diff --git a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/asks.go b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/asks.go
new file mode 100644
index 0000000..6fbe713
--- /dev/null
+++ b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/asks.go
@@ -0,0 +1,161 @@
+package main
+
+import (
+	"context"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/buildstep"
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"git.novox.be/novox/mesh-sdk/go/delivery/buildrun"
+
+	"mesh-delivery-assigner/internal/assign"
+)
+
+// AssignerModule is the asker the assigner's compose asks name.
+const AssignerModule = "mesh-delivery-assigner"
+
+// composeAsks are the compose asks the assigner put on the Go build seat's check stream (novox/hq ADR 0482 clause 4,
+// issues 1622 and 1623), by what each composes: one ask per key, put once and followed by a watch of its step key until
+// it ends, which wakes a pass. No timer: an ask that waits in the stream waits (its backlog is the view's and the
+// watchman's to show). A restart forgets them and asks again; the build module keeps a composer built for a commit, so a
+// second ask builds nothing.
+type composeAsks struct {
+	ctx   context.Context
+	asker buildrun.Asker
+	wake  func()
+	logf  func(string, ...any)
+	now   func() time.Time
+
+	mu   sync.Mutex
+	asks map[string]*composeAsk
+}
+
+type composeAsk struct {
+	id     string
+	cancel context.CancelFunc
+	ran    *assign.Ran
+}
+
+func newComposeAsks(ctx context.Context, asker buildrun.Asker, wake func(), logf func(string, ...any)) *composeAsks {
+	return &composeAsks{ctx: ctx, asker: asker, wake: wake, logf: logf, now: time.Now, asks: map[string]*composeAsk{}}
+}
+
+// get is the end of the ask that composes key, or nil and its id while it runs. The first get of a key puts the ask.
+// A job its layout refuses ends at once, failed with why: never silent.
+func (a *composeAsks) get(key string, job delivery.ComposeJob) (*assign.Ran, string) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if k, ok := a.asks[key]; ok {
+		return k.ran, k.id
+	}
+	ask := delivery.StepAsk{ID: buildstep.NewID(), Kind: delivery.AskCheck, Language: delivery.AskGo, Asker: AssignerModule,
+		At: a.now(), Compose: &job}
+	ctx, cancel := context.WithCancel(a.ctx)
+	k := &composeAsk{id: ask.ID, cancel: cancel}
+	a.asks[key] = k
+	if err := a.asker.Put(ctx, ask); err != nil {
+		a.logf("the compose ask %s for %s could not be put: %v", ask.ID, key, err)
+		k.ran = &assign.Ran{State: delivery.StepFailed, Reason: "the compose ask could not be put on the Go build seat: " + err.Error()}
+		cancel()
+		return k.ran, k.id
+	}
+	go func() {
+		step, err := a.asker.Wait(ctx, ask)
+		if ctx.Err() != nil {
+			return
+		}
+		ran := assign.RanOf(step)
+		if err != nil {
+			ran = assign.Ran{State: delivery.StepFailed, Reason: "the compose ask's step could not be followed: " + err.Error()}
+		}
+		a.mu.Lock()
+		k.ran = &ran
+		a.mu.Unlock()
+		a.wake()
+	}()
+	return nil, k.id
+}
+
+// sweep forgets, and stops following, every ask whose key no pass asked for this time: a candidate gone or moved on.
+func (a *composeAsks) sweep(seen map[string]bool) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	for key, k := range a.asks {
+		if !seen[key] {
+			k.cancel()
+			delete(a.asks, key)
+		}
+	}
+}
+
+// composeAtHead is the composition of a group with a mesh-sdk member: the composer at its head and at each reader's
+// mesh-sdk, asked of the Go build seat, and the verdict once every ask ended; until then the verdict waits, naming the
+// asks. The keys it used are marked in seen.
+func (p *passer) composeAtHead(ctx context.Context, m assign.Mesh, ch assign.Change, seen map[string]bool, began time.Time) delivery.CompositionVerdict {
+	waiting := func(words string) delivery.CompositionVerdict {
+		return delivery.CompositionVerdict{Group: ch.Group, Heads: ch.Heads, State: delivery.CompositionWaits, Words: words}
+	}
+	head, err := assign.HeadJob(m, ch)
+	if err != nil {
+		return delivery.CompositionVerdict{Group: ch.Group, Heads: ch.Heads, State: delivery.CompositionFail, SDK: "mesh-sdk " + ch.SDKHead,
+			Words: "the compose ask at the group's mesh-sdk head cannot be made: " + err.Error()}
+	}
+	var pending []string
+	key := "head." + ch.Group + "." + ch.Heads + "." + ch.SDKHead
+	seen[key] = true
+	headRan, id := p.asks.get(key, head)
+	if headRan == nil {
+		pending = append(pending, "mesh-sdk "+ch.SDKHead+" (ask "+id+")")
+	}
+	var readers []assign.Reader
+	var notJudged []string
+	for _, module := range sortedNames(assign.ReaderModules) {
+		repository := assign.ReaderModules[module]
+		var commits []string
+		member := false
+		if h := ch.Members[repository]; h != "" {
+			commits, member = []string{h}, true
+		} else {
+			commits = p.w.runningCommits(ctx, kvGetter(p.kvs[delivery.BuildsBucket]), module)
+		}
+		if len(commits) == 0 {
+			notJudged = append(notJudged, "not judged whether "+module+" reads the group's manifests: no build of it is assigned with a "+
+				"commit on record, and the group holds no member of "+repository)
+			continue
+		}
+		for _, commit := range commits {
+			job, err := assign.ReaderJob(ch, module, repository, commit)
+			if err != nil {
+				readers = append(readers, assign.Reader{Module: module, Commit: commit, Member: member,
+					Ran: assign.Ran{State: delivery.StepFailed, Reason: err.Error()}})
+				continue
+			}
+			rkey := "reader." + ch.Group + "." + ch.Heads + "." + module + "." + commit
+			seen[rkey] = true
+			ran, rid := p.asks.get(rkey, job)
+			if ran == nil {
+				pending = append(pending, module+" at "+commit+" (ask "+rid+")")
+				continue
+			}
+			readers = append(readers, assign.Reader{Module: module, Commit: commit, Member: member, Ran: *ran})
+		}
+	}
+	if len(pending) > 0 {
+		return waiting("composing on the Go build seat with the mesh-sdk of " + strings.Join(pending, ", "))
+	}
+	v := assign.FromComposer(m, ch, *headRan, readers, time.Since(began))
+	v.Warnings = append(v.Warnings, notJudged...)
+	return v
+}
+
+func sortedNames(m map[string]string) []string {
+	out := make([]string, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	sort.Strings(out)
+	return out
+}
diff --git a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/compose_integration_test.go b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/compose_integration_test.go
new file mode 100644
index 0000000..8bc0662
--- /dev/null
+++ b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/compose_integration_test.go
@@ -0,0 +1,180 @@
+//go:build integration
+
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/nats-io/nats.go/jetstream"
+
+	"git.novox.be/novox/mesh-sdk/go/composer"
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"git.novox.be/novox/mesh-sdk/go/provider"
+)
+
+// holder is a test holder of the Go build seat: it takes every ask from the seat's check worker, as mesh-build-go does,
+// and answers a compose ask by running mesh-sdk's composer over the request filled from the indexer's manifests (the
+// linked composer standing for the one the build module builds at the asked commit), writing its step on its node's key.
+type holder struct {
+	mu   sync.Mutex
+	asks []delivery.StepAsk
+}
+
+func (h *holder) taken() []delivery.StepAsk {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	return append([]delivery.StepAsk(nil), h.asks...)
+}
+
+func (b *bed) hold(t *testing.T) *holder {
+	t.Helper()
+	seat, _ := delivery.AskStream(delivery.AskGo)
+	worker, _ := delivery.AskWorker(delivery.AskGo, delivery.AskCheck)
+	subject, _ := delivery.AskSubject(delivery.AskGo, delivery.AskCheck)
+	cons, err := b.js.CreateOrUpdateConsumer(b.ctx, seat, jetstream.ConsumerConfig{Durable: worker, FilterSubject: subject, AckPolicy: jetstream.AckExplicitPolicy})
+	if err != nil {
+		t.Fatal(err)
+	}
+	steps := b.buckets[delivery.StepsBucket(goBuildModule)]
+	h := &holder{}
+	cc, err := cons.Consume(func(m jetstream.Msg) {
+		a, err := delivery.DecodeStepAsk(m.Data())
+		if err != nil || a.Compose == nil {
+			t.Errorf("the seat was put an ask that is not a compose: %v %s", err, m.Data())
+			m.Ack()
+			return
+		}
+		h.mu.Lock()
+		h.asks = append(h.asks, a)
+		h.mu.Unlock()
+		req, err := composer.Decode(a.Compose.Request)
+		if err == nil {
+			err = composer.Fill(&req, func(digest string) ([]byte, error) {
+				key, _ := delivery.ManifestsKey(digest)
+				e, err := b.buckets[delivery.ManifestsBucket].Get(b.ctx, key)
+				if err != nil {
+					return nil, err
+				}
+				man, err := delivery.DecodeManifest(e.Value())
+				return []byte(man.JSON), err
+			})
+		}
+		step := delivery.Step{ID: a.ID, Tool: "compose", Node: "ace", State: delivery.StepPassed, Started: time.Now().UTC(), Ended: time.Now().UTC()}
+		if err != nil {
+			step.State, step.Reason = delivery.StepFailed, err.Error()
+		} else if v, err := composer.Run(req); err != nil {
+			step.State, step.Reason = delivery.StepFailed, err.Error()
+		} else {
+			vr, _ := json.Marshal(v)
+			step.Result, _ = json.Marshal(delivery.ComposeAnswer{Built: true, Key: "composer-0123456789abcdef", Verdict: vr})
+		}
+		raw, err := delivery.EncodeStep(step)
+		if err != nil {
+			t.Errorf("the step cannot be written: %v", err)
+		}
+		key, _ := delivery.AskStepKey("ace", a.ID)
+		if _, err := steps.Put(b.ctx, key, raw); err != nil {
+			t.Errorf("the step %s: %v", key, err)
+		}
+		m.Ack()
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(cc.Stop)
+	return h
+}
+
+// 1622/AC1-AC3 and 1623 on the bus: started whole, the assigner composes a group holding a mesh-sdk pull request by
+// asking the Go build seat for the composer at the member's head, and for the composer at the mesh-sdk the running
+// controller pins; it waits naming the asks while they run, writes the verdict from their answers naming the mesh-sdk the
+// head's composer was built with, and asks nothing more on a later pass of the same heads.
+func TestAGroupWithAnSDKMemberIsComposedByTheComposerAtItsHead(t *testing.T) {
+	b := startBed(t, "")
+	trunk := b.manifest(t, "web", `{"module":"web","version":"1"}`)
+	changed := b.manifest(t, "web", `{"module":"web","version":"2"}`)
+	mod, err := delivery.EncodeModule(delivery.Module{FP2: delivery.SourcePrefix2 + strings.Repeat("a", 64), Manifest: trunk, Repository: "novox/web", Commit: strings.Repeat("1", 40), Path: delivery.RootPath})
+	if err != nil {
+		t.Fatal(err)
+	}
+	b.putJSON(t, delivery.ModulesBucket, "web", mod)
+	b.putJSON(t, provider.AssignmentsBucket, "g14.web", []byte(`{"node":"g14","module":"web","build":"build-1"}`))
+	running := strings.Repeat("3", 40)
+	b.putJSON(t, provider.AssignmentsBucket, "novox.mesh-controller", []byte(`{"node":"novox","module":"mesh-controller","build":"c-1"}`))
+	b.putJSON(t, delivery.BuildsBucket, delivery.BuildKey("mesh-controller", "c-1"), []byte(`{"id":"c-1","repository":"novox/mesh-controller","commit":"`+running+`"}`))
+	sdkHead := strings.Repeat("4", 40)
+	heads := []delivery.Head{{Repository: "novox/mesh-sdk", Number: 80, Head: sdkHead}, {Repository: "novox/web", Number: 7, Head: strings.Repeat("2", 40)}}
+	hash, _ := delivery.HeadsHash(heads)
+	pulls := []delivery.CandidatePull{{Head: heads[0]}, {Head: heads[1],
+		Modules: []string{"web"}, Fps: map[string]delivery.CandidateModule{"web": {FP2: delivery.SourcePrefix2 + strings.Repeat("b", 64), Manifest: changed}}}}
+	cand, err := delivery.EncodeCandidate(delivery.Candidate{Group: "feat/x", HeadsHash: hash, Pulls: pulls, FPSet: delivery.FingerprintSet(pulls)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	ckey, _ := delivery.CandidatesKey("feat/x")
+	b.putJSON(t, delivery.CandidatesBucket, ckey, cand)
+	b.start(t)
+	key, _ := delivery.CompositionKey("feat/x", hash)
+	// No holder takes the asks yet: the composition waits, naming them.
+	waits := b.verdict(t, key, func(v delivery.CompositionVerdict) bool { return v.State == delivery.CompositionWaits })
+	if !strings.Contains(waits.Words, "mesh-sdk "+sdkHead) || !strings.Contains(waits.Words, "mesh-controller at "+running) {
+		t.Fatalf("the waiting composition says %q, want the head's and the running controller's asks", waits.Words)
+	}
+	h := b.hold(t)
+	v := b.verdict(t, key, func(v delivery.CompositionVerdict) bool { return v.State != delivery.CompositionWaits })
+	if v.State != delivery.CompositionPass || len(v.Machines) != 1 || v.Machines[0] != "g14" || v.SDK == "" {
+		t.Fatalf("composition = %+v, want a pass on g14", v)
+	}
+	asks := h.taken()
+	if len(asks) != 2 {
+		t.Fatalf("the seat was asked %d composes, want two (the head, the running controller)", len(asks))
+	}
+	byRepo := map[string]delivery.StepAsk{}
+	for _, a := range asks {
+		byRepo[a.Compose.Fetch["repository"].(string)] = a
+		if a.Asker != AssignerModule || a.Kind != delivery.AskCheck || a.Language != delivery.AskGo {
+			t.Errorf("ask = %+v", a)
+		}
+	}
+	if a := byRepo["novox/mesh-sdk"]; a.Compose == nil || a.Compose.Fetch["commit"] != sdkHead || a.Compose.Dir != "go" {
+		t.Errorf("the head's ask = %+v", a.Compose)
+	}
+	if a := byRepo["novox/mesh-controller"]; a.Compose == nil || a.Compose.Fetch["commit"] != running || a.Compose.Dir != delivery.RootPath {
+		t.Errorf("the running controller's ask = %+v", a.Compose)
+	}
+	// A later pass of the same heads (woken by any change) asks nothing more.
+	on, _ := delivery.EncodePresence(delivery.Presence{Online: true})
+	b.putJSON(t, delivery.PresenceBucket, "ace", on)
+	time.Sleep(500 * time.Millisecond)
+	if n := len(h.taken()); n != 2 {
+		t.Errorf("a later pass of the same heads asked again: %d asks", n)
+	}
+}
+
+// verdict waits for the composition under key to satisfy ok, and answers it.
+func (b *bed) verdict(t *testing.T, key string, ok func(delivery.CompositionVerdict) bool) delivery.CompositionVerdict {
+	t.Helper()
+	var last delivery.CompositionVerdict
+	for deadline := time.Now().Add(10 * time.Second); time.Now().Before(deadline); time.Sleep(50 * time.Millisecond) {
+		e, err := b.buckets[delivery.AssignerCompositionsBucket].Get(b.ctx, key)
+		if errors.Is(err, jetstream.ErrKeyNotFound) {
+			continue
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		if last, err = delivery.DecodeCompositionVerdict(e.Value()); err != nil {
+			t.Fatal(err)
+		}
+		if ok(last) {
+			return last
+		}
+	}
+	t.Fatalf("no composition under %s as wanted after 10 s; last %+v", key, last)
+	return last
+}
diff --git a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/integration_test.go b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/integration_test.go
index f04d60c..d017c8d 100644
--- a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/integration_test.go
+++ b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/integration_test.go
@@ -73,6 +73,12 @@ func startBed(t *testing.T, leaveOut string) *bed {
 	if _, err := js.CreateStream(ctx, jetstream.StreamConfig{Name: "EVENTS", Subjects: []string{"mesh.mod.*.event.>"}}); err != nil {
 		t.Fatal(err)
 	}
+	// The Go build seat's work queue, as the broker derives it from the seat's accepted kinds (delivery.AskStream).
+	seat, _ := delivery.AskStream(delivery.AskGo)
+	if _, err := js.CreateStream(ctx, jetstream.StreamConfig{Name: seat, Subjects: []string{"mesh.seat.node-build-go.accept.>"},
+		Retention: jetstream.WorkQueuePolicy}); err != nil {
+		t.Fatal(err)
+	}
 	return b
 }
 
@@ -213,6 +219,13 @@ func TestAChangeIsComposedOnTheBus(t *testing.T) {
 	if took := time.Since(began); took > 5*time.Second {
 		t.Errorf("the composition took %s", took)
 	}
+	// 1622/AC3: a group without a mesh-sdk member builds no composer: no ask is put on the Go build seat.
+	seat, _ := delivery.AskStream(delivery.AskGo)
+	if st, err := b.js.Stream(b.ctx, seat); err != nil {
+		t.Fatal(err)
+	} else if info, err := st.Info(b.ctx); err != nil || info.State.Msgs != 0 {
+		t.Errorf("a group without a mesh-sdk member put %d asks on the Go build seat (%v)", info.State.Msgs, err)
+	}
 }
 
 // get waits for a key of one of the bed's buckets and answers its value.
diff --git a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/main.go b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/main.go
index 8900bd8..0ca3642 100644
--- a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/main.go
+++ b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/main.go
@@ -22,6 +22,7 @@ import (
 
 	"git.novox.be/novox/mesh-sdk/go/announce"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"git.novox.be/novox/mesh-sdk/go/delivery/buildrun"
 	"git.novox.be/novox/mesh-sdk/go/process"
 	"git.novox.be/novox/mesh-sdk/go/provider"
 )
@@ -47,8 +48,13 @@ var readBuckets = []string{
 	delivery.ModulesBucket,
 	delivery.PresenceBucket,
 	delivery.HaveBucket,
+	delivery.StepsBucket(goBuildModule),
 }
 
+// goBuildModule is the Go build module, which takes the assigner's compose asks on its seat's check stream and writes
+// their steps in its steps state (novox/hq ADR 0482 clause 4).
+const goBuildModule = "mesh-build-go"
+
 // ownBuckets are the assigner's own states, as its manifest declares them.
 var ownBuckets = []string{delivery.AssignerAsksBucket, delivery.AssignerCompositionsBucket, delivery.AssignerDifferencesBucket, delivery.AssignerWantsBucket}
 
@@ -124,7 +130,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 		logger.Printf("what the assigner keeps could not be said on the bus, so its readers are not granted until it is: %v", err)
 	}
 	logger.Printf("started on the account %s, reading %d buckets", account.User, len(kvs))
-	return follow(ctx, kvs, logger)
+	return follow(ctx, js, kvs, logger)
 }
 
 // watched are the buckets whose every value a pass reads; the manifests are read by digest when a pass needs one, and
@@ -145,7 +151,7 @@ var watched = []string{
 
 // follow watches every bucket a pass reads and makes a pass at start, once every watch handed what stands, and after
 // each change; changes that come during a pass make one more. It returns when ctx ends or a watch ends.
-func follow(ctx context.Context, kvs buckets, logger *log.Logger) error {
+func follow(ctx context.Context, js jetstream.JetStream, kvs buckets, logger *log.Logger) error {
 	w := newWorld()
 	dirty := make(chan struct{}, 1)
 	wake := func() {
@@ -210,6 +216,7 @@ func follow(ctx context.Context, kvs buckets, logger *log.Logger) error {
 		}
 	}
 	p := &passer{kvs: kvs, w: w, logf: logger.Printf}
+	p.asks = newComposeAsks(ctx, buildrun.Asker{Publisher: buildrun.JetStreamAsks{JS: js}, Waiter: buildrun.JetStreamAsks{JS: js}}, wake, logger.Printf)
 	wake()
 	for {
 		select {
diff --git a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/pass.go b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/pass.go
index a626f95..c5855fd 100644
--- a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/pass.go
+++ b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/pass.go
@@ -19,6 +19,7 @@ type passer struct {
 	w    *world
 	logf func(string, ...any)
 	sdk  string
+	asks *composeAsks
 }
 
 func (p *passer) pass(ctx context.Context) {
@@ -162,6 +163,10 @@ func (p *passer) compositions(ctx context.Context) {
 	written := p.w.read(delivery.AssignerCompositionsBucket)
 	trunk := p.w.trunk()
 	var mesh *assign.Mesh
+	seen := map[string]bool{}
+	if p.asks != nil {
+		defer p.asks.sweep(seen)
+	}
 	for key, value := range p.w.read(delivery.CandidatesBucket) {
 		c, err := delivery.DecodeCandidate(value)
 		if err != nil {
@@ -183,7 +188,12 @@ func (p *passer) compositions(ctx context.Context) {
 				m := p.w.mesh(ctx, kvGetter(p.kvs[delivery.ManifestsBucket]))
 				mesh = &m
 			}
-			v = assign.Compose(*mesh, ch, p.sdk, time.Since(began))
+			if ch.SDKHead != "" && p.asks != nil {
+				// A group with a mesh-sdk member is composed by the composer built at its head (ADR 0482 clauses 3 to 5).
+				v = p.composeAtHead(ctx, *mesh, ch, seen, began)
+			} else {
+				v = assign.Compose(*mesh, ch, p.sdk, time.Since(began))
+			}
 		}
 		if before, err := delivery.DecodeCompositionVerdict(written[out]); err == nil {
 			before.Took, before.V = v.Took, 0
diff --git a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/world.go b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/world.go
index fcd91d8..2ea9b7c 100644
--- a/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/world.go
+++ b/modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/world.go
@@ -193,12 +193,13 @@ func (w *world) trunk() map[string]delivery.Module {
 
 // mesh is the mesh as the trunk holds it, for the composition check.
 func (w *world) mesh(ctx context.Context, manifests getter) assign.Mesh {
-	m := assign.Mesh{Manifests: map[string]catalogue.Manifest{}, Unreadable: map[string]string{}}
+	m := assign.Mesh{Manifests: map[string]catalogue.Manifest{}, Unreadable: map[string]string{}, Digests: map[string]string{}}
 	m.Assigned, _ = w.assigned()
 	for name, mod := range w.trunk() {
 		if mod.Manifest == "" {
 			continue
 		}
+		m.Digests[name] = mod.Manifest
 		p := w.manifest(ctx, manifests, mod.Manifest)
 		if p.err != "" {
 			m.Unreadable[name] = p.err
@@ -211,12 +212,14 @@ func (w *world) mesh(ctx context.Context, manifests getter) assign.Mesh {
 
 // change is a candidate's change for the composition check, or why the composition waits for it.
 func (w *world) change(ctx context.Context, manifests getter, c delivery.Candidate, trunk map[string]delivery.Module) (assign.Change, string) {
-	ch := assign.Change{Group: c.Group, Heads: c.HeadsHash, Manifests: map[string]catalogue.Manifest{}, Unreadable: map[string]string{}, Twice: map[string][]string{}}
+	ch := assign.Change{Group: c.Group, Heads: c.HeadsHash, Manifests: map[string]catalogue.Manifest{}, Unreadable: map[string]string{},
+		Twice: map[string][]string{}, Digests: map[string]string{}, Members: map[string]string{}}
 	definedIn := map[string]map[string]bool{}
 	for _, p := range c.Pulls {
 		if p.Repository == assign.SDKRepository {
 			ch.SDKHead = p.Head.Head
 		}
+		ch.Members[p.Repository] = p.Head.Head
 		for _, name := range p.Modules {
 			fp, ok := p.Fps[name]
 			if !ok || fp.Manifest == "" {
@@ -233,6 +236,7 @@ func (w *world) change(ctx context.Context, manifests getter, c delivery.Candida
 			if strings.HasPrefix(got.err, "the indexer holds no manifest") {
 				return ch, got.err
 			}
+			ch.Digests[name] = fp.Manifest
 			if got.err != "" {
 				ch.Unreadable[name] = got.err
 				continue
@@ -368,3 +372,31 @@ func (w *world) sentFP(ctx context.Context, get getter, module, build string) st
 	w.mu.Unlock()
 	return rec.SourceFingerprint
 }
+
+// runningCommits is, of every open assignment of module, the commit of the build it names, read from the controller's
+// build record (`mesh-controller_builds` `<module>.build.<id>`), distinct and sorted: the commits a reader of the
+// manifests runs at (novox/hq ADR 0482 clause 5, issue 1623). A build whose record names no full commit is left out.
+func (w *world) runningCommits(ctx context.Context, get getter, module string) []string {
+	_, all := w.assigned()
+	seen := map[string]bool{}
+	for _, a := range all {
+		if a.Module != module || a.Build == "" {
+			continue
+		}
+		value, err := get(ctx, delivery.BuildKey(module, a.Build))
+		if err != nil {
+			continue
+		}
+		var rec delivery.BuildRecord
+		if json.Unmarshal(value, &rec) != nil || len(rec.Commit) != 40 {
+			continue
+		}
+		seen[rec.Commit] = true
+	}
+	out := make([]string, 0, len(seen))
+	for c := range seen {
+		out = append(out, c)
+	}
+	sort.Strings(out)
+	return out
+}
diff --git a/modules/mesh-delivery-assigner/go.mod b/modules/mesh-delivery-assigner/go.mod
index 1408845..892c1d3 100644
--- a/modules/mesh-delivery-assigner/go.mod
+++ b/modules/mesh-delivery-assigner/go.mod
@@ -3,7 +3,7 @@ module mesh-delivery-assigner
 go 1.26.0
 
 require (
-	git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023150321-a2c813a6b468
+	git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024021031-66bf26c0d721
 	github.com/nats-io/nats-server/v2 v2.11.17
 	github.com/nats-io/nats.go v1.54.0
 )
diff --git a/modules/mesh-delivery-assigner/go.sum b/modules/mesh-delivery-assigner/go.sum
index 2a3e9f3..8bc73fe 100644
--- a/modules/mesh-delivery-assigner/go.sum
+++ b/modules/mesh-delivery-assigner/go.sum
@@ -1,5 +1,9 @@
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023150321-a2c813a6b468 h1:UcMee+IV2ii5E4dRZdQUaSaeh9IedTFYumuQbBiB0k0=
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023150321-a2c813a6b468/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024015135-4784762a70ce h1:9pNnygyhuYIpVs0HPzgheGLNjPXigGGQpnFTp8lw+X4=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024015135-4784762a70ce/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024021031-66bf26c0d721 h1:N//dDGmry1F3HKeBTWd+z/SfK+wb8bUUMJF0OlgR0/g=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024021031-66bf26c0d721/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
 github.com/antithesishq/antithesis-sdk-go v0.7.0-default-no-op h1:Z/MZK75wC/NSrkgqeNIa7jexam9uWzhLmFTSCPI/kn0=
 github.com/antithesishq/antithesis-sdk-go v0.7.0-default-no-op/go.mod h1:FQyySiasQQM8735Ddel3MRojmy4dA1IqCeyJ5jmPMbI=
 github.com/google/go-tpm v0.9.8 h1:slArAR9Ft+1ybZu0lBwpSmpwhRXaa85hWtMinMyRAWo=
diff --git a/modules/mesh-delivery-assigner/internal/assign/compose.go b/modules/mesh-delivery-assigner/internal/assign/compose.go
index d609c87..b1f26e4 100644
--- a/modules/mesh-delivery-assigner/internal/assign/compose.go
+++ b/modules/mesh-delivery-assigner/internal/assign/compose.go
@@ -32,6 +32,9 @@ type Mesh struct {
 	Manifests  map[string]catalogue.Manifest
 	Unreadable map[string]string
 	Assigned   map[string][]string
+	// Digests is every trunk module's manifest digest in the indexer's manifests bucket, readable or not: what a
+	// composer at another mesh-sdk reads (issue 1622).
+	Digests map[string]string
 }
 
 // Change is a group's change as the indexer's candidate holds it: the manifest of every module it moves or adds, those
@@ -45,6 +48,10 @@ type Change struct {
 	Twice        map[string][]string
 	SDKHead      string
 	Rebuilt      int
+	// Digests is the manifest digest of every module the change moves or adds, readable by the assigner or not, and
+	// Members the head of each member by repository: what a composer at the group's mesh-sdk reads (issues 1622, 1623).
+	Digests map[string]string
+	Members map[string]string
 }
 
 // Touched is the machines a change touches (ADR 0446 clause 3): those running a module the change moves, adds or
@@ -167,13 +174,19 @@ func Compose(m Mesh, c Change, sdk string, took time.Duration) delivery.Composit
 			failAt(node, fmt.Sprintf("%s composes on the trunk and not with the change: %v", node, now))
 		}
 	}
-	if c.Rebuilt > delivery.CompositionRebuildWarning {
-		v.Warnings = append(v.Warnings, fmt.Sprintf("the merge rebuilds %d modules, more than %d", c.Rebuilt, delivery.CompositionRebuildWarning))
-	}
 	if c.SDKHead != "" {
 		v.Warnings = append(v.Warnings, fmt.Sprintf("composed with the assigner's own mesh-sdk %s, not the group's mesh-sdk head %s, "+
 			"and a field the running controller cannot read is not judged (issues 1621, 1622, 1623)", sdk, c.SDKHead))
 	}
+	return finish(v, m, c, machines, fails)
+}
+
+// finish adds what every composition says beside its own judgement (more modules rebuilt than CompositionRebuildWarning,
+// what is not judged, the machines not composed, a change that touches none) and its state from fails.
+func finish(v delivery.CompositionVerdict, m Mesh, c Change, machines []string, fails []string) delivery.CompositionVerdict {
+	if c.Rebuilt > delivery.CompositionRebuildWarning {
+		v.Warnings = append(v.Warnings, fmt.Sprintf("the merge rebuilds %d modules, more than %d", c.Rebuilt, delivery.CompositionRebuildWarning))
+	}
 	v.Warnings = append(v.Warnings, NotJudged)
 	if could := CouldRun(m, c, machines); len(could) > 0 {
 		v.Warnings = append(v.Warnings, "not judged until the facts snapshot is on the bus (issue 1461, ADR 0537 clause 1): "+
diff --git a/modules/mesh-delivery-assigner/internal/assign/composer.go b/modules/mesh-delivery-assigner/internal/assign/composer.go
new file mode 100644
index 0000000..b8ccb02
--- /dev/null
+++ b/modules/mesh-delivery-assigner/internal/assign/composer.go
@@ -0,0 +1,241 @@
+package assign
+
+import (
+	"encoding/json"
+	"fmt"
+	"slices"
+	"sort"
+	"strings"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/compose"
+	"git.novox.be/novox/mesh-sdk/go/composer"
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+// The cases of a group's composition at its mesh-sdk head: the trunk as it is, and the trunk with the change.
+const (
+	CaseTrunk  = "trunk"
+	CaseChange = "change"
+)
+
+// SDKDir is the directory of mesh-sdk's Go module in its repository, where the composer is built at a head; a reader's
+// composer is built at its repository's root, at the mesh-sdk its go.mod pins.
+const SDKDir = "go"
+
+// The modules whose mesh-sdk reads the manifests a plan sends (ADR 0482 clause 5): the controller composes and sends
+// them, the node-engine applies them. Each is judged at its repository.
+var ReaderModules = map[string]string{"mesh-controller": "novox/mesh-controller", "mesh-host": "novox/mesh-host"}
+
+// HeadJob is the compose job of a group with a mesh-sdk member (ADR 0482 clauses 3 and 4, issue 1622): the composer
+// built at the member's head composes the machines the change touches, as the trunk holds them and as the change leaves
+// them, every manifest named by its digest in the indexer's bucket, the seats held from the claims (the assigner's input).
+func HeadJob(m Mesh, c Change) (delivery.ComposeJob, error) {
+	trunk, change := map[string]composer.Manifest{}, map[string]composer.Manifest{}
+	for name, digest := range m.Digests {
+		trunk[name] = composer.Manifest{Digest: digest}
+		change[name] = composer.Manifest{Digest: digest}
+	}
+	for name, digest := range c.Digests {
+		change[name] = composer.Manifest{Digest: digest}
+	}
+	for _, name := range c.Removed {
+		delete(change, name)
+	}
+	req := composer.Request{V: composer.Version, Input: plainInput(m.Assigned), Depth: composer.DepthResolve,
+		HoldingsFromClaims: true, Machines: Touched(m, c),
+		Cases: []composer.Case{{Name: CaseTrunk, Manifests: trunk}, {Name: CaseChange, Manifests: change}}}
+	return job(SDKRepository, c.SDKHead, SDKDir, req, "the mesh-sdk head of "+c.Group)
+}
+
+// ReaderJob is the compose job that reads the manifests a group changes with the mesh-sdk a reader module pins at
+// commit (ADR 0482 clause 5, issue 1623): no machine is composed, every changed manifest is only read.
+func ReaderJob(c Change, module, repository, commit string) (delivery.ComposeJob, error) {
+	changed := map[string]composer.Manifest{}
+	for name, digest := range c.Digests {
+		changed[name] = composer.Manifest{Digest: digest}
+	}
+	req := composer.Request{V: composer.Version, Cases: []composer.Case{{Name: CaseChange, Manifests: changed}}}
+	return job(repository, commit, delivery.RootPath, req, module+" at "+commit)
+}
+
+func job(repository, commit, dir string, req composer.Request, reader string) (delivery.ComposeJob, error) {
+	raw, err := json.Marshal(req)
+	if err != nil {
+		return delivery.ComposeJob{}, err
+	}
+	return delivery.ComposeJob{Fetch: map[string]any{"repository": repository, "commit": commit}, Dir: dir,
+		Package: composer.Package, Request: raw, Reader: reader}, nil
+}
+
+// plainInput is the composition's input without its catalogue: the nodes by name with what is assigned to them (the
+// composer derives the seats held from the claims, as input does).
+func plainInput(assigned map[string][]string) compose.Input {
+	var in compose.Input
+	nodes := make([]string, 0, len(assigned))
+	for n := range assigned {
+		nodes = append(nodes, n)
+	}
+	sort.Strings(nodes)
+	for _, n := range nodes {
+		in.Nodes = append(in.Nodes, compose.Node{Name: n, Assigned: slices.Clone(assigned[n])})
+	}
+	return in
+}
+
+// Ran is one compose ask's end: its step's state and reason, and the composer's verdict when the step passed.
+type Ran struct {
+	State   string
+	Reason  string
+	Verdict *composer.Verdict
+}
+
+// RanOf reads a compose ask's end from its step value; a step that passed with an answer that cannot be read is a
+// failure, named.
+func RanOf(step delivery.Step) Ran {
+	r := Ran{State: step.State, Reason: step.Reason}
+	if step.State != delivery.StepPassed {
+		return r
+	}
+	var a delivery.ComposeAnswer
+	var v composer.Verdict
+	if err := json.Unmarshal(step.Result, &a); err != nil {
+		return Ran{State: delivery.StepFailed, Reason: "the compose step's answer cannot be read: " + err.Error()}
+	}
+	if err := json.Unmarshal(a.Verdict, &v); err != nil || v.V != composer.Version {
+		return Ran{State: delivery.StepFailed, Reason: fmt.Sprintf("the composer's verdict cannot be read (version %d): %v", v.V, err)}
+	}
+	r.Verdict = &v
+	return r
+}
+
+func caseOf(v *composer.Verdict, name string) (composer.CaseVerdict, bool) {
+	if v == nil {
+		return composer.CaseVerdict{}, false
+	}
+	for _, c := range v.Cases {
+		if c.Name == name {
+			return c, true
+		}
+	}
+	return composer.CaseVerdict{}, false
+}
+
+// Reader is one module whose mesh-sdk reads the group's manifests, at the commit judged: the group's member head when
+// the group holds one in its repository, else a build its node runs.
+type Reader struct {
+	Module, Commit string
+	Member         bool
+	Ran            Ran
+}
+
+// FromComposer is the composition check of a group with a mesh-sdk member, judged from the composer built at its head
+// (ADR 0482 clauses 3 to 5, issues 1622 and 1623): the same failures as Compose, read from the composer's two cases, the
+// verdict naming the mesh-sdk the composer was built with; a trunk manifest the head's mesh-sdk refuses fails too (the
+// group would deliver a reader that refuses it); and each reader that refuses a changed manifest the head reads fails
+// it, naming the module and its words, unless the reader's mesh-sdk has no composer to ask (before issue 1621), which is
+// said and judged nothing.
+func FromComposer(m Mesh, c Change, head Ran, readers []Reader, took time.Duration) delivery.CompositionVerdict {
+	v := delivery.CompositionVerdict{Group: c.Group, Heads: c.Heads, SDK: "mesh-sdk " + c.SDKHead, Took: took.Milliseconds()}
+	machines := Touched(m, c)
+	v.Machines = machines
+	var fails []string
+	if head.State != delivery.StepPassed {
+		v.Words = fmt.Sprintf("the composer at mesh-sdk %s did not run: %s %s", c.SDKHead, head.State, head.Reason)
+		v.State = delivery.CompositionFail
+		return v
+	}
+	if head.Verdict.SDK != "" {
+		v.SDK = head.Verdict.SDK
+	}
+	trunk, okT := caseOf(head.Verdict, CaseTrunk)
+	change, okC := caseOf(head.Verdict, CaseChange)
+	if !okT || !okC {
+		v.State, v.Words = delivery.CompositionFail, "the composer's verdict holds no trunk or no change case"
+		return v
+	}
+	failAt := func(machine, words string) {
+		if v.Machine == "" && machine != "" {
+			v.Machine = machine
+		}
+		fails = append(fails, words)
+	}
+	for _, name := range sortedKeys(c.Digests) {
+		if r := change.Manifests[name]; !r.Accepted {
+			failAt("", fmt.Sprintf("the manifest of %s cannot be read by mesh-sdk %s: %s", name, c.SDKHead, r.Refused))
+		}
+	}
+	for _, name := range sortedKeys(trunk.Manifests) {
+		if _, changed := c.Digests[name]; changed || slices.Contains(c.Removed, name) || trunk.Manifests[name].Accepted {
+			continue
+		}
+		if _, before := m.Unreadable[name]; before {
+			continue // already unreadable on the trunk: not the group's doing
+		}
+		failAt("", fmt.Sprintf("mesh-sdk %s refuses the trunk's manifest of %s, which the group does not change: %s", c.SDKHead, name, trunk.Manifests[name].Refused))
+	}
+	for _, name := range sortedKeys(c.Twice) {
+		failAt("", fmt.Sprintf("%s is defined twice, in %s", name, strings.Join(c.Twice[name], " and ")))
+	}
+	for _, node := range machines {
+		for _, name := range c.Removed {
+			if slices.Contains(m.Assigned[node], name) {
+				failAt(node, fmt.Sprintf("%s runs %s, which the change removes from its source", node, name))
+			}
+		}
+		before, now := trunk.Machines[node], change.Machines[node]
+		switch {
+		case now.Composes:
+		case !before.Composes:
+			v.Warnings = append(v.Warnings, fmt.Sprintf("%s does not compose on the trunk either, so the change is not blamed: %s", node, before.Reason))
+		default:
+			failAt(node, fmt.Sprintf("%s composes on the trunk and not with the change: %s", node, now.Reason))
+		}
+	}
+	readerFails, readerWarnings := judgeReaders(c, change, readers)
+	fails = append(fails, readerFails...)
+	v.Warnings = append(v.Warnings, readerWarnings...)
+	return finish(v, m, c, machines, fails)
+}
+
+// NoComposer says whether a compose step failed because the mesh-sdk it was built at has no composer command (a mesh-sdk
+// from before issue 1621): Go's words for a package its module does not hold.
+func NoComposer(reason string) bool {
+	for _, words := range []string{"cannot find package", "no required module provides package", "is not in std", "cannot find module providing package", "does not contain package"} {
+		if strings.Contains(reason, words) {
+			return true
+		}
+	}
+	return false
+}
+
+func judgeReaders(c Change, head composer.CaseVerdict, readers []Reader) (fails, warnings []string) {
+	for _, r := range readers {
+		at := r.Module + " at " + r.Commit
+		if r.Member {
+			at = r.Module + "'s member head " + r.Commit
+		}
+		if r.Ran.State != delivery.StepPassed {
+			if NoComposer(r.Ran.Reason) {
+				warnings = append(warnings, fmt.Sprintf("not judged whether %s reads the group's manifests: the mesh-sdk it pins has no "+
+					"composer command (before issue 1621)", at))
+				continue
+			}
+			fails = append(fails, fmt.Sprintf("the composer of the mesh-sdk %s pins did not run: %s %s", at, r.Ran.State, r.Ran.Reason))
+			continue
+		}
+		read, ok := caseOf(r.Ran.Verdict, CaseChange)
+		if !ok {
+			fails = append(fails, fmt.Sprintf("the composer of the mesh-sdk %s pins answered no change case", at))
+			continue
+		}
+		for _, name := range sortedKeys(c.Digests) {
+			if !head.Manifests[name].Accepted || read.Manifests[name].Accepted {
+				continue
+			}
+			fails = append(fails, fmt.Sprintf("the manifest of %s uses what %s (mesh-sdk %s) cannot read: %s; the group must move %s onto "+
+				"mesh-sdk %s", name, at, r.Ran.Verdict.SDK, read.Manifests[name].Refused, r.Module, c.SDKHead))
+		}
+	}
+	return fails, warnings
+}
diff --git a/modules/mesh-delivery-assigner/internal/assign/composer_test.go b/modules/mesh-delivery-assigner/internal/assign/composer_test.go
new file mode 100644
index 0000000..4b709c8
--- /dev/null
+++ b/modules/mesh-delivery-assigner/internal/assign/composer_test.go
@@ -0,0 +1,184 @@
+package assign
+
+import (
+	"encoding/json"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/catalogue"
+	"git.novox.be/novox/mesh-sdk/go/composer"
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+var testTime = time.Date(2026, 10, 24, 3, 0, 0, 0, time.UTC)
+
+const sdkHead = "1111111111111111111111111111111111111111"
+
+// raws are the manifests of meshOf as their repositories hold them, by digest, as the indexer keeps them; `web2` is web
+// with a field `shares` that the assigner's own mesh-sdk does not read (the shape of issue 1593).
+var raws = map[string]string{
+	"d-mail":  `{"module": "mail", "version": "1", "requires": ["postgres-database"]}`,
+	"d-store": `{"module": "store", "version": "1", "provides": [{"name": "postgres-database"}]}`,
+	"d-web":   `{"module": "web", "version": "1"}`,
+	"d-web2":  `{"module": "web", "version": "2", "shares": ["photos"]}`,
+}
+
+// asTheHeadReads is how mesh-sdk at the group's head reads each manifest: as the assigner's own reads it, but for the
+// field it adds, which it accepts. The head's composer is built and run by the Go build module (ADR 0482 clause 4); here
+// its verdict is made by mesh-sdk's composer.Run over the same request, with `web2` read as the head reads it.
+var asTheHeadReads = map[string]string{"d-web2": `{"module": "web", "version": "2"}`}
+
+func digestedMesh() Mesh {
+	m := meshOf()
+	m.Digests = map[string]string{"mail": "d-mail", "store": "d-store", "web": "d-web"}
+	return m
+}
+
+// webShares is a group of mesh-sdk and mesh-catalog whose catalogue member gives web the field only the SDK head reads:
+// the assigner's own reader refuses it (Unreadable), as the indexer's manifest is read in world.change.
+func webShares(members map[string]string) Change {
+	_, err := catalogue.ParseManifest([]byte(raws["d-web2"]))
+	return Change{Group: "g", Heads: "0123456789ab", SDKHead: sdkHead, Manifests: map[string]catalogue.Manifest{},
+		Unreadable: map[string]string{"web": err.Error()}, Digests: map[string]string{"web": "d-web2"}, Members: members}
+}
+
+// run is the composer's step for a job, as the build module answers it: the request filled from the indexer's digests
+// (with read standing in for how that mesh-sdk reads a digest), run, and its verdict put in a passed step.
+func run(t *testing.T, job delivery.ComposeJob, read map[string]string) Ran {
+	t.Helper()
+	req, err := composer.Decode(job.Request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := composer.Fill(&req, func(d string) ([]byte, error) {
+		if r, ok := read[d]; ok {
+			return []byte(r), nil
+		}
+		return []byte(raws[d]), nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+	v, err := composer.Run(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	v.SDK = "(devel) " + job.Fetch["commit"].(string)
+	vr, _ := json.Marshal(v)
+	ans, _ := json.Marshal(delivery.ComposeAnswer{Built: true, Key: "composer-0", Verdict: vr})
+	return RanOf(delivery.Step{State: delivery.StepPassed, Result: ans})
+}
+
+// 1622/AC1: a group holding a mesh-sdk pull request is composed by the composer built at the member's head: the job
+// fetches mesh-sdk at that head, builds mesh-sdk's composer command in its Go module, and hands it the trunk and the
+// change by digest, never the manifests' bytes, composing the machines the change touches.
+func TestAGroupWithAnSDKMemberAsksTheComposerAtItsHead(t *testing.T) {
+	c := webShares(nil)
+	job, err := HeadJob(digestedMesh(), c)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if job.Fetch["repository"] != SDKRepository || job.Fetch["commit"] != sdkHead || job.Dir != "go" || job.Package != composer.Package {
+		t.Fatalf("job = %+v", job)
+	}
+	ask := delivery.StepAsk{ID: "mh3k2x9qa1b2c5", Kind: delivery.AskCheck, Language: delivery.AskGo, Asker: "mesh-delivery-assigner",
+		At: testTime, Compose: &job}
+	if _, err := delivery.EncodeStepAsk(ask); err != nil {
+		t.Fatalf("the ask is refused by its layout: %v", err)
+	}
+	req, err := composer.Decode(job.Request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(req.Cases) != 2 || req.Cases[0].Name != CaseTrunk || req.Cases[1].Name != CaseChange {
+		t.Fatalf("cases = %+v", req.Cases)
+	}
+	if req.Cases[0].Manifests["web"].Digest != "d-web" || req.Cases[1].Manifests["web"].Digest != "d-web2" || len(req.Cases[1].Manifests["web"].Raw) != 0 {
+		t.Errorf("the cases do not name web by its trunk and its change digest: %+v", req.Cases)
+	}
+	if !slices.Equal(req.Machines, []string{"g14"}) || req.Depth != composer.DepthResolve || !req.HoldingsFromClaims {
+		t.Errorf("request = machines %v, depth %q, holdings from claims %v", req.Machines, req.Depth, req.HoldingsFromClaims)
+	}
+}
+
+// 1622/AC2, AC3: a catalogue member using a field only the SDK head reads passes with the head's composer, and the
+// verdict names the mesh-sdk the composer was built with; with the assigner's own mesh-sdk it fails.
+func TestAFieldTheGroupAddsPassesWithTheHeadsComposer(t *testing.T) {
+	c := webShares(map[string]string{"novox/mesh-controller": "2222222222222222222222222222222222222222"})
+	job, _ := HeadJob(digestedMesh(), c)
+	head := run(t, job, asTheHeadReads)
+	v := FromComposer(digestedMesh(), c, head, nil, 0)
+	if v.State != delivery.CompositionPass || v.SDK != "(devel) "+sdkHead || !slices.Equal(v.Machines, []string{"g14"}) {
+		t.Fatalf("with the head's composer: %+v", v)
+	}
+	if _, err := delivery.EncodeCompositionVerdict(v); err != nil {
+		t.Errorf("the verdict cannot be written: %v", err)
+	}
+	own := Compose(digestedMesh(), c, "v0.1.14-own", 0)
+	if own.State != delivery.CompositionFail || !strings.Contains(own.Words, "shares") {
+		t.Fatalf("with the assigner's own mesh-sdk: %+v, want a fail naming the field", own)
+	}
+}
+
+// 1623/AC1, AC2: the group of mesh-sdk and mesh-catalog adding and using a field, without a controller member, fails
+// naming the field and mesh-controller, whose running build pins an older mesh-sdk; with a controller member pinned to
+// the SDK head (its composer reads what the head reads) it passes.
+func TestAFieldTheRunningControllerCannotReadFailsTheGroup(t *testing.T) {
+	c := webShares(nil)
+	job, _ := HeadJob(digestedMesh(), c)
+	head := run(t, job, asTheHeadReads)
+	running := "3333333333333333333333333333333333333333"
+	rjob, err := ReaderJob(c, "mesh-controller", "novox/mesh-controller", running)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if rjob.Fetch["commit"] != running || rjob.Dir != delivery.RootPath {
+		t.Fatalf("reader job = %+v", rjob)
+	}
+	old := run(t, rjob, nil) // the running controller's mesh-sdk reads web2 as the assigner's own does: refused
+	v := FromComposer(digestedMesh(), c, head, []Reader{{Module: "mesh-controller", Commit: running, Ran: old}}, 0)
+	if v.State != delivery.CompositionFail || !strings.Contains(v.Words, "shares") || !strings.Contains(v.Words, "move mesh-controller onto mesh-sdk "+sdkHead) {
+		t.Fatalf("without a controller member: %+v, want a fail naming the field and mesh-controller", v)
+	}
+	member := "2222222222222222222222222222222222222222"
+	pinned := run(t, rjob, asTheHeadReads)
+	v = FromComposer(digestedMesh(), c, head, []Reader{{Module: "mesh-controller", Commit: member, Member: true, Ran: pinned}}, 0)
+	if v.State != delivery.CompositionPass {
+		t.Fatalf("with a controller member pinned to the head: %+v, want a pass", v)
+	}
+}
+
+// 1623: a reader whose mesh-sdk has no composer command (before issue 1621) is said, not failed; a reader's composer that
+// failed for another cause fails the group, loud.
+func TestAReaderWithNoComposerIsSaidAndAnotherFailureFails(t *testing.T) {
+	c := webShares(nil)
+	job, _ := HeadJob(digestedMesh(), c)
+	head := run(t, job, asTheHeadReads)
+	none := Ran{State: delivery.StepFailed, Reason: "go build: no required module provides package git.novox.be/novox/mesh-sdk/go/cmd/mesh-compose"}
+	v := FromComposer(digestedMesh(), c, head, []Reader{{Module: "mesh-host", Commit: "abc", Ran: none}}, 0)
+	if v.State != delivery.CompositionPass || !hasWarning(v, "mesh-host at abc") || !hasWarning(v, "before issue 1621") {
+		t.Fatalf("a reader with no composer: %+v", v)
+	}
+	broke := Ran{State: delivery.StepFailed, Reason: "the fetch could not reach the forge"}
+	v = FromComposer(digestedMesh(), c, head, []Reader{{Module: "mesh-host", Commit: "abc", Ran: broke}}, 0)
+	if v.State != delivery.CompositionFail || !strings.Contains(v.Words, "could not reach the forge") {
+		t.Fatalf("a reader whose composer failed: %+v", v)
+	}
+}
+
+// 1622: the head's composer that did not run fails the composition naming why, never passes it; a trunk manifest the
+// head's mesh-sdk refuses fails it too, since the group would deliver a reader that refuses it.
+func TestTheHeadsComposerFailingOrRefusingTheTrunkFails(t *testing.T) {
+	c := webShares(nil)
+	v := FromComposer(digestedMesh(), c, Ran{State: delivery.StepRefused, Reason: "the guard refuses it"}, nil, 0)
+	if v.State != delivery.CompositionFail || !strings.Contains(v.Words, "the guard refuses it") || !strings.Contains(v.Words, sdkHead) {
+		t.Fatalf("a head composer that did not run: %+v", v)
+	}
+	job, _ := HeadJob(digestedMesh(), c)
+	strict := map[string]string{"d-web2": asTheHeadReads["d-web2"], "d-mail": `{"module": "mail", "version": "1", "gone": true}`}
+	v = FromComposer(digestedMesh(), c, run(t, job, strict), nil, 0)
+	if v.State != delivery.CompositionFail || !strings.Contains(v.Words, "refuses the trunk's manifest of mail") {
+		t.Fatalf("a head that refuses a trunk manifest: %+v", v)
+	}
+}
diff --git a/modules/mesh-delivery-assigner/manifest_test.go b/modules/mesh-delivery-assigner/manifest_test.go
index 03b879a..db46536 100644
--- a/modules/mesh-delivery-assigner/manifest_test.go
+++ b/modules/mesh-delivery-assigner/manifest_test.go
@@ -32,7 +32,7 @@ func TestManifest(t *testing.T) {
 	}
 	reads := slices.Clone(m.Reads)
 	slices.Sort(reads)
-	want := []string{"mesh-controller.assignments", "mesh-controller.builds", "mesh-controller.grants",
+	want := []string{"mesh-build-go.steps", "mesh-controller.assignments", "mesh-controller.builds", "mesh-controller.grants",
 		"mesh-delivery-builder.builds", "mesh-delivery-indexer.candidates", "mesh-delivery-indexer.manifests",
 		"mesh-delivery-indexer.modules", "mesh-delivery-watchman.presence", "node-engine.have"}
 	if !reflect.DeepEqual(reads, want) {
@@ -51,6 +51,10 @@ func TestManifest(t *testing.T) {
 	if m.SecretsOwner != "mesh-delivery-assigner" {
 		t.Errorf("secrets-owner = %q", m.SecretsOwner)
 	}
+	// 1622: the assigner puts its compose asks on the Go build seat, as the checker puts its checks.
+	if !slices.Equal(m.Uses, []string{"node-build-go"}) {
+		t.Errorf("uses = %v, want exactly node-build-go", m.Uses)
+	}
 	if len(m.Claims) != 0 || len(m.Tools) != 0 || len(m.Invokes) != 0 {
 		t.Errorf("the assigner claims, serves or calls something: claims %v, tools %v, invokes %v", m.Claims, m.Tools, m.Invokes)
 	}
diff --git a/modules/mesh-delivery-assigner/module.json b/modules/mesh-delivery-assigner/module.json
index 418e8fb..b07feba 100644
--- a/modules/mesh-delivery-assigner/module.json
+++ b/modules/mesh-delivery-assigner/module.json
@@ -3,6 +3,7 @@
   "version": "1",
   "slug": "assigner",
   "reads": [
+    "mesh-build-go.steps",
     "mesh-controller.assignments",
     "mesh-controller.builds",
     "mesh-controller.grants",
@@ -13,6 +14,9 @@
     "mesh-delivery-watchman.presence",
     "node-engine.have"
   ],
+  "uses": [
+    "node-build-go"
+  ],
   "state": [
     { "name": "asks", "history": 5 },
     { "name": "compositions", "history": 5 },