fix(websocket): send resource deltas after initial state

Refs #1601
Refs #1665
Refs #1497

docs/release-control/v6/internal/subsystems/alerts.md

@@ -216,6 +216,7 @@ acknowledgement, history, and recovery reuse the normal alert pipeline.
 73. `frontend-modern/src/utils/alertsActivation.ts`
 74. `internal/operationaltrust/contracts.go`
 75. `internal/alerts/operational_contract.go`
+76. `internal/alerts/issue1497_test.go`
 
 ## Shared Boundaries
 

docs/release-control/v6/internal/subsystems/api-contracts.md

@@ -88,6 +88,11 @@ an enabled external-probe assignment.
 11. `pkg/pulsecli/root.go`
 12. `frontend-modern/src/types/api.ts`
     12a. `frontend-modern/src/types/operationalTrust.ts`
+    12b. `frontend-modern/src/stores/websocket.ts`
+    12c. `frontend-modern/src/stores/__tests__/websocket-unified.test.ts`
+    12d. `internal/websocket/hub.go`
+    12e. `internal/websocket/state_delta.go`
+    12f. `internal/websocket/state_delta_test.go`
 13. `frontend-modern/src/types/actionAudit.ts`
 14. `frontend-modern/src/api/actionAudit.ts`
     14a. `frontend-modern/src/api/patrolAttention.ts`
@@ -291,12 +296,20 @@ not a parallel API payload contract. `internal/api/agent_handlers_base.go` and
 or retain full frontend-state payloads at the handler boundary. The WebSocket
 hub owns tenant-aware state resolution after coalescing, through the same state
 getter that backs the canonical `/api/state` payload. The hub must serialize
-whole-state resolution and JSON construction across coalesced broadcasts,
+whole-state resolution across coalesced broadcasts, initial-client delivery,
-initial-client delivery, and explicit client data requests; cancel delayed or
+and explicit client data requests. Initial hydration and explicit resync return
-queued work when its client leaves; and join every state producer before
+the complete canonical state; subsequent invalidation broadcasts must derive
-closing client channels. Reconnect, request, or invalidation churn must not
+ordered, per-client resource merge patches from that same resolved state,
-multiply concurrent clones of that canonical payload or race shutdown sends
+including explicit removals and resource order, instead of repeatedly
-against channel closure. A client's lifecycle cancellation signal must be
+serialising the whole resource graph. A delta may be sent only after that
+client has accepted its full baseline, and the baseline may advance only after
+the ordered client queue accepts the delta. The frontend must apply JSON Merge
+Patch deletion semantics, preserve unchanged resource fields, and retain
+support for complete `rawData` snapshots during resync. The hub must cancel
+delayed or queued work when its client leaves and join every state producer
+before closing client channels. Reconnect, request, or invalidation churn must
+not multiply concurrent clones of that canonical payload or race shutdown
+sends against channel closure. A client's lifecycle cancellation signal must be
 initialized and read through one synchronized owner before registration,
 requested-state work, delayed initial-state delivery, or disconnect may race
 over it; every path must observe the same stable signal. Ordinary unregister

frontend-modern/browser-verification.json

@@ -1,38 +1,42 @@
 {
   "version": 1,
-  "base_sha": "1b8bb4e91c905de19931634ab79e9e95960f811a",
+  "base_sha": "662bb4f1946606e51122bfe06fd2d82aa33c6a15",
-  "verified_at": "2026-08-05T23:31:48Z",
+  "verified_at": "2026-08-05T23:44:24Z",
   "result": "passed",
-  "changed_paths": ["frontend-modern/src/components/shared/InlineDetailTableRow.tsx"],
+  "changed_paths": [
+    "frontend-modern/src/stores/websocket.ts",
+    "frontend-modern/src/types/api.ts"
+  ],
   "content_sha256": {
-    "frontend-modern/src/components/shared/InlineDetailTableRow.tsx": "aeb0e3d0d5946d5dc5929d2d5ce9ae1a39d1c48fc8cc35db2332059d577d5c9e"
+    "frontend-modern/src/stores/websocket.ts": "056eb325939f188c6095273441fd3b48c0f02cdfb4dd0a5b044c43b73868301a",
+    "frontend-modern/src/types/api.ts": "895ab5aa5ee5e1162008be6c8da83377169941d25b1655d73a1042d91076697b"
   },
   "routes": [
-    "/standalone/machines",
+    "/proxmox/overview",
-    "/proxmox/backups?view=coverage",
+    "/truenas/overview"
-    "/proxmox/ceph",
-    "/proxmox/mail"
   ],
   "viewports": [
-    { "width": 1920, "height": 1080 },
+    {
-    { "width": 1280, "height": 820 },
+      "width": 1280,
-    { "width": 768, "height": 820 },
+      "height": 720
-    { "width": 390, "height": 820 },
+    },
-    { "width": 320, "height": 820 }
+    {
+      "width": 390,
+      "height": 844
+    }
   ],
   "states": [
-    "collapsed and expanded Agent machine resource details",
+    "full initial WebSocket state hydration",
-    "collapsed and expanded Proxmox backup coverage evidence",
+    "live resource merge-patch refresh after the initial baseline",
-    "auto-opened and closed single-cluster Ceph details",
+    "full WebSocket resync after a page reload",
-    "collapsed, expanded, and closed Mail Gateway details",
+    "Proxmox node disk-array rendering after a delta refresh",
-    "desktop, tablet, narrow-phone, and minimum-width responsive table columns"
+    "TrueNAS system and health-alert rendering after desktop and narrow delta refreshes"
   ],
   "interactions": [
-    "opened and closed the Agent machine resource drawer and confirmed focus returned to its disclosure",
+    "signed in to the current local build and waited for the initial Proxmox resource workspace to hydrate",
-    "expanded and collapsed backup coverage evidence and confirmed focus remained on its disclosure",
+    "waited through a live refresh and confirmed changing metrics rendered without disconnecting or logging browser errors",
-    "closed the auto-opened Ceph cluster drawer and confirmed focus returned to its disclosure",
+    "opened the TrueNAS workspace and confirmed its system and alert projections remained intact after delta application",
-    "opened and closed the Mail Gateway drawer and confirmed focus returned to its disclosure",
+    "reloaded the TrueNAS route and confirmed the complete resync restored a connected, populated workspace",
-    "measured every visible table at all five viewports and confirmed table width matched its container with zero document overflow",
+    "repeated TrueNAS initial hydration and live refresh at 390 by 844 and visually inspected the responsive result"
-    "visually inspected the final Mail Gateway table at 1920 and 320 pixels"
   ]
 }

frontend-modern/src/stores/__tests__/websocket-resilience.test.ts

@@ -162,6 +162,57 @@ describe('websocket store resilience', () => {
     }
   });
 
+  it('preserves active alert state when a resource-only delta arrives', async () => {
+    const { store, dispose } = await createStoreHarness();
+    try {
+      vi.advanceTimersByTime(1);
+      const alert = {
+        id: 'agent:host-1-cpu',
+        type: 'cpu',
+        level: 'warning',
+        resourceId: 'agent:host-1',
+        resourceName: 'host-1',
+        node: 'host-1',
+        instance: 'host-1',
+        message: 'CPU above threshold',
+        value: 90,
+        threshold: 80,
+        startTime: '2026-05-14T07:59:00Z',
+        acknowledged: false,
+      };
+      currentInstance!.onmessage?.({
+        data: JSON.stringify({
+          type: 'initialState',
+          data: {
+            connectedInfrastructure: [],
+            resources: [{ id: 'agent:host-1', type: 'agent', name: 'host-1' }],
+            activeAlerts: [alert],
+            recentlyResolved: [],
+            lastUpdate: Date.now(),
+          },
+        }),
+      } as MessageEvent);
+
+      currentInstance!.onmessage?.({
+        data: JSON.stringify({
+          type: 'rawData',
+          data: {
+            resourceDelta: {
+              upserts: [{ id: 'agent:host-1', cpu: { current: 91 } }],
+            },
+            lastUpdate: Date.now() + 1_000,
+          },
+        }),
+      } as MessageEvent);
+
+      expect(store.activeAlerts[alert.id]).toMatchObject(alert);
+      expect(store.state.activeAlerts).toHaveLength(1);
+      expect(store.state.resources[0]?.cpu?.current).toBe(91);
+    } finally {
+      dispose();
+    }
+  });
+
   it('manual reconnect avoids duplicate reconnect scheduling', async () => {
     const { store, dispose } = await createStoreHarness();
     try {

frontend-modern/src/stores/__tests__/websocket-unified.test.ts

@@ -254,6 +254,72 @@ describe('websocket store unified resource contract', () => {
     }
   });
 
+  it('applies resource merge patches without replacing unchanged resource data', async () => {
+    const { store, dispose } = await createStoreHarness();
+    try {
+      await waitForOpenTick();
+
+      emitMessage({
+        type: 'initialState',
+        data: {
+          connectedInfrastructure: [],
+          resources: [
+            {
+              id: 'agent-1',
+              type: 'agent',
+              name: 'agent-1',
+              status: 'online',
+              lastSeen: 100,
+              cpu: { current: 10 },
+              platformData: {
+                agent: { osName: 'Debian', agentVersion: '6.2.0' },
+                disks: [{ name: 'sda', usage: 10 }],
+              },
+            },
+            { id: 'vm-1', type: 'vm', name: 'vm-1', status: 'running' },
+          ],
+          lastUpdate: 100,
+          activeAlerts: [],
+          recentlyResolved: [],
+        },
+      });
+
+      emitMessage({
+        type: 'rawData',
+        data: {
+          lastUpdate: 200,
+          resourceDelta: {
+            upserts: [
+              {
+                id: 'agent-1',
+                lastSeen: 200,
+                cpu: { current: 42 },
+                status: null,
+                platformData: { disks: [{ name: 'sda', usage: 20 }] },
+              },
+              { id: 'vm-2', type: 'vm', name: 'vm-2', status: 'running' },
+            ],
+            removed: ['vm-1'],
+            order: ['vm-2', 'agent-1'],
+          },
+        },
+      });
+
+      expect(store.state.resources.map((resource) => resource.id)).toEqual(['vm-2', 'agent-1']);
+      const agent = store.state.resources[1];
+      expect(agent?.cpu?.current).toBe(42);
+      expect(agent?.lastSeen).toBe(200);
+      expect(agent?.platformData).toMatchObject({
+        agent: { osName: 'Debian', agentVersion: '6.2.0' },
+        disks: [{ name: 'sda', usage: 20 }],
+      });
+      expect(agent).not.toHaveProperty('status');
+      expect(store.state.lastUpdate).toBe(200);
+    } finally {
+      dispose();
+    }
+  });
+
   it('preserves connected infrastructure when raw updates omit that projection', async () => {
     const { store, dispose } = await createStoreHarness();
     try {

frontend-modern/src/stores/websocket.ts

@@ -7,6 +7,7 @@ import type {
   ResolvedAlert,
   ConnectedInfrastructureItem,
 } from '@/types/api';
+import type { Resource } from '@/types/resource';
 import { logger } from '@/utils/logger';
 import { POLLING_INTERVALS, WEBSOCKET } from '@/constants';
 import { notificationStore } from './notifications';
@@ -40,9 +41,75 @@ const shownAutoRegisterNotifications = new Map<string, number>();
 
 const asRecord = (value: unknown): Record<string, unknown> | undefined =>
   value && typeof value === 'object' ? (value as Record<string, unknown>) : undefined;
+const asMergePatchRecord = (value: unknown): Record<string, unknown> | undefined =>
+  value && typeof value === 'object' && !Array.isArray(value)
+    ? (value as Record<string, unknown>)
+    : undefined;
 const asString = (value: unknown): string | undefined =>
   typeof value === 'string' && value.trim().length > 0 ? value.trim() : undefined;
 
+const isInboundPayloadWithinLimit = (payload: string): boolean => {
+  // JSON state is overwhelmingly ASCII. Avoid allocating a second multi-megabyte
+  // Blob for normal messages, while retaining an exact UTF-8 check for strings
+  // large enough that non-ASCII code points could cross the byte limit.
+  if (payload.length <= Math.floor(MAX_INBOUND_WEBSOCKET_MESSAGE_BYTES / 3)) return true;
+  if (payload.length > MAX_INBOUND_WEBSOCKET_MESSAGE_BYTES) return false;
+  return new TextEncoder().encode(payload).byteLength <= MAX_INBOUND_WEBSOCKET_MESSAGE_BYTES;
+};
+
+const applyJSONMergePatch = (current: unknown, patch: unknown): unknown => {
+  const patchRecord = asMergePatchRecord(patch);
+  if (!patchRecord) return patch;
+
+  const result: Record<string, unknown> = { ...(asMergePatchRecord(current) ?? {}) };
+  Object.entries(patchRecord).forEach(([key, value]) => {
+    if (value === null) {
+      delete result[key];
+      return;
+    }
+    result[key] = asMergePatchRecord(value) ? applyJSONMergePatch(result[key], value) : value;
+  });
+  return result;
+};
+
+const applyResourceStateDelta = (
+  current: readonly Resource[],
+  delta: { upserts?: unknown; removed?: unknown; order?: unknown },
+): Resource[] => {
+  const resourcesById = new Map(current.map((resource) => [resource.id, resource] as const));
+  const removed = Array.isArray(delta.removed)
+    ? new Set(delta.removed.filter((id): id is string => typeof id === 'string'))
+    : new Set<string>();
+  removed.forEach((id) => resourcesById.delete(id));
+
+  const addedIds: string[] = [];
+  if (Array.isArray(delta.upserts)) {
+    delta.upserts.forEach((patch) => {
+      const id = asString(asRecord(patch)?.id);
+      if (!id) return;
+      if (!resourcesById.has(id)) addedIds.push(id);
+      const next = applyJSONMergePatch(resourcesById.get(id), patch) as Resource;
+      if (next.id === id) resourcesById.set(id, next);
+    });
+  }
+
+  const requestedOrder = Array.isArray(delta.order)
+    ? delta.order.filter((id): id is string => typeof id === 'string')
+    : undefined;
+  const order = requestedOrder ?? [
+    ...current.map((resource) => resource.id).filter((id) => !removed.has(id)),
+    ...addedIds,
+  ];
+  const ordered = order
+    .map((id) => resourcesById.get(id))
+    .filter((resource): resource is Resource => Boolean(resource));
+  const included = new Set(ordered.map((resource) => resource.id));
+  resourcesById.forEach((resource, id) => {
+    if (!included.has(id)) ordered.push(resource);
+  });
+  return ordered;
+};
+
 const parseTimestampMs = (value: unknown): number | null => {
   if (typeof value === 'number' && Number.isFinite(value)) {
     return value < 1_000_000_000_000 ? value * 1000 : value;
@@ -407,10 +474,9 @@ export function createWebSocketStore(url: string) {
         return;
       }
 
-      const payloadSizeBytes = new Blob([event.data]).size;
+      if (!isInboundPayloadWithinLimit(event.data)) {
-      if (payloadSizeBytes > MAX_INBOUND_WEBSOCKET_MESSAGE_BYTES) {
         logger.warn('Ignoring oversized WebSocket payload', {
-          sizeBytes: payloadSizeBytes,
+          characterCount: event.data.length,
           maxBytes: MAX_INBOUND_WEBSOCKET_MESSAGE_BYTES,
         });
         return;
@@ -466,10 +532,21 @@ export function createWebSocketStore(url: string) {
                 setState('pveTagStyles', message.data.pveTagStyles ?? {});
               }
               // Handle unified resources
+              let nextResources: Resource[] | undefined;
               if (message.data.resources !== undefined) {
-                const nextResources = Array.isArray(message.data.resources)
+                nextResources = Array.isArray(message.data.resources)
                   ? mergeCanonicalResourceSnapshot(message.data.resources, state.resources)
                   : [];
+              } else if (
+                'resourceDelta' in message.data &&
+                message.data.resourceDelta !== undefined
+              ) {
+                nextResources = applyResourceStateDelta(
+                  state.resources,
+                  message.data.resourceDelta,
+                );
+              }
+              if (nextResources !== undefined) {
                 logger.debug('[WebSocket] Updating resources', {
                   count: nextResources.length,
                   types: [...new Set(nextResources.map((resource) => resource.type) || [])],

frontend-modern/src/types/api.ts

@@ -33,6 +33,16 @@ export interface State {
   resources: Resource[];
 }
 
+export interface ResourceStateDelta {
+  upserts?: Array<Partial<Resource> & Pick<Resource, 'id'>>;
+  removed?: string[];
+  order?: string[];
+}
+
+export type StateUpdate = Partial<State> & {
+  resourceDelta?: ResourceStateDelta;
+};
+
 export interface PVETagStyle {
   colors: Record<string, string>;
   caseSensitive: boolean;
@@ -1333,7 +1343,7 @@ export interface Incident {
 // WebSocket message types
 export type WSMessage =
   | { type: 'initialState'; data: State }
-  | { type: 'rawData'; data: State }
+  | { type: 'rawData'; data: StateUpdate }
   | { type: 'error'; error: string }
   | { type: 'ping'; data?: unknown }
   | { type: 'pong'; data?: unknown }

internal/alerts/issue1497_test.go

@@ -0,0 +1,80 @@
+package alerts
+
+import (
+	"testing"
+	"time"
+
+	"github.com/rcourtman/pulse-go-rewrite/internal/models"
+)
+
+func TestCheckHostHonoursCanonicalResourceIntentGrace(t *testing.T) {
+	m := newTestManager(t)
+	start := time.Date(2026, 8, 6, 0, 0, 0, 0, time.UTC)
+	now := start
+	var tick time.Duration
+	m.now = func() time.Time { return now }
+	m.intentClock = func() time.Duration { return tick }
+
+	m.mu.Lock()
+	m.config.TimeThresholds = map[string]int{}
+	m.config.AgentDefaults = ThresholdConfig{
+		CPU: &HysteresisThreshold{Trigger: 80, Clear: 75},
+	}
+	m.mu.Unlock()
+
+	document := NewAlertIntentPolicyDocument()
+	document.Resources["agent-canonical"] = map[string]AlertIntentRule{
+		MetricAlertIntentSignal("cpu"): {GraceSeconds: intPointer(240)},
+	}
+	if err := m.LoadIntentPolicies(document); err != nil {
+		t.Fatal(err)
+	}
+	m.SetResourceIntentIdentityResolver(func(resourceID string) (string, bool) {
+		if resourceID == "agent:host-1" {
+			return "agent-canonical", true
+		}
+		return "", false
+	})
+
+	host := models.Host{
+		ID:       "host-1",
+		Hostname: "host-1",
+		CPUUsage: 90,
+		Status:   "online",
+		LastSeen: start,
+	}
+	alertID := canonicalMetricStateID(hostResourceID(host.ID), "cpu")
+
+	m.CheckHost(host)
+	m.mu.RLock()
+	_, exists := m.activeAlerts[alertID]
+	m.mu.RUnlock()
+	if exists {
+		t.Fatal("alert activated before the canonical resource grace period started")
+	}
+
+	now = start.Add(239 * time.Second)
+	tick = 239 * time.Second
+	m.CheckHost(host)
+	m.mu.RLock()
+	_, exists = m.activeAlerts[alertID]
+	m.mu.RUnlock()
+	if exists {
+		t.Fatal("alert activated before the canonical resource grace period elapsed")
+	}
+
+	now = start.Add(240 * time.Second)
+	tick = 240 * time.Second
+	m.CheckHost(host)
+	m.mu.RLock()
+	alert := m.activeAlerts[alertID]
+	if alert == nil {
+		m.mu.RUnlock()
+		t.Fatal("alert did not activate when the canonical resource grace period elapsed")
+	}
+	alertStart := alert.StartTime
+	m.mu.RUnlock()
+	if !alertStart.Equal(start) {
+		t.Fatalf("alert StartTime = %s, want first match %s", alertStart, start)
+	}
+}

internal/websocket/hub.go

@@ -273,6 +273,8 @@ type Client struct {
 	lifecycleMu   sync.Mutex
 	lifecycleDone chan struct{}
 	lifecycleOnce sync.Once
+	stateMu       sync.Mutex
+	stateSnapshot *clientStateSnapshot
 }
 
 func (c *Client) lifecycleSignal() chan struct{} {
@@ -320,6 +322,57 @@ func (c *Client) safeSend(data []byte) (sent bool) {
 	}
 }
 
+func (c *Client) queueFullState(messageType string, state interface{}) (int, bool, error) {
+	snapshot, err := buildClientStateSnapshot(state)
+	if err != nil {
+		return 0, false, err
+	}
+	data, err := json.Marshal(Message{Type: messageType, Data: state})
+	if err != nil {
+		return 0, false, err
+	}
+
+	c.stateMu.Lock()
+	defer c.stateMu.Unlock()
+	if !c.safeSend(data) {
+		return len(data), false, nil
+	}
+	c.stateSnapshot = snapshot
+	return len(data), true, nil
+}
+
+func (c *Client) queueStateDelta(current *clientStateSnapshot) (int, bool, bool, error) {
+	if current == nil {
+		return 0, false, false, fmt.Errorf("current state snapshot is nil")
+	}
+
+	c.stateMu.Lock()
+	defer c.stateMu.Unlock()
+	if c.stateSnapshot == nil {
+		// Initial hydration owns the baseline. A client still waiting for that
+		// payload must not receive a delta based on state it has never seen.
+		return 0, false, false, nil
+	}
+
+	delta, err := buildClientStateDelta(c.stateSnapshot, current)
+	if err != nil {
+		return 0, true, false, err
+	}
+	if len(delta) == 0 {
+		c.stateSnapshot = current
+		return 0, false, true, nil
+	}
+	data, err := json.Marshal(Message{Type: "rawData", Data: delta})
+	if err != nil {
+		return 0, true, false, err
+	}
+	if !c.safeSend(data) {
+		return len(data), true, false, nil
+	}
+	c.stateSnapshot = current
+	return len(data), true, true, nil
+}
+
 // cloneAlertData returns a broadcast-safe copy of alert data to avoid data races when
 // downstream sanitization/encoding happens concurrently with alert manager mutations.
 func cloneAlertData(alert interface{}) interface{} {
@@ -753,15 +806,7 @@ func (h *Hub) sendInitialState(client *Client) {
 	log.Debug().Str("client", client.id).Msg("about to get state")
 	stateData := h.getStateForClient(client)
 	log.Debug().Str("client", client.id).Interface("stateType", fmt.Sprintf("%T", stateData)).Msg("got state for initial message")
-	initialMsg := Message{
+	stateData = h.prepareStateForBroadcast(stateData)
-		Type: "initialState",
-		Data: sanitizeData(h.prepareStateForBroadcast(stateData)),
-	}
-	data, err = json.Marshal(initialMsg)
-	if err != nil {
-		log.Error().Err(err).Str("client", client.id).Msg("failed to marshal initial state")
-		return
-	}
 
 	h.mu.RLock()
 	_, stillRegistered := h.clients[client]
@@ -771,8 +816,14 @@ func (h *Hub) sendInitialState(client *Client) {
 		return
 	}
 
-	log.Info().Str("client", client.id).Int("dataLen", len(data)).Int("dataKB", len(data)/1024).Msg("sending initial state to client")
+	dataLen, sent, err := client.queueFullState("initialState", stateData)
-	if client.safeSend(data) {
+	if err != nil {
+		log.Error().Err(err).Str("client", client.id).Msg("failed to marshal initial state")
+		return
+	}
+
+	log.Info().Str("client", client.id).Int("dataLen", dataLen).Int("dataKB", dataLen/1024).Msg("sending initial state to client")
+	if sent {
 		log.Info().Str("client", client.id).Msg("initial state sent successfully")
 	} else {
 		log.Warn().Str("client", client.id).Msg("client closed or buffer full, skipping initial state")
@@ -789,16 +840,15 @@ func (h *Hub) sendRequestedState(client *Client) {
 	}
 	defer h.releaseStateBuildSlot()
 
-	stateMsg := Message{
+	_, sent, err := client.queueFullState(
-		Type: "rawData",
+		"rawData",
-		Data: sanitizeData(h.prepareStateForBroadcast(h.getStateForClient(client))),
+		h.prepareStateForBroadcast(h.getStateForClient(client)),
-	}
+	)
-	data, err := json.Marshal(stateMsg)
 	if err != nil {
 		log.Error().Err(err).Str("client", client.id).Msg("failed to marshal state for requestData")
 		return
 	}
-	if !client.safeSend(data) {
+	if !sent {
 		log.Warn().Str("client", client.id).Msg("Failed to queue requestData state response; client channel closed or full")
 	}
 }
@@ -1126,6 +1176,14 @@ func (h *Hub) dispatchStateBroadcast(pending *Message, orgID string) {
 		return
 	}
 	defer h.releaseStateBuildSlot()
+	if _, currentStateRequest := h.messageHasCurrentStateRequest(*pending); currentStateRequest {
+		prepared, ok := h.prepareMessageForMarshal(*pending, orgID)
+		if !ok {
+			return
+		}
+		h.dispatchCurrentStateSnapshot(prepared.Data, orgID)
+		return
+	}
 	data, ok := h.marshalBroadcastMessage(*pending, orgID)
 	if !ok {
 		return
@@ -1137,6 +1195,46 @@ func (h *Hub) dispatchStateBroadcast(pending *Message, orgID string) {
 	h.dispatchToTenantClients(orgID, data, "Client send channel full, dropping tenant coalesced message and closing connection")
 }
 
+func (h *Hub) dispatchCurrentStateSnapshot(state interface{}, orgID string) {
+	snapshot, err := buildClientStateSnapshot(state)
+	if err != nil {
+		log.Error().Err(err).Str("org_id", normalizeOrgID(orgID)).Msg("Failed to build WebSocket state delta snapshot")
+		return
+	}
+
+	h.mu.RLock()
+	clients := make([]*Client, 0, len(h.clients))
+	if orgID == "" {
+		for client := range h.clients {
+			clients = append(clients, client)
+		}
+	} else if tenantClients := h.clientsByTenant[orgID]; tenantClients != nil {
+		for client := range tenantClients {
+			clients = append(clients, client)
+		}
+	}
+	h.mu.RUnlock()
+
+	for _, client := range clients {
+		dataLen, attempted, sent, queueErr := client.queueStateDelta(snapshot)
+		if queueErr != nil {
+			log.Error().Err(queueErr).Str("client", client.id).Str("org_id", client.orgID).Msg("Failed to marshal WebSocket state delta")
+			continue
+		}
+		if !attempted || sent {
+			if sent {
+				log.Debug().Str("client", client.id).Int("dataLen", dataLen).Msg("queued WebSocket state delta")
+			}
+			continue
+		}
+		h.mu.Lock()
+		if h.removeClientLocked(client) {
+			log.Warn().Str("client", client.id).Str("org_id", client.orgID).Msg("Client send channel full, dropping state delta and closing connection")
+		}
+		h.mu.Unlock()
+	}
+}
+
 func (h *Hub) runStateBroadcastWorker() {
 	defer close(h.stateBroadcastDone)
 	for {

internal/websocket/state_delta.go

@@ -0,0 +1,175 @@
+package websocket
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"sort"
+)
+
+const resourceDeltaField = "resourceDelta"
+
+type clientStateSnapshot struct {
+	fields        map[string]json.RawMessage
+	resources     map[string]json.RawMessage
+	resourceOrder []string
+}
+
+type resourceDeltaPayload struct {
+	Upserts []json.RawMessage `json:"upserts,omitempty"`
+	Removed []string          `json:"removed,omitempty"`
+	Order   []string          `json:"order,omitempty"`
+}
+
+func buildClientStateSnapshot(state interface{}) (*clientStateSnapshot, error) {
+	encoded, err := json.Marshal(state)
+	if err != nil {
+		return nil, fmt.Errorf("marshal state snapshot: %w", err)
+	}
+
+	fields := make(map[string]json.RawMessage)
+	if err := json.Unmarshal(encoded, &fields); err != nil {
+		return nil, fmt.Errorf("decode state snapshot: %w", err)
+	}
+
+	resources := make(map[string]json.RawMessage)
+	resourceOrder := make([]string, 0)
+	if encodedResources, ok := fields["resources"]; ok {
+		var entries []json.RawMessage
+		if err := json.Unmarshal(encodedResources, &entries); err != nil {
+			return nil, fmt.Errorf("decode state resources: %w", err)
+		}
+		for _, entry := range entries {
+			var identity struct {
+				ID string `json:"id"`
+			}
+			if err := json.Unmarshal(entry, &identity); err != nil {
+				return nil, fmt.Errorf("decode state resource identity: %w", err)
+			}
+			if identity.ID == "" {
+				return nil, fmt.Errorf("state resource is missing id")
+			}
+			if _, exists := resources[identity.ID]; exists {
+				return nil, fmt.Errorf("state resource id %q is duplicated", identity.ID)
+			}
+			resources[identity.ID] = append(json.RawMessage(nil), entry...)
+			resourceOrder = append(resourceOrder, identity.ID)
+		}
+		delete(fields, "resources")
+	}
+
+	return &clientStateSnapshot{
+		fields:        fields,
+		resources:     resources,
+		resourceOrder: resourceOrder,
+	}, nil
+}
+
+func buildClientStateDelta(previous, current *clientStateSnapshot) (map[string]interface{}, error) {
+	if previous == nil || current == nil {
+		return nil, fmt.Errorf("state delta requires previous and current snapshots")
+	}
+
+	delta := make(map[string]interface{})
+	for key, currentValue := range current.fields {
+		if previousValue, ok := previous.fields[key]; !ok || !bytes.Equal(previousValue, currentValue) {
+			delta[key] = currentValue
+		}
+	}
+	for key := range previous.fields {
+		if _, ok := current.fields[key]; !ok {
+			delta[key] = nil
+		}
+	}
+
+	resourceDelta := resourceDeltaPayload{}
+	for _, id := range current.resourceOrder {
+		currentResource := current.resources[id]
+		previousResource, exists := previous.resources[id]
+		if !exists {
+			resourceDelta.Upserts = append(resourceDelta.Upserts, currentResource)
+			continue
+		}
+		if bytes.Equal(previousResource, currentResource) {
+			continue
+		}
+		patch, err := createJSONMergePatch(previousResource, currentResource)
+		if err != nil {
+			return nil, fmt.Errorf("build resource %q patch: %w", id, err)
+		}
+		resourceDelta.Upserts = append(resourceDelta.Upserts, patch)
+	}
+	for id := range previous.resources {
+		if _, exists := current.resources[id]; !exists {
+			resourceDelta.Removed = append(resourceDelta.Removed, id)
+		}
+	}
+	sort.Strings(resourceDelta.Removed)
+	if !reflect.DeepEqual(previous.resourceOrder, current.resourceOrder) {
+		resourceDelta.Order = append([]string(nil), current.resourceOrder...)
+	}
+	if len(resourceDelta.Upserts) > 0 || len(resourceDelta.Removed) > 0 || len(resourceDelta.Order) > 0 {
+		delta[resourceDeltaField] = resourceDelta
+	}
+
+	return delta, nil
+}
+
+func createJSONMergePatch(previous, current json.RawMessage) (json.RawMessage, error) {
+	var previousValue interface{}
+	if err := json.Unmarshal(previous, &previousValue); err != nil {
+		return nil, err
+	}
+	var currentValue interface{}
+	if err := json.Unmarshal(current, &currentValue); err != nil {
+		return nil, err
+	}
+
+	patchValue, changed := diffJSONMergeValue(previousValue, currentValue)
+	if !changed {
+		return json.RawMessage(`{}`), nil
+	}
+	if patchObject, ok := patchValue.(map[string]interface{}); ok {
+		if currentObject, ok := currentValue.(map[string]interface{}); ok {
+			if id, ok := currentObject["id"]; ok {
+				patchObject["id"] = id
+			}
+		}
+	}
+	patch, err := json.Marshal(patchValue)
+	if err != nil {
+		return nil, err
+	}
+	return patch, nil
+}
+
+func diffJSONMergeValue(previous, current interface{}) (interface{}, bool) {
+	if reflect.DeepEqual(previous, current) {
+		return nil, false
+	}
+
+	previousObject, previousIsObject := previous.(map[string]interface{})
+	currentObject, currentIsObject := current.(map[string]interface{})
+	if !previousIsObject || !currentIsObject {
+		return current, true
+	}
+
+	patch := make(map[string]interface{})
+	for key := range previousObject {
+		if _, exists := currentObject[key]; !exists {
+			patch[key] = nil
+		}
+	}
+	for key, currentValue := range currentObject {
+		previousValue, exists := previousObject[key]
+		if !exists {
+			patch[key] = currentValue
+			continue
+		}
+		if nestedPatch, changed := diffJSONMergeValue(previousValue, currentValue); changed {
+			patch[key] = nestedPatch
+		}
+	}
+	return patch, len(patch) > 0
+}

internal/websocket/state_delta_test.go

@@ -0,0 +1,132 @@
+package websocket
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"github.com/rcourtman/pulse-go-rewrite/internal/models"
+)
+
+func TestClientStateDeltaOmitsUnchangedResourcePayload(t *testing.T) {
+	previousState := models.EmptyStateFrontend()
+	previousState.LastUpdate = 100
+	previousState.Resources = []models.ResourceFrontend{{
+		ID:           "agent:host-1",
+		Type:         "agent",
+		Name:         "host-1",
+		DisplayName:  "host-1",
+		Status:       "online",
+		LastSeen:     100,
+		CPU:          &models.ResourceMetricFrontend{Current: 40},
+		PlatformData: json.RawMessage(`{"static":"` + strings.Repeat("x", 16*1024) + `"}`),
+	}}
+	currentState := previousState
+	currentState.LastUpdate = 200
+	currentState.Resources = append([]models.ResourceFrontend(nil), previousState.Resources...)
+	currentState.Resources[0].LastSeen = 200
+	currentState.Resources[0].CPU = &models.ResourceMetricFrontend{Current: 41}
+
+	previous, err := buildClientStateSnapshot(previousState)
+	if err != nil {
+		t.Fatal(err)
+	}
+	current, err := buildClientStateSnapshot(currentState)
+	if err != nil {
+		t.Fatal(err)
+	}
+	delta, err := buildClientStateDelta(previous, current)
+	if err != nil {
+		t.Fatal(err)
+	}
+	encoded, err := json.Marshal(delta)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(string(encoded), strings.Repeat("x", 128)) {
+		t.Fatal("state delta repeated unchanged platform data")
+	}
+	if !strings.Contains(string(encoded), `"current":41`) || !strings.Contains(string(encoded), `"lastSeen":200`) {
+		t.Fatalf("state delta omitted changed telemetry: %s", encoded)
+	}
+	if len(encoded) >= 1024 {
+		t.Fatalf("state delta = %d bytes, want a bounded telemetry patch", len(encoded))
+	}
+}
+
+func TestClientStateDeltaTracksResourceRemovalAndOrder(t *testing.T) {
+	previousState := models.EmptyStateFrontend()
+	previousState.Resources = []models.ResourceFrontend{
+		{ID: "a", Type: "agent", Name: "a", DisplayName: "a"},
+		{ID: "b", Type: "agent", Name: "b", DisplayName: "b"},
+	}
+	currentState := models.EmptyStateFrontend()
+	currentState.Resources = []models.ResourceFrontend{
+		{ID: "c", Type: "agent", Name: "c", DisplayName: "c"},
+		{ID: "a", Type: "agent", Name: "a", DisplayName: "a"},
+	}
+
+	previous, err := buildClientStateSnapshot(previousState)
+	if err != nil {
+		t.Fatal(err)
+	}
+	current, err := buildClientStateSnapshot(currentState)
+	if err != nil {
+		t.Fatal(err)
+	}
+	delta, err := buildClientStateDelta(previous, current)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resources, ok := delta[resourceDeltaField].(resourceDeltaPayload)
+	if !ok {
+		t.Fatalf("resource delta type = %T", delta[resourceDeltaField])
+	}
+	if len(resources.Removed) != 1 || resources.Removed[0] != "b" {
+		t.Fatalf("removed = %#v, want [b]", resources.Removed)
+	}
+	if len(resources.Order) != 2 || resources.Order[0] != "c" || resources.Order[1] != "a" {
+		t.Fatalf("order = %#v, want [c a]", resources.Order)
+	}
+	if len(resources.Upserts) != 1 || !strings.Contains(string(resources.Upserts[0]), `"id":"c"`) {
+		t.Fatalf("upserts = %s, want full resource c", resources.Upserts)
+	}
+}
+
+func TestClientStateBaselineAdvancesOnlyAfterDeltaIsQueued(t *testing.T) {
+	initial := models.EmptyStateFrontend()
+	initial.LastUpdate = 100
+	initial.Resources = []models.ResourceFrontend{{
+		ID: "agent:host-1", Type: "agent", Name: "host-1", DisplayName: "host-1",
+		CPU: &models.ResourceMetricFrontend{Current: 10},
+	}}
+	current := initial
+	current.LastUpdate = 200
+	current.Resources = append([]models.ResourceFrontend(nil), initial.Resources...)
+	current.Resources[0].CPU = &models.ResourceMetricFrontend{Current: 20}
+
+	client := &Client{send: make(chan []byte, 1)}
+	if _, sent, err := client.queueFullState("initialState", initial); err != nil || !sent {
+		t.Fatalf("queueFullState() sent=%v error=%v", sent, err)
+	}
+	initialBaseline := client.stateSnapshot
+	currentSnapshot, err := buildClientStateSnapshot(current)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, attempted, sent, err := client.queueStateDelta(currentSnapshot); err != nil || !attempted || sent {
+		t.Fatalf("full-channel queueStateDelta() attempted=%v sent=%v error=%v", attempted, sent, err)
+	}
+	if client.stateSnapshot != initialBaseline {
+		t.Fatal("client baseline advanced after the delta queue rejected the payload")
+	}
+
+	<-client.send
+	if _, attempted, sent, err := client.queueStateDelta(currentSnapshot); err != nil || !attempted || !sent {
+		t.Fatalf("retry queueStateDelta() attempted=%v sent=%v error=%v", attempted, sent, err)
+	}
+	if client.stateSnapshot != currentSnapshot {
+		t.Fatal("client baseline did not advance after the delta was queued")
+	}
+}