diff --git a/modules/mesh-delivery-checker/cmd/mesh-delivery-checker/try_candidates.go b/modules/mesh-delivery-checker/cmd/mesh-delivery-checker/try_candidates.go
index 1e8de96e..d2feeece 100644
--- a/modules/mesh-delivery-checker/cmd/mesh-delivery-checker/try_candidates.go
+++ b/modules/mesh-delivery-checker/cmd/mesh-delivery-checker/try_candidates.go
@@ -9,6 +9,8 @@ import (
 
 	"git.novox.be/novox/mesh-delivery/contracts/go/clients/meshcatalogclient"
 	"git.novox.be/novox/mesh-delivery/contracts/go/seats/meshcatalog"
+
+	"mesh-delivery-checker/internal/check"
 )
 
 // tryCandidates answers the indexer's candidate of a try's group (novox/hq ADR 0593 §7, issue 1921): the candidate the
@@ -59,7 +61,7 @@ func (t tryCandidates) TryCandidate(ctx context.Context, group, heads string, bo
 func waitsOnPreview(c meshcatalog.CandidatesByGroupValue) bool {
 	for _, p := range c.Pulls {
 		for _, m := range p.Fps {
-			if m.None2 == meshcatalogclient.NoneNoPreview {
+			if check.WaitsOnPreview(m) {
 				return true
 			}
 		}
diff --git a/modules/mesh-delivery-checker/internal/check/builds.go b/modules/mesh-delivery-checker/internal/check/builds.go
index f26ae985..ef5bd073 100644
--- a/modules/mesh-delivery-checker/internal/check/builds.go
+++ b/modules/mesh-delivery-checker/internal/check/builds.go
@@ -124,21 +124,22 @@ func (j *Examiner) buildWait() time.Duration {
 
 // awaitBuilds waits for the candidate's builds before the check composes (ADR 0603 §10): the check builds what the builder
 // builds of the candidate and composes with it. A failed build fails the check, naming the module and the builder's words;
-// a build not ended within the bound is named, and the check composes without it. It answers the words of the composition's
-// warnings (what the check composes without waiting on its build, and why) and the failures of the builds that failed, for
-// the rules to take in (rules.JudgeComposition); failed says the check has failed already.
-func (st *state) awaitBuilds(ctx context.Context, failed bool) ([]string, []FailedFirst) {
+// a build not ended within the bound is named in late, and the check does not compose: an engine composes a module only at
+// a done build, so its answer would be the builder's lateness read as the change's refusal (issue 1921). It answers the
+// words of the composition's warnings (what the check composes without waiting on its build, and why) and the failures of
+// the builds that failed, for the rules to take in (rules.JudgeComposition); failed says the check has failed already.
+func (st *state) awaitBuilds(ctx context.Context, failed bool) (words []string, failedBuilds []FailedFirst, late string) {
 	wait, words := st.buildsOfTheCheck(ctx)
 	if len(wait) == 0 {
-		return words, nil
+		return words, nil, ""
 	}
 	modules := slices.Sorted(maps.Keys(wait))
 	if failed {
 		// The check has failed already: its red status is never held back by a build (the lead, 2026-11-03).
-		return append(words, fmt.Sprintf("the check failed before its builds were waited on, so it composes at once, without waiting on the builds of %s (ADR 0603 §10)", strings.Join(modules, ", "))), nil
+		return append(words, fmt.Sprintf("the check failed before its builds were waited on, so it composes at once, without waiting on the builds of %s (ADR 0603 §10)", strings.Join(modules, ", "))), nil, ""
 	}
 	if st.j.BuildEnds == nil {
-		return append(words, fmt.Sprintf("the builds of %s cannot be heard, so the check composes without waiting on them (ADR 0603 §10)", strings.Join(modules, ", "))), nil
+		return append(words, fmt.Sprintf("the builds of %s cannot be heard, so the check composes without waiting on them (ADR 0603 §10)", strings.Join(modules, ", "))), nil, ""
 	}
 	bound := st.j.buildWait()
 	waits := fmt.Sprintf("waits on the builder's builds of %s before it composes, at most %s (ADR 0603 §10)", strings.Join(modules, ", "), bound)
@@ -149,21 +150,20 @@ func (st *state) awaitBuilds(ctx context.Context, failed bool) ([]string, []Fail
 	cancel()
 	if err != nil {
 		st.j.logf("%s: the builds of %s cannot be heard: %v", st.c.Key, strings.Join(modules, ", "), err)
-		return append(words, fmt.Sprintf("the builds of %s cannot be heard (%v), so the check composes without waiting on them (ADR 0603 §10)", strings.Join(modules, ", "), err)), nil
+		return append(words, fmt.Sprintf("the builds of %s cannot be heard (%v), so the check composes without waiting on them (ADR 0603 §10)", strings.Join(modules, ", "), err)), nil, ""
 	}
-	var late []string
-	var failedBuilds []FailedFirst
+	var notEnded []string
 	for _, module := range modules {
 		b, done := ended[module]
 		switch {
 		case !done:
-			late = append(late, module)
+			notEnded = append(notEnded, module)
 		case b.State == builds.BuildFailed:
 			failedBuilds = append(failedBuilds, FailedFirst{Test: BuildFailedTest + module, Words: fmt.Sprintf("the candidate's build at %s failed (rebuild with builder_build_again; the check asks itself again when the build is done): %s", wait[module], b.Error)})
 		}
 	}
-	if len(late) > 0 && ctx.Err() == nil {
-		words = append(words, fmt.Sprintf("the builds of %s did not end within the build-wait-bound of %s, so the check composes without them (ADR 0603 §10)", strings.Join(late, ", "), bound))
+	if len(notEnded) > 0 && ctx.Err() == nil {
+		late = fmt.Sprintf("the builder's builds of %s did not end within the build-wait-bound of %s, so the change is not composed: an engine composes a module only at a done build (ADR 0603 §10)", strings.Join(notEnded, ", "), bound)
 	}
-	return words, failedBuilds
+	return words, failedBuilds, late
 }
diff --git a/modules/mesh-delivery-checker/internal/check/composition_test.go b/modules/mesh-delivery-checker/internal/check/composition_test.go
index 28b435d3..46437e5c 100644
--- a/modules/mesh-delivery-checker/internal/check/composition_test.go
+++ b/modules/mesh-delivery-checker/internal/check/composition_test.go
@@ -13,6 +13,7 @@ import (
 	nec "git.novox.be/novox/mesh-contracts/go/clients/nodeengineclient"
 	ne "git.novox.be/novox/mesh-contracts/go/seats/nodeengine"
 
+	"git.novox.be/novox/mesh-delivery/contracts/go/clients/meshcatalogclient"
 	"git.novox.be/novox/mesh-delivery/contracts/go/clients/meshdeliverycheckerclient"
 	"git.novox.be/novox/mesh-delivery/contracts/go/seats/meshcatalog"
 	"git.novox.be/novox/mesh-delivery/contracts/go/seats/meshdeliverychecker"
@@ -223,6 +224,8 @@ func aChangeToABuildModuleOrBaseNamesTheModulesBuiltOnIt(t *testing.T) {
 		failed   string   // the failure's test, when the check fails on a build
 		tests    bool     // the tests fail: the check fails before it would wait
 		served   []string // what the running builds serve (ADR 0614 §7); nil reads nothing of what runs
+		preview  bool     // the check began on the candidate the indexer wrote before its merge preview (issue 1921)
+		late     bool     // a build did not end within the bound: the change is not composed (issue 1921)
 	}{
 		{name: "the Go build module invokes a verb no running build serves",
 			manifest: map[string]catalogue.Manifest{"mesh-build-go": {Module: "mesh-build-go", Build: goBuild,
@@ -254,8 +257,10 @@ func aChangeToABuildModuleOrBaseNamesTheModulesBuiltOnIt(t *testing.T) {
 			builds: map[string]buildsrec.BuilderBuild{"web": {State: buildsrec.BuildBuilding}}, tests: true,
 			want: []string{"composes at once, without waiting on the builds of web"}},
 		{name: "a build does not end within the bound", manifest: map[string]catalogue.Manifest{"web": {Module: "web", Build: goBuild}},
-			builds: map[string]buildsrec.BuilderBuild{"web": {State: buildsrec.BuildBuilding}},
-			want:   []string{"the builds of web did not end within the build-wait-bound"}, waited: "web"},
+			builds: map[string]buildsrec.BuilderBuild{"web": {State: buildsrec.BuildBuilding}}, late: true,
+			want: []string{"the builder's builds of web did not end within the build-wait-bound", "the change is not composed"}, waited: "web"},
+		{name: "the check began before the merge preview", manifest: map[string]catalogue.Manifest{"web": {Module: "web", Build: goBuild}},
+			builds: map[string]buildsrec.BuilderBuild{"web": done}, preview: true, waited: "web"},
 		{name: "a module the trunk does not hold", manifest: map[string]catalogue.Manifest{"web": {Module: "web", Build: goBuild}, "new": {Module: "new", Build: goBuild}},
 			builds: map[string]buildsrec.BuilderBuild{"web": done}, runs: knownRuns{"web"},
 			want: []string{"new: the trunk holds no record of it yet"}, waited: "web"},
@@ -294,8 +299,20 @@ func aChangeToABuildModuleOrBaseNamesTheModulesBuiltOnIt(t *testing.T) {
 		b.ex.Read = read
 		p := pullOf(catalogueRepository, 9, head1)
 		p.Fps, p.Modules = fps, slices.Sorted(maps.Keys(fps))
+		if row.preview {
+			previewed := p
+			b.ex.Candidates = &fakeTryCandidates{cand: meshcatalog.CandidatesByGroupValue{Fpset: "fps", Pulls: []meshcatalog.CandidatesByGroupValuePullsItem{previewed}}}
+			p.Fps = map[string]meshcatalog.CandidatesByGroupValuePullsItemFpsValue{}
+			for module, m := range fps {
+				p.Fps[module] = meshcatalog.CandidatesByGroupValuePullsItemFpsValue{Manifest: m.Manifest, None2: meshcatalogclient.NoneNoPreview}
+			}
+		}
 		v := b.judge(t, caseOf(t, p)).Verdict
-		if v.Composition == nil || len(comps.asked) != 1 {
+		asks := 1
+		if row.late {
+			asks = 0 // never asked: an engine would read the builder's lateness as the change's refusal
+		}
+		if v.Composition == nil || len(comps.asked) != asks || row.late && v.Composition.State != meshdeliverycheckerclient.CompositionNotRun {
 			t.Fatalf("%s: composed %d times: %+v", row.name, len(comps.asked), v)
 		}
 		if ends != nil {
diff --git a/modules/mesh-delivery-checker/internal/check/judge.go b/modules/mesh-delivery-checker/internal/check/judge.go
index 8afd1362..bc6adc0a 100644
--- a/modules/mesh-delivery-checker/internal/check/judge.go
+++ b/modules/mesh-delivery-checker/internal/check/judge.go
@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"git.novox.be/novox/mesh-delivery/contracts/go/clients/meshcatalogclient"
 	"git.novox.be/novox/mesh-delivery/contracts/go/clients/meshdeliverycheckerclient"
 	"git.novox.be/novox/mesh-delivery/contracts/go/seats/meshcatalog"
 	"git.novox.be/novox/mesh-delivery/contracts/go/seats/meshdeliverychecker"
@@ -161,8 +162,9 @@ type Examiner struct {
 		Names(ctx context.Context) ([]string, error)
 	}
 	// Candidates answers the indexer's candidate of a try's group once it wrote it from the try's previews, the manifests
-	// and fingerprints a group try's manifest validator and composition judge (issue 1921); nil leaves a whole try with no
-	// manifest or composition judged, said.
+	// and fingerprints a group try's manifest validator and composition judge (issue 1921), and of a pull request's check
+	// that began before the indexer read its merge preview; nil leaves such a case with no manifest or composition judged,
+	// said.
 	Candidates TryCandidates
 	// Inputs keeps the checking program's inputs (the state InputsStateName, served on InputVerb); BeginBound is how long a run ask may wait for a
 	// build node to begin it (DefaultBeginBound when zero).
@@ -289,23 +291,52 @@ type state struct {
 	tags groupVersions
 	// outOfStep are, by pull request (pullKey), the generated files a level's compare step found out of step (design 26).
 	outOfStep map[string][]string
-	// 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 says why the case has no previewed candidate of its heads from the indexer within the bound: for a whole
+	// try its manifests were not judged and it was not composed; for a pull request's check that began on the indexer's
+	// first write, its manifests were judged from that write and it was not composed. Empty when it has one, and for a pull
+	// request's check whose case already holds its fingerprints.
 	noCandidate string
 }
 
 // narrowTry says the case is a try narrowed to a level or to some tests: it judges no manifest, graph or composition.
 func (st *state) narrowTry() bool { return st.c.Try != nil && !st.c.Try.Whole() }
 
-// takeTryCandidate gives a whole try's pulls the fingerprints and manifests the indexer's candidate of its group holds
-// (the group's pull-request check has them from its candidate, CaseOf), once the indexer wrote it from the try's previews.
-// A try with no such candidate is said so, in a note and in its composition, and is no pass (Tryer.run).
+// WaitsOnPreview says a candidate's module has no fingerprint yet because the indexer wrote it from a pull message alone,
+// before it read the head's merge preview (meshcatalogclient.NoneNoPreview): its fingerprint comes with the preview, on the
+// same heads, so a candidate that holds one is not yet the candidate of its heads previewed.
+func WaitsOnPreview(m meshcatalog.CandidatesByGroupValuePullsItemFpsValue) bool {
+	return m.None2 == meshcatalogclient.NoneNoPreview
+}
+
+// previewPending says a module of the case's pulls waits on its merge preview (WaitsOnPreview).
+func (st *state) previewPending() bool {
+	for _, p := range st.c.Pulls {
+		for _, m := range p.Fps {
+			if WaitsOnPreview(m) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// takeTryCandidate gives a whole try's pulls the fingerprints and manifests the indexer's candidate of its group holds,
+// once the indexer wrote it from the try's previews. A pull request's check has them from its candidate (CaseOf), but the
+// indexer writes a pull request's candidate first from its pull message alone and its fingerprints only with the merge
+// preview, on the same heads: a check that began on that first write takes the previewed candidate too, before it judges
+// the manifests, waits on the builds and composes (ADR 0603 §10; issue 1921: a check otherwise waited on no build and
+// the engines, reading the previewed candidate, refused its modules as not yet built). A case with no such candidate is
+// said so, in a note and in its composition, and is no pass (Tryer.run).
 func (st *state) takeTryCandidate(ctx context.Context) {
-	if st.c.Try == nil || !st.c.Try.Whole() {
+	if st.c.Try != nil && !st.c.Try.Whole() || st.c.Try == nil && !st.previewPending() {
 		return
 	}
 	no := func(why string) {
-		st.noCandidate = "the group's manifests and composition were not judged: " + why
+		what := "the group's manifests and composition were not judged: "
+		if st.c.Try == nil {
+			what = "the composition was not judged (the manifests were judged from the indexer's first write): "
+		}
+		st.noCandidate = what + why
 		st.c.note(st.noCandidate)
 	}
 	if st.j.Candidates == nil {
@@ -400,9 +431,13 @@ func (st *state) compose(ctx context.Context) {
 	// ADR 0603 §10: the check composes with the candidate's builds; whether it failed already is the running rules' word
 	// on the facts so far, an asking choice only (the verdict is the rules' of every fact).
 	failed := rules.Judge(st.factsOf()).State == meshdeliverycheckerclient.VerdictFail
-	words, builds := st.awaitBuilds(ctx, failed)
+	words, builds, late := st.awaitBuilds(ctx, failed)
 	c := &rules.Composition{Waits: words, BuildsFailed: builds, FPSet: st.c.FPSet}
 	st.composition = c
+	if late != "" {
+		c.NotRun = late // the builder's lateness, said by name, never the change's refusal (issue 1921)
+		return
+	}
 	// A composer built at the node-engine member's head is built against the group's tagged versions the check heard, as
 	// its other Go builds are (design 51 *Tagged versions*); it composes against the heads' seat list (seatList).
 	st.c.Tagged = st.tags.goPackages