diff --git a/Cargo.toml b/Cargo.toml
index 8228fb45..576f2c61 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -48,6 +48,9 @@ url             = {version = "2.5", features = ["serde"]}
 [workspace.metadata.release]
 tag = false
 
+[profile.dev]
+opt-level = 1
+
 [profile.dev.build-override]
 opt-level = 3
 
diff --git a/crates/languageserver/src/main.rs b/crates/languageserver/src/main.rs
index 9ddb4e56..a85487ca 100644
--- a/crates/languageserver/src/main.rs
+++ b/crates/languageserver/src/main.rs
@@ -1,11 +1,13 @@
 #![recursion_limit = "256"]
 
-use tower_lsp::{LspService, Server};
-
 mod backend;
 mod keyword;
 mod server;
+#[cfg(test)]
+mod tests;
+
 use backend::Backend;
+use tower_lsp::{LspService, Server};
 
 #[tokio::main]
 async fn main() {
diff --git a/crates/languageserver/src/tests.rs b/crates/languageserver/src/tests.rs
new file mode 100644
index 00000000..72098c85
--- /dev/null
+++ b/crates/languageserver/src/tests.rs
@@ -0,0 +1,243 @@
+use crate::Backend;
+use serde_json::{json, Value};
+use std::collections::VecDeque;
+use std::env;
+use std::path::PathBuf;
+use tokio::io::{AsyncReadExt, AsyncWriteExt, DuplexStream};
+use tower_lsp::jsonrpc::{Id, Request, Response};
+use tower_lsp::lsp_types::*;
+use tower_lsp::{Client, LanguageServer, LspService, Server};
+
+struct TestServer {
+    req_stream: DuplexStream,
+    res_stream: DuplexStream,
+    responses: VecDeque<String>,
+}
+
+impl TestServer {
+    fn new<F, S>(init: F) -> Self
+    where
+        F: FnOnce(Client) -> S,
+        S: LanguageServer,
+    {
+        let (req_client, req_server) = tokio::io::duplex(1024);
+        let (res_server, res_client) = tokio::io::duplex(1024);
+
+        let (service, socket) = LspService::new(init);
+
+        tokio::spawn(Server::new(req_server, res_server, socket).serve(service));
+
+        Self {
+            req_stream: req_client,
+            res_stream: res_client,
+            responses: VecDeque::new(),
+        }
+    }
+
+    fn encode(payload: &str) -> String {
+        format!("Content-Length: {}\r\n\r\n{}", payload.len(), payload)
+    }
+
+    fn decode(text: &str) -> Vec<String> {
+        let mut ret = Vec::new();
+        let mut temp = text;
+
+        while !temp.is_empty() {
+            let p = temp.find("\r\n\r\n").unwrap();
+            let (header, body) = temp.split_at(p + 4);
+            let len = header
+                .strip_prefix("Content-Length: ")
+                .unwrap()
+                .strip_suffix("\r\n\r\n")
+                .unwrap();
+            let len: usize = len.parse().unwrap();
+            let (body, rest) = body.split_at(len);
+            ret.push(body.to_string());
+            temp = rest;
+        }
+
+        ret
+    }
+
+    async fn send_request(&mut self, req: Request) {
+        let req = serde_json::to_string(&req).unwrap();
+        let req = Self::encode(&req);
+        self.req_stream.write_all(req.as_bytes()).await.unwrap();
+    }
+
+    async fn send_ack(&mut self, id: &Id) {
+        let req = Response::from_ok(id.clone(), None::<serde_json::Value>.into());
+        let req = serde_json::to_string(&req).unwrap();
+        let req = Self::encode(&req);
+        self.req_stream.write_all(req.as_bytes()).await.unwrap();
+    }
+
+    async fn recv_response(&mut self) -> Response {
+        if self.responses.is_empty() {
+            let mut buf = vec![0; 1024];
+            let n = self.res_stream.read(&mut buf).await.unwrap();
+            let ret = String::from_utf8(buf[..n].to_vec()).unwrap();
+            for x in Self::decode(&ret) {
+                self.responses.push_front(x);
+            }
+        }
+        let res = self.responses.pop_back().unwrap();
+        serde_json::from_str(&res).unwrap()
+    }
+
+    async fn recv_notification(&mut self) -> Request {
+        if self.responses.is_empty() {
+            let mut buf = vec![0; 1024];
+            let n = self.res_stream.read(&mut buf).await.unwrap();
+            let ret = String::from_utf8(buf[..n].to_vec()).unwrap();
+            for x in Self::decode(&ret) {
+                self.responses.push_front(x);
+            }
+        }
+        let res = self.responses.pop_back().unwrap();
+        serde_json::from_str(&res).unwrap()
+    }
+}
+
+fn build_initialize(id: i64) -> Request {
+    let params = InitializeParams::default();
+    Request::build("initialize")
+        .params(json!(params))
+        .id(id)
+        .finish()
+}
+
+fn build_initialized() -> Request {
+    let params = InitializedParams {};
+    Request::build("initialized").params(json!(params)).finish()
+}
+
+fn build_did_open(text: &str) -> Request {
+    let mut path = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
+    path.pop();
+    path.pop();
+    path.push("test.veryl");
+    let uri = Url::from_file_path(path).unwrap();
+    let text_document = TextDocumentItem {
+        uri,
+        language_id: "veryl".to_string(),
+        version: 0,
+        text: text.to_string(),
+    };
+
+    let params = DidOpenTextDocumentParams { text_document };
+
+    Request::build("textDocument/didOpen")
+        .params(json!(params))
+        .finish()
+}
+
+#[tokio::test]
+async fn did_open() {
+    let mut server = TestServer::new(Backend::new);
+
+    let req = build_initialize(1);
+    server.send_request(req).await;
+    let res = server.recv_response().await;
+    assert!(res.is_ok());
+
+    let req = build_initialized();
+    server.send_request(req).await;
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "window/logMessage");
+    assert_eq!(res.params().unwrap()["message"], "server initialized!");
+
+    let req = build_did_open("module A {}");
+    server.send_request(req).await;
+
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "window/logMessage");
+    assert_eq!(res.params().unwrap()["message"], "did_open");
+
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "textDocument/publishDiagnostics");
+    assert!(res.params().unwrap()["diagnostics"]
+        .as_array()
+        .unwrap()
+        .is_empty());
+}
+
+#[tokio::test]
+async fn diagnostics() {
+    let mut server = TestServer::new(Backend::new);
+
+    let req = build_initialize(1);
+    server.send_request(req).await;
+    let res = server.recv_response().await;
+    assert!(res.is_ok());
+
+    let req = build_initialized();
+    server.send_request(req).await;
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "window/logMessage");
+    assert_eq!(res.params().unwrap()["message"], "server initialized!");
+
+    let req = build_did_open("module A { var a: logic; }");
+    server.send_request(req).await;
+
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "window/logMessage");
+    assert_eq!(res.params().unwrap()["message"], "did_open");
+
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "textDocument/publishDiagnostics");
+    let diags = res.params().unwrap()["diagnostics"].as_array().unwrap();
+    assert_eq!(diags[0]["code"], Value::from("unused_variable"));
+    assert_eq!(diags[0]["range"]["start"]["character"], Value::from(15));
+    assert_eq!(diags[0]["range"]["start"]["line"], Value::from(0));
+    assert_eq!(diags[0]["range"]["end"]["character"], Value::from(16));
+    assert_eq!(diags[0]["range"]["end"]["line"], Value::from(0));
+}
+
+#[tokio::test]
+async fn progress() {
+    let mut server = TestServer::new(Backend::new);
+
+    let req = build_initialize(1);
+    server.send_request(req).await;
+    let res = server.recv_response().await;
+    assert!(res.is_ok());
+
+    let req = build_initialized();
+    server.send_request(req).await;
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "window/logMessage");
+    assert_eq!(res.params().unwrap()["message"], "server initialized!");
+
+    let req = build_did_open("module A {}");
+    server.send_request(req).await;
+
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "window/logMessage");
+    assert_eq!(res.params().unwrap()["message"], "did_open");
+
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "textDocument/publishDiagnostics");
+
+    let res = server.recv_notification().await;
+    assert_eq!(res.method(), "window/workDoneProgress/create");
+
+    server.send_ack(res.id().unwrap()).await;
+
+    let mut progress_finish = false;
+    let mut percentage = 0;
+    while !progress_finish {
+        let res = server.recv_notification().await;
+        dbg!(&res);
+        if res.method() == "$/progress" {
+            if res.params().unwrap()["value"]["kind"] == Value::from("end") {
+                progress_finish = true;
+            } else if res.params().unwrap()["value"]["kind"] == Value::from("report") {
+                percentage = res.params().unwrap()["value"]["percentage"]
+                    .as_i64()
+                    .unwrap();
+            }
+        }
+    }
+    assert_eq!(percentage, 100);
+}