channelhealth: controller->agent channel health-check (periodic probe + classified operator alert) v0.90.0

New internal/channelhealth Checker: ~60s probe via the PRODUCTION memoized client
(Server.ProbeAgentChannel, GET /storage), classifies failures (spike Q1 map), debounces transient
reasons (N>=2; construction error latches distinctly), seeds first obs, alerts operator+dashboard on
transition. Notifier.NotifyAgentChannelDown/Recovered (English, operator-only), AlertManager dashboard
banner (Hungarian). No agent/hub change. Spike-proven.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EPZ4GJ8L5Jqf8UiPwbn1kt

controller/cmd/controller/main.go

@@ -25,6 +25,7 @@ import (
 	"gitea.dooplex.hu/admin/felhom-controller/internal/assets"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/backup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/bootstrap"
+	"gitea.dooplex.hu/admin/felhom-controller/internal/channelhealth"
 	cf "gitea.dooplex.hu/admin/felhom-controller/internal/cloudflare"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/config"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/crypto"
@@ -253,9 +254,9 @@ func main() {
 			TLSAddr:        cfg.MailRelay.TLSListen,
 			PlainNoTLSAddr: cfg.MailRelay.PlainNoTLSListen,
 			ServiceName:    cfg.MailRelay.ShimHost,
-			Policy:      mailrelay.NewPolicy(cfg.MailRelay.FromDomains),
-			Forwarder:   mailrelay.NewHubForwarder(cfg.Hub.URL, cfg.Hub.APIKey),
-			Logger:      logger,
+			Policy:         mailrelay.NewPolicy(cfg.MailRelay.FromDomains),
+			Forwarder:      mailrelay.NewHubForwarder(cfg.Hub.URL, cfg.Hub.APIKey),
+			Logger:         logger,
 		})
 		if err := mailShim.Apply(sett.AppEmailEnabled()); err != nil {
 			logger.Printf("[ERROR] [mailrelay] could not start app-email shim: %v", err)
@@ -717,6 +718,17 @@ func main() {
 	}
 	webServer.SetStartTime(startTime)
 
+	// Controller→agent channel health (self-health slice). A ~60s probe of the local-API channel via
+	// the PRODUCTION memoized client (webServer.ProbeAgentChannel), classified + debounced, alerting the
+	// operator + dashboard on a transition. Only on a provisioned guest (endpoint set) — mirrors
+	// probeLocalAPI's guard. Registered after webServer (it owns the memoized client); sched.Every
+	// launches the job immediately since the scheduler is already started.
+	if cfg.LocalAPI.Endpoint != "" {
+		chSink := channelSink{notifier: notifier, alertMgr: alertMgr}
+		chChecker := channelhealth.New(webServer.ProbeAgentChannel, chSink, logger)
+		sched.Every("agent-channel-health", 60*time.Second, chChecker.Check)
+	}
+
 	// Wire debug callbacks (only in debug mode)
 	if cfg.Logging.Level == "debug" {
 		dc := &web.DebugCallbacks{}
@@ -1308,6 +1320,22 @@ func parseDurationOr(s string, def time.Duration) time.Duration {
 	return d
 }
 
+// channelSink adapts the notify.Notifier + web.AlertManager to the channelhealth.Sink seam (kept in
+// main.go so the channelhealth package imports neither — no cycle). SetDashboard reflects the current
+// state each probe (idempotent); NotifyDown/NotifyRecovered fire only on a transition.
+type channelSink struct {
+	notifier *notify.Notifier
+	alertMgr *web.AlertManager
+}
+
+func (s channelSink) SetDashboard(down bool, _ channelhealth.Reason, msg string) {
+	s.alertMgr.SetAgentChannelAlert(down, msg)
+}
+func (s channelSink) NotifyDown(reason channelhealth.Reason, eventType, severity, msg string) {
+	s.notifier.NotifyAgentChannelDown(string(reason), eventType, severity, msg)
+}
+func (s channelSink) NotifyRecovered() { s.notifier.NotifyAgentChannelRecovered() }
+
 // probeLocalAPI proves the controller↔agent local-API channel at startup and logs this guest's
 // mounts (slice 8A). Non-fatal: it only runs when a local-API endpoint is configured, and any
 // error is logged for diagnosis without affecting the controller's boot. The leaf SHA-256 from

controller/internal/channelhealth/checker.go

@@ -0,0 +1,189 @@
+// Package channelhealth periodically proves the controller→agent local-API channel and surfaces a
+// classified operator alert + dashboard entry on a state change. It closes the gap the R1
+// pin-mismatch incident exposed: the channel was only probed once at startup (probeLocalAPI) and only
+// logged, so a mid-life agent re-key went unnoticed until a user hit the dead disk UI.
+//
+// Design is spike-driven (SPIKE-controller-agent-channel-health-2026-06-29, GO):
+//   - Probe via the PRODUCTION memoized agent client (reused, not a fresh one): it self-heals after an
+//     agent restart, reflects exactly what the UI sees, and avoids the per-call transport leak the
+//     singleton fixed. The construction error (bad fingerprint → agentClient() fails, latching via
+//     sync.Once) is surfaced distinctly.
+//   - Classify the failure by the error substring (the spike Q1 map).
+//   - Debounce transient reasons (connection-refused / timeout): a clean agent restart shows a ~1s
+//     socket gap (one failed probe) that must NOT page — require N>=2 consecutive before alerting.
+//     Pin-mismatch / 401 / construction / DNS alert on first observation (they don't self-clear).
+//   - First scheduler observation SEEDS state without notifying (mirrors host_staleness/host_capability).
+package channelhealth
+
+import (
+	"context"
+	"log"
+	"strings"
+	"sync"
+	"time"
+)
+
+// debounceThreshold is the number of consecutive down-probes a TRANSIENT reason needs before it is
+// treated as a real outage (suppresses the ~1s agent-restart blip from the spike's Q2).
+const debounceThreshold = 2
+
+// probeTimeout mirrors probeLocalAPI's 15s budget for the GET /storage call.
+const probeTimeout = 15 * time.Second
+
+// Reason is the classified channel-down cause.
+type Reason string
+
+const (
+	ReasonPinMismatch   Reason = "pin_mismatch"
+	ReasonUnauthorized  Reason = "unauthorized"
+	ReasonUnreachable   Reason = "unreachable"
+	ReasonTimeout       Reason = "timeout"
+	ReasonMisconfigured Reason = "misconfigured"      // endpoint unresolvable (DNS)
+	ReasonConstruction  Reason = "construction_error" // agentClient() build error — LATCHES until restart
+	ReasonUnknown       Reason = "unknown"
+)
+
+// Probe runs one channel check via the production memoized client. It returns whether the failure was
+// a CONSTRUCTION error (agentClient() couldn't even build — a latching config fault) and the error
+// (nil = channel up). Production wires this to Server.ProbeAgentChannel; tests inject a fake.
+type Probe func(ctx context.Context) (constructionErr bool, err error)
+
+// Sink receives the checker's outputs. SetDashboard reflects the CURRENT state every check (idempotent
+// — a born-down channel shows immediately, with no notification). NotifyDown/NotifyRecovered fire ONLY
+// on a transition (the operator relay, with the hub-side cooldown). Implemented in main.go over the
+// notify.Notifier + web.AlertManager (kept out of this package to avoid an import cycle).
+type Sink interface {
+	SetDashboard(down bool, reason Reason, hungarianMsg string)
+	NotifyDown(reason Reason, eventType, severity, englishMsg string)
+	NotifyRecovered()
+}
+
+type classification struct {
+	reason    Reason
+	eventType string
+	severity  string // "error" (critical) | "warning" (transient)
+	english   string // operator alert message (relayed to the hub)
+	hungarian string // short dashboard line
+	debounce  bool   // transient → require N>=2 consecutive
+}
+
+// classify maps a probe result to a classification. constructionErr is authoritative for the latching
+// config fault; otherwise the error substring decides (spike Q1 map). Match substrings, not exact
+// strings (the wrapped Get "..." prefix varies).
+func classify(constructionErr bool, err error) classification {
+	if constructionErr {
+		return classification{ReasonConstruction, "agent_channel_construction_error", "error",
+			"Controller→agent channel down: agent client misconfigured — local_api.fingerprint is not a valid SHA-256, the controller cannot build the client (config fix + controller restart needed).",
+			"A tárolókezelő ügynök kapcsolata hibásan beállítva.", false}
+	}
+	s := err.Error()
+	switch {
+	case strings.Contains(s, "TLS pin mismatch"):
+		return classification{ReasonPinMismatch, "agent_channel_pin_mismatch", "error",
+			"Controller→agent channel down: TLS pin mismatch — the agent's leaf cert no longer matches the controller's bootstrap fingerprint (re-pin / re-bootstrap).",
+			"A tárolókezelő ügynök tanúsítványa megváltozott.", false}
+	case strings.Contains(s, "HTTP 401"):
+		return classification{ReasonUnauthorized, "agent_channel_unauthorized", "error",
+			"Controller→agent channel down: agent rejected the controller token (HTTP 401) — token stale/rotated (re-bootstrap).",
+			"A tárolókezelő ügynök elutasította a hozzáférést.", false}
+	case strings.Contains(s, "no such host") || strings.Contains(s, "lookup "):
+		return classification{ReasonMisconfigured, "agent_channel_misconfigured", "error",
+			"Controller→agent channel down: agent endpoint unresolvable — local_api.endpoint misconfigured.",
+			"A tárolókezelő ügynök címe nem feloldható.", false}
+	case strings.Contains(s, "connection refused"):
+		return classification{ReasonUnreachable, "agent_channel_unreachable", "warning",
+			"Controller→agent channel down: agent unreachable (connection refused) — felhom-agent down or :8443 closed.",
+			"A tárolókezelő ügynök nem elérhető.", true}
+	case strings.Contains(s, "context deadline exceeded") || strings.Contains(s, "i/o timeout") || strings.Contains(s, "Client.Timeout"):
+		return classification{ReasonTimeout, "agent_channel_timeout", "warning",
+			"Controller→agent channel down: timeout — host unreachable / packets dropped.",
+			"A tárolókezelő ügynök nem válaszol.", true}
+	default:
+		return classification{ReasonUnknown, "agent_channel_unknown", "warning",
+			"Controller→agent channel down: " + s,
+			"A tárolókezelő ügynök nem elérhető.", true}
+	}
+}
+
+// Checker holds the in-memory channel state. No persistence — the state is re-derived each run
+// (mirrors the AlertManager's state-based model). Safe for the single scheduler caller; the mutex
+// guards against an overlapping run.
+type Checker struct {
+	probe  Probe
+	sink   Sink
+	logger *log.Logger
+
+	mu              sync.Mutex
+	state           string // "" (unseeded) | "up" | "down:<reason>"
+	consecutiveDown int
+}
+
+// New builds a checker over the probe + sink seams.
+func New(probe Probe, sink Sink, logger *log.Logger) *Checker {
+	return &Checker{probe: probe, sink: sink, logger: logger}
+}
+
+// Check runs one probe cycle: classify, debounce, reflect on the dashboard, and notify on a real
+// transition. Best-effort + idempotent — safe to call on a timer. Never returns an error to the
+// scheduler (a probe failure IS the signal, not a job failure).
+func (c *Checker) Check(ctx context.Context) error {
+	pctx, cancel := context.WithTimeout(ctx, probeTimeout)
+	constructionErr, perr := c.probe(pctx)
+	cancel()
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	// Channel UP.
+	if perr == nil {
+		c.consecutiveDown = 0
+		c.sink.SetDashboard(false, "", "")
+		prev := c.state
+		c.state = "up"
+		if prev == "" || prev == "up" {
+			return nil // seed, or steady-up → no notify
+		}
+		c.logger.Printf("[INFO] [channel] agent channel recovered (was %s)", prev)
+		c.sink.NotifyRecovered()
+		return nil
+	}
+
+	// Channel DOWN — classify + debounce transient reasons.
+	cls := classify(constructionErr, perr)
+	c.consecutiveDown++
+	if cls.debounce && c.consecutiveDown < debounceThreshold {
+		// A transient blip (e.g. the ~1s agent-restart socket gap). Hold the previous state — do NOT
+		// flip the dashboard or notify. If we've never seen anything yet, assume up until confirmed.
+		if c.state == "" {
+			c.state = "up"
+		}
+		c.logger.Printf("[DEBUG] [channel] transient down (%s, %d/%d) — suppressed pending confirmation: %v",
+			cls.reason, c.consecutiveDown, debounceThreshold, perr)
+		return nil
+	}
+
+	newState := "down:" + string(cls.reason)
+	c.sink.SetDashboard(true, cls.reason, cls.hungarian) // dashboard reflects current state always
+	prev := c.state
+	c.state = newState
+	if prev == "" {
+		c.logger.Printf("[INFO] [channel] agent channel down at startup (%s) — seeded, no alert: %v", cls.reason, perr)
+		return nil // first observation seeds, no alert (dashboard already set above)
+	}
+	if prev == newState {
+		return nil // steady down, same reason → no duplicate notify (the dashboard stays set)
+	}
+	c.logger.Printf("[WARN] [channel] agent channel DOWN (%s→%s): %v", prev, newState, perr)
+	c.sink.NotifyDown(cls.reason, cls.eventType, cls.severity, cls.english)
+	return nil
+}
+
+// State returns the current channel state (for tests/diagnostics).
+func (c *Checker) State() string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.state == "" {
+		return "unseeded"
+	}
+	return c.state
+}

controller/internal/channelhealth/checker_test.go

@@ -0,0 +1,218 @@
+package channelhealth
+
+import (
+	"context"
+	"errors"
+	"io"
+	"log"
+	"testing"
+)
+
+type downCall struct {
+	reason    Reason
+	eventType string
+	severity  string
+}
+
+type fakeSink struct {
+	downs      []downCall
+	recovered  int
+	dashDown   bool
+	dashReason Reason
+}
+
+func (f *fakeSink) SetDashboard(down bool, reason Reason, _ string) {
+	f.dashDown = down
+	f.dashReason = reason
+}
+func (f *fakeSink) NotifyDown(reason Reason, eventType, severity, _ string) {
+	f.downs = append(f.downs, downCall{reason, eventType, severity})
+}
+func (f *fakeSink) NotifyRecovered() { f.recovered++ }
+
+// scriptedProbe returns the next (constructionErr, err) on each call; a fresh agentapi.New is NEVER
+// built here — the checker only calls this seam (proving it reuses the production client).
+type scriptedProbe struct {
+	steps []struct {
+		cons bool
+		err  error
+	}
+	i     int
+	calls int
+}
+
+func (p *scriptedProbe) fn(_ context.Context) (bool, error) {
+	p.calls++
+	s := p.steps[p.i]
+	if p.i < len(p.steps)-1 {
+		p.i++
+	}
+	return s.cons, s.err
+}
+
+func newChecker(t *testing.T, sink Sink) *Checker {
+	t.Helper()
+	return New(nil, sink, log.New(io.Discard, "", 0))
+}
+
+func step(cons bool, err error) struct {
+	cons bool
+	err  error
+} {
+	return struct {
+		cons bool
+		err  error
+	}{cons, err}
+}
+
+// run feeds the checker a sequence of probe results.
+func run(c *Checker, p *scriptedProbe) {
+	c.probe = p.fn
+	for range p.steps {
+		_ = c.Check(context.Background())
+	}
+}
+
+// §8 classification: each error → correct reason/event/severity. Seed up first so the down is a
+// transition (non-debounce reasons fire on the first down observation).
+func TestClassify_PerReason(t *testing.T) {
+	cases := []struct {
+		name      string
+		cons      bool
+		err       error
+		reason    Reason
+		eventType string
+		severity  string
+	}{
+		{"pin", false, errors.New(`agentapi: GET /storage: Get "https://x": agentapi: TLS pin mismatch: ...`), ReasonPinMismatch, "agent_channel_pin_mismatch", "error"},
+		{"401", false, errors.New("agentapi: GET /storage: HTTP 401"), ReasonUnauthorized, "agent_channel_unauthorized", "error"},
+		{"dns", false, errors.New(`agentapi: GET /storage: Get "https://x": dial tcp: lookup x: no such host`), ReasonMisconfigured, "agent_channel_misconfigured", "error"},
+		{"construction", true, errors.New("agentapi: fingerprint must be a SHA-256 (64 hex chars), got 10"), ReasonConstruction, "agent_channel_construction_error", "error"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			sink := &fakeSink{}
+			c := newChecker(t, sink)
+			p := &scriptedProbe{steps: []struct {
+				cons bool
+				err  error
+			}{step(false, nil), step(tc.cons, tc.err)}} // seed up → down
+			run(c, p)
+			if len(sink.downs) != 1 {
+				t.Fatalf("want 1 down alert, got %d", len(sink.downs))
+			}
+			d := sink.downs[0]
+			if d.reason != tc.reason || d.eventType != tc.eventType || d.severity != tc.severity {
+				t.Fatalf("got %+v, want reason=%s event=%s sev=%s", d, tc.reason, tc.eventType, tc.severity)
+			}
+			if !sink.dashDown {
+				t.Errorf("dashboard should be down")
+			}
+		})
+	}
+}
+
+// §7-C debounce RED-PROOF: a single connection-refused (the ~1s agent-restart blip) must NOT alert;
+// TWO consecutive must alert exactly once. A no-debounce impl fires on the single blip → fails this.
+func TestDebounce_TransientBlipSuppressed(t *testing.T) {
+	refused := errors.New(`agentapi: GET /storage: Get "https://x": dial tcp: connect: connection refused`)
+
+	// One refused sandwiched by up: NO alert.
+	sink := &fakeSink{}
+	c := newChecker(t, sink)
+	run(c, &scriptedProbe{steps: []struct {
+		cons bool
+		err  error
+	}{step(false, nil), step(false, refused), step(false, nil)}})
+	if len(sink.downs) != 0 {
+		t.Fatalf("single transient blip must NOT alert, got %d", len(sink.downs))
+	}
+	if sink.dashDown {
+		t.Errorf("single blip must not flip the dashboard down")
+	}
+
+	// Two consecutive refused: exactly ONE alert.
+	sink2 := &fakeSink{}
+	c2 := newChecker(t, sink2)
+	run(c2, &scriptedProbe{steps: []struct {
+		cons bool
+		err  error
+	}{step(false, nil), step(false, refused), step(false, refused)}})
+	if len(sink2.downs) != 1 || sink2.downs[0].reason != ReasonUnreachable {
+		t.Fatalf("two consecutive refused → want 1 unreachable alert, got %+v", sink2.downs)
+	}
+}
+
+// §7-A/B: up→down alerts once (not per cycle); §7: down→up recovers.
+func TestTransitions_NoDuplicate_AndRecovery(t *testing.T) {
+	pin := errors.New("agentapi: GET /storage: ...: agentapi: TLS pin mismatch: ...")
+	sink := &fakeSink{}
+	c := newChecker(t, sink)
+	// seed up, then 3× down (same reason), then up.
+	run(c, &scriptedProbe{steps: []struct {
+		cons bool
+		err  error
+	}{step(false, nil), step(false, pin), step(false, pin), step(false, pin), step(false, nil)}})
+	if len(sink.downs) != 1 {
+		t.Fatalf("steady down must alert once, got %d", len(sink.downs))
+	}
+	if sink.recovered != 1 {
+		t.Fatalf("want 1 recovered, got %d", sink.recovered)
+	}
+	if sink.dashDown {
+		t.Errorf("dashboard should be cleared after recovery")
+	}
+}
+
+// §7-E: a reason change (unreachable→pin_mismatch) re-alerts.
+func TestReasonChange_ReAlerts(t *testing.T) {
+	refused := errors.New("...: connect: connection refused")
+	pin := errors.New("...: agentapi: TLS pin mismatch: ...")
+	sink := &fakeSink{}
+	c := newChecker(t, sink)
+	// seed up; 2× refused (confirm unreachable → alert 1); then pin (reason change → alert 2).
+	run(c, &scriptedProbe{steps: []struct {
+		cons bool
+		err  error
+	}{step(false, nil), step(false, refused), step(false, refused), step(false, pin)}})
+	if len(sink.downs) != 2 {
+		t.Fatalf("reason change should re-alert (want 2), got %d: %+v", len(sink.downs), sink.downs)
+	}
+	if sink.downs[0].reason != ReasonUnreachable || sink.downs[1].reason != ReasonPinMismatch {
+		t.Fatalf("want unreachable then pin_mismatch, got %+v", sink.downs)
+	}
+}
+
+// §9.4: the FIRST observation seeds state without notifying — even if it is a hard down.
+func TestFirstObservation_SeedsNoAlert(t *testing.T) {
+	pin := errors.New("...: agentapi: TLS pin mismatch: ...")
+	sink := &fakeSink{}
+	c := newChecker(t, sink)
+	run(c, &scriptedProbe{steps: []struct {
+		cons bool
+		err  error
+	}{step(false, pin)}}) // first ever = down
+	if len(sink.downs) != 0 {
+		t.Fatalf("first observation must not notify, got %d", len(sink.downs))
+	}
+	if !sink.dashDown {
+		t.Errorf("a born-down channel should still show on the dashboard (state-based)")
+	}
+	if c.State() != "down:pin_mismatch" {
+		t.Errorf("state = %s, want down:pin_mismatch", c.State())
+	}
+}
+
+// The checker only uses the injected probe seam (never a fresh agentapi.New).
+func TestProbe_SeamOnly(t *testing.T) {
+	sink := &fakeSink{}
+	c := newChecker(t, sink)
+	p := &scriptedProbe{steps: []struct {
+		cons bool
+		err  error
+	}{step(false, nil)}}
+	run(c, p)
+	if p.calls != 1 {
+		t.Fatalf("checker should call the probe seam exactly once, got %d", p.calls)
+	}
+}

controller/internal/notify/notifier.go

@@ -335,6 +335,26 @@ func (n *Notifier) NotifyStorageReconnected(label string) {
 		fmt.Sprintf("Meghajtó újra csatlakoztatva: %s", label), StorageDetails{Label: label})
 }
 
+// AgentChannelDetails carries the classified reason for a controller→agent channel-down event.
+type AgentChannelDetails struct {
+	Reason string `json:"reason"`
+}
+
+// NotifyAgentChannelDown sends an OPERATOR-facing controller→agent channel-down event (the message is
+// English — the customer can't act on "the agent re-keyed", so this never reaches them: the event type
+// is not a customer notification toggle, same as the host_* operator events). eventType/severity/
+// message come from the channelhealth classifier (spike Q1 map).
+func (n *Notifier) NotifyAgentChannelDown(reason, eventType, severity, message string) {
+	n.PushEvent(eventType, severity, message, AgentChannelDetails{Reason: reason})
+}
+
+// NotifyAgentChannelRecovered sends the recovery event (info → logged, no email, mirrors
+// storage_reconnected).
+func (n *Notifier) NotifyAgentChannelRecovered() {
+	n.PushEvent("agent_channel_recovered", "info",
+		"Controller→agent channel recovered — local-API reachable again.", nil)
+}
+
 // NotifyAppDeployed sends an app deployment event.
 func (n *Notifier) NotifyAppDeployed(stackName, displayName string) {
 	n.PushEvent("app_deployed", "info",
@@ -630,4 +650,3 @@ func statusRank(status string) int {
 		return 0
 	}
 }
-

controller/internal/web/agent_disk_handlers.go

@@ -1,6 +1,7 @@
 package web
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"net/http"
@@ -59,6 +60,22 @@ func (s *Server) agentClient() (*agentapi.Client, error) {
 	return s.agentCli, s.agentCliErr
 }
 
+// ProbeAgentChannel runs one controller→agent channel health probe using the PRODUCTION memoized
+// client (NOT a fresh one — spike SPIKE-controller-agent-channel-health-2026-06-29: it self-heals,
+// reflects exactly what the disk UI sees, and avoids the per-call transport leak the singleton fixed).
+// It returns whether the failure was a CONSTRUCTION error (agentClient() couldn't build — a latching
+// config fault, distinct from a runtime channel failure) and the error (nil = channel up). The probe
+// is GET /storage (cheap, read-only — the same call probeLocalAPI uses at startup). This is the
+// channelhealth.Probe seam.
+func (s *Server) ProbeAgentChannel(ctx context.Context) (constructionErr bool, err error) {
+	client, cerr := s.agentClient()
+	if cerr != nil {
+		return true, cerr
+	}
+	_, serr := client.Storage(ctx)
+	return false, serr
+}
+
 // writeDiskJSON writes the standard {ok,data,error} envelope used by the disk API.
 func writeDiskJSON(w http.ResponseWriter, status int, ok bool, errMsg string, data interface{}) {
 	w.Header().Set("Content-Type", "application/json")

controller/internal/web/alerts.go

@@ -33,6 +33,10 @@ type AlertManager struct {
 	alerts          []Alert
 	logger          *log.Logger
 	hubPushStatusFn func() HubPushStatusData
+	// agentChannelAlert is set/cleared by the channel-health checker (out-of-band from Refresh, which
+	// is health-report-driven). nil = channel up. It is prepended in GetAlerts so a dead controller→
+	// agent link (disk/storage UI broken) is always visible regardless of the health-report cycle.
+	agentChannelAlert *Alert
 }
 
 // NewAlertManager creates a new AlertManager.
@@ -49,6 +53,25 @@ func (am *AlertManager) SetHubPushStatus(fn func() HubPushStatusData) {
 	am.mu.Unlock()
 }
 
+// SetAgentChannelAlert sets (down=true) or clears (down=false) the controller→agent channel-down
+// dashboard banner. Called by the channel-health checker each probe (idempotent). msg is the short
+// Hungarian display line.
+func (am *AlertManager) SetAgentChannelAlert(down bool, msg string) {
+	am.mu.Lock()
+	defer am.mu.Unlock()
+	if !down {
+		am.agentChannelAlert = nil
+		return
+	}
+	am.agentChannelAlert = &Alert{
+		ID:       "agent-channel-down",
+		Level:    "error",
+		Message:  msg,
+		Link:     "/settings",
+		LinkText: "Beállítások",
+	}
+}
+
 // Refresh regenerates alerts from the latest health check report and config state.
 // Called after each health check cycle (every 5 minutes) and on storage state changes.
 func (am *AlertManager) Refresh(report *monitor.HealthReport, cfg *config.Config, backupMgr *backup.Manager, updateAvailable bool, latestVersion string, storagePaths ...[]settings.StoragePath) {
@@ -159,7 +182,7 @@ func (am *AlertManager) GetAlerts(excludeIDs ...string) []Alert {
 	am.mu.RLock()
 	defer am.mu.RUnlock()
 
-	if len(am.alerts) == 0 {
+	if len(am.alerts) == 0 && am.agentChannelAlert == nil {
 		return nil
 	}
 
@@ -169,6 +192,10 @@ func (am *AlertManager) GetAlerts(excludeIDs ...string) []Alert {
 	}
 
 	var result []Alert
+	// Channel-down is prepended (highest priority — the agent link being dead breaks disk/storage UI).
+	if am.agentChannelAlert != nil && !exclude[am.agentChannelAlert.ID] {
+		result = append(result, *am.agentChannelAlert)
+	}
 	for _, a := range am.alerts {
 		if exclude[a.ID] {
 			continue
@@ -239,4 +266,3 @@ func countLevel(alerts []Alert, level string) int {
 	}
 	return n
 }
-