diff --git a/README.md b/README.md
index fd42762..b2fdfc3 100644
--- a/README.md
+++ b/README.md
@@ -64,10 +64,14 @@ let client = TAuthClient::new(
 
 ## Start Login
 
-Redirect the user to `client.auth_url(None)` from your web handler.
+Redirect the user to `client.auth_url(None, session_id)` from your web handler. The `session_id` is a `u64` you generate (e.g. from the current time in milliseconds) — it is passed through the TAuth flow and returned in the callback so you can correlate the login with a pending session.
 
 ```rust
-let login_url = client.auth_url(None);
+let session_id = std::time::SystemTime::now()
+    .duration_since(std::time::UNIX_EPOCH)
+    .map(|d| d.as_millis() as u64)
+    .unwrap_or(0);
+let login_url = client.auth_url(None, session_id);
 ```
 
 The URL includes:
@@ -75,6 +79,7 @@ The URL includes:
 - `identifier`: your app identifier
 - `redirect`: your callback URL
 - `public_key`: your app MTP public key bundle
+- `sessionId`: the session ID you generated, passed through to the callback
 - `challenge`: optional caller-supplied challenge
 
 ## Handle Callback
diff --git a/example/backend/src/main.rs b/example/backend/src/main.rs
index df46454..44ac4be 100644
--- a/example/backend/src/main.rs
+++ b/example/backend/src/main.rs
@@ -103,7 +103,11 @@ impl AppState {
 }
 
 async fn login(State(state): State<AppState>) -> Redirect {
-    Redirect::temporary(&state.tauth.auth_url(None))
+    let session_id = std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .map(|d| d.as_millis() as u64)
+        .unwrap_or(0);
+    Redirect::temporary(&state.tauth.auth_url(None, session_id))
 }
 
 async fn callback(State(state): State<AppState>, uri: axum::http::Uri) -> Result<Response, AppError> {
diff --git a/src/lib.rs b/src/lib.rs
index 4514353..63fe2fc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -231,17 +231,18 @@ impl TAuthClient {
         }
     }
 
-    pub fn generate_link(&self, challenge: Option<&str>) -> String {
-        self.auth_url(challenge)
+    pub fn generate_link(&self, challenge: Option<&str>, session_id: u64) -> String {
+        self.auth_url(challenge, session_id)
     }
 
-    pub fn auth_url(&self, challenge: Option<&str>) -> String {
+    pub fn auth_url(&self, challenge: Option<&str>, session_id: u64) -> String {
         let mut link = self.frontend_url.clone();
         {
             let mut pairs = link.query_pairs_mut();
             pairs.append_pair("identifier", &self.identifier);
             pairs.append_pair("redirect", self.redirect_url.as_str());
             pairs.append_pair("public_key", &self.public_key());
+            pairs.append_pair("sessionId", &session_id.to_string());
             if let Some(challenge) = challenge {
                 pairs.append_pair("challenge", challenge);
             }
@@ -500,7 +501,7 @@ mod tests {
         )
         .unwrap();
 
-        let link = Url::parse(&client.generate_link(Some("abc123"))).unwrap();
+        let link = Url::parse(&client.generate_link(Some("abc123"), 42)).unwrap();
         assert_eq!(
             link.query_pairs()
                 .find(|(key, _)| key == "identifier")
@@ -515,6 +516,13 @@ mod tests {
                 .1,
             "abc123"
         );
+        assert_eq!(
+            link.query_pairs()
+                .find(|(key, _)| key == "sessionId")
+                .unwrap()
+                .1,
+            "42"
+        );
         assert!(
             link.query_pairs()
                 .any(|(key, value)| key == "public_key" && !value.is_empty())