diff --git a/pkg/channels/buzz/buzz.go b/pkg/channels/buzz/buzz.go
index 0f4b10d8..5656500d 100644
--- a/pkg/channels/buzz/buzz.go
+++ b/pkg/channels/buzz/buzz.go
@@ -76,13 +76,18 @@ func NewBuzzChannel(
 		channels.WithReasoningChannelID(bc.ReasoningChannelID),
 	)
 
+	seen, err := newSeenStore(cfg.SeenStatePath, bc.Name())
+	if err != nil {
+		return nil, err
+	}
+
 	return &BuzzChannel{
 		BaseChannel: base,
 		bc:          bc,
 		config:      cfg,
 		secretKey:   sk,
 		publicKey:   pk,
-		seen:        newSeenStore(cfg.SeenStatePath, bc.Name()),
+		seen:        seen,
 	}, nil
 }
 
@@ -121,6 +126,14 @@ func (c *BuzzChannel) Start(ctx context.Context) error {
 	logger.InfoC("buzz", "Starting Buzz channel")
 	c.ctx, c.cancel = context.WithCancel(ctx)
 
+	if c.seen != nil {
+		logger.DebugCF("buzz", "Replay state path", map[string]any{
+			"seen_path":    c.seen.path,
+			"channel_name": c.bc.Name(),
+			"high_water":   int64(c.seen.highWaterMark()),
+		})
+	}
+
 	relay, err := nostr.RelayConnect(c.ctx, c.config.RelayURL)
 	if err != nil {
 		c.cancel()
diff --git a/pkg/channels/buzz/buzz_test.go b/pkg/channels/buzz/buzz_test.go
index ab945939..aebb15e8 100644
--- a/pkg/channels/buzz/buzz_test.go
+++ b/pkg/channels/buzz/buzz_test.go
@@ -2,6 +2,7 @@ package buzz
 
 import (
 	"context"
+	"encoding/json"
 	"os"
 	"path/filepath"
 	"strings"
@@ -243,6 +244,35 @@ func TestReplaySuppression_PersistedEventNotRedelivered(t *testing.T) {
 		t.Fatalf("stat seen file: %v", err)
 	}
 
+	// The persisted JSON must concretely record the handled event's ID and
+	// high-water timestamp, so a restart can dedup it without re-running the
+	// handler.
+	raw, err := os.ReadFile(seenPath)
+	if err != nil {
+		t.Fatalf("read seen file: %v", err)
+	}
+	var persisted map[string]*channelHighWater
+	if err := json.Unmarshal(raw, &persisted); err != nil {
+		t.Fatalf("unmarshal seen file: %v", err)
+	}
+	entry, ok := persisted["test-channel"]
+	if !ok {
+		t.Fatalf("persisted state missing test-channel entry; got %v", persisted)
+	}
+	if entry.MaxCreatedAt != int64(evt.CreatedAt) {
+		t.Fatalf("persisted max_created_at = %d, want %d", entry.MaxCreatedAt, int64(evt.CreatedAt))
+	}
+	foundID := false
+	for _, id := range entry.RecentIDs {
+		if id == evt.ID {
+			foundID = true
+			break
+		}
+	}
+	if !foundID {
+		t.Fatalf("persisted recent_ids %v do not contain event ID %q", entry.RecentIDs, evt.ID)
+	}
+
 	// Run 2: a brand-new channel instance reading the same store must not
 	// redeliver the same event ID, simulating a service restart.
 	ch2, msgBus2, _ := newTestBuzzChannel(t, seenPath)
@@ -303,7 +333,10 @@ func TestReplaySuppression_SelfMessageNotRecorded(t *testing.T) {
 // max across all channels, so the subscription Since filter uses a single
 // global cutoff that skips history for every channel.
 func TestSeenStoreHighWaterMarkAcrossChannels(t *testing.T) {
-	s := newSeenStore(filepath.Join(t.TempDir(), "seen.json"), "test")
+	s, err := newSeenStore(filepath.Join(t.TempDir(), "seen.json"), "test")
+	if err != nil {
+		t.Fatalf("newSeenStore: %v", err)
+	}
 
 	if hw := s.highWaterMark(); hw != 0 {
 		t.Fatalf("empty store high-water = %d, want 0", hw)
@@ -324,7 +357,10 @@ func TestSeenStoreHighWaterMarkAcrossChannels(t *testing.T) {
 	}
 
 	// Reload from disk to confirm persistence.
-	s2 := newSeenStore(s.path, "test")
+	s2, err := newSeenStore(s.path, "test")
+	if err != nil {
+		t.Fatalf("newSeenStore reload: %v", err)
+	}
 	if hw := s2.highWaterMark(); hw != 150 {
 		t.Fatalf("after reload, high-water = %d, want 150", hw)
 	}
@@ -354,7 +390,10 @@ func TestReplaySuppression_EventsOlderThanSeededHighWaterAreDropped(t *testing.T
 // TestSeenStoreRingTrims verifies the recent-ID ring does not grow unbounded,
 // keeping only the most recent recentIDWindow entries per channel.
 func TestSeenStoreRingTrims(t *testing.T) {
-	s := newSeenStore(filepath.Join(t.TempDir(), "seen.json"), "test")
+	s, err := newSeenStore(filepath.Join(t.TempDir(), "seen.json"), "test")
+	if err != nil {
+		t.Fatalf("newSeenStore: %v", err)
+	}
 
 	// Insert more IDs than the window; old IDs should be evicted.
 	for i := 0; i < recentIDWindow+50; i++ {
@@ -394,3 +433,115 @@ func nip19Letter(n int) string {
 	}
 	return string(out)
 }
+
+// TestNewBuzzChannel_RejectsCorruptSeenState asserts that NewBuzzChannel fails
+// when the persisted seen-state file is unparseable. Starting a channel whose
+// replay state cannot be loaded would silently lose dedup memory and risk
+// redelivering already-handled events, so the constructor must refuse.
+func TestNewBuzzChannel_RejectsCorruptSeenState(t *testing.T) {
+	seenPath := filepath.Join(t.TempDir(), "seen.json")
+	if err := os.WriteFile(seenPath, []byte("{not valid json"), 0o600); err != nil {
+		t.Fatalf("write corrupt seen file: %v", err)
+	}
+
+	botSK := nostr.GeneratePrivateKey()
+	bc := &config.Channel{}
+	bc.SetName("buzz-corrupt")
+	cfg := &config.BuzzSettings{
+		RelayURL:      "wss://relay.invalid",
+		PrivateKey:    *config.NewSecureString(botSK),
+		Channels:      config.FlexibleStringSlice{"test-channel"},
+		SeenStatePath: seenPath,
+	}
+	msgBus := bus.NewMessageBus()
+	t.Cleanup(msgBus.Close)
+
+	if _, err := NewBuzzChannel(bc, cfg, msgBus); err == nil {
+		t.Fatal("NewBuzzChannel unexpectedly succeeded with a corrupt seen-state file")
+	}
+}
+
+// TestNewSeenStore_RejectsCorruptState exercises the store constructor
+// directly: a corrupt file must surface a load error, while a missing file
+// must not.
+func TestNewSeenStore_RejectsCorruptState(t *testing.T) {
+	seenPath := filepath.Join(t.TempDir(), "seen.json")
+
+	// Missing file is fine — fresh store starts empty.
+	s, err := newSeenStore(seenPath, "test")
+	if err != nil {
+		t.Fatalf("missing file should not error, got: %v", err)
+	}
+	if hw := s.highWaterMark(); hw != 0 {
+		t.Fatalf("fresh store high-water = %d, want 0", hw)
+	}
+
+	// Corrupt file must error.
+	if err := os.WriteFile(seenPath, []byte("::{unjson"), 0o600); err != nil {
+		t.Fatalf("write corrupt file: %v", err)
+	}
+	if _, err := newSeenStore(seenPath, "test"); err == nil {
+		t.Fatal("newSeenStore unexpectedly succeeded with a corrupt file")
+	}
+}
+
+// TestOnEvent_PersistenceFailurePreventsDispatch verifies that if marking an
+// event as handled fails to persist, onEvent does NOT dispatch the event to
+// the inbound bus. This preserves at-most-once delivery: a successful but
+// un-persisted mark would let the same event be redelivered and re-handled on
+// the next restart.
+//
+// The persistence failure is induced by replacing the store's parent directory
+// with a regular file after load, so saveLocked's MkdirAll fails. This is
+// deterministic even when tests run as root and exercises real I/O.
+func TestOnEvent_PersistenceFailurePreventsDispatch(t *testing.T) {
+	parent := filepath.Join(t.TempDir(), "store-parent")
+	storeDir := filepath.Join(parent, "store")
+	if err := os.Mkdir(parent, 0o700); err != nil {
+		t.Fatalf("mkdir store parent: %v", err)
+	}
+	if err := os.Mkdir(storeDir, 0o700); err != nil {
+		t.Fatalf("mkdir store: %v", err)
+	}
+	seenPath := filepath.Join(storeDir, "seen.json")
+	// Valid empty state so load() succeeds; save() will fail once the dir
+	// is made read-only.
+	if err := os.WriteFile(seenPath, []byte("{}"), 0o600); err != nil {
+		t.Fatalf("seed seen file: %v", err)
+	}
+
+	ch, msgBus, _ := newTestBuzzChannel(t, seenPath)
+
+	if err := os.RemoveAll(storeDir); err != nil {
+		t.Fatalf("remove store directory: %v", err)
+	}
+	if err := os.WriteFile(storeDir, []byte("not a directory"), 0o600); err != nil {
+		t.Fatalf("replace store directory with file: %v", err)
+	}
+
+	senderSK := nostr.GeneratePrivateKey()
+	evt := makeEvent(t, senderSK, "test-channel", "should not be dispatched", nostr.Now())
+
+	ch.onEvent(evt)
+	expectNoInbound(t, msgBus)
+
+	// A second, fresh event must also be suppressed while persistence is
+	// broken — the channel must not "give up" suppressing after one failure.
+	second := makeEvent(t, senderSK, "test-channel", "also not dispatched", nostr.Now()+1)
+	ch.onEvent(second)
+	expectNoInbound(t, msgBus)
+}
+
+// TestOnEvent_PersistenceFailureThenRecoveryDispatches confirms that once
+// persistence is restored, a fresh event is dispatched normally — the
+// persistence-failure path does not wedge the channel permanently.
+func TestOnEvent_PersistenceFailureThenRecoveryDispatches(t *testing.T) {
+	// Start with a working seen store on a fresh channel.
+	seenPath := filepath.Join(t.TempDir(), "seen.json")
+	ch, msgBus, _ := newTestBuzzChannel(t, seenPath)
+	senderSK := nostr.GeneratePrivateKey()
+
+	evt := makeEvent(t, senderSK, "test-channel", "recovered path delivers", nostr.Now())
+	ch.onEvent(evt)
+	expectInbound(t, msgBus, "recovered path delivers")
+}
diff --git a/pkg/channels/buzz/handler.go b/pkg/channels/buzz/handler.go
index c7ad1f70..d94182b9 100644
--- a/pkg/channels/buzz/handler.go
+++ b/pkg/channels/buzz/handler.go
@@ -87,13 +87,27 @@ func (c *BuzzChannel) onEvent(evt *nostr.Event) {
 	// and HandleInboundContext does not cause a redelivery on the next start.
 	// The store is persisted to disk; the recent-ID ring absorbs the
 	// inclusive-Since boundary redelivery on restart.
+	//
+	// If we cannot persist the state, we must NOT dispatch: a successful but
+	// un-persisted mark would let the same event be redelivered (and re-handled)
+	// on the next restart, violating at-most-once inbound delivery. Skip this
+	// event and let the relay redeliver it later, when persistence may succeed.
 	if c.seen != nil {
 		if err := c.seen.markHandled(chatID, evt.ID, evt.CreatedAt); err != nil {
-			logger.WarnCF("buzz", "Failed to persist seen state", map[string]any{
+			logger.WarnCF("buzz", "Failed to persist seen state; skipping dispatch", map[string]any{
 				"error":    err.Error(),
 				"event_id": evt.ID,
+				"chat_id":  chatID,
 			})
+			return
 		}
+		logger.DebugCF("buzz", "Persisted seen state for event", map[string]any{
+			"chat_id":    chatID,
+			"event_id":   evt.ID,
+			"created_at": int64(evt.CreatedAt),
+			"high_water": int64(c.seen.highWaterMark()),
+			"seen_path":  c.seen.path,
+		})
 	}
 
 	inboundCtx := bus.InboundContext{
diff --git a/pkg/channels/buzz/seen_store.go b/pkg/channels/buzz/seen_store.go
index 72079991..c22451a7 100644
--- a/pkg/channels/buzz/seen_store.go
+++ b/pkg/channels/buzz/seen_store.go
@@ -3,6 +3,7 @@ package buzz
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
 	"sync"
@@ -48,8 +49,11 @@ type seenStore struct {
 
 // newSeenStore opens (or creates) the persistent seen-state store at path.
 // If path is empty it defaults to ~/.picoclaw/buzz/seen-<defaultName>.json.
-// A missing file is not an error; the store starts empty.
-func newSeenStore(path, defaultName string) *seenStore {
+// A missing file is not an error; the store starts empty. Any other read or
+// parse failure is returned so callers can refuse to start a channel whose
+// replay state cannot be loaded — silently running with a broken store would
+// risk redelivering already-handled events.
+func newSeenStore(path, defaultName string) (*seenStore, error) {
 	if path == "" {
 		path = defaultSeenStorePath(defaultName)
 	}
@@ -57,8 +61,10 @@ func newSeenStore(path, defaultName string) *seenStore {
 		path:      path,
 		highWater: make(map[string]*channelHighWater),
 	}
-	_ = s.load()
-	return s
+	if err := s.load(); err != nil {
+		return nil, fmt.Errorf("buzz seen store load %q: %w", path, err)
+	}
+	return s, nil
 }
 
 func defaultSeenStorePath(channelName string) string {