diff --git a/modules/mesh-build-go/README.md b/modules/mesh-build-go/README.md
index 69d8cfa2..f2493e35 100644
--- a/modules/mesh-build-go/README.md
+++ b/modules/mesh-build-go/README.md
@@ -105,6 +105,16 @@ fingerprint once, as the version the build nodes agree on (ADR 0477 §3), and th
 version's. A package with no tests at the level has no digest and is named in `without_tests`. A directory that vendors
 is listed and tested from its vendor directory (`-mod=vendor`).
 
+## A change to this module is judged by its candidate
+
+`build_go_digest` and `build_go_test` take `runner`: `installed` (the default) runs the step as this program;
+`workspace` compiles the workspace's own `modules/mesh-build-go` (`go build` with `GOPROXY=off`, `GOTOOLCHAIN=local`, in a
+run step shut off) into the workspace's `candidate/mesh-build-go`, once per workspace, and runs the step as that program:
+`<candidate> level <spec>` in one bubblewrap, the spec naming the step (`digest`, `unit` or `integration`). This program
+keeps the fetch, the forge key and the bus login (ADR 0535 §1). A candidate that does not compile fails the step with the
+compiler's first error, and nothing runs (§3). Every digest and level answer says `judged_by`: `installed`, or
+`candidate <commit of the tree>`. The checker asks `workspace` when a merge changes `modules/mesh-build-go/` (issue 1898).
+
 ## Build
 
 `build_go_build {workspace, module, artifact}` builds one Go bundle artifact of a module of the workspace with the build
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 9f14bd9c..3682a61b 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
@@ -11,37 +11,72 @@ import (
 	"git.novox.be/novox/mesh-sdk/go/delivery"
 )
 
-// The lead's decision L5 (the checker keeps a verdict of a core repository): the fetch writes the siblings it placed
-// beside the workspace, and the digest folds each one's repository and commit into every package's digest and echoes
-// them, so a verdict is kept only for the siblings it was judged with.
-func TestTheSiblingsCommitsAreInEveryDigestAndEchoed(t *testing.T) {
+// writeSiblings records the siblings the fetch placed, as the fetch does.
+func writeSiblings(t *testing.T, m *Module, placed []Sibling) {
+	t.Helper()
+	raw, _ := json.Marshal(placed)
+	if err := os.WriteFile(filepath.Join(m.Work, "ws1", besideRecord), raw, 0o600); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// Issue 1896/AC1 and AC3 (ADR 0536 §1): a sibling's commit changes no digest of a package whose tests reach no file
+// naming MESH_CHECK_BESIDE; the answer still echoes every placed sibling, in form 4.
+func TestASiblingsCommitLeavesAPackageThatDoesNotReadTheSiblingsAsItWas(t *testing.T) {
 	t.Parallel()
 	m, _ := digestFixture(t)
 	alone := digests(t, m, "unit")
 	if len(alone.Beside) != 0 {
 		t.Errorf("a workspace without siblings echoes %v", alone.Beside)
 	}
-	write := func(placed []Sibling) {
-		raw, _ := json.Marshal(placed)
-		if err := os.WriteFile(filepath.Join(m.Work, "ws1", besideRecord), raw, 0o600); err != nil {
-			t.Fatal(err)
-		}
-	}
 	other := strings.Repeat("b", 40)
-	write([]Sibling{{Repository: "novox/mesh-sdk", Commit: commitSHA, Dir: "mesh-sdk"}, {Repository: "novox/mesh-catalog", Commit: commitSHA, Dir: "mesh-catalog"}})
+	writeSiblings(t, m, []Sibling{{Repository: "novox/mesh-sdk", Commit: commitSHA, Dir: "mesh-sdk"}, {Repository: "novox/mesh-catalog", Commit: commitSHA, Dir: "mesh-catalog"}})
 	with := digests(t, m, "unit")
 	want := []string{"novox/mesh-catalog " + commitSHA, "novox/mesh-sdk " + commitSHA}
 	if !slices.Equal(with.Beside, want) {
 		t.Errorf("echoed %v, want %v", with.Beside, want)
 	}
-	write([]Sibling{{Repository: "novox/mesh-sdk", Commit: other, Dir: "mesh-sdk"}, {Repository: "novox/mesh-catalog", Commit: commitSHA, Dir: "mesh-catalog"}})
+	writeSiblings(t, m, []Sibling{{Repository: "novox/mesh-sdk", Commit: other, Dir: "mesh-sdk"}, {Repository: "novox/mesh-catalog", Commit: commitSHA, Dir: "mesh-catalog"}})
 	moved := digests(t, m, "unit")
 	for p, d := range alone.Packages {
-		if with.Packages[p] == d {
-			t.Errorf("%s: the siblings left the digest as it was without them", p)
+		if with.Packages[p] != d || moved.Packages[p] != d {
+			t.Errorf("%s: a sibling left its digest changed though no test of it reads the siblings", p)
 		}
-		if moved.Packages[p] == with.Packages[p] {
-			t.Errorf("%s: a sibling's new commit left the digest as it was", p)
+	}
+	if !slices.Equal(moved.Beside, []string{"novox/mesh-catalog " + commitSHA, "novox/mesh-sdk " + other}) {
+		t.Errorf("echoed %v after the move", moved.Beside)
+	}
+}
+
+// Issue 1896/AC2: a package whose test imports an in-tree package naming MESH_CHECK_BESIDE is judged again when a
+// sibling moves; a package beside it that does not import it is not.
+func TestASiblingsCommitChangesTheDigestOfAPackageWhoseTestsReachAReaderOfTheSiblings(t *testing.T) {
+	t.Parallel()
+	m, tree := digestFixture(t)
+	for name, text := range map[string]string{
+		"f/f.go":      "package f\n\n// Beside is where the siblings lie.\nconst Beside = \"MESH_CHECK_BESIDE\"\n",
+		"e/e_test.go": "package e\n\nimport (\n\t\"testing\"\n\n\t\"fixture/digest/f\"\n)\n\nfunc TestE(t *testing.T) {\n\tif f.Beside == \"\" {\n\t\tt.Fatal(\"e\")\n\t}\n}\n",
+		"e/e.go":      "package e\n",
+		"g/g_test.go": "package g\n\nimport \"testing\"\n\nfunc TestG(t *testing.T) {\n\t// MESH_CHECK_BESIDE named in the test's own file\n}\n",
+		"g/g.go":      "package g\n",
+	} {
+		if err := os.MkdirAll(filepath.Dir(filepath.Join(tree, name)), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		edit(t, tree, name, text)
+	}
+	writeSiblings(t, m, []Sibling{{Repository: "novox/mesh-sdk", Commit: commitSHA, Dir: "mesh-sdk"}})
+	before := digests(t, m, "unit")
+	writeSiblings(t, m, []Sibling{{Repository: "novox/mesh-sdk", Commit: strings.Repeat("b", 40), Dir: "mesh-sdk"}})
+	after := digests(t, m, "unit")
+	for _, p := range []string{"fixture/digest/e", "fixture/digest/g"} {
+		if len(before.Packages[p]) != 64 || before.Packages[p] == after.Packages[p] {
+			t.Errorf("%s reads the siblings and its digest stayed (%q, %q)", p, before.Packages[p], after.Packages[p])
+		}
+	}
+	for _, p := range []string{"fixture/digest/a", "fixture/digest/c"} {
+		if before.Packages[p] != after.Packages[p] {
+			t.Errorf("%s does not read the siblings and its digest changed", p)
 		}
 	}
 }
@@ -81,7 +116,7 @@ func TestTheIntegrationLevelDigestsUnderBothTags(t *testing.T) {
 			t.Errorf("the integration digest listed %q", s.command)
 		}
 	}
-	if DigestVersion != "3" {
+	if DigestVersion != "4" {
 		t.Errorf("the digest's form changed in kind and is still version %s", DigestVersion)
 	}
 }
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/candidate.go b/modules/mesh-build-go/cmd/mesh-build-go/candidate.go
new file mode 100644
index 00000000..38a27ad6
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/candidate.go
@@ -0,0 +1,132 @@
+package main
+
+// A change to this module is judged by its own candidate (novox/hq ADR 0535; issue 1898): asked with the runner
+// workspace, the digest and test steps compile the program of mesh-build-go from the workspace's merged tree in a run
+// step, shut off, and run the step as that program in one shut-off process (`<candidate> level <spec>`, level.go), where
+// this program would run the step's commands. This program keeps the fetch, the forge key and the bus login; the
+// candidate's program holds none of them. Each answer says which program ran it (judged_by).
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+)
+
+// The runners of a digest or test step: this installed program, or the candidate's compiled from the workspace.
+const (
+	RunnerInstalled = "installed"
+	RunnerWorkspace = "workspace"
+)
+
+// runnerProperty is the runner argument as the digest and test tools announce it: a checker reads it there, and an
+// installed module that does not announce it judges as it always did (ADR 0535 §2).
+var runnerProperty = map[string]any{"type": "string", "enum": []string{RunnerInstalled, RunnerWorkspace},
+	"description": "optional: installed (the default) runs the step as this installed program; workspace compiles the " +
+		"workspace's own mesh-build-go (modules/mesh-build-go of the merged tree) shut off and runs the step as it, for a " +
+		"change to this module (ADR 0535); the answer's judged_by names the program that ran"}
+
+// candidateModule is the directory of this module's go.mod in the catalogue's tree, and candidateMain its program's
+// package there.
+const (
+	candidateModule = "modules/mesh-build-go"
+	candidateMain   = "./cmd/mesh-build-go"
+)
+
+// runnerArg reads the runner argument: "" (the installed program), installed or workspace.
+func runnerArg(v any) (string, error) {
+	if v == nil {
+		return "", nil
+	}
+	s, ok := v.(string)
+	if !ok || (s != "" && s != RunnerInstalled && s != RunnerWorkspace) {
+		return "", fmt.Errorf("the runner %v is neither %s nor %s", v, RunnerInstalled, RunnerWorkspace)
+	}
+	return s, nil
+}
+
+// workspaceThere refuses a workspace id that is not one, or a workspace this node never fetched.
+func (m *Module) workspaceThere(workspace string) error {
+	if !workspaceOK.MatchString(workspace) {
+		return fmt.Errorf("%q is not a workspace id", workspace)
+	}
+	if _, err := os.Stat(filepath.Join(m.Work, workspace, "tree")); err != nil {
+		return fmt.Errorf("no workspace %s: it was never fetched here, or it is older than a day and was removed", workspace)
+	}
+	return nil
+}
+
+// candidateLocks hold one mutex per workspace, so its candidate is compiled once.
+var candidateLocks sync.Map
+
+// candidate is the program of mesh-build-go compiled from the workspace's tree, in the workspace beside its tree
+// (candidate/mesh-build-go), compiled once per workspace, in a run step shut off with the caches read-only and nothing
+// fetched (GOPROXY=off: the fetch step downloaded every module the tree's go.mod files name). A candidate that does not
+// compile is the step's failure, naming the compiler's first error (ADR 0535 §3).
+func (m *Module) candidate(ctx context.Context, workspace string) (string, error) {
+	exe := filepath.Join(m.Work, workspace, "candidate", "mesh-build-go")
+	mu, _ := candidateLocks.LoadOrStore(exe, &sync.Mutex{})
+	mu.(*sync.Mutex).Lock()
+	defer mu.(*sync.Mutex).Unlock()
+	if st, err := os.Stat(exe); err == nil && st.Mode().IsRegular() {
+		return exe, nil
+	}
+	dir := filepath.Join(m.Work, workspace, "tree", candidateModule)
+	if _, err := os.Stat(filepath.Join(dir, "go.mod")); err != nil {
+		return "", fmt.Errorf("the workspace holds no %s/go.mod, so it has no candidate mesh-build-go to run", candidateModule)
+	}
+	if err := os.MkdirAll(filepath.Dir(exe), 0o700); err != nil {
+		return "", err
+	}
+	run, err := m.stepRunner(workspace, false)
+	if err != nil {
+		return "", err
+	}
+	env := append(m.goEnvFor(dir), "GOPROXY=off", "GOTOOLCHAIN=local")
+	r := run(ctx, env, "go", "-C", dir, "build", "-o", exe, candidateMain)
+	if r.Status != 0 || r.Err != "" {
+		return "", fmt.Errorf("the candidate's mesh-build-go does not compile: %s", compileError(r.Err+"\n"+r.Stderr+"\n"+r.Stdout))
+	}
+	return exe, nil
+}
+
+// compileError is the first line of the compiler's output that says an error: not the `# package` heading go build
+// prints above a package's errors, nor Go's progress.
+func compileError(out string) string {
+	for _, l := range strings.Split(out, "\n") {
+		if l = strings.TrimSpace(l); l != "" && !strings.HasPrefix(l, "#") && !strings.HasPrefix(l, "go: downloading ") {
+			return l
+		}
+	}
+	return firstLine(out)
+}
+
+// mergeCommit is the commit the workspace's tree holds: the merge the fetch made, or the head it fetched.
+func (m *Module) mergeCommit(ctx context.Context, workspace string) (string, error) {
+	r := m.Run(ctx, nil, "git", "-C", filepath.Join(m.Work, workspace, "tree"), "rev-parse", "HEAD")
+	commit := strings.TrimSpace(r.Stdout)
+	if r.Status != 0 || r.Err != "" || commit == "" {
+		return "", fmt.Errorf("the workspace's commit: git rev-parse: %s", firstLine(r.Err+"\n"+r.Stderr))
+	}
+	return commit, nil
+}
+
+// asCandidate runs the spec's step as the candidate's program, shut off, and answers what it said and the judged_by of
+// the answer: "candidate <merge commit>".
+func (m *Module) asCandidate(ctx context.Context, spec levelSpec) (levelAnswer, string, error) {
+	commit, err := m.mergeCommit(ctx, spec.Workspace)
+	if err != nil {
+		return levelAnswer{}, "", err
+	}
+	exe, err := m.candidate(ctx, spec.Workspace)
+	if err != nil {
+		return levelAnswer{}, "", err
+	}
+	a, err := m.shutOff(ctx, exe, spec)
+	if err != nil {
+		return levelAnswer{}, "", fmt.Errorf("the candidate's mesh-build-go: %w", err)
+	}
+	return a, "candidate " + commit, nil
+}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/candidate_integration_test.go b/modules/mesh-build-go/cmd/mesh-build-go/candidate_integration_test.go
new file mode 100644
index 00000000..fe6dc280
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/candidate_integration_test.go
@@ -0,0 +1,127 @@
+//go:build integration
+
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// candidateMainSource is a candidate's program that answers a mark: asked `level <spec>`, it tries what a run step must
+// not reach (the account's key and login, a name, a socket of the machine) and answers a passing level of 42 tests whose
+// why is "mark" and what it reached.
+const candidateMainSource = `package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"net"
+	"os"
+	"time"
+)
+
+func main() {
+	if len(os.Args) != 3 || os.Args[1] != "level" {
+		os.Exit(2)
+	}
+	seen := map[string]bool{}
+	_, err := os.ReadFile(%q)
+	seen["key"] = err == nil
+	_, err = os.ReadFile(%q)
+	seen["login"] = err == nil
+	_, err = net.LookupHost("git.novox.be")
+	seen["resolved"] = err == nil
+	c, err := net.DialTimeout("tcp", %q, 2*time.Second)
+	if err == nil {
+		c.Close()
+	}
+	seen["tcp"] = err == nil
+	raw, _ := json.Marshal(seen)
+	var spec struct {
+		Step string ` + "`json:\"step\"`" + `
+	}
+	json.Unmarshal([]byte(os.Args[2]), &spec)
+	out, _ := json.Marshal(map[string]any{"result": map[string]any{"level": spec.Step, "passed": true, "tests": 42,
+		"packages": []any{}, "why": "mark " + string(raw)}})
+	fmt.Println(string(out))
+}
+`
+
+// 1898/AC4 (ADR 0535 §1): a tree whose mesh-build-go program answers a mark: the unit level with the runner workspace
+// compiles that program and runs it shut off, answers its mark judged by the candidate at the tree's commit, and the
+// program reached no key, no login, no name and no socket of the machine.
+func TestTheUnitLevelUnderTheWorkspaceRunnerRunsTheTreesProgramShutOff(t *testing.T) {
+	needTools(t, "the candidate's program shut off", "go", "git", "bwrap")
+	home := t.TempDir()
+	key, login := filepath.Join(home, ".ssh", "mesh_ed25519"), filepath.Join(home, "broker")
+	for _, f := range []string{key, login} {
+		if err := os.MkdirAll(filepath.Dir(f), 0o700); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(f, []byte("secret"), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	tcp, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer tcp.Close()
+
+	work, err := os.MkdirTemp("", "w")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { os.RemoveAll(work) })
+	tree := filepath.Join(work, "ws1", "tree")
+	dir := filepath.Join(tree, candidateModule)
+	if err := os.MkdirAll(filepath.Join(dir, "cmd", "mesh-build-go"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "go.mod"), []byte("module meshbuildgo\n\ngo 1.25.0\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	src := fmt.Sprintf(candidateMainSource, key, login, tcp.Addr().String())
+	if err := os.WriteFile(filepath.Join(dir, "cmd", "mesh-build-go", "main.go"), []byte(src), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	for _, c := range [][]string{{"init", "-q"}, {"add", "-A"}, {"-c", "user.email=a@b", "-c", "user.name=x", "commit", "-qm", "x"}} {
+		if out, err := exec.Command("git", append([]string{"-C", tree}, c...)...).CombinedOutput(); err != nil {
+			t.Fatalf("git %v: %v %s", c, err, out)
+		}
+	}
+	head, err := exec.Command("git", "-C", tree, "rev-parse", "HEAD").Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	m := &Module{Recipe: Current(), Home: home, Work: work, Cache: t.TempDir()}
+	m.Run = StepRunner(home)
+	out, err := m.test(context.Background(), map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": RunnerWorkspace})
+	if err != nil {
+		t.Fatal(err)
+	}
+	res := out.(*TestResult)
+	if res.Tests != 42 || !strings.HasPrefix(res.Why, "mark ") {
+		t.Fatalf("result = %+v, want the candidate's mark", res)
+	}
+	if want := "candidate " + strings.TrimSpace(string(head)); res.JudgedBy != want {
+		t.Errorf("judged by %q, want %q", res.JudgedBy, want)
+	}
+	var seen map[string]bool
+	if err := json.Unmarshal([]byte(strings.TrimPrefix(res.Why, "mark ")), &seen); err != nil || len(seen) != 4 {
+		t.Fatalf("the program said %q: %v", res.Why, err)
+	}
+	for what, reached := range seen {
+		if reached {
+			t.Errorf("the candidate's program reached %s", what)
+		}
+	}
+}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/candidate_test.go b/modules/mesh-build-go/cmd/mesh-build-go/candidate_test.go
new file mode 100644
index 00000000..cf12dadd
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/candidate_test.go
@@ -0,0 +1,262 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
+)
+
+// Issue 1898 K1, K2 (ADR 0535): asked with the runner workspace, the digest and test steps compile the workspace's own
+// mesh-build-go shut off and run the step as it, in one shut-off process; the answer says which program ran.
+
+const candidateSHA = "fedcba9876543210fedcba9876543210fedcba98"
+
+// candidateRig is a module over a workspace whose tree holds modules/mesh-build-go, with a runner standing for git, go
+// and the candidate's program; what ran shut off is in seen, every command in ran.
+type candidateRig struct {
+	m        *Module
+	states   *keptStates
+	seen     []shutOff
+	ran      []string
+	compile  forgekey.Ran                     // what go build answers; Status 0 writes the program
+	answer   func(spec levelSpec) levelAnswer // what the candidate's program answers
+	specs    []levelSpec
+	compiled int
+}
+
+func newCandidateRig(t *testing.T) *candidateRig {
+	t.Helper()
+	cr := &candidateRig{states: newStates()}
+	cr.m = moduleWith(cr.states, "go1.27.1")
+	cr.m.Home, cr.m.Work, cr.m.Cache = t.TempDir(), t.TempDir(), t.TempDir()
+	cr.m.Executable = "/opt/bundle/mesh-build-go"
+	dir := filepath.Join(cr.m.Work, "ws1", "tree", candidateModule)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "go.mod"), []byte("module meshbuildgo\n\ngo 1.26.0\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	cr.answer = func(spec levelSpec) levelAnswer {
+		if spec.Step == stepDigest {
+			return levelAnswer{Digest: &DigestResult{Level: spec.Level, Packages: map[string]string{"meshbuildgo/cmd/mesh-build-go": "mark"}}}
+		}
+		return levelAnswer{Result: &TestResult{Level: spec.Step, Passed: true, Tests: 42, Packages: []PackageResult{}, Why: "mark"}}
+	}
+	exe := filepath.Join(cr.m.Work, "ws1", "candidate", "mesh-build-go")
+	cr.m.Run = throughBubblewrap(t, func(ctx context.Context, env []string, name string, args ...string) forgekey.Ran {
+		line := name + " " + strings.Join(args, " ")
+		cr.ran = append(cr.ran, line)
+		switch {
+		case name == "git" && slices.Contains(args, "rev-parse"):
+			return forgekey.Ran{Stdout: candidateSHA + "\n"}
+		case name == "git" && slices.Contains(args, "ls-files"):
+			return forgekey.Ran{Stdout: "go.mod\n"}
+		case name == "postgres":
+			return forgekey.Ran{Stdout: "postgres (PostgreSQL) 18.6\n"}
+		case name == "go" && slices.Contains(args, "build"):
+			cr.compiled++
+			for _, want := range []string{"GOPROXY=off", "GOTOOLCHAIN=local", "GOFLAGS=-mod=mod"} {
+				if !slices.Contains(env, want) {
+					t.Errorf("the candidate compiled without %s: %v", want, env)
+				}
+			}
+			if cr.compile.Status == 0 && cr.compile.Err == "" {
+				if err := os.WriteFile(args[slices.Index(args, "-o")+1], []byte("program"), 0o755); err != nil {
+					t.Fatal(err)
+				}
+			}
+			return cr.compile
+		case name == exe && len(args) == 2 && args[0] == levelArg:
+			var spec levelSpec
+			if err := json.Unmarshal([]byte(args[1]), &spec); err != nil {
+				t.Errorf("the spec %s: %v", args[1], err)
+			}
+			cr.specs = append(cr.specs, spec)
+			raw, _ := json.Marshal(cr.answer(spec))
+			return forgekey.Ran{Stdout: string(raw) + "\n"}
+		}
+		return forgekey.Ran{Status: 1, Err: "not expected here: " + line}
+	}, &cr.seen)
+	return cr
+}
+
+func (cr *candidateRig) test(t *testing.T, args map[string]any) (TestResult, delivery.Step) {
+	t.Helper()
+	if _, err := stepTool(t, cr.m, "build_go_test")(args); err != nil {
+		t.Fatal(err)
+	}
+	st := cr.states.waitEnd(t)
+	var res TestResult
+	if st.State == delivery.StepPassed {
+		if err := json.Unmarshal(st.Result, &res); err != nil {
+			t.Fatalf("result %s: %v", st.Result, err)
+		}
+	}
+	return res, st
+}
+
+func TestTheDigestAndTestToolsAnnounceTheRunner(t *testing.T) { // 1898 K1, ADR 0535 §2
+	m := moduleWith(newStates(), "go1.27.1")
+	for _, s := range m.StepTools() {
+		if s.Name != "build_go_digest" && s.Name != "build_go_test" {
+			continue
+		}
+		p, _ := s.Input["properties"].(map[string]any)["runner"].(map[string]any)
+		if p == nil || !slices.Equal(p["enum"].([]string), []string{RunnerInstalled, RunnerWorkspace}) {
+			t.Errorf("%s announces the runner %v, want installed and workspace", s.Name, p)
+		}
+	}
+}
+
+func TestTheUnitLevelUnderTheWorkspaceRunnerRunsTheCandidateShutOff(t *testing.T) { // 1898 K1, K2 (AC4's unit part)
+	cr := newCandidateRig(t)
+	res, st := cr.test(t, map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": RunnerWorkspace, "packages": []any{"./..."}})
+	if st.State != delivery.StepPassed {
+		t.Fatalf("the step ended %s: %s", st.State, st.Reason)
+	}
+	if res.Tests != 42 || res.Why != "mark" || res.JudgedBy != "candidate "+candidateSHA {
+		t.Errorf("result = %+v, want the candidate's mark judged by candidate %s", res, candidateSHA)
+	}
+	if len(cr.specs) != 1 || cr.specs[0].Step != stepUnit || cr.specs[0].Workspace != "ws1" || !slices.Equal(cr.specs[0].Packages, []string{"./..."}) || cr.specs[0].Bound != UnitBound {
+		t.Errorf("the candidate was asked %+v, want the unit level of ws1 with its packages and bound", cr.specs)
+	}
+	for _, l := range cr.ran {
+		if strings.HasPrefix(l, "go ") && strings.Contains(l, " test ") {
+			t.Errorf("the installed program ran %q: the level is the candidate's to run", l)
+		}
+	}
+	// Two commands shut off: the compile, then the candidate's process; each as a run step is (ADR 0448 §2).
+	if len(cr.seen) != 2 || !strings.Contains(cr.seen[0].command, " build ") || !strings.Contains(cr.seen[0].command, "-C "+filepath.Join(cr.m.Work, "ws1", "tree", candidateModule)) ||
+		!strings.HasPrefix(cr.seen[1].command, filepath.Join(cr.m.Work, "ws1", "candidate", "mesh-build-go")+" level ") {
+		t.Fatalf("shut off %+v, want the compile and the candidate's process", cr.seen)
+	}
+	for _, s := range cr.seen {
+		checkShutOff(t, cr.m, s, false)
+	}
+}
+
+func TestTheIntegrationLevelUnderTheWorkspaceRunnerRunsTheCandidate(t *testing.T) { // 1898 K2
+	cr := newCandidateRig(t)
+	res, st := cr.test(t, map[string]any{"workspace": "ws1", "level": LevelIntegration, "runner": RunnerWorkspace})
+	if st.State != delivery.StepPassed || res.JudgedBy != "candidate "+candidateSHA {
+		t.Fatalf("the step ended %s (%s): %+v", st.State, st.Reason, res)
+	}
+	if len(cr.specs) != 1 || cr.specs[0].Step != stepIntegration || cr.specs[0].Bound != IntegrationRun || cr.specs[0].Setup != IntegrationSetup {
+		t.Errorf("the candidate was asked %+v, want the integration level with its bounds", cr.specs)
+	}
+}
+
+func TestTheDigestUnderTheWorkspaceRunnerIsTheCandidatesAndTheCandidateIsCompiledOnce(t *testing.T) { // 1898 K1, K2
+	cr := newCandidateRig(t)
+	out, err := stepTool(t, cr.m, "build_go_digest")(map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": RunnerWorkspace})
+	if err != nil {
+		t.Fatal(err)
+	}
+	d := out.(DigestResult)
+	if d.Packages["meshbuildgo/cmd/mesh-build-go"] != "mark" || d.JudgedBy != "candidate "+candidateSHA {
+		t.Errorf("digest = %+v, want the candidate's, judged by candidate %s", d, candidateSHA)
+	}
+	if len(cr.specs) != 1 || cr.specs[0].Step != stepDigest || cr.specs[0].Level != LevelUnit {
+		t.Errorf("the candidate was asked %+v", cr.specs)
+	}
+	cr.test(t, map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": RunnerWorkspace})
+	if cr.compiled != 1 {
+		t.Errorf("compiled the candidate %d times for two steps of one workspace, want once", cr.compiled)
+	}
+}
+
+func TestACandidateThatDoesNotCompileFailsTheStepWithTheCompilersFirstErrorAndRunsNoLevel(t *testing.T) { // 1898/AC5
+	cr := newCandidateRig(t)
+	cr.compile = forgekey.Ran{Status: 1, Stderr: "# meshbuildgo/cmd/mesh-build-go\ncmd/mesh-build-go/digest.go:12:2: undefined: x\ncmd/mesh-build-go/digest.go:13:2: undefined: y\n"}
+	_, st := cr.test(t, map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": RunnerWorkspace})
+	if st.State == delivery.StepPassed || !strings.Contains(st.Reason, "does not compile: cmd/mesh-build-go/digest.go:12:2: undefined: x") || strings.Contains(st.Reason, "undefined: y") {
+		t.Fatalf("the step ended %s: %q, want it failed naming the first compile error", st.State, st.Reason)
+	}
+	if len(cr.specs) != 0 {
+		t.Errorf("the candidate ran %+v after it failed to compile", cr.specs)
+	}
+	for _, l := range cr.ran {
+		if strings.Contains(l, " test ") {
+			t.Errorf("a level ran: %q", l)
+		}
+	}
+	if _, err := stepTool(t, cr.m, "build_go_digest")(map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": RunnerWorkspace}); err == nil ||
+		!strings.Contains(err.Error(), "undefined: x") {
+		t.Errorf("the digest answered %v, want the compile error", err)
+	}
+}
+
+func TestTheInstalledProgramSaysItJudged(t *testing.T) { // 1898 K1, ADR 0535 §5
+	m, states, ws, _ := workspaceFor(t, "slow")
+	for _, runner := range []any{nil, RunnerInstalled} {
+		args := map[string]any{"workspace": ws, "level": LevelUnit}
+		if runner != nil {
+			args["runner"] = runner
+		}
+		res, st := runUnitWith(t, m, states, args)
+		if st.State != delivery.StepPassed || res.JudgedBy != RunnerInstalled {
+			t.Errorf("runner %v: the step ended %s, judged by %q, want installed", runner, st.State, res.JudgedBy)
+		}
+	}
+	dm, _ := digestFixture(t)
+	if d := digests(t, dm, LevelUnit); d.JudgedBy != RunnerInstalled {
+		t.Errorf("the installed digest is judged by %q, want installed", d.JudgedBy)
+	}
+}
+
+func TestAnUnknownRunnerIsRefused(t *testing.T) { // 1898 K1
+	cr := newCandidateRig(t)
+	if _, err := cr.m.digest(context.Background(), map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": "elsewhere"}); err == nil {
+		t.Error("the digest took the runner elsewhere")
+	}
+	if _, err := cr.m.test(context.Background(), map[string]any{"workspace": "ws1", "level": LevelUnit, "runner": "elsewhere"}); err == nil {
+		t.Error("the test step took the runner elsewhere")
+	}
+}
+
+func TestTheShutOffProcessRunsTheStepItIsAsked(t *testing.T) { // 1898 K2: the candidate's side of the spec
+	m, _, ws, calls := workspaceFor(t, "slow")
+	m.Inside = true
+	a := m.answer(context.Background(), levelSpec{Step: stepUnit, Workspace: ws, Packages: []string{"./..."}, Bound: UnitBound})
+	if a.Error != "" || a.Result == nil || a.Result.Level != LevelUnit || a.Result.Tests == 0 {
+		t.Fatalf("the unit step answered %+v", a)
+	}
+	ranTest := false
+	for _, c := range *calls {
+		ranTest = ranTest || strings.HasPrefix(c.line, "go -C ") && strings.Contains(c.line, " test -json ")
+	}
+	if !ranTest {
+		t.Errorf("the process ran no go test: %+v", *calls)
+	}
+	dm, _ := digestFixture(t)
+	dm.Inside = true
+	if a := dm.answer(context.Background(), levelSpec{Step: stepDigest, Level: LevelUnit, Workspace: "ws1"}); a.Error != "" || a.Digest == nil || len(a.Digest.Packages) == 0 {
+		t.Errorf("the digest step answered %+v", a)
+	}
+	if a := m.answer(context.Background(), levelSpec{Step: "deploy", Workspace: ws}); a.Error == "" {
+		t.Error("the process took a step that is not one")
+	}
+}
+
+func runUnitWith(t *testing.T, m *Module, states *keptStates, args map[string]any) (TestResult, delivery.Step) {
+	t.Helper()
+	if _, err := stepTool(t, m, "build_go_test")(args); err != nil {
+		t.Fatal(err)
+	}
+	st := states.waitEnd(t)
+	var res TestResult
+	if st.State == delivery.StepPassed {
+		if err := json.Unmarshal(st.Result, &res); err != nil {
+			t.Fatalf("result %s: %v", st.Result, err)
+		}
+	}
+	return res, st
+}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/digest.go b/modules/mesh-build-go/cmd/mesh-build-go/digest.go
index 55412464..1c92d318 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/digest.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/digest.go
@@ -5,6 +5,7 @@ package main
 // digest is unchanged is not run again. The tool answers in its call (it is short, ADR 0469 §2).
 
 import (
+	"bytes"
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
@@ -22,7 +23,12 @@ import (
 // DigestVersion is the form of the digest's text: changed whenever what the digest hashes changes in kind.
 // 2: no Go version in the text; the toolchain enters a fingerprint as the agreed version (ADR 0477 §3). 3: the siblings'
 // commits (the lead's decision L5) and the integration level's tags integration,isolation (the checker's agreement).
-const DigestVersion = "3"
+// 4: the siblings' commits enter only the digest of a package whose tests reach a file naming MESH_CHECK_BESIDE (ADR 0536 §1).
+const DigestVersion = "4"
+
+// besideVariable is the name a file bears when it reads where the siblings lie; a package whose tests reach such a file
+// depends on the siblings' commits.
+const besideVariable = "MESH_CHECK_BESIDE"
 
 // integrationTags are the build tags of the integration level, its run and its digest alike: integration, and isolation
 // for the bubblewrap proofs (mesh-sdk's and the build modules'), which a build node can run.
@@ -83,6 +89,9 @@ type DigestResult struct {
 	// only when they are the siblings it gave the fetch.
 	Beside       []string `json:"beside,omitempty"`
 	WithoutTests []string `json:"without_tests,omitempty"`
+	// JudgedBy is the program that ran the digest (ADR 0535 §5): "installed", or "candidate <merge commit>" under the
+	// runner workspace.
+	JudgedBy string `json:"judged_by,omitempty"`
 }
 
 func (m *Module) digestTool() StepTool {
@@ -97,6 +106,7 @@ func (m *Module) digestTool() StepTool {
 			"properties": map[string]any{
 				"workspace": map[string]any{"type": "string", "description": "the workspace id build_go_fetch answered"},
 				"level":     map[string]any{"type": "string", "enum": []string{LevelUnit, LevelIntegration}, "description": "the level"},
+				"runner":    runnerProperty,
 			},
 			"required": []string{"workspace", "level"},
 		},
@@ -107,12 +117,49 @@ func (m *Module) digestTool() StepTool {
 func (m *Module) digest(ctx context.Context, args map[string]any) (any, error) {
 	workspace, _ := args["workspace"].(string)
 	level, _ := args["level"].(string)
+	runner, err := runnerArg(args["runner"])
+	if err != nil {
+		return nil, err
+	}
+	if runner == RunnerWorkspace {
+		if err := m.workspaceThere(workspace); err != nil {
+			return nil, err
+		}
+		switch level {
+		case LevelUnit:
+		case LevelIntegration:
+			if _, err := m.checkPostgres(ctx); err != nil {
+				return nil, err // a *buildstep.Refusal, as the installed program's digest says it
+			}
+		default:
+			return nil, fmt.Errorf("the level %q is neither unit nor integration", level)
+		}
+		a, judged, err := m.asCandidate(ctx, levelSpec{Step: stepDigest, Level: level, Workspace: workspace})
+		if err != nil {
+			return nil, err
+		}
+		if a.Digest == nil {
+			return nil, fmt.Errorf("the candidate's mesh-build-go answered no digest")
+		}
+		a.Digest.JudgedBy = judged
+		return *a.Digest, nil
+	}
+	res, err := m.digestHere(ctx, workspace, level)
+	if err != nil {
+		return nil, err
+	}
+	res.JudgedBy = RunnerInstalled
+	return res, nil
+}
+
+// digestHere is the digest step as this program runs it.
+func (m *Module) digestHere(ctx context.Context, workspace, level string) (DigestResult, error) {
 	if !workspaceOK.MatchString(workspace) {
-		return nil, fmt.Errorf("%q is not a workspace id", workspace)
+		return DigestResult{}, fmt.Errorf("%q is not a workspace id", workspace)
 	}
 	tree := filepath.Join(m.Work, workspace, "tree")
 	if _, err := os.Stat(tree); err != nil {
-		return nil, fmt.Errorf("no workspace %s: it was never fetched here, or it is older than a day and was removed", workspace)
+		return DigestResult{}, fmt.Errorf("no workspace %s: it was never fetched here, or it is older than a day and was removed", workspace)
 	}
 	var tags []string
 	header := []string{"digest " + DigestVersion, "level " + level}
@@ -120,34 +167,35 @@ func (m *Module) digest(ctx context.Context, args map[string]any) (any, error) {
 	case LevelUnit:
 	case LevelIntegration:
 		if _, err := m.checkPostgres(ctx); err != nil {
-			return nil, err
+			return DigestResult{}, err
 		}
 		tags = integrationTags
 		header = append(header, "postgres "+strconv.Itoa(m.Recipe.PostgresMajor), "nats-server "+m.Recipe.NatsServer)
 	default:
-		return nil, fmt.Errorf("the level %q is neither unit nor integration", level)
+		return DigestResult{}, fmt.Errorf("the level %q is neither unit nor integration", level)
 	}
 	header = append(header, "tags "+strings.Join(tags, ","))
 	beside, err := m.besideLines(workspace)
 	if err != nil {
-		return nil, err
+		return DigestResult{}, err
 	}
+	besideHeader := make([]string, 0, len(beside))
 	for _, b := range beside {
-		header = append(header, "beside "+b)
+		besideHeader = append(besideHeader, "beside "+b)
 	}
 
 	dirs, err := m.goModDirs(ctx, tree)
 	if err != nil {
-		return nil, err
+		return DigestResult{}, err
 	}
 	res := DigestResult{Level: level, Packages: map[string]string{}, Beside: beside}
 	for _, d := range dirs {
 		pkgs, err := m.listPackages(ctx, workspace, tree, d, tags)
 		if err != nil {
-			return nil, err
+			return DigestResult{}, err
 		}
-		if err := digestModule(&res, pkgs, tree, header); err != nil {
-			return nil, err
+		if err := digestModule(&res, pkgs, tree, header, besideHeader); err != nil {
+			return DigestResult{}, err
 		}
 	}
 	sort.Strings(res.WithoutTests)
@@ -190,7 +238,7 @@ func parsePackages(out string) (map[string]*goPackage, error) {
 
 // digestModule adds to res the digest of each root package of one go.mod directory. A root is a package of the main
 // module that is listed, not only a dependency, and not a variant of another.
-func digestModule(res *DigestResult, pkgs map[string]*goPackage, tree string, header []string) error {
+func digestModule(res *DigestResult, pkgs map[string]*goPackage, tree string, header, beside []string) error {
 	roots := []string{}
 	for path, p := range pkgs {
 		if p.ForTest == "" && !p.DepOnly && !p.Standard && !strings.HasSuffix(path, ".test") && p.Module != nil && p.Module.Main {
@@ -208,7 +256,7 @@ func digestModule(res *DigestResult, pkgs map[string]*goPackage, tree string, he
 		if testMain == nil {
 			return fmt.Errorf("go list gave no test binary for %s", path)
 		}
-		d, err := packageDigest(root, testMain, pkgs, tree, header)
+		d, err := packageDigest(root, testMain, pkgs, tree, header, beside)
 		if err != nil {
 			return fmt.Errorf("%s: %w", path, err)
 		}
@@ -219,8 +267,9 @@ func digestModule(res *DigestResult, pkgs map[string]*goPackage, tree string, he
 
 // packageDigest is the digest of one package: a hash over a sorted text of the header, the files of every package its
 // tests reach in the module's own tree (by their content, under their path in the tree), the version and checksum of
-// every module outside it, and the package's own test data. A package of the standard library is the Go version's.
-func packageDigest(root, testMain *goPackage, pkgs map[string]*goPackage, tree string, header []string) (string, error) {
+// every module outside it, and the package's own test data. The beside lines (the siblings' commits) join the header
+// only when a file the tests reach names MESH_CHECK_BESIDE. A package of the standard library is the Go version's.
+func packageDigest(root, testMain *goPackage, pkgs map[string]*goPackage, tree string, header, beside []string) (string, error) {
 	reached := map[string]*goPackage{root.ImportPath: root}
 	for _, dep := range testMain.Deps {
 		if p := pkgs[dep]; p != nil {
@@ -228,6 +277,7 @@ func packageDigest(root, testMain *goPackage, pkgs map[string]*goPackage, tree s
 		}
 	}
 	lines := []string{}
+	readsBeside := false
 	for _, p := range reached {
 		if p.Standard || p.Module == nil {
 			continue
@@ -246,6 +296,11 @@ func packageDigest(root, testMain *goPackage, pkgs map[string]*goPackage, tree s
 				return "", err
 			}
 			lines = append(lines, l)
+			if !readsBeside {
+				if raw, err := os.ReadFile(filepath.Join(p.Dir, f)); err == nil && bytes.Contains(raw, []byte(besideVariable)) {
+					readsBeside = true
+				}
+			}
 		}
 	}
 	data := filepath.Join(root.Dir, "testdata")
@@ -263,6 +318,9 @@ func packageDigest(root, testMain *goPackage, pkgs map[string]*goPackage, tree s
 		}
 	}
 	sort.Strings(lines)
+	if readsBeside {
+		header = append(append([]string{}, header...), beside...)
+	}
 	h := sha256.New()
 	for _, l := range append(append([]string{}, header...), lines...) {
 		h.Write([]byte(l + "\n"))
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/digest_test.go b/modules/mesh-build-go/cmd/mesh-build-go/digest_test.go
index ff1e8e6e..5e2d81d0 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/digest_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/digest_test.go
@@ -173,7 +173,7 @@ func TestAModuleOutsideTheTreeCountsByVersionAndChecksum(t *testing.T) {
 	testMain := &goPackage{ImportPath: "x/r.test", Deps: []string{ext.ImportPath}}
 	pkgs[root.ImportPath] = root
 	digest := func() string {
-		d, err := packageDigest(root, testMain, pkgs, dir, []string{"digest 1"})
+		d, err := packageDigest(root, testMain, pkgs, dir, []string{"digest 1"}, nil)
 		if err != nil {
 			t.Fatal(err)
 		}
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 435def4d..e5a1b49b 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/fetch.go
@@ -6,12 +6,15 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
 	"os"
 	"path/filepath"
 	"regexp"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 
 	"git.novox.be/novox/mesh-sdk/go/buildstep"
@@ -114,7 +117,7 @@ func (m *Module) fetchInto(ctx context.Context, req buildstep.FetchRequest, id s
 		return nil, err
 	}
 	remote := m.remote(repository)
-	if err := buildstep.FetchCommit(ctx, m.Run, tree, remote, commit, ref, m.FetchOptions...); err != nil {
+	if err := buildstep.FetchCommit(ctx, m.Run, tree, remote, commit, ref, m.fetchOptions(ctx, repository, commit, ref)...); err != nil {
 		return nil, err
 	}
 	res := FetchResult{Workspace: id, Commit: commit}
@@ -123,6 +126,9 @@ func (m *Module) fetchInto(ctx context.Context, req buildstep.FetchRequest, id s
 			return nil, err
 		}
 	}
+	if err := m.dissociate(ctx, tree); err != nil {
+		return nil, err
+	}
 	if res.Beside, err = m.placeSiblings(ctx, root, siblings); err != nil {
 		return nil, err
 	}
@@ -143,11 +149,27 @@ func (m *Module) fetchInto(ctx context.Context, req buildstep.FetchRequest, id s
 	if base != "" && res.Merge == "" {
 		return res, nil // a conflict: the tree is at the trunk, nothing of the head is built or tested
 	}
-	for _, d := range dirs {
+	// The directories download side by side, up to the bound (ADR 0536 §2); the first failure in directory order is the
+	// step's. Resolving a pin places a repository beside the workspace, which two directories may share: one at a time.
+	var pins sync.Mutex
+	errs := make([]error, len(dirs))
+	slots := make(chan struct{}, m.parallel())
+	var wg sync.WaitGroup
+	for i, d := range dirs {
 		if _, err := os.Stat(filepath.Join(tree, d, "vendor", "modules.txt")); err == nil {
 			continue // a module that vendors resolves nothing
 		}
-		if err := m.downloadModules(ctx, root, filepath.Join(tree, d), d); err != nil {
+		wg.Add(1)
+		slots <- struct{}{}
+		go func() {
+			defer wg.Done()
+			defer func() { <-slots }()
+			errs[i] = m.downloadModules(ctx, root, filepath.Join(tree, d), d, &pins)
+		}()
+	}
+	wg.Wait()
+	for _, err := range errs {
+		if err != nil {
 			return nil, err
 		}
 	}
@@ -157,13 +179,15 @@ func (m *Module) fetchInto(ctx context.Context, req buildstep.FetchRequest, id s
 // downloadModules downloads and verifies the Go modules of the directory dir (d in the tree). A pin of a forge module
 // that Go cannot fetch is looked for among the forge's pull request heads and handed to Go by a modfile (pins.go); a
 // failure that names no such pin is the first failure.
-func (m *Module) downloadModules(ctx context.Context, root, dir, d string) error {
+func (m *Module) downloadModules(ctx context.Context, root, dir, d string, pins *sync.Mutex) error {
 	for {
 		failure := m.downloadAndVerify(ctx, dir, d)
 		if failure == nil {
 			return nil
 		}
+		pins.Lock()
 		progress, err := m.resolvePins(ctx, root, dir, failure.Error())
+		pins.Unlock()
 		if err != nil {
 			return err
 		}
@@ -173,8 +197,41 @@ func (m *Module) downloadModules(ctx context.Context, root, dir, d string) error
 	}
 }
 
+// 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
+
+// verifiedMark is the file naming a pass of go mod verify for a go.mod and go.sum pair, "" when a pass is not kept:
+// the module cache is only read by steps that are shut off with it read-only (ADR 0448 §3), so the pass holds only then,
+// and a directory with a pin modfile verifies another file.
+func (m *Module) verifiedMark(dir string) string {
+	if m.Cache == "" || m.Bubblewrap == nil {
+		return ""
+	}
+	if _, ok := m.pinModfile(dir); ok {
+		return ""
+	}
+	h := sha256.New()
+	for _, name := range []string{"go.mod", "go.sum"} {
+		raw, _ := os.ReadFile(filepath.Join(dir, name))
+		fmt.Fprintf(h, "%s %d\n", name, len(raw))
+		h.Write(raw)
+	}
+	return filepath.Join(m.Cache, "verified", hex.EncodeToString(h.Sum(nil)))
+}
+
 func (m *Module) downloadAndVerify(ctx context.Context, dir, d string) error {
 	for _, verb := range []string{"download", "verify"} {
+		mark := ""
+		if verb == "verify" {
+			if mark = m.verifiedMark(dir); mark != "" {
+				mu, _ := verifyLocks.LoadOrStore(mark, &sync.Mutex{})
+				mu.(*sync.Mutex).Lock()
+				defer mu.(*sync.Mutex).Unlock()
+				if _, err := os.Stat(mark); err == nil {
+					continue // this go.mod and go.sum passed before
+				}
+			}
+		}
 		cmd, env := []string{"-C", dir, "mod", verb}, m.goEnvFor(dir)
 		if modfile, ok := m.pinModfile(dir); ok && verb == "verify" {
 			// go mod verify takes no -modfile: it is run on the modfile itself, which names the placed repositories.
@@ -184,6 +241,12 @@ func (m *Module) downloadAndVerify(ctx context.Context, dir, d string) error {
 		if r.Status != 0 || r.Err != "" {
 			return fmt.Errorf("go mod %s in %s: %s", verb, d, causeLine(r.Err+"\n"+r.Stdout+"\n"+r.Stderr))
 		}
+		if mark != "" {
+			// Said after the pass; a mark that cannot be written only means the next fetch verifies again.
+			if os.MkdirAll(filepath.Dir(mark), 0o700) == nil {
+				os.WriteFile(mark, nil, 0o600)
+			}
+		}
 	}
 	return nil
 }
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 d9d092c5..04874659 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
@@ -30,6 +30,7 @@ type fakeStep struct {
 	gomods  string
 	gate    chan struct{} // when set, the first git command waits for it
 	goRule  func(line string) forgekey.Ran
+	files   map[string]string                      // files (relative to the tree) written at checkout: go.mod and go.sum of the fixture
 	vendors []string                               // directories (relative to the tree) that get a vendor/modules.txt at checkout
 	rule    func(line string) (forgekey.Ran, bool) // answers a command before the defaults do
 }
@@ -55,6 +56,10 @@ func (f *fakeStep) run(ctx context.Context, env []string, name string, args ...s
 	case name == "git" && strings.Contains(line, " checkout "):
 		for i, a := range args {
 			if a == "-C" {
+				for name, text := range f.files {
+					os.MkdirAll(filepath.Dir(filepath.Join(args[i+1], name)), 0o755)
+					os.WriteFile(filepath.Join(args[i+1], name), []byte(text), 0o644)
+				}
 				for _, v := range f.vendors {
 					os.MkdirAll(filepath.Join(args[i+1], v, "vendor"), 0o755)
 					os.WriteFile(filepath.Join(args[i+1], v, "vendor", "modules.txt"), nil, 0o644)
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/integration.go b/modules/mesh-build-go/cmd/mesh-build-go/integration.go
index 03de39ab..231714a0 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/integration.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/integration.go
@@ -75,8 +75,16 @@ func (m *Module) integrationHere(ctx context.Context, workspace, tree string, pa
 	env := append([]string{"MESH_TEST_POSTGRES=" + postgres, "MESH_TEST_NATS=" + nats}, m.besideEnv(workspace)...)
 	// Compiling counts as setup: the tests are built once, here, with the addresses a TestMain may read, and the run
 	// below finds them built. Whether they compile is for the run to say; its failure is not read here.
+	// Only the directories the selected packages lie in compile (ADR 0536 §3); no selection is every directory.
+	byDir, err := packagesByDir(tree, dirs, packages)
+	if err != nil {
+		return nil, err
+	}
 	for _, d := range dirs {
-		m.Run(ctx, append(m.goEnvFor(filepath.Join(tree, d)), env...), "go", "-C", filepath.Join(tree, d), "test", "-tags", strings.Join(integrationTags, ","), "-run", "^$", "-count=1", "./...")
+		if len(byDir[d]) == 0 {
+			continue
+		}
+		m.Run(ctx, append(m.goEnvFor(filepath.Join(tree, d)), env...), "go", append([]string{"-C", filepath.Join(tree, d), "test", "-tags", strings.Join(integrationTags, ","), "-run", "^$", "-count=1"}, byDir[d]...)...)
 	}
 	setup := m.clock().Sub(began)
 
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/integration_level_test.go b/modules/mesh-build-go/cmd/mesh-build-go/integration_level_test.go
index 9d2aafa1..fafb7bda 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/integration_level_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/integration_level_test.go
@@ -31,6 +31,10 @@ type integFake struct {
 	busDown bool
 	runDirs []string // run directories seen when the store was started, to be gone at the end
 	panicOn string   // a phase that panics
+	// gomods is what git ls-files answers ("go.mod\n" when empty); onTest is called, unlocked, as each go test -json
+	// command runs, with its line.
+	gomods string
+	onTest func(line string)
 }
 
 func newIntegFake(t *testing.T, captured string) *integFake {
@@ -63,10 +67,16 @@ func (f *integFake) run(ctx context.Context, env []string, name string, args ...
 	case name == "postgres":
 		return forgekey.Ran{Stdout: f.postgres + "\n"}
 	case name == "git" && len(args) > 2 && args[2] == "ls-files":
+		if f.gomods != "" {
+			return forgekey.Ran{Stdout: f.gomods}
+		}
 		return forgekey.Ran{Stdout: "go.mod\n"}
 	case name == "go" && strings.Contains(line, "-run ^$"):
 		f.tick("compile")
 	case name == "go":
+		if f.onTest != nil {
+			f.onTest(line)
+		}
 		f.tick("test")
 		status := 0
 		if strings.Contains(f.captured, `"Action":"fail"`) {
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/level.go b/modules/mesh-build-go/cmd/mesh-build-go/level.go
index 520962dc..6ff18aa6 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/level.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/level.go
@@ -18,11 +18,22 @@ import (
 	"git.novox.be/novox/mesh-sdk/go/buildstep"
 )
 
-// levelArg is the first argument that makes this binary an integration level's process.
+// levelArg is the first argument that makes this binary a step's shut-off process: the integration level's, and, when
+// the merge changes this module, the candidate's program running a digest or a level (ADR 0535 §1, candidate.go).
 const levelArg = "level"
 
-// levelSpec is what the shut-off process is asked: the workspace, the packages and the -run pattern, the level's bounds.
+// The steps a shut-off process runs (levelSpec.Step); empty is the integration level, as a spec said before the runner.
+const (
+	stepDigest      = "digest"
+	stepUnit        = LevelUnit
+	stepIntegration = LevelIntegration
+)
+
+// levelSpec is what the shut-off process is asked: the step, the workspace, the packages and the -run pattern, the
+// level's bounds; for a digest, its level.
 type levelSpec struct {
+	Step      string        `json:"step,omitempty"`
+	Level     string        `json:"level,omitempty"`
 	Work      string        `json:"work"`
 	Cache     string        `json:"cache"`
 	Workspace string        `json:"workspace"`
@@ -32,10 +43,11 @@ type levelSpec struct {
 	Bound     time.Duration `json:"bound"`
 }
 
-// levelAnswer is the one line the process prints: the level's result, or why it could not run.
+// levelAnswer is the one line the process prints: the level's result or the digest, or why it could not run.
 type levelAnswer struct {
-	Result *TestResult `json:"result,omitempty"`
-	Error  string      `json:"error,omitempty"`
+	Result *TestResult   `json:"result,omitempty"`
+	Digest *DigestResult `json:"digest,omitempty"`
+	Error  string        `json:"error,omitempty"`
 }
 
 // executable is this program's path, bound read-only into the step: Executable when a test set it.
@@ -50,35 +62,63 @@ func (m *Module) executable() (string, error) {
 	return filepath.EvalSymlinks(exe)
 }
 
-// integrationShutOff runs the integration level in one shut-off process and reads its answer.
+// integrationShutOff runs the integration level in one shut-off process of this program and reads its answer.
 func (m *Module) integrationShutOff(ctx context.Context, workspace string, packages []string, pattern string) (*TestResult, error) {
 	exe, err := m.executable()
 	if err != nil {
 		return nil, fmt.Errorf("the integration level's program: %w", err)
 	}
 	b := m.integrationBounds()
-	spec, err := json.Marshal(levelSpec{Work: m.Work, Cache: m.Cache, Workspace: workspace, Packages: packages, Run: pattern, Setup: b.Setup, Bound: b.Run})
+	a, err := m.shutOff(ctx, exe, levelSpec{Step: stepIntegration, Workspace: workspace, Packages: packages, Run: pattern, Setup: b.Setup, Bound: b.Run})
 	if err != nil {
 		return nil, err
 	}
+	if a.Result == nil {
+		return nil, fmt.Errorf("the integration level's shut-off process answered no result")
+	}
+	return a.Result, nil
+}
+
+// shutOff runs the program exe as one shut-off process of the workspace (`exe level <spec>`) and reads its answer: the
+// workspace read-write, the program alone read-only, the shared caches beneath a throwaway layer, the Node build
+// module's toolchains read-only for the integration level.
+func (m *Module) shutOff(ctx context.Context, exe string, spec levelSpec) (levelAnswer, error) {
+	spec.Work, spec.Cache = m.Work, m.Cache
+	raw, err := json.Marshal(spec)
+	if err != nil {
+		return levelAnswer{}, err
+	}
 	for _, c := range m.caches() {
 		if err := os.MkdirAll(c, 0o700); err != nil {
-			return nil, fmt.Errorf("the shared cache %s: %w", c, err)
+			return levelAnswer{}, fmt.Errorf("the shared cache %s: %w", c, err)
 		}
 	}
-	iso := m.isolation(workspace, false)
+	iso := m.isolation(spec.Workspace, false)
 	iso.ReadOnly = append(iso.ReadOnly, exe) // the program alone, not the directory it lies in
-	iso.ReadOnly = append(iso.ReadOnly, m.nodeToolchains()...)
-	r := buildstep.Isolated(m.Run, iso)(ctx, nil, exe, levelArg, string(spec))
+	if spec.Step == stepIntegration || spec.Step == "" {
+		iso.ReadOnly = append(iso.ReadOnly, m.nodeToolchains()...)
+	}
+	r := buildstep.Isolated(m.Run, iso)(ctx, nil, exe, levelArg, string(raw))
 	var answer levelAnswer
 	line := lastLine(r.Stdout)
-	if err := json.Unmarshal([]byte(line), &answer); err != nil || (answer.Result == nil && answer.Error == "") {
-		return nil, fmt.Errorf("the integration level's shut-off process answered nothing it says (status %d %s): %s", r.Status, r.Err, firstLine(r.Stderr+"\n"+r.Stdout))
+	if err := json.Unmarshal([]byte(line), &answer); err != nil || (answer.Result == nil && answer.Digest == nil && answer.Error == "") {
+		return levelAnswer{}, fmt.Errorf("the %s step's shut-off process answered nothing it says (status %d %s): %s", stepName(spec.Step), r.Status, r.Err, firstLine(r.Stderr+"\n"+r.Stdout))
 	}
 	if answer.Error != "" {
-		return nil, fmt.Errorf("%s", answer.Error)
+		return levelAnswer{}, fmt.Errorf("%s", answer.Error)
 	}
-	return answer.Result, nil
+	return answer, nil
+}
+
+// stepName names a spec's step in words.
+func stepName(step string) string {
+	switch step {
+	case stepDigest:
+		return "digest"
+	case stepUnit:
+		return "unit level's"
+	}
+	return "integration level's"
 }
 
 // NodeToolchains is the Node build module's toolchains directory on this node: its cache directory (module.json
@@ -124,18 +164,40 @@ func runLevelProcess(args []string) int {
 	if !workspaceOK.MatchString(spec.Workspace) {
 		return say(levelAnswer{Error: fmt.Sprintf("%q is not a workspace id", spec.Workspace)})
 	}
-	m := &Module{Inside: true, Recipe: Current(), Work: spec.Work, Cache: spec.Cache, Home: os.Getenv("HOME"),
+	m := &Module{Inside: true, Recipe: Current(), Work: spec.Work, Cache: spec.Cache, Home: os.Getenv("HOME"), UnitBound: spec.Bound,
 		IntegrationBounds: Bounds{Setup: spec.Setup, Run: spec.Bound}, Say: func(string, ...any) {}}
 	m.Run = StepRunner(m.Home)
-	ctx := context.Background()
-	major, err := m.checkPostgres(ctx)
-	if err != nil {
-		return say(levelAnswer{Error: err.Error()})
+	return say(m.answer(context.Background(), spec))
+}
+
+// answer runs the spec's step in this process, already shut off: a digest of a level, the unit level or the integration
+// level of the workspace.
+func (m *Module) answer(ctx context.Context, spec levelSpec) levelAnswer {
+	tree := filepath.Join(m.Work, spec.Workspace, "tree")
+	switch spec.Step {
+	case stepDigest:
+		d, err := m.digestHere(ctx, spec.Workspace, spec.Level)
+		if err != nil {
+			return levelAnswer{Error: err.Error()}
+		}
+		return levelAnswer{Digest: &d}
+	case stepUnit:
+		res, err := m.runLevel(ctx, tree, levelOpts{workspace: spec.Workspace, level: LevelUnit, packages: spec.Packages, run: spec.Run,
+			bounds: Bounds{Run: m.unitBound()}, env: m.besideEnv(spec.Workspace)})
+		if err != nil {
+			return levelAnswer{Error: err.Error()}
+		}
+		return levelAnswer{Result: res}
+	case stepIntegration, "":
+		major, err := m.checkPostgres(ctx)
+		if err != nil {
+			return levelAnswer{Error: err.Error()}
+		}
+		res, err := m.integrationHere(ctx, spec.Workspace, tree, spec.Packages, spec.Run, major)
+		if err != nil {
+			return levelAnswer{Error: err.Error()}
+		}
+		return levelAnswer{Result: res}
 	}
-	tree := filepath.Join(spec.Work, spec.Workspace, "tree")
-	res, err := m.integrationHere(ctx, spec.Workspace, tree, spec.Packages, spec.Run, major)
-	if err != nil {
-		return say(levelAnswer{Error: err.Error()})
-	}
-	return say(levelAnswer{Result: res})
+	return levelAnswer{Error: fmt.Sprintf("the step %q is not digest, unit or integration", spec.Step)}
 }
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/levels.go b/modules/mesh-build-go/cmd/mesh-build-go/levels.go
index 9631252a..455ca95f 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/levels.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/levels.go
@@ -83,6 +83,9 @@ type TestResult struct {
 	OverWhich string `json:"over_which,omitempty"`
 	Store     string `json:"store,omitempty"`
 	Bus       string `json:"bus,omitempty"`
+	// JudgedBy is the program that ran the level (ADR 0535 §5): "installed", or "candidate <merge commit>" under the
+	// runner workspace.
+	JudgedBy string `json:"judged_by,omitempty"`
 }
 
 // event is one line of `go test -json` (test2json, and the build events go test adds).
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/mirror.go b/modules/mesh-build-go/cmd/mesh-build-go/mirror.go
new file mode 100644
index 00000000..0ad17fda
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/mirror.go
@@ -0,0 +1,73 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+	"syscall"
+
+	"git.novox.be/novox/mesh-sdk/go/buildstep"
+)
+
+// mirrorLocks hold one mutex per mirror of this process; the lock file beside the mirror holds off another process.
+var mirrorLocks sync.Map
+
+// mirrorDir is where the bare mirror of a repository lies: one per repository in the module's cache (ADR 0536 §2).
+func (m *Module) mirrorDir(repository string) string {
+	return filepath.Join(m.Cache, "mirror", filepath.FromSlash(repository)+".git")
+}
+
+// fetchOptions are the options a fetch of repository runs with: the module's own, and the mirror of the repository
+// brought to the commit, so git transfers only the objects the mirror lacks. A mirror that cannot be brought to the
+// commit is said and left out: the fetch is then the full one, as it was.
+func (m *Module) fetchOptions(ctx context.Context, repository, commit, ref string) []buildstep.Option {
+	opts := m.FetchOptions
+	if m.Cache == "" || !repositoryOK.MatchString(repository) {
+		return opts
+	}
+	mirror := m.mirrorDir(repository)
+	if err := m.updateMirror(ctx, mirror, m.remote(repository), commit, ref); err != nil {
+		if m.Say != nil {
+			m.Say("the mirror of %s is not used: %v", repository, err)
+		}
+		return opts
+	}
+	return append(append([]buildstep.Option{}, opts...), buildstep.WithMirror(mirror))
+}
+
+// updateMirror brings the mirror to the commit under the mirror's lock: a mutex for the checks of this process, a lock
+// file for another process of the node.
+func (m *Module) updateMirror(ctx context.Context, mirror, remote, commit, ref string) error {
+	mu, _ := mirrorLocks.LoadOrStore(mirror, &sync.Mutex{})
+	mu.(*sync.Mutex).Lock()
+	defer mu.(*sync.Mutex).Unlock()
+	if err := os.MkdirAll(mirror, 0o700); err != nil { // git init --bare takes an empty directory
+		return err
+	}
+	lock, err := os.OpenFile(mirror+".lock", os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return err
+	}
+	defer lock.Close()
+	if err := syscall.Flock(int(lock.Fd()), syscall.LOCK_EX); err != nil {
+		return fmt.Errorf("lock %s: %w", mirror, err)
+	}
+	defer syscall.Flock(int(lock.Fd()), syscall.LOCK_UN)
+	return buildstep.UpdateMirror(ctx, m.Run, mirror, remote, commit, ref, m.FetchOptions...)
+}
+
+// dissociate makes the repository in dir self-contained again, as git clone --dissociate does: the objects it borrowed
+// from the mirror are packed into it and its alternates file goes, so a step shut off with the workspace alone (the
+// mirror is not bound into it) finds every object. The network saving stays: the borrowing was for the transfer.
+func (m *Module) dissociate(ctx context.Context, dir string) error {
+	alternates := filepath.Join(dir, ".git", "objects", "info", "alternates")
+	if _, err := os.Stat(alternates); err != nil {
+		return nil // no mirror was used
+	}
+	if r := m.Run(ctx, []string{"GIT_TERMINAL_PROMPT=0"}, "git", "-C", dir, "repack", "-a", "-d", "-q"); r.Status != 0 || r.Err != "" {
+		return fmt.Errorf("git repack in %s: %s", dir, firstLine(r.Err+" "+r.Stderr))
+	}
+	return os.Remove(alternates)
+}
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
new file mode 100644
index 00000000..6721155e
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/mirror_test.go
@@ -0,0 +1,190 @@
+package main
+
+import (
+	"encoding/json"
+	"git.novox.be/novox/mesh-sdk/go/buildstep"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+func fetchOnce(t *testing.T, m *Module, states *keptStates, args map[string]any) FetchResult {
+	t.Helper()
+	if _, err := fetchTool(t, m)(args); err != nil {
+		t.Fatal(err)
+	}
+	st := states.waitEnd(t)
+	if st.State != delivery.StepPassed {
+		t.Fatalf("the step ended %s: %s", st.State, st.Reason)
+	}
+	var res FetchResult
+	if err := json.Unmarshal(st.Result, &res); err != nil {
+		t.Fatal(err)
+	}
+	return res
+}
+
+// Issue 890/AC4 (ADR 0536 §2): two fetches of one repository on one module bring the same mirror to their commit, and
+// each workspace's alternates file names the mirror's objects.
+func TestTwoFetchesOfARepositoryUseOneMirrorAndReferenceIt(t *testing.T) {
+	f := &fakeStep{gomods: "go.mod\n"}
+	m, states := fetchModule(t, f)
+	mirror := filepath.Join(m.Cache, "mirror", "novox", "mesh-sdk.git")
+	var workspaces []string
+	for i := 0; i < 2; i++ {
+		workspaces = append(workspaces, fetchOnce(t, m, states, fetchArgs).Workspace)
+	}
+	updates := 0
+	for _, l := range f.lines() {
+		if strings.Contains(l, "-C "+mirror+" fetch --no-tags origin "+commitSHA) {
+			updates++
+		}
+		if strings.Contains(l, "mirror") && !strings.Contains(l, mirror) {
+			t.Errorf("a command names another mirror: %s", l)
+		}
+	}
+	if updates != 2 {
+		t.Errorf("the mirror %s was updated %d times, want 2:\n%s", mirror, updates, strings.Join(f.lines(), "\n"))
+	}
+	// The fake git writes nothing, so the alternates file the SDK wrote stands until the fetch dissociates: it is gone
+	// at the end and git repack -a -d ran in each tree.
+	for _, ws := range workspaces {
+		tree := filepath.Join(m.Work, ws, "tree")
+		if _, err := os.Stat(filepath.Join(tree, ".git", "objects", "info", "alternates")); err == nil {
+			t.Errorf("the workspace %s still borrows from the mirror", ws)
+		}
+		if !strings.Contains(strings.Join(f.lines(), "\n"), "-C "+tree+" repack -a -d") {
+			t.Errorf("git repack -a -d did not run in %s", tree)
+		}
+	}
+}
+
+// A sibling is placed through its own repository's mirror.
+func TestASiblingIsPlacedThroughItsOwnMirror(t *testing.T) {
+	f := &fakeStep{gomods: "go.mod\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}}}
+	ws := fetchOnce(t, m, states, args).Workspace
+	for _, repo := range []string{"mesh-controller", "mesh-sdk"} {
+		mirror := filepath.Join(m.Cache, "mirror", "novox", repo+".git")
+		if !strings.Contains(strings.Join(f.lines(), "\n"), "-C "+mirror+" fetch --no-tags origin "+commitSHA) {
+			t.Errorf("the mirror of %s was not updated", repo)
+		}
+	}
+	if _, err := os.Stat(filepath.Join(m.Work, ws, "beside", "mesh-sdk", ".git", "objects", "info", "alternates")); err == nil {
+		t.Errorf("the sibling still borrows from the mirror")
+	}
+}
+
+// Two checks at once on one node update one mirror one after the other.
+func TestTwoUpdatesOfOneMirrorNeverRunAtOnce(t *testing.T) {
+	var mu sync.Mutex
+	inside, most := 0, 0
+	f := &fakeStep{gomods: "go.mod\n"}
+	f.rule = func(line string) (r runAnswer, ok bool) {
+		if !strings.Contains(line, "fetch --no-tags origin") || strings.Contains(line, "--depth") {
+			return r, false
+		}
+		mu.Lock()
+		inside++
+		if inside > most {
+			most = inside
+		}
+		mu.Unlock()
+		sleepABit()
+		mu.Lock()
+		inside--
+		mu.Unlock()
+		return r, true
+	}
+	m, states := fetchModule(t, f)
+	var wg sync.WaitGroup
+	for i := 0; i < 2; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			fetchTool(t, m)(fetchArgs)
+		}()
+	}
+	wg.Wait()
+	states.waitEnd(t)
+	states.waitEnd(t)
+	if most != 1 {
+		t.Errorf("%d updates of the mirror ran at once", most)
+	}
+}
+
+// A mirror that cannot be updated does not fail the fetch: the fetch goes on without it.
+func TestAMirrorThatCannotBeUpdatedLetsTheFetchGoOnWithoutIt(t *testing.T) {
+	f := &fakeStep{gomods: "go.mod\n"}
+	f.rule = func(line string) (r runAnswer, ok bool) {
+		if strings.Contains(line, "mirror") && strings.Contains(line, " fetch ") {
+			return runAnswer{Status: 1, Stderr: "fatal: no space left\n"}, true
+		}
+		return r, false
+	}
+	m, states := fetchModule(t, f)
+	ws := fetchOnce(t, m, states, fetchArgs).Workspace
+	if _, err := os.Stat(filepath.Join(m.Work, ws, "tree", ".git", "objects", "info", "alternates")); err == nil {
+		t.Errorf("the workspace borrows from a mirror that is not there")
+	}
+}
+
+func sleepABit() { time.Sleep(30 * time.Millisecond) }
+
+// The workspace is self-contained after the fetch: no alternates file, and git finds the commit with the mirror gone,
+// as a step shut off with the workspace alone needs. Real git, a local repository as the forge.
+func TestTheWorkspaceIsSelfContainedAfterAFetchThroughTheMirror(t *testing.T) {
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skip("no git")
+	}
+	forge := t.TempDir()
+	src := filepath.Join(forge, "novox", "mesh-sdk.git")
+	if err := os.MkdirAll(src, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	git := func(dir string, args ...string) string {
+		cmd := exec.Command("git", append([]string{"-C", dir, "-c", "user.name=t", "-c", "user.email=t@example.org", "-c", "commit.gpgsign=false"}, args...)...)
+		cmd.Env = append(os.Environ(), "GIT_CONFIG_GLOBAL=/dev/null")
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			t.Fatalf("git %v: %v %s", args, err, out)
+		}
+		return strings.TrimSpace(string(out))
+	}
+	git(src, "init", "-q")
+	if err := os.WriteFile(filepath.Join(src, "f.txt"), []byte("one\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	git(src, "add", ".")
+	git(src, "commit", "-q", "-m", "one")
+	commit := git(src, "rev-parse", "HEAD")
+
+	states := newStates()
+	m := moduleWith(states, "go1.27.1")
+	m.Home, m.Work, m.Cache = t.TempDir(), t.TempDir(), t.TempDir()
+	m.Run = StepRunner(m.Home)
+	m.RemoteBase = "file://" + forge + "/"
+	m.FetchOptions = []buildstep.Option{buildstep.AllowFileRemote()}
+	ws := fetchOnce(t, m, states, map[string]any{"repository": "novox/mesh-sdk", "commit": commit}).Workspace
+	tree := filepath.Join(m.Work, ws, "tree")
+	if _, err := os.Stat(filepath.Join(tree, ".git", "objects", "info", "alternates")); err == nil {
+		t.Errorf("the workspace still has an alternates file")
+	}
+	if err := os.RemoveAll(filepath.Join(m.Cache, "mirror")); err != nil {
+		t.Fatal(err)
+	}
+	if out, err := exec.Command("git", "-C", tree, "cat-file", "-e", commit+"^{commit}").CombinedOutput(); err != nil {
+		t.Errorf("git does not find the commit with the mirror gone: %v %s", err, out)
+	}
+	if out, err := exec.Command("git", "-C", tree, "fsck", "--connectivity-only").CombinedOutput(); err != nil {
+		t.Errorf("the workspace is not whole: %v %s", err, out)
+	}
+}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/module.go b/modules/mesh-build-go/cmd/mesh-build-go/module.go
index fffb3e1f..0f306017 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/module.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/module.go
@@ -54,6 +54,9 @@ type Module struct {
 	// Bubblewrap reads the installed bubblewrap's version (ProbeBubblewrap): a step is refused, and the module is
 	// unhealthy, while it is missing or older than BubblewrapLeast. Nil probes nothing.
 	Bubblewrap func(ctx context.Context) (string, error)
+	// Parallel is how many go.mod directories a level runs at once; zero is half the processors, and at least one
+	// (ADR 0536 §3: a node runs two checks at once).
+	Parallel int
 	// Inside is the integration level's own process, started shut off in bubblewrap by a test step (level.go): its
 	// commands run as they are, never shut off a second time.
 	Inside bool
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/pins_test.go b/modules/mesh-build-go/cmd/mesh-build-go/pins_test.go
index bbdc0ff7..a29e4fac 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/pins_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/pins_test.go
@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"sync"
 	"testing"
 
 	"git.novox.be/novox/mesh-sdk/go/delivery"
@@ -28,6 +29,7 @@ type pinHarness struct {
 	pulls   string // what git ls-remote answers
 	onTrunk bool   // the pin is fetchable as it is: go mod download never says unknown revision
 	cmds    []cmdRecord
+	mu      sync.Mutex
 }
 
 func newPinHarness(t *testing.T, gomod string, vendor bool, pulls string) (*pinHarness, *keptStates) {
@@ -36,7 +38,9 @@ func newPinHarness(t *testing.T, gomod string, vendor bool, pulls string) (*pinH
 	h := &pinHarness{t: t, m: m, f: f, pulls: pulls}
 	m.Run = func(ctx context.Context, env []string, name string, args ...string) forgekey.Ran {
 		line := name + " " + strings.Join(args, " ")
+		h.mu.Lock()
 		h.cmds = append(h.cmds, cmdRecord{env: env, line: line})
+		h.mu.Unlock()
 		switch {
 		case name == "git" && strings.Contains(line, " checkout ") && !strings.Contains(line, "pinned"):
 			for i, a := range args {
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 87ae1066..067dcfff 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/preview.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/preview.go
@@ -96,7 +96,10 @@ func (m *Module) placeSiblings(ctx context.Context, root string, siblings []sibl
 		if err := os.MkdirAll(dir, 0o700); err != nil {
 			return nil, err
 		}
-		if err := buildstep.FetchCommit(ctx, m.Run, dir, m.remote(s.repository), s.commit, s.ref, m.FetchOptions...); err != nil {
+		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 != "" {
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/selected_dirs_test.go b/modules/mesh-build-go/cmd/mesh-build-go/selected_dirs_test.go
new file mode 100644
index 00000000..fa86bb54
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/selected_dirs_test.go
@@ -0,0 +1,172 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+// threeModules gives the workspace three go.mod directories, a, b and c, whose modules are fixture/a, fixture/b and
+// fixture/c.
+func threeModules(t *testing.T, f *integFake) (*Module, *keptStates) {
+	t.Helper()
+	m, states := integModule(t, f)
+	f.gomods = "a/go.mod\nb/go.mod\nc/go.mod\n"
+	for _, d := range []string{"a", "b", "c"} {
+		dir := filepath.Join(m.Work, "ws1", "tree", d)
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filepath.Join(dir, "go.mod"), []byte("module fixture/"+d+"\n\ngo 1.25.0\n"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return m, states
+}
+
+func runIntegrationOf(t *testing.T, m *Module, states *keptStates, packages ...any) (TestResult, delivery.Step) {
+	t.Helper()
+	for _, tool := range m.Tools(m.StepTools()) {
+		if tool.Name != "build_go_test" {
+			continue
+		}
+		args := map[string]any{"workspace": "ws1", "level": "integration"}
+		if len(packages) > 0 {
+			args["packages"] = packages
+		}
+		if _, err := tool.Run(args); err != nil {
+			t.Fatal(err)
+		}
+	}
+	st := states.waitEnd(t)
+	var res TestResult
+	if st.State == delivery.StepPassed {
+		if err := json.Unmarshal(st.Result, &res); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return res, st
+}
+
+// Issue 890/AC1 (ADR 0536 §3): with packages selected in one of three go.mod directories, the integration level
+// compiles and runs go test in that directory only.
+func TestOnlyTheSelectedDirectoryCompilesAndRuns(t *testing.T) {
+	f := newIntegFake(t, "slow")
+	m, states := threeModules(t, f)
+	if _, st := runIntegrationOf(t, m, states, "fixture/b/x"); st.State != delivery.StepPassed {
+		t.Fatalf("the step ended %s: %s", st.State, st.Reason)
+	}
+	tree := filepath.Join(m.Work, "ws1", "tree")
+	compiled, ran := 0, 0
+	for _, l := range strings.Split(f.lines(), "\n") {
+		if !strings.HasPrefix(l, "go ") || !strings.Contains(l, " test ") {
+			continue
+		}
+		if !strings.HasPrefix(l, "go -C "+filepath.Join(tree, "b")+" test ") {
+			t.Errorf("go test ran outside the selected directory: %s", l)
+		}
+		if strings.Contains(l, "-run ^$") {
+			compiled++
+		} else {
+			ran++
+		}
+	}
+	if compiled != 1 || ran != 1 {
+		t.Errorf("compiled %d times and ran %d times, want once each:\n%s", compiled, ran, f.lines())
+	}
+}
+
+// Issue 890/AC1: no selection keeps every directory, compiled and run.
+func TestWithoutASelectionEveryDirectoryCompilesAndRuns(t *testing.T) {
+	f := newIntegFake(t, "slow")
+	m, states := threeModules(t, f)
+	runIntegrationOf(t, m, states)
+	compiled, ran := 0, 0
+	for _, l := range strings.Split(f.lines(), "\n") {
+		if strings.HasPrefix(l, "go ") && strings.Contains(l, " test ") {
+			if strings.Contains(l, "-run ^$") {
+				compiled++
+			} else {
+				ran++
+			}
+		}
+	}
+	if compiled != 3 || ran != 3 {
+		t.Errorf("compiled %d and ran %d, want 3 and 3:\n%s", compiled, ran, f.lines())
+	}
+}
+
+// Issue 890/AC2: with two directories selected and a runner that blocks, both go test commands run at once; the
+// result lists the packages in directory order.
+func TestSelectedDirectoriesRunAtOnceAndAnswerInDirectoryOrder(t *testing.T) {
+	f := newIntegFake(t, "slow")
+	var mu sync.Mutex
+	running, most := 0, 0
+	release := make(chan struct{})
+	f.onTest = func(line string) {
+		mu.Lock()
+		running++
+		if running > most {
+			most = running
+		}
+		if running == 2 {
+			close(release)
+		}
+		mu.Unlock()
+		select {
+		case <-release:
+		case <-time.After(10 * time.Second):
+		}
+		mu.Lock()
+		running--
+		mu.Unlock()
+	}
+	m, states := threeModules(t, f)
+	m.Parallel = 3
+	res, st := runIntegrationOf(t, m, states, "fixture/c/x", "fixture/a/x")
+	if st.State != delivery.StepPassed {
+		t.Fatalf("the step ended %s: %s", st.State, st.Reason)
+	}
+	if most != 2 {
+		t.Errorf("at most %d go test commands ran at once, want 2", most)
+	}
+	dirs := []string{}
+	for _, p := range res.Packages {
+		dirs = append(dirs, p.Dir)
+	}
+	if got := fmt.Sprint(dirs); !strings.HasPrefix(got, "[a ") || !strings.HasSuffix(got, " c]") {
+		t.Errorf("the packages lie in directories %s, want a's before c's", got)
+	}
+}
+
+// The bound holds: with one directory at a time, no two go test commands run at once.
+func TestTheDirectoriesAtOnceNeverPassTheBound(t *testing.T) {
+	f := newIntegFake(t, "slow")
+	var mu sync.Mutex
+	running, most := 0, 0
+	f.onTest = func(string) {
+		mu.Lock()
+		running++
+		if running > most {
+			most = running
+		}
+		mu.Unlock()
+		time.Sleep(20 * time.Millisecond)
+		mu.Lock()
+		running--
+		mu.Unlock()
+	}
+	m, states := threeModules(t, f)
+	m.Parallel = 1
+	runIntegrationOf(t, m, states)
+	if most != 1 {
+		t.Errorf("%d go test commands ran at once with a bound of 1", most)
+	}
+}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/test.go b/modules/mesh-build-go/cmd/mesh-build-go/test.go
index d129c277..c2b8167b 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/test.go
@@ -10,8 +10,12 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strings"
+	"sync"
 	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
 )
 
 // testTimeout is go test's own timeout for a level: a bound on a test that never ends, far above any level's bound.
@@ -33,7 +37,8 @@ func (m *Module) testTool() StepTool {
 				"level":     map[string]any{"type": "string", "enum": []string{LevelUnit, LevelIntegration}, "description": "the level"},
 				"packages": map[string]any{"type": "array", "items": map[string]any{"type": "string"},
 					"description": "package patterns of each module, ./... when none"},
-				"run": map[string]any{"type": "string", "description": "optional go test -run pattern: only the tests it matches run (a try's tests)"},
+				"run":    map[string]any{"type": "string", "description": "optional go test -run pattern: only the tests it matches run (a try's tests)"},
+				"runner": runnerProperty,
 			},
 			"required": []string{"workspace", "level"},
 		},
@@ -68,13 +73,48 @@ func (m *Module) test(ctx context.Context, args map[string]any) (any, error) {
 	if _, err := os.Stat(tree); err != nil {
 		return nil, fmt.Errorf("no workspace %s: it was never fetched here, or it is older than a day and was removed", workspace)
 	}
-	switch level {
-	case LevelUnit:
-		return m.runLevel(ctx, tree, levelOpts{workspace: workspace, level: LevelUnit, packages: packages, run: pattern, bounds: Bounds{Run: m.unitBound()}, env: m.besideEnv(workspace)})
-	case LevelIntegration:
-		return m.integration(ctx, workspace, tree, packages, pattern)
+	runner, err := runnerArg(args["runner"])
+	if err != nil {
+		return nil, err
 	}
-	return nil, fmt.Errorf("the level %q is neither unit nor integration", level)
+	var res *TestResult
+	switch {
+	case runner == RunnerWorkspace && (level == LevelUnit || level == LevelIntegration):
+		return m.testAsCandidate(ctx, workspace, level, packages, pattern)
+	case level == LevelUnit:
+		res, err = m.runLevel(ctx, tree, levelOpts{workspace: workspace, level: LevelUnit, packages: packages, run: pattern, bounds: Bounds{Run: m.unitBound()}, env: m.besideEnv(workspace)})
+	case level == LevelIntegration:
+		res, err = m.integration(ctx, workspace, tree, packages, pattern)
+	default:
+		return nil, fmt.Errorf("the level %q is neither unit nor integration", level)
+	}
+	if err != nil {
+		return nil, err
+	}
+	res.JudgedBy = RunnerInstalled
+	return res, nil
+}
+
+// testAsCandidate runs a level as the candidate's program (ADR 0535 §1): the integration level is refused first as the
+// installed program would refuse it (no postgres), then the whole level runs in the candidate's one shut-off process.
+func (m *Module) testAsCandidate(ctx context.Context, workspace, level string, packages []string, pattern string) (*TestResult, error) {
+	spec := levelSpec{Step: stepUnit, Workspace: workspace, Packages: packages, Run: pattern, Bound: m.unitBound()}
+	if level == LevelIntegration {
+		if _, err := m.checkPostgres(ctx); err != nil {
+			return nil, err // a *buildstep.Refusal: this level only
+		}
+		b := m.integrationBounds()
+		spec = levelSpec{Step: stepIntegration, Workspace: workspace, Packages: packages, Run: pattern, Setup: b.Setup, Bound: b.Run}
+	}
+	a, judged, err := m.asCandidate(ctx, spec)
+	if err != nil {
+		return nil, err
+	}
+	if a.Result == nil {
+		return nil, fmt.Errorf("the candidate's mesh-build-go answered no result of the %s level", level)
+	}
+	a.Result.JudgedBy = judged
+	return a.Result, nil
 }
 
 // besideEnv is MESH_CHECK_BESIDE, naming the directory build_go_fetch placed the workspace's siblings in (ADR 0484
@@ -143,26 +183,71 @@ func (m *Module) runLevel(ctx context.Context, tree string, o levelOpts) (*TestR
 		return nil, err
 	}
 	began := m.clock()
+	// The directories that hold a selected package run side by side, up to the bound, each into a result of its own;
+	// the results join in directory order, so the answer does not depend on which ended first (ADR 0536 §3).
+	type dirRun struct {
+		res   *TestResult
+		rd    *reader
+		r     runAnswer
+		noise []string
+	}
+	var todo []string
 	for _, d := range dirs {
-		packages := byDir[d]
-		if len(packages) == 0 {
-			continue // no package asked for lies in this module
+		if len(byDir[d]) > 0 { // no package asked for lies in the others
+			todo = append(todo, d)
 		}
-		args := []string{"-C", filepath.Join(tree, d), "test", "-json", "-count=1", "-timeout", testTimeout}
-		if len(o.tags) > 0 {
-			args = append(args, "-tags", strings.Join(o.tags, ","))
-		}
-		if o.run != "" {
-			args = append(args, "-run", o.run)
-		}
-		args = append(args, packages...)
-		r := run(ctx, append(m.goEnvFor(filepath.Join(tree, d)), o.env...), "go", args...)
-		rd := newReader(res, d)
-		noise := rd.read(r.Stdout)
-		rd.finish()
-		if (r.Status != 0 || r.Err != "") && len(rd.pkgs) == 0 {
+	}
+	runs := make([]dirRun, len(todo))
+	slots := make(chan struct{}, m.parallel())
+	var wg sync.WaitGroup
+	var crash any // a panic of a run goes on in the caller, which cleans up after it
+	var crashMu sync.Mutex
+	for i, d := range todo {
+		wg.Add(1)
+		slots <- struct{}{}
+		go func() {
+			defer wg.Done()
+			defer func() { <-slots }()
+			defer func() {
+				if p := recover(); p != nil {
+					crashMu.Lock()
+					crash = p
+					crashMu.Unlock()
+				}
+			}()
+			args := []string{"-C", filepath.Join(tree, d), "test", "-json", "-count=1", "-timeout", testTimeout}
+			if len(o.tags) > 0 {
+				args = append(args, "-tags", strings.Join(o.tags, ","))
+			}
+			if o.run != "" {
+				args = append(args, "-run", o.run)
+			}
+			args = append(args, byDir[d]...)
+			dr := &runs[i]
+			dr.res = &TestResult{}
+			dr.r = run(ctx, append(m.goEnvFor(filepath.Join(tree, d)), o.env...), "go", args...)
+			dr.rd = newReader(dr.res, d)
+			dr.noise = dr.rd.read(dr.r.Stdout)
+			dr.rd.finish()
+		}()
+	}
+	wg.Wait()
+	if crash != nil {
+		panic(crash)
+	}
+	for i, d := range todo {
+		dr := runs[i]
+		if (dr.r.Status != 0 || dr.r.Err != "") && len(dr.rd.pkgs) == 0 {
 			// go test said nothing a test said: it could not run (a go.mod it cannot read, a toolchain it cannot find).
-			return nil, fmt.Errorf("go test in %s: %s", d, firstLine(r.Err+"\n"+strings.Join(noise, "\n")+"\n"+r.Stderr))
+			return nil, fmt.Errorf("go test in %s: %s", d, firstLine(dr.r.Err+"\n"+strings.Join(dr.noise, "\n")+"\n"+dr.r.Stderr))
+		}
+		res.Tests += dr.res.Tests
+		res.Skipped += dr.res.Skipped
+		res.Failed = append(res.Failed, dr.res.Failed...)
+		res.Packages = append(res.Packages, dr.res.Packages...)
+		res.Slowest = append(res.Slowest, dr.res.Slowest...)
+		if dr.res.Elapsed > res.Elapsed {
+			res.Elapsed = dr.res.Elapsed
 		}
 	}
 	if o.wall {
@@ -172,6 +257,17 @@ func (m *Module) runLevel(ctx context.Context, tree string, o levelOpts) (*TestR
 	return res, nil
 }
 
+// runAnswer is what a step runner answers.
+type runAnswer = forgekey.Ran
+
+// parallel is how many directories a level runs at once.
+func (m *Module) parallel() int {
+	if m.Parallel > 0 {
+		return m.Parallel
+	}
+	return max(1, runtime.NumCPU()/2)
+}
+
 // packagesByDir says which patterns run in which go.mod directory. A relative pattern (./...) runs in every directory, as
 // it always did; an import path, as build_go_digest keys its packages, runs only in the directory whose module holds it
 // (the longest module path that is the path or its prefix), as another module's path would fail the step. No pattern
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/verify_once_test.go b/modules/mesh-build-go/cmd/mesh-build-go/verify_once_test.go
new file mode 100644
index 00000000..6d9776b4
--- /dev/null
+++ b/modules/mesh-build-go/cmd/mesh-build-go/verify_once_test.go
@@ -0,0 +1,112 @@
+package main
+
+import (
+	"context"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
+)
+
+func shutOffModule(t *testing.T, f *fakeStep) (*Module, *keptStates) {
+	t.Helper()
+	m, states := fetchModule(t, f)
+	// A module with bubblewrap runs its steps shut off, with the module cache read-only: a pass of go mod verify holds.
+	m.Bubblewrap = func(context.Context) (string, error) { return "bubblewrap 0.13.0", nil }
+	return m, states
+}
+
+func countVerifies(f *fakeStep) int {
+	n := 0
+	for _, l := range f.lines() {
+		if strings.HasPrefix(l, "go -C ") && strings.HasSuffix(l, " mod verify") {
+			n++
+		}
+	}
+	return n
+}
+
+// Issue 890/AC5 (ADR 0536 §2): two directories with the same go.mod and go.sum verify once, and a later fetch with the
+// same pair verifies no time; a different go.mod verifies again.
+func TestGoModVerifyRunsOncePerGoModAndGoSumDigest(t *testing.T) {
+	same := map[string]string{"a/go.mod": "module a\n", "a/go.sum": "x\n", "b/go.mod": "module a\n", "b/go.sum": "x\n"}
+	f := &fakeStep{gomods: "a/go.mod\nb/go.mod\n", files: same}
+	m, states := shutOffModule(t, f)
+	fetchOnce(t, m, states, fetchArgs)
+	if n := countVerifies(f); n != 1 {
+		t.Errorf("two directories with one go.mod and go.sum verified %d times, want 1", n)
+	}
+	fetchOnce(t, m, states, fetchArgs)
+	if n := countVerifies(f); n != 1 {
+		t.Errorf("a later fetch with the same pair verified again (%d in all)", n)
+	}
+
+	g := &fakeStep{gomods: "a/go.mod\nb/go.mod\n", files: map[string]string{"a/go.mod": "module a\n", "a/go.sum": "x\n", "b/go.mod": "module b\n", "b/go.sum": "x\n"}}
+	m, states = shutOffModule(t, g)
+	fetchOnce(t, m, states, fetchArgs)
+	if n := countVerifies(g); n != 2 {
+		t.Errorf("two different go.mod verified %d times, want 2", n)
+	}
+}
+
+// Without bubblewrap the module cache is not read-only to a step: a pass is never reused.
+func TestAPassOfVerifyIsNotReusedWhenStepsAreNotShutOff(t *testing.T) {
+	f := &fakeStep{gomods: "a/go.mod\nb/go.mod\n", files: map[string]string{"a/go.mod": "m\n", "b/go.mod": "m\n"}}
+	m, states := fetchModule(t, f) // no Bubblewrap
+	fetchOnce(t, m, states, fetchArgs)
+	fetchOnce(t, m, states, fetchArgs)
+	if n := countVerifies(f); n != 4 {
+		t.Errorf("verified %d times, want every directory every fetch (4)", n)
+	}
+}
+
+// A verify that fails leaves no mark: the next fetch verifies again.
+func TestAFailedVerifyLeavesNoMark(t *testing.T) {
+	fail := true
+	f := &fakeStep{gomods: "go.mod\n", files: map[string]string{"go.mod": "m\n"}}
+	f.goRule = func(line string) forgekey.Ran {
+		if strings.HasSuffix(line, " mod verify") && fail {
+			return forgekey.Ran{Status: 1, Stdout: "x v1: dir has been modified\n"}
+		}
+		return forgekey.Ran{}
+	}
+	m, states := shutOffModule(t, f)
+	fetchTool(t, m)(fetchArgs)
+	states.waitEnd(t)
+	fail = false
+	fetchOnce(t, m, states, fetchArgs)
+	if n := countVerifies(f); n != 2 {
+		t.Errorf("verified %d times, want 2 (the failed one and the retry)", n)
+	}
+}
+
+// Issue 890 (ADR 0536 §2): the downloads of the directories run side by side, never more at once than the bound.
+func TestDownloadsRunSideBySideUpToTheBound(t *testing.T) {
+	var mu sync.Mutex
+	running, most := 0, 0
+	f := &fakeStep{gomods: "a/go.mod\nb/go.mod\nc/go.mod\nd/go.mod\n", files: map[string]string{
+		"a/go.mod": "a\n", "b/go.mod": "b\n", "c/go.mod": "c\n", "d/go.mod": "d\n"}}
+	f.goRule = func(line string) forgekey.Ran {
+		if strings.HasSuffix(line, " mod download") {
+			mu.Lock()
+			running++
+			if running > most {
+				most = running
+			}
+			mu.Unlock()
+			time.Sleep(40 * time.Millisecond)
+			mu.Lock()
+			running--
+			mu.Unlock()
+		}
+		return forgekey.Ran{}
+	}
+	m, states := fetchModule(t, f)
+	m.Parallel = 2
+	fetchOnce(t, m, states, fetchArgs)
+	if most != 2 {
+		t.Errorf("at most %d downloads ran at once, want 2", most)
+	}
+}
diff --git a/modules/mesh-build-go/go.mod b/modules/mesh-build-go/go.mod
index 999b31b9..e63fc0a5 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 v0.1.14-0.20261023062828-6147a2045467
+	git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023121526-2248b985bfac
 	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 ccb75980..868be86a 100644
--- a/modules/mesh-build-go/go.sum
+++ b/modules/mesh-build-go/go.sum
@@ -2,6 +2,10 @@ git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261022190740-104533e8c0c9 h1:Ufmd2cYE
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261022190740-104533e8c0c9/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023062828-6147a2045467 h1:i85+fFT0wZiTZ9CRVfMzWg1+Zr7oOLl3UH7hEaKcOIc=
 git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023062828-6147a2045467/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023120306-780be2fae5a1 h1:MqP6mWWVxM+niUGPxSSh4Q8lduYkkQ7oTAkDeC9nyv4=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023120306-780be2fae5a1/go.mod h1:ouERNpZizzwZ0HLEtAypsY9FR20FyUgUYRni6J7kVCY=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023121526-2248b985bfac h1:xx601bHwpO/aq5KyE08t4ymSXGHGnk+D9VHfmOq+5P8=
+git.novox.be/novox/mesh-sdk/go v0.1.14-0.20261023121526-2248b985bfac/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=
diff --git a/modules/mesh-delivery-gate/cmd/mesh-delivery-gate/bus_test.go b/modules/mesh-delivery-gate/cmd/mesh-delivery-gate/bus_test.go
index 76f46626..77bc463a 100644
--- a/modules/mesh-delivery-gate/cmd/mesh-delivery-gate/bus_test.go
+++ b/modules/mesh-delivery-gate/cmd/mesh-delivery-gate/bus_test.go
@@ -520,7 +520,7 @@ func TestAPassingVerdictAsksTheApproveAndItsApproveMergesWithinSecondsOverARealB
 	passed := b.publish(t, "mesh.seat."+ControllerSeat+".event.checked", judgedAt("novox/app", 7, head, "pass", "t1t1t1t1t1t1"))
 
 	// No review names the head, so nothing is asked (ADR 0496, issue 1803): the delivery waits for a review.
-	rt.until("the delivery to wait for a review", 5*time.Second, func() bool {
+	rt.until("the delivery to wait for a review", eventWithin, func() bool {
 		e, err := b.kvs["pulls"].Get(ctx, "novox.app.7")
 		var k PullKey
 		return err == nil && json.Unmarshal(e.Value(), &k) == nil && k.Verdict == "pass"
@@ -537,7 +537,7 @@ func TestAPassingVerdictAsksTheApproveAndItsApproveMergesWithinSecondsOverARealB
 		"title": announced.Title, "head_sha": head, "base": "main", "body": "Reviewed-by: lead@shanks at " + head})
 	_ = passed
 	var key string
-	rt.until("the ask to be written", 5*time.Second, func() bool { key = onlyAsk(b); return key != "" })
+	rt.until("the ask to be written", eventWithin, func() bool { key = onlyAsk(b); return key != "" })
 	ask := askAt(t, b, key)
 	t.Logf("the ask was written %s after the edit's event", time.Since(edit))
 	if !strings.Contains(ask.Ask.Explanation, "reviewed by lead on shanks") || ask.Ask.ID != sdk.AskID(key) {
@@ -547,7 +547,7 @@ func TestAPassingVerdictAsksTheApproveAndItsApproveMergesWithinSecondsOverARealB
 		t.Fatalf("merged before the Approve")
 	}
 	// The pull request's key holds its head, the trunk commit it was asked against and the verdict.
-	rt.until("the pull request's key to hold head, trunk and verdict", 5*time.Second, func() bool {
+	rt.until("the pull request's key to hold head, trunk and verdict", eventWithin, func() bool {
 		e, err := b.kvs["pulls"].Get(ctx, "novox.app.7")
 		var k PullKey
 		return err == nil && json.Unmarshal(e.Value(), &k) == nil && k == PullKey{Head: head, Trunk: "t1t1t1t1t1t1", Verdict: "pass"}
@@ -559,7 +559,7 @@ func TestAPassingVerdictAsksTheApproveAndItsApproveMergesWithinSecondsOverARealB
 			t.Fatalf("the gate asked %s %d times: the bridge writes to the forge", tool, n)
 		}
 	}
-	rt.until("the owed keys to be written", 5*time.Second, func() bool {
+	rt.until("the owed keys to be written", eventWithin, func() bool {
 		keys, err := b.kvs["owed"].Keys(ctx)
 		return err == nil && len(keys) > 0
 	})
@@ -567,7 +567,7 @@ func TestAPassingVerdictAsksTheApproveAndItsApproveMergesWithinSecondsOverARealB
 	// seconds, with the Approve's heads, and the gate asks the forge nothing.
 	approved := b.answer(t, key, OptionApprove)
 	var handed sdk.Merge
-	rt.until("the merge to be handed to the bridge", 5*time.Second, func() bool { handed = mergeAt(b, "fix/x"); return len(handed.Pulls) > 0 })
+	rt.until("the merge to be handed to the bridge", eventWithin, func() bool { handed = mergeAt(b, "fix/x"); return len(handed.Pulls) > 0 })
 	if len(handed.Pulls) != 1 || handed.Pulls[0].Head.Head != head || handed.Pulls[0].Number != 7 || handed.Pulls[0].State != sdk.MergeMerging ||
 		handed.Approved[sdk.ApprovedKey("novox/app", 7)] != head || handed.Ask != key {
 		t.Fatalf("the merge handed is %+v; wanted pull request 7 at the approved head, merging, under the ask %s", handed, key)
@@ -579,7 +579,7 @@ func TestAPassingVerdictAsksTheApproveAndItsApproveMergesWithinSecondsOverARealB
 	// The bridge says it merged; the mesh's source then holds the merge, in one write.
 	b.forgeSays(t, sdk.Forge{Kind: sdk.ForgeMerged, Repository: "novox/app", Number: 7, Head: head, Merge: strings.Repeat("b", 40),
 		Root: "r", Trees: map[string]string{"app": "t"}})
-	rt.until("source.main to hold the merge", 5*time.Second, func() bool {
+	rt.until("source.main to hold the merge", eventWithin, func() bool {
 		e, err := b.kvs["source"].Get(ctx, sdk.SourceMainKey)
 		if err != nil {
 			return false
@@ -743,7 +743,7 @@ func TestAGroupOfTwoWithGreenChecksGetsOneAskAndOnItsApproveBothMergeOneAfterThe
 	// One member is handed, with both heads approved, within seconds of the answer; the second only after the bridge says the
 	// first merged.
 	var first sdk.Merge
-	rt.until("the first member to be handed", 5*time.Second, func() bool { first = mergeAt(b, "feat/x"); return len(first.Pulls) > 0 })
+	rt.until("the first member to be handed", eventWithin, func() bool { first = mergeAt(b, "feat/x"); return len(first.Pulls) > 0 })
 	if len(first.Pulls) != 1 || first.Pulls[0].State != sdk.MergeMerging || len(first.Approved) != 2 ||
 		first.Approved[sdk.ApprovedKey("novox/mesh-controller", 1)] != ctlHead || first.Approved[sdk.ApprovedKey("novox/mesh-catalog", 2)] != catHead {
 		t.Fatalf("the first merge handed is %+v", first)
@@ -757,14 +757,14 @@ func TestAGroupOfTwoWithGreenChecksGetsOneAskAndOnItsApproveBothMergeOneAfterThe
 	b.forgeSays(t, sdk.Forge{Kind: sdk.ForgeMerged, Repository: one.Repository, Number: one.Number, Head: one.Head.Head,
 		Merge: strings.Repeat("a", 40), Root: "r", Trees: map[string]string{"x": "t"}})
 	var both sdk.Merge
-	rt.until("the second member to be handed", 5*time.Second, func() bool { both = mergeAt(b, "feat/x"); return len(both.Pulls) == 2 })
+	rt.until("the second member to be handed", eventWithin, func() bool { both = mergeAt(b, "feat/x"); return len(both.Pulls) == 2 })
 	if both.Pulls[0].State != sdk.MergeMerged || both.Pulls[1].State != sdk.MergeMerging || both.Pulls[1].Repository == one.Repository {
 		t.Fatalf("the second merge handed is %+v", both.Pulls)
 	}
 	two := both.Pulls[1]
 	b.forgeSays(t, sdk.Forge{Kind: sdk.ForgeMerged, Repository: two.Repository, Number: two.Number, Head: two.Head.Head,
 		Merge: strings.Repeat("b", 40), Root: "r", Trees: map[string]string{"x": "t"}})
-	rt.until("source.main to hold both merges", 5*time.Second, func() bool {
+	rt.until("source.main to hold both merges", eventWithin, func() bool {
 		e, err := b.kvs["source"].Get(ctx, sdk.SourceMainKey)
 		if err != nil {
 			return false
@@ -942,3 +942,9 @@ func mustDecode[T any](t *testing.T, raw []byte) T {
 	}
 	return v
 }
+
+// eventWithin is how long a test waits for one step of the gate's path over the real bus. On an idle machine each step
+// takes milliseconds (the tests log it), but a slow, loaded build node ran the ask step in 6.69 s against the old 5 s
+// (issue 1404). A failed first ask is asked again only after askAgainAfter (30 s, react.go), and 20 s deliberately does not
+// cover that: a failed first ask still fails the test.
+const eventWithin = 20 * time.Second
diff --git a/modules/nats/cmd/mesh-nats-snapshot/dockerguard_test.go b/modules/nats/cmd/mesh-nats-snapshot/dockerguard_test.go
new file mode 100644
index 00000000..7605d15c
--- /dev/null
+++ b/modules/nats/cmd/mesh-nats-snapshot/dockerguard_test.go
@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+// dockerVerdict decides what a test that starts containers does: run, skip or fail. found is the result of looking
+// the docker program up, probeErr the result of asking the daemon for its version (nil when it answered). The checker's
+// integration level runs shut off (no network, no /run), so on a build node the program is found and the daemon is not
+// reachable; that is a skip, not a failure, except where docker is required (MESH_REQUIRE_DOCKER=1).
+func dockerVerdict(found bool, lookErr, probeErr error, required bool) (action, reason string) {
+	var why string
+	switch {
+	case !found:
+		why = fmt.Sprintf("there is no docker in this toolchain (%v)", lookErr)
+	case probeErr != nil:
+		why = fmt.Sprintf("no docker daemon is reachable from here (%v)", probeErr)
+	default:
+		return "run", ""
+	}
+	if required {
+		return "fail", "MESH_REQUIRE_DOCKER=1 and " + why
+	}
+	return "skip", why
+}
+
+func TestDockerVerdict(t *testing.T) {
+	boom := fmt.Errorf("cannot connect")
+	cases := []struct {
+		name        string
+		found       bool
+		look, probe error
+		required    bool
+		want        string
+	}{
+		{"answers", true, nil, nil, false, "run"},
+		{"answers, required", true, nil, nil, true, "run"},
+		{"no program", false, boom, nil, false, "skip"},
+		{"no program, required", false, boom, nil, true, "fail"},
+		{"program but no daemon", true, nil, boom, false, "skip"},
+		{"program but no daemon, required", true, nil, boom, true, "fail"},
+	}
+	for _, c := range cases {
+		got, why := dockerVerdict(c.found, c.look, c.probe, c.required)
+		if got != c.want {
+			t.Errorf("%s: got %s, want %s", c.name, got, c.want)
+		}
+		if got != "run" && why == "" {
+			t.Errorf("%s: no reason given", c.name)
+		}
+	}
+}
diff --git a/modules/nats/cmd/mesh-nats-snapshot/live_test.go b/modules/nats/cmd/mesh-nats-snapshot/live_test.go
index 99af6599..a9918d6c 100644
--- a/modules/nats/cmd/mesh-nats-snapshot/live_test.go
+++ b/modules/nats/cmd/mesh-nats-snapshot/live_test.go
@@ -60,14 +60,24 @@ var snapshotGrants = struct{ publish, subscribe []string }{
 
 func needDocker(t *testing.T) {
 	t.Helper()
-	if _, err := exec.LookPath("docker"); err != nil {
-		// The Go toolchain container of the merge check holds no docker. Where docker is expected
-		// (MESH_REQUIRE_DOCKER=1, which merge-check.sh sets for its docker step) its absence is a failure.
-		if os.Getenv("MESH_REQUIRE_DOCKER") == "1" {
-			t.Fatalf("MESH_REQUIRE_DOCKER=1 and there is no docker here: %v", err)
+	_, lookErr := exec.LookPath("docker")
+	var probeErr error
+	if lookErr == nil {
+		// The binary alone proves nothing: the checker's integration level runs shut off (no network, no /run), so on
+		// a build node docker is found and its daemon is not reachable (issue 1823).
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		if out, err := exec.CommandContext(ctx, "docker", "version", "--format", "{{.Server.Version}}").CombinedOutput(); err != nil {
+			probeErr = fmt.Errorf("%v: %s", err, strings.TrimSpace(string(out)))
 		}
-		t.Skipf("NOT RUN: this test starts a throwaway nats container and there is no docker in this toolchain (%v); "+
-			"it is proof only where merge-check.sh's docker step runs it (MESH_CHECK_DOCKER=1)", err)
+	}
+	// Where docker is expected (MESH_REQUIRE_DOCKER=1, which merge-check.sh sets for its docker step) its absence is a failure.
+	switch action, why := dockerVerdict(lookErr == nil, lookErr, probeErr, os.Getenv("MESH_REQUIRE_DOCKER") == "1"); action {
+	case "fail":
+		t.Fatal(why)
+	case "skip":
+		t.Skipf("NOT RUN: this test starts a throwaway nats container and %s; "+
+			"it is proof only where merge-check.sh's docker step runs it (MESH_CHECK_DOCKER=1)", why)
 	}
 }