diff --git a/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/main.go b/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/main.go
index 0b4258d..43b411e 100644
--- a/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/main.go
+++ b/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/main.go
@@ -253,6 +253,10 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 	if err != nil {
 		return err
 	}
+	taken, err := process.EssentialBucket(ctx, js, "indexer", delivery.TakenBucket, "say the merge a repository's modules were taken in at")
+	if err != nil {
+		return err
+	}
 	// The controller's build records only name the directory of a module whose key names none (fillPaths): without them
 	// such a module waits for its path, every other is indexed (issue 1945).
 	builds := process.OptionalBucket(ctx, js, delivery.BuildsBucket, "a module whose key names no directory is not given one from the controller's build records until the indexer starts again", logger.Printf)
@@ -286,7 +290,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 		return err
 	}
 	defer calls.Close()
-	ix := &index.Indexer{Store: kvStore{candidates}, Modules: kvStore{modules}, Manifests: kvStore{manifests}, Builds: known, Logf: logger.Printf,
+	ix := &index.Indexer{Store: kvStore{candidates}, Modules: kvStore{modules}, Manifests: kvStore{manifests}, Taken: kvStore{taken}, Builds: known, Logf: logger.Printf,
 		Named: namedVia(calls.Call), Trunk: trunkVia(calls.Call), Contains: containsVia(calls.Call), Read: readVia(calls.Call)}
 	// The gate's source bucket (mesh-delivery-gate_source) comes with the gate (release 3e, novox/hq ADR 0450 §1): its key
 	// main is the mesh's source revision, which stamps the modules (issue 1344). Where it does not exist yet the indexer does
diff --git a/modules/mesh-delivery-indexer/internal/index/candidates.go b/modules/mesh-delivery-indexer/internal/index/candidates.go
index 592b309..b3fd257 100644
--- a/modules/mesh-delivery-indexer/internal/index/candidates.go
+++ b/modules/mesh-delivery-indexer/internal/index/candidates.go
@@ -9,6 +9,7 @@ import (
 	"strings"
 	"sync"
 	"sync/atomic"
+	"time"
 
 	"git.novox.be/novox/mesh-sdk/go/delivery"
 )
@@ -47,7 +48,11 @@ type Indexer struct {
 	Modules Store
 	// Manifests is the manifests bucket; nil writes no manifests (issue 1449).
 	Manifests Store
-	Builds    *Builds
+	// Taken is the taken bucket: per repository the last merge taken in (taken.go, ADR 0556); nil writes none.
+	Taken Store
+	// Now is the clock of a taken value; nil is time.Now.
+	Now    func() time.Time
+	Builds *Builds
 	// Named asks the forge for the git object of each named path of the module directories given, at a commit
 	// (gitea.gitea_commit_trees with paths; novox/hq ADR 0499 clauses 2 and 3): the indexer is the one reader of a
 	// manifest's paths (ADR 0495 clause 1, the lead's decision L8 of R4e). Nil asks nothing, and a module naming paths
@@ -94,6 +99,9 @@ func (ix *Indexer) Apply(ctx context.Context, f delivery.Forge) error {
 		if err := ix.applyMerged(ctx, f); err != nil {
 			return err
 		}
+		if err := ix.writeTaken(ctx, f); err != nil {
+			return err
+		}
 		ix.kickRetry(ctx) // a registered module the forge could not read is read again on a merge, in the background (issue 1945)
 	}
 	var digests map[string]string
diff --git a/modules/mesh-delivery-indexer/internal/index/modules_test.go b/modules/mesh-delivery-indexer/internal/index/modules_test.go
index 18987ef..c11182b 100644
--- a/modules/mesh-delivery-indexer/internal/index/modules_test.go
+++ b/modules/mesh-delivery-indexer/internal/index/modules_test.go
@@ -94,22 +94,39 @@ func TestAnUnchangedModuleWritesNothing(t *testing.T) { // 1239 (unchanged value
 		// writes is the values written by then; commit is each module's commit after.
 		writes int
 		commit map[string]string
+		// taken is the merge the repository's taken key names after then (issue 1280, ADR 0556 clause 1).
+		taken string
 	}{
-		{name: "the same merge again", writes: 0, commit: map[string]string{"a": merge, "b": merge, "c": merge},
+		{name: "the same merge again", writes: 0, commit: map[string]string{"a": merge, "b": merge, "c": merge}, taken: merge,
 			then: func(ix *Indexer) error { return ix.Apply(context.Background(), mergedMsg(repo, three)) }},
-		{name: "a merge touching one of three modules", writes: 1, commit: map[string]string{"a": next, "b": merge, "c": merge},
+		{name: "a merge touching one of three modules", writes: 1, commit: map[string]string{"a": next, "b": merge, "c": merge}, taken: next,
 			then: func(ix *Indexer) error {
 				m := mergedMsg(repo, map[string]string{"modules/a": "treeA2", "modules/b": "treeB", "modules/c": "treeC"})
 				m.Merge = next
 				return ix.Apply(context.Background(), m)
 			}},
-		{name: "a later source revision of another repository's merge", writes: 0, commit: map[string]string{"a": merge, "b": merge, "c": merge},
+		{name: "a merge touching none of them is still taken in", writes: 0, commit: map[string]string{"a": merge, "b": merge, "c": merge}, taken: next,
+			then: func(ix *Indexer) error {
+				m := mergedMsg(repo, three)
+				m.Merge = next
+				return ix.Apply(context.Background(), m)
+			}},
+		{name: "an earlier merge heard again after a later one leaves taken", writes: 0, commit: map[string]string{"a": merge, "b": merge, "c": merge}, taken: merge,
+			then: func(ix *Indexer) error {
+				ix.Contains = func(_ context.Context, _, commit, in string) (bool, error) { return commit == next && in == merge, nil }
+				m := mergedMsg(repo, three)
+				m.Merge = next
+				return ix.Apply(context.Background(), m)
+			}},
+		{name: "a later source revision of another repository's merge", writes: 0, commit: map[string]string{"a": merge, "b": merge, "c": merge}, taken: merge,
 			then: func(ix *Indexer) error {
 				return ix.ApplySource(context.Background(), 9, delivery.Source{Group: "g", Heads: map[string]string{repo: merge, "novox/mesh-catalog": otherMerge}})
 			}},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			ix, mods := modulesIndexer(t)
+			taken := newMem()
+			ix.Taken = taken
 			ctx := context.Background()
 			if err := ix.Apply(ctx, mergedMsg(repo, three)); err != nil {
 				t.Fatal(err)
@@ -129,6 +146,10 @@ func TestAnUnchangedModuleWritesNothing(t *testing.T) { // 1239 (unchanged value
 					t.Errorf("%s = %+v (%v), want commit %.8s", module, m, ok, want)
 				}
 			}
+			key, _ := delivery.RepositoryKey(repo)
+			if got, err := delivery.DecodeTaken(taken.vals[key]); err != nil || got.Merge != tc.taken || got.Repository != repo {
+				t.Errorf("taken.%s = %+v (%v), want merge %.8s", key, got, err, tc.taken)
+			}
 		})
 	}
 }
diff --git a/modules/mesh-delivery-indexer/internal/index/taken.go b/modules/mesh-delivery-indexer/internal/index/taken.go
new file mode 100644
index 0000000..bcc9a10
--- /dev/null
+++ b/modules/mesh-delivery-indexer/internal/index/taken.go
@@ -0,0 +1,51 @@
+package index
+
+import (
+	"context"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+// What the indexer took in (novox/hq ADR 0556 clause 1, issue 1280): after a merged message is applied whole, the key of
+// its repository in the state `taken` names that merge, so a reader of the modules knows they say the repository at that
+// merge or later. The indexer is the key's only writer. A merge heard again after a later one (a redelivery) does not put
+// the key back: the key moves only to a merge that contains the one it names, as the forge says it; a forge that cannot
+// say keeps the key as it is and says so.
+
+// writeTaken writes the repository's taken key for the merge f carries, unless it names that merge or a later one.
+func (ix *Indexer) writeTaken(ctx context.Context, f delivery.Forge) error {
+	if ix.Taken == nil || f.Merge == "" {
+		return nil
+	}
+	key, err := delivery.RepositoryKey(f.Repository)
+	if err != nil {
+		ix.logf("%s at %s: no taken key is written: %v", f.Repository, f.Merge, err)
+		return nil
+	}
+	return write(ctx, ix.Taken, key, func(value []byte, found bool) ([]byte, bool, error) {
+		if found {
+			if cur, err := delivery.DecodeTaken(value); err == nil {
+				if cur.Merge == f.Merge {
+					return nil, false, nil
+				}
+				if ix.Contains != nil {
+					later, err := ix.Contains(ctx, f.Repository, f.Merge, cur.Merge)
+					if err != nil {
+						ix.logf("%s: whether %s is in %s cannot be read, so taken.%s stays at %s: %v", f.Repository, f.Merge, cur.Merge, key, cur.Merge, err)
+						return nil, false, nil
+					}
+					if later {
+						return nil, false, nil // a merge heard again after a later one
+					}
+				}
+			}
+		}
+		now := time.Now
+		if ix.Now != nil {
+			now = ix.Now
+		}
+		data, err := delivery.EncodeTaken(delivery.Taken{Repository: delivery.RepositoryName(f.Repository), Merge: f.Merge, At: now().UTC()})
+		return data, false, err
+	})
+}
diff --git a/modules/mesh-delivery-indexer/manifest_test.go b/modules/mesh-delivery-indexer/manifest_test.go
index 46b3589..28f0816 100644
--- a/modules/mesh-delivery-indexer/manifest_test.go
+++ b/modules/mesh-delivery-indexer/manifest_test.go
@@ -80,7 +80,7 @@ func TestManifestDeclarations(t *testing.T) { // 1238/AC1, 1444 (reads the build
 	if m.Module != "mesh-delivery-indexer" {
 		t.Errorf("module = %q", m.Module)
 	}
-	if want := []state{{Name: "candidates", History: 10}, {Name: "manifests", History: 1}, {Name: "modules", History: 20}, {Name: "takeins", History: 1}}; !reflect.DeepEqual(m.State, want) {
+	if want := []state{{Name: "candidates", History: 10}, {Name: "manifests", History: 1}, {Name: "modules", History: 20}, {Name: "taken", History: 1}, {Name: "takeins", History: 1}}; !reflect.DeepEqual(m.State, want) {
 		t.Errorf("state = %+v, want %+v", m.State, want)
 	}
 	if want := []string{"mesh-build-go.toolchains", "mesh-build-node.toolchains", "mesh-controller.assignments", "mesh-controller.builds", "mesh-delivery-checker.previews", "mesh-delivery-gate.source"}; !reflect.DeepEqual(sorted(m.Reads), want) {
diff --git a/modules/mesh-delivery-indexer/module.json b/modules/mesh-delivery-indexer/module.json
index a30c24e..a2d545b 100644
--- a/modules/mesh-delivery-indexer/module.json
+++ b/modules/mesh-delivery-indexer/module.json
@@ -15,6 +15,7 @@
     { "name": "candidates", "history": 10 },
     { "name": "manifests", "history": 1 },
     { "name": "modules", "history": 20 },
+    { "name": "taken", "history": 1 },
     { "name": "takeins", "history": 1 }
   ],
   "reads": [