internal/broker/jetstream.go

@@ -42,6 +42,12 @@ func Dial(url string, opts ...nats.Option) (*JetStream, error) {
 		}
 		opts = append(opts, nats.Secure(pinned))
 	}
+	// **Its own inbox, and nothing wider.** Every principal is granted `_INBOX.<its user>.>` and
+	// no other inbox; the client's default prefix is random, and the server refused the first
+	// subscription to it (2026-09-28). The user is in the URL, so the prefix follows from it.
+	if user, _, _ := CredentialIn(url); user != "" {
+		opts = append(opts, nats.CustomInboxPrefix("_INBOX."+user))
+	}
 	// The address in an error is the address alone. The URL carries this controller's password,
 	// and an error here is written on the assumption it will be logged.
 	where := BareAddress(url)

internal/broker/nats.go

@@ -521,7 +521,10 @@ func ComposeAccounts(principals []Principal) (string, error) {
 	// One account for the mesh: accounts in NATS isolate subject spaces entirely, and the mesh is
 	// one space (design 25 §4). The cost of that — that permissions are the only isolation — is
 	// paid in the scoping of every inbox and every ack subject.
-	b.WriteString("accounts {\n  MESH {\n    users = [\n")
+	// JetStream is enabled per account once accounts exist at all: with only the global block set,
+	// a user in MESH is told "JetStream not enabled for account" the first time it binds a
+	// consumer, which is the first thing every host does (2026-09-28).
+	b.WriteString("accounts {\n  MESH {\n    jetstream: enabled\n    users = [\n")
 	for _, p := range sorted {
 		perms, err := PermissionsFor(p)
 		if err != nil {

internal/broker/testdata/composed.conf

@@ -21,6 +21,7 @@ jetstream {
 
 accounts {
   MESH {
+    jetstream: enabled
     users = [
       { user: "controller", password: "$2a$11$cccccccccccccccccccccc", permissions: {
         publish: { allow: ["$JS.ACK.CONTROL.controller.>", "$JS.ACK.EVENTS.controller.>", "$JS.API.>", "_INBOX.enrol.>", "mesh.control.>", "mesh.node.>", "mesh.seat.mesh-build-machine.accept.>"] }

internal/broker/users_test.go

@@ -186,7 +186,13 @@ func TestWhatTheMeshWritesIsUsersAndNothingAboutTheServer(t *testing.T) {
 	}
 	// None of the server's own settings. Each of these in the mesh's file is a value the controller
 	// would then own, and the module could no longer change its own image without the mesh agreeing.
-	for _, absent := range []string{"port:", "http:", "jetstream", "tls {", "store_dir", "cert_file"} {
+	// `jetstream {` is the server's block (its store, its limits); `jetstream: enabled` inside the
+	// account is the account's, and the mesh owns the account — a user in it is told "JetStream
+	// not enabled for account" without it (2026-09-28).
+	if !strings.Contains(got, "jetstream: enabled") {
+		t.Errorf("the account does not enable JetStream, so no user in it can bind a consumer")
+	}
+	for _, absent := range []string{"port:", "http:", "jetstream {", "tls {", "store_dir", "cert_file"} {
 		if strings.Contains(got, absent) {
 			t.Errorf("the accounts file contains %q, which belongs to the module that raises the "+
 				"server, not to the mesh", absent)