diff --git a/go/cmd/mesh-compose/main.go b/go/cmd/mesh-compose/main.go
new file mode 100644
index 0000000..e871b0a
--- /dev/null
+++ b/go/cmd/mesh-compose/main.go
@@ -0,0 +1,144 @@
+// mesh-compose is mesh-sdk's composer command (novox/hq ADR 0482 clause 4, issue 1621): it reads a composer.Request on
+// standard input, validates every manifest with this mesh-sdk's reader, composes the machines named in each case and
+// writes one composer.Verdict on standard output. Built at any commit of mesh-sdk (or at the mesh-sdk a module pins),
+// it answers what that commit's composition answers, so a program that links one mesh-sdk can have another's judged.
+//
+//	mesh-compose < request.json > verdict.json
+//	mesh-compose -catalogue <dir> [-node <name>]... [-depth resolve|compose] [-declarations] [< request.json]
+//
+// -catalogue reads <dir>/modules/*/module.json and a module.json at <dir>'s root as the first case's manifests (a
+// checkout or `git archive` of the catalogue repository at a ref: the command fetches nothing). -node adds a machine to
+// compose; -declarations answers each machine's declared paths with their owners and modes.
+//
+// It opens no network connection, bus, store or file beyond its standard input and the -catalogue directory, and reads
+// no environment. Exit codes: 0 the verdict is written (a machine that does not compose is a verdict, not an error);
+// 1 the request cannot be read or run, said on standard error; 2 the flags are wrong.
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"git.novox.be/novox/mesh-sdk/go/composer"
+)
+
+func main() {
+	os.Exit(run(os.Args[1:], os.Stdin, os.Stdout, os.Stderr))
+}
+
+type nodes []string
+
+func (n *nodes) String() string     { return strings.Join(*n, ",") }
+func (n *nodes) Set(v string) error { *n = append(*n, v); return nil }
+
+// run is the command: its arguments, standard input, output and error; it answers the exit code.
+func run(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
+	fs := flag.NewFlagSet("mesh-compose", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	catalogueDir := fs.String("catalogue", "", "a catalogue checkout: <dir>/modules/*/module.json are the first case's manifests")
+	var machines nodes
+	fs.Var(&machines, "node", "a machine to compose (repeatable)")
+	depth := fs.String("depth", "", "resolve or compose (default: the request's, else compose)")
+	declarations := fs.Bool("declarations", false, "answer each composed machine's declared paths, owners and modes")
+	if err := fs.Parse(args); err != nil {
+		return 2
+	}
+	if fs.NArg() > 0 {
+		fmt.Fprintf(stderr, "mesh-compose: unexpected arguments %q\n", fs.Args())
+		return 2
+	}
+	raw, err := io.ReadAll(stdin)
+	if err != nil {
+		fmt.Fprintln(stderr, "mesh-compose: standard input cannot be read:", err)
+		return 1
+	}
+	var req composer.Request
+	if strings.TrimSpace(string(raw)) == "" {
+		if *catalogueDir == "" {
+			fmt.Fprintln(stderr, "mesh-compose: no request on standard input and no -catalogue")
+			return 1
+		}
+		req = composer.Request{V: composer.Version}
+	} else if req, err = composer.Decode(raw); err != nil {
+		fmt.Fprintln(stderr, "mesh-compose:", err)
+		return 1
+	}
+	if *catalogueDir != "" {
+		read, err := readCatalogue(*catalogueDir)
+		if err != nil {
+			fmt.Fprintln(stderr, "mesh-compose:", err)
+			return 1
+		}
+		if len(req.Cases) == 0 {
+			req.Cases = []composer.Case{{Name: "catalogue", Manifests: map[string]composer.Manifest{}}}
+		}
+		if req.Cases[0].Manifests == nil {
+			req.Cases[0].Manifests = map[string]composer.Manifest{}
+		}
+		for name, m := range read {
+			if _, given := req.Cases[0].Manifests[name]; !given {
+				req.Cases[0].Manifests[name] = m
+			}
+		}
+	}
+	req.Machines = append(req.Machines, machines...)
+	if *depth != "" {
+		req.Depth = *depth
+	}
+	if *declarations {
+		req.Declarations = true
+	}
+	v, err := composer.Run(req)
+	if err != nil {
+		fmt.Fprintln(stderr, "mesh-compose:", err)
+		return 1
+	}
+	enc := json.NewEncoder(stdout)
+	if err := enc.Encode(v); err != nil {
+		fmt.Fprintln(stderr, "mesh-compose: the verdict cannot be written:", err)
+		return 1
+	}
+	return 0
+}
+
+// readCatalogue reads every module.json of a catalogue checkout by its module's name: the root's and each of
+// modules/*/. A manifest whose name cannot be read is kept under its directory's name, so the reader's refusal is said.
+func readCatalogue(dir string) (map[string]composer.Manifest, error) {
+	paths, err := filepath.Glob(filepath.Join(dir, "modules", "*", "module.json"))
+	if err != nil {
+		return nil, err
+	}
+	if _, err := os.Stat(filepath.Join(dir, "module.json")); err == nil {
+		paths = append(paths, filepath.Join(dir, "module.json"))
+	}
+	if len(paths) == 0 {
+		return nil, fmt.Errorf("the catalogue %s holds no module.json", dir)
+	}
+	sort.Strings(paths)
+	out := map[string]composer.Manifest{}
+	for _, p := range paths {
+		raw, err := os.ReadFile(p)
+		if err != nil {
+			return nil, err
+		}
+		var probe struct {
+			Module string `json:"module"`
+		}
+		name := filepath.Base(filepath.Dir(p))
+		if json.Unmarshal(raw, &probe) == nil && probe.Module != "" {
+			name = probe.Module
+		}
+		if _, twice := out[name]; twice {
+			return nil, errors.New("the catalogue " + dir + " defines " + name + " twice")
+		}
+		out[name] = composer.Manifest{Raw: raw}
+	}
+	return out, nil
+}
diff --git a/go/cmd/mesh-compose/main_test.go b/go/cmd/mesh-compose/main_test.go
new file mode 100644
index 0000000..358e743
--- /dev/null
+++ b/go/cmd/mesh-compose/main_test.go
@@ -0,0 +1,293 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"go/build"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"sort"
+	"strings"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/catalogue"
+	"git.novox.be/novox/mesh-sdk/go/compose"
+	"git.novox.be/novox/mesh-sdk/go/composer"
+	"git.novox.be/novox/mesh-sdk/go/internal/beside"
+)
+
+// captured is every manifest of the catalogue captured in testdata/beside (internal/beside), raw, by module name.
+func captured(t *testing.T) map[string]composer.Manifest {
+	t.Helper()
+	read, err := readCatalogue(filepath.Dir(beside.Catalogue(t)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(read) < 100 {
+		t.Fatalf("the captured catalogue holds %d manifests, fewer than the catalogue's", len(read))
+	}
+	return read
+}
+
+// meshOf is a mesh of three machines over the captured catalogue, each assigned real modules, the input the assigner
+// composes with (nodes and their assignments, holdings from the claims): anchor runs the bus and the store, laptop a
+// desktop's modules, ace a module whose requirement is provided elsewhere.
+func meshOf() compose.Input {
+	return compose.Input{Nodes: []compose.Node{
+		{Name: "anchor", Assigned: []string{"nats", "postgres"}},
+		{Name: "laptop", Assigned: []string{"sudo", "zsh"}},
+		{Name: "ace", Assigned: []string{"mesh-reads", "nats"}},
+	}}
+}
+
+func request(t *testing.T, depth string) composer.Request {
+	t.Helper()
+	return composer.Request{V: composer.Version, Input: meshOf(), Depth: depth, HoldingsFromClaims: true,
+		Machines: []string{"anchor", "laptop", "ace"},
+		Cases:    []composer.Case{{Name: "trunk", Manifests: captured(t)}}}
+}
+
+func runJSON(t *testing.T, args []string, in []byte) (int, composer.Verdict, string) {
+	t.Helper()
+	var out, errOut bytes.Buffer
+	code := run(args, bytes.NewReader(in), &out, &errOut)
+	var v composer.Verdict
+	if code == 0 {
+		if err := json.Unmarshal(out.Bytes(), &v); err != nil {
+			t.Fatalf("the verdict is not JSON: %v\n%s", err, out.String())
+		}
+	}
+	return code, v, errOut.String()
+}
+
+func encode(t *testing.T, r composer.Request) []byte {
+	t.Helper()
+	raw, err := json.Marshal(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return raw
+}
+
+// 1621/AC1: the command reads the plain input and the manifests on standard input, validates every manifest, composes
+// the machines named and writes one verdict: per machine composes or the reason, per manifest accepted or the field
+// refused.
+func TestTheCommandWritesOneVerdictPerManifestAndMachine(t *testing.T) {
+	r := request(t, composer.DepthResolve)
+	r.Cases[0].Manifests["broken"] = composer.Manifest{Raw: json.RawMessage(`{"module":"broken","version":"1","shares":["x"]}`)}
+	code, v, stderr := runJSON(t, nil, encode(t, r))
+	if code != 0 {
+		t.Fatalf("exit %d: %s", code, stderr)
+	}
+	if len(v.Cases) != 1 || v.Cases[0].Name != "trunk" || v.SDK == "" {
+		t.Fatalf("verdict = %+v", v)
+	}
+	c := v.Cases[0]
+	if got := c.Manifests["broken"]; got.Accepted || !strings.Contains(got.Refused, `"shares"`) {
+		t.Errorf("the manifest with an unknown field reads %+v, want refused naming the field", got)
+	}
+	if got := c.Manifests["nats"]; !got.Accepted {
+		t.Errorf("nats reads %+v, want accepted", got)
+	}
+	if len(c.Machines) != 3 {
+		t.Fatalf("machines = %v, want the three named", c.Machines)
+	}
+	for name, m := range c.Machines {
+		if !m.Composes && m.Reason == "" {
+			t.Errorf("%s does not compose and says no reason", name)
+		}
+	}
+}
+
+// 1621/AC3: over the captured catalogue and a mesh of three machines, the command's verdict equals, machine for
+// machine and manifest for manifest, what the linked reader, resolution and composition answer, at both depths.
+func TestTheCommandAnswersWhatTheLinkedCompositionAnswers(t *testing.T) {
+	for _, depth := range []string{composer.DepthResolve, composer.DepthCompose} {
+		r := request(t, depth)
+		code, v, stderr := runJSON(t, nil, encode(t, r))
+		if code != 0 {
+			t.Fatalf("%s: exit %d: %s", depth, code, stderr)
+		}
+		in := meshOf()
+		in.Catalogue = map[string]catalogue.Manifest{}
+		for name, m := range r.Cases[0].Manifests {
+			parsed, err := catalogue.ParseManifest(m.Raw)
+			got := v.Cases[0].Manifests[name]
+			if (err == nil) != got.Accepted || (err != nil && err.Error() != got.Refused) {
+				t.Errorf("%s: the manifest of %s: linked %v, command %+v", depth, name, err, got)
+			}
+			if err == nil {
+				in.Catalogue[name] = parsed
+			}
+		}
+		in.Holdings = composer.HoldingsOf(in.Catalogue, in.Nodes)
+		composed := 0
+		for _, node := range r.Machines {
+			got := v.Cases[0].Machines[node]
+			var want composer.Machine
+			if depth == composer.DepthResolve {
+				if _, err := compose.Resolve(in, node); err != nil {
+					want = composer.Machine{Reason: err.Error()}
+				} else {
+					want = composer.Machine{Composes: true}
+				}
+			} else {
+				d, problems, err := compose.Compose(in, node)
+				switch {
+				case err != nil:
+					want = composer.Machine{Reason: err.Error()}
+				default:
+					want = composer.Machine{Composes: len(problems) == 0, Problems: problems, Refusals: d.Refusals()}
+					if len(problems) > 0 {
+						want.Reason = "the node-engine's validator refuses the declaration"
+					}
+				}
+			}
+			if !reflect.DeepEqual(got, want) {
+				t.Errorf("%s: %s: command %+v, linked %+v", depth, node, got, want)
+			}
+			if got.Composes {
+				composed++
+			} else {
+				t.Logf("%s: %s does not compose: %s %v", depth, node, got.Reason, got.Problems)
+			}
+		}
+		t.Logf("%s: %d of %d machines compose", depth, composed, len(r.Machines))
+	}
+}
+
+// 1621 note 6: a person composes a catalogue checkout (repository@ref checked out or archived: the command fetches
+// nothing) for a node and reads its declaration: each path with its kind, owner and mode.
+func TestACatalogueCheckoutIsComposedForANode(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(dir, "modules", "demo"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	manifest := `{"module": "demo", "version": "1", "resources": [{"id": "state", "type": "directory", ` +
+		`"path": "/var/lib/demo", "mode": "0750", "owner": "root"}]}`
+	if err := os.WriteFile(filepath.Join(dir, "modules", "demo", "module.json"), []byte(manifest), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	in := encode(t, composer.Request{V: composer.Version, Input: compose.Input{Nodes: []compose.Node{{Name: "laptop", Assigned: []string{"demo"}}}}})
+	code, v, stderr := runJSON(t, []string{"-catalogue", dir, "-node", "laptop", "-declarations"}, in)
+	if code != 0 {
+		t.Fatalf("exit %d: %s", code, stderr)
+	}
+	m := v.Cases[0].Machines["laptop"]
+	want := []composer.Path{{Path: "/var/lib/demo", Kind: "directory", Owner: "root", Mode: "0750", Module: ""}}
+	if !m.Composes || len(m.Declared) != 1 || m.Declared[0].Path != want[0].Path || m.Declared[0].Mode != "0750" ||
+		m.Declared[0].Owner != "root" || m.Declared[0].Kind != "directory" {
+		t.Fatalf("laptop = %+v, want it composed declaring %+v", m, want)
+	}
+}
+
+// The exit codes: 1 for a request that cannot be read or run, 2 for wrong flags, each said on standard error.
+func TestTheExitCodesSayWhatFailed(t *testing.T) {
+	unfilled := encode(t, composer.Request{V: composer.Version, Cases: []composer.Case{{Name: "c", Manifests: map[string]composer.Manifest{"x": {Digest: "abc"}}}}})
+	for _, c := range []struct {
+		name string
+		args []string
+		in   string
+		code int
+		says string
+	}{
+		{"not JSON", nil, "{", 1, "not JSON"},
+		{"another version", nil, `{"v":2}`, 1, "version 2"},
+		{"a digest left unfilled", nil, string(unfilled), 1, "digest"},
+		{"an unknown depth", nil, `{"v":1,"depth":"deep"}`, 1, "depth"},
+		{"nothing to read", nil, "", 1, "no request"},
+		{"a wrong flag", []string{"-nope"}, "", 2, "nope"},
+		{"an argument", []string{"extra"}, "", 2, "unexpected"},
+	} {
+		var out, errOut bytes.Buffer
+		if got := run(c.args, strings.NewReader(c.in), &out, &errOut); got != c.code || !strings.Contains(errOut.String(), c.says) {
+			t.Errorf("%s: exit %d saying %q, want %d saying %q", c.name, got, errOut.String(), c.code, c.says)
+		}
+	}
+}
+
+// sdkPackages is every package of this module the command imports, directly or not, with the packages each imports.
+func sdkPackages(t *testing.T) map[string][]string {
+	t.Helper()
+	_, file, _, _ := runtime.Caller(0)
+	root := filepath.Join(filepath.Dir(file), "..", "..")
+	const module = "git.novox.be/novox/mesh-sdk/go"
+	out := map[string][]string{}
+	var walk func(path string)
+	walk = func(path string) {
+		if _, done := out[path]; done {
+			return
+		}
+		rel := strings.TrimPrefix(strings.TrimPrefix(path, module), "/")
+		pkg, err := build.Default.ImportDir(filepath.Join(root, rel), 0)
+		if err != nil {
+			t.Fatalf("%s: %v", path, err)
+		}
+		out[path] = pkg.Imports
+		for _, imp := range pkg.Imports {
+			if strings.HasPrefix(imp, module+"/") {
+				walk(imp)
+			}
+		}
+	}
+	walk(composer.Package)
+	return out
+}
+
+// 1621/AC2 and AC4: the command imports, through every package of mesh-sdk it reaches, only the standard library and
+// mesh-sdk's own packages (so `go build` from mesh-sdk's module needs nothing beyond its lock), and none that reaches
+// a bus, a store, a network service or another process.
+func TestTheCommandImportsNoBusStoreNetworkOrProcess(t *testing.T) {
+	forbidden := []string{"net/http", "net/rpc", "net/smtp", "os/exec", "database/sql", "plugin", "syscall"}
+	for path, imports := range sdkPackages(t) {
+		for _, imp := range imports {
+			std := !strings.Contains(strings.SplitN(imp, "/", 2)[0], ".")
+			if !std && !strings.HasPrefix(imp, "git.novox.be/novox/mesh-sdk/go/") {
+				t.Errorf("%s imports %s, which is neither the standard library nor mesh-sdk", path, imp)
+			}
+			for _, f := range forbidden {
+				if imp == f {
+					t.Errorf("%s imports %s", path, imp)
+				}
+			}
+		}
+	}
+}
+
+// 1621/AC2: run with an empty network namespace, the command still writes its verdict: it needs no network. The test
+// runs its own binary again under `unshare --net` as an unprivileged user's namespace; where the kernel or the sandbox
+// allows no new namespace (a build module's run step is already one with no network), it says so and skips.
+func TestTheCommandRunsWithNoNetwork(t *testing.T) {
+	if os.Getenv("MESH_COMPOSE_HELPER") == "1" {
+		os.Exit(run(nil, os.Stdin, os.Stdout, os.Stderr))
+	}
+	unshare, err := exec.LookPath("unshare")
+	if err != nil {
+		t.Skip("no unshare on this machine, so no empty network namespace can be made here")
+	}
+	if out, err := exec.Command(unshare, "--net", "--map-root-user", "true").CombinedOutput(); err != nil {
+		t.Skipf("this machine makes no unprivileged network namespace (%v: %s); a run step's own has no network", err, out)
+	}
+	r := request(t, composer.DepthResolve)
+	cmd := exec.Command(unshare, "--net", "--map-root-user", os.Args[0], "-test.run=^TestTheCommandRunsWithNoNetwork$")
+	cmd.Env = []string{"MESH_COMPOSE_HELPER=1"}
+	cmd.Stdin = bytes.NewReader(encode(t, r))
+	var out, errOut bytes.Buffer
+	cmd.Stdout, cmd.Stderr = &out, &errOut
+	if err := cmd.Run(); err != nil {
+		t.Fatalf("the command in an empty network namespace: %v: %s", err, errOut.String())
+	}
+	var v composer.Verdict
+	if err := json.Unmarshal(out.Bytes(), &v); err != nil || len(v.Cases) != 1 || len(v.Cases[0].Machines) != 3 {
+		t.Fatalf("no verdict from the command with no network: %v\n%s", err, out.String())
+	}
+	names := make([]string, 0, len(v.Cases[0].Machines))
+	for n := range v.Cases[0].Machines {
+		names = append(names, n)
+	}
+	sort.Strings(names)
+	t.Logf("composed with no network: %v", names)
+}
diff --git a/go/composer/composer.go b/go/composer/composer.go
new file mode 100644
index 0000000..5ab962a
--- /dev/null
+++ b/go/composer/composer.go
@@ -0,0 +1,332 @@
+// Package composer is mesh-sdk's composition and validator as one function over a plain request (novox/hq ADR 0482
+// clause 4, issue 1621): what the command mesh-compose runs, so a program that links any one mesh-sdk can have the
+// composition of another commit judged by building that commit's command and reading its verdict.
+//
+// **It reads its request and nothing else.** No bus, store, network, file or environment: the manifests come inside the
+// request (raw, as their repository holds them), the rest of the mesh is a compose.Input, and the answer is a value.
+// A manifest named only by its digest in the delivery indexer's bucket is filled before the run by Fill, by whoever
+// holds the bus (the Go build module's compose job); Run refuses a digest left unfilled.
+package composer
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"slices"
+	"sort"
+
+	"git.novox.be/novox/mesh-sdk/go/catalogue"
+	"git.novox.be/novox/mesh-sdk/go/compose"
+	"git.novox.be/novox/mesh-sdk/go/sdkversion"
+)
+
+// Version is the version of a Request and of a Verdict; a reader refuses another.
+const Version = 1
+
+// Package is the import path of the command, which a compose job builds at a commit (in mesh-sdk's own module, or in
+// a module that requires mesh-sdk, at the version it pins).
+const Package = "git.novox.be/novox/mesh-sdk/go/cmd/mesh-compose"
+
+// The depths of a composition.
+const (
+	// DepthResolve resolves each machine's set over the rest of the mesh (compose.Resolve): what the assigner judges
+	// while the facts snapshot is not on the bus (issue 1461).
+	DepthResolve = "resolve"
+	// DepthCompose composes each machine's whole declaration and runs the node-engine's validator over it
+	// (compose.Compose).
+	DepthCompose = "compose"
+)
+
+// Request is everything one run composes: the rest of the mesh, one or more cases of manifests composed over it, and
+// the machines composed in each case.
+type Request struct {
+	V int `json:"v"`
+	// Input is the composition's input without its catalogue (and without each node's kept manifests): the cases give
+	// those. A Catalogue or Kept it holds is ignored.
+	Input compose.Input `json:"input"`
+	// Cases are composed one after the other over Input, each with its own manifests; their names are unique.
+	Cases []Case `json:"cases"`
+	// Machines are the nodes composed in every case; none means the manifests are only read.
+	Machines []string `json:"machines,omitempty"`
+	// Depth is DepthResolve or DepthCompose; empty is DepthCompose.
+	Depth string `json:"depth,omitempty"`
+	// HoldingsFromClaims adds to Input's holdings every seat an assigned module claims, held on its node (the
+	// assigner's input, which has no holdings of record).
+	HoldingsFromClaims bool `json:"holdings_from_claims,omitempty"`
+	// Declarations asks each composed machine's declared paths (DepthCompose only).
+	Declarations bool `json:"declarations,omitempty"`
+}
+
+// Case is one catalogue composed: every module's manifest by name, and per node the manifests kept at the build it runs.
+type Case struct {
+	Name      string                         `json:"name"`
+	Manifests map[string]Manifest            `json:"manifests"`
+	Kept      map[string]map[string]Manifest `json:"kept,omitempty"`
+}
+
+// Manifest is a module's manifest as its repository holds it (Raw, the bytes), or the digest the delivery indexer keeps
+// it under (Digest), which Fill replaces by the bytes. On the wire it is the manifest itself, or {"digest": "..."}.
+type Manifest struct {
+	Raw    json.RawMessage
+	Digest string
+}
+
+// MarshalJSON writes the raw manifest, or {"digest": ...} when only the digest is known.
+func (m Manifest) MarshalJSON() ([]byte, error) {
+	if len(m.Raw) > 0 {
+		return m.Raw, nil
+	}
+	return json.Marshal(map[string]string{"digest": m.Digest})
+}
+
+// UnmarshalJSON reads {"digest": "..."} (an object with that one key) as a digest, and any other value as the raw
+// manifest.
+func (m *Manifest) UnmarshalJSON(raw []byte) error {
+	var probe map[string]json.RawMessage
+	if json.Unmarshal(raw, &probe) == nil && len(probe) == 1 {
+		if d, ok := probe["digest"]; ok {
+			var digest string
+			if err := json.Unmarshal(d, &digest); err != nil {
+				return fmt.Errorf("a manifest's digest is not a string: %w", err)
+			}
+			*m = Manifest{Digest: digest}
+			return nil
+		}
+	}
+	*m = Manifest{Raw: slices.Clone(raw)}
+	return nil
+}
+
+// Verdict is what one run answers: the mesh-sdk the command was built with, and each case's verdict in the order asked.
+type Verdict struct {
+	V     int           `json:"v"`
+	SDK   string        `json:"sdk"`
+	Cases []CaseVerdict `json:"cases"`
+}
+
+// CaseVerdict is one case's: every manifest read or refused, every machine composed or not.
+type CaseVerdict struct {
+	Name      string                  `json:"name"`
+	Manifests map[string]ManifestRead `json:"manifests"`
+	Machines  map[string]Machine      `json:"machines,omitempty"`
+}
+
+// ManifestRead is one manifest as this mesh-sdk reads it: accepted, or refused with the reader's words.
+type ManifestRead struct {
+	Accepted bool   `json:"accepted"`
+	Refused  string `json:"refused,omitempty"`
+}
+
+// Machine is one machine composed: whether it composes (resolves, at DepthResolve), why not, the node-engine
+// validator's problems and the composition's refusals (DepthCompose), and its declared paths when asked.
+type Machine struct {
+	Composes bool     `json:"composes"`
+	Reason   string   `json:"reason,omitempty"`
+	Problems []string `json:"problems,omitempty"`
+	Refusals []string `json:"refusals,omitempty"`
+	Declared []Path   `json:"declared,omitempty"`
+}
+
+// Path is one path a declaration puts on the machine, with its kind (the resource's type), owner, group and mode as
+// declared, and the module it is of.
+type Path struct {
+	Path   string `json:"path"`
+	Kind   string `json:"kind"`
+	Owner  string `json:"owner,omitempty"`
+	Group  string `json:"group,omitempty"`
+	Mode   string `json:"mode,omitempty"`
+	Module string `json:"module,omitempty"`
+}
+
+// ErrUnfilled is a manifest named by a digest that nobody replaced by its bytes.
+var ErrUnfilled = errors.New("composer: a manifest is named by its digest and was not filled")
+
+// Decode reads a request; it refuses one that is not JSON or of another version.
+func Decode(data []byte) (Request, error) {
+	var r Request
+	if err := json.Unmarshal(data, &r); err != nil {
+		return Request{}, fmt.Errorf("composer: the request is not JSON of its shape: %w", err)
+	}
+	if r.V != Version {
+		return Request{}, fmt.Errorf("composer: a request of version %d, this composer reads %d", r.V, Version)
+	}
+	return r, nil
+}
+
+// Fill replaces every manifest named by its digest by the bytes get answers for that digest; a digest get cannot answer
+// stops it, named.
+func Fill(r *Request, get func(digest string) ([]byte, error)) error {
+	fill := func(m map[string]Manifest) error {
+		for name, man := range m {
+			if len(man.Raw) > 0 {
+				continue
+			}
+			raw, err := get(man.Digest)
+			if err != nil {
+				return fmt.Errorf("composer: the manifest of %s (digest %s): %w", name, man.Digest, err)
+			}
+			m[name] = Manifest{Raw: raw}
+		}
+		return nil
+	}
+	for _, c := range r.Cases {
+		if err := fill(c.Manifests); err != nil {
+			return err
+		}
+		for _, kept := range c.Kept {
+			if err := fill(kept); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// Run composes every case of r and answers the verdict, its SDK this binary's mesh-sdk. An error is a request it
+// cannot run (an unknown depth, a case named twice, a manifest left as a digest); a machine that does not compose is a
+// verdict.
+func Run(r Request) (Verdict, error) {
+	depth := r.Depth
+	if depth == "" {
+		depth = DepthCompose
+	}
+	if depth != DepthResolve && depth != DepthCompose {
+		return Verdict{}, fmt.Errorf("composer: the depth %q is not resolve or compose", r.Depth)
+	}
+	v := Verdict{V: Version, SDK: sdkversion.Version()}
+	seen := map[string]bool{}
+	for _, c := range r.Cases {
+		if seen[c.Name] {
+			return Verdict{}, fmt.Errorf("composer: the case %q is named twice", c.Name)
+		}
+		seen[c.Name] = true
+		cv, err := runCase(r, c, depth)
+		if err != nil {
+			return Verdict{}, err
+		}
+		v.Cases = append(v.Cases, cv)
+	}
+	return v, nil
+}
+
+func parse(m map[string]Manifest, reads map[string]ManifestRead) (map[string]catalogue.Manifest, error) {
+	out := map[string]catalogue.Manifest{}
+	for name, man := range m {
+		if len(man.Raw) == 0 {
+			return nil, fmt.Errorf("%w: %s (digest %s)", ErrUnfilled, name, man.Digest)
+		}
+		parsed, err := catalogue.ParseManifest(man.Raw)
+		if reads != nil {
+			if err != nil {
+				reads[name] = ManifestRead{Refused: err.Error()}
+			} else {
+				reads[name] = ManifestRead{Accepted: true}
+			}
+		}
+		if err == nil {
+			out[name] = parsed
+		}
+	}
+	return out, nil
+}
+
+func runCase(r Request, c Case, depth string) (CaseVerdict, error) {
+	cv := CaseVerdict{Name: c.Name, Manifests: map[string]ManifestRead{}}
+	shelf, err := parse(c.Manifests, cv.Manifests)
+	if err != nil {
+		return CaseVerdict{}, err
+	}
+	in := r.Input
+	in.Catalogue = shelf
+	in.Nodes = slices.Clone(in.Nodes)
+	in.Holdings = slices.Clone(in.Holdings)
+	for i := range in.Nodes {
+		in.Nodes[i].Kept = nil
+		if kept := c.Kept[in.Nodes[i].Name]; len(kept) > 0 {
+			k, err := parse(kept, nil)
+			if err != nil {
+				return CaseVerdict{}, err
+			}
+			in.Nodes[i].Kept = k
+		}
+	}
+	if r.HoldingsFromClaims {
+		in.Holdings = append(in.Holdings, HoldingsOf(shelf, in.Nodes)...)
+	}
+	if len(r.Machines) == 0 {
+		return cv, nil
+	}
+	cv.Machines = map[string]Machine{}
+	for _, node := range r.Machines {
+		cv.Machines[node] = machine(in, node, depth, r.Declarations)
+	}
+	return cv, nil
+}
+
+// HoldingsOf is every seat an assigned module claims, held on the node it is assigned to, in node order: the holdings
+// of an input that has none of record (the assigner's, mesh-delivery-assigner internal/assign input).
+func HoldingsOf(shelf map[string]catalogue.Manifest, nodes []compose.Node) []catalogue.Held {
+	var out []catalogue.Held
+	sorted := slices.Clone(nodes)
+	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Name < sorted[j].Name })
+	for _, n := range sorted {
+		for _, mod := range n.Assigned {
+			for _, cl := range shelf[mod].Claims {
+				scope := cl.Scope
+				if seat, ok := catalogue.SeatNamed(cl.Name); ok && scope == "" {
+					scope = seat.Scope
+				}
+				out = append(out, catalogue.Held{Claim: cl.Name, Scope: scope, Node: n.Name, Module: mod})
+			}
+		}
+	}
+	return out
+}
+
+func machine(in compose.Input, node, depth string, declarations bool) Machine {
+	if depth == DepthResolve {
+		if _, err := compose.Resolve(in, node); err != nil {
+			return Machine{Reason: err.Error()}
+		}
+		return Machine{Composes: true}
+	}
+	d, problems, err := compose.Compose(in, node)
+	if err != nil {
+		return Machine{Reason: err.Error()}
+	}
+	m := Machine{Composes: len(problems) == 0, Problems: problems, Refusals: d.Refusals()}
+	if len(problems) > 0 {
+		m.Reason = "the node-engine's validator refuses the declaration"
+	}
+	if declarations {
+		m.Declared = pathsOf(d)
+	}
+	return m
+}
+
+// pathsOf is every resource of a declaration that names a path, with its owner, group and mode as declared, sorted by
+// path.
+func pathsOf(d compose.Declared) []Path {
+	var out []Path
+	str := func(r map[string]any, key string) string {
+		if v, ok := r[key]; ok && v != nil {
+			return fmt.Sprint(v)
+		}
+		return ""
+	}
+	for _, r := range d.Resources {
+		p := str(r, "path")
+		if p == "" {
+			continue
+		}
+		out = append(out, Path{Path: p, Kind: str(r, "type"), Owner: str(r, "owner"), Group: str(r, "group"),
+			Mode: str(r, "mode"), Module: d.ModuleOf[str(r, "id")]})
+	}
+	sort.Slice(out, func(i, j int) bool {
+		if out[i].Path != out[j].Path {
+			return out[i].Path < out[j].Path
+		}
+		return out[i].Kind < out[j].Kind
+	})
+	return out
+}
diff --git a/go/composer/composer_test.go b/go/composer/composer_test.go
new file mode 100644
index 0000000..175894a
--- /dev/null
+++ b/go/composer/composer_test.go
@@ -0,0 +1,106 @@
+package composer
+
+import (
+	"encoding/json"
+	"errors"
+	"reflect"
+	"strings"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/catalogue"
+	"git.novox.be/novox/mesh-sdk/go/compose"
+)
+
+const demo = `{"module": "demo", "version": "1", "claims": [{"name": "node-files"}]}`
+
+// A manifest named by its digest goes on the wire as {"digest": ...}, and Fill puts the bytes the indexer keeps in its
+// place; a raw manifest goes as itself.
+func TestAManifestIsItsBytesOrItsDigest(t *testing.T) {
+	r := Request{V: Version, Cases: []Case{{Name: "change", Manifests: map[string]Manifest{
+		"demo": {Digest: "d1"}, "raw": {Raw: json.RawMessage(`{"module":"raw","version":"1"}`)}},
+		Kept: map[string]map[string]Manifest{"laptop": {"demo": {Digest: "d1"}}}}}}
+	wire, err := json.Marshal(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(wire), `"demo":{"digest":"d1"}`) || !strings.Contains(string(wire), `"raw":{"module":"raw","version":"1"}`) {
+		t.Fatalf("on the wire: %s", wire)
+	}
+	back, err := Decode(wire)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := Run(back); !errors.Is(err, ErrUnfilled) {
+		t.Fatalf("a digest left unfilled runs: %v", err)
+	}
+	asked := 0
+	if err := Fill(&back, func(d string) ([]byte, error) {
+		asked++
+		if d != "d1" {
+			return nil, errors.New("no such digest")
+		}
+		return []byte(demo), nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if asked != 2 || string(back.Cases[0].Manifests["demo"].Raw) != demo || string(back.Cases[0].Kept["laptop"]["demo"].Raw) != demo {
+		t.Fatalf("filled %d times: %+v", asked, back.Cases[0])
+	}
+	bad := Request{V: Version, Cases: []Case{{Name: "c", Manifests: map[string]Manifest{"x": {Digest: "nope"}}}}}
+	if err := Fill(&bad, func(string) ([]byte, error) { return nil, errors.New("not there") }); err == nil || !strings.Contains(err.Error(), "nope") {
+		t.Fatalf("a digest the indexer does not hold: %v", err)
+	}
+}
+
+// The input travels as JSON and comes back equal, so the command composes what its caller meant.
+func TestTheInputTravelsAsJSON(t *testing.T) {
+	in := compose.Input{
+		Nodes: []compose.Node{{Name: "anchor", Assigned: []string{"nats"}, Capabilities: map[string]bool{"systemd": true},
+			Place:    compose.Place{Endpoint: "anchor.example:51820", Site: "here", Hub: true, Address: "10.77.0.1"},
+			Pins:     map[string]catalogue.Chosen{"bus": {Node: "anchor", Module: "nats"}},
+			Settings: map[string][]catalogue.Layer{"nats": {{Values: map[string]any{"port": float64(4222)}}}}}},
+		Holdings: []catalogue.Held{{Claim: "mesh-broker", Scope: "mesh", Node: "anchor", Module: "nats"}},
+		Issued:   []compose.Issued{{Provision: "bus", Consumer: "laptop", ConsumerModule: "x", Provider: "anchor"}},
+	}
+	raw, err := json.Marshal(Request{V: Version, Input: in})
+	if err != nil {
+		t.Fatal(err)
+	}
+	back, err := Decode(raw)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(back.Input, in) {
+		t.Fatalf("the input came back as\n%+v\nwant\n%+v", back.Input, in)
+	}
+}
+
+// The seats a node's assigned modules claim are held there when the request says the holdings come from the claims.
+func TestHoldingsComeFromTheClaims(t *testing.T) {
+	m, err := catalogue.ParseManifest([]byte(demo))
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := HoldingsOf(map[string]catalogue.Manifest{"demo": m}, []compose.Node{{Name: "b", Assigned: []string{"demo"}}, {Name: "a", Assigned: []string{"demo"}}})
+	if len(got) != 2 || got[0].Node != "a" || got[0].Claim != "node-files" || got[0].Scope != "node" {
+		t.Fatalf("holdings = %+v", got)
+	}
+}
+
+// A request the command cannot run is refused, never answered as a verdict: another version, a case named twice, an
+// unknown depth.
+func TestARequestItCannotRunIsRefused(t *testing.T) {
+	if _, err := Decode([]byte(`{"v": 2}`)); err == nil {
+		t.Error("a request of version 2 is read")
+	}
+	if _, err := Run(Request{V: Version, Cases: []Case{{Name: "a"}, {Name: "a"}}}); err == nil {
+		t.Error("a case named twice runs")
+	}
+	if _, err := Run(Request{V: Version, Depth: "deep"}); err == nil {
+		t.Error("an unknown depth runs")
+	}
+	v, err := Run(Request{V: Version, Cases: []Case{{Name: "only-read", Manifests: map[string]Manifest{"demo": {Raw: json.RawMessage(demo)}}}}})
+	if err != nil || len(v.Cases) != 1 || !v.Cases[0].Manifests["demo"].Accepted || v.Cases[0].Machines != nil || v.SDK == "" {
+		t.Fatalf("a request with no machines: %+v, %v", v, err)
+	}
+}