diff --git a/.gitignore b/.gitignore
index b3e17c411e..a52be32a1a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,4 +18,3 @@ target/
 .DS_Store
 .vscode/
 .idea/
-.port_sessions/
diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index ab05e218c0..62d5110fb2 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -1,3 +1,8 @@
+## 2025-04-11 - Path Traversal Vulnerability in Session IDs
+**Vulnerability:** The Python `src/session_store.py` and Rust `rust/crates/runtime/src/session_control.rs` allow path traversal (e.g. `../` or `/`) via the `session_id` property when saving and loading sessions.
+**Learning:** This exposes a critical vulnerability where an attacker controlling the `session_id` can write or read arbitrary `.json` / `.jsonl` files on the filesystem. The session identifiers must be strictly validated to exclude path separators ('/', '\') and special segments ('.', '..') to mitigate path traversal vulnerabilities.
+**Prevention:** Explicitly validate input IDs to ensure they only contain safe characters (e.g., alphanumeric, hyphens, underscores) or strictly reject path separators and directory traversal sequences.
+
 ## 2024-04-10 - Path Traversal in Session Storage
 **Vulnerability:** Path traversal existed in both the Python (`src/session_store.py`) and Rust (`rust/crates/runtime/src/session_control.rs`) implementations because unsanitized `session_id` strings were used directly in file paths.
 **Learning:** Both reference implementations lacked central validation logic for system identifiers derived from external/user input.
diff --git a/rust/crates/runtime/src/session_control.rs b/rust/crates/runtime/src/session_control.rs
index 11b7905f00..9136cc828f 100644
--- a/rust/crates/runtime/src/session_control.rs
+++ b/rust/crates/runtime/src/session_control.rs
@@ -74,12 +74,17 @@ impl SessionStore {
         &self.workspace_root
     }
 
-    pub fn create_handle(&self, session_id: &str) -> Result<SessionHandle, SessionControlError> {
+    pub fn validate_session_id(session_id: &str) -> Result<(), SessionControlError> {
         if !is_valid_session_id(session_id) {
-            return Err(SessionControlError::Format(format!(
-                "Invalid session ID: {session_id}"
-            )));
+            return Err(SessionControlError::Format(
+                "Invalid session ID".to_string(),
+            ));
         }
+        Ok(())
+    }
+
+    pub fn create_handle(&self, session_id: &str) -> Result<SessionHandle, SessionControlError> {
+        Self::validate_session_id(session_id)?;
         let id = session_id.to_string();
         let path = self
             .sessions_root
@@ -120,11 +125,7 @@ impl SessionStore {
     }
 
     pub fn resolve_managed_path(&self, session_id: &str) -> Result<PathBuf, SessionControlError> {
-        if !is_valid_session_id(session_id) {
-            return Err(SessionControlError::Format(format!(
-                "Invalid session ID: {session_id}"
-            )));
-        }
+        Self::validate_session_id(session_id)?;
         for extension in [PRIMARY_SESSION_EXTENSION, LEGACY_SESSION_EXTENSION] {
             let path = self.sessions_root.join(format!("{session_id}.{extension}"));
             if path.exists() {
@@ -353,9 +354,9 @@ pub fn create_managed_session_handle_for(
     session_id: &str,
 ) -> Result<SessionHandle, SessionControlError> {
     if !is_valid_session_id(session_id) {
-        return Err(SessionControlError::Format(format!(
-            "Invalid session ID: {session_id}"
-        )));
+        return Err(SessionControlError::Format(
+            "Invalid session ID".to_string(),
+        ));
     }
     let id = session_id.to_string();
     let path =
@@ -420,9 +421,9 @@ pub fn resolve_managed_session_path_for(
     session_id: &str,
 ) -> Result<PathBuf, SessionControlError> {
     if !is_valid_session_id(session_id) {
-        return Err(SessionControlError::Format(format!(
-            "Invalid session ID: {session_id}"
-        )));
+        return Err(SessionControlError::Format(
+            "Invalid session ID".to_string(),
+        ));
     }
     let directory = managed_sessions_dir_for(base_dir)?;
     for extension in [PRIMARY_SESSION_EXTENSION, LEGACY_SESSION_EXTENSION] {
@@ -742,7 +743,7 @@ mod tests {
             .expect("session message should save");
         let handle = store
             .create_handle(&session.session_id)
-            .expect("handle should create");
+            .expect("should create handle");
         let session = session.with_persistence_path(handle.path.clone());
         session
             .save_to_path(&handle.path)
diff --git a/src/session_store.py b/src/session_store.py
index a2f1600aa4..b0fbd122a4 100644
--- a/src/session_store.py
+++ b/src/session_store.py
@@ -17,14 +17,14 @@ class StoredSession:
 
 
 def validate_session_id(session_id: str) -> None:
+    if not session_id:
+        raise ValueError("Invalid session ID: cannot be empty")
     if "/" in session_id or "\\" in session_id:
-        raise ValueError(f"Invalid session ID: contains path separators ({session_id})")
+        raise ValueError("Invalid session ID: contains path separators")
     if session_id in (".", ".."):
-        raise ValueError(f"Invalid session ID: cannot be '.' or '..' ({session_id})")
+        raise ValueError("Invalid session ID: cannot be '.' or '..'")
     if ".." in session_id:
-        raise ValueError(
-            f"Invalid session ID: contains directory traversal ('..') ({session_id})"
-        )
+        raise ValueError("Invalid session ID: contains directory traversal ('..')")
 
 
 def save_session(session: StoredSession, directory: Path | None = None) -> Path: