Imported from BuilderIO/agent-native (templates/slides/.agents/skills/real-time-sync/SKILL.md). Install upstream with npx skills add BuilderIO/agent-native --skill real-time-sync. Copyright stays with the author.

Real-Time Sync

Rule

The UI stays in sync with agent/script changes through useDbSync(). In-process writes stream over /_agent-native/events first; /_agent-native/poll remains the cross-process/serverless fallback. When the agent writes to the database, the UI detects the change and updates automatically — no manual refresh needed.

Why

The agent modifies data in SQL, but the UI runs in the browser. SSE bridges same-process writes immediately; polling bridges anything SSE cannot see, such as another serverless invocation, cron job, or external script. Every visible write increments a version counter, useDbSync() receives the change, and React Query invalidates the relevant caches. This is what makes database writes feel real-time without relying on aggressive polling.

How It Works

1. Server increments a version counter on every database write. In-process events stream through the authenticated /_agent-native/events endpoint.

2. Client listens for sync events and updates per-source change counters:

   import { useDbSync } from "@agent-native/core/client/hooks";
   useDbSync({ queryClient });

   For each non-own event, useDbSync bumps a per-source counter (e.g. dashboards, analyses, settings, action) and invalidates a small fixed list of framework-internal prefixes (["action"], ["app-state"], ["__set_url__"], etc.). It does not blanket-invalidate templates' own data queries — that caused request storms in production. A successful mutating action refreshes active useActionQuery observers under the ["action"] prefix. Browser actions carry a tab id so their originating tab ignores the sync echo while other tabs refresh. Legacy apps can opt into broader compatibility with actionInvalidatePredicate, but first-party apps should use action-backed or source-versioned query keys. Idle fallback polling runs once per minute; active agent work temporarily uses the faster cadence.

3. Templates fold per-source counters into their query keys. This is the pattern that makes "agent writes show up without a manual refresh" reliable:

   import { useChangeVersion } from "@agent-native/core/client/hooks";
   import { useQuery } from "@tanstack/react-query";
   
   const v = useChangeVersion("dashboards");
   const dashboard = useQuery({
     queryKey: ["dashboard", id, v],
     queryFn: () => fetchDashboard(id),
     placeholderData: (prev) => prev, // no flicker on refetch
   });

   When the agent writes (update-dashboard action → server emits source: "dashboards"), the counter advances, the queryKey changes, and React Query refetches that one query. The old data stays on screen during the refetch thanks to placeholderData.

   For list/sidebar queries, use the same pattern — pass the counter into the queryKey of every list query you want to keep fresh.

4. Fallback polling calls /_agent-native/poll?since=N. It polls every 2 seconds only while an agent run is in flight; otherwise it idles at 60 seconds whether or not SSE is connected (SSE_FALLBACK_INTERVAL_MS and IDLE_POLL_INTERVAL_MS, both 60 s). A hidden tab polls no faster than every 10 seconds (HIDDEN_POLL_INTERVAL_MS). Polling is the universal serverless fallback: new framework writes are read from the durable sync_events log, while the older DB timestamp scan remains as a slower safety net for direct SQL writes and older processes.

5. When the agent writes to the database, the version increments, SSE/polling detects it, and React Query refetches the affected queries.

Don't

• Don't create manual polling loops — useDbSync() handles SSE plus fallback polling

• Don't create your own fetch-based polling alongside useDbSync — use the onEvent callback for custom handling

• Don't open your own EventSource to /_agent-native/events. A tab must hold exactly ONE SSE connection no matter how many features listen — extra streams eat the browser's per-origin connection budget and can starve ordinary data fetches (worst on HTTP/1.1 dev servers). Subscribe to the shared transport instead:

  import { subscribeSyncEvents } from "@agent-native/core/client/hooks";
  
  const unsubscribe = subscribeSyncEvents({
    onEvents: (events) => {
      // filter by event.source and handle push-style updates
    },
    // Optional: relax your own fallback cadence while push is healthy.
    onSseStateChange: (connected) => {},
  });

  useDbSync and every subscribeSyncEvents subscriber share one EventSource and one fallback poll loop per tab — this is how collaborative documents receive doc updates and cursor/awareness events. Hidden tabs keep that shared transport alive by default: SSE stays connected and active fallback polling relaxes to a 10-second floor (idle polling remains once per minute). Pass pauseWhenHidden: true only for a consumer that explicitly must stop all hidden-tab sync.

Which sources to depend on

Common sources you'll fold into query keys:

If a query reads data the agent can mutate via more than one path, depend on multiple sources with useChangeVersions:

const v = useChangeVersions(["dashboards", "action"]);
useQuery({ queryKey: ["dashboard", id, v], ... });

useChangeVersions returns a single integer that advances whenever any of the listed sources advance.

Tuning refetch behavior

To prevent cache thrashing during rapid agent writes, set staleTime on your queries:

useQuery({
  queryKey: ["items"],
  queryFn: fetchItems,
  staleTime: 2000, // don't refetch within 2 seconds
});

Troubleshooting

Jitter Prevention

When the agent writes to application-state via script helpers (writeAppState, deleteAppState), the write is automatically tagged with requestSource: "agent". This prevents the UI from overwriting active user edits when it receives the change event.

How it works

1. Agent writes are tagged: the script helpers in @agent-native/core/application-state pass { requestSource: "agent" } to the store.
2. UI writes are tagged: templates send a per-tab ID via the X-Request-Source header on PUT/DELETE requests to application-state endpoints.
3. Sync filters: useDbSync() accepts an ignoreSource option. The UI passes its own tab ID so it ignores events from its own writes — but still picks up events from agents, other tabs, and scripts.

Template setup

// app/lib/tab-id.ts — already scaffolded, do not redefine
import { getBrowserTabId } from "@agent-native/core/client/hooks";
export const TAB_ID = getBrowserTabId();

// app/root.tsx
import { TAB_ID } from "@/lib/tab-id";

useDbSync({
  queryClient,
  ignoreSource: TAB_ID,
});

getBrowserTabId() is the id the server resolves tab-scoped application_state and WebMCP calls against — see context-awareness.

The use-navigation-state.ts hook sends the same TAB_ID in the X-Request-Source header when writing navigation state, so the tab that wrote the state does not refetch it.

Why this matters

Without jitter prevention, a cycle occurs: the UI writes state, sync detects the change, the UI refetches and re-renders, potentially overwriting what the user is actively editing. With ignoreSource, the UI only reacts to changes from other sources (agent scripts, other browser tabs, other users).

Action Routes and Live Sync

Actions work with the same sync system. When a mutating action writes to the database, the version counter increments and useDbSync picks up the change. Frontend mutations via useActionMutation automatically invalidate ["action"] query keys on success, triggering refetches of useActionQuery hooks. Client components should call actions through those hooks, not with raw action-route fetches.

For custom apps, the best out-of-the-box path is:

1. Put read actions in actions/ with defineAction({ http: { method: "GET" } }).
2. Put write actions in actions/ with the default POST/PUT/DELETE behavior.
3. Call reads from React with useActionQuery and writes with useActionMutation.

This avoids duplicate /api/* JSON CRUD routes and makes agent-created records show up automatically. Raw useQuery can still work, but it should include useChangeVersions(["action", "<domain-source>"]) in the query key for targeted refreshes.

Auto-emit on mutating actions

The framework emits a change event with source: "action" whenever any non-read-only action runs to completion — whether called via HTTP (/_agent-native/actions/:name) or as an agent tool call. Read-only actions (http: { method: "GET" } or explicit readOnly: true) are skipped.

This means UIs don't need the agent to remember to call refresh-screen after every mutation. A listener like this (used in the macros template) will refresh after any mutating agent call:

useDbSync({
  queryClient,
  ignoreSource: TAB_ID,
  onEvent: (data) => {
    if (data.requestSource === TAB_ID) return;
    // Invalidate all useActionQuery caches so list-*, get-*, etc. refetch
    queryClient.invalidateQueries({ queryKey: ["action"] });
  },
});

refresh-screen remains available for unusual cases — e.g. the agent mutated data via a path the framework can't see (external system the app mirrors), or the agent wants to pass a scope hint for narrower invalidation.

Keeping Stateful Components In Sync

The useChangeVersion / useActionQuery pattern above keeps the query layer fresh. But components that copy a server value into local React state still go stale on agent edits — refetching the query updates the prop, yet the local copy never re-adopts it. This is a recurring bug.

Never do this for a value the agent can mutate:

// BUG: `title` is captured once and never re-reads the prop.
const [title, setTitle] = useState(props.title);

When the agent renames the record, the query refetches, props.title updates, but the input still shows the stale value until the component remounts.

Derived-state surfaces (form fields, inline editors, popovers): use useReconciledState. It re-adopts the authoritative external value when it changes, except while the user is actively editing that field — so agent mutations show up live without clobbering in-progress typing:

import { useReconciledState } from "@agent-native/core/client/hooks";

// `active` = true while the user is editing this field (focused / dirty).
const [title, setTitle] = useReconciledState(props.title, { active: isEditing });

Collaborative rich-text editors are different — they don't copy a value into useState. They reconcile authoritative SQL content into a shared Y.Doc under an updatedAt gate with lead-client election. See real-time-collab → "Agent edits as a real-time peer editor". Don't reach for useReconciledState for a Yjs-backed editor.

Granular server-side merge for non-body fields

For structured documents (slide decks, form builders, design files) where the Yjs body collab would cause LWW conflicts at the container level, pair the change-sync updatedAt bump with a granular server-side merge action that accepts targeted per-item operations (add/patch/delete/reorder). Concurrent edits to different items both survive at the action level; the collab source version bump then propagates the merged state to all open clients. See real-time-collab for the pattern and examples.

Related Skills

• storing-data — Application-state and settings are data stores that sync through change events
• context-awareness — Navigation state writes use jitter prevention to avoid overwriting active edits
• actions — Mutating actions trigger change events
• client-methods — Route details belong in helpers/hooks, not components
• self-modifying-code — Agent code edits trigger change events; rapid edits can cause event storms
• real-time-collab — Collaborative editors reconcile agent edits into a shared Y.Doc, driven by the same change-sync updatedAt bump; also the granular server-side merge pattern for structured data