the engine as shipped, which is exactly why it is easy to forget.

session.datamodel keeps its keys. Variable names come from the chart, not from the run, and this message's stated job is to name the datamodel's variables reliably - which it still does.

session.start's fixtures is a datamodel bundle by construction (ADR-0003), so it is replaced whole rather than descended into. A host that supplied no fixtures still omits the key, as today, and the two states stay distinguishable.

Redaction replaces at a key that is already present; it never creates one. Nine of these positions are conditionally absent, and writing a sentinel into an absent key would assert something the run never did: an eventless round (trace.transitions_selected with no event) becoming evented, a first write (effect.datamodel_change with no prior_value) acquiring a prior value, or a host that supplied no fixtures acquiring a bundle.

What is never projected

type, session, seq, macrostep, microstep, round, otel; state indexes, t_index, c_index, d_index, invokeid, send_id, state_index, invoke_index; every session.start table (states, transitions, contents, data) and every location and value_location object in them; configurations; exit and entry sequences in their engine order; selected t_index lists; kind and type discriminators; owner and origin objects; event names; transition event descriptors; label on effect.log; src and invoke_type on effect.invoke; target and send_type on the send family; location_path and location_source on effect.datamodel_change; session.halted's reason (a closed three-value set); effect.done's configuration; and, on trace.event_dequeued's error object, kind, span, location, and location_kind.

That list is what leaves a projected stream worth rendering at all: the timeline, the diagram highlighting, the click-through to source, and the run's outcome are built entirely from fields in it.

otel is in that list because a trace id and a span id are randomly generated identifiers carrying no chart vocabulary, no datamodel value, and no source text - the same category as seq and macrostep. The residual is real and stated rather than waved at: a trace id is a join key into a backend that may hold what this stream withheld. The control for that is attaching no correlation context to a subscription a projected audience reads, which makes the key absent; there is no projection knob for it, and ADR-0013 records why not.

session.start's data table is already deliberately identity-only - it carries d_index, id, location, and value_location, and no representation of the declared value - so it needs no rule.

Allowlists: two shapes, because there are two kinds of position

Within projected mode the default is deny.

Located positions - the datamodel - are allowlisted by path prefix, written as arrays of segments in the same encoding effect.datamodel_change's location_path already uses:

allow_paths: [["authorization", "status"], ["account", "currency"]]

So ["authorization"] allows the whole authorization subtree and ["authorization", "status"] allows one leaf. The same prefixes apply to session.datamodel, whose keys are the first segment of every path. That is why the datamodel allowlist is expressed by path and not by variable name: one expression covers both messages, and a host that may show authorization.status but not authorization.amount_cents can say so.

Three cases, and the third is the operator ruling of 2026-08-29 recorded on ADR-0012:

• An allowed prefix matches the write's leading segments (the prefix is no longer than the write's path): the whole value passes.
• No allowed prefix relates to the path: the whole value is redacted.
• An allowed prefix is longer than the write's path and extends it - the write is shallower than the prefix, so the written value contains both the allowed leaf and its withheld siblings. The projection descends into the value and redacts selectively, so the allowed leaf passes and every sibling is redacted. Allowing the whole write would leak a sibling the profile withheld; denying it would withhold a leaf the profile allowed.

Descent applies identically to session.datamodel's snapshot values. Where a value cannot be descended into because it is a scalar rather than an object or an array, the allowed leaf is unreachable and the value is redacted whole.

Unlocated positions - payloads, which have no path - are allowlisted by naming the position, from a closed set:

allow_positions: [:event_data, :log_value, :send_data,
                  :invoke_params, :invoke_content, :donedata]

Naming a position allows it wholesale, at every message that carries it. There is deliberately no per-key allowlist inside a payload: an event payload has no stable schema the way a datamodel location does, so a key-level rule there would be a guess that silently stops matching when a chart changes - and a redaction rule that silently stops matching is the failure this whole section exists to avoid. A host needing finer control over a payload narrows what the chart puts in it.

allow_source

session.start's source is verbatim SCXML. It is authored rather than run data, and it is what the whole inspector resolves indexes against, so projection retains it by default.

The honest caveat: the data table's value_location exists so a consumer can recover a <data> element's declared initial value by slicing source. So retaining source retains declared initial values, even under a profile that redacts everything else. Runtime values are unaffected - the binding fold's results arrive as effect.datamodel_change and are redacted normally - but a chart with a literal secret in a <data expr="..."> is not protected by projection.

A profile may set allow_source: false, which replaces source with the sentinel. The cost is stated plainly: without source, location objects still resolve to line and column but nothing can display the text at them, so click-through degrades to coordinates.

trace.event_dequeued's error.expression is chart text under the same reasoning - it is the failing expression's entity-expanded source, not run data - so allow_source: false redacts it alongside source. error's other keys (kind, span, location, location_kind) are unaffected.

What projection is not

It is not anonymization. Structure leaks. Which branch a run took, how many rounds a macrostep needed, which transition fired on which event - these imply things about the values that produced them, and an observer with the chart in hand can infer a great deal from a fully redacted stream. The guarantee is narrow and worth stating in those terms: no datamodel value crosses the producer boundary. A host needing more than that needs to withhold structure, which this format does not provide.

It is not access control. It shapes one stream. It does not authenticate a subscriber, does not decide which sessions a subscriber may see, and does not stop a host attaching a full-fidelity subscriber alongside a projected one. Choosing the profile correctly per tenant is the host's job, and this format cannot check it.

A projected capture is not byte-comparable to a full one. Golden-trace conformance does not transfer: a profile whose output matters is tested against its own golden, produced under that profile. The existing goldens stay valid because the default is untouched.

location_path's integer segments are resolved index expressions, so a projected stream still reveals which array index a write landed on - a narrow inference channel about a value the same stream redacts. It is retained because a consumer that cannot see the path cannot fold the write at all, which would cost the whole datamodel-shape view for very little.

Worked example

The complete JSON Lines trace of a two-state chart with one external transition. The producer ran with session_id: "sess_golden", attached early (Statifier.Trace.Subscriber's :subscribers-at-start_link path, docs/plans/260817-sui-t36.3-session-subscriber-and-trace-normalizer.md phase 5), so it sees the initialize burst that runs to quiescence before Statifier.Session.start_link/2 returns. This is the same fixture phase 5's golden test compares against byte-for-byte (test/support/trace/two_state.jsonl).

Chart:

<scxml xmlns="http://www.w3.org/2005/07/scxml" initial="a" version="1.0">
    <state id="a">
        <transition event="go" target="b"/>
    </state>
    <state id="b"/>
</scxml>

xmlns and version are required attributes on the root element (Statifier.Validator); a chart missing either fails to compile, so an example chart must carry both. They do show up in the trace, in the root state row's attribute_locations, alongside initial - written attributes all, and the map records what the author wrote rather than what the format finds interesting. Note also what is absent from the transition row's map: type, because this chart does not write it, which is exactly the distinction that field exists to preserve.

Driven with one external event, "go", with no data, after the session has already come up and settled into a on its own. Trace (one JSON object per line, each shown here with lexicographic keys exactly as the wire form produces - the actual bytes, not a reformatting):

{"contents":[],"data":[],"seq":0,"session":"sess_golden","states":[{"attribute_locations":{"initial":{"end_column":58,"end_line":1,"end_offset":57,"start_column":57,"start_line":1,"start_offset":56},"version":{"end_column":72,"end_line":1,"end_offset":71,"start_column":69,"start_line":1,"start_offset":68},"xmlns":{"end_column":46,"end_line":1,"end_offset":45,"start_column":15,"start_line":1,"start_offset":14}},"children":[1,2],"index":0,"kind":"scxml","location":{"end_column":9,"end_line":6,"end_offset":178,"start_column":1,"start_line":1,"start_offset":0},"transitions":[]},{"attribute_locations":{"id":{"end_column":17,"end_line":2,"end_offset":90,"start_column":16,"start_line":2,"start_offset":89}},"children":[],"id":"a","index":1,"kind":"state","location":{"end_column":13,"end_line":4,"end_offset":149,"start_column":5,"start_line":2,"start_offset":78},"parent":0,"transitions":[0]},{"attribute_locations":{"id":{"end_column":17,"end_line":5,"end_offset":166,"start_column":16,"start_line":5,"start_offset":165}},"children":[],"id":"b","index":2,"kind":"state","location":{"end_column":20,"end_line":5,"end_offset":169,"start_column":5,"start_line":5,"start_offset":154},"parent":0,"transitions":[]}],"transitions":[{"attribute_locations":{"event":{"end_column":30,"end_line":3,"end_offset":122,"start_column":28,"start_line":3,"start_offset":120},"target":{"end_column":41,"end_line":3,"end_offset":133,"start_column":40,"start_line":3,"start_offset":132}},"content":[],"events":[["go"]],"location":{"end_column":44,"end_line":3,"end_offset":136,"start_column":9,"start_line":3,"start_offset":101},"source":1,"t_index":0,"targets":[2],"type":"external"}],"type":"session.start","version":1}
{"datamodel":{"_event":{"$undefined":true},"_ioprocessors":{"http://www.w3.org/TR/scxml/#SCXMLEventProcessor":{"location":"#_scxml_sess_golden"}},"_name":{"$undefined":true},"_sessionid":"sess_golden"},"seq":1,"session":"sess_golden","type":"session.datamodel"}
{"indexes":[0,1],"macrostep":1,"microstep":1,"round":0,"seq":2,"session":"sess_golden","type":"trace.entry_set"}
{"macrostep":1,"microstep":1,"round":1,"seq":3,"session":"sess_golden","t_indexes":[],"type":"trace.transitions_selected"}
{"invoke_ids":[],"macrostep":1,"microstep":1,"round":1,"seq":4,"session":"sess_golden","state_indexes":[0,1],"type":"trace.invoke_pass"}
{"configuration":[0,1],"macrostep":1,"microstep":1,"round":1,"seq":5,"session":"sess_golden","type":"trace.macrostep_stable"}
{"event":{"name":"go","type":"external"},"from":"external","macrostep":2,"microstep":0,"round":0,"seq":6,"session":"sess_golden","type":"trace.event_dequeued"}
{"event":{"name":"go","type":"external"},"finalized":[],"forwarded":[],"macrostep":2,"microstep":0,"round":0,"seq":7,"session":"sess_golden","type":"trace.finalize_autoforward"}
{"event":{"name":"go","type":"external"},"macrostep":2,"microstep":0,"round":0,"seq":8,"session":"sess_golden","t_indexes":[0],"type":"trace.transitions_selected"}
{"indexes":[1],"macrostep":2,"microstep":1,"round":0,"seq":9,"session":"sess_golden","type":"trace.exit_set"}
{"c_indexes":[],"macrostep":2,"microstep":1,"owner":{"kind":"transition","t_index":0},"round":0,"seq":10,"session":"sess_golden","type":"trace.content_executed"}
{"indexes":[2],"macrostep":2,"microstep":1,"round":0,"seq":11,"session":"sess_golden","type":"trace.entry_set"}
{"macrostep":2,"microstep":1,"round":1,"seq":12,"session":"sess_golden","t_indexes":[],"type":"trace.transitions_selected"}
{"invoke_ids":[],"macrostep":2,"microstep":1,"round":1,"seq":13,"session":"sess_golden","state_indexes":[2],"type":"trace.invoke_pass"}
{"configuration":[0,2],"macrostep":2,"microstep":1,"round":1,"seq":14,"session":"sess_golden","type":"trace.macrostep_stable"}

seq starts at 0 on session.start and increments by one across every subsequent message, with no gaps - this chart never touches the <invoke>/ internal-<send> reordering seam, so a producer's live delivery order and (macrostep, round) order agree throughout, and the trace above is byte-comparable run to run. session.datamodel at seq 1 always follows session.start and always precedes the first trace.* message, carrying no macrostep/microstep/round of its own. macrostep 1 is the session's own initialize burst (Appendix D's initialize procedure, entering a before any event is ever sent - configuration on trace.macrostep_stable is [0, 2] later, not [2], because the synthesized root at index 0 is always a member); macrostep 2 is the driven "go" transition into b. trace.content_executed appears once, for the transition's own (empty) executable content list; trace.done does not appear because this run never halts through a <final> state - a chart reaching one would show it in the shape its schema above describes.

Persistence and the v1 round-trip

A trace is written to storage as JSON Lines: one message object per line, UTF-8, each line newline-terminated, with no enclosing array and no header line. The worked example above is exactly that format, and test/support/trace/two_state.jsonl is a file in it. Nothing else about the file is specified - a .jsonl extension is convention, not conformance, and a stream may be split across files or held in a database column as long as message order is preserved.

Ordering in a file is seq order, which is the order the producer emitted (see "Ordering"). A reader may rely on that; it does not have to sort.

The round-trip law. For any conformant v1 stream, decoding a message to its envelope and payload and re-encoding it reproduces the bytes it was read from. Two properties make that hold, and both are deliberate:

• Encoding is canonical. Object keys are emitted in lexicographic order at every nesting level and arrays in producer order (see "Canonical order"), so re-encoding cannot permute anything.
• Decoding is structural only. The seven reserved envelope keys are lifted out and everything else is the payload, kept in wire shape. In particular a decoder does not interpret $-tagged values (see "Value encoding"); those are decoded by whichever consumer needs Elixir-side, or language-side, terms, at the point it needs them. A decoder that eagerly decoded values could not round-trip, because more than one wire form can decode to the same in-memory value.

The law matters because it is what lets a capture be compared: a golden trace, a regression fixture, and a bug report attached to a ticket are all the same bytes reproduced, so a diff between two runs is a diff of behavior rather than of formatting.

A self-describing capture. A stream whose session.start carries source needs no other artefact to be reopened: the chart it ran against is recompiled from the message itself. A stream captured without source is equally conformant, and a reader must be given the chart another way. A projected stream may withhold source deliberately (see "allow_source"), which is the case where the reader always has to supply it.

In this implementation the three legs are StatifierUI.Trace.Capture.record/3, save/2, and load/1, over StatifierUI.Trace.Json's encode_lines/1 and decode_lines/1. The law above is executable: test/statifier_ui/trace/round_trip_test.exs decodes the checked-in golden and asserts the re-encoded bytes are identical to the file, and the golden test proves a live session produces those bytes in the first place.

Type index

One row per type this document defines - 24 rows: 9 trace.*, 10 effect.*, and 5 session.* (the four lifecycle types plus session.datamodel). This table's first column is a machine boundary: a drift test parses exactly this table's backtick-quoted type strings and asserts them equal to the producer's own emitted type set, so a type documented here and not emitted, or emitted and not documented here, fails that test rather than drifting silently.

References

• ADR-0005 (docs/adr/0005-language-neutral-trace-wire-format.md) - the envelope, the nine trace.* type names, the session.start role, the JSON discipline this document restates by reference, versioning, and the clause naming this document as the format's normative home.
• ADR-0013 (docs/adr/0013-otel-correlation-in-the-wire-format.md) - the otel key: its name, its shape, the messages that may carry it, its projection rule, and the host-supplied seam that fills it in.
• statifier docs/opentelemetry.md and statifier ADR-0062 - the span topology the otel key points into, and the separate bridge package that produces it. This document depends on neither at runtime; it points at their identifiers.
• ADR-0003 (docs/adr/0003-fixtures-as-the-example-data-contract.md) - the fixtures sidecar object session.start's fixtures field carries verbatim.
• ADR-0006 (docs/adr/0006-datasets-and-expression-fixtures.md) - the datasets and expressions keys the same carried object may additionally contain.
• st-nbmj - the upstream gap behind effect.* messages once carrying no round, superseded by st-xb2b, whose ADR-0046 settled it (the two ids name one thread of work: st-nbmj filed the gap, st-xb2b decided it); this producer's own propagation of round onto effect.* messages was sui-67d, above.
• st-r6l9 - the upstream reordering seam behind the old ordering warning above, closed by ADR-0044.
• st-1xwh - the upstream effect (Statifier.Effect.DatamodelInit) behind session.datamodel, above.
• st-oef3 - the upstream effect (Statifier.Effect.DatamodelChange) behind effect.datamodel_change - a per-write value change, distinct from session.datamodel's one-time starting snapshot; serialized here by sui-h92.
• sui-qay - attribute_locations on session.start's states and transitions rows, above. Closes the gap this entry used to name: the location tables carried no attribute-level spans until statifier's st-9i5r carried the map onto the compiled Machine, which is the layer this document's producer reads.