diff --git a/Cargo.toml b/Cargo.toml
index 1293ac44f..1e2851ea5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -31,6 +31,7 @@ repository = "https://github.com/erg-lang/erg"
 homepage = "https://erg-lang.org/"
 
 [features]
+default = ["parallel"]
 # when "debug" feature is turned on, that of the following crates will also be turned on.
 debug = ["erg_common/debug", "erg_parser/debug", "erg_compiler/debug", "erg_linter/debug"] # "els/debug"
 backtrace = ["erg_common/backtrace", "els/backtrace"]
@@ -69,9 +70,8 @@ gal = ["erg_common/gal", "erg_compiler/gal"]
 els = ["erg_common/els", "erg_compiler/els", "dep:els"]
 full-repl = ["erg_common/full-repl"]
 full = ["els", "full-repl", "unicode", "pretty"]
-experimental = ["erg_common/experimental", "erg_parser/experimental", "erg_compiler/experimental", "erg_linter/experimental", "parallel"]
+experimental = ["erg_common/experimental", "erg_parser/experimental", "erg_compiler/experimental", "erg_linter/experimental"]
 log-level-error = ["erg_common/log-level-error", "erg_parser/log-level-error", "erg_compiler/log-level-error", "erg_linter/log-level-error"]
-# The parallelizing compiler was found to contain a bug that caused it to hang in complex dependencies, so it is disabled by default.
 parallel = ["erg_common/parallel", "erg_parser/parallel", "erg_compiler/parallel", "erg_linter/parallel"]
 
 [workspace.dependencies]
diff --git a/crates/erg_common/Cargo.toml b/crates/erg_common/Cargo.toml
index f9f61c7c6..ebef03e16 100644
--- a/crates/erg_common/Cargo.toml
+++ b/crates/erg_common/Cargo.toml
@@ -10,6 +10,7 @@ repository.workspace = true
 homepage.workspace = true
 
 [features]
+default = ["parallel"]
 debug = ["dep:backtrace-on-stack-overflow", "dep:w-boson"]
 backtrace = ["dep:backtrace-on-stack-overflow", "dep:w-boson"]
 japanese = []
@@ -23,7 +24,7 @@ py_compat = []
 gal = []
 no_std = []
 full-repl = ["dep:crossterm"]
-experimental = ["parallel"]
+experimental = []
 pylib = ["dep:pyo3"]
 log-level-error = []
 parallel = []
diff --git a/crates/erg_compiler/Cargo.toml b/crates/erg_compiler/Cargo.toml
index a5c4c8ec9..c789218df 100644
--- a/crates/erg_compiler/Cargo.toml
+++ b/crates/erg_compiler/Cargo.toml
@@ -11,6 +11,7 @@ repository.workspace = true
 homepage.workspace = true
 
 [features]
+default = ["parallel"]
 # when "debug" feature is turned on, that of parser will also be turned on.
 debug = ["erg_common/debug", "erg_parser/debug"]
 japanese = ["erg_common/japanese", "erg_parser/japanese"]
@@ -39,7 +40,7 @@ gal = ["erg_common/gal"]
 els = ["erg_common/els"]
 no_std = ["erg_common/no_std"]
 full-repl = ["erg_common/full-repl"]
-experimental = ["erg_common/experimental", "erg_parser/experimental", "parallel"]
+experimental = ["erg_common/experimental", "erg_parser/experimental"]
 pylib = ["dep:pyo3", "erg_common/pylib", "erg_parser/pylib"]
 pylib_compiler = ["pylib"]
 log-level-error = ["erg_common/log-level-error", "erg_parser/log-level-error"]
diff --git a/crates/erg_compiler/module/promise.rs b/crates/erg_compiler/module/promise.rs
index 268ad978f..db84b9543 100644
--- a/crates/erg_compiler/module/promise.rs
+++ b/crates/erg_compiler/module/promise.rs
@@ -17,7 +17,9 @@ pub enum Promise {
         parent: ThreadId,
         handle: JoinHandle<()>,
     },
-    Joining,
+    Joining {
+        id: ThreadId,
+    },
     Joined,
 }
 
@@ -27,7 +29,7 @@ impl fmt::Display for Promise {
             Self::Running { handle, .. } => {
                 write!(f, "running on thread {:?}", handle.thread().id())
             }
-            Self::Joining => write!(f, "joining"),
+            Self::Joining { id } => write!(f, "joining (thread {id:?})"),
             Self::Joined => write!(f, "joined"),
         }
     }
@@ -44,36 +46,40 @@ impl Promise {
     /// can be joined if `true`
     pub fn is_finished(&self) -> bool {
         match self {
-            Self::Joined => true,
-            Self::Joining => false,
+            Self::Joined { .. } => true,
+            Self::Joining { .. } => false,
             Self::Running { handle, .. } => handle.is_finished(),
         }
     }
 
     pub const fn is_joined(&self) -> bool {
-        matches!(self, Self::Joined)
+        matches!(self, Self::Joined { .. })
     }
 
     pub const fn is_joining(&self) -> bool {
-        matches!(self, Self::Joining)
+        matches!(self, Self::Joining { .. })
     }
 
     pub fn thread_id(&self) -> Option<ThreadId> {
         match self {
-            Self::Joined | Self::Joining => None,
+            Self::Joined => None,
+            Self::Joining { id } => Some(*id),
             Self::Running { handle, .. } => Some(handle.thread().id()),
         }
     }
 
     pub fn parent_thread_id(&self) -> Option<ThreadId> {
         match self {
-            Self::Joined | Self::Joining => None,
+            Self::Joined { .. } | Self::Joining { .. } => None,
             Self::Running { parent, .. } => Some(*parent),
         }
     }
 
     pub fn take(&mut self) -> Self {
-        std::mem::replace(self, Self::Joining)
+        let joining = Self::Joining {
+            id: self.thread_id().unwrap(),
+        };
+        std::mem::replace(self, joining)
     }
 }
 
@@ -197,7 +203,12 @@ impl SharedPromises {
                 return Ok(());
             }
         }
-        while let Some(Promise::Joining) | None = self.promises.borrow().get(path) {
+        // prevent deadlock
+        if self.thread_id(path).is_some_and(|id| id == current().id()) {
+            *self.promises.borrow_mut().get_mut(path).unwrap() = Promise::Joined;
+            return Ok(());
+        }
+        while let Some(Promise::Joining { .. }) | None = self.promises.borrow().get(path) {
             safe_yield();
         }
         if self.is_joined(path) {
@@ -209,10 +220,6 @@ impl SharedPromises {
             self.wait_until_finished(path);
             return Ok(());
         };
-        if handle.thread().id() == current().id() {
-            *self.promises.borrow_mut().get_mut(path).unwrap() = Promise::Joined;
-            return Ok(());
-        }
         let res = handle.join();
         *self.promises.borrow_mut().get_mut(path).unwrap() = Promise::Joined;
         res
@@ -239,6 +246,13 @@ impl SharedPromises {
         }
     }
 
+    pub fn thread_id(&self, path: &NormalizedPathBuf) -> Option<ThreadId> {
+        self.promises
+            .borrow()
+            .get(path)
+            .and_then(|promise| promise.thread_id())
+    }
+
     pub fn progress(&self) -> Progress {
         let mut total = 0;
         let mut running = 0;
@@ -246,8 +260,8 @@ impl SharedPromises {
         for promise in self.promises.borrow().values() {
             match promise {
                 Promise::Running { .. } => running += 1,
-                Promise::Joining => finished += 1,
-                Promise::Joined => finished += 1,
+                Promise::Joining { .. } => finished += 1,
+                Promise::Joined { .. } => finished += 1,
             }
             total += 1;
         }
diff --git a/crates/erg_linter/Cargo.toml b/crates/erg_linter/Cargo.toml
index be3bd31be..7d9a1efa0 100644
--- a/crates/erg_linter/Cargo.toml
+++ b/crates/erg_linter/Cargo.toml
@@ -10,6 +10,7 @@ repository.workspace = true
 homepage.workspace = true
 
 [features]
+default = ["parallel"]
 debug = ["erg_common/debug", "erg_parser/debug", "erg_compiler/debug"]
 backtrace = ["erg_common/backtrace"]
 japanese = [
diff --git a/crates/erg_parser/Cargo.toml b/crates/erg_parser/Cargo.toml
index 1d240e958..5f18bdffe 100644
--- a/crates/erg_parser/Cargo.toml
+++ b/crates/erg_parser/Cargo.toml
@@ -10,6 +10,7 @@ repository.workspace = true
 homepage.workspace = true
 
 [features]
+default = ["parallel"]
 debug = ["erg_common/debug"]
 japanese = ["erg_common/japanese"]
 simplified_chinese = ["erg_common/simplified_chinese"]
@@ -17,7 +18,7 @@ traditional_chinese = ["erg_common/traditional_chinese"]
 unicode = ["erg_common/unicode"]
 pretty = ["erg_common/pretty"]
 large_thread = ["erg_common/large_thread"]
-experimental = ["erg_common/experimental", "parallel"]
+experimental = ["erg_common/experimental"]
 pylib = ["dep:pyo3", "erg_common/pylib"]
 pylib_parser = ["pylib"]
 log-level-error = ["erg_common/log-level-error"]
diff --git a/doc/EN/dev_guide/build_features.md b/doc/EN/dev_guide/build_features.md
index 52a4df21e..3c27ae566 100644
--- a/doc/EN/dev_guide/build_features.md
+++ b/doc/EN/dev_guide/build_features.md
@@ -43,7 +43,7 @@ Enable Python-compatible mode, which makes parts of the APIs and syntax compatib
 
 ## experimental
 
-Enable experimental features (contains `parallel`).
+Enable experimental features.
 
 ## log-level-error
 
@@ -51,4 +51,4 @@ Only display error logs.
 
 ## parallel
 
-Enable compiler parallelization. Unstable feature.
+Enable compiler parallelization. Default is enabled.
diff --git a/doc/JA/dev_guide/build_features.md b/doc/JA/dev_guide/build_features.md
index 27e72bd6d..768da6b14 100644
--- a/doc/JA/dev_guide/build_features.md
+++ b/doc/JA/dev_guide/build_features.md
@@ -45,7 +45,7 @@ Python互換モードを有効にする。APIや文法の一部がPythonと互
 
 ## experimental
 
-実験的な機能を有効にする。`parallel`も有効化される。
+実験的な機能を有効にする。
 
 ## log-level-error
 
@@ -53,4 +53,4 @@ Python互換モードを有効にする。APIや文法の一部がPythonと互
 
 ## parallel
 
-コンパイラの並列化を有効にする。不安定機能。
+コンパイラの並列化を有効にする。デフォルトは有効。