diff --git a/packages/core/js-client/src/api.ts b/packages/core/js-client/src/api.ts
index 031066c1..2b831f30 100644
--- a/packages/core/js-client/src/api.ts
+++ b/packages/core/js-client/src/api.ts
@@ -133,7 +133,6 @@ export const v5_callFunction = async (
     peer: peerOrArg,
     args: callArgs,
     config,
-    fireAndForget: returnTypeVoid,
   });
 
   if (returnTypeVoid) {
diff --git a/packages/core/js-client/src/compilerSupport/callFunction.ts b/packages/core/js-client/src/compilerSupport/callFunction.ts
index cf1272cd..6d8f177e 100644
--- a/packages/core/js-client/src/compilerSupport/callFunction.ts
+++ b/packages/core/js-client/src/compilerSupport/callFunction.ts
@@ -61,8 +61,6 @@ export const callAquaFunction = async ({
   config = {},
   peer,
   args,
-  // TODO: remove after LNG-286 is done
-  fireAndForget = false,
 }: CallAquaFunctionArgs) => {
   log.trace("calling aqua function %j", { script, config, args });
 
@@ -82,12 +80,7 @@ export const callAquaFunction = async ({
       registerParticleScopeService(peer, particle, service);
     }
 
-    // If fireAndForget is enabled, then function call completed when one of the two conditions is met:
-    //  1. The particle is sent to the network
-    //  2. All CallRequests are executed, e.g., all variable loading and local function calls are completed
-    if (!fireAndForget) {
-      registerParticleScopeService(peer, particle, responseService(resolve));
-    }
+    registerParticleScopeService(peer, particle, responseService(resolve));
 
     registerParticleScopeService(peer, particle, injectRelayService(peer));
 
diff --git a/packages/core/js-client/src/jsPeer/FluencePeer.ts b/packages/core/js-client/src/jsPeer/FluencePeer.ts
index 0adb4dbd..20cc88cc 100644
--- a/packages/core/js-client/src/jsPeer/FluencePeer.ts
+++ b/packages/core/js-client/src/jsPeer/FluencePeer.ts
@@ -539,17 +539,6 @@ export abstract class FluencePeer {
                       "id %s. send successful",
                       newParticle.id,
                     );
-
-                    if (
-                      this.jsServiceHost.getHandler(
-                        "callbackSrv",
-                        "response",
-                        item.particle.id,
-                      ) == null
-                    ) {
-                      // try to finish script if fire-and-forget enabled
-                      item.onSuccess(null);
-                    }
                   })
                   .catch((e: unknown) => {
                     log_particle.error(
@@ -627,21 +616,6 @@ export abstract class FluencePeer {
                       });
                     });
                 }
-              } else {
-                // TODO: remove after LNG-286 is done
-                // eslint-disable-next-line eqeqeq
-                if (
-                  this.jsServiceHost.getHandler(
-                    "callbackSrv",
-                    "response",
-                    item.particle.id,
-                  ) == null
-                ) {
-                  // try to finish script if fire-and-forget enabled
-                  setTimeout(() => {
-                    item.onSuccess(null);
-                  }, 0);
-                }
               }
 
               return connectionPromise;
diff --git a/packages/core/js-client/src/services/builtins.ts b/packages/core/js-client/src/services/builtins.ts
index b5e23c61..3dc6a781 100644
--- a/packages/core/js-client/src/services/builtins.ts
+++ b/packages/core/js-client/src/services/builtins.ts
@@ -312,10 +312,11 @@ export const builtInServices: Record<
     }),
 
     concat: withSchema(z.array(z.array(z.unknown())))((args) => {
-      // concat accepts only 'never' type
+      // Schema is used with unknown type to prevent useless runtime check
       // eslint-disable-next-line @typescript-eslint/consistent-type-assertions
       const arr = args as never[][];
-      return success([].concat(...arr));
+
+      return success(arr.flat());
     }),
 
     string_to_b58: withSchema(z.tuple([z.string()]))(([input]) => {