diff --git a/modules/mesh-build-go/cmd/mesh-build-go/beside_digest_test.go b/modules/mesh-build-go/cmd/mesh-build-go/beside_digest_test.go
index f59242f7c..c5a7bccf7 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/beside_digest_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/beside_digest_test.go
@@ -83,7 +83,8 @@ func TestASiblingsCommitChangesTheDigestOfAPackageWhoseTestsReachAReaderOfTheSib
 
 // The fetch writes the siblings it placed, which the digest reads.
 func TestTheFetchRecordsTheSiblingsItPlaced(t *testing.T) {
-	f := &fakeStep{gomods: "go.mod\n"}
+	// Every checkout writes this go.mod: the placed sibling is a Go module at its root, so the fetch downloads its graph.
+	f := &fakeStep{gomods: "go.mod\n", files: map[string]string{"go.mod": "module git.novox.be/novox/mesh-sdk/go\n\ngo 1.26.0\n"}}
 	m, states := fetchModule(t, f)
 	args := map[string]any{"repository": "novox/mesh-controller", "commit": commitSHA, "beside": []any{
 		map[string]any{"repository": "novox/mesh-sdk", "commit": commitSHA}}}
@@ -101,6 +102,20 @@ func TestTheFetchRecordsTheSiblingsItPlaced(t *testing.T) {
 	if err != nil || json.Unmarshal(raw, &placed) != nil || len(placed) != 1 || placed[0].Repository != "novox/mesh-sdk" || placed[0].Commit != commitSHA {
 		t.Errorf("the record is %s (%v)", raw, err)
 	}
+	// The placed sibling's graph is downloaded once, over the workspace the fetch laid for it (downloadBeside).
+	want := "go -C " + filepath.Join(m.Work, res.Workspace, "beside", "mesh-sdk") + " mod download"
+	n := 0
+	for _, c := range f.calls {
+		if c.line == want {
+			n++
+			if !strings.Contains(strings.Join(c.env, "\n"), "GOWORK="+filepath.Join(m.Work, res.Workspace, "beside-0.work")) {
+				t.Errorf("%q ran without the sibling's workspace: %q", want, c.env)
+			}
+		}
+	}
+	if n != 1 {
+		t.Errorf("%q ran %d times, want once:\n%s", want, n, strings.Join(f.lines(), "\n"))
+	}
 }
 
 // The checker's agreement: the integration level runs and digests under the tags integration and isolation, so the
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 a1988224a..e8707c2f5 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
@@ -189,6 +189,9 @@ func (m *Module) fetchInto(ctx context.Context, req sdkstep.FetchRequest, id str
 			return nil, err
 		}
 	}
+	if err := m.downloadBeside(ctx, root, tree, dirs); err != nil {
+		return nil, err
+	}
 	for _, t := range taken {
 		res.Packages = append(res.Packages, t...)
 	}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/workspace.go b/modules/mesh-build-go/cmd/mesh-build-go/workspace.go
new file mode 100644
index 000000000..5b46f23f4
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/workspace.go
@@ -0,0 +1,145 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// zeroVersion is the version a mesh package is required at in every go.mod of the mesh (ADR 0573 §2, §4).
+const zeroVersion = "v0.0.0-00010101000000-000000000000"
+
+// workspaceOf is the go.work of the module in first and of every module it requires at the zero version, transitively,
+// each found among the modules in candidates by its module path: each used, and each also replaced at the zero version by
+// its own directory. Go (from 1.27) looks the zero version of a required module up even where the module is a member of
+// the workspace, once a build loads the module graph (any import outside the workspace); a versioned replace is the one
+// form it takes without a lookup, and an unversioned one is refused for a member of the workspace (issue 1921: the
+// composer's build fetched mesh-contracts from the network). Only what first needs is laid (issue 1921, the group check
+// of 8a80e8e86858): a module laid beside that requires one no tree holds (mesh-grants requiring a catalogue module, the
+// catalogue being the repository under check) made Go fetch that module's zero version and fail, though the composer
+// needs neither. A requirement at the zero version no candidate holds is an error naming it and who requires it.
+func workspaceOf(first string, candidates []string) (string, error) {
+	paths := map[string]string{}
+	for _, d := range candidates {
+		path, _, err := goModOf(d)
+		if err != nil {
+			return "", err
+		}
+		if _, seen := paths[path]; !seen { // the same module laid twice: the first is kept
+			paths[path] = d
+		}
+	}
+	var use, replace strings.Builder
+	laid := map[string]bool{}
+	queue := []string{first}
+	for len(queue) > 0 {
+		d := queue[0]
+		queue = queue[1:]
+		path, needs, err := goModOf(d)
+		if err != nil {
+			return "", err
+		}
+		if laid[path] {
+			continue
+		}
+		laid[path] = true
+		fmt.Fprintf(&use, "\t%s\n", d)
+		fmt.Fprintf(&replace, "replace %s %s => %s\n", path, zeroVersion, d)
+		for _, need := range needs {
+			if laid[need] {
+				continue
+			}
+			dir, ok := paths[need]
+			if !ok {
+				return "", fmt.Errorf("%s (%s) requires %s at the zero version, and no tree laid beside holds it", path, d, need)
+			}
+			queue = append(queue, dir)
+		}
+	}
+	return "go 1.26.0\n\nuse (\n" + use.String() + ")\n\n" + replace.String(), nil
+}
+
+// goModOf is the module path of the go.mod in d and the modules it requires at the zero version.
+func goModOf(d string) (path string, needs []string, err error) {
+	raw, err := os.ReadFile(filepath.Join(d, "go.mod"))
+	if err != nil {
+		return "", nil, err
+	}
+	for _, line := range strings.Split(string(raw), "\n") {
+		f := strings.Fields(strings.TrimPrefix(strings.TrimSpace(line), "require "))
+		switch {
+		case len(f) == 2 && f[0] == "module" && path == "":
+			path = strings.Trim(f[1], `"`)
+		case len(f) >= 2 && f[1] == zeroVersion:
+			needs = append(needs, strings.Trim(f[0], `"`))
+		}
+	}
+	if path == "" {
+		return "", nil, fmt.Errorf("%s/go.mod names no module", d)
+	}
+	return path, needs, nil
+}
+
+// downloadBeside downloads, where the network is, the Go modules of the graph that the tests of a workspace's siblings
+// build: a sibling that is a Go module at its root (the node-engine's repository, whose composer a catalogue test builds)
+// is laid with what it requires at the zero version, found in the fetched tree and the other siblings (workspaceOf), as
+// a go.work, and its graph is downloaded into the module cache. The integration level runs shut off from the network and
+// builds with GOPROXY=off, so a module this fetch did not download is one it cannot get. A sibling whose requirement no
+// tree holds is said and left out: the test that builds it fails naming that module. A download that fails fails the fetch.
+func (m *Module) downloadBeside(ctx context.Context, root, tree string, dirs []string) error {
+	beside := filepath.Join(root, "beside")
+	entries, err := os.ReadDir(beside)
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil // no siblings placed
+	}
+	if err != nil {
+		return err
+	}
+	var candidates, roots []string
+	for _, d := range dirs {
+		candidates = append(candidates, filepath.Join(tree, filepath.FromSlash(d)))
+	}
+	for _, e := range entries {
+		dir := filepath.Join(beside, e.Name())
+		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
+			roots = append(roots, dir)
+		}
+		err := filepath.WalkDir(dir, func(p string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+			if d.IsDir() && (d.Name() == ".git" || d.Name() == "testdata" || d.Name() == "node_modules") {
+				return filepath.SkipDir
+			}
+			if !d.IsDir() && d.Name() == "go.mod" {
+				candidates = append(candidates, filepath.Dir(p))
+			}
+			return nil
+		})
+		if err != nil {
+			return err
+		}
+	}
+	for i, r := range roots {
+		work, err := workspaceOf(r, candidates)
+		if err != nil {
+			if m.Say != nil {
+				m.Say("beside %s: its modules are not downloaded: %v", filepath.Base(r), err)
+			}
+			continue
+		}
+		file := filepath.Join(root, fmt.Sprintf("beside-%d.work", i))
+		if err := os.WriteFile(file, []byte(work), 0o600); err != nil {
+			return err
+		}
+		env := append(m.goEnv(), "GOWORK="+file, "GOFLAGS=-mod=readonly") // a workspace takes no -mod=mod
+		if res := m.Run(ctx, env, "go", "-C", r, "mod", "download"); res.Status != 0 || res.Err != "" {
+			return fmt.Errorf("go mod download of the graph of %s beside: %s", filepath.Base(r), causeLine(res.Err+"\n"+res.Stdout+"\n"+res.Stderr))
+		}
+	}
+	return nil
+}