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 8eb13073a..aa9024e38 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
@@ -13,6 +13,7 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -212,7 +213,9 @@ func (m *Module) downloadModules(ctx context.Context, root, repository, dir, d s
 	for {
 		failure := m.downloadAndVerify(ctx, dir, d)
 		if failure == nil {
-			return taken, nil
+			// The proof that what was downloaded is all the shut-off steps load: else the fetch fails here, naming the
+			// package, and not a vet, a test or a build later with less to say (issue 1966).
+			return taken, m.shutOffLoad(ctx, m.goEnvFor(dir), dir, d)
 		}
 		// A requirement at the zero version names no commit: it is taken from the asked versions or the trunk's latest.
 		took, err := m.takeZeroPins(ctx, root, repository, dir, d, failure.Error(), asked, pins)
@@ -232,6 +235,27 @@ func (m *Module) downloadModules(ctx context.Context, root, repository, dir, d s
 	}
 }
 
+// shutOffLoadFormat is what go list -e says of each package it loads: nothing, or the package and Go's words of its error.
+const shutOffLoadFormat = `{{with .Error}}{{$.ImportPath}}: {{.Err}}{{"\n"}}{{end}}`
+
+// shutOffLoad loads the packages of dir, with their tests and every package they import, as a shut-off step does
+// (GOPROXY=off, the step's own environment env), and fails naming each package whose load would reach the network, in
+// Go's words. A package that fails to load for another cause (a compile error, an import of a package of the module
+// that does not exist) is the shut-off steps' to say, with their own verdict lines, and is not the fetch's.
+func (m *Module) shutOffLoad(ctx context.Context, env []string, dir, d string) error {
+	r := m.Run(ctx, append(env, "GOPROXY=off"), "go", "-C", dir, "list", "-e", "-deps", "-test", "-f", shutOffLoadFormat, "./...")
+	var lost []string
+	for _, l := range strings.Split(r.Stdout+"\n"+r.Stderr, "\n") {
+		if l = strings.TrimSpace(l); strings.Contains(l, "GOPROXY=off") && !slices.Contains(lost, l) {
+			lost = append(lost, l)
+		}
+	}
+	if len(lost) > 0 {
+		return fmt.Errorf("the packages of %s load shut off only with what the fetch did not download: %s", d, strings.Join(lost, "; "))
+	}
+	return nil
+}
+
 // verifyLocks hold one mutex per digest of a go.mod and go.sum, so two directories with the same pair verify once.
 var verifyLocks sync.Map
 
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 d76472a6f..7b91bd94a 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
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"sync"
 	"testing"
@@ -211,22 +212,54 @@ func TestNoCommandOfTheStepSeesAPasswordOrAToken(t *testing.T) {
 
 // 1485/AC3: go mod verify failing ends the step failed naming the directory and the first line of its output.
 func TestAVerifyThatFailsEndsTheStepNamingTheDirectoryAndTheLine(t *testing.T) {
-	f := &fakeStep{gomods: "go.mod\ntools/go.mod\n", goRule: func(line string) forgekey.Ran {
-		if strings.HasSuffix(line, "tools mod verify") || strings.Contains(line, "/tools mod verify") {
-			return forgekey.Ran{Status: 1, Stdout: "\nexample.org/x v1.0.0: dir has been modified\nsecond line\n"}
-		}
-		return forgekey.Ran{}
-	}}
-	m, states := fetchModule(t, f)
-	if _, err := fetchTool(t, m)(fetchArgs); err != nil {
-		t.Fatal(err)
-	}
-	st := states.waitEnd(t)
-	if st.State != sdkstep.StepFailed {
-		t.Fatalf("the step ended %s, want failed", st.State)
-	}
-	if !strings.Contains(st.Reason, "tools") || !strings.Contains(st.Reason, "example.org/x v1.0.0: dir has been modified") || strings.Contains(st.Reason, "second line") {
-		t.Errorf("reason = %q", st.Reason)
+	const seatPkg = "git.novox.be/novox/mesh-delivery/contracts/go/seats/nodebuildgo"
+	for _, c := range []struct {
+		name, ran, out string
+		want           []string
+		notWant        string
+	}{
+		{name: "go mod verify fails", ran: "tools mod verify", out: "\nexample.org/x v1.0.0: dir has been modified\nsecond line\n",
+			want: []string{"tools", "example.org/x v1.0.0: dir has been modified"}, notWant: "second line"},
+		// 1966/AC178-2: what the fetch downloaded misses a package the directory's code imports: the shut-off package load
+		// fails the fetch naming the package in Go's words, not a shut-off step later.
+		{name: "the shut-off package load would reach the network", ran: "tools list -e -deps -test",
+			out:  "fixture/tools/cmd: \n" + seatPkg + ": module lookup disabled by GOPROXY=off\n",
+			want: []string{"tools", "shut off", seatPkg + ": module lookup disabled by GOPROXY=off"}, notWant: "fixture/tools/cmd"},
+	} {
+		t.Run(c.name, func(t *testing.T) {
+			f := &fakeStep{gomods: "go.mod\ntools/go.mod\n", goRule: func(line string) forgekey.Ran {
+				if strings.Contains(line, "/"+c.ran) {
+					if strings.Contains(line, " list ") {
+						return forgekey.Ran{Stdout: c.out} // go list -e says a package's error and exits 0
+					}
+					return forgekey.Ran{Status: 1, Stdout: c.out}
+				}
+				return forgekey.Ran{}
+			}}
+			m, states := fetchModule(t, f)
+			if _, err := fetchTool(t, m)(fetchArgs); err != nil {
+				t.Fatal(err)
+			}
+			st := states.waitEnd(t)
+			if st.State != sdkstep.StepFailed {
+				t.Fatalf("the step ended %s, want failed", st.State)
+			}
+			for _, w := range c.want {
+				if !strings.Contains(st.Reason, w) {
+					t.Errorf("reason = %q, want it to hold %q", st.Reason, w)
+				}
+			}
+			if strings.Contains(st.Reason, c.notWant) {
+				t.Errorf("reason = %q holds %q", st.Reason, c.notWant)
+			}
+			if c.ran == "tools list -e -deps -test" {
+				for _, call := range f.calls {
+					if strings.Contains(call.line, " list -e -deps") && !slices.Contains(call.env, "GOPROXY=off") {
+						t.Errorf("the package load ran with the network: %s", call.env)
+					}
+				}
+			}
+		})
 	}
 }
 
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/workspace.go b/modules/mesh-build-go/cmd/mesh-build-go/workspace.go
index 2fc825525..b9184fdf8 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/workspace.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/workspace.go
@@ -171,6 +171,14 @@ func (m *Module) downloadBeside(ctx context.Context, root, tree string, dirs []s
 		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))
 		}
+		// What the packages' load needs beyond the module graph's download (the go.mod of a version a dependency requires,
+		// which a build of the composer reads: issue 1966), fetched with the network, then proven shut off.
+		if res := m.Run(ctx, env, "go", "-C", r, "list", "-e", "-deps", "-test", "-f", shutOffLoadFormat, "./..."); res.Status != 0 || res.Err != "" {
+			return fmt.Errorf("go list of the packages of %s beside: %s", filepath.Base(r), causeLine(res.Err+"\n"+res.Stdout+"\n"+res.Stderr))
+		}
+		if err := m.shutOffLoad(ctx, env, r, "beside "+filepath.Base(r)); err != nil {
+			return err
+		}
 	}
 	return nil
 }
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/workspace_test.go b/modules/mesh-build-go/cmd/mesh-build-go/workspace_test.go
index 49990aeb5..1ad94163e 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/workspace_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/workspace_test.go
@@ -6,6 +6,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 
@@ -72,7 +73,7 @@ func TestTheFetchDownloadsTheGraphOfASiblingModuleLaidWithTheTreesItRequires(t *
 	put(filepath.Join(root, "beside", "mesh-sdk", "go"), "module git.novox.be/novox/mesh-sdk/go\n\ngo 1.26.0\n")
 	var said []string
 	var lines, envs []string
-	fail := false
+	fail, lost := false, ""
 	m := &Module{Say: func(f string, a ...any) { said = append(said, fmt.Sprintf(f, a...)) }}
 	m.Run = func(ctx context.Context, env []string, name string, args ...string) forgekey.Ran {
 		lines = append(lines, name+" "+strings.Join(args, " "))
@@ -80,14 +81,26 @@ func TestTheFetchDownloadsTheGraphOfASiblingModuleLaidWithTheTreesItRequires(t *
 		if fail {
 			return forgekey.Ran{Status: 1, Stderr: "dial tcp: network is unreachable"}
 		}
+		if slices.Contains(env, "GOPROXY=off") {
+			return forgekey.Ran{Stdout: lost}
+		}
 		return forgekey.Ran{}
 	}
 	if err := m.downloadBeside(context.Background(), root, tree, []string{"modules/x"}); err != nil {
 		t.Fatal(err)
 	}
-	if len(lines) != 1 || lines[0] != "go -C "+host+" mod download" || !strings.Contains(envs[0], "GOWORK="+filepath.Join(root, "beside-0.work")) {
-		t.Fatalf("commands %q env %q, want one go mod download of the node-engine's module over its workspace", lines, envs)
+	// The graph's download, the packages' load with the network, then the same load shut off (issue 1966).
+	load := "go -C " + host + " list -e -deps -test -f " + shutOffLoadFormat + " ./..."
+	if len(lines) != 3 || lines[0] != "go -C "+host+" mod download" || lines[1] != load || lines[2] != load ||
+		!strings.Contains(envs[0], "GOWORK="+filepath.Join(root, "beside-0.work")) || strings.HasSuffix(envs[1], "GOPROXY=off") || !strings.HasSuffix(envs[2], "GOPROXY=off") {
+		t.Fatalf("commands %q env %q, want go mod download of the node-engine's module over its workspace, then its packages loaded with the network and shut off", lines, envs)
 	}
+	// 1966/AC178-3 (its unit half): the shut-off load misses a go.mod the composer's build reads: the fetch fails naming it.
+	lines, lost = nil, "github.com/nats-io/nkeys: github.com/nats-io/nkeys@v0.4.16 requires golang.org/x/crypto@v0.52.0: module lookup disabled by GOPROXY=off\n"
+	if err := m.downloadBeside(context.Background(), root, tree, []string{"modules/x"}); err == nil || !strings.Contains(err.Error(), "beside mesh-host") || !strings.Contains(err.Error(), "golang.org/x/crypto@v0.52.0: module lookup disabled") {
+		t.Errorf("a shut-off load that would reach the network = %v, want the fetch failed naming it", err)
+	}
+	lost = ""
 	work, _ := os.ReadFile(filepath.Join(root, "beside-0.work"))
 	for _, want := range []string{filepath.Join(tree, "modules", "x"), filepath.Join(root, "beside", "mesh-sdk", "go"), host} {
 		if !strings.Contains(string(work), want) {