Attribute violations to the causing step and render witness evidence (#59)

* feat(ltl): attribute violations to the obligation origin step

* feat(verifier): label evaluator observations with the runner step index

* feat(trace): carry the causing step in violation witnesses and summary

* feat(replay): move the violation marker to the causing step

* feat(replay-ui): render witness evidence in the violations panel

* feat(replay-ui): wire witnesses and step jump into violation panels

* fix(ltl): treat next obligations as vacuous at run end

* fix(runner): give the finalize trace record its own step index

internal/ltl/evaluator.go

@@ -33,17 +33,27 @@ func (v Verdict) String() string {
 // violates, the overall verdict latches to Violated.
 type Evaluator struct {
 	root      Formula
-	pending   []Formula
+	pending   []obligation
 	violated  bool
 	steps     int
 	violation *Violation
 }
 
+// obligation pairs a residual formula with the step that spawned it, so a
+// deferred check (a next, a pending eventually) that fails on a later step can
+// be attributed to the step that created the obligation.
+type obligation struct {
+	formula Formula
+	origin  int
+}
+
 // Violation is the witness for a latched verdict: the failing sub-formula, a
-// human-readable reason, and the observation step it fired at. A thrown
-// predicate carries the goja error text as its reason and sets IsError; a plain
-// false carries "predicate false"; Finalize fills it for liveness obligations
-// that never discharged.
+// human-readable reason, and the step the failed obligation originated at. For
+// an immediate predicate failure that is the observation step itself; for a
+// deferred obligation (next, eventually) it is the earlier step that spawned
+// it, the one that caused the violation. A thrown predicate carries the goja
+// error text as its reason and sets IsError; a plain false carries "predicate
+// false"; Finalize fills it for liveness obligations that never discharged.
 type Violation struct {
 	Formula Formula
 	Reason  string
@@ -62,19 +72,29 @@ func (e *Evaluator) Observe() Verdict {
 	return e.ObserveAt(time.Now())
 }
 
-// ObserveAt is like Observe but takes the current step time explicitly.
+// ObserveAt is like Observe but takes the current step time explicitly. Steps
+// are numbered by an internal counter starting at 1; callers whose step
+// numbering can skip observations should use ObserveAtStep instead.
 func (e *Evaluator) ObserveAt(now time.Time) Verdict {
+	return e.ObserveAtStep(now, e.steps+1)
+}
+
+// ObserveAtStep is like ObserveAt but labels the observation with the caller's
+// step index, so violation witnesses carry the caller's numbering even when
+// some steps were never observed (for example transitional steps the verifier
+// skips).
+func (e *Evaluator) ObserveAtStep(now time.Time, step int) Verdict {
 	if e.violated {
 		return VerdictViolated
 	}
-	e.steps++
+	e.steps = step
 
-	fresh := rootObligation(e.root)
+	fresh := obligation{formula: rootObligation(e.root), origin: step}
 	obligations := append(e.pending, fresh)
 	e.pending = e.pending[:0]
 
-	for _, obligation := range obligations {
-		result := reduce(obligation, now)
+	for _, entry := range obligations {
+		result := reduce(entry.formula, now)
 		switch result.status {
 		case statusHolds:
 			// drop
@@ -83,11 +103,11 @@ func (e *Evaluator) ObserveAt(now time.Time) Verdict {
 			e.pending = nil
 			e.violation = result.witness
 			if e.violation != nil {
-				e.violation.Step = e.steps
+				e.violation.Step = entry.origin
 			}
 			return VerdictViolated
 		case statusPending:
-			e.pending = append(e.pending, result.formula)
+			e.pending = append(e.pending, obligation{formula: result.formula, origin: entry.origin})
 		}
 	}
 
@@ -100,41 +120,43 @@ func (e *Evaluator) ObserveAt(now time.Time) Verdict {
 }
 
 // collapse removes structurally-identical obligations, keeping the first
-// occurrence in order. Distinct predicates never merge because ThunkFormula's
-// name participates in its describe() key, so deduping cannot hide a violation.
-func collapse(obligations []Formula) []Formula {
+// occurrence in order so the surviving entry carries the earliest origin step.
+// Distinct predicates never merge because ThunkFormula's name participates in
+// its describe() key, so deduping cannot hide a violation.
+func collapse(obligations []obligation) []obligation {
 	if len(obligations) < 2 {
 		return obligations
 	}
 	seen := make(map[string]struct{}, len(obligations))
 	result := obligations[:0]
-	for _, obligation := range obligations {
-		key := obligation.describe()
+	for _, entry := range obligations {
+		key := entry.formula.describe()
 		if _, ok := seen[key]; ok {
 			continue
 		}
 		seen[key] = struct{}{}
-		result = append(result, obligation)
+		result = append(result, entry)
 	}
 	return result
 }
 
-// Finalize reports the terminal verdict for the run. Pending obligations that
-// can never be discharged by a future step (an unbounded eventually that never
-// fired, a strong next with no successor) resolve to Violated; safety
-// obligations that were never breached resolve to Holds.
+// Finalize reports the terminal verdict for the run. A liveness promise that
+// never discharged (an eventually that never fired) resolves to Violated. A
+// deferred state check (the residue of a next) has no successor state to
+// evaluate against, so it is indefinite and resolves vacuously to Holds: the
+// run ended before the obligation could be checked, which is not a failure.
 func (e *Evaluator) Finalize() Verdict {
 	if e.violated {
 		return VerdictViolated
 	}
-	for _, obligation := range e.pending {
-		if finalize(obligation) == statusViolated {
+	for _, entry := range e.pending {
+		if finalize(entry.formula) == statusViolated {
 			e.violated = true
 			e.pending = nil
 			e.violation = &Violation{
-				Formula: obligation,
-				Reason:  finalizeReason(obligation),
-				Step:    e.steps,
+				Formula: entry.formula,
+				Reason:  finalizeReason(entry.formula),
+				Step:    entry.origin,
 			}
 			return VerdictViolated
 		}
@@ -156,17 +178,18 @@ func finalizeReason(formula Formula) string {
 	switch formula.(type) {
 	case EventuallyFormula:
 		return "eventually never satisfied"
-	case NextFormula:
-		return "next obligation unmet at run end"
-	case ThunkFormula:
-		return "obligation unmet at run end"
 	default:
 		return "liveness obligation unmet at run end"
 	}
 }
 
 // finalize collapses a pending obligation to its terminal status assuming no
-// further steps will occur.
+// further steps will occur. Three-valued: a pending thunk or next is the
+// residue of a deferred state check with no state left to check, so it is
+// indefinite (statusPending) rather than violated; only a liveness promise
+// (an eventually that never fired) is a definite end-of-run violation.
+// Connectives combine with Kleene semantics so an indefinite sub-formula
+// never manufactures a definite verdict.
 func finalize(formula Formula) residualStatus {
 	switch concrete := formula.(type) {
 	case PureFormula:
@@ -175,11 +198,11 @@ func finalize(formula Formula) residualStatus {
 		}
 		return statusViolated
 	case ThunkFormula:
-		return statusViolated
+		return statusPending
 	case EventuallyFormula:
 		return statusViolated
 	case NextFormula:
-		return statusViolated
+		return statusPending
 	case AlwaysFormula:
 		return statusHolds
 	case NowFormula:
@@ -188,24 +211,34 @@ func finalize(formula Formula) residualStatus {
 		switch finalize(concrete.Inner) {
 		case statusViolated:
 			return statusHolds
-		default:
+		case statusHolds:
 			return statusViolated
+		default:
+			return statusPending
 		}
 	case AndFormula:
-		if finalize(concrete.Left) == statusViolated || finalize(concrete.Right) == statusViolated {
+		left, right := finalize(concrete.Left), finalize(concrete.Right)
+		if left == statusViolated || right == statusViolated {
 			return statusViolated
 		}
+		if left == statusPending || right == statusPending {
+			return statusPending
+		}
 		return statusHolds
 	case OrFormula:
-		if finalize(concrete.Left) == statusHolds || finalize(concrete.Right) == statusHolds {
+		left, right := finalize(concrete.Left), finalize(concrete.Right)
+		if left == statusHolds || right == statusHolds {
 			return statusHolds
 		}
+		if left == statusPending || right == statusPending {
+			return statusPending
+		}
 		return statusViolated
 	case ImpliesFormula:
-		if finalize(concrete.Antecedent) == statusViolated {
-			return statusHolds
-		}
-		return finalize(concrete.Consequent)
+		return finalize(OrFormula{
+			Left:  NotFormula{Inner: concrete.Antecedent},
+			Right: concrete.Consequent,
+		})
 	default:
 		return statusHolds
 	}
@@ -224,9 +257,9 @@ func (e *Evaluator) Residual() Formula {
 	if len(e.pending) == 0 {
 		return PureFormula{Value: true}
 	}
-	combined := e.pending[0]
-	for _, formula := range e.pending[1:] {
-		combined = AndFormula{Left: combined, Right: formula}
+	combined := e.pending[0].formula
+	for _, entry := range e.pending[1:] {
+		combined = AndFormula{Left: combined, Right: entry.formula}
 	}
 	return combined
 }
@@ -258,11 +291,6 @@ type reduceResult struct {
 
 func holds() reduceResult { return reduceResult{status: statusHolds} }
 
-// violated reports a violation without an attached witness. Used where the
-// failing sub-formula is recovered from a child result whose own witness is
-// carried up by violatedFrom.
-func violated() reduceResult { return reduceResult{status: statusViolated} }
-
 // violatedWith reports a violation that originates at the given sub-formula
 // with the given reason. The reason distinguishes a thrown predicate from a
 // plain false so callers (and the replay UI) can render the cause.

internal/ltl/finalize_test.go

@@ -18,13 +18,32 @@ func TestFinalize_UnboundedEventuallyUnmetIsViolated(t *testing.T) {
 	}
 }
 
-func TestFinalize_FinalStepNextIsViolated(t *testing.T) {
+func TestFinalize_FinalStepNextIsVacuouslyHolds(t *testing.T) {
+	// A next obligation pending at run end has no successor state to check;
+	// the run ending before the check is not a failure (weak next at the
+	// trace boundary).
 	evaluator := NewEvaluator(Next(ThunkNamed("p", func() (bool, error) { return true, nil })))
 	if got := evaluator.Observe(); got != VerdictPending {
 		t.Fatalf("step 1: got %v, want pending", got)
 	}
-	if got := evaluator.Finalize(); got != VerdictViolated {
-		t.Errorf("Finalize = %v, want violated", got)
+	if got := evaluator.Finalize(); got != VerdictHolds {
+		t.Errorf("Finalize = %v, want holds", got)
+	}
+	if witness := evaluator.Violation(); witness != nil {
+		t.Errorf("Violation = %+v, want nil for a vacuous next", witness)
+	}
+}
+
+func TestFinalize_AlwaysNextNeverReportsAtRunEnd(t *testing.T) {
+	// always(next(p)): every step spawns a deferred check and the last one is
+	// always pending when the run ends. That residue must not surface as an
+	// end-of-run violation.
+	evaluator := NewEvaluator(Always(Next(ThunkNamed("p", func() (bool, error) { return true, nil }))))
+	for index := range 3 {
+		evaluator.ObserveAt(time.Unix(int64(index), 0))
+	}
+	if got := evaluator.Finalize(); got != VerdictHolds {
+		t.Errorf("Finalize = %v, want holds", got)
 	}
 }
 
@@ -128,20 +147,23 @@ func TestViolationLatchIsMonotonic(t *testing.T) {
 }
 
 func TestCollapse_IdenticalObligationsMerge(t *testing.T) {
-	merged := collapse([]Formula{
-		Next(Pure(true)),
-		Next(Pure(true)),
-		Next(Pure(true)),
+	merged := collapse([]obligation{
+		{formula: Next(Pure(true)), origin: 1},
+		{formula: Next(Pure(true)), origin: 2},
+		{formula: Next(Pure(true)), origin: 3},
 	})
 	if len(merged) != 1 {
 		t.Errorf("expected 1 obligation after collapse, got %d", len(merged))
 	}
+	if merged[0].origin != 1 {
+		t.Errorf("collapse must keep the earliest origin, got %d", merged[0].origin)
+	}
 }
 
 func TestCollapse_DistinctPredicatesDoNotMerge(t *testing.T) {
-	merged := collapse([]Formula{
-		Eventually(ThunkNamed("p3", func() (bool, error) { return false, nil })),
-		Eventually(ThunkNamed("p4", func() (bool, error) { return false, nil })),
+	merged := collapse([]obligation{
+		{formula: Eventually(ThunkNamed("p3", func() (bool, error) { return false, nil }))},
+		{formula: Eventually(ThunkNamed("p4", func() (bool, error) { return false, nil }))},
 	})
 	if len(merged) != 2 {
 		t.Errorf("distinct predicates must not merge, got %d", len(merged))

internal/ltl/witness_test.go

@@ -71,6 +71,83 @@ func TestViolation_FinalizeFillsWitness(t *testing.T) {
 	}
 }
 
+func TestViolation_NextAttributesOriginStep(t *testing.T) {
+	// always(next(p)): the obligation spawned at step 2 is checked against
+	// step 3's state; the violation belongs to step 2, the step that caused it.
+	values := []bool{true, false}
+	step := 0
+	evaluator := NewEvaluator(Always(Next(ThunkNamed("p", func() (bool, error) {
+		current := values[step]
+		step++
+		return current, nil
+	}))))
+	if got := evaluator.ObserveAt(time.Unix(0, 0)); got != VerdictPending {
+		t.Fatalf("step 1: got %v, want pending", got)
+	}
+	if got := evaluator.ObserveAt(time.Unix(1, 0)); got != VerdictPending {
+		t.Fatalf("step 2: got %v, want pending", got)
+	}
+	if got := evaluator.ObserveAt(time.Unix(2, 0)); got != VerdictViolated {
+		t.Fatalf("step 3: got %v, want violated", got)
+	}
+	witness := evaluator.Violation()
+	if witness == nil {
+		t.Fatal("Violation = nil, want non-nil")
+	}
+	if witness.Step != 2 {
+		t.Errorf("Step = %d, want 2 (the step that spawned the next obligation)", witness.Step)
+	}
+}
+
+func TestViolation_FinalizeAttributesOriginStep(t *testing.T) {
+	// An eventually that never fires is reported by Finalize at the step the
+	// obligation was first spawned (collapse keeps the earliest origin).
+	evaluator := NewEvaluator(Eventually(ThunkNamed("p", func() (bool, error) {
+		return false, nil
+	})))
+	evaluator.ObserveAt(time.Unix(0, 0))
+	evaluator.ObserveAt(time.Unix(1, 0))
+	evaluator.ObserveAt(time.Unix(2, 0))
+	if got := evaluator.Finalize(); got != VerdictViolated {
+		t.Fatalf("Finalize = %v, want violated", got)
+	}
+	witness := evaluator.Violation()
+	if witness == nil {
+		t.Fatal("Violation = nil after Finalize, want non-nil")
+	}
+	if witness.Step != 1 {
+		t.Errorf("Step = %d, want 1 (the step the eventually obligation was spawned)", witness.Step)
+	}
+}
+
+func TestViolation_ObserveAtStepUsesCallerNumbering(t *testing.T) {
+	// The caller skips step 5 (a transitional step the verifier never saw); the
+	// origin must carry the caller's labels, not a contiguous internal count.
+	values := []bool{true, false}
+	step := 0
+	evaluator := NewEvaluator(Always(Next(ThunkNamed("p", func() (bool, error) {
+		current := values[step]
+		step++
+		return current, nil
+	}))))
+	if got := evaluator.ObserveAtStep(time.Unix(0, 0), 3); got != VerdictPending {
+		t.Fatalf("step 3: got %v, want pending", got)
+	}
+	if got := evaluator.ObserveAtStep(time.Unix(1, 0), 4); got != VerdictPending {
+		t.Fatalf("step 4: got %v, want pending", got)
+	}
+	if got := evaluator.ObserveAtStep(time.Unix(2, 0), 6); got != VerdictViolated {
+		t.Fatalf("step 6: got %v, want violated", got)
+	}
+	witness := evaluator.Violation()
+	if witness == nil {
+		t.Fatal("Violation = nil, want non-nil")
+	}
+	if witness.Step != 4 {
+		t.Errorf("Step = %d, want 4 (caller-labeled origin, not detection step 6)", witness.Step)
+	}
+}
+
 func TestViolation_NilBeforeViolation(t *testing.T) {
 	evaluator := NewEvaluator(Always(Pure(true)))
 	evaluator.Observe()

internal/replay/runs_cache.go

@@ -91,7 +91,7 @@ func scanSteps(tracePath string) ([]StepSummary, []int64, int, time.Time, error)
 	reader := bufio.NewReaderSize(file, 64*1024)
 	steps := []StepSummary{}
 	offsets := []int64{}
-	violationCount := 0
+	attributions := []violationAttribution{}
 	var offset int64
 	for {
 		lineStart := offset
@@ -102,19 +102,20 @@ func scanSteps(tracePath string) ([]StepSummary, []int64, int, time.Time, error)
 			trimmed = trimmed[:len(trimmed)-1]
 		}
 		if len(trimmed) > 0 {
-			summary, partial, decodeErr := decodeStepSummary(trimmed)
+			summary, lineAttributions, decodeErr := decodeStepSummary(trimmed)
 			if decodeErr != nil {
 				return nil, nil, 0, time.Time{}, decodeErr
 			}
 			steps = append(steps, summary)
 			offsets = append(offsets, lineStart)
-			violationCount += partial
+			attributions = append(attributions, lineAttributions...)
 		}
 		if err != nil {
 			break
 		}
 	}
-	return steps, offsets, violationCount, info.ModTime(), nil
+	markViolations(steps, attributions)
+	return steps, offsets, len(attributions), info.ModTime(), nil
 }
 
 // Step decodes the full Step record at index n (1-based, matching trace.Step.Index).

internal/replay/runs_decode.go

@@ -56,7 +56,16 @@ func tallyTrace(tracePath string) (steps, violations int, err error) {
 	return steps, violations, nil
 }
 
-func decodeStepSummary(line []byte) (StepSummary, int, error) {
+// violationAttribution maps one recorded violation to the step the marker
+// belongs on: the causing step its witness names, falling back to the step
+// whose trace line carries it (the detection step) for witnesses written
+// before the step field existed.
+type violationAttribution struct {
+	attributedStep int
+	detectedStep   int
+}
+
+func decodeStepSummary(line []byte) (StepSummary, []violationAttribution, error) {
 	var partial struct {
 		Index     int       `json:"step"`
 		Timestamp time.Time `json:"timestamp"`
@@ -76,15 +85,28 @@ func decodeStepSummary(line []byte) (StepSummary, int, error) {
 		} `json:"next_action,omitempty"`
 		Exceptions []json.RawMessage `json:"exceptions,omitempty"`
 		Violations []string          `json:"violations,omitempty"`
+		Witnesses  map[string]struct {
+			Step int `json:"step"`
+		} `json:"witnesses,omitempty"`
 	}
 	if err := json.Unmarshal(line, &partial); err != nil {
-		return StepSummary{}, 0, fmt.Errorf("decode step: %w", err)
+		return StepSummary{}, nil, fmt.Errorf("decode step: %w", err)
+	}
+	var attributions []violationAttribution
+	for _, name := range partial.Violations {
+		attribution := violationAttribution{
+			attributedStep: partial.Index,
+			detectedStep:   partial.Index,
+		}
+		if witness, ok := partial.Witnesses[name]; ok && witness.Step > 0 {
+			attribution.attributedStep = witness.Step
+		}
+		attributions = append(attributions, attribution)
 	}
 	summary := StepSummary{
 		Index:         partial.Index,
 		Timestamp:     partial.Timestamp,
 		Screen:        partial.Screen,
-		HasViolations: len(partial.Violations) > 0,
 		HasExceptions: len(partial.Exceptions) > 0,
 	}
 	if partial.NextAction != nil {
@@ -113,7 +135,33 @@ func decodeStepSummary(line []byte) (StepSummary, int, error) {
 			}
 		}
 	}
-	return summary, len(partial.Violations), nil
+	return summary, attributions, nil
+}
+
+// markViolations sets HasViolations on the step each attribution points at.
+// The marker goes on the causing step; when that index is missing from the
+// trace the detection step keeps it. Duplicate indices (a finalize line echoes
+// the last step's index) resolve to the first occurrence, the real step.
+func markViolations(steps []StepSummary, attributions []violationAttribution) {
+	if len(attributions) == 0 {
+		return
+	}
+	positionOf := make(map[int]int, len(steps))
+	for position, step := range steps {
+		if _, ok := positionOf[step.Index]; !ok {
+			positionOf[step.Index] = position
+		}
+	}
+	for _, attribution := range attributions {
+		position, ok := positionOf[attribution.attributedStep]
+		if !ok {
+			position, ok = positionOf[attribution.detectedStep]
+			if !ok {
+				continue
+			}
+		}
+		steps[position].HasViolations = true
+	}
 }
 
 func swipeDirectionLabel(fromX, fromY, toX, toY int) string {

internal/replay/runs_test.go

@@ -138,6 +138,66 @@ func TestCacheStep_LazyDecodeReturnsFullStep(t *testing.T) {
 	}
 }
 
+func TestCacheOpen_ViolationMarkerMovesToCausingStep(t *testing.T) {
+	// The violation is detected at step 3 but its witness attributes it to
+	// step 2 (the step that spawned the next obligation). The action-list
+	// marker belongs on step 2; the full step 3 payload keeps the violation
+	// record itself.
+	root := t.TempDir()
+	startedAt := time.Now().UTC()
+	steps := []trace.Step{
+		{Index: 1, Timestamp: startedAt},
+		{Index: 2, Timestamp: startedAt.Add(time.Second)},
+		{
+			Index:      3,
+			Timestamp:  startedAt.Add(2 * time.Second),
+			Violations: []string{"prop1"},
+			Witnesses:  map[string]trace.Witness{"prop1": {Reason: "predicate false", Step: 2}},
+		},
+	}
+	writeRun(t, root, "r1", trace.Meta{StartedAt: startedAt, EndedAt: timePointer(startedAt.Add(3 * time.Second))}, steps)
+
+	cache := NewCache(root)
+	run, err := cache.Open("r1")
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	if !run.Steps[1].HasViolations {
+		t.Error("step 2 (causing step) should carry the violation marker")
+	}
+	if run.Steps[2].HasViolations {
+		t.Error("step 3 (detection step) should not carry the marker")
+	}
+	full, err := cache.Step(run, 3)
+	if err != nil {
+		t.Fatalf("Step(3): %v", err)
+	}
+	if len(full.Violations) != 1 || full.Violations[0] != "prop1" {
+		t.Errorf("step 3 payload violations = %v, want [prop1]", full.Violations)
+	}
+}
+
+func TestCacheOpen_ViolationWithoutWitnessStepKeepsDetectionStep(t *testing.T) {
+	// Traces written before witnesses carried a step field fall back to
+	// marking the detection step, the old behavior.
+	root := t.TempDir()
+	startedAt := time.Now().UTC()
+	steps := []trace.Step{
+		{Index: 1, Timestamp: startedAt},
+		{Index: 2, Timestamp: startedAt.Add(time.Second), Violations: []string{"prop1"}},
+	}
+	writeRun(t, root, "r1", trace.Meta{StartedAt: startedAt, EndedAt: timePointer(startedAt.Add(2 * time.Second))}, steps)
+
+	cache := NewCache(root)
+	run, err := cache.Open("r1")
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	if !run.Steps[1].HasViolations {
+		t.Error("step 2 should keep the marker when the witness has no step")
+	}
+}
+
 func TestDecodeStepSummary_ActionLabelPerKind(t *testing.T) {
 	cases := []struct {
 		line      string

internal/runner/runner.go

@@ -8,6 +8,8 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"maps"
+	"slices"
 	"strings"
 	"time"
 
@@ -185,6 +187,7 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 				Tree:       tree,
 				LastAction: lastAction,
 				StepTime:   stepStart,
+				StepIndex:  stepIndex,
 				RunStart:   summary.StartTime,
 				Logs:       logs,
 			}); err != nil {
@@ -263,10 +266,7 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 		}
 		summary.Steps = stepIndex
 		if len(violations) > 0 {
-			summary.Violations = append(summary.Violations, ViolationRecord{
-				StepIndex:  stepIndex,
-				Properties: violations,
-			})
+			summary.Violations = append(summary.Violations, violationRecords(violations, witnesses, stepIndex)...)
 		}
 		// Wait actions are themselves a settling: skip the idle poll. Actions
 		// that mutate the UI fall through to WaitForIdle so the next step's
@@ -284,17 +284,17 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 	}
 
 	// Finalize each evaluator once the loop ends so liveness obligations that
-	// never discharged (an unbounded eventually that never fired, a strong
-	// next with no successor) are reported as violations rather than silently
-	// left pending. Properties already violated mid-run are not re-reported.
+	// never discharged (an eventually that never fired) are reported as
+	// violations rather than silently left pending. Properties already
+	// violated mid-run are not re-reported. The synthetic record gets its own
+	// step index so no two trace lines share one; witnesses still attribute
+	// the violation to the step that spawned the obligation.
 	if ended := options.Verifier.Finalize(); len(ended) > 0 {
-		witnesses := collectWitnesses(options.Verifier, ended, logger, stepIndex)
-		summary.Violations = append(summary.Violations, ViolationRecord{
-			StepIndex:  stepIndex,
-			Properties: ended,
-		})
+		finalIndex := stepIndex + 1
+		witnesses := collectWitnesses(options.Verifier, ended, logger, finalIndex)
+		summary.Violations = append(summary.Violations, violationRecords(ended, witnesses, finalIndex)...)
 		finalStep := trace.Step{
-			Index:      stepIndex,
+			Index:      finalIndex,
 			Timestamp:  time.Now(),
 			Violations: ended,
 			Witnesses:  witnesses,
@@ -806,6 +806,26 @@ func captureMetrics(ctx context.Context, options Options, logger *slog.Logger, s
 	}
 }
 
+// violationRecords groups newly-violated properties by the step their witness
+// attributes the violation to (the causing step), falling back to the
+// detection step for properties without a witness. Records are ordered by
+// step; properties keep the sorted order NewlyViolatedProperties produced.
+func violationRecords(properties []string, witnesses map[string]trace.Witness, detectionStep int) []ViolationRecord {
+	byStep := map[int][]string{}
+	for _, name := range properties {
+		step := detectionStep
+		if witness, ok := witnesses[name]; ok && witness.Step > 0 {
+			step = witness.Step
+		}
+		byStep[step] = append(byStep[step], name)
+	}
+	records := make([]ViolationRecord, 0, len(byStep))
+	for _, step := range slices.Sorted(maps.Keys(byStep)) {
+		records = append(records, ViolationRecord{StepIndex: step, Properties: byStep[step]})
+	}
+	return records
+}
+
 // collectWitnesses gathers the violation witness for each newly-violated
 // property, logs its cause, and returns them keyed by property name for the
 // trace. Properties without a captured witness are skipped.
@@ -820,10 +840,12 @@ func collectWitnesses(verifierInstance *verifier.Verifier, properties []string,
 			continue
 		}
 		logger.Warn("property violated",
-			"step", stepIndex, "property", name, "reason", witness.Reason, "error", witness.IsError)
+			"step", witness.Step, "detected_step", stepIndex,
+			"property", name, "reason", witness.Reason, "error", witness.IsError)
 		witnesses[name] = trace.Witness{
 			Reason:     witness.Reason,
 			IsError:    witness.IsError,
+			Step:       witness.Step,
 			Extractors: witness.Extractors,
 		}
 	}

internal/runner/runner_test.go

@@ -253,6 +253,186 @@ func TestRunner_ViolationSurfacesOnlyOnOnsetStep(t *testing.T) {
 	}
 }
 
+func TestRunner_NextViolationAttributedToCausingStep(t *testing.T) {
+	// always(next(p)): the obligation spawned at step 2 fails against step 3's
+	// state. The summary record and the trace witness must attribute the
+	// violation to step 2 (the causing step); the trace line that carries it is
+	// still step 3, where the failure was detected.
+	const nextViolationSpec = `
+import { actions, always, next, extract } from "@sanderling/spec";
+let observed = 0;
+const tick = extract(() => ++observed);
+globalThis.properties = {
+  nextHolds: always(next(() => tick.current < 3)),
+};
+globalThis.actions = actions(() => []);
+`
+	state := newHarnessWithSpec(t, nextViolationSpec)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	summary, err := Run(ctx, Options{
+		Duration:    time.Hour,
+		IdleTimeout: 20 * time.Millisecond,
+		MaxSteps:    5,
+		Driver:      state.mock,
+		Verifier:    state.verifier,
+		TraceWriter: state.writer,
+	})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	if len(summary.Violations) != 1 {
+		t.Fatalf("expected exactly one ViolationRecord, got %d: %v",
+			len(summary.Violations), summary.Violations)
+	}
+	if summary.Violations[0].StepIndex != 2 {
+		t.Errorf("summary step: got %d, want 2 (the step that spawned the next obligation)",
+			summary.Violations[0].StepIndex)
+	}
+	if !slices.Equal(summary.Violations[0].Properties, []string{"nextHolds"}) {
+		t.Errorf("properties: got %v, want [nextHolds]", summary.Violations[0].Properties)
+	}
+
+	file, err := os.Open(filepath.Join(state.writer.Directory(), "trace.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer file.Close()
+
+	type traceLine struct {
+		Step       int                      `json:"step"`
+		Violations []string                 `json:"violations"`
+		Witnesses  map[string]trace.Witness `json:"witnesses"`
+	}
+	found := false
+	scanner := bufio.NewScanner(file)
+	scanner.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
+	for scanner.Scan() {
+		var line traceLine
+		if err := json.Unmarshal(scanner.Bytes(), &line); err != nil {
+			t.Fatalf("trace line decode: %v", err)
+		}
+		if len(line.Violations) == 0 {
+			continue
+		}
+		found = true
+		if line.Step != 3 {
+			t.Errorf("violation detected on step %d, want 3", line.Step)
+		}
+		witness, ok := line.Witnesses["nextHolds"]
+		if !ok {
+			t.Fatalf("step %d carries no witness for nextHolds", line.Step)
+		}
+		if witness.Step != 2 {
+			t.Errorf("witness step: got %d, want 2 (causing step)", witness.Step)
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		t.Fatalf("scan trace: %v", err)
+	}
+	if !found {
+		t.Error("no trace line carried the violation")
+	}
+}
+
+func TestRunner_AlwaysNextLeavesNoEndOfRunViolation(t *testing.T) {
+	// always(next(p)) ends every run with a pending deferred check. That
+	// residue is vacuous (no successor state to check), so neither the
+	// summary nor the trace may report an end-of-run violation.
+	const spec = `
+import { actions, always, next } from "@sanderling/spec";
+globalThis.properties = {
+  nextHolds: always(next(() => true)),
+};
+globalThis.actions = actions(() => []);
+`
+	state := newHarnessWithSpec(t, spec)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	summary, err := Run(ctx, Options{
+		Duration:    time.Hour,
+		IdleTimeout: 20 * time.Millisecond,
+		MaxSteps:    3,
+		Driver:      state.mock,
+		Verifier:    state.verifier,
+		TraceWriter: state.writer,
+	})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	if len(summary.Violations) != 0 {
+		t.Errorf("expected no violations, got %v", summary.Violations)
+	}
+}
+
+func TestRunner_FinalizeRecordUsesDistinctStepIndex(t *testing.T) {
+	// An eventually that never fires is reported at run end through a
+	// synthetic trace record. That record must carry its own step index so no
+	// two trace lines share one (duplicate indices made the replay UI select
+	// two rows at once).
+	const spec = `
+import { actions, eventually } from "@sanderling/spec";
+globalThis.properties = {
+  neverFires: eventually(() => false),
+};
+globalThis.actions = actions(() => []);
+`
+	state := newHarnessWithSpec(t, spec)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	summary, err := Run(ctx, Options{
+		Duration:    time.Hour,
+		IdleTimeout: 20 * time.Millisecond,
+		MaxSteps:    3,
+		Driver:      state.mock,
+		Verifier:    state.verifier,
+		TraceWriter: state.writer,
+	})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	if !containsProperty(summary.Violations, "neverFires") {
+		t.Fatalf("expected neverFires in violations: %v", summary.Violations)
+	}
+
+	file, err := os.Open(filepath.Join(state.writer.Directory(), "trace.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer file.Close()
+
+	type traceLine struct {
+		Step       int      `json:"step"`
+		Violations []string `json:"violations"`
+	}
+	seen := map[int]bool{}
+	finalizeStep := 0
+	scanner := bufio.NewScanner(file)
+	scanner.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
+	for scanner.Scan() {
+		var line traceLine
+		if err := json.Unmarshal(scanner.Bytes(), &line); err != nil {
+			t.Fatalf("trace line decode: %v", err)
+		}
+		if seen[line.Step] {
+			t.Errorf("duplicate step index %d in trace", line.Step)
+		}
+		seen[line.Step] = true
+		if slices.Contains(line.Violations, "neverFires") {
+			finalizeStep = line.Step
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		t.Fatalf("scan trace: %v", err)
+	}
+	if finalizeStep != summary.Steps+1 {
+		t.Errorf("finalize record step = %d, want %d (steps+1)", finalizeStep, summary.Steps+1)
+	}
+}
+
 func TestRunner_ThrowingPredicateIsLoggedNotPanic(t *testing.T) {
 	const throwingSpec = `
 import { actions, always, Tap } from "@sanderling/spec";

internal/trace/writer.go

@@ -45,6 +45,11 @@ type Step struct {
 type Witness struct {
 	Reason  string `json:"reason,omitempty"`
 	IsError bool   `json:"is_error,omitempty"`
+	// Step is the step the failed obligation originated at: the step that
+	// caused the violation. For a deferred obligation (a next, an eventually)
+	// this is earlier than the step whose record carries the witness, which is
+	// where the failure was detected.
+	Step       int                        `json:"step,omitempty"`
 	Extractors map[string]json.RawMessage `json:"extractors,omitempty"`
 }
 

internal/verifier/verifier_test.go

@@ -782,6 +782,46 @@ globalThis.properties = {
 	}
 }
 
+// A next obligation spawned at step k and checked against step k+1's state is
+// attributed to step k, the step that caused it. StepIndex from SnapshotInput
+// labels the evaluator observations so the witness carries runner step numbers.
+func TestEvaluateProperties_NextViolationCarriesOriginStepIndex(t *testing.T) {
+	const spec = `
+globalThis.flag = __sanderling__.extract(state => state.snapshots["flag"] ?? false, "flag");
+globalThis.properties = {
+  flagStaysTrue: __sanderling__.always(__sanderling__.next(() => flag.current === true)),
+};
+`
+	verifier := newVerifier(t)
+	mustLoad(t, verifier, spec)
+
+	push := func(stepIndex int, flag string) map[string]ltl.Verdict {
+		t.Helper()
+		if err := verifier.PushSnapshot(SnapshotInput{
+			Snapshots: Snapshots{"flag": json.RawMessage(flag)},
+			StepIndex: stepIndex,
+		}); err != nil {
+			t.Fatal(err)
+		}
+		return verifier.EvaluateProperties()
+	}
+
+	push(7, `true`)
+	push(8, `true`)
+	verdicts := push(9, `false`)
+	if got := verdicts["flagStaysTrue"]; got != ltl.VerdictViolated {
+		t.Fatalf("step 9: verdict = %v, want violated", got)
+	}
+
+	witness := verifier.Witness("flagStaysTrue")
+	if witness == nil {
+		t.Fatal("Witness = nil, want non-nil")
+	}
+	if witness.Step != 8 {
+		t.Errorf("Witness.Step = %d, want 8 (the step that spawned the next obligation)", witness.Step)
+	}
+}
+
 func TestLoad_AcceptsSpecWithoutPropertiesOrActions(t *testing.T) {
 	verifier := newVerifier(t)
 	if err := verifier.Load(`const noop = 1;`); err != nil {

internal/verifier/worker.go

@@ -39,6 +39,7 @@ type Verifier struct {
 	lastLogs       []LogEntry
 	lastExceptions []Exception
 	stepTime       time.Time
+	stepIndex      int
 	runStart       time.Time
 
 	appPackage string
@@ -259,6 +260,7 @@ func (v *Verifier) PushSnapshot(input SnapshotInput) error {
 	v.lastLogs = input.Logs
 	v.lastExceptions = input.Exceptions
 	v.stepTime = input.StepTime
+	v.stepIndex = input.StepIndex
 	if v.runStart.IsZero() {
 		v.runStart = input.RunStart
 	}
@@ -385,6 +387,11 @@ type SnapshotInput struct {
 	Tree       *hierarchy.Tree
 	LastAction *Action
 	StepTime   time.Time
+	// StepIndex is the runner's step number for this snapshot. Evaluators label
+	// observations with it so violation witnesses carry runner step numbers even
+	// when transitional steps were skipped. Zero means unlabeled; evaluators
+	// then fall back to their internal counter.
+	StepIndex  int
 	RunStart   time.Time
 	Logs       []LogEntry
 	Exceptions []Exception
@@ -404,8 +411,12 @@ func (v *Verifier) EvaluateProperties() map[string]ltl.Verdict {
 		stepTime = time.Now()
 	}
 	for name, evaluator := range v.evaluators {
+		if v.stepIndex > 0 {
+			verdicts[name] = evaluator.ObserveAtStep(stepTime, v.stepIndex)
+		} else {
 			verdicts[name] = evaluator.ObserveAt(stepTime)
 		}
+	}
 
 	var onset []string
 	for name, verdict := range verdicts {

replay-ui/src/panels/ViolationsPanel.css

@@ -97,6 +97,57 @@
   font-size: 13px;
 }
 
+.violations-panel-witness {
+  margin-top: 6px;
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  font-size: 12px;
+}
+
+.violations-panel-witness-line {
+  display: flex;
+  align-items: baseline;
+  gap: 8px;
+  margin: 0;
+}
+
+.violations-panel-witness-key {
+  flex-shrink: 0;
+  min-width: 96px;
+  color: var(--text-muted);
+  font-size: 11px;
+  text-transform: lowercase;
+}
+
+.violations-panel-witness-value {
+  word-break: break-all;
+  white-space: pre-wrap;
+  margin: 0;
+}
+
+.violations-panel-witness-step {
+  font-family: var(--font-mono);
+  font-size: 11px;
+  padding: 1px 6px;
+  background: var(--surface);
+  color: var(--text-primary);
+  border: 1px solid var(--border);
+  border-radius: 3px;
+  cursor: pointer;
+}
+
+.violations-panel-witness-step:hover {
+  border-color: var(--border-strong);
+}
+
+.violations-panel-witness-evidence {
+  display: flex;
+  flex-direction: column;
+  gap: 3px;
+  margin: 0;
+}
+
 .violations-panel-residual {
   margin-top: 6px;
   font-size: 12px;

replay-ui/src/panels/ViolationsPanel.tsx

@@ -1,5 +1,5 @@
 import { useMemo } from "react";
-import type { ResidualNode } from "../types";
+import type { ResidualNode, Witness } from "../types";
 import ResidualNodeView from "../components/ResidualNode";
 import "./ViolationsPanel.css";
 
@@ -7,8 +7,10 @@ export interface ViolationsPanelProps {
   propertyNames: string[];
   violations: string[];
   residuals?: Record<string, ResidualNode>;
+  witnesses?: Record<string, Witness>;
   onJumpToFirstViolation: () => void;
   hasFirstViolation: boolean;
+  onJumpToStep?: (step: number) => void;
   /** When true, only render violated rows and hide the header button row. */
   violationsOnly?: boolean;
 }
@@ -36,12 +38,74 @@ function statusFor(
   return "pending";
 }
 
+function formatValue(value: unknown): string {
+  const encoded = JSON.stringify(value);
+  return encoded === undefined ? String(value) : encoded;
+}
+
+function WitnessView({
+  witness,
+  onJumpToStep,
+  open,
+}: {
+  witness: Witness;
+  onJumpToStep?: (step: number) => void;
+  open: boolean;
+}) {
+  const evidence = Object.entries(witness.extractors ?? {})
+    .filter(([, value]) => value !== null && value !== undefined)
+    .sort(([a], [b]) => a.localeCompare(b));
+  return (
+    <div className="violations-panel-witness">
+      {witness.reason ? (
+        <div className="violations-panel-witness-line">
+          <span className="violations-panel-witness-key">
+            {witness.is_error ? "error" : "reason"}
+          </span>
+          <span className="violations-panel-witness-value">{witness.reason}</span>
+        </div>
+      ) : null}
+      {witness.step ? (
+        <div className="violations-panel-witness-line">
+          <span className="violations-panel-witness-key">caused at</span>
+          {onJumpToStep ? (
+            <button
+              type="button"
+              className="violations-panel-witness-step"
+              onClick={() => onJumpToStep(witness.step as number)}
+            >
+              step {witness.step}
+            </button>
+          ) : (
+            <span className="violations-panel-witness-value">step {witness.step}</span>
+          )}
+        </div>
+      ) : null}
+      {evidence.length > 0 ? (
+        <details className="violations-panel-residual" open={open}>
+          <summary>witness</summary>
+          <dl className="violations-panel-witness-evidence">
+            {evidence.map(([name, value]) => (
+              <div key={name} className="violations-panel-witness-line">
+                <dt className="violations-panel-witness-key">{name}</dt>
+                <dd className="violations-panel-witness-value">{formatValue(value)}</dd>
+              </div>
+            ))}
+          </dl>
+        </details>
+      ) : null}
+    </div>
+  );
+}
+
 export default function ViolationsPanel({
   propertyNames,
   violations,
   residuals,
+  witnesses,
   onJumpToFirstViolation,
   hasFirstViolation,
+  onJumpToStep,
   violationsOnly = false,
 }: ViolationsPanelProps) {
   const violationSet = useMemo(() => new Set(violations), [violations]);
@@ -84,6 +148,7 @@ export default function ViolationsPanel({
       <ul className="violations-panel-list">
         {rows.map(({ name, status }) => {
           const residual = residuals?.[name];
+          const witness = status === "violated" ? witnesses?.[name] : undefined;
           return (
             <li key={name} className="violations-panel-row" data-status={status}>
               <div className="violations-panel-row-head">
@@ -96,6 +161,9 @@ export default function ViolationsPanel({
                 </span>
                 <span className="violations-panel-name">{name}</span>
               </div>
+              {witness ? (
+                <WitnessView witness={witness} onJumpToStep={onJumpToStep} open={violationsOnly} />
+              ) : null}
               {residual ? (
                 <details className="violations-panel-residual">
                   <summary>residual</summary>

replay-ui/src/routes/RunDetail.tsx

@@ -126,6 +126,8 @@ export default function RunDetail() {
   const violationsAfter = nextStep?.violations ?? violationsBefore;
   const residualsBefore = currentStep?.residuals;
   const residualsAfter = nextStep?.residuals ?? residualsBefore;
+  const witnessesBefore = currentStep?.witnesses;
+  const witnessesAfter = nextStep?.witnesses ?? witnessesBefore;
   const exceptionsForStep = currentStep?.exceptions;
 
   const beforeTabs: TabDefinition[] = [
@@ -157,8 +159,10 @@ export default function RunDetail() {
           propertyNames={history?.names ?? []}
           violations={violationsBefore}
           residuals={residualsBefore}
+          witnesses={witnessesBefore}
           onJumpToFirstViolation={jumpToFirstViolation}
           hasFirstViolation={history?.firstViolationStep !== undefined}
+          onJumpToStep={goTo}
         />
       ),
     },
@@ -176,8 +180,10 @@ export default function RunDetail() {
           propertyNames={history?.names ?? []}
           violations={violationsBefore}
           residuals={residualsBefore}
+          witnesses={witnessesBefore}
           onJumpToFirstViolation={jumpToFirstViolation}
           hasFirstViolation={history?.firstViolationStep !== undefined}
+          onJumpToStep={goTo}
           violationsOnly
         />
       ),
@@ -213,8 +219,10 @@ export default function RunDetail() {
           propertyNames={history?.names ?? []}
           violations={violationsAfter}
           residuals={residualsAfter}
+          witnesses={witnessesAfter}
           onJumpToFirstViolation={jumpToFirstViolation}
           hasFirstViolation={history?.firstViolationStep !== undefined}
+          onJumpToStep={goTo}
         />
       ),
     },
@@ -232,8 +240,10 @@ export default function RunDetail() {
           propertyNames={history?.names ?? []}
           violations={violationsAfter}
           residuals={residualsAfter}
+          witnesses={witnessesAfter}
           onJumpToFirstViolation={jumpToFirstViolation}
           hasFirstViolation={history?.firstViolationStep !== undefined}
+          onJumpToStep={goTo}
           violationsOnly
         />
       ),

replay-ui/src/types.ts

@@ -121,6 +121,16 @@ export interface ExtractorChange {
   curr: unknown;
 }
 
+export interface Witness {
+  reason?: string;
+  is_error?: boolean;
+  // step is the step the failed obligation originated at: the causing step,
+  // which for deferred obligations (next, eventually) is earlier than the
+  // step whose record carries the witness.
+  step?: number;
+  extractors?: Record<string, unknown>;
+}
+
 export interface Step {
   step: number;
   timestamp: string;
@@ -134,4 +144,5 @@ export interface Step {
   residuals?: Record<string, ResidualNode>;
   metrics?: Metrics;
   extractor_changes?: Record<string, ExtractorChange>;
+  witnesses?: Record<string, Witness>;
 }