diff --git a/CHANGELOG.md b/CHANGELOG.md
index 058f9a9a..974040b3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,6 +40,11 @@ night report (`audits/night-2026-10-28/`).
   and said once; no agent (9202) = Docker's flag only, as before. The inspect now reads `{{.Id}}` too. Tests
   `TestR528_*` (stacks, cmd/controller), red before the fix (the host-counted kill was not seen) or red-proved by
   mutation (the back-off, the wiring).
+- **Found walking the capability map's B2 on 9202 (small, fixed in the session):** a Stop/Start of a name that is no
+  app on the box (the bootstrap-managed `felhom-controller`) answered 500 „…az alkalmazás beállításai nem menthetők." —
+  the desired-state write's „not found" read as a save failure. `stacks.SetDesiredState` returns the
+  `ErrStackNotFound` kind with the household's sentence (`err.stacks.alkalmazas_nem_talalhato`); `actionStack` answers
+  404 before recording any intent. Nothing was ever stopped. Test `TestAnActionOnANameThatIsNoAppIsA404` (red first).
 - **H8 — `internal/backup/restore_copy_sweep.go`:** an unused restore copy goes after 14 days. Every 6 hours
   (`restore-copy-sweep`) a copy is removed only when the app has no restore hold of any kind, the app-stop marker is
   readable and does not name the app, no restore runs, and the copy is older than 14 days. Tests `TestR1256_H8_*`
diff --git a/controller/internal/api/router.go b/controller/internal/api/router.go
index 72f24d79..2d2c4d7b 100644
--- a/controller/internal/api/router.go
+++ b/controller/internal/api/router.go
@@ -771,6 +771,12 @@ func (r *Router) actionStack(w http.ResponseWriter, req *http.Request, action, n
 	// above, so nothing is recorded for an action that was never going to happen.
 	if desired, ok := desiredStateForAction(action); ok {
 		if derr := r.stackMgr.SetDesiredState(name, desired); derr != nil {
+			if errors.Is(derr, stacks.ErrStackNotFound) {
+				// no app by that name on this box (e.g. the bootstrap-managed controller): a 404, not a save failure
+				r.logger.Printf("[WARN] [api] %s for %s refused: no such app", action, name)
+				writeJSON(w, stackOpStatusFor(derr), apiResponse{OK: false, Error: r.errText(req, derr)})
+				return
+			}
 			r.logger.Printf("[ERROR] [api] %s for %s refused: could not record desired state: %v", action, name, derr)
 			writeJSON(w, http.StatusInternalServerError, apiResponse{
 				OK:    false,
diff --git a/controller/internal/api/unknown_stack_action_test.go b/controller/internal/api/unknown_stack_action_test.go
new file mode 100644
index 00000000..386038d8
--- /dev/null
+++ b/controller/internal/api/unknown_stack_action_test.go
@@ -0,0 +1,29 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// Found walking the capability map's B2 („App lifecycle") on 9202, 2026-10-28 night: a Stop of a name that is no app
+// on this box (`felhom-controller` — bootstrap-managed, not a stack) answered 500 „…az alkalmazás beállításai nem
+// menthetők." — the desired-state write's „not found" read as a save failure. It is a 404 now, before any intent is
+// recorded. RED-PROOF: drop the ErrStackNotFound branch in actionStack → 500 → FAILS.
+func TestAnActionOnANameThatIsNoAppIsA404(t *testing.T) {
+	r, _, _, _ := newSlice4Router(t)
+	for _, action := range []string{"stop", "start"} {
+		w := httptest.NewRecorder()
+		r.actionStack(w, httptest.NewRequest("POST", "/api/stacks/nosuchapp/"+action, nil), action, "nosuchapp")
+		var resp apiResponse
+		_ = json.Unmarshal(w.Body.Bytes(), &resp)
+		if w.Code != http.StatusNotFound {
+			t.Fatalf("%s of a name that is no app: %d %q — want 404", action, w.Code, resp.Error)
+		}
+		if strings.Contains(strings.ToLower(resp.Error), "mentheto") || strings.Contains(resp.Error, "menthetők") {
+			t.Fatalf("%s: the answer still blames a save: %q", action, resp.Error)
+		}
+	}
+}
diff --git a/controller/internal/stacks/desiredstate.go b/controller/internal/stacks/desiredstate.go
index a65bc318..32caa22c 100644
--- a/controller/internal/stacks/desiredstate.go
+++ b/controller/internal/stacks/desiredstate.go
@@ -3,6 +3,8 @@ package stacks
 import (
 	"fmt"
 	"path/filepath"
+
+	"gitea.dooplex.hu/admin/felhom-controller/internal/util"
 )
 
 // Desired-state values for AppConfig.DesiredState (R-166, decision D-b).
@@ -57,7 +59,8 @@ func (m *Manager) SetDesiredState(name, desired string) error {
 
 	stack, ok := m.GetStack(name)
 	if !ok {
-		return fmt.Errorf("stack %q not found", name)
+		// a KIND (404 at the API) and the household's sentence — not a save failure
+		return util.MsgErrorf(ErrStackNotFound, "err.stacks.alkalmazas_nem_talalhato", name)
 	}
 	stackDir := filepath.Dir(stack.ComposePath)