diff --git a/scripts/localnet/app-synchronizer.sc b/scripts/localnet/app-synchronizer.sc
new file mode 100644
index 000000000..fb679af6e
--- /dev/null
+++ b/scripts/localnet/app-synchronizer.sc
@@ -0,0 +1,54 @@
+// Copyright (c) 2025-2026 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+bootstrap.synchronizer(
+  synchronizerName = "app-synchronizer",
+  sequencers = Seq(`app-sequencer`),
+  mediators = Seq(`app-mediator`),
+  synchronizerOwners = Seq(`app-sequencer`),
+  synchronizerThreshold = 1,
+  staticSynchronizerParameters = StaticSynchronizerParameters.defaultsWithoutKMS(ProtocolVersion.latest),
+)
+
+// Connect app-provider to the new synchronizer.
+// Note: app-user is intentionally NOT connected to app-synchronizer so that
+// the SDK (which picks connectedSynchronizers[0]) always selects global-domain.
+`app-provider`.synchronizers.connect_local(`app-sequencer`, "app-synchronizer")
+
+// Wait for app-provider to be active on app-synchronizer
+utils.retry_until_true {
+  `app-provider`.synchronizers.active("app-synchronizer")
+}
+
+// Vet packages on app-synchronizer for app-provider.
+// The Splice app already uploaded DARs and vetted them on global-domain.
+// We replicate the vetting from the authorized store to app-synchronizer
+// so that the synchronizer is fully functional.
+val appSyncId = `app-provider`.synchronizers.list_connected()
+  .find(_.synchronizerAlias.unwrap == "app-synchronizer")
+  .getOrElse(throw new RuntimeException("app-synchronizer not found in connected synchronizers"))
+  .synchronizerId
+
+for (participant <- Seq(`app-provider`)) {
+  val vettedFromAuthorized = participant.topology.vetted_packages
+    .list(store = Some(TopologyStoreId.Authorized), filterParticipant = participant.id.filterString)
+    .flatMap(_.item.packages)
+
+  if (vettedFromAuthorized.nonEmpty) {
+    logger.info(s"Vetting ${vettedFromAuthorized.size} packages on app-synchronizer for ${participant.name}")
+    participant.topology.vetted_packages.propose_delta(
+      participant = participant.id,
+      store = appSyncId,
+      adds = vettedFromAuthorized.toSeq,
+    )
+  }
+}
+
+// Wait for vetting topology to propagate
+utils.retry_until_true {
+  val providerVetted = `app-provider`.topology.vetted_packages
+    .list(store = Some(appSyncId), filterParticipant = `app-provider`.id.filterString)
+  providerVetted.nonEmpty && providerVetted.head.item.packages.nonEmpty
+}
+
+logger.info("app-synchronizer bootstrap with package vetting completed successfully")
diff --git a/scripts/src/start-localnet.ts b/scripts/src/start-localnet.ts
index e2b19d97b..142fc3cb0 100644
--- a/scripts/src/start-localnet.ts
+++ b/scripts/src/start-localnet.ts
@@ -17,6 +17,10 @@ const GENERATED_COMPOSE_OVERRIDE = path.join(
 )
 
 const CANTON_MAX_COMMANDS_IN_FLIGHT = 256
+const CUSTOM_APP_SYNCHRONIZER_SC = path.join(
+    rootDir,
+    'scripts/localnet/app-synchronizer.sc'
+)
 
 function ensureComposeOverride() {
     fs.mkdirSync(path.dirname(GENERATED_COMPOSE_OVERRIDE), { recursive: true })
@@ -30,6 +34,9 @@ function ensureComposeOverride() {
             `        canton.participants.app-provider.ledger-api.command-service.max-commands-in-flight = ${CANTON_MAX_COMMANDS_IN_FLIGHT}`,
             `        canton.participants.app-user.ledger-api.command-service.max-commands-in-flight = ${CANTON_MAX_COMMANDS_IN_FLIGHT}`,
             `        canton.participants.sv.ledger-api.command-service.max-commands-in-flight = ${CANTON_MAX_COMMANDS_IN_FLIGHT}`,
+            '  multi-sync-startup:',
+            '    volumes:',
+            `      - ${CUSTOM_APP_SYNCHRONIZER_SC}:/app/app-synchronizer.sc`,
             '',
         ].join('\n'),
         'utf8'
@@ -55,8 +62,8 @@ const composeBase = [
     'app-provider',
     '--profile',
     'app-user',
-    // '--profile',
-    // 'multi-sync',
+    '--profile',
+    'multi-sync',
 ]
 
 const network = getNetworkArg()