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 7728190..5fa7dc5 100644
--- a/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/main.go
+++ b/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/main.go
@@ -363,6 +363,15 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 			go func() { sourceErrs <- follow(ctx, sw, each) }()
 		}
 	}
+	// Every repository the source names gets its taken key at start, and again when the state appears, from the source's
+	// head and through the path a merge uses (issue 1280): a merge heard before the state existed wrote none.
+	taken.Ready(func(jetstream.KeyValue) {
+		go func() {
+			if err := ix.FillTaken(ctx); err != nil {
+				logger.Printf("filling the taken keys from the source: %v", err)
+			}
+		}()
+	})
 	// Only the toolchain reports of nodes the build module is assigned to count (issue 1940): the controller's
 	// assignments are settled first, so the toolchains settle through the gate already knowing them.
 	gate := newToolchainGate(sortedModules(), ix.PutToolchain, logger.Printf)
diff --git a/modules/mesh-delivery-indexer/internal/index/modules_test.go b/modules/mesh-delivery-indexer/internal/index/modules_test.go
index 71bf565..a61073b 100644
--- a/modules/mesh-delivery-indexer/internal/index/modules_test.go
+++ b/modules/mesh-delivery-indexer/internal/index/modules_test.go
@@ -119,6 +119,33 @@ func TestAnUnchangedModuleWritesNothing(t *testing.T) { // 1239 (unchanged value
 				m.Merge = next
 				return ix.Apply(context.Background(), m)
 			}},
+		{name: "a merge heard while the taken state was missing is filled from the source when it appears", writes: 2, commit: map[string]string{"a": next, "b": merge, "c": merge}, taken: next,
+			then: func(ix *Indexer) error { // issue 1280: the key was missing, the merge was taken in without it
+				on := ix.Taken.(*memStore)
+				key, _ := delivery.RepositoryKey(repo)
+				delete(on.vals, key)
+				ix.Taken = &memStore{unbound: process.ErrNotYet}
+				m := mergedMsg(repo, map[string]string{"modules/a": "treeA2", "modules/b": "treeB", "modules/c": "treeC"})
+				m.Merge = next
+				if err := ix.Apply(context.Background(), m); err != nil {
+					return err
+				}
+				if err := ix.ApplySource(context.Background(), 8, sourceOf(repo, next)); err != nil {
+					return err
+				}
+				ix.Taken = on
+				return ix.FillTaken(context.Background())
+			}},
+		{name: "filling from an older source head leaves a later key", writes: 0, commit: map[string]string{"a": merge, "b": merge, "c": merge}, taken: next,
+			then: func(ix *Indexer) error {
+				ix.Contains = func(_ context.Context, _, commit, in string) (bool, error) { return commit == merge && in == next, nil }
+				m := mergedMsg(repo, three)
+				m.Merge = next
+				if err := ix.Apply(context.Background(), m); err != nil {
+					return err
+				}
+				return ix.FillTaken(context.Background()) // the source still names merge
+			}},
 		{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 }
diff --git a/modules/mesh-delivery-indexer/internal/index/taken.go b/modules/mesh-delivery-indexer/internal/index/taken.go
index 576b2a4..325afde 100644
--- a/modules/mesh-delivery-indexer/internal/index/taken.go
+++ b/modules/mesh-delivery-indexer/internal/index/taken.go
@@ -3,6 +3,9 @@ package index
 import (
 	"context"
 	"errors"
+	"fmt"
+	"maps"
+	"slices"
 	"time"
 
 	"git.novox.be/novox/mesh-sdk/go/delivery"
@@ -58,3 +61,21 @@ func (ix *Indexer) writeTaken(ctx context.Context, f delivery.Forge) error {
 	}
 	return err
 }
+
+// FillTaken writes the taken key of every repository the gate's source names, at the head it names, through writeTaken as a
+// live merge does (issue 1280): a merge heard while the state was not on the bus left its repository without a key until
+// that repository's next merge. It runs at start and when the state appears. A repository whose key names that head or a
+// later one is left; a repository the source does not name is left; the first error is returned after every repository
+// was tried.
+func (ix *Indexer) FillTaken(ctx context.Context) error {
+	ix.mu.Lock()
+	heads := maps.Clone(ix.source.heads)
+	ix.mu.Unlock()
+	var first error
+	for _, repository := range slices.Sorted(maps.Keys(heads)) {
+		if err := ix.writeTaken(ctx, delivery.Forge{Repository: repository, Merge: heads[repository]}); err != nil && first == nil {
+			first = fmt.Errorf("%s: %w", repository, err)
+		}
+	}
+	return first
+}