diff --git a/internal/keycloak/client_test.go b/internal/keycloak/client_test.go
index 08ad32d..8452ace 100644
--- a/internal/keycloak/client_test.go
+++ b/internal/keycloak/client_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -24,6 +25,7 @@ type stubKC struct {
 	emailCalls  atomic.Int32
 	healthCalls atomic.Int32
 	syncCalls   atomic.Int32
+	lastOrgBody string
 
 	tokenFails atomic.Bool // when true, /token returns 401 once
 }
@@ -53,6 +55,8 @@ func newStubKC(t *testing.T) *stubKC {
 	mux.HandleFunc("/admin/realms/test-realm/organizations", func(w http.ResponseWriter, r *http.Request) {
 		s.orgCalls.Add(1)
 		if r.Method == http.MethodPost {
+			body, _ := io.ReadAll(r.Body)
+			s.lastOrgBody = string(body)
 			w.Header().Set("Location", s.srv.URL+"/admin/realms/test-realm/organizations/org-xyz")
 			w.WriteHeader(http.StatusCreated)
 			return
@@ -135,6 +139,12 @@ func TestHTTPAdapter_createOrgAndInvite(t *testing.T) {
 		t.Errorf("call counts: org=%d user=%d member=%d email=%d",
 			s.orgCalls.Load(), s.userCalls.Load(), s.memberCalls.Load(), s.emailCalls.Load())
 	}
+	// KC 26 rejects a domainless org; the adapter must send a synthetic
+	// per-slug domain so onboarding actually provisions.
+	if !strings.Contains(s.lastOrgBody, `"domains"`) ||
+		!strings.Contains(s.lastOrgBody, "acme.tenant.breakpilot.com") {
+		t.Errorf("org create body missing synthetic domain: %s", s.lastOrgBody)
+	}
 }
 
 func TestHTTPAdapter_emailMissingAdminEmailRejected(t *testing.T) {
diff --git a/internal/keycloak/orgs.go b/internal/keycloak/orgs.go
index d4fb4cd..9fc0f8b 100644
--- a/internal/keycloak/orgs.go
+++ b/internal/keycloak/orgs.go
@@ -13,6 +13,11 @@ import (
 
 // ─── organizations API ───────────────────────────────────────────────────
 
+// orgDomainSuffix namespaces the synthetic org domain. The slug is unique in
+// the registry, so "<slug>.tenant.breakpilot.com" is unique per organization
+// and never a real deliverable mail domain we might clash with.
+const orgDomainSuffix = ".tenant.breakpilot.com"
+
 type orgCreate struct {
 	Name        string              `json:"name"`
 	Alias       string              `json:"alias"`
@@ -50,6 +55,16 @@ func (a *HTTPAdapter) CreateOrgAndInvite(ctx context.Context, in InviteInput) (*
 		Name:        in.Name,
 		Alias:       in.Slug,
 		Description: fmt.Sprintf("Auto-provisioned from tenant-registry %s", in.TenantID),
+		// Keycloak 26 rejects an organization with no domain ("You must
+		// provide at least one domain"). Membership is registry-authoritative
+		// (model B2), so we do NOT use Keycloak's email-domain auto-join; a
+		// synthetic per-tenant domain derived from the unique slug satisfies
+		// the constraint without depending on the customer's real mail domain
+		// (which may be a shared public domain and would collide across
+		// tenants). Unverified is fine — verification only gates auto-join.
+		Domains: []map[string]any{
+			{"name": in.Slug + orgDomainSuffix, "verified": false},
+		},
 		Attributes: map[string][]string{
 			"tenant_id": {in.TenantID},
 		},