diff --git a/REUSE.md b/REUSE.md
index 75fa8d7..2f80a82 100644
--- a/REUSE.md
+++ b/REUSE.md
@@ -311,6 +311,7 @@
 | `stacks.Manager.execCommand` / `composeExecCustomEnv` for NEW long-running calls | No context/timeout — a hung docker CLI blocks forever | `exec.CommandContext` + explicit timeout (copy `rsyncCopy` or appexport `composeExecEnv`) |
 | `config.LoadPermissive` | Skips validation — setup-mode only (customer.id/domain may be unset) | `config.Load` everywhere else |
 | `ExportDataMounts` / `ParseComposeHDDMounts` as **backup-classification** input | `ExportDataMounts` unions the `${USERDATA_PATH}` ROOT (export-capture logic, not per-bind); `ParseComposeHDDMounts` resolves absolutes AND drops the `:ro` flag — classification needs `${VAR}`-relative paths + read-only awareness | `ParseComposeClassifiableBinds` (controller/internal/stacks/classify_binds.go) |
+| any settings getter/setter or helper that ends in one (`m.note`, `boxLang`) INSIDE a `settings.UpdateOffboxStatus`/`UpdateCrossDriveStatus` callback (R-578) | The callback runs under the settings WRITE lock and `sync.RWMutex` is not reentrant — it DEADLOCKS holding the lock and wedges settings.json for the box | Resolve the value BEFORE the callback (`lang := m.boxLang()`); `TestR578NothingTakesTheSettingsLockInsideASettingsCallback` (controller/internal/settings/r578_callback_lock_gate_test.go) convicts it in every package |
 | `docker compose restart` (any wrapper) | Does not pick up new images or env | `RedeployFromEnv` / composeExec `up -d` |
 | `backup.WholeOnTier` vs `UpdateCopyHolds` (R-659) | `UpdateCopyHolds` says what a copy HOLDS; it does not say the restore will ACCEPT it — a file app's second-drive copy holds the files and its unit restore still refuses | `WholeOnTier` (asks `DeclaredDriveFileLegs`, the refusal's own predicate) before a sentence names a copy as a way back |
 
diff --git a/controller/internal/backup/offsite_diag_test.go b/controller/internal/backup/offsite_diag_test.go
index 46a99e2..6ceb81f 100644
--- a/controller/internal/backup/offsite_diag_test.go
+++ b/controller/internal/backup/offsite_diag_test.go
@@ -203,6 +203,9 @@ func TestR570SentenceStaysHungarian(t *testing.T) {
 //
 // RED-PROOF (REPORT): put `m.note(...)` back inside the final UpdateOffboxStatus callback → this test
 // names the file and the line, in a second, instead of the suite hanging for 25 minutes.
+//
+// R-578: the controller-wide successor is TestR578NothingTakesTheSettingsLockInsideASettingsCallback
+// (internal/settings) — it derives the lockers and helpers from source in every package.
 func TestNoteHelpersAreNotCalledUnderTheSettingsLock(t *testing.T) {
 	callback := regexp.MustCompile(`\.Update\w*\(func\(`)
 	note := regexp.MustCompile(`\b(m|s)\.(note|noteErr|boxLang)\(`)
diff --git a/controller/internal/settings/r578_callback_lock_gate_test.go b/controller/internal/settings/r578_callback_lock_gate_test.go
new file mode 100644
index 0000000..ac0f172
--- /dev/null
+++ b/controller/internal/settings/r578_callback_lock_gate_test.go
@@ -0,0 +1,355 @@
+package settings
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+// R-578 — the settings-callback deadlock, as a gate over EVERY package of the controller.
+//
+// `UpdateOffboxStatus` (and every other *Settings method that takes a func and the settings lock)
+// runs its callback while holding the settings WRITE lock. sync.RWMutex is not reentrant: anything
+// inside that callback that takes the settings lock again — a getter, a setter, or a package helper
+// that ends in one (`m.note` → `m.boxLang` → `GetLanguage`) — DEADLOCKS while holding the lock, and
+// wedges everything else on the box that touches settings.json. Release C of localisation slice 2
+// shipped exactly that; the only symptom was a 25-minute test timeout. The backup-only predecessor,
+// TestNoteHelpersAreNotCalledUnderTheSettingsLock, knew three helper names in one package.
+//
+// This test derives everything from source instead of a list:
+//   - lockers:   *Settings methods that take s.mu (directly or through another *Settings method);
+//   - takers:    lockers that have a func-typed parameter (the callback runners);
+//   - tainted:   per package, every func/method whose body calls an exported locker or another
+//     tainted name of the same package (fixpoint) — so a new helper is covered the day it is written;
+//   - violation: a call to a locker or a tainted name inside a func literal passed to a taker, OR a
+//     taker given something that is not a literal (the gate could not see inside it).
+//
+// The matching is by NAME (no type checker — this must stay fast inside `go test ./...`). A false
+// positive is possible if an unrelated type has a method named like a settings locker and is called
+// inside a callback; rename it or hoist the call — both are cheap, a hang is not.
+//
+// RED-PROOF (R-578): put `lang := m.note("x")` inside the final UpdateOffboxStatus callback in
+// internal/backup/offbox.go → this test names internal/backup/offbox.go:<line> and the call chain.
+func TestR578NothingTakesTheSettingsLockInsideASettingsCallback(t *testing.T) {
+	root := filepath.Join("..", "..") // controller/
+	pkgs := parseControllerPackages(t, root)
+
+	own, ok := pkgs[filepath.Join(root, "internal", "settings")]
+	if !ok {
+		t.Fatal("internal/settings was not parsed — the walk no longer finds this package")
+	}
+	lockers, takers := settingsLockers(own)
+	if len(lockers) < 20 || len(takers) == 0 {
+		t.Fatalf("found %d settings lockers and %d callback runners — the pattern no longer matches the code", len(lockers), len(takers))
+	}
+	exported := map[string]bool{}
+	for name := range lockers {
+		if ast.IsExported(name) {
+			exported[name] = true
+		}
+	}
+
+	var bad []string
+	callbacks := 0
+	backupTaint := map[string]bool{}
+	for dir, files := range pkgs {
+		tainted := taintedFuncs(files, exported)
+		if dir == filepath.Join(root, "internal", "backup") {
+			backupTaint = tainted
+		}
+		for _, pf := range files {
+			ast.Inspect(pf.file, func(n ast.Node) bool {
+				call, ok := n.(*ast.CallExpr)
+				if !ok {
+					return true
+				}
+				sel, ok := call.Fun.(*ast.SelectorExpr)
+				if !ok {
+					return true
+				}
+				idx, isTaker := takers[sel.Sel.Name]
+				if !isTaker {
+					return true
+				}
+				for _, i := range idx {
+					if i >= len(call.Args) {
+						continue
+					}
+					arg := call.Args[i]
+					if lit, ok := arg.(*ast.FuncLit); ok {
+						callbacks++
+						for _, hit := range lockingCalls(lit.Body, exported, tainted) {
+							bad = append(bad, pf.pos(hit.pos)+"  "+hit.name+"() inside "+sel.Sel.Name+"(func…)")
+						}
+						continue
+					}
+					bad = append(bad, pf.pos(arg.Pos())+"  "+sel.Sel.Name+" is passed a non-literal callback — the gate cannot see inside it; pass a func literal")
+				}
+				return true
+			})
+		}
+	}
+
+	// Positive controls: the instrument is looking at something, and the transitive taint reaches
+	// the helpers that actually caused the 2026-09-18 deadlock.
+	if callbacks < 10 {
+		t.Fatalf("found only %d settings callbacks across the controller — the walk no longer matches the code", callbacks)
+	}
+	for _, h := range []string{"boxLang", "note", "noteErr"} {
+		if !backupTaint[h] {
+			t.Errorf("internal/backup %s() is not seen as taking the settings lock — the taint analysis lost the chain that deadlocked release C", h)
+		}
+	}
+	if len(bad) > 0 {
+		sort.Strings(bad)
+		t.Errorf("a settings callback (which holds the settings WRITE lock) calls something that takes "+
+			"the settings lock again — sync.RWMutex is not reentrant, so this DEADLOCKS and wedges "+
+			"settings.json for the whole box. Hoist the call before the callback (%d):\n  %s",
+			len(bad), strings.Join(bad, "\n  "))
+	}
+	t.Logf("%d packages, %d settings lockers, %d callback runners, %d callbacks examined", len(pkgs), len(lockers), len(takers), callbacks)
+}
+
+// TestR578GateConvictsAHelperChain is the decoy: a synthetic package whose callback reaches the
+// settings lock only through two local helpers. If the taint fixpoint stops following calls, this
+// fails before the real gate quietly goes blind.
+func TestR578GateConvictsAHelperChain(t *testing.T) {
+	src := `package decoy
+type M struct{ settings *S }
+func (m *M) lang() string { return m.settings.GetLanguage() }
+func (m *M) msg() string  { return "x" + m.lang() }
+func (m *M) clean() string { return "y" }
+func (m *M) run() {
+	_ = m.settings.UpdateOffboxStatus(func(o *T) { o.A = m.msg(); o.B = m.clean() })
+}`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "decoy.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	files := []parsedFile{{fset: fset, file: f, path: "decoy.go"}}
+	exported := map[string]bool{"GetLanguage": true}
+	tainted := taintedFuncs(files, exported)
+	if !tainted["msg"] || !tainted["lang"] || tainted["clean"] {
+		t.Fatalf("taint = %v; want lang+msg tainted, clean not", tainted)
+	}
+	var lit *ast.FuncLit
+	ast.Inspect(f, func(n ast.Node) bool {
+		if l, ok := n.(*ast.FuncLit); ok {
+			lit = l
+		}
+		return true
+	})
+	hits := lockingCalls(lit.Body, exported, tainted)
+	if len(hits) != 1 || hits[0].name != "msg" {
+		t.Fatalf("hits = %+v; want exactly msg()", hits)
+	}
+}
+
+type parsedFile struct {
+	fset *token.FileSet
+	file *ast.File
+	path string
+}
+
+func (p parsedFile) pos(pos token.Pos) string {
+	ps := p.fset.Position(pos)
+	return filepath.ToSlash(p.path) + ":" + strconv.Itoa(ps.Line)
+}
+
+// parseControllerPackages parses every non-test .go file under controller/, grouped by directory.
+func parseControllerPackages(t *testing.T, root string) map[string][]parsedFile {
+	t.Helper()
+	fset := token.NewFileSet()
+	out := map[string][]parsedFile{}
+	err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if info.IsDir() {
+			base := info.Name()
+			if path != root && (strings.HasPrefix(base, ".") || base == "testdata" || base == "vendor") {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if !strings.HasSuffix(path, ".go") || strings.HasSuffix(path, "_test.go") {
+			return nil
+		}
+		f, perr := parser.ParseFile(fset, path, nil, 0)
+		if perr != nil {
+			return perr
+		}
+		dir := filepath.Dir(path)
+		rel, _ := filepath.Rel(root, path)
+		out[dir] = append(out[dir], parsedFile{fset: fset, file: f, path: rel})
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return out
+}
+
+// settingsLockers returns the *Settings methods that take s.mu (directly, or by calling another
+// *Settings method that does), and the subset that also take a func-typed parameter (with the
+// argument positions of those parameters).
+func settingsLockers(files []parsedFile) (lockers map[string]bool, takers map[string][]int) {
+	type method struct {
+		recv string
+		decl *ast.FuncDecl
+	}
+	var methods []method
+	for _, pf := range files {
+		for _, d := range pf.file.Decls {
+			fd, ok := d.(*ast.FuncDecl)
+			if !ok || fd.Recv == nil || len(fd.Recv.List) != 1 || fd.Body == nil {
+				continue
+			}
+			star, ok := fd.Recv.List[0].Type.(*ast.StarExpr)
+			if !ok {
+				continue
+			}
+			if id, ok := star.X.(*ast.Ident); !ok || id.Name != "Settings" {
+				continue
+			}
+			recv := "_"
+			if len(fd.Recv.List[0].Names) == 1 {
+				recv = fd.Recv.List[0].Names[0].Name
+			}
+			methods = append(methods, method{recv, fd})
+		}
+	}
+	lockers = map[string]bool{}
+	for changed := true; changed; {
+		changed = false
+		for _, m := range methods {
+			if lockers[m.decl.Name.Name] {
+				continue
+			}
+			found := false
+			ast.Inspect(m.decl.Body, func(n ast.Node) bool {
+				call, ok := n.(*ast.CallExpr)
+				if !ok || found {
+					return !found
+				}
+				sel, ok := call.Fun.(*ast.SelectorExpr)
+				if !ok {
+					return true
+				}
+				// s.mu.Lock() / s.mu.RLock()
+				if (sel.Sel.Name == "Lock" || sel.Sel.Name == "RLock") && isRecvField(sel.X, m.recv, "mu") {
+					found = true
+				}
+				// s.OtherLockingMethod()
+				if id, ok := sel.X.(*ast.Ident); ok && id.Name == m.recv && lockers[sel.Sel.Name] {
+					found = true
+				}
+				return !found
+			})
+			if found {
+				lockers[m.decl.Name.Name] = true
+				changed = true
+			}
+		}
+	}
+	// takers: locker -> the argument positions that are func-typed.
+	takers = map[string][]int{}
+	for _, m := range methods {
+		if !lockers[m.decl.Name.Name] {
+			continue
+		}
+		pos := 0
+		for _, p := range m.decl.Type.Params.List {
+			n := len(p.Names)
+			if n == 0 {
+				n = 1
+			}
+			if _, ok := p.Type.(*ast.FuncType); ok {
+				for k := 0; k < n; k++ {
+					takers[m.decl.Name.Name] = append(takers[m.decl.Name.Name], pos+k)
+				}
+			}
+			pos += n
+		}
+	}
+	return lockers, takers
+}
+
+func isRecvField(x ast.Expr, recv, field string) bool {
+	sel, ok := x.(*ast.SelectorExpr)
+	if !ok || sel.Sel.Name != field {
+		return false
+	}
+	id, ok := sel.X.(*ast.Ident)
+	return ok && id.Name == recv
+}
+
+// calledName is the name a call expression invokes: `f(…)` → f, `x.y.f(…)` → f.
+func calledName(call *ast.CallExpr) string {
+	switch fn := call.Fun.(type) {
+	case *ast.Ident:
+		return fn.Name
+	case *ast.SelectorExpr:
+		return fn.Sel.Name
+	}
+	return ""
+}
+
+// taintedFuncs returns, for one package, the names of funcs/methods whose body (transitively, within
+// the package) calls an exported settings locker.
+func taintedFuncs(files []parsedFile, exported map[string]bool) map[string]bool {
+	bodies := map[string][]*ast.BlockStmt{}
+	for _, pf := range files {
+		for _, d := range pf.file.Decls {
+			if fd, ok := d.(*ast.FuncDecl); ok && fd.Body != nil {
+				bodies[fd.Name.Name] = append(bodies[fd.Name.Name], fd.Body)
+			}
+		}
+	}
+	tainted := map[string]bool{}
+	for changed := true; changed; {
+		changed = false
+		for name, bs := range bodies {
+			if tainted[name] {
+				continue
+			}
+			for _, b := range bs {
+				if len(lockingCalls(b, exported, tainted)) > 0 {
+					tainted[name] = true
+					changed = true
+					break
+				}
+			}
+		}
+	}
+	return tainted
+}
+
+type lockHit struct {
+	name string
+	pos  token.Pos
+}
+
+func lockingCalls(body ast.Node, exported, tainted map[string]bool) []lockHit {
+	var hits []lockHit
+	ast.Inspect(body, func(n ast.Node) bool {
+		call, ok := n.(*ast.CallExpr)
+		if !ok {
+			return true
+		}
+		name := calledName(call)
+		_, isSel := call.Fun.(*ast.SelectorExpr)
+		if (isSel && exported[name]) || tainted[name] {
+			hits = append(hits, lockHit{name, call.Pos()})
+		}
+		return true
+	})
+	return hits
+}