diff --git a/spec/bench/README.md b/spec/bench/README.md
new file mode 100644
index 0000000..8a73738
--- /dev/null
+++ b/spec/bench/README.md
@@ -0,0 +1,40 @@
+# Benchmark of the engines
+
+Same work, four engines, five runs, median. Docker is the mock of
+`spec/support/bin`, the catalogue is `myos-stacks` reached through
+`$HOME/.local/share/myos/stack`, the environment is `env.sh`.
+
+    sh spec/bench/run.sh
+
+`justfile` is a prototype of just as the engine: shebang recipes that source
+`lib/*.sh` once. `go/main.go` is a prototype of the core in Go: stack path,
+groups, compose files, project name, dry-run command; `export` runs one `sh`
+per stack directory to evaluate the shell hooks.
+
+## Results, 2026-09-05, Mac Studio M2 Ultra
+
+| work | make | sh (bin/myos) | just | go |
+|---|---:|---:|---:|---:|
+| fixed cost, empty target | 312 ms | 50 ms | 170 ms | 23 ms |
+| `up` 1 / 3 stacks, no hooks in the stack | 792 / 2201 | 177 / 332 | 204 / 268 | 23 / 23 |
+| `up` 1 / 3 stacks, real catalogue with hooks | — | 353 / 855 | (prototype does not load hooks) | (idem) |
+| `export`, 80 settings of the `host` group | — | 1477 | 1314 | 720 (1 sh) |
+| same, `MYOS_VAR_MEMO=1` | — | 1503 | 1318 | 774 |
+| one computed setting (`HOST_FABIO_SERVICE_9998_TAGS`) | — | ~43 ms net (69 − 26) | | |
+
+Reference points: `sh -c :` 24 ms, `just --version` 27 ms, sourcing `lib/*.sh` +2 ms.
+
+## What it says
+
+- The engine's own cost: go flat at 23 ms; sh 177 ms + ~63 ms per stack; just
+  204 ms + ~32 ms per stack; make 792 ms + ~700 ms per stack (it re-reads
+  itself for every stack).
+- The shell hooks cost ~40 ms per computed setting, on every engine: 80
+  settings ≈ 0.7 s even from Go, which runs the very same `sh`. Memoisation
+  changes nothing, because the cost is not repeated lookups: each `tagprefix`
+  spawns 15-20 command substitutions for distinct, mostly empty, variables.
+- `bin/myos` doubles that to 1.5 s by loading the hooks of a directory once
+  per stack reference instead of once per directory: `host/consul`,
+  `host/fabio` and `host/registrator` share `stack/host/_stack.sh`.
+- just's fixed cost (170 ms for a shebang recipe, against 27 ms for `just
+  --version`) is its own overhead of writing and running the recipe script.
diff --git a/spec/bench/bench.sh b/spec/bench/bench.sh
new file mode 100755
index 0000000..183b40f
--- /dev/null
+++ b/spec/bench/bench.sh
@@ -0,0 +1,16 @@
+#!/bin/sh
+# bench.sh LABEL N -- CMD...  run CMD N times, print the median wall time in ms
+# Hermetic: docker is the mock of spec/support/bin, config comes from the
+# environment only, HOME points at the fixture catalogue.
+set -u
+label=$1; n=$2; shift 2; [ "$1" = "--" ] && shift
+i=0; times=""
+while [ "$i" -lt "$n" ]; do
+  s=$(python3 -c 'import time;print(int(time.time()*1e6))')
+  "$@" >/dev/null 2>&1
+  e=$(python3 -c 'import time;print(int(time.time()*1e6))')
+  times="$times $(( (e - s) / 1000 ))"
+  i=$((i + 1))
+done
+median=$(printf '%s\n' $times | sort -n | awk '{a[NR]=$1} END {print a[int((NR+1)/2)]}')
+printf '%-44s %6s ms   (runs:%s)\n' "$label" "$median" "$times"
diff --git a/spec/bench/env.sh b/spec/bench/env.sh
new file mode 100644
index 0000000..893517d
--- /dev/null
+++ b/spec/bench/env.sh
@@ -0,0 +1,5 @@
+# the hermetic environment every engine runs in
+export PATH=/Users/aya/dev/myos/spec/support/bin:/Users/aya/.local/bin:/usr/bin:/bin
+export HOME=/tmp/myos-bench/home WORKDIR=/tmp/myos-bench/wd MYOS_ROOT=/Users/aya/dev/myos
+export USER=tester HOSTNAME=testhost DOMAIN=example.test ENV=local DRYRUN=true
+export MYOS_CONF=/dev/null MYOS_PROJECT_FORMAT=user-app-env DOCKER_MACHINE=x86_64 DOCKER_SYSTEM=Linux
diff --git a/spec/bench/go/main.go b/spec/bench/go/main.go
new file mode 100644
index 0000000..64113a7
--- /dev/null
+++ b/spec/bench/go/main.go
@@ -0,0 +1,238 @@
+// A prototype of the myos core in Go, just large enough to be benchmarked
+// fairly against the other engines: stack path, group expansion, compose file
+// resolution across every directory of the path, project name, and the
+// dry-run compose command. Same rules as lib/stack.sh and lib/naming.sh.
+//
+// export delegates the shell hooks to ONE sh per stack directory, which is
+// what a Go engine would do to keep the developer contract in shell.
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+func env(k, def string) string {
+	if v := os.Getenv(k); v != "" {
+		return v
+	}
+	return def
+}
+
+// stackPath: the directories stacks are looked up in, project first
+func stackPath(workdir string) []string {
+	home := env("HOME", "/nonexistent")
+	root := env("MYOS_ROOT", ".")
+	prefix := filepath.Dir(filepath.Dir(root))
+	var out []string
+	seen := map[string]bool{}
+	for _, d := range []string{workdir, filepath.Join(workdir, ".."), filepath.Join(home, ".local/share"), filepath.Join(prefix, "share"), "/usr/local/share", "/usr/share"} {
+		for _, c := range []string{filepath.Join(d, "stack"), filepath.Join(d, "myos/stack")} {
+			if st, err := os.Stat(c); err == nil && st.IsDir() {
+				if r, err := filepath.EvalSymlinks(c); err == nil {
+					c = r
+				}
+				if !seen[c] {
+					seen[c] = true
+					out = append(out, c)
+				}
+			}
+		}
+	}
+	return out
+}
+
+// groupValue: the list a lowercase group name expands to, from <g>.env,
+// <g>/<g>.env or <g>/_stack.env along the path
+func groupValue(path []string, name string) string {
+	if strings.ContainsAny(name, "/:.") || strings.ToLower(name) != name {
+		return ""
+	}
+	if v := os.Getenv(name); v != "" {
+		return v
+	}
+	for _, d := range path {
+		for _, f := range []string{filepath.Join(d, name+".env"), filepath.Join(d, name, name+".env"), filepath.Join(d, name, "_stack.env")} {
+			b, err := os.ReadFile(f)
+			if err != nil {
+				continue
+			}
+			for _, line := range strings.Split(string(b), "\n") {
+				if strings.HasPrefix(line, name+"=") {
+					return strings.Trim(strings.TrimPrefix(line, name+"="), "\"")
+				}
+			}
+		}
+	}
+	return ""
+}
+
+func expand(path []string, refs []string, depth int) []string {
+	var out []string
+	for _, r := range refs {
+		if v := groupValue(path, r); v != "" && depth < 16 {
+			out = append(out, expand(path, strings.Fields(v), depth+1)...)
+		} else {
+			out = append(out, r)
+		}
+	}
+	return out
+}
+
+func stackName(ref string) string {
+	r := strings.TrimSuffix(ref, "/")
+	if i := strings.LastIndex(r, ":"); i >= 0 {
+		r = r[:i]
+	}
+	return strings.TrimSuffix(filepath.Base(r), ".yml")
+}
+
+// stackDirs: every directory of the path holding the stack, least specific first
+func stackDirs(path []string, ref string) []string {
+	r := strings.TrimSuffix(ref, "/")
+	if i := strings.LastIndex(r, ":"); i >= 0 {
+		r = r[:i]
+	}
+	name := stackName(ref)
+	var found []string
+	for _, d := range path {
+		var hit string
+		if st, err := os.Stat(filepath.Join(d, r)); err == nil && st.IsDir() {
+			hit = filepath.Join(d, r)
+		} else if _, err := os.Stat(filepath.Join(d, r+".yml")); err == nil {
+			hit = filepath.Dir(filepath.Join(d, r))
+		} else if st, err := os.Stat(filepath.Join(d, name)); err == nil && st.IsDir() {
+			hit = filepath.Join(d, name)
+		}
+		if hit != "" {
+			found = append([]string{hit}, found...)
+		}
+	}
+	return found
+}
+
+func exists(p string) bool { _, err := os.Stat(p); return err == nil }
+
+// composeFiles: the files that exist, in the order the framework loads them
+func composeFiles(dir string, names, suffixes []string, envName string) []string {
+	var out []string
+	for _, e := range []string{"yml", "yaml"} {
+		for _, n := range names {
+			for _, f := range []string{
+				filepath.Join(dir, n+"."+e), filepath.Join(dir, n+"."+envName+"."+e),
+				filepath.Join(dir, envName, n+"."+e), filepath.Join(dir, envName, n+"."+envName+"."+e)} {
+				if exists(f) {
+					out = append(out, f)
+				}
+			}
+			for _, s := range suffixes {
+				for _, f := range []string{filepath.Join(dir, n+"."+s+"."+e), filepath.Join(dir, n+"."+s+"."+envName+"."+e)} {
+					if exists(f) {
+						out = append(out, f)
+					}
+				}
+			}
+		}
+	}
+	return out
+}
+
+func scope(ref string) string {
+	switch strings.SplitN(ref, "/", 2)[0] {
+	case "host":
+		return "host"
+	case "User", "user":
+		return "user"
+	case "cluster":
+		return "cluster"
+	}
+	return "app"
+}
+
+func projectName(sc, user, envName, app string) string {
+	switch sc {
+	case "host":
+		return env("HOST_COMPOSE_PROJECT_NAME", env("HOSTNAME", "localhost"))
+	case "user":
+		return user
+	case "cluster":
+		return strings.ToLower(app)
+	}
+	n := strings.NewReplacer(".", "", "-", "", "_", "").Replace(strings.ToLower(app))
+	if env("MYOS_PROJECT_FORMAT", "user-env-app") == "user-app-env" {
+		return user + "-" + n + "-" + envName
+	}
+	return user + "-" + envName + "-" + n
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintln(os.Stderr, "usage: myos-go <noop|up|export> [stack...]")
+		os.Exit(2)
+	}
+	workdir := env("WORKDIR", ".")
+	envName := env("ENV", "local")
+	user := env("USER", "tester")
+	path := stackPath(workdir)
+	suffixes := []string{"app", "labels", "networks", "ssh", "volumes", "latest"}
+
+	switch os.Args[1] {
+	case "noop":
+		return
+	case "up":
+		refs := expand(path, os.Args[2:], 0)
+		byProject := map[string][]string{}
+		var order []string
+		for _, ref := range refs {
+			app := stackName(ref)
+			var files []string
+			for _, d := range stackDirs(path, ref) {
+				files = append(files, composeFiles(d, []string{"docker-compose", app}, suffixes, envName)...)
+			}
+			p := projectName(scope(ref), user, envName, app)
+			if _, ok := byProject[p]; !ok {
+				order = append(order, p)
+			}
+			byProject[p] = append(byProject[p], files...)
+		}
+		for _, p := range order {
+			files := append(byProject[p], filepath.Join(env("MYOS_ROOT", "."), "share/compose/networks.yml"))
+			var b strings.Builder
+			b.WriteString("docker compose")
+			for _, f := range files {
+				b.WriteString(" -f " + f)
+			}
+			fmt.Printf("%s -p %s up -d\n", b.String(), p)
+		}
+	case "export":
+		// one sh per stack directory evaluates its hooks and prints every value
+		refs := expand(path, os.Args[2:], 0)
+		seen := map[string]bool{}
+		var dirs []string
+		for _, ref := range refs {
+			for _, d := range stackDirs(path, ref) {
+				if !seen[d] {
+					seen[d] = true
+					dirs = append(dirs, d)
+				}
+			}
+		}
+		sort.Strings(dirs)
+		root := env("MYOS_ROOT", ".")
+		for _, d := range dirs {
+			script := fmt.Sprintf(`for m in core str var tags naming stack config compose hooks; do . %s/lib/$m.sh; done
+[ -f %s/_stack.sh ] || exit 0
+myos_stack_hooks %s _
+for v in $(sed -n 's/^myos_default_\([A-Za-z_][A-Za-z0-9_]*\)().*/\1/p' %s/_stack.sh | sort -u); do printf '%%s=%%s\n' "$v" "$(myos_var "$v")"; done`, root, d, d, d)
+			cmd := exec.Command("sh", "-c", script)
+			cmd.Env = os.Environ()
+			cmd.Stdout = os.Stdout
+			cmd.Stderr = os.Stderr
+			_ = cmd.Run()
+		}
+	}
+}
diff --git a/spec/bench/justfile b/spec/bench/justfile
new file mode 100644
index 0000000..4131fb2
--- /dev/null
+++ b/spec/bench/justfile
@@ -0,0 +1,52 @@
+# just as the engine: the interface is a justfile, the logic stays in lib/*.sh.
+# Recipes are shebang recipes, so a whole body runs in ONE sh that sources
+# lib/ once; what is measured is just's own overhead on top of the shell.
+
+set export
+
+MYOS_ROOT := env_var_or_default("MYOS_ROOT", "/Users/aya/dev/myos")
+WORKDIR   := env_var_or_default("WORKDIR", justfile_directory())
+ENV       := env_var_or_default("ENV", "local")
+USER      := env_var_or_default("USER", "tester")
+HOSTNAME  := env_var_or_default("HOSTNAME", "testhost")
+DOMAIN    := env_var_or_default("DOMAIN", "example.test")
+DRYRUN    := env_var_or_default("DRYRUN", "true")
+
+# a recipe that does nothing: the fixed cost of just + one sh + sourcing lib/
+noop:
+    #!/bin/sh
+    for m in core str var tags naming stack config compose hooks; do . $MYOS_ROOT/lib/$m.sh; done
+
+# up STACKS: resolve every stack of the groups, group by compose project,
+# print one compose command per project (what bin/myos does)
+up +stacks:
+    #!/bin/sh
+    for m in core str var tags naming stack config compose hooks; do . $MYOS_ROOT/lib/$m.sh; done
+    rows=""
+    for ref in $(myos_group_expand {{stacks}}); do
+      files=""
+      for d in $(myos_stack_dirs "$ref"); do
+        files="$files $(myos_compose_files "$d" "docker-compose $(myos_stack_name "$ref")" "$(myos_compose_suffixes)" "$ENV" | tr '\n' ' ')"
+      done
+      app=$(myos_stack_name "$ref")
+      project=$(myos_project_name "$(myos_scope "$ref")" "$USER" "$ENV" "$app")
+      rows="$rows
+    $project|$files"
+    done
+    for project in $(printf '%s\n' "$rows" | sed '/^$/d' | cut -d'|' -f1 | awk '!s[$0]++'); do
+      files=$(printf '%s\n' "$rows" | awk -F'|' -v p="$project" '$1==p {print $2}' | tr ' ' '\n' | sed '/^$/d' | awk '!s[$0]++')
+      fargs=""; for f in $files $MYOS_ROOT/share/compose/networks.yml; do fargs="$fargs -f $f"; done
+      echo "docker compose$fargs -p $project up -d"
+    done
+
+# export STACKS: every setting the hooks of the stacks declare
+export +stacks:
+    #!/bin/sh
+    for m in core str var tags naming stack config compose hooks; do . $MYOS_ROOT/lib/$m.sh; done
+    refs=$(myos_group_expand {{stacks}})
+    for ref in $refs; do
+      for d in $(myos_stack_dirs "$ref"); do myos_stack_hooks "$d" "$(myos_stack_name "$ref")"; done
+    done
+    names=$(for ref in $refs; do for d in $(myos_stack_dirs "$ref"); do
+      [ -f "$d/_stack.sh" ] && sed -n 's/^myos_default_\([A-Za-z_][A-Za-z0-9_]*\)().*/\1/p' "$d/_stack.sh"; done; done | sort -u)
+    for v in $names; do printf '%s=%s\n' "$v" "$(myos_var "$v")"; done
diff --git a/spec/bench/run.sh b/spec/bench/run.sh
new file mode 100755
index 0000000..04cfb96
--- /dev/null
+++ b/spec/bench/run.sh
@@ -0,0 +1,39 @@
+#!/bin/sh
+# the full matrix: 5 runs each, median, every engine on the same work
+. /tmp/myos-bench/env.sh; cd "$WORKDIR"
+B=/tmp/myos-bench/bench.sh; N=5
+MK="make -esC $MYOS_ROOT MYOS=. WORKDIR=$WORKDIR"
+SH="$MYOS_ROOT/bin/myos"
+JU="just --justfile /tmp/myos-bench/justfile"
+GO=/tmp/myos-bench/myos-go
+S1="host/consul"; S2="host/consul host/fabio"; S3="host/consul host/fabio host/registrator"
+
+echo "== cout fixe : demarrage + cible vide"
+$B "make  noop" $N -- $MK FORCE
+$B "sh    noop (myos version)" $N -- $SH version
+$B "just  noop (parse + 1 sh + source lib/)" $N -- $JU noop
+$B "go    noop" $N -- $GO noop
+echo
+echo "== up : 1 / 2 / 3 stacks, dry-run"
+$B "make  up 1" $N -- $MK up STACK="$S1"
+$B "make  up 2" $N -- $MK up STACK="$S2"
+$B "make  up 3" $N -- $MK up STACK="$S3"
+$B "sh    up 1" $N -- $SH up host/consul
+$B "sh    up 2" $N -- $SH up host/consul host/fabio
+$B "sh    up 3" $N -- $SH up host/consul host/fabio host/registrator
+$B "just  up 1" $N -- $JU up host/consul
+$B "just  up 2" $N -- $JU up host/consul host/fabio
+$B "just  up 3" $N -- $JU up host/consul host/fabio host/registrator
+$B "go    up 1" $N -- $GO up host/consul
+$B "go    up 2" $N -- $GO up host/consul host/fabio
+$B "go    up 3" $N -- $GO up host/consul host/fabio host/registrator
+echo
+echo "== export : les 80 reglages du groupe host (evaluation des hooks shell)"
+$B "sh    export, hooks tels quels" $N -- $SH export STACK=host
+$B "just  export, hooks tels quels" $N -- $JU export host
+$B "go    export, hooks tels quels (1 sh/repertoire)" $N -- $GO export host
+echo
+echo "== export : memes hooks, evalues en une passe (MYOS_VAR_MEMO=1)"
+MYOS_VAR_MEMO=1 $B "sh    export, memoise" $N -- $SH export STACK=host
+MYOS_VAR_MEMO=1 $B "just  export, memoise" $N -- $JU export host
+MYOS_VAR_MEMO=1 $B "go    export, memoise" $N -- $GO export host