diff --git a/go/buscompose/buscompose_test.go b/go/buscompose/buscompose_test.go
index fbb033f..7825671 100644
--- a/go/buscompose/buscompose_test.go
+++ b/go/buscompose/buscompose_test.go
@@ -3,11 +3,6 @@ package buscompose
 import (
 	"encoding/json"
 	"errors"
-	"time"
-
-	"git.novox.be/novox/mesh-sdk/go/announce"
-	"git.novox.be/novox/mesh-sdk/go/delivery"
-	"github.com/nats-io/nats.go/jetstream"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -15,8 +10,11 @@ import (
 	"slices"
 	"strings"
 	"testing"
+	"time"
 
+	"git.novox.be/novox/mesh-sdk/go/announce"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"github.com/nats-io/nats.go/jetstream"
 )
 
 // Defends issue 1377 and novox/hq ADR 0547 §5: this is the controller's bus composer copied, and it composes byte for
@@ -439,7 +437,7 @@ func TestTheCopiedComposerComposesAsTheControllers(t *testing.T) {
 			}
 		}
 		announced := map[string]announce.States{ControllerSeat: ControllerAnnouncement("novox")}
-		for reader, want := range map[string]bool{VaultSeat: true, GrantsModule: true, AssignerModule: true, "mesh-delivery-view": false} {
+		for reader, want := range map[string]bool{VaultSeat: true, GrantsModule: true, "mesh-delivery-assigner": true, "mesh-delivery-view": false} {
 			granted, _ := ReadsAnnouncedBy(reader, []string{ControllerLease}, announced)
 			if (len(granted) == 1) != want {
 				t.Errorf("%s reads the lease: %v, want %v", reader, granted, want)
diff --git a/go/buscompose/userlist_switch.go b/go/buscompose/userlist_switch.go
index 5829ff2..6e2cb07 100644
--- a/go/buscompose/userlist_switch.go
+++ b/go/buscompose/userlist_switch.go
@@ -79,13 +79,10 @@ const VaultBusAccounts = "mesh-vault.bus-accounts"
 
 // NamedReaders are the states only the modules named may read, by `<owner>.<name>`: the grants part's wanted accounts
 // are read by the vault alone (novox/hq ADR 0561 §2).
-// The controller's lease is read by the two parts that turn on at its write (ADR 0561 §3, §6) and by the assigner, which
-// reads the grants from the grants part from then on (ADR 0547 §4, §5; delivery.GrantSource).
+// The controller's lease is read by the parts that turn on at its write (ADR 0561 §3, §6), and by the assigner, which
+// reads the catalogue's placement alone from then (ADR 0549 §5, issue 774).
 var NamedReaders = map[string][]string{VaultBusAccounts: {GrantsModule}, GrantsAccounts: {VaultSeat},
-	ControllerLease: {VaultSeat, GrantsModule, AssignerModule}}
-
-// AssignerModule is the assigner, which reads the lease to choose the source of the grants.
-const AssignerModule = "mesh-delivery-assigner"
+	ControllerLease: {VaultSeat, GrantsModule, delivery.AssignerModule}}
 
 // ReaderMay says whether module may read the state read (`<owner>.<name>`) by the named readers: always, for a state
 // that names none.
diff --git a/go/delivery/buildrun/standing.go b/go/delivery/buildrun/standing.go
new file mode 100644
index 0000000..f230d0f
--- /dev/null
+++ b/go/delivery/buildrun/standing.go
@@ -0,0 +1,225 @@
+package buildrun
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"maps"
+	"slices"
+	"sync"
+	"time"
+
+	"github.com/nats-io/nats.go/jetstream"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"git.novox.be/novox/mesh-sdk/go/process"
+)
+
+// A build module writes <node>.<language> of its toolchains state on every node it runs on, and nothing deletes the key
+// when the module leaves a node: no part writes another module's state, and the module is gone. A report of a node the
+// build module no longer runs on is therefore no builder's (issue 1940: mesh-build-node taken off ace on 2026-10-23 left
+// ace.typescript at v26.12.0, and every Node fingerprint said "builders disagree").
+//
+// ToolchainGate stands between the toolchains buckets and what takes their entries in (the indexer's PutToolchain, a
+// ToolchainSet): an entry is passed on only while the catalogue's placement (the indexer's state `placement`, read by
+// delivery.Placed; ADR 0549 §3) places its build module on its node, and only once it was written since that placement;
+// it is passed on as deleted otherwise, said once, and passed on when its node writes it again. A report written before
+// the placement is one an earlier run of the module left (issue 1940, 2026-10-25 20:56: mesh-build-node put on ace again
+// brought ace.typescript v26.12.0 of 2026-10-23 back into the agreement before the module ran there). Until the
+// placement is settled, and while no node's placement is held (before the first placement file reaches the indexer, or
+// where the state cannot be read), every entry is passed on. Builders that run and report different versions still
+// disagree: that refusal is the guard of novox/hq ADR 0465, and it stays.
+//
+// This replaces the copies of the gate the indexer, the builder and the checker each kept over the controller's
+// assignments (issue 1940's subtask, issue 774: those go with the controller's lease, ADR 0549 §6).
+type ToolchainGate struct {
+	mu      sync.Mutex
+	modules map[string]bool                                           // the build modules whose toolchains pass the gate
+	forward func(module, key string, value []byte, deleted bool) bool // answers whether it changed what it holds
+	logf    func(format string, a ...any)
+	placed  *delivery.Placed
+	known   bool                         // the placement is settled
+	raw     map[string]map[string][]byte // module -> toolchains key -> value, as the bucket holds it
+	left    map[string]string            // <module>/<key> -> why it was said left out, so each reason is said once
+}
+
+// NewToolchainGate is a gate over the toolchains of modules, passing what stands on to forward and saying with logf what
+// it leaves out.
+func NewToolchainGate(modules []string, forward func(module, key string, value []byte, deleted bool) bool, logf func(string, ...any)) *ToolchainGate {
+	g := &ToolchainGate{modules: map[string]bool{}, forward: forward, logf: logf, placed: delivery.NewPlaced(),
+		raw: map[string]map[string][]byte{}, left: map[string]string{}}
+	for _, m := range modules {
+		g.modules[m] = true
+	}
+	return g
+}
+
+// Toolchain takes one change of a build module's toolchains bucket in and answers whether what it passed on changed.
+func (g *ToolchainGate) Toolchain(module, key string, value []byte, deleted bool) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	if g.raw[module] == nil {
+		g.raw[module] = map[string][]byte{}
+	}
+	if deleted {
+		delete(g.raw[module], key)
+	} else {
+		g.raw[module][key] = value
+	}
+	return g.pass(module, key)
+}
+
+// Placement takes one revision of the indexer's placement in (at is when the bus wrote it) and answers whether what it
+// passed on changed. A value that does not read is said, and places nothing on its node.
+func (g *ToolchainGate) Placement(key string, value []byte, deleted bool, at time.Time) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	was := g.placed.Seeded()
+	node, changed, err := g.placed.Put(key, value, deleted, at)
+	if err != nil {
+		g.logf("%s: %v (issue 1940)", delivery.PlacementBucket, err)
+	}
+	if !g.known {
+		return false
+	}
+	if was != g.placed.Seeded() {
+		// The first placement came, or the last went: every report's standing changed.
+		return g.all()
+	}
+	return g.again(node, changed)
+}
+
+// Settled says the placement's history is read: from now on only the reports of placed nodes pass, once a node's
+// placement is held. It answers whether what it passed on changed.
+func (g *ToolchainGate) Settled() bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.placed.Settle()
+	g.known = true
+	return g.all()
+}
+
+// all passes on every report again. The caller holds mu.
+func (g *ToolchainGate) all() bool {
+	changed := false
+	for _, module := range slices.Sorted(maps.Keys(g.raw)) {
+		for _, k := range slices.Sorted(maps.Keys(g.raw[module])) {
+			changed = g.pass(module, k) || changed
+		}
+	}
+	return changed
+}
+
+// again passes on every report of node by a build module among modules. The caller holds mu.
+func (g *ToolchainGate) again(node string, modules []string) bool {
+	changed := false
+	for _, module := range modules {
+		if !g.modules[module] {
+			continue
+		}
+		for _, k := range slices.Sorted(maps.Keys(g.raw[module])) {
+			if n, _, err := delivery.ParseToolchainKey(k); err == nil && n == node {
+				changed = g.pass(module, k) || changed
+			}
+		}
+	}
+	return changed
+}
+
+// pass hands one key on as it stands: its value when its node is placed and wrote it since (or the placement is not
+// known), deleted otherwise. The caller holds mu.
+func (g *ToolchainGate) pass(module, key string) bool {
+	value, held := g.raw[module][key]
+	if !held {
+		return g.forward(module, key, nil, true)
+	}
+	node, _, err := delivery.ParseToolchainKey(key)
+	why := ""
+	if err == nil && g.known && g.placed.Seeded() {
+		why = g.leftOut(module, node, value)
+	}
+	if why == "" {
+		delete(g.left, module+"/"+key)
+		return g.forward(module, key, value, false)
+	}
+	if g.left[module+"/"+key] != why {
+		g.left[module+"/"+key] = why
+		g.logf("%s.%s is left out of the agreed toolchain: %s (issue 1940)", delivery.ToolchainsBucket(module), key, why)
+	}
+	return g.forward(module, key, nil, true)
+}
+
+// leftOut is why a node's report is no builder's, or empty when it counts: the catalogue does not place the module on its
+// node, or the report was written before it did. A value that is no toolchain is passed on for its reader to refuse.
+// The caller holds mu.
+func (g *ToolchainGate) leftOut(module, node string, value []byte) string {
+	since, placed := g.placed.Since(node, module)
+	if !placed {
+		return fmt.Sprintf("the catalogue does not place %s on %s, so it is no builder", module, node)
+	}
+	tc, err := delivery.DecodeToolchain(value)
+	if err == nil && tc.At.Before(since) {
+		return fmt.Sprintf("it was written at %s, before %s was placed on %s at %s: it counts once %s writes it again",
+			tc.At.UTC().Format(time.RFC3339), module, node, since.UTC().Format(time.RFC3339), node)
+	}
+	return ""
+}
+
+// FollowPlacement reads the indexer's placement with its history into gate, says it settled, and follows it; changed
+// is told whenever what the gate passes on changed. Where the state cannot be read or watched (not there, or this part
+// may not read it) it says so and the gate passes every report on: not fatal, since an indexer, builder or checker that
+// stops for it stops every delivery (issue 1945). An error of the watch after that ends on the channel.
+func FollowPlacement(ctx context.Context, js process.BucketOpener, gate *ToolchainGate, changed func(context.Context), logf func(string, ...any)) (<-chan error, error) {
+	errs := make(chan error, 1)
+	kv, err := js.KeyValue(ctx, delivery.PlacementBucket)
+	if err != nil {
+		logf("the state %s cannot be read, so every node's toolchain report counts, a build node's the catalogue does not place too (issue 1940): %v", delivery.PlacementBucket, err)
+		return errs, nil
+	}
+	w, err := kv.WatchAll(ctx, jetstream.IncludeHistory())
+	if err != nil {
+		logf("the state %s cannot be watched, so every node's toolchain report counts, a build node's the catalogue does not place too (issue 1940): %v", delivery.PlacementBucket, err)
+		return errs, nil
+	}
+	put := func(e jetstream.KeyValueEntry) bool {
+		return gate.Placement(e.Key(), e.Value(), e.Operation() != jetstream.KeyValuePut, e.Created())
+	}
+	for settled := false; !settled; {
+		select {
+		case <-ctx.Done():
+			_ = w.Stop()
+			return errs, ctx.Err()
+		case e, ok := <-w.Updates():
+			if !ok {
+				return errs, errors.New("the watch of " + delivery.PlacementBucket + " ended before it settled")
+			}
+			if e == nil {
+				settled = true
+				continue
+			}
+			put(e)
+		}
+	}
+	if gate.Settled() {
+		changed(ctx)
+	}
+	go func() {
+		defer func() { _ = w.Stop() }()
+		for {
+			select {
+			case <-ctx.Done():
+				errs <- ctx.Err()
+				return
+			case e, ok := <-w.Updates():
+				if !ok {
+					errs <- errors.New("the watch of " + delivery.PlacementBucket + " ended")
+					return
+				}
+				if e != nil && put(e) {
+					changed(ctx)
+				}
+			}
+		}
+	}()
+	return errs, nil
+}
diff --git a/go/delivery/buildrun/standing_test.go b/go/delivery/buildrun/standing_test.go
new file mode 100644
index 0000000..6377d36
--- /dev/null
+++ b/go/delivery/buildrun/standing_test.go
@@ -0,0 +1,235 @@
+package buildrun
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/nats-io/nats.go/jetstream"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+func gateReport(t *testing.T, node, version string, at time.Time) (string, []byte) {
+	t.Helper()
+	key, err := delivery.ToolchainKey(node, "typescript")
+	if err != nil {
+		t.Fatal(err)
+	}
+	value, err := delivery.EncodeToolchain(delivery.Toolchain{Language: "typescript", Version: version, Recipe: "f59571cf4be75adf", At: at})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return key, value
+}
+
+// placing is a value of the state `placement` placing modules on node.
+func placing(t *testing.T, node string, modules ...string) (string, []byte) {
+	t.Helper()
+	p := delivery.Placement{Node: node}
+	for _, m := range modules {
+		p.Modules = append(p.Modules, delivery.PlacedModule{Module: m})
+	}
+	key, err := delivery.PlacementKey(node)
+	if err != nil {
+		t.Fatal(err)
+	}
+	value, err := delivery.EncodePlacement(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return key, value
+}
+
+func agreedOf(set *ToolchainSet) (string, string) {
+	tc, none := delivery.AgreedToolchain(set.Entries("typescript"))
+	return tc.Version, none
+}
+
+// Issue 1940, the gate over the catalogue's placement (issue 774, ADR 0549 §3): a build node's report counts only while
+// the placement places its build module on its node, and only once written since; before the placement is settled, and
+// while no node's placement is held, every report counts. The live cases: mesh-build-node taken off ace left
+// ace.typescript at v26.12.0 against v26.12.1 on the three others; put on ace again, its report of 2026-10-23 06:29 came
+// back before the module ran there.
+func TestToolchainGate(t *testing.T) {
+	old := time.Date(2026, 10, 23, 6, 29, 18, 0, time.UTC)     // ace's report from its earlier run
+	placed := time.Date(2026, 10, 25, 19, 56, 45, 0, time.UTC) // mesh-build-node placed on ace again
+	three := []string{"g14", "novox", "shanks"}
+	start := func(t *testing.T, seeded bool) (*ToolchainGate, *ToolchainSet, *[]string) {
+		set := &ToolchainSet{}
+		said := &[]string{}
+		gate := NewToolchainGate([]string{GoBuildModule, NodeBuildModule}, set.Put, func(f string, a ...any) { *said = append(*said, fmt.Sprintf(f, a...)) })
+		for _, node := range three {
+			if seeded {
+				key, value := placing(t, node, NodeBuildModule)
+				gate.Placement(key, value, false, old.Add(-time.Hour))
+			}
+			key, value := gateReport(t, node, "v26.12.1", placed.Add(time.Minute))
+			gate.Toolchain(NodeBuildModule, key, value, false)
+		}
+		return gate, set, said
+	}
+
+	t.Run("not placed on ace: its report is no builder's", func(t *testing.T) {
+		gate, set, said := start(t, true)
+		key, value := gateReport(t, "ace", "v26.12.0", old)
+		gate.Toolchain(NodeBuildModule, key, value, false)
+		if _, none := agreedOf(set); !strings.HasPrefix(none, "builders disagree") {
+			t.Fatalf("before the placement is settled every report counts; got %q", none)
+		}
+		k, v := placing(t, "ace", GoBuildModule) // another build module on ace: no matter
+		gate.Placement(k, v, false, old)
+		if !gate.Settled() {
+			t.Fatal("settling left ace's report out, so what was passed on changed")
+		}
+		if v, none := agreedOf(set); v != "v26.12.1" || none != "" {
+			t.Fatalf("the placed nodes agree on v26.12.1; got %q, %q", v, none)
+		}
+		if len(*said) != 1 || !strings.Contains((*said)[0], "does not place mesh-build-node on ace") {
+			t.Fatalf("ace's report is said left out once; said %q", *said)
+		}
+		if gate.Toolchain(NodeBuildModule, key, value, false) {
+			t.Fatal("a report of a node not placed changed what was passed on")
+		}
+		// Placed on ace with a report written since: it counts, and the builders really disagree.
+		k, v = placing(t, "ace", GoBuildModule, NodeBuildModule)
+		gate.Placement(k, v, false, old.Add(-time.Minute))
+		if _, none := agreedOf(set); !strings.HasPrefix(none, "builders disagree: v26.12.0 recipe f59571cf4be75adf on ace") {
+			t.Fatalf("builders that run different versions still disagree; got %q", none)
+		}
+		// Taken off ace again (its key deleted): its report goes again.
+		if !gate.Placement(k, nil, true, placed) {
+			t.Fatal("taking ace's placement away passed nothing on")
+		}
+		if v, none := agreedOf(set); v != "v26.12.1" || none != "" {
+			t.Fatalf("got %q, %q", v, none)
+		}
+		// A report deleted from the bucket goes whether its node is placed or not.
+		key, _ = gateReport(t, "g14", "v26.12.1", placed)
+		if !gate.Toolchain(NodeBuildModule, key, nil, true) || len(set.Entries("typescript")) != 2 {
+			t.Fatalf("deleting g14's report: %d entries left, want novox and shanks", len(set.Entries("typescript")))
+		}
+	})
+
+	for _, c := range []struct {
+		name   string
+		at     time.Time // when ace wrote its report
+		counts bool
+	}{
+		{"a report left before the placement", old, false},
+		{"a report written since the placement", placed.Add(30 * time.Second), true},
+	} {
+		t.Run(c.name, func(t *testing.T) {
+			gate, set, said := start(t, true)
+			gate.Settled()
+			k, v := placing(t, "ace", NodeBuildModule)
+			if gate.Placement(k, v, false, placed) {
+				t.Fatal("a placement with no report passed something on")
+			}
+			key, value := gateReport(t, "ace", "v26.12.0", c.at)
+			if got := gate.Toolchain(NodeBuildModule, key, value, false); got != c.counts {
+				t.Fatalf("ace's report passed on: %v, want %v", got, c.counts)
+			}
+			// ace's placement written again with another module beside: its standing does not move, nothing is said again.
+			k, v = placing(t, "ace", GoBuildModule, NodeBuildModule)
+			if gate.Placement(k, v, false, placed.Add(time.Minute)) {
+				t.Fatal("the same placement of mesh-build-node changed what was passed on")
+			}
+			if c.counts {
+				return
+			}
+			if v, none := agreedOf(set); v != "v26.12.1" || none != "" {
+				t.Fatalf("ace's left report is no builder's; got %q, %q", v, none)
+			}
+			if len(*said) != 1 || !strings.Contains((*said)[0], "before mesh-build-node was placed on ace at 2026-10-25T19:56:45Z") {
+				t.Fatalf("said %q, want once why ace's report is left out", *said)
+			}
+			key, value = gateReport(t, "ace", "v26.12.1", placed.Add(time.Minute))
+			if !gate.Toolchain(NodeBuildModule, key, value, false) || len(set.Entries("typescript")) != 4 {
+				t.Fatal("ace's new report was not passed on")
+			}
+		})
+	}
+
+	t.Run("no placement held: every report counts", func(t *testing.T) {
+		gate, set, _ := start(t, false)
+		key, value := gateReport(t, "ace", "v26.12.0", old)
+		gate.Toolchain(NodeBuildModule, key, value, false)
+		gate.Settled()
+		if n := len(set.Entries("typescript")); n != 4 {
+			t.Fatalf("before the first placement file every report counts; got %d", n)
+		}
+		k, v := placing(t, "g14", NodeBuildModule)
+		if !gate.Placement(k, v, false, old.Add(-time.Hour)) || len(set.Entries("typescript")) != 1 {
+			t.Fatalf("the first placement leaves every node it does not place out; got %d", len(set.Entries("typescript")))
+		}
+		if !gate.Placement(k, nil, true, placed) || len(set.Entries("typescript")) != 4 {
+			t.Fatal("the last placement gone: every report counts again")
+		}
+	})
+
+	t.Run("the history: a full history's oldest revision stands since before it", func(t *testing.T) {
+		p := delivery.NewPlaced()
+		k, v := placing(t, "ace", NodeBuildModule)
+		for i := range delivery.PlacementHistory {
+			p.Put(k, v, false, old.Add(time.Duration(i)*time.Minute))
+		}
+		k2, v2 := placing(t, "g14", NodeBuildModule)
+		p.Put(k2, v2, false, old)
+		if _, _, err := p.Put("placement.novox", []byte("not json"), false, old); err == nil {
+			t.Error("a value that does not read was not said")
+		}
+		p.Settle()
+		if s, ok := p.Since("ace", NodeBuildModule); !ok || !s.IsZero() {
+			t.Errorf("ace's module stands since %v, %v; want the zero time", s, ok)
+		}
+		if s, ok := p.Since("g14", NodeBuildModule); !ok || !s.Equal(old) {
+			t.Errorf("g14's module stands since %v, %v; want its first revision", s, ok)
+		}
+		if got := p.PlacedNodes(NodeBuildModule); strings.Join(got, ",") != "ace,g14" || len(p.ByNode()) != 2 || !p.Seeded() {
+			t.Errorf("PlacedNodes = %v, ByNode = %v", got, p.ByNode())
+		}
+	})
+
+	t.Run("a placement the bus refuses is said, and the part goes on", func(t *testing.T) {
+		var logs bytes.Buffer
+		logger := log.New(&logs, "", 0)
+		refused := errors.New("nats: permissions violation for publish to $JS.API.CONSUMER.CREATE.KV_mesh-delivery-indexer_placement")
+		gate, _, _ := start(t, false)
+		for _, bus := range []refusingBus{{err: refused}, {refuseOpen: true, err: refused}} {
+			if _, err := FollowPlacement(context.Background(), bus, gate, func(context.Context) {}, logger.Printf); err != nil {
+				t.Errorf("a refused placement stopped the start: %v", err)
+			}
+		}
+		if !strings.Contains(logs.String(), "cannot be watched") || !strings.Contains(logs.String(), "cannot be read") {
+			t.Errorf("the refusals were not said: %q", logs.String())
+		}
+	})
+}
+
+// refusingBus is a bus that refuses: every open (refuseOpen), or every watch of a bucket it opens, with err.
+type refusingBus struct {
+	refuseOpen bool
+	err        error
+}
+
+func (r refusingBus) KeyValue(context.Context, string) (jetstream.KeyValue, error) {
+	if r.refuseOpen {
+		return nil, r.err
+	}
+	return refusedWatch{err: r.err}, nil
+}
+
+type refusedWatch struct {
+	jetstream.KeyValue
+	err error
+}
+
+func (r refusedWatch) WatchAll(context.Context, ...jetstream.WatchOpt) (jetstream.KeyWatcher, error) {
+	return nil, r.err
+}
diff --git a/go/delivery/buildrun/toolchains_test.go b/go/delivery/buildrun/toolchains_test.go
index e434b8a..4df0d38 100644
--- a/go/delivery/buildrun/toolchains_test.go
+++ b/go/delivery/buildrun/toolchains_test.go
@@ -35,10 +35,8 @@ func TestToolchainSetKeepsEachNodesEntryByLanguageAndSaysWhenOneChanged(t *testi
 	if !s.Put(GoBuildModule, "ace.go", nil, true) || len(s.Entries("go")) != 1 {
 		t.Error("a delete did not remove the node's entry")
 	}
-}
-
-func TestToolchainSetLeavesOutWhatIsNotAToolchainOfTheModulesLanguage(t *testing.T) { // 1607/AC6
-	var s ToolchainSet
+	// What is not a toolchain of the module's language is left out.
+	s = ToolchainSet{}
 	if s.Put(GoBuildModule, "novox.typescript", tcValue(t, "typescript", "x"), false) {
 		t.Error("a typescript toolchain in the go build module's bucket counted")
 	}
diff --git a/go/delivery/placed.go b/go/delivery/placed.go
new file mode 100644
index 0000000..e73937a
--- /dev/null
+++ b/go/delivery/placed.go
@@ -0,0 +1,150 @@
+package delivery
+
+import (
+	"fmt"
+	"maps"
+	"slices"
+	"sync"
+	"time"
+)
+
+// Placed is where the indexer's placement (the state `placement`, ADR 0549 §2, §3) places each module, and since when:
+// the one reader of that state for every part that asks "does module stand on node" (issue 774, design 47's third
+// milestone), in place of each part reading the controller's assignments (`mesh-controller_assignments`, which goes
+// with the controller's lease, ADR 0549 §6).
+//
+// Its input is the state's revisions in the order the bus wrote them, as a watch with the history (jetstream's
+// IncludeHistory) hands them: Put takes each, and Settle ends the history. A module stands since the revision of its
+// node's key that first named it in an unbroken run; a module in the oldest revision of a key whose history is full
+// (PlacementHistory) stands since before what the bus still keeps (the zero time), since an earlier revision may have
+// named it. A key's value that does not read places nothing on its node, and Put says why.
+//
+// Safe for use from a watch and from a reader at once.
+type Placed struct {
+	mu      sync.Mutex
+	since   map[string]map[string]time.Time // node -> module -> since when it stands
+	seen    map[string]int                  // node -> revisions read before Settle
+	first   map[string]time.Time            // node -> when its first revision read was written
+	settled bool
+}
+
+// NewPlaced is a Placed that holds no placement yet.
+func NewPlaced() *Placed {
+	return &Placed{since: map[string]map[string]time.Time{}, seen: map[string]int{}, first: map[string]time.Time{}}
+}
+
+// Put takes one revision of a key of the state `placement` (its value, whether it is a delete, and when the bus wrote
+// it). It answers the node the key is of and the modules whose standing on it changed (placed or removed), in name
+// order, and an error for a key or a value that does not read (the node then places nothing).
+func (p *Placed) Put(key string, value []byte, deleted bool, at time.Time) (node string, changed []string, err error) {
+	node, err = ParsePlacementKey(key)
+	if err != nil {
+		return "", nil, err
+	}
+	now := map[string]bool{}
+	if !deleted {
+		pl, derr := DecodePlacement(value)
+		switch {
+		case derr != nil:
+			err = fmt.Errorf("the placement of %s does not read, so it places nothing there: %w", node, derr)
+		case pl.Node != node:
+			err = fmt.Errorf("the placement under %s names the node %q, so it places nothing there", key, pl.Node)
+		default:
+			for _, m := range pl.Assigned() {
+				now[m] = true
+			}
+		}
+	}
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if !p.settled {
+		if p.seen[node] == 0 {
+			p.first[node] = at
+		}
+		p.seen[node]++
+	}
+	was := p.since[node]
+	next := map[string]time.Time{}
+	for m := range now {
+		if s, ok := was[m]; ok {
+			next[m] = s
+		} else {
+			next[m] = at
+			changed = append(changed, m)
+		}
+	}
+	for m := range was {
+		if !now[m] {
+			changed = append(changed, m)
+		}
+	}
+	if len(next) == 0 {
+		delete(p.since, node)
+	} else {
+		p.since[node] = next
+	}
+	slices.Sort(changed)
+	return node, changed, err
+}
+
+// Settle ends the history: from now on every Put is a new revision. A module in the oldest kept revision of a key whose
+// history is full stands since the zero time.
+func (p *Placed) Settle() {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.settled {
+		return
+	}
+	p.settled = true
+	for node, n := range p.seen {
+		if n < PlacementHistory {
+			continue
+		}
+		for m, s := range p.since[node] {
+			if s.Equal(p.first[node]) {
+				p.since[node][m] = time.Time{}
+			}
+		}
+	}
+}
+
+// Seeded says some node's placement is held: before the first placement file reaches the indexer nothing is placed,
+// and a reader keeps what it did before (ADR 0549 §5).
+func (p *Placed) Seeded() bool {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	return len(p.since) > 0
+}
+
+// Since says whether module stands on node, and since when (the zero time: since before the revisions kept).
+func (p *Placed) Since(node, module string) (time.Time, bool) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	s, ok := p.since[node][module]
+	return s, ok
+}
+
+// PlacedNodes is every node module is placed on, in name order.
+func (p *Placed) PlacedNodes(module string) []string {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	var out []string
+	for node, mods := range p.since {
+		if _, ok := mods[module]; ok {
+			out = append(out, node)
+		}
+	}
+	slices.Sort(out)
+	return out
+}
+
+// ByNode is, by node, the modules placed on it in name order.
+func (p *Placed) ByNode() map[string][]string {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	out := make(map[string][]string, len(p.since))
+	for node, mods := range p.since {
+		out[node] = slices.Sorted(maps.Keys(mods))
+	}
+	return out
+}