diff --git a/crates/backend/src/lib.rs b/crates/backend/src/lib.rs
index e63426f..082b4b5 100644
--- a/crates/backend/src/lib.rs
+++ b/crates/backend/src/lib.rs
@@ -106,6 +106,8 @@ pub mod openai_tools;
 pub mod stream;
 mod ticket_dto;
 
+pub use infrastructure::{acquire_app_data_dir_lock, AppDataDirLock, AppDataDirLockError};
+
 use crate::mcp_endpoint::{mcp_endpoint, AppMcpRuntimeProvider, McpEndpoint};
 use crate::openai_tools::{AppOpenAiToolInvoker, LateBoundOpenAiToolInvoker};
 
diff --git a/crates/infrastructure/src/lock.rs b/crates/infrastructure/src/lock.rs
new file mode 100644
index 0000000..ed28e58
--- /dev/null
+++ b/crates/infrastructure/src/lock.rs
@@ -0,0 +1,199 @@
+//! Process-wide advisory lock for the effective IdeA app-data directory.
+
+use std::fs::{self, File, OpenOptions};
+use std::io;
+use std::path::{Path, PathBuf};
+
+use fs4::fs_std::FileExt;
+use thiserror::Error;
+
+const LOCK_FILE_NAME: &str = "idea.lock";
+
+/// RAII guard holding the exclusive advisory lock on `<app_data_dir>/idea.lock`.
+///
+/// The lock is released when this value is dropped or the process exits.
+#[derive(Debug)]
+pub struct AppDataDirLock {
+    lock_path: PathBuf,
+    file: File,
+}
+
+impl AppDataDirLock {
+    /// Path of the lock file currently held by this guard.
+    #[must_use]
+    pub fn lock_path(&self) -> &Path {
+        &self.lock_path
+    }
+}
+
+impl Drop for AppDataDirLock {
+    fn drop(&mut self) {
+        let _ = self.file.unlock();
+    }
+}
+
+/// Error returned while acquiring the app-data lock.
+#[derive(Debug, Error)]
+pub enum AppDataDirLockError {
+    /// Another process already holds the lock for this app-data directory.
+    #[error("app-data lock already held: {lock_path}")]
+    Contended {
+        /// Lock file path.
+        lock_path: PathBuf,
+    },
+    /// The lock file or its parent directory could not be accessed.
+    #[error("failed to access app-data lock {lock_path}: {source}")]
+    Io {
+        /// Lock file path, or the intended path when directory creation failed.
+        lock_path: PathBuf,
+        /// Underlying I/O error.
+        #[source]
+        source: io::Error,
+    },
+}
+
+/// Acquires an exclusive non-blocking advisory lock for `app_data_dir`.
+///
+/// The effective lock file is `<app_data_dir>/idea.lock`; different app-data
+/// directories therefore do not block each other.
+///
+/// # Errors
+/// [`AppDataDirLockError::Contended`] when another process already holds the
+/// same lock, or [`AppDataDirLockError::Io`] for filesystem access failures.
+pub fn acquire_app_data_dir_lock(
+    app_data_dir: impl AsRef<Path>,
+) -> Result<AppDataDirLock, AppDataDirLockError> {
+    let app_data_dir = app_data_dir.as_ref();
+    let lock_path = app_data_dir.join(LOCK_FILE_NAME);
+    fs::create_dir_all(app_data_dir).map_err(|source| AppDataDirLockError::Io {
+        lock_path: lock_path.clone(),
+        source,
+    })?;
+    let file = OpenOptions::new()
+        .read(true)
+        .write(true)
+        .create(true)
+        .open(&lock_path)
+        .map_err(|source| AppDataDirLockError::Io {
+            lock_path: lock_path.clone(),
+            source,
+        })?;
+    match file.try_lock_exclusive() {
+        Ok(()) => Ok(AppDataDirLock { lock_path, file }),
+        Err(source) if source.kind() == io::ErrorKind::WouldBlock => {
+            Err(AppDataDirLockError::Contended { lock_path })
+        }
+        Err(source) => Err(AppDataDirLockError::Io { lock_path, source }),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::env;
+    use std::process::{Command, Stdio};
+    use std::thread;
+    use std::time::{Duration, Instant};
+
+    use super::*;
+
+    struct TempDir(PathBuf);
+
+    impl TempDir {
+        fn new(label: &str) -> Self {
+            let path =
+                env::temp_dir().join(format!("idea-app-lock-{label}-{}", uuid::Uuid::new_v4()));
+            fs::create_dir_all(&path).unwrap();
+            Self(path)
+        }
+
+        fn path(&self) -> &Path {
+            &self.0
+        }
+    }
+
+    impl Drop for TempDir {
+        fn drop(&mut self) {
+            let _ = fs::remove_dir_all(&self.0);
+        }
+    }
+
+    #[test]
+    fn contention_on_same_path_is_reported() {
+        let tmp = TempDir::new("same-path");
+        let _guard = acquire_app_data_dir_lock(tmp.path()).unwrap();
+
+        let err = acquire_app_data_dir_lock(tmp.path()).unwrap_err();
+
+        assert!(matches!(err, AppDataDirLockError::Contended { .. }));
+    }
+
+    #[test]
+    fn drop_releases_lock_and_allows_reacquire() {
+        let tmp = TempDir::new("reacquire");
+        let guard = acquire_app_data_dir_lock(tmp.path()).unwrap();
+        drop(guard);
+
+        let reacquired = acquire_app_data_dir_lock(tmp.path()).unwrap();
+
+        assert_eq!(reacquired.lock_path(), tmp.path().join(LOCK_FILE_NAME));
+    }
+
+    #[test]
+    fn different_app_data_dirs_can_coexist() {
+        let a = TempDir::new("a");
+        let b = TempDir::new("b");
+
+        let lock_a = acquire_app_data_dir_lock(a.path()).unwrap();
+        let lock_b = acquire_app_data_dir_lock(b.path()).unwrap();
+
+        assert_ne!(lock_a.lock_path(), lock_b.lock_path());
+    }
+
+    #[test]
+    fn child_process_holds_app_data_lock_until_killed() {
+        let Some(dir) = env::var_os("IDEA_LOCK_TEST_CHILD_DIR") else {
+            return;
+        };
+        let Some(ready) = env::var_os("IDEA_LOCK_TEST_CHILD_READY") else {
+            return;
+        };
+
+        let _guard = acquire_app_data_dir_lock(PathBuf::from(dir)).unwrap();
+        fs::write(ready, b"ready").unwrap();
+        thread::sleep(Duration::from_secs(30));
+    }
+
+    #[test]
+    fn second_os_process_fails_and_brutal_kill_releases_lock() {
+        let tmp = TempDir::new("process");
+        let ready = tmp.path().join("child-ready");
+        let exe = env::current_exe().unwrap();
+        let mut child = Command::new(exe)
+            .arg("--exact")
+            .arg("lock::tests::child_process_holds_app_data_lock_until_killed")
+            .arg("--nocapture")
+            .env("IDEA_LOCK_TEST_CHILD_DIR", tmp.path())
+            .env("IDEA_LOCK_TEST_CHILD_READY", &ready)
+            .stdin(Stdio::null())
+            .stdout(Stdio::null())
+            .stderr(Stdio::null())
+            .spawn()
+            .unwrap();
+
+        let deadline = Instant::now() + Duration::from_secs(5);
+        while !ready.exists() && Instant::now() < deadline {
+            thread::sleep(Duration::from_millis(20));
+        }
+        assert!(ready.exists(), "child did not acquire the lock in time");
+
+        let err = acquire_app_data_dir_lock(tmp.path()).unwrap_err();
+        assert!(matches!(err, AppDataDirLockError::Contended { .. }));
+
+        child.kill().unwrap();
+        let status = child.wait().unwrap();
+        assert!(!status.success());
+
+        let reacquired = acquire_app_data_dir_lock(tmp.path()).unwrap();
+        assert_eq!(reacquired.lock_path(), tmp.path().join(LOCK_FILE_NAME));
+    }
+}
diff --git a/crates/web-server/src/lib.rs b/crates/web-server/src/lib.rs
index 6853d1f..f714198 100644
--- a/crates/web-server/src/lib.rs
+++ b/crates/web-server/src/lib.rs
@@ -128,6 +128,13 @@ pub fn run_from_args(args: Vec<String>) -> ExitCode {
             return ExitCode::from(2);
         }
     };
+    let _app_data_lock = match backend::acquire_app_data_dir_lock(&config.app_data_dir) {
+        Ok(lock) => lock,
+        Err(err) => {
+            eprintln!("idea --serve: {}", serve_app_data_lock_message(&err));
+            return ExitCode::from(1);
+        }
+    };
 
     match tokio::runtime::Builder::new_multi_thread()
         .enable_all()
@@ -147,6 +154,19 @@ pub fn run_from_args(args: Vec<String>) -> ExitCode {
     }
 }
 
+fn serve_app_data_lock_message(err: &backend::AppDataDirLockError) -> String {
+    match err {
+        backend::AppDataDirLockError::Contended { lock_path } => format!(
+            "app-data dir is already locked at {}; another IdeA instance is using this app-data-dir",
+            lock_path.display()
+        ),
+        backend::AppDataDirLockError::Io { lock_path, source } => format!(
+            "failed to access app-data lock at {}: {source}",
+            lock_path.display()
+        ),
+    }
+}
+
 /// Configuration shared by the standalone and embedded web-server entry points.
 #[derive(Clone, Debug)]
 pub struct ServerConfig {
@@ -5036,6 +5056,31 @@ mod tests {
         }
     }
 
+    #[test]
+    fn idea_serve_lock_failure_stops_before_backend_store_writes() {
+        let app_data_dir =
+            std::env::temp_dir().join(format!("idea-server-lock-{}", Uuid::new_v4()));
+        let web_root = create_web_root();
+        let _guard = backend::acquire_app_data_dir_lock(&app_data_dir).unwrap();
+
+        let code = run_from_args(vec![
+            "--app-data-dir".to_owned(),
+            app_data_dir.to_string_lossy().into_owned(),
+            "--web-root".to_owned(),
+            web_root.to_string_lossy().into_owned(),
+        ]);
+
+        assert_eq!(code, ExitCode::from(1));
+        let mut entries = std::fs::read_dir(&app_data_dir)
+            .unwrap()
+            .map(|entry| entry.unwrap().file_name().to_string_lossy().into_owned())
+            .collect::<Vec<_>>();
+        entries.sort();
+        assert_eq!(entries, vec!["idea.lock"]);
+        let _ = std::fs::remove_dir_all(app_data_dir);
+        let _ = std::fs::remove_dir_all(web_root);
+    }
+
     #[test]
     fn default_app_data_dir_uses_tauri_identifier_with_env_precedence() {
         let _lock = ENV_LOCK.lock().unwrap();