diff --git a/Cargo.lock b/Cargo.lock
index 7cbd0b8..028103b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1550,6 +1550,7 @@ dependencies = [
  "getrandom 0.4.3",
  "hex",
  "k256",
+ "libc",
  "omachat-crypto",
  "omachat-nostr",
  "omachat-proto",
diff --git a/crates/omachatd/Cargo.toml b/crates/omachatd/Cargo.toml
index 3ffcf09..10b9016 100644
--- a/crates/omachatd/Cargo.toml
+++ b/crates/omachatd/Cargo.toml
@@ -10,6 +10,7 @@ repository.workspace = true
 ed25519-dalek = "=2.2.0"
 getrandom = "=0.4.3"
 hex = "=0.4.3"
+libc = "=0.2.189"
 omachat-crypto = { path = "../omachat-crypto", version = "=0.0.1" }
 omachat-nostr = { path = "../omachat-nostr", version = "=0.0.1" }
 omachat-proto = { path = "../omachat-proto", version = "=0.0.1" }
diff --git a/crates/omachatd/src/ipc_server.rs b/crates/omachatd/src/ipc_server.rs
index 34d0430..4fde46e 100644
--- a/crates/omachatd/src/ipc_server.rs
+++ b/crates/omachatd/src/ipc_server.rs
@@ -6,8 +6,9 @@ use serde_json::to_value;
 use std::{
     error::Error,
     fmt, fs,
+    fs::{File, OpenOptions},
     future::Future,
-    os::unix::fs::{FileTypeExt, PermissionsExt},
+    os::unix::fs::{FileTypeExt, OpenOptionsExt, PermissionsExt},
     path::{Path, PathBuf},
     pin::Pin,
     sync::{Arc, Mutex},
@@ -72,6 +73,8 @@ impl EventHub {
 pub struct IpcServer<H> {
     listener: UnixListener,
     socket_path: PathBuf,
+    lock_path: PathBuf,
+    _instance_lock: File,
     handler: Arc<H>,
     events: EventHub,
 }
@@ -83,6 +86,21 @@ impl<H: RequestHandler> IpcServer<H> {
         events: EventHub,
     ) -> Result<Self, ServerError> {
         let socket_path = socket_path.as_ref().to_owned();
+        let lock_path = socket_path.with_extension("lock");
+        let instance_lock = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .mode(0o600)
+            .custom_flags(libc::O_CLOEXEC | libc::O_NOFOLLOW)
+            .open(&lock_path)
+            .map_err(ServerError::Io)?;
+        instance_lock.try_lock().map_err(|error| match error {
+            fs::TryLockError::WouldBlock => ServerError::AlreadyRunning,
+            fs::TryLockError::Error(error) => ServerError::Io(error),
+        })?;
+        fs::set_permissions(&lock_path, fs::Permissions::from_mode(0o600))
+            .map_err(ServerError::Io)?;
         if socket_path.exists() {
             let metadata = fs::symlink_metadata(&socket_path).map_err(ServerError::Io)?;
             if !metadata.file_type().is_socket() {
@@ -96,6 +114,8 @@ impl<H: RequestHandler> IpcServer<H> {
         Ok(Self {
             listener,
             socket_path,
+            lock_path,
+            _instance_lock: instance_lock,
             handler: Arc::new(handler),
             events,
         })
@@ -167,6 +187,7 @@ impl<H: RequestHandler> IpcServer<H> {
 impl<H> Drop for IpcServer<H> {
     fn drop(&mut self) {
         let _ = fs::remove_file(&self.socket_path);
+        let _ = fs::remove_file(&self.lock_path);
     }
 }
 
@@ -273,6 +294,7 @@ pub enum ServerError {
     Io(std::io::Error),
     Protocol(IpcError),
     OccupiedPath,
+    AlreadyRunning,
 }
 
 impl fmt::Display for ServerError {
@@ -281,6 +303,7 @@ impl fmt::Display for ServerError {
             Self::Io(error) => write!(formatter, "IPC I/O failed: {error}"),
             Self::Protocol(error) => write!(formatter, "IPC protocol failed: {error}"),
             Self::OccupiedPath => formatter.write_str("IPC path exists and is not a socket"),
+            Self::AlreadyRunning => formatter.write_str("another daemon owns the IPC endpoint"),
         }
     }
 }
@@ -290,7 +313,7 @@ impl Error for ServerError {
         match self {
             Self::Io(error) => Some(error),
             Self::Protocol(error) => Some(error),
-            Self::OccupiedPath => None,
+            Self::OccupiedPath | Self::AlreadyRunning => None,
         }
     }
 }
diff --git a/crates/omachatd/tests/ipc_server.rs b/crates/omachatd/tests/ipc_server.rs
index 9734395..a1b38bc 100644
--- a/crates/omachatd/tests/ipc_server.rs
+++ b/crates/omachatd/tests/ipc_server.rs
@@ -425,3 +425,17 @@ async fn assert_client_eof(reader: &mut BufReader<tokio::net::unix::OwnedReadHal
         .expect("read client EOF");
     assert_eq!(count, 0, "server closed client after flushing response");
 }
+
+#[tokio::test]
+async fn second_server_cannot_replace_a_live_ipc_endpoint() {
+    let temporary = tempdir().expect("temporary directory");
+    let socket = temporary.path().join("omachat.sock");
+    let first = IpcServer::bind(&socket, Handler, EventHub::default()).expect("first bind");
+
+    let second = IpcServer::bind(&socket, Handler, EventHub::default());
+    assert!(matches!(second, Err(ServerError::AlreadyRunning)));
+    assert!(socket.exists(), "the live endpoint remains addressable");
+
+    drop(first);
+    assert!(!socket.exists(), "the owner removes its endpoint on drop");
+}
diff --git a/docs/installation.md b/docs/installation.md
index 7be2eaf..c2b5225 100644
--- a/docs/installation.md
+++ b/docs/installation.md
@@ -5,6 +5,14 @@ inputs for the Omarchy v4.0.1 clean-install gate.
 
 ## User service
 
+OmaChat's local IPC socket is an account-local control boundary, not an
+application sandbox. Socket permissions exclude other Unix users, and the
+daemon refuses a concurrent instance, but processes already running as the
+same account are inside this trust boundary. Run untrusted desktop software
+under a separate OS identity or sandbox that cannot access the account's
+runtime directory. In particular, do not describe the `0600` socket as
+authenticating individual same-user applications.
+
 After a package installs the binaries and `omachatd.service`:
 
 ```sh