diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8a2d138..086f3a9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,32 @@
 All notable changes to **felhom-agent** are recorded here. Update on every code
 change that gets pushed.
 
+## v0.11.0 — slice 8B: app-consistent backup — /backup/due policy + /backup/status phases (2026-06-10)
+
+The agent half of slice 8B (doc 03 §8). Turns the 8A thin backup stubs into the real policy the
+in-guest controller's quiesce loop drives (controller half: `felhom-controller` v0.36.0). No hub
+change. The downtime optimization (`vzdump --mode snapshot` + a `snapshotted` phase) is the 8B.2
+fast-follow; the hub-served per-guest policy is slice 10.
+
+### Changed (`internal/localapi`)
+- **`GET /backup/due`** — real **cadence** policy (replaces the 8A "never backed up" stub): a guest
+  is due when no **successful** backup is recorded OR the newest one is older than the agent-local
+  cadence (`backup.backup_cadence_seconds`, default 24h). A successful `POST /backup` flips due to
+  **false** for the window, so the controller won't re-quiesce in a loop. A failed backup does not
+  satisfy the cadence. Returns `age_seconds` for diagnosis.
+- **`GET /backup/status`** — real **phases** `idle | running | done | failed` + the job id, so the
+  controller can poll a backup to completion (was: just the latest stored backup).
+- **`POST /backup`** — returns a **job id** + `running` phase; tracks the in-flight job and is
+  **single-flight per guest** (a second POST while one runs returns the same job — no concurrent
+  vzdump). On completion the job transitions done/failed and the result is recorded to the store.
+- Config: `backup.backup_cadence_seconds` + `BackupCadence()`; the local-API server takes the cadence.
+
+### Tests
+- `/backup/due`: due when stale / no backup, **not due within the window after a success**, due again
+  past the cadence, **a failed backup does not count**. `/backup/status`: running→done and
+  running→failed (gated fake to observe the running phase). `POST /backup` single-flight (one vzdump
+  for concurrent POSTs). All still self-scoped (token→guest).
+
 ## v0.10.0 — slice 8A: agent local-API server + provisioning back-half (2026-06-10)
 
 The host-agent half of slice 8A (doc 03 §6). Adds the per-guest **local API** the in-guest
diff --git a/cmd/felhom-agent/main.go b/cmd/felhom-agent/main.go
index c2b8cb4..e47f5b2 100644
--- a/cmd/felhom-agent/main.go
+++ b/cmd/felhom-agent/main.go
@@ -40,7 +40,7 @@ import (
 
 // version is the agent version. Overridable at build time with
 // -ldflags "-X main.version=<v>"; defaults to the in-repo CHANGELOG version.
-var version = "0.10.0"
+var version = "0.11.0"
 
 func main() {
 	var (
@@ -485,14 +485,15 @@ func buildLocalAPIServer(cfg config.Config, px *proxmox.Client, store *backup.St
 	logger.Info("local-api leaf ready", "fingerprint_sha256", fp, "cert", cfg.LocalAPI.CertPath())
 	runner := backup.NewBackupRunner(px, cfg.Backup.LocalBackupTarget, "", "felhom local-api", logger)
 	srv, err := localapi.NewServer(localapi.Options{
-		ListenAddr: cfg.LocalAPI.ListenAddr,
-		Cert:       cert,
-		Guests:     px,
-		Backups:    runner,
-		Store:      store,
-		Storage:    observer,
-		Tokens:     tokens,
-		Logger:     logger,
+		ListenAddr:    cfg.LocalAPI.ListenAddr,
+		Cert:          cert,
+		Guests:        px,
+		Backups:       runner,
+		Store:         store,
+		Storage:       observer,
+		Tokens:        tokens,
+		BackupCadence: cfg.Backup.BackupCadence(),
+		Logger:        logger,
 	})
 	if err != nil {
 		logger.Warn("daemon: local-api disabled (server build)", "err", err)
diff --git a/configs/agent.example.json b/configs/agent.example.json
index 422ed48..0d3a5b3 100644
--- a/configs/agent.example.json
+++ b/configs/agent.example.json
@@ -43,7 +43,8 @@
     "scratch_vmid_min": 990000,
     "scratch_vmid_max": 990009,
     "pbs_verify_cadence_seconds": 0,
-    "pbs_secret_dir": "/etc/pve/priv/storage"
+    "pbs_secret_dir": "/etc/pve/priv/storage",
+    "backup_cadence_seconds": 0
   },
   "local_api": {
     "enable": true,
diff --git a/internal/config/config.go b/internal/config/config.go
index d4b9462..f91bf5f 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -139,6 +139,19 @@ type BackupConfig struct {
 	// PBSSecretDir holds the per-storage PBS token secret files (<id>.pw). Default
 	// /etc/pve/priv/storage (PVE-managed, 0600). The agent reads it at runtime; never logged.
 	PBSSecretDir string `json:"pbs_secret_dir"`
+
+	// BackupCadenceSeconds drives the local-API GET /backup/due (slice 8B): a guest is "due" when
+	// its newest successful backup is older than this (or none exists). 0 → default (24h). The
+	// hub-served per-guest policy is slice 10; this is the agent-local cadence.
+	BackupCadenceSeconds int `json:"backup_cadence_seconds"`
+}
+
+// BackupCadence returns the per-guest /backup/due window: positive as-is, else 24h default.
+func (b BackupConfig) BackupCadence() time.Duration {
+	if b.BackupCadenceSeconds > 0 {
+		return time.Duration(b.BackupCadenceSeconds) * time.Second
+	}
+	return 24 * time.Hour
 }
 
 // Default scratch VMID band + restore-test cadence.
diff --git a/internal/localapi/server.go b/internal/localapi/server.go
index bc4eb33..90116b2 100644
--- a/internal/localapi/server.go
+++ b/internal/localapi/server.go
@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"gitea.dooplex.hu/admin/felhom-agent/internal/hub"
@@ -60,7 +61,33 @@ type Options struct {
 	Store      BackupStore
 	Storage    StorageView
 	Tokens     TokenAuthority
-	Logger     *slog.Logger
+	// BackupCadence is the per-guest backup interval driving GET /backup/due (slice 8B). A guest
+	// is "due" when no successful backup is recorded OR the newest one is older than this. 0 → a
+	// safe default (24h). The hub-served policy is slice 10; this is the agent-local cadence.
+	BackupCadence time.Duration
+	Logger        *slog.Logger
+}
+
+// defaultBackupCadence is the fallback /backup/due window when none is configured.
+const defaultBackupCadence = 24 * time.Hour
+
+// Backup phase vocabulary reported by GET /backup/status (slice 8B). The 8B.2 fast-follow adds a
+// `snapshotted` phase (vzdump --mode snapshot) so the controller can unquiesce at snapshot-taken.
+const (
+	PhaseIdle    = "idle"
+	PhaseRunning = "running"
+	PhaseDone    = "done"
+	PhaseFailed  = "failed"
+)
+
+// backupJob is the in-flight/last backup job for one guest (drives /backup/status phases).
+type backupJob struct {
+	JobID      string
+	Phase      string
+	StartedAt  time.Time
+	FinishedAt time.Time
+	Archive    string
+	Error      string
 }
 
 // Server is the per-guest local API (doc 03 §6). It serves the agent's pinned self-signed leaf
@@ -73,7 +100,12 @@ type Server struct {
 	store   BackupStore
 	storage StorageView
 	tokens  TokenAuthority
+	cadence time.Duration
 	logger  *slog.Logger
+	now     func() time.Time
+
+	jobsMu sync.Mutex
+	jobs   map[int]*backupJob // per-guest backup job state (slice 8B)
 
 	baseCtx context.Context // for fire-and-forget backups; set in Run
 }
@@ -89,6 +121,10 @@ func NewServer(o Options) (*Server, error) {
 	if o.Logger == nil {
 		o.Logger = slog.Default()
 	}
+	cadence := o.BackupCadence
+	if cadence <= 0 {
+		cadence = defaultBackupCadence
+	}
 	return &Server{
 		addr:    o.ListenAddr,
 		cert:    o.Cert,
@@ -97,7 +133,10 @@ func NewServer(o Options) (*Server, error) {
 		store:   o.Store,
 		storage: o.Storage,
 		tokens:  o.Tokens,
+		cadence: cadence,
 		logger:  o.Logger,
+		now:     func() time.Time { return time.Now().UTC() },
+		jobs:    map[int]*backupJob{},
 	}, nil
 }
 
@@ -313,6 +352,13 @@ type backupRequest struct {
 	VMID int `json:"vmid"`
 }
 
+// BackupResponse is POST /backup. The controller polls GET /backup/status on job_id to completion.
+type BackupResponse struct {
+	VMID  int    `json:"vmid"`
+	JobID string `json:"job_id"`
+	Phase string `json:"phase"`
+}
+
 func (s *Server) handleBackup(w http.ResponseWriter, r *http.Request, vmid int) {
 	// Body is optional; if present its vmid must match the token's guest.
 	if r.ContentLength != 0 {
@@ -324,10 +370,24 @@ func (s *Server) handleBackup(w http.ResponseWriter, r *http.Request, vmid int)
 			return
 		}
 	}
-	// Enqueue: a vzdump runs for minutes, so we do not block the request. The backup runs on
-	// the server's base context (cancelled on daemon shutdown) and records into the store; the
-	// controller polls GET /backup/status. This is the crash-consistent path (8A); the
-	// app-consistent quiesce-then-backup loop is 8B.
+	// Single-flight per guest: if a backup is already running for this guest, return that job
+	// (don't start a second concurrent vzdump). The controller polls /backup/status on it.
+	s.jobsMu.Lock()
+	if cur := s.jobs[vmid]; cur != nil && cur.Phase == PhaseRunning {
+		job := *cur
+		s.jobsMu.Unlock()
+		writeStatus(w, http.StatusAccepted, true, BackupResponse{VMID: vmid, JobID: job.JobID, Phase: job.Phase}, "")
+		return
+	}
+	jobID := "backup-" + strconv.Itoa(vmid) + "-" + strconv.FormatInt(s.now().UnixNano(), 10)
+	s.jobs[vmid] = &backupJob{JobID: jobID, Phase: PhaseRunning, StartedAt: s.now()}
+	s.jobsMu.Unlock()
+
+	// Enqueue: a vzdump runs for minutes, so we do not block the request. The backup runs on the
+	// server's base context (cancelled on daemon shutdown), updates the job phase, and records
+	// into the store; the controller polls GET /backup/status. This is the host-side half of the
+	// 8B app-consistent path — the controller quiesces (stops its stacks) BEFORE calling this, so
+	// the vzdump captures a clean-shutdown-consistent state.
 	base := s.baseCtx
 	if base == nil {
 		base = context.Background()
@@ -342,36 +402,94 @@ func (s *Server) handleBackup(w http.ResponseWriter, r *http.Request, vmid int)
 			if b.Error == "" {
 				b.Error = err.Error()
 			}
-			s.logger.Error("local-api: enqueued backup failed", "vmid", vmid, "err", err)
+			s.logger.Error("local-api: backup job failed", "vmid", vmid, "job", jobID, "err", err)
 		} else {
-			s.logger.Info("local-api: enqueued backup complete", "vmid", vmid, "archive", b.Archive)
+			s.logger.Info("local-api: backup job complete", "vmid", vmid, "job", jobID, "archive", b.Archive)
 		}
 		s.store.RecordBackup(b)
+		s.finishJob(vmid, jobID, b)
 	}()
-	writeStatus(w, http.StatusAccepted, true, map[string]any{"vmid": vmid, "enqueued": true}, "")
+	writeStatus(w, http.StatusAccepted, true, BackupResponse{VMID: vmid, JobID: jobID, Phase: PhaseRunning}, "")
 }
 
-// BackupDueResponse is GET /backup/due. Thin in 8A: a guest with no successful backup recorded
-// is "due"; otherwise not. Policy-scheduled cadence (hub manifest) lands in slice 10, and the
-// quiesce-on-due consumer is 8B — both noted in the response.
+// finishJob transitions the guest's job to done/failed (only if it is still the current job — a
+// later job started after a single-flight gap must not be overwritten by an older one's result).
+func (s *Server) finishJob(vmid int, jobID string, b hub.Backup) {
+	s.jobsMu.Lock()
+	defer s.jobsMu.Unlock()
+	cur := s.jobs[vmid]
+	if cur == nil || cur.JobID != jobID {
+		return
+	}
+	cur.FinishedAt = s.now()
+	if b.Success {
+		cur.Phase = PhaseDone
+		cur.Archive = b.Archive
+	} else {
+		cur.Phase = PhaseFailed
+		cur.Error = b.Error
+	}
+}
+
+// jobSnapshot returns a copy of the guest's current job (ok=false if none).
+func (s *Server) jobSnapshot(vmid int) (backupJob, bool) {
+	s.jobsMu.Lock()
+	defer s.jobsMu.Unlock()
+	if j := s.jobs[vmid]; j != nil {
+		return *j, true
+	}
+	return backupJob{}, false
+}
+
+// BackupDueResponse is GET /backup/due (slice 8B). A guest is due when no successful backup is
+// recorded OR the newest successful one is older than the agent-local cadence. A successful
+// POST /backup flips this to false for the window, so the controller won't re-quiesce in a loop.
+// The hub-served policy is slice 10.
 type BackupDueResponse struct {
-	VMID   int    `json:"vmid"`
-	Due    bool   `json:"due"`
-	Reason string `json:"reason"`
+	VMID    int    `json:"vmid"`
+	Due     bool   `json:"due"`
+	Reason  string `json:"reason"`
+	AgeSecs *int64 `json:"age_seconds,omitempty"` // age of the newest successful backup; null if none
 }
 
 func (s *Server) handleBackupDue(w http.ResponseWriter, r *http.Request, vmid int) {
-	latest := s.latestBackupFor(r.Context(), vmid)
-	if latest == nil || !latest.Success {
+	latest := s.latestSuccessfulBackupFor(r.Context(), vmid)
+	if latest == nil {
 		writeOK(w, BackupDueResponse{VMID: vmid, Due: true, Reason: "no successful backup recorded yet"})
 		return
 	}
-	writeOK(w, BackupDueResponse{VMID: vmid, Due: false, Reason: "policy-scheduled cadence lands in slice 10"})
+	age, ok := backupAge(latest.StartedAt, s.now())
+	if !ok {
+		// Unparseable timestamp: fail safe toward "due" so a backup still happens.
+		writeOK(w, BackupDueResponse{VMID: vmid, Due: true, Reason: "last backup time unparseable — treating as due"})
+		return
+	}
+	ageSecs := int64(age.Seconds())
+	if age >= s.cadence {
+		writeOK(w, BackupDueResponse{VMID: vmid, Due: true, Reason: "older than cadence", AgeSecs: &ageSecs})
+		return
+	}
+	writeOK(w, BackupDueResponse{VMID: vmid, Due: false, Reason: "within cadence window", AgeSecs: &ageSecs})
+}
+
+// BackupStatusResponse is GET /backup/status (slice 8B): the current/last job phase + the latest
+// recorded backup. Phase is idle when no job has run this process lifetime.
+type BackupStatusResponse struct {
+	VMID    int         `json:"vmid"`
+	Phase   string      `json:"phase"`             // idle | running | done | failed
+	JobID   string      `json:"job_id,omitempty"`
+	Error   string      `json:"error,omitempty"`
+	Backup  *hub.Backup `json:"backup,omitempty"`  // latest recorded backup for this guest
 }
 
 func (s *Server) handleBackupStatus(w http.ResponseWriter, r *http.Request, vmid int) {
-	latest := s.latestBackupFor(r.Context(), vmid)
-	writeOK(w, map[string]any{"vmid": vmid, "backup": latest})
+	resp := BackupStatusResponse{VMID: vmid, Phase: PhaseIdle, Backup: s.latestBackupFor(r.Context(), vmid)}
+	if job, ok := s.jobSnapshot(vmid); ok {
+		resp.Phase = job.Phase
+		resp.JobID = job.JobID
+		resp.Error = job.Error
+	}
+	writeOK(w, resp)
 }
 
 func (s *Server) handleRestoreTestStatus(w http.ResponseWriter, r *http.Request, vmid int) {
@@ -389,9 +507,19 @@ func (s *Server) handleRestoreTestStatus(w http.ResponseWriter, r *http.Request,
 
 // latestBackupFor returns this guest's most recent backup from the store (nil if none).
 func (s *Server) latestBackupFor(ctx context.Context, vmid int) *hub.Backup {
+	return s.pickLatestBackup(ctx, vmid, false)
+}
+
+// latestSuccessfulBackupFor returns this guest's most recent SUCCESSFUL backup (nil if none) —
+// the basis for /backup/due (a failed backup must not satisfy the cadence).
+func (s *Server) latestSuccessfulBackupFor(ctx context.Context, vmid int) *hub.Backup {
+	return s.pickLatestBackup(ctx, vmid, true)
+}
+
+func (s *Server) pickLatestBackup(ctx context.Context, vmid int, successOnly bool) *hub.Backup {
 	var latest *hub.Backup
 	for _, b := range s.store.Backups(ctx) {
-		if b.VMID != vmid {
+		if b.VMID != vmid || (successOnly && !b.Success) {
 			continue
 		}
 		bb := b
@@ -402,6 +530,15 @@ func (s *Server) latestBackupFor(ctx context.Context, vmid int) *hub.Backup {
 	return latest
 }
 
+// backupAge parses an RFC3339 backup start time and returns its age relative to now.
+func backupAge(startedAt string, now time.Time) (time.Duration, bool) {
+	t, err := time.Parse(time.RFC3339, startedAt)
+	if err != nil {
+		return 0, false
+	}
+	return now.Sub(t), true
+}
+
 // classByStorage builds a storage-id → class(fast|slow|"") map from the host storage view. A
 // view error is logged and yields an empty map (class falls back to "" — a hint, not load-bearing).
 func (s *Server) classByStorage(ctx context.Context) map[string]string {
diff --git a/internal/localapi/server_test.go b/internal/localapi/server_test.go
index a93eb6a..eaaef54 100644
--- a/internal/localapi/server_test.go
+++ b/internal/localapi/server_test.go
@@ -64,14 +64,24 @@ func (f *fakeGuests) WaitTask(_ context.Context, _ string, _ proxmox.WaitOptions
 }
 
 type fakeBackups struct {
-	mu    sync.Mutex
-	vmids []int
+	mu      sync.Mutex
+	vmids   []int
+	gate    chan struct{} // if non-nil, Backup blocks until it is closed (observe the running phase)
+	failErr string        // if set, Backup returns this error (drives the failed phase)
 }
 
 func (f *fakeBackups) Backup(_ context.Context, vmid int) (hub.Backup, error) {
 	f.mu.Lock()
 	f.vmids = append(f.vmids, vmid)
+	gate := f.gate
+	failErr := f.failErr
 	f.mu.Unlock()
+	if gate != nil {
+		<-gate
+	}
+	if failErr != "" {
+		return hub.Backup{VMID: vmid}, fmt.Errorf("%s", failErr)
+	}
 	return hub.Backup{VMID: vmid, Success: true, Archive: "local:backup/vzdump-x", StartedAt: "2026-06-10T00:00:00Z"}, nil
 }
 func (f *fakeBackups) called() []int { f.mu.Lock(); defer f.mu.Unlock(); return append([]int(nil), f.vmids...) }
@@ -106,25 +116,35 @@ func (m staticTokens) Lookup(tok string) (int, bool) { v, ok := m[tok]; return v
 
 // ---- harness ----------------------------------------------------------------------------
 
-func newTestServer(t *testing.T, g *fakeGuests, b *fakeBackups, st *fakeStore, sv StorageView) http.Handler {
+// testNow is the fixed clock the test server uses, so /backup/due cadence math is deterministic.
+var testNow = time.Date(2026, 6, 10, 12, 0, 0, 0, time.UTC)
+
+func newTestServerS(t *testing.T, g *fakeGuests, b *fakeBackups, st *fakeStore, sv StorageView) *Server {
 	t.Helper()
 	if sv == nil {
 		sv = fakeStorage{}
 	}
 	srv, err := NewServer(Options{
-		ListenAddr: "127.0.0.1:0",
-		Guests:     g,
-		Backups:    b,
-		Store:      st,
-		Storage:    sv,
-		Tokens:     staticTokens{"A": 8200, "B": 9300},
-		Logger:     slog.New(slog.NewTextHandler(io.Discard, nil)),
+		ListenAddr:    "127.0.0.1:0",
+		Guests:        g,
+		Backups:       b,
+		Store:         st,
+		Storage:       sv,
+		Tokens:        staticTokens{"A": 8200, "B": 9300},
+		BackupCadence: 24 * time.Hour,
+		Logger:        slog.New(slog.NewTextHandler(io.Discard, nil)),
 	})
 	if err != nil {
 		t.Fatalf("new server: %v", err)
 	}
 	srv.baseCtx = context.Background()
-	return srv.Handler()
+	srv.now = func() time.Time { return testNow }
+	return srv
+}
+
+func newTestServer(t *testing.T, g *fakeGuests, b *fakeBackups, st *fakeStore, sv StorageView) http.Handler {
+	t.Helper()
+	return newTestServerS(t, g, b, st, sv).Handler()
 }
 
 func do(t *testing.T, h http.Handler, method, path, token, body string) *httptest.ResponseRecorder {
@@ -304,26 +324,126 @@ func TestBackup_EnqueuesForTokenGuest(t *testing.T) {
 	}
 }
 
-func TestBackupDue_ThinHeuristic(t *testing.T) {
-	st := &fakeStore{}
-	h := newTestServer(t, &fakeGuests{}, &fakeBackups{}, st, nil)
-
-	// no backup recorded → due
+func dueOf(t *testing.T, h http.Handler) BackupDueResponse {
+	t.Helper()
 	w := do(t, h, "GET", "/backup/due", "A", "")
 	var resp struct {
 		Data BackupDueResponse `json:"data"`
 	}
-	_ = json.Unmarshal(w.Body.Bytes(), &resp)
-	if !resp.Data.Due {
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("decode due: %v", err)
+	}
+	return resp.Data
+}
+
+// testNow is 2026-06-10T12:00:00Z, cadence 24h.
+func TestBackupDue_Cadence(t *testing.T) {
+	st := &fakeStore{}
+	h := newTestServer(t, &fakeGuests{}, &fakeBackups{}, st, nil)
+
+	// no backup recorded → due
+	if d := dueOf(t, h); !d.Due {
 		t.Fatal("expected due=true with no backup recorded")
 	}
-	// a successful backup for this guest → not due
-	st.backups = []hub.Backup{{VMID: 8200, Success: true, StartedAt: "2026-06-10T00:00:00Z"}}
-	w = do(t, h, "GET", "/backup/due", "A", "")
-	_ = json.Unmarshal(w.Body.Bytes(), &resp)
-	if resp.Data.Due {
-		t.Fatal("expected due=false after a successful backup")
+	// a successful backup 1h ago → NOT due (within the 24h window)
+	st.backups = []hub.Backup{{VMID: 8200, Success: true, StartedAt: "2026-06-10T11:00:00Z"}}
+	if d := dueOf(t, h); d.Due {
+		t.Fatalf("expected due=false 1h after a successful backup; reason=%q", d.Reason)
 	}
+	// a successful backup >24h ago → due again
+	st.backups = []hub.Backup{{VMID: 8200, Success: true, StartedAt: "2026-06-08T00:00:00Z"}}
+	if d := dueOf(t, h); !d.Due {
+		t.Fatal("expected due=true when the newest backup is older than the cadence")
+	}
+	// a FAILED backup 1h ago does NOT satisfy the cadence → still due
+	st.backups = []hub.Backup{{VMID: 8200, Success: false, StartedAt: "2026-06-10T11:00:00Z"}}
+	if d := dueOf(t, h); !d.Due {
+		t.Fatal("expected due=true: a failed backup must not count as a successful one")
+	}
+}
+
+// POST /backup returns a running phase; the job transitions running→done; a successful backup then
+// flips /backup/due to false (the controller won't re-quiesce in a loop).
+func TestBackupStatus_RunningToDone(t *testing.T) {
+	st := &fakeStore{}
+	b := &fakeBackups{}
+	srv := newTestServerS(t, &fakeGuests{}, b, st, nil)
+	h := srv.Handler()
+
+	// gate the backup goroutine so we can observe the running phase deterministically
+	b.gate = make(chan struct{})
+
+	w := do(t, h, "POST", "/backup", "A", "")
+	if w.Code != http.StatusAccepted {
+		t.Fatalf("POST /backup: got %d want 202", w.Code)
+	}
+	var br struct {
+		Data BackupResponse `json:"data"`
+	}
+	_ = json.Unmarshal(w.Body.Bytes(), &br)
+	if br.Data.Phase != PhaseRunning || br.Data.JobID == "" {
+		t.Fatalf("expected running phase + job id, got %+v", br.Data)
+	}
+	if ph := statusPhase(t, h); ph != PhaseRunning {
+		t.Fatalf("status while running: got %q want running", ph)
+	}
+	close(b.gate) // let the backup complete
+	// wait for done
+	deadline := time.Now().Add(time.Second)
+	for time.Now().Before(deadline) {
+		if statusPhase(t, h) == PhaseDone {
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	if ph := statusPhase(t, h); ph != PhaseDone {
+		t.Fatalf("status after completion: got %q want done", ph)
+	}
+}
+
+func TestBackupStatus_RunningToFailed(t *testing.T) {
+	b := &fakeBackups{failErr: "vzdump exploded"}
+	srv := newTestServerS(t, &fakeGuests{}, b, &fakeStore{}, nil)
+	h := srv.Handler()
+
+	if do(t, h, "POST", "/backup", "A", "").Code != http.StatusAccepted {
+		t.Fatal("POST /backup not accepted")
+	}
+	deadline := time.Now().Add(time.Second)
+	for time.Now().Before(deadline) {
+		if statusPhase(t, h) == PhaseFailed {
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	if ph := statusPhase(t, h); ph != PhaseFailed {
+		t.Fatalf("status after a failed backup: got %q want failed", ph)
+	}
+}
+
+// A second POST /backup while one is running does NOT start a second vzdump (single-flight).
+func TestBackup_SingleFlight(t *testing.T) {
+	b := &fakeBackups{gate: make(chan struct{})}
+	srv := newTestServerS(t, &fakeGuests{}, b, &fakeStore{}, nil)
+	h := srv.Handler()
+
+	do(t, h, "POST", "/backup", "A", "")
+	do(t, h, "POST", "/backup", "A", "") // should be coalesced onto the running job
+	close(b.gate)
+	time.Sleep(30 * time.Millisecond)
+	if c := b.called(); len(c) != 1 {
+		t.Fatalf("expected a single vzdump for concurrent POSTs, got %d", len(c))
+	}
+}
+
+func statusPhase(t *testing.T, h http.Handler) string {
+	t.Helper()
+	w := do(t, h, "GET", "/backup/status", "A", "")
+	var resp struct {
+		Data BackupStatusResponse `json:"data"`
+	}
+	_ = json.Unmarshal(w.Body.Bytes(), &resp)
+	return resp.Data.Phase
 }
 
 func TestBackupStatus_FiltersToThisGuest(t *testing.T) {