Say S17 clears once the new bus is sent, as it does

The comment and the signals table's bound said it cleared once the bus's machine runs the new build; it
clears once that machine has been sent it, when no send is refused for the bus any more.

cmd/mesh-controller/bus_waits.go

@@ -0,0 +1,195 @@
+package main
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/conditions"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// A send held for the bus's planned step (novox/hq issue 336).
+//
+// **A wait only a person can end is said to that person at once.** No send replaces the bus but its planned
+// step (sendToEach, ADR 0236), and the step is a person's `bus upgrade`. So while a new bus build waits, every
+// walk whose tier sends to the bus's machine is refused, and tries again each tick. The refusal was kept only
+// as the walk's note; nothing was raised, and the operator found the hold by asking why a walk did not move.
+// Row S17 raises it on the first tick after the first refusal, in the bus's scope, for the operator: what
+// waits, behind which bus build, since when, and the verb. A walk held only by it is not late, so S3 leaves it
+// out, as it leaves out a walk under a paused build seat. It clears once the bus's machine has been sent the
+// build the mesh holds — the step was taken, and no send is refused for the bus any more — whatever the walks'
+// notes still say. Whether the new bus came up healthy is the step's own condition (probe DB).
+
+// kindBusStepWaiting is S17's kind: bus.<module>.step-waiting.
+const kindBusStepWaiting = "bus-step-waiting"
+
+// busFacts is a new bus build waiting for its planned step, and the sends held for it.
+type busFacts struct {
+	module, to string
+	// from is the build each machine of the bus runs; machines those whose bus the step would replace.
+	from     map[string]string
+	machines []string
+	waits    []busWaitFacts
+}
+
+// busWaitFacts is one walk whose send was refused because it would replace the bus.
+type busWaitFacts struct {
+	plan, repository, commit string
+	// modules are what its tier sends: what waits.
+	modules []string
+	// since is the first refusal: as this controller saw it, or, read back, the save that kept the refusal.
+	since time.Time
+}
+
+// busRefusedFirst is when this controller first saw each walk refused for the bus's step. The walk's note
+// keeps the refusal across a restart; this keeps its moment more exactly than the walk's last save.
+var busRefusedFirst = &firstSeen{at: map[string]time.Time{}}
+
+type firstSeen struct {
+	mu sync.Mutex
+	at map[string]time.Time
+}
+
+// mark keeps the first moment an id was seen.
+func (s *firstSeen) mark(id string, at time.Time) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if _, seen := s.at[id]; !seen {
+		s.at[id] = at
+	}
+}
+
+// of is when an id was first seen; zero when it was not.
+func (s *firstSeen) of(id string) time.Time {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.at[id]
+}
+
+// keepOnly forgets every id not given: a walk no longer held is not held since then.
+func (s *firstSeen) keepOnly(ids map[string]bool) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for id := range s.at {
+		if !ids[id] {
+			delete(s.at, id)
+		}
+	}
+}
+
+// refusedForTheBus is whether an error is the refusal of a send for the bus's planned step.
+func refusedForTheBus(err error) bool {
+	return err != nil && strings.Contains(err.Error(), errBusWaits.Error())
+}
+
+// busWaitsOf is the sends held for the bus's step, from the open walks: each walk not waiting for its
+// delivery's word whose note keeps a refusal for this very bus build, while the build would still replace
+// the bus on a machine. first is when this controller first saw a walk refused, zero when it did not.
+func busWaitsOf(plans []inventory.Plan, b busPending, first func(string) time.Time) busFacts {
+	f := busFacts{module: b.module, to: b.to, from: b.from}
+	for _, n := range b.machines {
+		if b.moves(n) {
+			f.machines = append(f.machines, n)
+		}
+	}
+	if len(f.machines) == 0 {
+		return f
+	}
+	for _, p := range plans {
+		if !p.Open() || p.Waiting() || !strings.Contains(p.Note, errBusWaits.Error()) || !strings.Contains(p.Note, short(b.to)) {
+			continue
+		}
+		since := p.Updated
+		if seen := first(p.ID); !seen.IsZero() && (since.IsZero() || seen.Before(since)) {
+			since = seen
+		}
+		var modules []string
+		if p.Tier >= 0 && p.Tier < len(p.Tiers) {
+			modules = append(modules, p.Tiers[p.Tier]...)
+		} else {
+			modules = planModules(p)
+		}
+		sort.Strings(modules)
+		f.waits = append(f.waits, busWaitFacts{plan: p.ID, repository: p.Repository, commit: p.Commit, modules: modules,
+			since: since})
+	}
+	sort.Slice(f.waits, func(i, j int) bool { return f.waits[i].since.Before(f.waits[j].since) })
+	return f
+}
+
+// heldByTheBus is the walks of a bus's facts, by id: what S3 leaves out.
+func (b busFacts) heldByTheBus() map[string]bool {
+	out := map[string]bool{}
+	for _, w := range b.waits {
+		out[w.plan] = true
+	}
+	return out
+}
+
+// busUpgradeVerb is the call that ends the wait, as the summary names it: through the mesh MCP server, with
+// why, saying whether the new version can be undone ("reversible": "true") or not ("irreversible": "true").
+const busUpgradeVerb = "`mesh_call mesh-controller.bus {\"upgrade\": \"true\", \"why\": \"…\", \"reversible\" or \"irreversible\": \"true\"}`"
+
+// watchBusWaits is S17: a send held for the bus's planned step, said at once to the operator.
+func watchBusWaits(f *signalFacts) []conditions.Observation {
+	b := f.bus
+	if len(b.waits) == 0 || len(b.machines) == 0 {
+		return nil
+	}
+	since := b.waits[0].since
+	var walks, what []string
+	for _, w := range b.waits {
+		walks = append(walks, fmt.Sprintf("%s (%s %s, tier: %s)", w.plan, w.repository, short(w.commit),
+			strings.Join(w.modules, ", ")))
+		what = append(what, deliveryWhat(w.modules, w.repository))
+	}
+	var from []string
+	for _, n := range b.machines {
+		from = append(from, short(orNotKnown(b.from[n])))
+	}
+	machines := namesWords(b.machines, 3)
+	in := f.now.Sub(since)
+	return []conditions.Observation{{Scope: conditions.ScopeBus, ID: b.module, Token: "step-waiting",
+		Kind: kindBusStepWaiting, Severity: conditions.Warning, Resolver: conditions.ResolverOperator,
+		Machine: b.machines[0], Also: b.machines[1:],
+		Summary: fmt.Sprintf("sends to %s wait for the bus's planned step: a new bus build (%s %s → %s) would replace "+
+			"the bus there, which only a person's %s does; waiting since %s: %s", strings.Join(b.machines, ", "),
+			b.module, strings.Join(sortedUnique(from), ", "), short(b.to), busUpgradeVerb,
+			since.UTC().Format(time.RFC3339), strings.Join(walks, "; ")),
+		Said:     fmt.Sprintf("%d walk(s) refused since %s", len(b.waits), since.UTC().Format(time.RFC3339)),
+		Headline: clipWords("Sends to "+machines+" wait for a bus upgrade", conditions.HeadlineMax),
+		Explanation: clipWords(fmt.Sprintf("A new version of the mesh's message system is built, and only a person "+
+			"installs it. Until then nothing else is sent to %s: %s waits, for %s so far.", machines,
+			namesWords(sortedUnique(what), 3), humanDuration(in)), conditions.ExplanationMax),
+		Needs:    "start the bus upgrade " + FromMeshMCPServer,
+		Resolved: "Resolved: the bus upgrade started, and sends go on"}}
+}
+
+// sortedUnique is a list sorted, each once.
+func sortedUnique(xs []string) []string {
+	seen := map[string]bool{}
+	var out []string
+	for _, x := range xs {
+		if !seen[x] {
+			seen[x] = true
+			out = append(out, x)
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
+// clipWords keeps a plain sentence within a bound, at a word.
+func clipWords(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	cut := strings.LastIndex(s[:n-1], " ")
+	if cut <= 0 {
+		cut = n - 1
+	}
+	return s[:cut] + "…"
+}

cmd/mesh-controller/bus_waits_test.go

@@ -0,0 +1,159 @@
+package main
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/conditions"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// novox/hq issue 336: a send held because it would replace the bus outside its planned step waits for a
+// person, so a person is told at once — what waits, behind which bus build, since when, and the verb that
+// ends it — and the wait is not read as a walk running late.
+
+// aBusOnAnchor is a new bus build waiting for its step on anchor: nats 88135ad0 there, 32307bd1 held.
+func aBusOnAnchor() busPending {
+	return busPending{module: "nats", machines: []string{"anchor"}, from: map[string]string{"anchor": "88135ad0aaaa"},
+		to: "32307bd1bbbb", same: map[string]bool{}}
+}
+
+// refusedNote is the note a walk keeps when its send was refused for the bus, as advanceHeld writes it.
+func refusedNote(b busPending, tier int) string {
+	held := "sending anchor would replace the bus (nats " + short(b.from["anchor"]) + " → " + short(b.to) +
+		"), which is a planned step: `bus upgrade --why …` snapshots its streams first and checks them after (novox/hq ADR 0236)"
+	return "tier " + string(rune('0'+tier)) + ": " + errBusWaits.Error() + ": " + held + " — tried again"
+}
+
+func TestASendHeldForTheBusStepIsFoundFromTheWalksItHolds(t *testing.T) {
+	now := time.Date(2026, 10, 8, 18, 30, 0, 0, time.UTC)
+	b := aBusOnAnchor()
+	walk := func(id, repo, note string, updated time.Time) inventory.Plan {
+		return inventory.Plan{ID: id, Repository: repo, Commit: "c0ffee001122", State: inventory.PlanRolling, Tier: 0,
+			Tiers: [][]string{{"mesh-host"}}, Modules: map[string]*inventory.PlanModule{"mesh-host": {}}, Note: note,
+			Updated: updated}
+	}
+	for _, c := range []struct {
+		name  string
+		bus   busPending
+		plans []inventory.Plan
+		first map[string]time.Time
+		want  []string
+		since time.Time
+	}{
+		{"a walk refused for the bus", b,
+			[]inventory.Plan{walk("plan-1", "novox/mesh-host", refusedNote(b, 0), now.Add(-27*time.Minute))}, nil,
+			[]string{"plan-1"}, now.Add(-27 * time.Minute)},
+		{"refused earlier than its last save, as this controller saw it", b,
+			[]inventory.Plan{walk("plan-1", "novox/mesh-host", refusedNote(b, 0), now.Add(-5*time.Minute))},
+			map[string]time.Time{"plan-1": now.Add(-28 * time.Minute)}, []string{"plan-1"}, now.Add(-28 * time.Minute)},
+		{"a walk refused for another reason", b,
+			[]inventory.Plan{walk("plan-1", "novox/mesh-host", "tier 0: the build seat is paused — tried again", now)}, nil,
+			nil, time.Time{}},
+		{"refused for an older bus build than the one held now", func() busPending { o := b; o.to = "99999999cccc"; return o }(),
+			[]inventory.Plan{walk("plan-1", "novox/mesh-host", refusedNote(b, 0), now)}, nil, nil, time.Time{}},
+		{"the bus already runs the build held: its step started", func() busPending {
+			o := aBusOnAnchor()
+			o.from = map[string]string{"anchor": o.to}
+			return o
+		}(), []inventory.Plan{walk("plan-1", "novox/mesh-host", refusedNote(b, 0), now)}, nil, nil, time.Time{}},
+		{"a walk waiting for its delivery's word asks no send", b, func() []inventory.Plan {
+			p := walk("plan-1", "novox/mesh-host", refusedNote(b, 0), now)
+			p.Delivery = &inventory.PlanDelivery{Awaits: "mesh-delivery"}
+			return []inventory.Plan{p}
+		}(), nil, nil, time.Time{}},
+	} {
+		t.Run(c.name, func(t *testing.T) {
+			got := busWaitsOf(c.plans, c.bus, func(id string) time.Time { return c.first[id] })
+			var ids []string
+			for _, w := range got.waits {
+				ids = append(ids, w.plan)
+			}
+			if strings.Join(ids, ",") != strings.Join(c.want, ",") {
+				t.Fatalf("held for the bus: %v, want %v", ids, c.want)
+			}
+			if len(c.want) > 0 {
+				if !got.waits[0].since.Equal(c.since) {
+					t.Errorf("waiting since %s, want %s", got.waits[0].since, c.since)
+				}
+				if strings.Join(got.machines, ",") != "anchor" || got.to != b.to || got.module != "nats" {
+					t.Errorf("behind %+v", got)
+				}
+			}
+		})
+	}
+}
+
+// **Said at once, not as lateness, and cleared when the step starts**: the walk held only for the bus raises
+// the bus's condition on the first tick, before any tier bound, naming what waits, the bus build from and to,
+// since when and `bus upgrade`, for the operator; S3 says nothing of it even past its bound; and the
+// condition clears once the bus's machine runs the new build.
+func TestASendHeldForTheBusStepIsSaidAtOnceAndNotAsLateness(t *testing.T) {
+	now := time.Date(2026, 10, 8, 18, 30, 0, 0, time.UTC)
+	b := aBusOnAnchor()
+	held := func(entered time.Duration) *signalFacts {
+		f := calm(now)
+		f.plans = []planFacts{{id: "plan-1", repository: "novox/mesh-host", commit: "c0ffee00", tier: 0, tiers: 1,
+			entered: now.Add(-entered), bound: 30 * time.Minute, waiting: refusedNote(b, 0), bus: true}}
+		f.bus = busFacts{module: "nats", to: b.to, from: b.from, machines: []string{"anchor"},
+			waits: []busWaitFacts{{plan: "plan-1", repository: "novox/mesh-host", commit: "c0ffee001122",
+				modules: []string{"mesh-host"}, since: now.Add(-time.Minute)}}}
+		return f
+	}
+
+	f := held(time.Minute)
+	got := watchBusWaits(f)
+	if len(got) != 1 {
+		t.Fatalf("a send held for the bus a minute ago raised %+v", got)
+	}
+	o := got[0]
+	if o.Key() != "bus.nats.step-waiting" || o.Kind != kindBusStepWaiting || o.Resolver != conditions.ResolverOperator {
+		t.Fatalf("raised %s (%s), resolver %q", o.Key(), o.Kind, o.Resolver)
+	}
+	for _, want := range []string{"anchor", "mesh-host", "88135ad0", "32307bd1", "mesh_call mesh-controller.bus",
+		`"upgrade": "true"`, `"reversible"`, `"irreversible"`, "2026-10-08T18:29:00Z", "plan-1"} {
+		if !strings.Contains(o.Summary, want) {
+			t.Errorf("its summary does not say %q: %s", want, o.Summary)
+		}
+	}
+	if !strings.Contains(o.Explanation, "mesh-host") || !strings.Contains(o.Headline, "bus upgrade") || o.Needs == "" {
+		t.Errorf("its words do not say what waits and what the operator does: %+v", o)
+	}
+	if why, ok := conditions.PlainWords(conditions.Words{Headline: o.Headline, Explanation: o.Explanation,
+		Resolved: o.Resolved, Needs: o.Needs}, "anchor"); !ok {
+		t.Errorf("its words are not plain: %s", why)
+	}
+
+	// Past S3's bound: still the bus's wait, never a stalled walk.
+	late := held(45 * time.Minute)
+	if s3 := watchPlans(late); len(s3) != 0 {
+		t.Fatalf("a walk held only by the bus step was said stalled: %+v", s3)
+	}
+	// A walk held for something else past its bound is still stalled.
+	other := held(45 * time.Minute)
+	other.plans[0].bus = false
+	if s3 := watchPlans(other); len(s3) != 1 {
+		t.Fatalf("a walk held for something else past its bound raised %+v", s3)
+	}
+
+	// Through the keeper: open at the first tick, cleared when the step started.
+	store := conditions.NewInMemory()
+	k := conditions.NewKeeper(t.Context(), conditions.Options{Store: store, History: store, Teller: &conditions.Told{},
+		Now: func() time.Time { return now }})
+	defer k.Close(context.Background())
+	w := &watchdogs{keeper: k, started: now.Add(-time.Hour)}
+	w.see(t.Context(), held(time.Minute))
+	open, err := k.Open(t.Context())
+	if err != nil || len(open) != 1 || open[0].Key != "bus.nats.step-waiting" {
+		t.Fatalf("after the first tick, open: %+v (%v)", open, err)
+	}
+	started := held(time.Minute)
+	started.bus = busFacts{module: "nats", to: b.to}
+	started.plans[0].bus = false
+	w.see(t.Context(), started)
+	if open, _ := k.Open(t.Context()); len(open) != 0 {
+		t.Fatalf("the step started, and open: %+v", open)
+	}
+}

cmd/mesh-controller/changeplan.go

@@ -5,6 +5,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/novox/mesh-controller/internal/catalogue"
 	"github.com/novox/mesh-controller/internal/inventory"
 	"github.com/novox/mesh-controller/internal/link"
 )
@@ -50,7 +51,13 @@ func changePlanOf(repository, base, head string, r mergeReach, entries []invento
 				}
 			}
 			switch {
-			case name == "nats":
+			case (name == "nats" || catalogue.ProvidesBus(e.Manifest)) && len(e.On) > 0:
+				// Said before the merge (novox/hq issue 336): a new bus build holds every send to the bus's machine
+				// until a person takes the step, so merging it is a promise to take it.
+				p.Steps = append(p.Steps, fmt.Sprintf("%s: %s is never sent by an ordinary send, and until %s runs, "+
+					"nothing else is sent to %s either — unless the new build turns out the same as the one running there "+
+					"(issue 280)", busUpgradeNeeded, name, busUpgradeVerb, strings.Join(e.On, ", ")))
+			case name == "nats" || catalogue.ProvidesBus(e.Manifest):
 				p.Steps = append(p.Steps, "a planned bus step: the bus is upgraded by `bus upgrade`, never by an ordinary send")
 			case waits && len(e.On) > 0:
 				p.Steps = append(p.Steps, fmt.Sprintf("%s waits for a person: its policy records (%s)", name,
@@ -81,6 +88,9 @@ func changePlanOf(repository, base, head string, r mergeReach, entries []invento
 	return p
 }
 
+// busUpgradeNeeded is how a change plan says that merging it holds the bus's machine for a person's step.
+const busUpgradeNeeded = "merging this needs a person's bus upgrade"
+
 // summaryOf is a change plan in one line: what it builds, where it goes, and whether the bus moves.
 func summaryOf(p link.ChangePlan) string {
 	if len(p.Moved) == 0 && len(p.New) == 0 {
@@ -110,7 +120,10 @@ func summaryOf(p link.ChangePlan) string {
 	}
 	bus := "no bus step"
 	for _, s := range p.Steps {
-		if strings.HasPrefix(s, "a planned bus step") {
+		switch {
+		case strings.HasPrefix(s, busUpgradeNeeded):
+			bus = busUpgradeNeeded
+		case strings.HasPrefix(s, "a planned bus step") && bus == "no bus step":
 			bus = "a bus step"
 		}
 	}

cmd/mesh-controller/changeplan_test.go

@@ -45,7 +45,7 @@ func TestAChangePlanSaysWhatEachMachineReceives(t *testing.T) {
 	}
 
 	p = plan("modules/nats/Dockerfile", "modules/photos/x.js")
-	if !strings.Contains(p.Summary, "a bus step") || !strings.Contains(p.Summary, "2 wait(s) for a person") ||
+	if !strings.Contains(p.Summary, "needs a person's bus upgrade") || !strings.Contains(p.Summary, "2 wait(s) for a person") ||
 		!strings.Contains(p.Summary, "(+1 dependent(s))") {
 		t.Errorf("the bus and a held module read %q", p.Summary)
 	}
@@ -58,7 +58,11 @@ func TestAChangePlanSaysWhatEachMachineReceives(t *testing.T) {
 		t.Errorf("the deploy plan reads %v", got)
 	}
 	text := strings.Join(p.Steps, "\n")
-	for _, want := range []string{"a planned bus step", "photos waits for a person", "nats provides mesh-bus"} {
+	// Said before the merge (novox/hq issue 336): merging it holds every send to the bus's machine until a
+	// person runs the bus's step, and the verb that does.
+	for _, want := range []string{"merging this needs a person's bus upgrade", "nothing else is sent to anchor",
+		"mesh_call mesh-controller.bus", "the same as the one running there", "photos waits for a person",
+		"nats provides mesh-bus"} {
 		if !strings.Contains(text, want) {
 			t.Errorf("the steps do not say %q:\n%s", want, text)
 		}

cmd/mesh-controller/plain_words.go

@@ -523,6 +523,13 @@ var plainWordings = map[string]func(conditions.Observation) words{
 			Explanation: "The mesh's message system is being upgraded; some things pause until it is done.",
 			Resolved:    "The bus upgrade is done"}
 	}),
+	kindBusStepWaiting: worded(func(o conditions.Observation) words {
+		return words{Headline: "Sends wait for a bus upgrade",
+			Needs: "start the bus upgrade " + FromMeshMCPServer,
+			Explanation: "A new version of the mesh's message system is built, and only a person installs it. Until " +
+				"then nothing else is sent to the machine that runs it.",
+			Resolved: "Resolved: the bus upgrade started, and sends go on"}
+	}),
 	kindBusUpgradeFailed: worded(func(o conditions.Observation) words {
 		return words{Headline: "The bus upgrade failed",
 			Needs:       "decide whether to put the bus back to the version before; the details say how.",

cmd/mesh-controller/release_plan.go

@@ -533,6 +533,10 @@ func advanceHeld(ctx context.Context, open *stores) {
 				// the state is left as it was and the step is tried again on the next tick. Said and
 				// kept when it is new: the same refusal on every tick is one fact, not one per tick.
 				p.Note = "tier " + fmt.Sprint(p.Tier) + ": " + err.Error() + " — tried again"
+				// A refusal for the bus's planned step is a person's to end: S17 says it from its first moment.
+				if refusedForTheBus(err) {
+					busRefusedFirst.mark(p.ID, time.Now())
+				}
 				if planSnapshot(*p) != before {
 					fmt.Printf("%s: %v\n", p.ID, err)
 					if err := inv.SavePlan(ctx, p); err != nil {

cmd/mesh-controller/replay336_test.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+	"github.com/novox/mesh-controller/internal/conditions"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// TestReplay336 replays novox/hq issue 336 (2026-10-08): a catalogue merge built a new bus build for the
+// control-node; from then on every send to that machine was refused for the bus's planned step, which only a
+// person starts, and for 28 minutes nothing but the walks' notes said so. The outcome asserted: the first
+// watchdog tick after the first refusal opens a condition for the operator that names what waits and the verb
+// that ends it, and it clears once the bus's machine runs the new build. Written with only what the controller
+// had before its fix — the stores, register, assign, a plan saved and advanced, the watchdogs' gathering and
+// seeing — so it is laid over the commit before.
+func TestReplay336(t *testing.T) {
+	open := aMesh(t)
+	ctx := t.Context()
+	inv := open.inventory
+	bus := catalogue.Manifest{Module: "nats", Version: "2", Provides: []catalogue.Offer{{Name: "mesh-bus"}}}
+	if err := inv.RegisterModule(ctx, bus, inventory.Source{Repository: "novox/mesh-catalog", Seat: "git",
+		Path: "modules/nats", BuiltFrom: "32307bd1", Head: "32307bd1"}); err != nil {
+		t.Fatal(err)
+	}
+	register(t, open, catalogue.Manifest{Module: "engine", Version: "1"})
+	for _, m := range []string{"nats", "engine"} {
+		if _, err := inv.Assign(ctx, "anchor", m); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// The control-node runs the bus build it was last sent; the mesh holds a newer one, which waits for its step.
+	if err := inv.RecordSent(ctx, nodeID(t, open, "anchor"), "d-anchor", map[string]string{"nats": "88135ad0"}); err != nil {
+		t.Fatal(err)
+	}
+	// A walk of the engine, built, whose tier sends to the control-node.
+	now := time.Now().UTC()
+	plan := inventory.Plan{ID: "plan-engine", Repository: "novox/mesh-host", Commit: "e1e1e1e1", Created: now,
+		State: inventory.PlanBuilding, Tiers: [][]string{{"engine"}},
+		Modules: map[string]*inventory.PlanModule{"engine": {State: "built", BuiltAt: &now, Commit: "e1e1e1e1", Build: "b"}}}
+	if err := inv.SavePlan(ctx, &plan); err != nil {
+		t.Fatal(err)
+	}
+	advancePlans(ctx, open)
+	p, err := inv.PlanByID(ctx, "plan-engine")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(p.Note, errBusWaits.Error()) {
+		t.Fatalf("the walk's send to the bus's machine was not refused for the bus: %s %q", p.State, p.Note)
+	}
+
+	store := conditions.NewInMemory()
+	k := conditions.NewKeeper(ctx, conditions.Options{Store: store, History: store, Teller: &conditions.Told{}})
+	defer k.Close(context.Background())
+	w := &watchdogs{open: open, keeper: k, started: now.Add(-time.Hour)}
+	waiting := func() []conditions.Condition {
+		t.Helper()
+		w.see(ctx, w.gather(ctx))
+		all, err := k.Open(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var out []conditions.Condition
+		for _, c := range all {
+			if strings.HasPrefix(c.Key, "bus.") && c.Resolver == conditions.ResolverOperator {
+				out = append(out, c)
+			}
+		}
+		return out
+	}
+	got := waiting()
+	if len(got) != 1 {
+		t.Fatalf("the first tick after the refusal told the operator nothing about the bus's step: %d condition(s)", len(got))
+	}
+	for _, want := range []string{"anchor", "engine", "mesh-controller.bus", "upgrade", "32307bd1"} {
+		if !strings.Contains(got[0].Summary, want) {
+			t.Errorf("the condition does not say %q: %s", want, got[0].Summary)
+		}
+	}
+	// The step taken: the control-node is sent the new bus build, and the wait is over.
+	if err := inv.RecordSent(ctx, nodeID(t, open, "anchor"), "d-anchor-2", map[string]string{"nats": "32307bd1"}); err != nil {
+		t.Fatal(err)
+	}
+	if got := waiting(); len(got) != 0 {
+		t.Fatalf("the bus's machine runs the new build, and the operator is still asked: %+v", got)
+	}
+}

cmd/mesh-controller/signals.go

@@ -210,6 +210,20 @@ var signalsTable = []signalRow{
 		newest: func(f *signalFacts) time.Time {
 			return newestOf(f.waits, func(w waitFacts) time.Time { return w.since })
 		}},
+	{Row: "S17", Signal: "a send held for the bus's planned step is told to a person", Emitter: "controller's plan",
+		Trigger: "each send refused because it would replace the bus outside its step (novox/hq issue 336)",
+		Bound: "none: raised at the first refusal, for the operator, naming what waits, the bus build from and to, " +
+			"since when and the mesh-controller.bus call; cleared once the bus's machine has been sent the build the mesh holds",
+		Kind: kindBusStepWaiting, Severity: conditions.Warning, Phase: 3,
+		needs: func(f *signalFacts) error {
+			if f.plansErr != nil {
+				return f.plansErr
+			}
+			return f.busErr
+		}, watch: watchBusWaits,
+		newest: func(f *signalFacts) time.Time {
+			return newestOf(f.bus.waits, func(w busWaitFacts) time.Time { return w.since })
+		}},
 }
 
 // watchFacts is S14: the snapshot a merge check is fed is older than its bound, or none was kept since
@@ -315,7 +329,9 @@ func watchReports(f *signalFacts) []conditions.Observation {
 func watchPlans(f *signalFacts) []conditions.Observation {
 	var out []conditions.Observation
 	for _, p := range f.plans {
-		if p.paused {
+		// Under a paused build seat, or held only by the bus's planned step (S17, novox/hq issue 336): a wait
+		// for a person, said as itself, not a walk running late.
+		if p.paused || p.bus {
 			continue
 		}
 		in := f.now.Sub(p.entered)

cmd/mesh-controller/signals_test.go

@@ -142,6 +142,19 @@ var suppressions = map[string]suppression{
 				since: f.now.Add(-31 * time.Minute)}}
 		},
 	},
+	// A send refused because it would replace the bus outside its planned step: said at its first refusal,
+	// whatever the bound (novox/hq issue 336). Inside: a new bus build waits, and no send was refused for it.
+	"S17": {
+		inside: func(f *signalFacts) {
+			f.bus = busFacts{module: "nats", to: "32307bd1bbbb", from: map[string]string{"anchor": "88135ad0aaaa"},
+				machines: []string{"anchor"}}
+		},
+		past: func(f *signalFacts) {
+			f.bus = busFacts{module: "nats", to: "32307bd1bbbb", from: map[string]string{"anchor": "88135ad0aaaa"},
+				machines: []string{"anchor"}, waits: []busWaitFacts{{plan: "plan-1", repository: "novox/app",
+					commit: "c0ffee001122", modules: []string{"app"}, since: f.now.Add(-time.Second)}}}
+		},
+	},
 	// Twice by hand within a fortnight is a healer wanted; once, or the first of two a day too old, is not.
 	"S15": {
 		inside: func(f *signalFacts) {

cmd/mesh-controller/watchdogs.go

@@ -97,6 +97,9 @@ type signalFacts struct {
 
 	// facts is the snapshot this controller keeps for merge checks (S14).
 	facts factsFacts
+
+	bus    busFacts
+	busErr error
 }
 
 // factsFacts is when the newest facts snapshot was taken, when this controller began keeping it, and
@@ -147,6 +150,7 @@ type planFacts struct {
 	bound                  time.Duration
 	waiting                string
 	paused                 bool
+	bus                    bool
 }
 
 // waitFacts is one walk waiting for its delivery's word: since its merge opened it.
@@ -320,7 +324,8 @@ func (w *watchdogs) gather(ctx context.Context) *signalFacts {
 		}
 	}
 	f.machines, f.machinesErr = w.gatherMachines(ctx, inv, now)
-	f.plans, f.waits, f.plansErr = gatherPlans(ctx, inv, now)
+	f.bus, f.busErr = gatherBus(ctx, inv)
+	f.plans, f.waits, f.plansErr = gatherPlans(ctx, inv, now, f.bus.heldByTheBus())
 	f.loop, f.loopErr = w.gatherLoop()
 	f.mergesPassed, f.merges, f.mergesErr = watchedMerges.last()
 	if f.mergesErr == nil && !f.mergesPassed.IsZero() && now.Sub(f.mergesPassed) > 3*mergeCatchUpEvery {
@@ -438,8 +443,24 @@ func (w *watchdogs) gatherMachines(ctx context.Context, inv *inventory.Inventory
 	return out, nil
 }
 
-// gatherPlans is every open plan, its tier's bound from what was measured, and what it waits on.
+// gatherBus is a new bus build waiting for its step and the walks refused for it (S17, novox/hq issue 336).
-func gatherPlans(ctx context.Context, inv *inventory.Inventory, now time.Time) ([]planFacts, []waitFacts, error) {
+func gatherBus(ctx context.Context, inv *inventory.Inventory) (busFacts, error) {
+	b, err := pendingBus(ctx, inv)
+	if err != nil {
+		return busFacts{}, fmt.Errorf("what a bus upgrade would do cannot be read: %w", err)
+	}
+	plans, err := inv.OpenPlans(ctx)
+	if err != nil {
+		return busFacts{}, fmt.Errorf("the open plans cannot be read: %w", err)
+	}
+	f := busWaitsOf(plans, b, busRefusedFirst.of)
+	busRefusedFirst.keepOnly(f.heldByTheBus())
+	return f, nil
+}
+
+// gatherPlans is every open plan, its tier's bound from what was measured, and what it waits on; byTheBus are
+// the walks held only by the bus's planned step, which S3 leaves to S17.
+func gatherPlans(ctx context.Context, inv *inventory.Inventory, now time.Time, byTheBus map[string]bool) ([]planFacts, []waitFacts, error) {
 	plans, err := inv.OpenPlans(ctx)
 	if err != nil {
 		return nil, nil, fmt.Errorf("the open plans cannot be read: %w", err)
@@ -466,7 +487,7 @@ func gatherPlans(ctx context.Context, inv *inventory.Inventory, now time.Time) (
 		bound := bounds.of(p.Repository)
 		out = append(out, planFacts{id: p.ID, repository: p.Repository, commit: p.Commit, tier: p.Tier,
 			tiers: len(p.Tiers), entered: inTierSince(p), bound: bound,
-			waiting: planLineWith(p, now, pause, bound), paused: paused})
+			waiting: planLineWith(p, now, pause, bound), paused: paused, bus: byTheBus[p.ID]})
 	}
 	return out, waits, nil
 }