diff --git a/relay_client/src/http.rs b/relay_client/src/http.rs
index 87e7af3..2e3df43 100644
--- a/relay_client/src/http.rs
+++ b/relay_client/src/http.rs
@@ -128,9 +128,22 @@ impl Client {
         .map(|_| ())
     }
 
-    /// Subscribes on topic to receive messages.
+    /// Subscribes on topic to receive messages. The request is resolved
+    /// optimistically as soon as the relay receives it.
     pub async fn subscribe(&self, topic: Topic) -> Response<rpc::Subscribe> {
-        self.request(rpc::Subscribe { topic }).await
+        self.request(rpc::Subscribe {
+            topic,
+            block: false,
+        })
+        .await
+    }
+
+    /// Subscribes on topic to receive messages. The request is resolved only
+    /// when fully processed by the relay.
+    /// Note: This function is experimental and will likely be removed in the
+    /// future.
+    pub async fn subscribe_blocking(&self, topic: Topic) -> Response<rpc::Subscribe> {
+        self.request(rpc::Subscribe { topic, block: true }).await
     }
 
     /// Unsubscribes from a topic.
@@ -223,13 +236,30 @@ impl Client {
         self.request(payload).await
     }
 
-    /// Subscribes on multiple topics to receive messages.
+    /// Subscribes on multiple topics to receive messages. The request is
+    /// resolved optimistically as soon as the relay receives it.
     pub async fn batch_subscribe(
         &self,
         topics: impl Into<Vec<Topic>>,
     ) -> Response<rpc::BatchSubscribe> {
         self.request(rpc::BatchSubscribe {
             topics: topics.into(),
+            block: false,
+        })
+        .await
+    }
+
+    /// Subscribes on multiple topics to receive messages. The request is
+    /// resolved only when fully processed by the relay.
+    /// Note: This function is experimental and will likely be removed in the
+    /// future.
+    pub async fn batch_subscribe_blocking(
+        &self,
+        topics: impl Into<Vec<Topic>>,
+    ) -> Response<rpc::BatchSubscribe> {
+        self.request(rpc::BatchSubscribe {
+            topics: topics.into(),
+            block: true,
         })
         .await
     }
diff --git a/relay_client/src/websocket.rs b/relay_client/src/websocket.rs
index 4805f1d..c3f71c4 100644
--- a/relay_client/src/websocket.rs
+++ b/relay_client/src/websocket.rs
@@ -165,9 +165,25 @@ impl Client {
         EmptyResponseFuture::new(response)
     }
 
-    /// Subscribes on topic to receive messages.
+    /// Subscribes on topic to receive messages. The request is resolved
+    /// optimistically as soon as the relay receives it.
     pub fn subscribe(&self, topic: Topic) -> ResponseFuture<Subscribe> {
-        let (request, response) = create_request(Subscribe { topic });
+        let (request, response) = create_request(Subscribe {
+            topic,
+            block: false,
+        });
+
+        self.request(request);
+
+        response
+    }
+
+    /// Subscribes on topic to receive messages. The request is resolved only
+    /// when fully processed by the relay.
+    /// Note: This function is experimental and will likely be removed in the
+    /// future.
+    pub fn subscribe_blocking(&self, topic: Topic) -> ResponseFuture<Subscribe> {
+        let (request, response) = create_request(Subscribe { topic, block: true });
 
         self.request(request);
 
@@ -204,10 +220,30 @@ impl Client {
         FetchMessageStream::new(self.clone(), topics.into())
     }
 
-    /// Subscribes on multiple topics to receive messages.
+    /// Subscribes on multiple topics to receive messages. The request is
+    /// resolved optimistically as soon as the relay receives it.
     pub fn batch_subscribe(&self, topics: impl Into<Vec<Topic>>) -> ResponseFuture<BatchSubscribe> {
         let (request, response) = create_request(BatchSubscribe {
             topics: topics.into(),
+            block: false,
+        });
+
+        self.request(request);
+
+        response
+    }
+
+    /// Subscribes on multiple topics to receive messages. The request is
+    /// resolved only when fully processed by the relay.
+    /// Note: This function is experimental and will likely be removed in the
+    /// future.
+    pub fn batch_subscribe_blocking(
+        &self,
+        topics: impl Into<Vec<Topic>>,
+    ) -> ResponseFuture<BatchSubscribe> {
+        let (request, response) = create_request(BatchSubscribe {
+            topics: topics.into(),
+            block: true,
         });
 
         self.request(request);
diff --git a/relay_rpc/src/rpc.rs b/relay_rpc/src/rpc.rs
index ebbf799..e50564d 100644
--- a/relay_rpc/src/rpc.rs
+++ b/relay_rpc/src/rpc.rs
@@ -306,6 +306,11 @@ pub struct ErrorData {
 pub struct Subscribe {
     /// The topic to subscribe to.
     pub topic: Topic,
+
+    /// Whether to disable optimistic response. By default optimistic response
+    /// is enabled.
+    #[serde(default)]
+    pub block: bool,
 }
 
 impl RequestPayload for Subscribe {
@@ -403,6 +408,11 @@ pub struct FetchResponse {
 pub struct BatchSubscribe {
     /// The topics to subscribe to.
     pub topics: Vec<Topic>,
+
+    /// Whether to disable optimistic response. By default optimistic response
+    /// is enabled.
+    #[serde(default)]
+    pub block: bool,
 }
 
 impl RequestPayload for BatchSubscribe {
diff --git a/relay_rpc/src/rpc/tests.rs b/relay_rpc/src/rpc/tests.rs
index 6fc6a01..074c492 100644
--- a/relay_rpc/src/rpc/tests.rs
+++ b/relay_rpc/src/rpc/tests.rs
@@ -31,6 +31,7 @@ fn subscribe() {
         1659980684711969.into(),
         Params::Subscribe(Subscribe {
             topic: "c4163cf65859106b3f5435fc296e7765411178ed452d1c30337a6230138c9840".into(),
+            block: false,
         }),
     ));
 
@@ -38,7 +39,7 @@ fn subscribe() {
 
     assert_eq!(
         &serialized,
-        r#"{"id":1659980684711969,"jsonrpc":"2.0","method":"irn_subscribe","params":{"topic":"c4163cf65859106b3f5435fc296e7765411178ed452d1c30337a6230138c9840"}}"#
+        r#"{"id":1659980684711969,"jsonrpc":"2.0","method":"irn_subscribe","params":{"topic":"c4163cf65859106b3f5435fc296e7765411178ed452d1c30337a6230138c9840","block":false}}"#
     );
 
     let deserialized: Payload = serde_json::from_str(&serialized).unwrap();
@@ -206,7 +207,8 @@ fn deserialize_batch_methods() {
                 topics: vec![
                     Topic::from("c4163cf65859106b3f5435fc296e7765411178ed452d1c30337a6230138c9840"),
                     Topic::from("c4163cf65859106b3f5435fc296e7765411178ed452d1c30337a6230138c9841")
-                ]
+                ],
+                block: false
             })
         })
     );
@@ -332,6 +334,7 @@ fn validation() {
         jsonrpc: jsonrpc.clone(),
         params: Params::Subscribe(Subscribe {
             topic: topic.clone(),
+            block: false,
         }),
     };
     assert_eq!(request.validate(), Ok(()));
@@ -342,6 +345,7 @@ fn validation() {
         jsonrpc: jsonrpc.clone(),
         params: Params::Subscribe(Subscribe {
             topic: Topic::from("invalid"),
+            block: false,
         }),
     };
     assert_eq!(
@@ -459,6 +463,7 @@ fn validation() {
         jsonrpc: jsonrpc.clone(),
         params: Params::BatchSubscribe(BatchSubscribe {
             topics: vec![topic.clone()],
+            block: false,
         }),
     };
     assert_eq!(request.validate(), Ok(()));
@@ -467,7 +472,10 @@ fn validation() {
     let request = Request {
         id,
         jsonrpc: jsonrpc.clone(),
-        params: Params::BatchSubscribe(BatchSubscribe { topics: vec![] }),
+        params: Params::BatchSubscribe(BatchSubscribe {
+            topics: vec![],
+            block: false,
+        }),
     };
     assert_eq!(request.validate(), Err(ValidationError::BatchEmpty));
 
@@ -478,7 +486,10 @@ fn validation() {
     let request = Request {
         id,
         jsonrpc: jsonrpc.clone(),
-        params: Params::BatchSubscribe(BatchSubscribe { topics }),
+        params: Params::BatchSubscribe(BatchSubscribe {
+            topics,
+            block: false,
+        }),
     };
     assert_eq!(
         request.validate(),
@@ -496,6 +507,7 @@ fn validation() {
             topics: vec![Topic::from(
                 "c4163cf65859106b3f5435fc296e7765411178ed452d1c30337a6230138c98401",
             )],
+            block: false,
         }),
     };
     assert_eq!(