diff --git a/controller/internal/stacks/deploy.go b/controller/internal/stacks/deploy.go
index a5ca9c5..a213cac 100644
--- a/controller/internal/stacks/deploy.go
+++ b/controller/internal/stacks/deploy.go
@@ -1337,6 +1337,14 @@ func (m *Manager) InjectMissingFields(stackNames []string) {
 
 			switch field.Type {
 			case "secret":
+				// R-757: a value the app must have been GIVEN — read only by the one-time after_install
+				// command, never by the compose file — cannot be invented for an app already installed:
+				// after_install does not run again, so the app never received it, and the reveal would
+				// then answer a login (calibre-web's ADMIN_USER, measured 2026-10-01) the app does not have.
+				if onlyGivenAtInstall(&meta, stackDir, field.EnvVar) {
+					m.logger.Printf("[INFO] [stacks] Stack %s: new field %s is read only by after_install, which ran (or not) at install — not invented for an installed app (R-757)", name, field.EnvVar)
+					continue
+				}
 				if field.Generate == "" {
 					m.logger.Printf("[WARN] [stacks] Stack %s: new secret field %s has no generator — skipping", name, field.EnvVar)
 					continue
@@ -1391,6 +1399,24 @@ func (m *Manager) InjectMissingFields(stackNames []string) {
 	m.logger.Printf("[INFO] [stacks] InjectMissingFields: processed %d stacks", count)
 }
 
+// onlyGivenAtInstall reports whether envVar reaches the app ONLY through the one-time after_install
+// command: it is in after_install's env list and no compose definition the app runs from (the catalog
+// mirror, or the pinned applied copy) names it. Unreadable compose files count as NOT naming it, which is
+// the side that skips the injection — inventing nothing is the safe error. Pinned by
+// TestR757_AfterInstallOnlyFieldIsNotInvented.
+func onlyGivenAtInstall(meta *Metadata, stackDir, envVar string) bool {
+	if meta == nil || meta.AfterInstall == nil || !containsStr(meta.AfterInstall.Env, envVar) {
+		return false
+	}
+	ref := regexp.MustCompile(`\$\{?` + regexp.QuoteMeta(envVar) + `\b`)
+	for _, p := range []string{filepath.Join(stackDir, "docker-compose.yml"), AppliedComposePath(stackDir)} {
+		if b, err := os.ReadFile(p); err == nil && ref.Match(b) {
+			return false
+		}
+	}
+	return true
+}
+
 func containsStr(slice []string, s string) bool {
 	for _, v := range slice {
 		if v == s {
diff --git a/controller/internal/stacks/r757_inject_test.go b/controller/internal/stacks/r757_inject_test.go
new file mode 100644
index 0000000..a786764
--- /dev/null
+++ b/controller/internal/stacks/r757_inject_test.go
@@ -0,0 +1,48 @@
+package stacks
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// R-757 — the consequence: after a template gains a generated secret that only after_install reads,
+// an INSTALLED app does not get an invented value (calibre-web's ADMIN_USER, 2026-10-01); a new secret
+// the compose file reads is still generated as before.
+func TestR757_AfterInstallOnlyFieldIsNotInvented(t *testing.T) {
+	compose := "services:\n  app:\n    image: nginx:1.27\n    environment:\n      - NEW_KEY=${NEW_KEY}\n"
+	m, dir, _ := newR442Manager(t, "app", compose, "deployed: true\nenv:\n  KEEP: x\n", "")
+	meta := "display_name: App\nslug: app\n" +
+		"deploy_fields:\n" +
+		"  - env_var: ADMIN_USER\n    label: U\n    type: secret\n    generate: \"hex:5\"\n" +
+		"  - env_var: NEW_KEY\n    label: K\n    type: secret\n    generate: \"hex:8\"\n" +
+		"after_install:\n  service: app\n  env: [ADMIN_USER]\n  command: [\"true\", \"${ADMIN_USER}\"]\n  success: OK\n"
+	if err := os.WriteFile(filepath.Join(dir, ".felhom.yml"), []byte(meta), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if md := LoadMetadata(dir); md.AfterInstall == nil || len(md.DeployFields) != 2 {
+		t.Fatalf("fixture metadata did not parse: after_install=%v fields=%d", md.AfterInstall, len(md.DeployFields))
+	}
+
+	m.InjectMissingFields([]string{"app"})
+
+	cfg := LoadAppConfig(dir)
+	if cfg == nil {
+		t.Fatal("app.yaml unreadable after injection")
+	}
+	env := cfg.Env
+	if _, ok := env["ADMIN_USER"]; ok {
+		t.Errorf("ADMIN_USER was INVENTED for an installed app (only after_install reads it): %v", keysOf(env))
+	}
+	if v := env["NEW_KEY"]; v == "" {
+		t.Errorf("a new secret the compose file reads must still be generated: %v", keysOf(env))
+	}
+}
+
+func keysOf(m map[string]string) []string {
+	var out []string
+	for k := range m {
+		out = append(out, k)
+	}
+	return out
+}