diff --git a/src/shared/context/FestipodDataContext.tsx b/src/shared/context/FestipodDataContext.tsx
index d9d3a77..8eabba1 100644
--- a/src/shared/context/FestipodDataContext.tsx
+++ b/src/shared/context/FestipodDataContext.tsx
@@ -244,12 +244,86 @@ function useNgData(): FestipodDataContextValue {
   const eventQuery = useShapeQuery(FpEventShapeType, 'public');
   const userQuery = useShapeQuery(FpUserProfileShapeType, 'protected');
   const partQuery = useShapeQuery(FpParticipationShapeType, 'protected');
-  const events = React.useMemo(() => adaptEvents(eventQuery.data), [eventQuery.data]);
   const users = React.useMemo(() => adaptUsers(userQuery.data), [userQuery.data]);
-  const participations = React.useMemo(
+  // The RAW reactive sets straight from `watchShape` (before the optimistic
+  // overlay). The exposed `events`/`participations` merge these with the pending
+  // overlay below — see the "OPTIMISTIC OVERLAY" block.
+  const reactiveEvents = React.useMemo(() => adaptEvents(eventQuery.data), [eventQuery.data]);
+  const reactiveParticipations = React.useMemo(
     () => adaptParticipations(partQuery.data),
     [partQuery.data],
   );
+
+  // --- OPTIMISTIC OVERLAY (standard useQuery-mutation pattern; NO polling) ------
+  // `watchShape` only surfaces a doc AFTER the broker push (a real latency), so a
+  // mutation's effect is invisible in the reactive set for a beat. To keep the
+  // screens' local-first feel, each mutation reflects its effect IMMEDIATELY in a
+  // pure app-side React overlay laid OVER the SDK surface (never touching the SDK
+  // internals — see rule_app-uses-sdk-surface-only), then RECONCILED by the push:
+  //   • `pendingAddEvents` / `pendingAddParticipations` — entities created
+  //     optimistically, keyed by their `@id` (= the doc NURI `writeEntity` returns,
+  //     the SAME id `watchShape` surfaces as `s.subject` → they match exactly).
+  //   • `pendingRemoveIds` — participation ids removed optimistically (leave).
+  // The exposed set = merge(reactive, pendingAdds) minus pendingRemoves, DEDUPED by
+  // id. Reconciliation (effects below) auto-cleans the overlay the moment the
+  // reactive read catches up: an add whose id now appears in the reactive set is
+  // dropped from pendingAdds; a remove whose id is no longer in the reactive set is
+  // dropped from pendingRemoves. No re-query, no interval — the overlay only reacts
+  // to `watchShape`'s own pushes.
+  const [pendingAddEvents, setPendingAddEvents] = useState<FpEventData[]>([]);
+  const [pendingAddParticipations, setPendingAddParticipations] = useState<FpParticipationData[]>([]);
+  const [pendingRemoveIds, setPendingRemoveIds] = useState<Set<string>>(() => new Set());
+
+  const events = React.useMemo(() => {
+    if (pendingAddEvents.length === 0) return reactiveEvents;
+    const seen = new Set(reactiveEvents.map(e => e.id));
+    const extra = pendingAddEvents.filter(e => !seen.has(e.id));
+    return extra.length ? [...reactiveEvents, ...extra] : reactiveEvents;
+  }, [reactiveEvents, pendingAddEvents]);
+
+  const participations = React.useMemo(() => {
+    let base = reactiveParticipations;
+    if (pendingAddParticipations.length) {
+      const seen = new Set(base.map(p => p.id));
+      const extra = pendingAddParticipations.filter(p => !seen.has(p.id));
+      if (extra.length) base = [...base, ...extra];
+    }
+    if (pendingRemoveIds.size) base = base.filter(p => !pendingRemoveIds.has(p.id));
+    return base;
+  }, [reactiveParticipations, pendingAddParticipations, pendingRemoveIds]);
+
+  // RECONCILIATION — drop each optimistic add once the reactive read carries its id
+  // (the push arrived), and each optimistic remove once the reactive read no longer
+  // carries its id (the SPARQL delete propagated). Guarded to no-op when there is
+  // nothing to reconcile, so a stable reactive set does not churn state.
+  useEffect(() => {
+    if (pendingAddEvents.length === 0) return;
+    const live = new Set(reactiveEvents.map(e => e.id));
+    setPendingAddEvents(prev => {
+      const next = prev.filter(e => !live.has(e.id));
+      return next.length === prev.length ? prev : next;
+    });
+  }, [reactiveEvents, pendingAddEvents]);
+
+  useEffect(() => {
+    if (pendingAddParticipations.length === 0) return;
+    const live = new Set(reactiveParticipations.map(p => p.id));
+    setPendingAddParticipations(prev => {
+      const next = prev.filter(p => !live.has(p.id));
+      return next.length === prev.length ? prev : next;
+    });
+  }, [reactiveParticipations, pendingAddParticipations]);
+
+  useEffect(() => {
+    if (pendingRemoveIds.size === 0) return;
+    const live = new Set(reactiveParticipations.map(p => p.id));
+    setPendingRemoveIds(prev => {
+      let changed = false;
+      const next = new Set(prev);
+      for (const id of prev) if (!live.has(id)) { next.delete(id); changed = true; }
+      return changed ? next : prev;
+    });
+  }, [reactiveParticipations, pendingRemoveIds]);
   // The read is "settled" once every scope has reached its sync barrier
   // (`isSuccess`). A synced-but-empty scope reads `isSuccess` with `data: []` — the
   // distinction the auto-seed relies on to tell "still syncing" from "truly empty".
@@ -281,6 +355,11 @@ function useNgData(): FestipodDataContextValue {
     // its own (scope re-resolution keyed on `getCurrentUser()`).
     setOwnedEventIds([]);
     joinUidsRef.current.clear();
+    // Drop the optimistic overlay too: it belongs to the OLD identity's session
+    // and must not bleed into the new identity's reads (isolation).
+    setPendingAddEvents([]);
+    setPendingAddParticipations([]);
+    setPendingRemoveIds(new Set());
     resetCaps();
     resetRegistryCache();
   }, [username]);
@@ -557,6 +636,13 @@ function useNgData(): FestipodDataContextValue {
     // OPTION B: this event's doc is MINE (I just created it), so track it as owned
     // → the owner-materializer subscribes to its inbox and maintains its count.
     setOwnedEventIds(prev => (prev.includes(eventGraph) ? prev : [...prev, eventGraph]));
+    // OPTIMISTIC OVERLAY: surface the created event IMMEDIATELY (its id = eventId =
+    // the doc NURI, the SAME id `watchShape('public')` will surface as `s.subject`,
+    // so the reconciliation effect drops this add once the push arrives). Without
+    // it, `getEvent(eventId)` right after create returns undefined until the push.
+    // participantCount starts at 0 (creator does not auto-participate).
+    const optimisticEvent: FpEventData = { ...event, id: eventId, participantCount: event.participantCount || 0 };
+    setPendingAddEvents(prev => (prev.some(e => e.id === eventId) ? prev : [...prev, optimisticEvent]));
     // The creator does NOT auto-participate (no host notion — settled product
     // decision): NO participation is written on create. The creator sees "J'y
     // serai" and may join/leave their own event like anyone else.
@@ -635,6 +721,14 @@ function useNgData(): FestipodDataContextValue {
     await writeEntity(partGraph, ENTITY_TYPE.participation, {
       event: iri(eventId), user: iri(uid), isConfirmed: bool(true),
     });
+    // OPTIMISTIC OVERLAY: surface the participation IMMEDIATELY so the joiner shows
+    // up in the list and `isParticipating` is true before the broker push. Its id =
+    // partGraph = the doc NURI, the SAME id `watchShape('protected')` will surface
+    // as `s.subject` (adaptParticipations sets `id: s.subject`) → the reconciliation
+    // effect drops this add once the push arrives. If a stale pendingRemove targeted
+    // this exact id (re-join same doc — never happens, ids are fresh), clear it too.
+    const optimisticPart: FpParticipationData = { id: partGraph, eventId, userId: uid, isConfirmed: true };
+    setPendingAddParticipations(prev => (prev.some(p => p.id === partGraph) ? prev : [...prev, optimisticPart]));
     // OPTION B (brief §B): the joiner does NOT write `participantCount` on the
     // EVENT doc — that doc belongs to the OWNER, and a non-owner write there is an
     // isolation violation (NextGraph write is membership-bound; there is no append
@@ -706,6 +800,23 @@ function useNgData(): FestipodDataContextValue {
       console.error(msg);
       throw new Error(msg);
     }
+    // OPTIMISTIC OVERLAY: the delete is broker-confirmed (remaining === 0) but the
+    // reactive `watchShape('protected')` set only drops the participation on the
+    // NEXT push (a beat later). Mark its id removed NOW so the leaver disappears
+    // from the list immediately; the reconciliation effect clears this pendingRemove
+    // once the reactive set no longer carries the id. Also drop it from
+    // pendingAddParticipations in case it was still only-optimistic (join+leave in
+    // the same session before the join's push landed).
+    setPendingRemoveIds(prev => {
+      if (prev.has(part.id)) return prev;
+      const next = new Set(prev);
+      next.add(part.id);
+      return next;
+    });
+    setPendingAddParticipations(prev => {
+      const next = prev.filter(p => p.id !== part.id);
+      return next.length === prev.length ? prev : next;
+    });
     // OPTION B, symmetric (brief §B "Désinscription"): the leaver does NOT write
     // `participantCount` on the EVENT doc (owner-owned — same isolation violation
     // as the join). Instead it DEPOSITS a `leave-participant` marker into the