diff --git a/modules/mesh-build-node/cmd/mesh-build-node/build.go b/modules/mesh-build-node/cmd/mesh-build-node/build.go
index a5f9519a..65b0ea83 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/build.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/build.go
@@ -63,6 +63,7 @@ func (m *Module) buildTool() StepTool {
 				"workspace": map[string]any{"type": "string", "description": "the workspace id a fetch step answered"},
 				"module":    map[string]any{"type": "string", "description": "the module's directory in the workspace, '.' when it is the root"},
 				"artifact":  map[string]any{"type": "string", "description": "the name of the bundle in the module's module.json"},
+				"publish":   map[string]any{"type": "boolean", "description": "true for a merged trunk commit alone: the build writes the shared cache; without it (a candidate) the cache is read beneath a throwaway layer"},
 			},
 			"required": []string{"workspace", "artifact"},
 		},
@@ -75,6 +76,7 @@ func (m *Module) build(ctx context.Context, args map[string]any) (any, error) {
 	workspace, _ := args["workspace"].(string)
 	module, _ := args["module"].(string)
 	name, _ := args["artifact"].(string)
+	publish, _ := args["publish"].(bool)
 	root, dir, err := m.moduleDirOf(workspace, module)
 	if err != nil {
 		return nil, err
@@ -112,14 +114,14 @@ func (m *Module) build(ctx context.Context, args map[string]any) (any, error) {
 	tsc := []string{m.tsc(), "--module", "NodeNext", "--moduleResolution", "NodeNext", "--target", "ES2022",
 		"--rootDir", ".", "--outDir", out}
 	tsc = append(tsc, sourcesFor(a.Entrypoints, out, ".ts")...)
-	if why := ranFailed(m.runIn(ctx, dir, nil, "node", tsc...)); why != "" {
+	if why := ranFailed(m.runIn(ctx, root, publish, dir, nil, "node", tsc...)); why != "" {
 		return nil, fmt.Errorf("tsc for %s: %s", a.Name, why)
 	}
 	launchers, err := writeLaunchers(filepath.Join(dir, out), a)
 	if err != nil {
 		return nil, err
 	}
-	if err := m.bundle(ctx, dir, a, launchers); err != nil {
+	if err := m.bundle(ctx, root, publish, dir, a, launchers); err != nil {
 		return nil, err
 	}
 	bundleDir := filepath.Join(dir, final)
@@ -150,7 +152,7 @@ func (m *Module) build(ctx context.Context, args map[string]any) (any, error) {
 
 // bundle makes every entrypoint and every launcher ONE file, with esbuild (builder.go:1971-2039): an entrypoint is
 // bundled in place under its own name, a launcher keeps its name with the extension .mjs.
-func (m *Module) bundle(ctx context.Context, dir string, a artifact, launchers map[string]string) error {
+func (m *Module) bundle(ctx context.Context, root string, publish bool, dir string, a artifact, launchers map[string]string) error {
 	out, final := Out(a.Name), Out(a.Name)+".bundled"
 	if err := os.RemoveAll(filepath.Join(dir, final)); err != nil {
 		return err
@@ -177,7 +179,7 @@ func (m *Module) bundle(ctx context.Context, dir string, a artifact, launchers m
 			return nil
 		}
 		args := append(append(append([]string{}, entries...), common...), extra...)
-		if why := ranFailed(m.runIn(ctx, dir, nil, m.esbuild(), args...)); why != "" {
+		if why := ranFailed(m.runIn(ctx, root, publish, dir, nil, m.esbuild(), args...)); why != "" {
 			return fmt.Errorf("esbuild for %s: %s", a.Name, why)
 		}
 		return nil
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/check.go b/modules/mesh-build-node/cmd/mesh-build-node/check.go
index 672a3539..02463848 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/check.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/check.go
@@ -139,7 +139,7 @@ func (m *Module) test(ctx context.Context, args map[string]any) (any, error) {
 	}
 	if len(sources) > 0 {
 		start := m.now()
-		r := m.runIn(ctx, dir, nil, "node", m.typeCheck(dir, sources)...)
+		r := m.runIn(ctx, root, false, dir, nil, "node", m.typeCheck(dir, sources)...)
 		check = m.now().Sub(start)
 		if err := ran("type-check", r); err != nil {
 			return nil, err
@@ -149,7 +149,7 @@ func (m *Module) test(ctx context.Context, args map[string]any) (any, error) {
 	if hasScript && len(failed) == 0 {
 		start := m.now()
 		env := []string{"PATH=" + binDir(root) + ":" + pathOrDefault(), "npm_config_cache=" + filepath.Join(m.Toolchain.Paths.Cache, "npm")}
-		r := m.runIn(ctx, dir, env, "npm", scriptArgs(script)...)
+		r := m.runIn(ctx, root, false, dir, env, "npm", scriptArgs(script)...)
 		run = m.now().Sub(start)
 		if err := ran(script, r); err != nil {
 			return nil, err
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/check_test.go b/modules/mesh-build-node/cmd/mesh-build-node/check_test.go
index dc3e79fa..5a95a94a 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/check_test.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/check_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 	"time"
@@ -122,8 +123,9 @@ func TestTheStepsCommandsGetAnEnvironmentOfTheirOwn(t *testing.T) {
 				t.Errorf("%s was given %s", joined(c), e)
 			}
 		}
-		if len(c.env) < 3 || !strings.HasPrefix(c.env[0], "PATH=") || !strings.HasPrefix(c.env[1], "HOME=") {
-			t.Errorf("%s env = %v, want PATH, HOME and the locale", c.name, c.env)
+		// 1309: the HOME of a run step is its own, inside its /tmp, never the account's.
+		if len(c.env) < 3 || !strings.HasPrefix(c.env[0], "PATH=") || !slices.Contains(c.env, "HOME=/tmp/home") || !slices.Contains(c.env, "LC_ALL=C") {
+			t.Errorf("%s env = %v, want PATH, the step's own HOME and the locale", c.name, c.env)
 		}
 	}
 }
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/harness_test.go b/modules/mesh-build-node/cmd/mesh-build-node/harness_test.go
index da417017..b991462d 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/harness_test.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/harness_test.go
@@ -27,7 +27,9 @@ type call struct {
 type fakeMachine struct {
 	mu    sync.Mutex
 	calls []call
-	now   time.Time
+	// shut are the commands that ran in bubblewrap, read back (isolate_test.go).
+	shut []shutOff
+	now  time.Time
 	// cost is how long a program takes, by the base name of its path; answer is what it prints and its status;
 	// effect runs the program's effects on the disk.
 	cost   map[string]time.Duration
@@ -74,7 +76,9 @@ func newMachine(t *testing.T) (*Module, *fakeMachine, string) {
 		Toolchain: &Toolchain{Paths: Paths{Home: home, Cache: cache}, Registry: func() (Registry, error) {
 			return Registry{URL: "https://git.novox.be/api/packages/novox/npm/"}, nil
 		}},
-		Work: work, Run: f.run, Clock: f.clock, Ctx: context.Background()}
+		Work: work, Clock: f.clock, Ctx: context.Background()}
+	// A run step's commands run in bubblewrap (1309); the fake reads each back and runs its command.
+	m.Run = throughBubblewrap(t, f.run, &f.shut)
 	// The toolchain directory as npm ci leaves it, with no bin links.
 	nm := filepath.Join(m.toolchainDir(), "node_modules")
 	for _, p := range []string{"typescript/bin/tsc", "esbuild/bin/esbuild", "@esbuild/linux-x64/bin/esbuild",
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/isolate.go b/modules/mesh-build-node/cmd/mesh-build-node/isolate.go
new file mode 100644
index 00000000..b6222da9
--- /dev/null
+++ b/modules/mesh-build-node/cmd/mesh-build-node/isolate.go
@@ -0,0 +1,110 @@
+package main
+
+// The run steps shut off (novox/hq ADR 0448; issue 1309, lead decision L2 of R4e): the type-check, the package's test
+// script, tsc and esbuild run the change's code, so each runs through mesh-sdk's buildstep.Isolated, in bubblewrap: no
+// network, none of the account (its home, its ssh key, its registry secret, its bus login, the runtime directory), the
+// workspace read-write, the toolchain read-only, the shared npm cache read-only beneath a throwaway writable layer, or
+// read-write for a trunk build alone (publish). The fetch step (git, npm ci of the module's own packages) holds the key
+// and the network and runs none of the change's code, so it runs as before.
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+
+	"git.novox.be/novox/mesh-sdk/go/buildstep"
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
+)
+
+// BubblewrapLeast is the oldest bubblewrap a run step is shut off with: --overlay-src and --tmp-overlay, which keep the
+// shared cache unchanged by a candidate, came in 0.10.
+const BubblewrapLeast = "0.10"
+
+// npmCache is the shared npm cache a package's test script reads.
+func (m *Module) npmCache() string { return filepath.Join(m.Toolchain.Paths.Cache, "npm") }
+
+// isolation is how a run step in the workspace root is shut off.
+func (m *Module) isolation(root string, write bool) buildstep.Isolation {
+	return buildstep.Isolation{
+		Workspace:   root,
+		ReadOnly:    []string{m.toolchainDir()},
+		Caches:      []string{m.npmCache()},
+		WriteCaches: write,
+		Env:         []string{"PATH=" + pathOrDefault(), "LC_ALL=C"},
+	}
+}
+
+// stepRunner is the runner of a run step in the workspace root: buildstep.Isolated over m.Run, every command shut off;
+// write is a trunk build's alone.
+func (m *Module) stepRunner(root string, write bool) (forgekey.Runner, error) {
+	// bubblewrap binds only a directory that is there.
+	if err := os.MkdirAll(m.npmCache(), 0o700); err != nil {
+		return nil, fmt.Errorf("the shared cache %s: %w", m.npmCache(), err)
+	}
+	return buildstep.Isolated(m.Run, m.isolation(root, write)), nil
+}
+
+// ProbeBubblewrap reads the installed bubblewrap's version (`bwrap --version`: "bubblewrap 0.13.0").
+func ProbeBubblewrap(ctx context.Context) (string, error) {
+	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
+	defer cancel()
+	r := forgekey.ExecRunnerClean(0)(ctx, []string{"PATH=" + defaultPath}, buildstep.Bubblewrap, "--version")
+	if r.Status != 0 || r.Err != "" {
+		return "", fmt.Errorf("`%s --version` failed: %s", buildstep.Bubblewrap, firstLine(r.Err+"\n"+r.Stderr))
+	}
+	return parseBubblewrapVersion(r.Stdout)
+}
+
+var bubblewrapVersion = regexp.MustCompile(`^bubblewrap (\d+)\.(\d+)(\.\d+)?\s*$`)
+
+func parseBubblewrapVersion(out string) (string, error) {
+	f := bubblewrapVersion.FindStringSubmatch(strings.TrimSpace(out))
+	if f == nil {
+		return "", fmt.Errorf("`%s --version` answered %q", buildstep.Bubblewrap, strings.TrimSpace(out))
+	}
+	return f[1] + "." + f[2] + f[3], nil
+}
+
+// bubblewrapProblem says why the installed bubblewrap cannot shut a run step off, or "".
+func bubblewrapProblem(version string, err error) string {
+	if err != nil {
+		return "bubblewrap cannot be run, and every build and test step runs in it (ADR 0448): " + err.Error()
+	}
+	if !atLeast(version, BubblewrapLeast) {
+		return fmt.Sprintf("bubblewrap %s installed, and a build or test step needs %s or later (ADR 0448)", version, BubblewrapLeast)
+	}
+	return ""
+}
+
+// atLeast says whether the dotted version v is least or later.
+func atLeast(v, least string) bool {
+	a, b := strings.Split(v, "."), strings.Split(least, ".")
+	for i := range b {
+		var x int
+		if i < len(a) {
+			x, _ = strconv.Atoi(a[i])
+		}
+		y, _ := strconv.Atoi(b[i])
+		if x != y {
+			return x > y
+		}
+	}
+	return true
+}
+
+// checkBubblewrap refuses a step when bubblewrap cannot shut it off; nil Bubblewrap probes nothing.
+func (m *Module) checkBubblewrap(ctx context.Context) error {
+	if m.Bubblewrap == nil {
+		return nil
+	}
+	v, err := m.Bubblewrap(ctx)
+	if why := bubblewrapProblem(v, err); why != "" {
+		return &buildstep.Refusal{Text: why, Want: BubblewrapLeast + " or later", Have: v}
+	}
+	return nil
+}
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/isolate_test.go b/modules/mesh-build-node/cmd/mesh-build-node/isolate_test.go
new file mode 100644
index 00000000..7f3f7391
--- /dev/null
+++ b/modules/mesh-build-node/cmd/mesh-build-node/isolate_test.go
@@ -0,0 +1,178 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/buildstep"
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
+)
+
+// shutOff is one command a step ran in bubblewrap: what it bound read-write, read-only and beneath a throwaway layer,
+// its bubblewrap flags, and the command.
+type shutOff struct {
+	flags                    []string
+	binds, roBinds, overlays []string
+	env                      []string
+	command                  string
+}
+
+// bwrapArity is how many words each bubblewrap option the SDK writes takes.
+var bwrapArity = map[string]int{"--unshare-all": 0, "--die-with-parent": 0, "--new-session": 0, "--clearenv": 0,
+	"--proc": 1, "--dev": 1, "--tmpfs": 1, "--dir": 1, "--chdir": 1, "--overlay-src": 1, "--tmp-overlay": 1,
+	"--ro-bind": 2, "--ro-bind-try": 2, "--bind": 2, "--symlink": 2, "--setenv": 2}
+
+// throughBubblewrap reads a bubblewrap command back (recorded in seen) and hands its command to run with the environment
+// bubblewrap sets (a name set twice keeps its last value, as bubblewrap's --setenv does); any other command goes to run
+// as it is.
+func throughBubblewrap(t *testing.T, run forgekey.Runner, seen *[]shutOff) forgekey.Runner {
+	t.Helper()
+	var mu sync.Mutex
+	return func(ctx context.Context, env []string, name string, args ...string) forgekey.Ran {
+		if name != buildstep.Bubblewrap {
+			return run(ctx, env, name, args...)
+		}
+		var s shutOff
+		var names []string
+		values := map[string]string{}
+		i := 0
+		for ; i < len(args) && args[i] != "--"; i++ {
+			n, ok := bwrapArity[args[i]]
+			if !ok || i+n >= len(args) {
+				t.Errorf("bubblewrap was given %q, which the test does not know: %v", args[i], args)
+				return forgekey.Ran{Status: 1, Err: "unknown bubblewrap option"}
+			}
+			s.flags = append(s.flags, args[i])
+			switch args[i] {
+			case "--bind":
+				s.binds = append(s.binds, args[i+1])
+			case "--ro-bind", "--ro-bind-try":
+				s.roBinds = append(s.roBinds, args[i+1])
+			case "--overlay-src":
+				s.overlays = append(s.overlays, args[i+1])
+			case "--setenv":
+				if _, set := values[args[i+1]]; !set {
+					names = append(names, args[i+1])
+				}
+				values[args[i+1]] = args[i+2]
+			}
+			i += n
+		}
+		if i+1 >= len(args) {
+			t.Errorf("bubblewrap was given no command: %v", args)
+			return forgekey.Ran{Status: 1, Err: "no command"}
+		}
+		for _, n := range names {
+			s.env = append(s.env, n+"="+values[n])
+		}
+		s.command = args[i+1] + " " + strings.Join(args[i+2:], " ")
+		mu.Lock()
+		*seen = append(*seen, s)
+		mu.Unlock()
+		return run(ctx, s.env, args[i+1], args[i+2:]...)
+	}
+}
+
+// checkShutOff fails the test unless s ran with no network and none of the account, the toolchain read-only, and the
+// shared cache beneath a throwaway layer unless write.
+func checkShutOff(t *testing.T, m *Module, root string, s shutOff, write bool) {
+	t.Helper()
+	for _, f := range []string{"--unshare-all", "--clearenv", "--die-with-parent"} {
+		if !slices.Contains(s.flags, f) {
+			t.Errorf("%s ran without %s", s.command, f)
+		}
+	}
+	home := m.Toolchain.Paths.Home
+	for _, b := range append(append([]string{}, s.binds...), s.roBinds...) {
+		if b == home || strings.HasPrefix(b, home+"/") || strings.HasPrefix(b, "/run") {
+			t.Errorf("%s was given %s: the account's home or the runtime directory", s.command, b)
+		}
+	}
+	for _, e := range s.env {
+		if e == "HOME="+home {
+			t.Errorf("%s was told the account's home", s.command)
+		}
+	}
+	if !slices.Contains(s.roBinds, m.toolchainDir()) || slices.Contains(s.binds, m.toolchainDir()) {
+		t.Errorf("%s was not given the toolchain read-only: %v %v", s.command, s.roBinds, s.binds)
+	}
+	c := m.npmCache()
+	bound, layered := slices.Contains(s.binds, c), slices.Contains(s.overlays, c)
+	if write != bound || write == layered {
+		t.Errorf("%s: the shared cache bound %v, layered %v; a trunk build (%v) alone writes it", s.command, bound, layered, write)
+	}
+	if !slices.Contains(s.binds, root) {
+		t.Errorf("%s was not given its workspace %s read-write: %v", s.command, root, s.binds)
+	}
+}
+
+// 1309-S7: the type-check and the test script run shut off, the cache beneath a throwaway layer.
+func TestTheTestStepRunsItsCommandsShutOff(t *testing.T) {
+	m, f, work := newMachine(t)
+	workspaceWith(t, work, map[string]string{"index.ts": `export const a = 1;`, "package.json": `{"scripts":{"test":"x"}}`})
+	if _, err := m.test(context.Background(), testArgs("unit")); err != nil {
+		t.Fatal(err)
+	}
+	if len(f.shut) != 2 || len(f.calls) != 2 {
+		t.Fatalf("shut off %d of %d commands, want the type-check and the script: %v", len(f.shut), len(f.calls), f.calls)
+	}
+	for _, s := range f.shut {
+		checkShutOff(t, m, work+"/w1", s, false)
+	}
+}
+
+// 1309-S8: tsc and esbuild of a candidate's build run shut off with the cache beneath a throwaway layer; a trunk build
+// (publish) writes it.
+func TestTheBuildStepRunsShutOffAndOnlyATrunkBuildWritesTheCache(t *testing.T) {
+	for _, publish := range []bool{false, true} {
+		m, f, work := builtMachine(t, "")
+		ws := "w1"
+		args := buildArgs()
+		if publish {
+			args["publish"] = true
+		}
+		if _, err := m.build(context.Background(), args); err != nil {
+			t.Fatalf("publish %v: %v", publish, err)
+		}
+		if len(f.shut) == 0 || len(f.shut) != len(f.calls) {
+			t.Fatalf("publish %v: shut off %d of %d commands", publish, len(f.shut), len(f.calls))
+		}
+		for _, s := range f.shut {
+			checkShutOff(t, m, work+"/"+ws, s, publish)
+		}
+	}
+}
+
+// 1309-S1: a step is refused, and the module is unhealthy, while bubblewrap is missing or older than 0.10.
+func TestAnOldOrMissingBubblewrapRefusesTheStepsAndIsUnhealthy(t *testing.T) {
+	for _, c := range []struct {
+		version string
+		err     error
+		ok      bool
+	}{{"0.13.0", nil, true}, {"0.10", nil, true}, {"0.9.0", nil, false}, {"", errors.New("bwrap: not found"), false}} {
+		m, _, _ := newMachine(t)
+		m.Probe = func(context.Context) (string, error) { return "v22.20.0", nil }
+		m.Toolchains = func(string, any) (uint64, error) { return 1, nil }
+		m.Bubblewrap = func(context.Context) (string, error) { return c.version, c.err }
+		err := m.Check(context.Background())
+		raw, _ := json.Marshal(m.Health(context.Background()))
+		if c.ok {
+			if err != nil || strings.Contains(string(raw), "unhealthy") {
+				t.Errorf("bubblewrap %q: refused %v, health %s", c.version, err, raw)
+			}
+			continue
+		}
+		var ref *buildstep.Refusal
+		if !errors.As(err, &ref) || !strings.Contains(err.Error(), "bubblewrap") {
+			t.Errorf("bubblewrap %q %v: Check answered %v", c.version, c.err, err)
+		}
+		if !strings.Contains(string(raw), "unhealthy") || !strings.Contains(string(raw), "bubblewrap") {
+			t.Errorf("bubblewrap %q %v: health %s", c.version, c.err, raw)
+		}
+	}
+}
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/isolation_integration_test.go b/modules/mesh-build-node/cmd/mesh-build-node/isolation_integration_test.go
new file mode 100644
index 00000000..095f16cf
--- /dev/null
+++ b/modules/mesh-build-node/cmd/mesh-build-node/isolation_integration_test.go
@@ -0,0 +1,91 @@
+//go:build integration
+
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"net"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"testing"
+)
+
+// probeScript is a package's test script that tries what a change's code must not reach and writes what it saw.
+const probeScript = `import fs from "node:fs";
+import dns from "node:dns/promises";
+import net from "node:net";
+const [key, login, port, sock, cache] = process.argv.slice(2);
+const seen = {};
+const read = (p) => { try { fs.readFileSync(p); return true; } catch { return false; } };
+seen.key = read(key);
+seen.login = read(login);
+try { await dns.lookup("git.novox.be"); seen.resolved = true; } catch { seen.resolved = false; }
+const reach = (opts) => new Promise((ok) => { const c = net.connect(opts); c.on("connect", () => { c.destroy(); ok(true); }); c.on("error", () => ok(false)); });
+seen.tcp = await reach({ host: "127.0.0.1", port: Number(port) });
+seen.socket = await reach({ path: sock });
+try { fs.writeFileSync(cache + "/written-by-a-candidate", "x"); seen.wroteCache = true; } catch { seen.wroteCache = false; }
+fs.writeFileSync("seen.json", JSON.stringify(seen));
+`
+
+// 1309-S9 (ADR 0448's check): a package's test script, run by the test step, reads no key and no login of the account,
+// resolves no name, reaches no socket of the machine (a TCP port on its loopback, a unix socket), and leaves the
+// shared cache unchanged.
+func TestARunStepReachesNoKeyLoginNameOrSocketAndLeavesTheCacheUnchanged(t *testing.T) {
+	if _, err := exec.LookPath("bwrap"); err != nil {
+		t.Skip("NOT RUN: no bubblewrap installed where this check runs; every build node has it (the module declares it)")
+	}
+	m, work := realMachine(t)
+	home := m.Toolchain.Paths.Home
+	key, login := filepath.Join(home, ".ssh", "mesh_ed25519"), filepath.Join(home, "broker")
+	write(t, key, "a key", 0o600)
+	write(t, login, "a login", 0o600)
+	tcp, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer tcp.Close()
+	sockDir, err := os.MkdirTemp("", "sock")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(sockDir)
+	sock := filepath.Join(sockDir, "s")
+	unix, err := net.Listen("unix", sock)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer unix.Close()
+	if err := os.MkdirAll(m.npmCache(), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	port := strconv.Itoa(tcp.Addr().(*net.TCPAddr).Port)
+	dir := filepath.Join(work, "w1", "tree", "modules", "m")
+	write(t, filepath.Join(dir, "probe.mjs"), probeScript, 0o644)
+	write(t, filepath.Join(dir, "package.json"), `{"name":"m","type":"module","scripts":{"test":"node probe.mjs `+key+` `+login+` `+port+` `+sock+` `+m.npmCache()+`"}}`, 0o644)
+	out, err := m.test(context.Background(), map[string]any{"workspace": "w1", "module": "modules/m", "level": "unit"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if r := out.(*TestResult); !r.Passed {
+		t.Fatalf("the probe did not run: %+v", r)
+	}
+	raw, err := os.ReadFile(filepath.Join(dir, "seen.json"))
+	if err != nil {
+		t.Fatalf("the probe wrote nothing: %v", err)
+	}
+	var seen map[string]bool
+	if err := json.Unmarshal(raw, &seen); err != nil {
+		t.Fatal(err)
+	}
+	for what, reached := range seen {
+		if reached && what != "wroteCache" {
+			t.Errorf("the run step reached %s: %s", what, raw)
+		}
+	}
+	if _, err := os.Stat(filepath.Join(m.npmCache(), "written-by-a-candidate")); err == nil {
+		t.Errorf("a candidate's step changed the shared cache")
+	}
+}
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/main.go b/modules/mesh-build-node/cmd/mesh-build-node/main.go
index 1d334cac..fcca5746 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/main.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/main.go
@@ -51,6 +51,7 @@ func main() {
 		Run:                 forgekey.ExecRunnerClean(0),
 		Ctx:                 ctx,
 		Say:                 logf,
+		Bubblewrap:          ProbeBubblewrap,
 	}
 	// Outside a launch by the runtime — a build, a check — there is no node and no state to write.
 	if m.Node != "" {
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/manifest_test.go b/modules/mesh-build-node/cmd/mesh-build-node/manifest_test.go
index efc359e9..d523c627 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/manifest_test.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/manifest_test.go
@@ -66,7 +66,7 @@ func TestManifestIsValidAndDeclaresTheAccountTheCapabilityAndThePackages(t *test
 		}
 	}
 	slices.Sort(packages)
-	if want := []string{"git", "openssh"}; !slices.Equal(packages, want) {
+	if want := []string{"bubblewrap", "git", "openssh"}; !slices.Equal(packages, want) {
 		t.Errorf("packages = %v, want %v", packages, want)
 	}
 }
@@ -194,3 +194,15 @@ func TestTheToolchainFilesAreAnArchiveUnpackedIntoTheHome(t *testing.T) {
 		t.Errorf("the module carries no toolchain/package-lock.json: %v", err)
 	}
 }
+
+// 1309-S1: every run step runs in bubblewrap, so the module declares its package (the lead's decision: each build module
+// that runs a step through it declares it), beside the user-namespaces capability bubblewrap needs.
+func TestTheModuleDeclaresBubblewrapAndTheCapabilityItNeeds(t *testing.T) {
+	m := readManifest(t)
+	if r := resource(t, m, "bubblewrap"); r["type"] != "package" || r["package"] != "bubblewrap" {
+		t.Errorf("the bubblewrap resource is %v, want the package bubblewrap", r)
+	}
+	if !slices.Contains(m.Capabilities, "user-namespaces") {
+		t.Errorf("capabilities %v lack user-namespaces", m.Capabilities)
+	}
+}
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/module.go b/modules/mesh-build-node/cmd/mesh-build-node/module.go
index 1f9e60d0..f277560b 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/module.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/module.go
@@ -48,6 +48,9 @@ type Module struct {
 	// Ctx is the process's context, which a step runs under (the tool call's is cut at 30 s, the step's is not).
 	Ctx context.Context
 	Say func(format string, a ...any)
+	// 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)
 
 	mu      sync.Mutex // guards Recipe
 	install sync.Mutex // one install at a time
@@ -87,7 +90,10 @@ func (m *Module) check(ctx context.Context) (string, error) {
 		m.mu.Unlock()
 	}
 	installed, err := m.Probe(ctx)
-	return installed, m.guard(m.recipe(), installed, err).Check(ctx)
+	if err := m.guard(m.recipe(), installed, err).Check(ctx); err != nil {
+		return installed, err
+	}
+	return installed, m.checkBubblewrap(ctx)
 }
 
 // Prepare is Check and then the toolchain directory of the recipe in force: what every step runs behind.
diff --git a/modules/mesh-build-node/cmd/mesh-build-node/workspace.go b/modules/mesh-build-node/cmd/mesh-build-node/workspace.go
index 001dec70..c49f6cc7 100644
--- a/modules/mesh-build-node/cmd/mesh-build-node/workspace.go
+++ b/modules/mesh-build-node/cmd/mesh-build-node/workspace.go
@@ -168,10 +168,16 @@ func copyFile(from, to string) error {
 	return out.Close()
 }
 
-// runIn runs a command with dir as its working directory (`env -C`), so it is the plain line the build agent runs from
-// the module's root, with the same relative paths. The command's environment is baseEnv and extra.
-func (m *Module) runIn(ctx context.Context, dir string, extra []string, name string, args ...string) forgekey.Ran {
-	return m.Run(ctx, append(m.baseEnv(), extra...), "env", append([]string{"-C", dir, name}, args...)...)
+// runIn runs a command of a run step shut off (stepRunner) with dir as its working directory (`env -C`), so it is the
+// plain line the build agent runs from the module's root, with the same relative paths. The command's environment is
+// a PATH, a locale and extra: never the account's HOME (the step's is its own) nor this process's environment. root is
+// the workspace; write is a trunk build's (publish), which alone writes the shared cache.
+func (m *Module) runIn(ctx context.Context, root string, write bool, dir string, extra []string, name string, args ...string) forgekey.Ran {
+	run, err := m.stepRunner(root, write)
+	if err != nil {
+		return forgekey.Ran{Status: 1, Err: err.Error()}
+	}
+	return run(ctx, extra, "env", append([]string{"-C", dir, name}, args...)...)
 }
 
 // ranFailed says why a command did not pass, or "" when it did.
diff --git a/modules/mesh-build-node/module.json b/modules/mesh-build-node/module.json
index b92212aa..f0b078d1 100644
--- a/modules/mesh-build-node/module.json
+++ b/modules/mesh-build-node/module.json
@@ -139,6 +139,11 @@
       "id": "openssh",
       "type": "package",
       "package": "openssh"
+    },
+    {
+      "id": "bubblewrap",
+      "type": "package",
+      "package": "bubblewrap"
     }
   ],
   "build": {