diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/gone_integration_test.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/gone_integration_test.go
index 58395d16..30195bbc 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/gone_integration_test.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/gone_integration_test.go
@@ -12,6 +12,7 @@ import (
 	"github.com/nats-io/nats.go"
 	"github.com/nats-io/nats.go/jetstream"
 
+	sweep "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep"
 	"git.novox.be/novox/mesh-sdk/go/catalogue"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
 )
@@ -35,7 +36,7 @@ func (h *heldVerb) serve(t *testing.T, b *bed) {
 			References []string `json:"references"`
 		}
 		_ = json.Unmarshal(m.Data, &args)
-		a := delivery.HeldAnswer{Store: "store.test:5100", Held: []string{}, Gone: []string{}}
+		a := sweep.HeldAnswer{Store: "store.test:5100", Held: []string{}, Gone: []string{}}
 		h.mu.Lock()
 		for _, r := range args.References {
 			h.asked = append(h.asked, r)
@@ -65,11 +66,11 @@ func (h *heldVerb) asks() int {
 // from the manifest's consumes.
 func eventsConsumer(t *testing.T, b *bed, node string) {
 	t.Helper()
-	if _, err := b.js.CreateStream(b.ctx, jetstream.StreamConfig{Name: eventsStream, Subjects: []string{delivery.ArtifactsGoneSubject}}); err != nil {
+	if _, err := b.js.CreateStream(b.ctx, jetstream.StreamConfig{Name: eventsStream, Subjects: []string{sweep.SubjectArtifactsGone}}); err != nil {
 		t.Fatal(err)
 	}
 	if _, err := b.js.CreateOrUpdateConsumer(b.ctx, eventsStream, jetstream.ConsumerConfig{Durable: node + "_mesh-delivery-builder",
-		AckPolicy: jetstream.AckExplicitPolicy, FilterSubjects: []string{delivery.ArtifactsGoneSubject}}); err != nil {
+		AckPolicy: jetstream.AckExplicitPolicy, FilterSubjects: []string{sweep.SubjectArtifactsGone}}); err != nil {
 		t.Fatal(err)
 	}
 }
@@ -168,12 +169,13 @@ func TestArtifactsGoneLeadsTheBuilderToBuildTheKeyAgain(t *testing.T) {
 	collected = true
 	mu.Unlock()
 	key, _ := delivery.BuilderBuildKey("ca-trust", fp)
-	body, err := delivery.EncodeArtifactsGone(delivery.ArtifactsGone{Module: "ca-trust", Key: key, Fingerprint: fp,
-		Gone: []string{ref}, At: time.Now().UTC()})
+	at := time.Now().UTC()
+	body, err := json.Marshal(sweep.ArtifactsGone{Event: sweep.EventArtifactsGone, Module: "ca-trust", Key: key, Fingerprint: fp,
+		Gone: []string{ref}, At: &at})
 	if err != nil {
 		t.Fatal(err)
 	}
-	if _, err := b.js.Publish(b.ctx, delivery.ArtifactsGoneSubject, body); err != nil {
+	if _, err := b.js.Publish(b.ctx, sweep.SubjectArtifactsGone, body); err != nil {
 		t.Fatal(err)
 	}
 	v := b.await(t, "ca-trust", func(v delivery.BuilderBuild) bool {
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go
index eee4c068..32901723 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go
@@ -28,6 +28,7 @@ import (
 	runner "git.novox.be/novox/mesh-catalog/modules/mesh-build-go/buildstep"
 	"git.novox.be/novox/mesh-contracts/go/seats/gopackageregistry"
 	"git.novox.be/novox/mesh-contracts/go/seats/ociartifactregistry"
+	sweep "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep"
 	"git.novox.be/novox/mesh-sdk/go/buildstep"
 	"git.novox.be/novox/mesh-sdk/go/buscompose"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
@@ -1191,7 +1192,7 @@ func (b *bed) registry(t *testing.T, node string, seatVersions ...string) *regis
 	mk(versions.FloorWaitsBucket)
 	// EVENTS with two seats' outcomes, and the forge bridge's stream (a branch's end): each with the builder's durable consumer.
 	for stream, subjects := range map[string][]string{
-		eventsStream: {delivery.ArtifactsGoneSubject, ociartifactregistry.SubjectPublished, ociartifactregistry.SubjectRefused, ociartifactregistry.SubjectRemoved,
+		eventsStream: {sweep.SubjectArtifactsGone, ociartifactregistry.SubjectPublished, ociartifactregistry.SubjectRefused, ociartifactregistry.SubjectRemoved,
 			gopackageregistry.SubjectPublished, gopackageregistry.SubjectRefused, gopackageregistry.SubjectRemoved},
 		forgeStream: {process.ModuleStreamSubject(delivery.ForgeBridgeModule, ">")},
 	} {
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go
index 0289ec8f..54414870 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go
@@ -25,6 +25,7 @@ import (
 	"github.com/nats-io/nats.go"
 	"github.com/nats-io/nats.go/jetstream"
 
+	sweep "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
 	"git.novox.be/novox/mesh-sdk/go/process"
 	"git.novox.be/novox/mesh-sdk/go/views"
@@ -329,7 +330,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 	floor := floorWake{b: b, waits: floorWaits, logf: logger.Printf}
 	// And the four registry seats' published, refused and removed (DLV-B4), which move a numbered version.
 	seats := seatHearer{n: numberer, b: b, logf: logger.Printf, pub: pubs}
-	hear(eventsStream, delivery.ArtifactsGoneEvent+", what wakes a floor, the registry seats' words and manifest-reads-changed are not heard until the builder starts again", func(m jetstream.Msg) {
+	hear(eventsStream, sweep.EventArtifactsGone+", what wakes a floor, the registry seats' words and manifest-reads-changed are not heard until the builder starts again", func(m jetstream.Msg) {
 		if reads.heard(m, logger.Printf) || floor.heard(ctx, m) || seats.heard(ctx, m) { // reads: the node-engine seat's manifest-reads-changed (ADR 0618 §4)
 			return
 		}
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/refused_test.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/refused_test.go
index 0e53e2f2..cc000338 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/refused_test.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/refused_test.go
@@ -21,6 +21,7 @@ import (
 	"github.com/nats-io/nats.go/jetstream"
 
 	"git.novox.be/novox/mesh-contracts/go/seats/nodeengine"
+	sweep "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep"
 	"git.novox.be/novox/mesh-delivery/modules/mesh-delivery-builder/buildrun"
 	"git.novox.be/novox/mesh-delivery/sdk/enginereads"
 	"git.novox.be/novox/mesh-delivery/sdk/forms"
@@ -223,7 +224,7 @@ func aFloorsWakeIsHeard(t *testing.T) {
 	if !f.heard(context.Background(), cleared) || !strings.Contains(logs.String(), "the floor of m/img is asked again: condition-cleared") {
 		t.Fatalf("condition-cleared: log %q", logs.String())
 	}
-	if f.heard(context.Background(), &wakeMsg{subject: delivery.ArtifactsGoneSubject}) {
+	if f.heard(context.Background(), &wakeMsg{subject: sweep.SubjectArtifactsGone}) {
 		t.Fatal("artifacts-gone was taken as a floor's wake")
 	}
 	// DLV-B4: a registry seat's outcome on the same consumer moves a version; a published wakes a name whose seat was
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/stored.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/stored.go
index c5fc876a..7c1a023e 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/stored.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/stored.go
@@ -9,8 +9,8 @@ import (
 
 	"github.com/nats-io/nats.go/jetstream"
 
+	sweep "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep"
 	"git.novox.be/novox/mesh-sdk/go/catalogue"
-	"git.novox.be/novox/mesh-sdk/go/delivery"
 
 	"mesh-delivery-builder/internal/build"
 )
@@ -35,7 +35,7 @@ func storedVia(call toolCaller) func(ctx context.Context, references []string) (
 		if err != nil {
 			return nil, fmt.Errorf("asking held: %w", err)
 		}
-		a, err := delivery.DecodeHeldAnswer(raw, references)
+		a, err := decodeHeld(raw, references)
 		if err != nil {
 			return nil, err
 		}
@@ -54,13 +54,62 @@ func storedVia(call toolCaller) func(ctx context.Context, references []string) (
 // heardGone takes one artifacts-gone event in: acknowledged, then handed to the builder, which asks the store and builds
 // the key again. A body that is not one is said and acknowledged, so it is not handed over again.
 func heardGone(ctx context.Context, b *build.Builder, m jetstream.Msg, logf func(string, ...any)) {
-	e, err := delivery.DecodeArtifactsGone(m.Data())
+	e, err := decodeGone(m.Data())
 	if ackErr := m.Ack(); ackErr != nil {
 		logf("acknowledging %s: %v", m.Subject(), ackErr)
 	}
 	if err != nil {
-		logf("%s holds what is not %s, left: %v", m.Subject(), delivery.ArtifactsGoneEvent, err)
+		logf("%s holds what is not %s, left: %v", m.Subject(), sweep.EventArtifactsGone, err)
 		return
 	}
 	b.ArtifactsGone(ctx, e)
 }
+
+// decodeHeld reads the seat's answer to held: the verb answers {output, ok, answer}, the answer being the seat's
+// HeldAnswer. A reference asked that the answer names in none of held, gone and unknown is an error: an answer that
+// leaves one out would let a key with a gone blob be reused.
+func decodeHeld(raw []byte, asked []string) (sweep.HeldAnswer, error) {
+	var wrapped struct {
+		Output string          `json:"output"`
+		OK     bool            `json:"ok"`
+		Answer json.RawMessage `json:"answer"`
+	}
+	if err := json.Unmarshal(raw, &wrapped); err != nil {
+		return sweep.HeldAnswer{}, fmt.Errorf("the answer of %s is not {output, ok, answer}: %w", SeatHeld, err)
+	}
+	if !wrapped.OK || len(wrapped.Answer) == 0 {
+		return sweep.HeldAnswer{}, fmt.Errorf("%s did not answer: %s", SeatHeld, wrapped.Output)
+	}
+	var a sweep.HeldAnswer
+	if err := json.Unmarshal(wrapped.Answer, &a); err != nil {
+		return sweep.HeldAnswer{}, fmt.Errorf("the answer of %s is not {held, gone}: %w", SeatHeld, err)
+	}
+	named := map[string]bool{}
+	for _, r := range a.Held {
+		named[r] = true
+	}
+	for _, r := range a.Gone {
+		named[r] = true
+	}
+	for r := range a.Unknown {
+		named[r] = true
+	}
+	for _, r := range asked {
+		if !named[r] {
+			return sweep.HeldAnswer{}, fmt.Errorf("the answer of %s names nothing of %s", SeatHeld, r)
+		}
+	}
+	return a, nil
+}
+
+// decodeGone reads the event's body, refusing one that names no module, fingerprint or reference.
+func decodeGone(raw []byte) (sweep.ArtifactsGone, error) {
+	var e sweep.ArtifactsGone
+	if err := json.Unmarshal(raw, &e); err != nil {
+		return sweep.ArtifactsGone{}, fmt.Errorf("%s is not one: %w", sweep.EventArtifactsGone, err)
+	}
+	if e.Event != sweep.EventArtifactsGone || e.Module == "" || e.Fingerprint == "" || len(e.Gone) == 0 {
+		return sweep.ArtifactsGone{}, fmt.Errorf("%s names no module, fingerprint or gone reference: %s", sweep.EventArtifactsGone, raw)
+	}
+	return e, nil
+}
diff --git a/modules/mesh-delivery-builder/go.mod b/modules/mesh-delivery-builder/go.mod
index 47f4e6af..ea243f5a 100644
--- a/modules/mesh-delivery-builder/go.mod
+++ b/modules/mesh-delivery-builder/go.mod
@@ -44,3 +44,7 @@ require (
 )
 
 replace git.novox.be/novox/mesh-delivery/sdk => ../../sdk
+
+require git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep v0.0.0-00010101000000-000000000000
+
+replace git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep => ../../contracts/go/seats/meshartifactsweep
diff --git a/modules/mesh-delivery-builder/internal/build/gone_test.go b/modules/mesh-delivery-builder/internal/build/gone_test.go
index 499cdbc9..c114010a 100644
--- a/modules/mesh-delivery-builder/internal/build/gone_test.go
+++ b/modules/mesh-delivery-builder/internal/build/gone_test.go
@@ -9,6 +9,7 @@ import (
 	"sync"
 	"testing"
 
+	sweep "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
 )
 
@@ -184,14 +185,14 @@ func TestArtifactsGoneMakesTheBuilderAskAndBuildAgain(t *testing.T) {
 	held.mu.Lock()
 	held.gone[goneRef] = true // collected after the builder's look; the controller's take-in found it
 	held.mu.Unlock()
-	r.b.ArtifactsGone(context.Background(), delivery.ArtifactsGone{Event: delivery.ArtifactsGoneEvent, Module: "m",
+	r.b.ArtifactsGone(context.Background(), sweep.ArtifactsGone{Event: sweep.EventArtifactsGone, Module: "m",
 		Fingerprint: "src2:" + strings.Repeat("9", 64), Gone: []string{goneRef}})
 	r.b.Wait()
 	if r.st.revs[r.key] != rev || !log.has("a fingerprint the module does not have now") {
 		t.Fatalf("an event for another fingerprint moved the key (revision %d, was %d): %v", r.st.revs[r.key], rev, log.lines)
 	}
 
-	r.b.ArtifactsGone(context.Background(), delivery.ArtifactsGone{Event: delivery.ArtifactsGoneEvent, Module: "m",
+	r.b.ArtifactsGone(context.Background(), sweep.ArtifactsGone{Event: sweep.EventArtifactsGone, Module: "m",
 		Key: r.key, Fingerprint: fp1, Gone: []string{goneRef}})
 	r.b.Wait()
 	v := valueOf(t, r.st, r.key)
diff --git a/modules/mesh-delivery-builder/internal/build/run.go b/modules/mesh-delivery-builder/internal/build/run.go
index fecf91c8..e651e436 100644
--- a/modules/mesh-delivery-builder/internal/build/run.go
+++ b/modules/mesh-delivery-builder/internal/build/run.go
@@ -13,6 +13,7 @@ import (
 	"sync"
 	"time"
 
+	sweep "git.novox.be/novox/mesh-delivery/contracts/go/seats/meshartifactsweep"
 	"git.novox.be/novox/mesh-delivery/modules/mesh-delivery-builder/buildrun"
 	"git.novox.be/novox/mesh-delivery/sdk/forms"
 	"git.novox.be/novox/mesh-sdk/go/buildstep"
@@ -281,7 +282,7 @@ func (b *Builder) storedOnce(key string) func(ctx context.Context, references []
 // that the store does not hold what the module's key names. The module is looked at again, which asks the store (ClaimOptions.Stored) and
 // builds the key again; an event for a fingerprint the module no longer has is said and left. The event only wakes the
 // builder: what is built rests on the store's answer, never on the event alone.
-func (b *Builder) ArtifactsGone(ctx context.Context, e delivery.ArtifactsGone) {
+func (b *Builder) ArtifactsGone(ctx context.Context, e sweep.ArtifactsGone) {
 	b.init()
 	b.mu.Lock()
 	m, ok := b.known[e.Module]
@@ -295,11 +296,11 @@ func (b *Builder) ArtifactsGone(ctx context.Context, e delivery.ArtifactsGone) {
 	b.mu.Unlock()
 	if !ok || m.FP2 != e.Fingerprint {
 		b.logf("%s: heard %s for %s, a fingerprint the module does not have now: nothing to build", e.Module,
-			delivery.ArtifactsGoneEvent, e.Fingerprint)
+			sweep.EventArtifactsGone, e.Fingerprint)
 		return
 	}
 	b.logf("%s: heard %s (%s): asking the artifact store and building the key again", e.Module,
-		delivery.ArtifactsGoneEvent, strings.Join(e.Gone, ", "))
+		sweep.EventArtifactsGone, strings.Join(e.Gone, ", "))
 	b.consider(ctx, e.Module, m)
 }