diff --git a/service.go b/service.go
index 4b7d524..a295f24 100644
--- a/service.go
+++ b/service.go
@@ -37,6 +37,11 @@ type ServiceConfig struct {
 	// misbehaving peer or sustained inbound load fills the operator's
 	// disk indefinitely.
 	InboxMaxFiles int
+	// InboxMaxBytes caps the total on-disk bytes used by the inbox.
+	// When > 0, saveInboxMessage checks the accumulated size before
+	// every write and evictExpandOverflow uses bytes instead of file
+	// count. Zero ⇒ no byte cap (backward-compatible).
+	InboxMaxBytes int64
 }
 
 // inboxEvictCheckEvery: only run the eviction-scan once every N saves
@@ -259,6 +264,35 @@ func (s *Service) saveInboxMessage(frame *Frame, from protocol.Addr) error {
 		return fmt.Errorf("mkdir: %w", err)
 	}
 
+	// Byte-budget check: if InboxMaxBytes is set, confirm there is room
+	// BEFORE writing. Evict if over, then re-check.
+	if s.cfg.InboxMaxBytes > 0 {
+		current, _ := inboxTotalBytes(dir)
+		// Estimate the JSON overhead: type, from, data, bytes, received_at,
+		// and possibly data_b64.
+		estimated := int64(len(frame.Payload)) + 256
+		if current+estimated > s.cfg.InboxMaxBytes {
+			s.evictInboxOverflowByBytes(dir)
+			after, _ := inboxTotalBytes(dir)
+			if after+estimated > s.cfg.InboxMaxBytes {
+				slog.Warn("inbox byte budget exceeded after eviction",
+					"current_bytes", after,
+					"max_bytes", s.cfg.InboxMaxBytes,
+					"frame_bytes", len(frame.Payload))
+				if s.deps.Events != nil {
+					s.deps.Events.Publish("inbox.full", map[string]any{
+						"from":         from.String(),
+						"type":         TypeName(frame.Type),
+						"frame_bytes":  len(frame.Payload),
+						"max_bytes":    s.cfg.InboxMaxBytes,
+					})
+				}
+				return fmt.Errorf("inbox byte budget exceeded: %d + %d > %d",
+					after, estimated, s.cfg.InboxMaxBytes)
+			}
+		}
+	}
+
 	ts := time.Now()
 	msg := map[string]interface{}{
 		"type":        TypeName(frame.Type),
@@ -298,10 +332,16 @@ func (s *Service) saveInboxMessage(frame *Frame, from protocol.Addr) error {
 }
 
 // evictInboxOverflow trims the inbox to at most cfg.InboxMaxFiles by
-// deleting the oldest files (by mtime). Best-effort: I/O errors are
-// logged and the loop continues. Called periodically from
-// saveInboxMessage.
+// deleting the oldest files (by mtime). When InboxMaxBytes > 0, the
+// eviction target is total bytes rather than file count. Best-effort:
+// I/O errors are logged and the loop continues. Called periodically
+// from saveInboxMessage.
 func (s *Service) evictInboxOverflow(dir string) {
+	// Byte-based eviction when InboxMaxBytes is configured.
+	if s.cfg.InboxMaxBytes > 0 {
+		s.evictInboxOverflowByBytes(dir)
+		return
+	}
 	maxFiles := s.cfg.InboxMaxFiles
 	if maxFiles <= 0 {
 		maxFiles = 10000
@@ -340,3 +380,66 @@ func (s *Service) evictInboxOverflow(dir string) {
 	}
 	slog.Info("inbox eviction", "dir", dir, "evicted", toEvict, "remaining", maxFiles)
 }
+
+// evictInboxOverflowByBytes trims total inbox size to InboxMaxBytes.
+func (s *Service) evictInboxOverflowByBytes(dir string) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		slog.Debug("inbox evict: readdir", "dir", dir, "err", err)
+		return
+	}
+	type aged struct {
+		name string
+		mod  time.Time
+		size int64
+	}
+	files := make([]aged, 0, len(entries))
+	var total int64
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		info, err := e.Info()
+		if err != nil {
+			continue
+		}
+		sz := info.Size()
+		files = append(files, aged{name: e.Name(), mod: info.ModTime(), size: sz})
+		total += sz
+	}
+	if total <= s.cfg.InboxMaxBytes {
+		return
+	}
+	// Oldest first.
+	sort.Slice(files, func(i, j int) bool { return files[i].mod.Before(files[j].mod) })
+	evicted := 0
+	for i := 0; i < len(files) && total > s.cfg.InboxMaxBytes; i++ {
+		p := filepath.Join(dir, files[i].name)
+		if err := os.Remove(p); err != nil {
+			continue
+		}
+		total -= files[i].size
+		evicted++
+	}
+	slog.Info("inbox eviction (bytes)", "dir", dir, "evicted", evicted, "total_bytes_after", total, "max_bytes", s.cfg.InboxMaxBytes)
+}
+
+// inboxTotalBytes sums the on-disk size of all regular files in dir.
+func inboxTotalBytes(dir string) (int64, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return 0, err
+	}
+	var total int64
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		info, err := e.Info()
+		if err != nil {
+			continue
+		}
+		total += info.Size()
+	}
+	return total, nil
+}
diff --git a/zz_service_errors_test.go b/zz_service_errors_test.go
index 718d2d0..42beb64 100644
--- a/zz_service_errors_test.go
+++ b/zz_service_errors_test.go
@@ -424,6 +424,108 @@ func (e *capturingEvents) Subscribe(pattern string) (<-chan coreapi.Event, func(
 	return ch, func() {}
 }
 
+// ---- PILOT-276: inbox byte-budget cap ----------------------------------
+
+// TestSaveInboxMessage_ByteBudgetEnforced verifies that when InboxMaxBytes is
+// set, saveInboxMessage rejects writes that would exceed the budget.
+func TestSaveInboxMessage_ByteBudgetEnforced(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	s := NewService(ServiceConfig{InboxDir: tmp, InboxMaxBytes: 512})
+
+	// First message (small) must succeed.
+	frame := &Frame{Type: TypeText, Payload: []byte("hello")}
+	if err := s.saveInboxMessage(frame, protocol.Addr{Node: 1}); err != nil {
+		t.Fatalf("first save under budget must succeed: %v", err)
+	}
+
+	// Second message — the inbox JSON overhead + data should push over 512.
+	// Force the test by writing a payload that fills the rest of the budget.
+	// After the first save, let's check how many files exist.
+	entries, _ := os.ReadDir(tmp)
+	t.Logf("entries after first save: %d", len(entries))
+
+	// Save enough messages to exceed 512 bytes total.
+	for i := 0; i < 20; i++ {
+		frame := &Frame{Type: TypeText, Payload: []byte(strings.Repeat("x", 50))}
+		err := s.saveInboxMessage(frame, protocol.Addr{Node: 2})
+		if err != nil {
+			t.Logf("saveInboxMessage #%d failed as expected: %v", i+2, err)
+			return // success — budget enforced
+		}
+	}
+	t.Error("expected saveInboxMessage to eventually reject when over byte budget")
+}
+
+// TestSaveInboxMessage_NoByteBudget_Unbounded documents that with
+// InboxMaxBytes unset (zero), old behaviour is preserved — writes are
+// never rejected on byte count.
+func TestSaveInboxMessage_NoByteBudget_Unbounded(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	s := NewService(ServiceConfig{InboxDir: tmp}) // InboxMaxBytes=0 (unset)
+
+	for i := 0; i < 5; i++ {
+		frame := &Frame{Type: TypeText, Payload: []byte(strings.Repeat("y", 200))}
+		if err := s.saveInboxMessage(frame, protocol.Addr{Node: 3}); err != nil {
+			t.Fatalf("save #%d with no byte budget must not reject: %v", i+1, err)
+		}
+	}
+	entries, _ := os.ReadDir(tmp)
+	if len(entries) != 5 {
+		t.Errorf("got %d files, want 5", len(entries))
+	}
+}
+
+// TestEvictInboxOverflow_ByteBasedEvictsOldest verifies that when
+// InboxMaxBytes is set, evictInboxOverflow uses total bytes (not file
+// count) to decide what to delete.
+func TestEvictInboxOverflow_ByteBasedEvictsOldest(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	s := NewService(ServiceConfig{InboxDir: tmp, InboxMaxBytes: 200})
+
+	now := time.Now()
+	// Write 3 files totalling > 200 bytes. Oldest should get evicted.
+	files := []struct {
+		name    string
+		content string
+		age     time.Duration
+	}{
+		{"old.json", strings.Repeat("a", 100), -10 * time.Second},
+		{"mid.json", strings.Repeat("b", 80), -5 * time.Second},
+		{"new.json", strings.Repeat("c", 60), -1 * time.Second},
+	}
+	for _, f := range files {
+		p := filepath.Join(tmp, f.name)
+		if err := os.WriteFile(p, []byte(f.content), 0600); err != nil {
+			t.Fatalf("write: %v", err)
+		}
+		mt := now.Add(f.age)
+		_ = os.Chtimes(p, mt, mt)
+	}
+
+	s.evictInboxOverflow(tmp)
+
+	entries, _ := os.ReadDir(tmp)
+	names := make([]string, len(entries))
+	for i, e := range entries {
+		names[i] = e.Name()
+	}
+	t.Logf("remaining entries: %v", names)
+
+	// old.json (oldest, 100 bytes) may have been evicted to get under 200.
+	// At a minimum, the total remaining bytes must not exceed 200.
+	var total int64
+	for _, e := range entries {
+		info, _ := e.Info()
+		total += info.Size()
+	}
+	if total > 200 {
+		t.Errorf("total bytes after eviction = %d, want <= 200", total)
+	}
+}
+
 // Ensure capturingEvents satisfies coreapi.EventBus at compile time.
 var _ coreapi.EventBus = (*capturingEvents)(nil)