diff --git a/modules/mesh-build-go/cmd/mesh-build-go/fetch.go b/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
index 91fd3176..92b64dd5 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
@@ -184,6 +184,12 @@ func (m *Module) fetchPhases(ctx context.Context, req buildstep.FetchRequest, id
 	}
 	remote := m.remote(repository)
 	opts := m.fetchOptions(ctx, repository, commit, ref)
+	mergeOpts := m.FetchOptions
+	if base != "" {
+		// The mirror is brought to the trunk commit too, so the merge preview deepens the tree from it and asks the forge
+		// for no history the mirror holds (issue 1921, try speed S6).
+		mergeOpts = m.fetchOptions(ctx, repository, base, "")
+	}
 	clock.lap(&took.Mirror)
 	if err := buildstep.FetchCommit(ctx, m.Run, tree, remote, commit, ref, opts...); err != nil {
 		return FetchResult{}, err
@@ -191,7 +197,7 @@ func (m *Module) fetchPhases(ctx context.Context, req buildstep.FetchRequest, id
 	clock.lap(&took.Fetch)
 	res := FetchResult{Workspace: id, Commit: commit, Took: took}
 	if base != "" {
-		if err := m.merge(ctx, &res, tree, remote, base, commit); err != nil {
+		if err := m.merge(ctx, &res, tree, remote, base, commit, mergeOpts); err != nil {
 			return FetchResult{}, err
 		}
 	}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/fetch_test.go b/modules/mesh-build-go/cmd/mesh-build-go/fetch_test.go
index a6734ac3..e3589532 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/fetch_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/fetch_test.go
@@ -349,7 +349,8 @@ func TestTheFetchSaysTheTimeOfEachPhaseAndTheyAddUpToTheTotal(t *testing.T) {
 		t.Fatalf("the result says no phase times: %s", st.Result)
 	}
 	got := *res.Took
-	want := FetchTook{Mirror: 1, Fetch: 2, Merge: 3, Dissociate: 4, Modules: 5, Total: 15}
+	// the mirror is brought to the head and to the base (1 s each), and the preview deepens from it (1 s) before merging
+	want := FetchTook{Mirror: 2, Fetch: 2, Merge: 4, Dissociate: 4, Modules: 5, Total: 17}
 	if got != want {
 		t.Errorf("phases = %+v, want %+v", got, want)
 	}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/mirror_test.go b/modules/mesh-build-go/cmd/mesh-build-go/mirror_test.go
index 6721155e..2a431c33 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/mirror_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/mirror_test.go
@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
 )
 
 func fetchOnce(t *testing.T, m *Module, states *keptStates, args map[string]any) FetchResult {
@@ -188,3 +189,61 @@ func TestTheWorkspaceIsSelfContainedAfterAFetchThroughTheMirror(t *testing.T) {
 		t.Errorf("the workspace is not whole: %v %s", err, out)
 	}
 }
+
+// 1921/S6: with a base, the mirror is brought to the trunk commit too, and the merge preview deepens the tree from the
+// mirror: it asks the forge for no history the mirror holds.
+func TestTheMirrorIsBroughtToTheBaseAndThePreviewFetchesFromIt(t *testing.T) {
+	f := &fakeStep{gomods: "go.mod\n", rule: previewRules(false)}
+	m, states := fetchModule(t, f)
+	mirror := filepath.Join(m.Cache, "mirror", "novox", "mesh-sdk.git")
+	f.files = map[string]string{"modules/m/module.json": "{}\n", "modules/m/go.sum": "sum\n", "go.mod": "module x\n", "go.sum": ""}
+	fetchOnce(t, m, states, map[string]any{"repository": "novox/mesh-sdk", "commit": commitSHA, "base": baseSHA})
+	lines := strings.Join(f.lines(), "\n")
+	if !strings.Contains(lines, "-C "+mirror+" fetch --no-tags origin "+baseSHA) {
+		t.Errorf("the mirror was not brought to the base:\n%s", lines)
+	}
+	if !strings.Contains(lines, "fetch --no-tags --unshallow "+mirror+" "+baseSHA) || strings.Contains(lines, "--unshallow origin") {
+		t.Errorf("the preview fetched the trunk's history from the forge:\n%s", lines)
+	}
+}
+
+// 1921/S6: the siblings are placed side by side: each sibling's fetch has started before any of them ends.
+func TestTheSiblingsArePlacedAtOnce(t *testing.T) {
+	var mu sync.Mutex
+	started := map[string]bool{}
+	both := make(chan struct{})
+	f := &fakeStep{gomods: "go.mod\n"}
+	f.rule = func(line string) (forgekey.Ran, bool) {
+		if !strings.Contains(line, "/beside/") || !strings.Contains(line, " fetch --no-tags --depth 1 ") {
+			return forgekey.Ran{}, false
+		}
+		mu.Lock()
+		for _, name := range []string{"mesh-catalog", "mesh-host"} {
+			if strings.Contains(line, "/beside/"+name) {
+				started[name] = true
+			}
+		}
+		if len(started) == 2 {
+			select {
+			case <-both:
+			default:
+				close(both)
+			}
+		}
+		mu.Unlock()
+		select {
+		case <-both:
+		case <-time.After(2 * time.Second):
+			return forgekey.Ran{Status: 1, Stderr: "the other sibling's fetch never started"}, true
+		}
+		return forgekey.Ran{}, false
+	}
+	m, states := fetchModule(t, f)
+	res := fetchOnce(t, m, states, map[string]any{"repository": "novox/mesh-controller", "commit": commitSHA, "beside": []any{
+		map[string]any{"repository": "novox/mesh-host", "commit": commitSHA},
+		map[string]any{"repository": "novox/mesh-catalog", "commit": commitSHA},
+	}})
+	if len(res.Beside) != 2 || res.Beside[0].Dir != "mesh-catalog" || res.Beside[1].Dir != "mesh-host" {
+		t.Errorf("beside = %+v, want both, in name order", res.Beside)
+	}
+}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/preview.go b/modules/mesh-build-go/cmd/mesh-build-go/preview.go
index 067dcfff..f5fa5b9f 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/preview.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/preview.go
@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"sync"
 
 	"git.novox.be/novox/mesh-sdk/go/buildstep"
 )
@@ -34,8 +35,9 @@ func (m *Module) remote(repository string) string {
 }
 
 // merge makes the merge preview of the head onto base and writes the answer into res.
-func (m *Module) merge(ctx context.Context, res *FetchResult, tree, remote, base, commit string) error {
-	mr, err := buildstep.MergeInto(ctx, m.Run, tree, remote, base, commit, m.FetchOptions...)
+// opts are the fetch's options, with the mirror when it holds base.
+func (m *Module) merge(ctx context.Context, res *FetchResult, tree, remote, base, commit string, opts []buildstep.Option) error {
+	mr, err := buildstep.MergeInto(ctx, m.Run, tree, remote, base, commit, opts...)
 	if err != nil {
 		return err
 	}
@@ -89,23 +91,22 @@ func (m *Module) besideDir(workspace string) string {
 // placeSiblings fetches each sibling into beside/<name> of the workspace root, as a plain directory. A symlink anywhere
 // in a placed tree fails the step naming it: the mesh creates none, and a test must not be led out of its sibling.
 func (m *Module) placeSiblings(ctx context.Context, root string, siblings []siblingArg) ([]Sibling, error) {
-	var placed []Sibling
-	for _, s := range siblings {
-		name := s.repository[strings.Index(s.repository, "/")+1:]
-		dir := filepath.Join(root, "beside", name)
-		if err := os.MkdirAll(dir, 0o700); err != nil {
+	// The siblings are placed side by side (issue 1921, try speed S6); the first failure in the order given is the step's.
+	placed := make([]Sibling, len(siblings))
+	errs := make([]error, len(siblings))
+	var wg sync.WaitGroup
+	for i, s := range siblings {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			placed[i], errs[i] = m.placeSibling(ctx, root, s)
+		}()
+	}
+	wg.Wait()
+	for _, err := range errs {
+		if err != nil {
 			return nil, err
 		}
-		if err := buildstep.FetchCommit(ctx, m.Run, dir, m.remote(s.repository), s.commit, s.ref, m.fetchOptions(ctx, s.repository, s.commit, s.ref)...); err != nil {
-			return nil, fmt.Errorf("beside %s: %w", s.repository, err)
-		}
-		if err := m.dissociate(ctx, dir); err != nil {
-			return nil, fmt.Errorf("beside %s: %w", s.repository, err)
-		}
-		if link := firstSymlink(dir); link != "" {
-			return nil, fmt.Errorf("beside %s holds a symlink, %s", s.repository, link)
-		}
-		placed = append(placed, Sibling{Repository: s.repository, Commit: s.commit, Dir: name})
 	}
 	sort.Slice(placed, func(i, j int) bool { return placed[i].Dir < placed[j].Dir })
 	if len(placed) > 0 {
@@ -121,6 +122,25 @@ func (m *Module) placeSiblings(ctx context.Context, root string, siblings []sibl
 	return placed, nil
 }
 
+// placeSibling fetches one sibling into beside/<name> of the workspace root.
+func (m *Module) placeSibling(ctx context.Context, root string, s siblingArg) (Sibling, error) {
+	name := s.repository[strings.Index(s.repository, "/")+1:]
+	dir := filepath.Join(root, "beside", name)
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return Sibling{}, err
+	}
+	if err := buildstep.FetchCommit(ctx, m.Run, dir, m.remote(s.repository), s.commit, s.ref, m.fetchOptions(ctx, s.repository, s.commit, s.ref)...); err != nil {
+		return Sibling{}, fmt.Errorf("beside %s: %w", s.repository, err)
+	}
+	if err := m.dissociate(ctx, dir); err != nil {
+		return Sibling{}, fmt.Errorf("beside %s: %w", s.repository, err)
+	}
+	if link := firstSymlink(dir); link != "" {
+		return Sibling{}, fmt.Errorf("beside %s holds a symlink, %s", s.repository, link)
+	}
+	return Sibling{Repository: s.repository, Commit: s.commit, Dir: name}, nil
+}
+
 // besideRecord is the file in a workspace's root naming the siblings the fetch placed beside it.
 const besideRecord = "beside.json"
 
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/preview_test.go b/modules/mesh-build-go/cmd/mesh-build-go/preview_test.go
index 7771689a..e1612833 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/preview_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/preview_test.go
@@ -85,7 +85,8 @@ func TestFetchWithABaseMergesAndAnswersThePreview(t *testing.T) {
 		t.Errorf("trees = %+v", res.Trees)
 	}
 	lines := strings.Join(f.lines(), "\n")
-	for _, want := range []string{"fetch --no-tags --unshallow origin " + baseSHA, "checkout -q --detach " + baseSHA,
+	mirror := filepath.Join(m.Cache, "mirror", "novox", "mesh-sdk.git")
+	for _, want := range []string{"fetch --no-tags --unshallow " + mirror + " " + baseSHA, "checkout -q --detach " + baseSHA,
 		"core.hooksPath=/dev/null", "merge --no-ff --no-edit --no-verify"} {
 		if !strings.Contains(lines, want) {
 			t.Errorf("git was not asked %q:\n%s", want, lines)
diff --git a/modules/mesh-build-go/go.mod b/modules/mesh-build-go/go.mod
index ebde537b..339169cc 100644
--- a/modules/mesh-build-go/go.mod
+++ b/modules/mesh-build-go/go.mod
@@ -3,7 +3,7 @@ module meshbuildgo
 go 1.26.0
 
 require (
-	git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024225846-cb548a6a588c
+	git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024230839-4a12ccb78d19
 	github.com/distribution/distribution/v3 v3.1.2
 	github.com/nats-io/nats-server/v2 v2.11.17
 	github.com/nats-io/nats.go v1.54.0
diff --git a/modules/mesh-build-go/go.sum b/modules/mesh-build-go/go.sum
index 6df07410..3eb57f08 100644
--- a/modules/mesh-build-go/go.sum
+++ b/modules/mesh-build-go/go.sum
@@ -2,6 +2,8 @@ git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024214154-bc16740feb0d h1:oJREw0gg
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024214154-bc16740feb0d/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024225846-cb548a6a588c h1:GWHP9ozfkFXHVEL2gE4Xa5kc5e8uPO+UL9+mau6iaIs=
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024225846-cb548a6a588c/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024230839-4a12ccb78d19 h1:NqU4hkk/jHdTEcGFnnXvdOJwD8dH5u+qWaCdQQ+JYMc=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261024230839-4a12ccb78d19/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
 github.com/alicebob/miniredis/v2 v2.35.0 h1:QwLphYqCEAo1eu1TqPRN2jgVMPBweeQcR21jeqDCONI=
 github.com/alicebob/miniredis/v2 v2.35.0/go.mod h1:TcL7YfarKPGDAthEtl5NBeHZfeUQj6OXMm/+iu5cLMM=
 github.com/antithesishq/antithesis-sdk-go v0.7.0-default-no-op h1:Z/MZK75wC/NSrkgqeNIa7jexam9uWzhLmFTSCPI/kn0=