diff --git a/Cargo.toml b/Cargo.toml
index 8105ee7..8890f35 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,3 +14,6 @@ ratatui = "0.30.0"
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.149"
 uuid = { version = "1.22.0", features = ["v4", "serde"] }
+
+[dev-dependencies]
+tempfile = "3"
diff --git a/src/core/commit.rs b/src/core/commit.rs
index 5a7a7b1..6482407 100644
--- a/src/core/commit.rs
+++ b/src/core/commit.rs
@@ -3,6 +3,7 @@ use std::process::{Command, ExitStatus};
 
 use crate::core::git::{self, RepoContext};
 use crate::core::restack::{self, RestackPreview};
+use crate::core::store::lock::StoreLock;
 use crate::core::store::{
     BranchDivergenceState, PendingCommitEntry, PendingCommitOperation, PendingOperationKind,
     PendingOperationState, StoreSession, dagger_paths, load_config, load_state, open_initialized,
@@ -299,6 +300,7 @@ fn maybe_restack_after_commit_inner(
         return Ok(PostCommitRestackOutcome::default());
     }
 
+    let lock = StoreLock::acquire(&store_paths.root)?;
     let state = load_state(&store_paths)?;
     let Some(node) = state.find_branch_by_name(current_branch).cloned() else {
         return Ok(PostCommitRestackOutcome::default());
@@ -306,12 +308,7 @@ fn maybe_restack_after_commit_inner(
 
     let actions =
         restack::plan_after_branch_advance(&state, node.id, &node.branch_name, old_head_oid)?;
-    let mut session = StoreSession {
-        repo: context.repo.clone(),
-        paths: store_paths,
-        config,
-        state,
-    };
+    let mut session = StoreSession::from_lock(context.repo.clone(), store_paths, config, state, lock);
     record_branch_divergence_state(&mut session, node.id, BranchDivergenceState::Diverged)?;
     let restack_outcome = match workflow::execute_resumable_restack_operation(
         &mut session,
diff --git a/src/core/store/lock.rs b/src/core/store/lock.rs
new file mode 100644
index 0000000..3fc1603
--- /dev/null
+++ b/src/core/store/lock.rs
@@ -0,0 +1,83 @@
+use std::fs;
+use std::io;
+use std::path::{Path, PathBuf};
+
+/// An advisory lock file that prevents concurrent dagger operations.
+/// The lock is released (file deleted) when the guard is dropped.
+pub struct StoreLock {
+    lock_path: PathBuf,
+}
+
+impl StoreLock {
+    /// Acquire an advisory lock by creating a lock file.
+    /// Returns an error if another process holds the lock.
+    pub fn acquire(dagger_root: &Path) -> io::Result<Self> {
+        fs::create_dir_all(dagger_root)?;
+        let lock_path = dagger_root.join("lock");
+
+        // Try to create the lock file exclusively.
+        // O_CREAT | O_EXCL ensures atomic creation — fails if file already exists.
+        match fs::OpenOptions::new()
+            .write(true)
+            .create_new(true)
+            .open(&lock_path)
+        {
+            Ok(mut file) => {
+                use std::io::Write;
+                let _ = writeln!(file, "{}", std::process::id());
+                Ok(Self { lock_path })
+            }
+            Err(e) if e.kind() == io::ErrorKind::AlreadyExists => Err(io::Error::other(format!(
+                "another dgr process appears to be running; \
+                 if this is incorrect, delete '{}'",
+                lock_path.display()
+            ))),
+            Err(e) => Err(e),
+        }
+    }
+}
+
+impl Drop for StoreLock {
+    fn drop(&mut self) {
+        let _ = fs::remove_file(&self.lock_path);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn acquire_creates_lock_file() {
+        let dir = tempfile::tempdir().unwrap();
+        let lock = StoreLock::acquire(dir.path()).unwrap();
+        assert!(dir.path().join("lock").exists());
+        drop(lock);
+    }
+
+    #[test]
+    fn drop_removes_lock_file() {
+        let dir = tempfile::tempdir().unwrap();
+        let lock = StoreLock::acquire(dir.path()).unwrap();
+        drop(lock);
+        assert!(!dir.path().join("lock").exists());
+    }
+
+    #[test]
+    fn second_acquire_fails_while_held() {
+        let dir = tempfile::tempdir().unwrap();
+        let _lock = StoreLock::acquire(dir.path()).unwrap();
+        let result = StoreLock::acquire(dir.path());
+        assert!(result.is_err());
+        let err = result.unwrap_err();
+        assert!(err.to_string().contains("another dgr process"));
+    }
+
+    #[test]
+    fn acquire_succeeds_after_release() {
+        let dir = tempfile::tempdir().unwrap();
+        let lock = StoreLock::acquire(dir.path()).unwrap();
+        drop(lock);
+        let _lock2 = StoreLock::acquire(dir.path()).unwrap();
+    }
+}
diff --git a/src/core/store/mod.rs b/src/core/store/mod.rs
index 3a40e50..3ed48f9 100644
--- a/src/core/store/mod.rs
+++ b/src/core/store/mod.rs
@@ -2,6 +2,7 @@ pub(crate) mod bootstrap;
 pub(crate) mod config;
 pub(crate) mod events;
 pub(crate) mod fs;
+pub(crate) mod lock;
 pub(crate) mod mutations;
 pub(crate) mod operation;
 pub(crate) mod session;
diff --git a/src/core/store/session.rs b/src/core/store/session.rs
index 736546b..dafc9ea 100644
--- a/src/core/store/session.rs
+++ b/src/core/store/session.rs
@@ -2,24 +2,56 @@ use std::io;
 
 use crate::core::git::{self, RepoContext};
 
+use super::lock::StoreLock;
 use super::{
     DaggerConfig, StoreInitialization, dagger_paths, initialize_store, load_config, load_state,
 };
 use crate::core::store::fs::DaggerPaths;
 use crate::core::store::types::DaggerState;
 
-#[derive(Debug, Clone)]
 pub struct StoreSession {
     pub repo: RepoContext,
     pub paths: DaggerPaths,
     pub config: DaggerConfig,
     pub state: DaggerState,
+    _lock: StoreLock,
+}
+
+impl StoreSession {
+    /// Build a session from pre-loaded parts with an already-acquired lock.
+    pub fn from_lock(
+        repo: RepoContext,
+        paths: DaggerPaths,
+        config: DaggerConfig,
+        state: DaggerState,
+        lock: StoreLock,
+    ) -> Self {
+        Self {
+            repo,
+            paths,
+            config,
+            state,
+            _lock: lock,
+        }
+    }
+
+    /// Build a session from pre-loaded parts, acquiring the lock.
+    pub fn from_parts(
+        repo: RepoContext,
+        paths: DaggerPaths,
+        config: DaggerConfig,
+        state: DaggerState,
+    ) -> io::Result<Self> {
+        let lock = StoreLock::acquire(&paths.root)?;
+        Ok(Self::from_lock(repo, paths, config, state, lock))
+    }
 }
 
 pub fn open_initialized(missing_message: &str) -> io::Result<StoreSession> {
     let repo = git::resolve_repo_context()?;
     let paths = dagger_paths(&repo.git_dir);
     let config = load_config(&paths)?.ok_or_else(|| io::Error::other(missing_message))?;
+    let lock = StoreLock::acquire(&paths.root)?;
     let state = load_state(&paths)?;
 
     Ok(StoreSession {
@@ -27,12 +59,14 @@ pub fn open_initialized(missing_message: &str) -> io::Result<StoreSession> {
         paths,
         config,
         state,
+        _lock: lock,
     })
 }
 
 pub fn open_or_initialize(trunk_branch: &str) -> io::Result<(StoreSession, StoreInitialization)> {
     let repo = git::resolve_repo_context()?;
     let paths = dagger_paths(&repo.git_dir);
+    let lock = StoreLock::acquire(&paths.root)?;
     let store_initialization = initialize_store(&paths, trunk_branch)?;
     let config =
         load_config(&paths)?.ok_or_else(|| io::Error::other("dagger config is missing"))?;
@@ -44,6 +78,7 @@ pub fn open_or_initialize(trunk_branch: &str) -> io::Result<(StoreSession, Store
             paths,
             config,
             state,
+            _lock: lock,
         },
         store_initialization,
     ))