The compose test downloads the composer's modules as the fetch step does, so no cache of this machine decides (issue 1921, milestone 47/G, the group check of c5d32074c945)

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

modules/mesh-build-go/cmd/mesh-build-go/compose_integration_test.go

@@ -5,6 +5,7 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"io/fs"
 	"os"
 	"os/exec"
@@ -190,6 +191,14 @@ func TestAComposeAskIsBuiltOnceAndAnsweredOnTheBus(t *testing.T) {
 						return nil, err
 					}
 					workFile = filepath.Join(m.Work, ws, "go.work")
+					// What the real fetch does before any build (fetchInto, downloadAndVerify): every module the graph
+					// names is downloaded into the cache, so the build with GOPROXY=off needs none, however much of the
+					// graph this machine's cache (filled by this module's own build) lacks. Go here is the same go
+					// command in the same environment, with the workspace in place of the tree's own go.mod.
+					r := m.Run(ctx, m.goEnv(), "go", "-C", tree, "mod", "download")
+					if r.Status != 0 || r.Err != "" {
+						return nil, fmt.Errorf("NOT CHECKED: the composer's modules cannot be downloaded as the fetch step downloads them: %s", causeLine(r.Err+"\n"+r.Stdout+"\n"+r.Stderr))
+					}
 				}
 				return map[string]any{"workspace": ws}, nil
 			}