diff --git a/modules/mesh-delivery-checker/internal/check/case.go b/modules/mesh-delivery-checker/internal/check/case.go
index cb60980f..882fafaf 100644
--- a/modules/mesh-delivery-checker/internal/check/case.go
+++ b/modules/mesh-delivery-checker/internal/check/case.go
@@ -1,6 +1,7 @@
 package check
 
 import (
+	"encoding/json"
 	"fmt"
 	"maps"
 	"slices"
@@ -45,6 +46,9 @@ type Case struct {
 	Queued time.Time
 	// Try narrows a try's check (its level, its tests); nil for a pull request's.
 	Try *TryOptions
+	// Seats is the seat list the check composes against (state.checkSeats), raw, handed to the composer of a node-engine
+	// member's head; nil when the checker's reader has not read its list.
+	Seats []json.RawMessage
 
 	notesMu sync.Mutex
 	// warnings are, by pull request, what part of its check did not run (Case.warn).
diff --git a/modules/mesh-delivery-checker/internal/check/composition_test.go b/modules/mesh-delivery-checker/internal/check/composition_test.go
index 20a810a0..bafc264a 100644
--- a/modules/mesh-delivery-checker/internal/check/composition_test.go
+++ b/modules/mesh-delivery-checker/internal/check/composition_test.go
@@ -2,6 +2,7 @@ package check
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"maps"
 	"slices"
@@ -22,12 +23,15 @@ type fakeCompositions struct {
 	v     meshwants.CompositionsByGroupHeadsValue
 	err   error
 	asked []string
+	// seats are the seat lists the cases asked carried (Case.Seats), one per ask.
+	seats [][]json.RawMessage
 	// waits answers only when its context ends.
 	waits bool
 }
 
 func (f *fakeCompositions) Compose(ctx context.Context, c *Case, _ buildstep.PackageVersions) (meshwants.CompositionsByGroupHeadsValue, error) {
 	f.asked = append(f.asked, c.Key)
+	f.seats = append(f.seats, c.Seats)
 	if f.waits {
 		<-ctx.Done()
 		return meshwants.CompositionsByGroupHeadsValue{}, ctx.Err()
@@ -57,6 +61,26 @@ func TestAPassingCheckTakesTheAssignersComposition(t *testing.T) {
 	if v.Composition.Fpset == "" || v.Composition.Fpset != cs.FPSet {
 		t.Errorf("the composition names the fingerprint set %q, want the candidate's %q (ADR 0548 §5)", v.Composition.Fpset, cs.FPSet)
 	}
+	// Issue 1966 (the group try of 2026-11-06 10:07): the composition is handed the check's seat list, the judge's, once the
+	// checker's reader read it; a list not read hands none (the composer then judges no machine, ADR 0636 §1).
+	if len(f.seats) != 1 || f.seats[0] != nil {
+		t.Errorf("with no seat list reader the composition was handed %v, want none", f.seats)
+	}
+	for _, read := range []bool{true, false} {
+		b.ex.SeatList = fakeSeatList{read: read}
+		b.judge(t, caseOf(t, pullOf(repoA, 7, head1)))
+		got := f.seats[len(f.seats)-1]
+		if want := 0; read {
+			want = 2
+			if len(got) != want || string(got[0]) != `{"seat":"a"}` {
+				t.Errorf("with the list read the composition was handed %s, want the reader's two definitions", got)
+			}
+		} else if len(got) != want {
+			t.Errorf("with the list not read the composition was handed %s, want none", got)
+		}
+	}
+	b.ex.SeatList = nil
+	f.asked, f.seats = f.asked[:1], f.seats[:1]
 	// ADR 0548 §5: a carry whose fingerprints did not change keeps the composition it passed and asks the assigner nothing.
 	carry := caseOf(t, pullOf(repoA, 7, head1))
 	carry.CarriedFrom, carry.Composition = map[string]string{repoA: trunkA2}, v.Composition
diff --git a/modules/mesh-delivery-checker/internal/check/engines.go b/modules/mesh-delivery-checker/internal/check/engines.go
index a4b0bb1c..600556df 100644
--- a/modules/mesh-delivery-checker/internal/check/engines.go
+++ b/modules/mesh-delivery-checker/internal/check/engines.go
@@ -237,7 +237,7 @@ func (e EngineCompositions) inSandbox(ctx context.Context, c *Case, tagged build
 		return a, fmt.Errorf("the engine of %s answered a composition input that cannot be encoded again: %w", node, err)
 	}
 	req := composer.Request{V: composer.Version, Input: input, Depth: composer.DepthCompose, Machines: []string{node},
-		Cases: []composer.Case{{Name: "change", Manifests: manifests, Verdicts: verdicts}}}
+		Cases: []composer.Case{{Name: "change", Manifests: manifests, Verdicts: verdicts}}, Seats: c.Seats}
 	raw, err := json.Marshal(req)
 	if err != nil {
 		return a, err
diff --git a/modules/mesh-delivery-checker/internal/check/engines_test.go b/modules/mesh-delivery-checker/internal/check/engines_test.go
index 1587dab0..5f41d060 100644
--- a/modules/mesh-delivery-checker/internal/check/engines_test.go
+++ b/modules/mesh-delivery-checker/internal/check/engines_test.go
@@ -152,6 +152,8 @@ func TestTheCompositionIsMadeFromTheEnginesAnswers(t *testing.T) {
 		host := CaseOf(c, map[string]string{repoA: trunkA, nodeEngineRepo: trunkA})
 		withInput := composed("anchor")
 		withInput.Input = map[string]any{"nodes": []any{map[string]any{"name": "anchor"}}}
+		// Issue 1966 (the group try of 2026-11-06 10:07): the composer at the head composes against the check's seat list.
+		host.Seats = []json.RawMessage{json.RawMessage(`{"format":1,"seat":"node-uplink"}`)}
 		e := &fakeEngines{answers: map[string]ne.ComposeCandidateAnswer{"anchor": withInput}}
 		sb := &fakeSandbox{v: func(node string) composer.Verdict {
 			return composer.Verdict{V: composer.Version, Cases: []composer.CaseVerdict{{Name: "change",
@@ -170,7 +172,8 @@ func TestTheCompositionIsMadeFromTheEnginesAnswers(t *testing.T) {
 		var req composer.Request
 		_ = json.Unmarshal(job.Request, &req)
 		if job.Fetch.Repository != nodeEngineRepo || job.Fetch.Commit != head2 || job.Package != composer.Package ||
-			string(req.Input) != `{"nodes":[{"name":"anchor"}]}` || req.Cases[0].Manifests["web"].Digest == "" {
+			string(req.Input) != `{"nodes":[{"name":"anchor"}]}` || req.Cases[0].Manifests["web"].Digest == "" ||
+			len(req.Seats) != 1 || string(req.Seats[0]) != `{"format":1,"seat":"node-uplink"}` {
 			t.Errorf("job = %+v, request %+v", job, req)
 		}
 		// Issue 1921: the composer is built at the group's heads, also a member that publishes no package yet.
diff --git a/modules/mesh-delivery-checker/internal/check/judge.go b/modules/mesh-delivery-checker/internal/check/judge.go
index c4326d2e..f138bddc 100644
--- a/modules/mesh-delivery-checker/internal/check/judge.go
+++ b/modules/mesh-delivery-checker/internal/check/judge.go
@@ -304,6 +304,9 @@ type state struct {
 	// noCandidate says why a whole try has no candidate of its group from the indexer, so its manifests were not judged and
 	// it was not composed; empty when it has one, and for a pull request's check.
 	noCandidate string
+	// seats is the check's seat list once read (checkSeats), and seatsErr why it is not.
+	seats    *checkSeats
+	seatsErr error
 }
 
 // narrowTry says the case is a try narrowed to a level or to some tests: it judges no manifest, graph or composition.
@@ -400,6 +403,14 @@ func (st *state) composed(ctx context.Context, v *meshdeliverychecker.VerdictsBy
 		return
 	}
 	words := st.awaitBuilds(ctx, v) // ADR 0603 §10: the check composes with the candidate's builds
+	// The composer of a node-engine member's head composes against the check's seat list, the judge's (ADR 0635 §1, ADR 0636
+	// §4); only a list the reader read is handed: the heads' alone are no list, and the composer then judges no machine.
+	st.c.Seats = nil
+	if seats, err := st.checkSeats(ctx); err != nil {
+		st.c.note("the composition is given no seat list: " + err.Error())
+	} else if seats.listed {
+		st.c.Seats = seats.defs
+	}
 	comp, err := st.j.Compositions.Compose(ctx, st.c, st.tags.goPackages)
 	if err != nil {
 		// Said in the verdict, never left as a composition not yet run (review finding 4).
diff --git a/modules/mesh-delivery-checker/internal/check/manifests_judge.go b/modules/mesh-delivery-checker/internal/check/manifests_judge.go
index da9c5e6d..0ec6cba7 100644
--- a/modules/mesh-delivery-checker/internal/check/manifests_judge.go
+++ b/modules/mesh-delivery-checker/internal/check/manifests_judge.go
@@ -132,19 +132,11 @@ func (st *state) judgeManifests(ctx context.Context) *meshdeliverychecker.Verdic
 		st.c.note("the manifests are judged against the predefined keys of " + from)
 	}
 	req.Packages = st.knownPackages(ctx)
-	if st.j.SeatList != nil {
-		seats, err := st.j.SeatList.Definitions()
-		if err != nil {
-			return fail(goModule, "the change's manifests cannot be judged: the seat list is not read ("+err.Error()+")")
-		}
-		req.Seats = seats
-	}
-	// The members' heads' definitions over the list's (ADR 0636 §4): the trunk's manifest validator of a new indexer as well.
-	heads, err := st.headSeats(ctx)
+	seats, err := st.checkSeats(ctx)
 	if err != nil {
 		return fail(goModule, "the change's manifests cannot be judged: "+err.Error())
 	}
-	req.Seats = overSeats(req.Seats, heads)
+	req.Seats = seats.defs
 	cv, trunkCase, v := st.runJudge(ctx, req, touched, commit, at, st.packagesOf(DeliveryRepository))
 	if v != nil {
 		return v
@@ -435,3 +427,37 @@ func (st *state) knownPackages(ctx context.Context) []string {
 func (st *state) trunkJudgeVersions() buildstep.PackageVersions {
 	return packagesFor(st.j.Published, &Case{}, nil, DeliveryRepository)
 }
+
+// checkSeats is the seat list a check judges and composes against, read once per check: the list the checker's reader holds
+// with the members' heads' definitions laid over it (ADR 0636 §4), the trunk's manifest validator of a new indexer as well.
+// listed says the reader's list was read; without a reader it is the heads' alone. An error is a list that is not read or a
+// head whose definitions are not taken.
+func (st *state) checkSeats(ctx context.Context) (checkSeats, error) {
+	if st.seats != nil {
+		return *st.seats, st.seatsErr
+	}
+	var out checkSeats
+	err := func() error {
+		if st.j.SeatList != nil {
+			defs, err := st.j.SeatList.Definitions()
+			if err != nil {
+				return fmt.Errorf("the seat list is not read (%s)", err.Error())
+			}
+			out.defs, out.listed = defs, true
+		}
+		heads, err := st.headSeats(ctx)
+		if err != nil {
+			return err
+		}
+		out.defs = overSeats(out.defs, heads)
+		return nil
+	}()
+	st.seats, st.seatsErr = &out, err
+	return out, err
+}
+
+// checkSeats is a check's seat list (state.checkSeats): the raw definitions, and whether the reader's list is among them.
+type checkSeats struct {
+	defs   []json.RawMessage
+	listed bool
+}