diff --git a/USAGE.md b/USAGE.md
index 1d6394b..0f51cb0 100644
--- a/USAGE.md
+++ b/USAGE.md
@@ -308,6 +308,9 @@ vallic domain list production
 vallic domain primary shop.example.com production
 vallic domain canonical shop.example.com production
 
+# Where the repository declares a frontend: which application a hostname reaches
+vallic domain application shop.example.com frontend production
+
 # Give one up
 vallic domain delete old.example.com production
 ```
diff --git a/internal/api/domains_promote_test.go b/internal/api/domains_promote_test.go
index 85fbdfb..464a811 100644
--- a/internal/api/domains_promote_test.go
+++ b/internal/api/domains_promote_test.go
@@ -6,6 +6,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -55,3 +56,33 @@ func TestPromoteDomainSaysWhetherTheOthersRedirect(t *testing.T) {
 		})
 	}
 }
+
+// The application is sent on its own, so nothing else about the hostname moves.
+func TestSetDomainApplicationSendsOnlyThat(t *testing.T) {
+	var sent map[string]any
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if err := json.NewDecoder(r.Body).Decode(&sent); err != nil {
+			t.Errorf("decoding: %v", err)
+		}
+		if r.Method != http.MethodPatch || !strings.HasSuffix(r.URL.Path, "/domains/5") {
+			t.Errorf("sent to %s %s", r.Method, r.URL.Path)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"domain":{"id":5,"application":"frontend"},"message":"ok"}`))
+	}))
+	defer server.Close()
+
+	client := New(server.URL, anonymous{}, "test")
+
+	updated, err := client.SetDomainApplication(context.Background(), 5, "frontend")
+	if err != nil {
+		t.Fatalf("SetDomainApplication: %v", err)
+	}
+	if updated.Domain.Application != "frontend" {
+		t.Errorf("application = %q", updated.Domain.Application)
+	}
+	if len(sent) != 1 || sent["application"] != "frontend" {
+		t.Errorf("sent %v", sent)
+	}
+}
diff --git a/internal/api/models.go b/internal/api/models.go
index 0acff64..0085a44 100644
--- a/internal/api/models.go
+++ b/internal/api/models.go
@@ -692,6 +692,11 @@ type Domain struct {
 	// environment and cannot be removed on its own.
 	Kind string `json:"kind"`
 
+	// Application is which of the environment's applications it asks to reach:
+	// "backend" or "frontend". With no frontend deployed every hostname reaches
+	// the backend, whatever this says.
+	Application string `json:"application"`
+
 	// IsPrimary is the site's address, which the platform hostname redirects
 	// to; IsCanonical, only ever on a primary custom domain, redirects every
 	// other hostname to it as well.
@@ -1075,6 +1080,18 @@ type Manifest struct {
 	// BackupExclude is what the files backup leaves out beside the
 	// framework's own caches, as the file wrote it.
 	BackupExclude []string `json:"backup_exclude"`
+
+	// Frontend is a Node.js or Bun application beside this one, in a directory
+	// of its own. Nil where the repository declares none.
+	Frontend *ManifestFrontend `json:"frontend"`
+}
+
+// ManifestFrontend is the application a repository runs beside its backend.
+type ManifestFrontend struct {
+	Root    string            `json:"root"`
+	Runtime map[string]string `json:"runtime"`
+	Start   string            `json:"start"`
+	Port    int               `json:"port"`
 }
 
 // ManifestService is one service a repository asked to run beside it.
diff --git a/internal/api/routes.go b/internal/api/routes.go
index f6cdb1e..882cc86 100644
--- a/internal/api/routes.go
+++ b/internal/api/routes.go
@@ -490,6 +490,23 @@ func (c *Client) PromoteDomain(ctx context.Context, domainID int, canonical bool
 	return &out, nil
 }
 
+// SetDomainApplication says which of the environment's applications a hostname
+// reaches: "backend" or "frontend".
+func (c *Client) SetDomainApplication(ctx context.Context, domainID int, application string) (*DomainUpdated, error) {
+	body, err := json.Marshal(map[string]any{"application": application})
+	if err != nil {
+		return nil, err
+	}
+
+	var out DomainUpdated
+
+	if err := c.do(ctx, http.MethodPatch, fmt.Sprintf("/domains/%d", domainID), body, &out); err != nil {
+		return nil, err
+	}
+
+	return &out, nil
+}
+
 // DeleteDomain removes a hostname.
 func (c *Client) DeleteDomain(ctx context.Context, domainID int) (*DomainRemoved, error) {
 	var out DomainRemoved
diff --git a/internal/cli/domain.go b/internal/cli/domain.go
index f781485..a6e8edf 100644
--- a/internal/cli/domain.go
+++ b/internal/cli/domain.go
@@ -28,6 +28,7 @@ actually being routed.`,
 			domainVerifyCommand(),
 			domainPrimaryCommand(),
 			domainCanonicalCommand(),
+			domainApplicationCommand(),
 			domainDeleteCommand(),
 		},
 	}
@@ -271,6 +272,57 @@ hostname cannot, because your own domains would redirect to it.`,
 	}
 }
 
+func domainApplicationCommand() *Command {
+	return &Command{
+		Name:    "application",
+		Summary: "send a hostname to the backend or the frontend",
+		Usage:   "domain application <hostname> <backend|frontend> [<env>]",
+		Long: `Says which of the environment's applications one of your own hostnames
+reaches, where the repository declares a frontend beside its backend. Your
+hostnames reach the backend until you send them to the frontend.
+
+It stops being primary or canonical as it moves: those are of the application
+it leaves. Choose again among the hostnames of the one it joins.`,
+		Run: func(ctx context.Context, env *Env, args []string) error {
+			if len(args) < 2 {
+				return Usagef(nil, "name a hostname, and backend or frontend")
+			}
+
+			if args[1] != "backend" && args[1] != "frontend" {
+				return Usagef(nil, "%q is neither backend nor frontend", args[1])
+			}
+
+			target, err := env.ResolveEnvironment(ctx, firstOf(args[2:]))
+			if err != nil {
+				return err
+			}
+
+			client, err := env.Client()
+			if err != nil {
+				return err
+			}
+
+			domain, err := findDomain(ctx, client, target.Environment.ID, args[0])
+			if err != nil {
+				return err
+			}
+
+			updated, err := client.SetDomainApplication(ctx, domain.ID, args[1])
+			if err != nil {
+				return err
+			}
+
+			if env.Printer.Structured() {
+				return env.Printer.Value(updated)
+			}
+
+			env.Printer.Good("%s", updated.Message)
+
+			return nil
+		},
+	}
+}
+
 // promoteDomain is domain primary and domain canonical: the same request,
 // with or without the other hostnames redirecting.
 func promoteDomain(ctx context.Context, env *Env, args []string, canonical bool) error {
diff --git a/internal/cli/validate.go b/internal/cli/validate.go
index d3af64a..6b1e7f1 100644
--- a/internal/cli/validate.go
+++ b/internal/cli/validate.go
@@ -331,6 +331,7 @@ func manifestEntries(manifest *api.Manifest) []manifestEntry {
 	add("mounts", strings.Join(manifest.Mounts, ", "))
 	add("extras", manifestExtras(manifest.Extras))
 	add("backup exclude", strings.Join(manifest.BackupExclude, ", "))
+	add("frontend", manifestFrontend(manifest.Frontend))
 
 	return entries
 }
@@ -412,6 +413,15 @@ func manifestWorkers(workers []api.Worker) string {
 	return strings.Join(parts, ", ")
 }
 
+// manifestFrontend renders the application beside the backend, or nothing.
+func manifestFrontend(frontend *api.ManifestFrontend) string {
+	if frontend == nil {
+		return ""
+	}
+
+	return frontend.Root + " (" + settingsLine(frontend.Runtime) + "), " + frontend.Start
+}
+
 // manifestExtras renders the customer's own machines and how to reach them.
 //
 // The ports are the point. An extra is a machine running software this platform