Instructions per chat (system prompt addition; UI dialog and Telegram /prompt)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/api.rs

@@ -574,6 +574,7 @@ async fn create_session(Extension(user): Extension<User>, State(app): State<AppS
         pinned: false,
         archived: false,
         folder: None,
+        instructions: String::new(),
     };
     let s = app.save_session(s)?;
     Ok(Json(json!(s)))
@@ -782,12 +783,17 @@ struct UpdateSession {
     archived: Option<bool>,
     /// "" removes the folder.
     folder: Option<String>,
+    /// Instructions for this chat ("" clears them).
+    instructions: Option<String>,
 }
 
 async fn update_session(Extension(user): Extension<User>, State(app): State<AppState>, Path(id): Path<String>, Json(body): Json<UpdateSession>) -> ApiResult {
     writable(&app, &user, &id)?;
     // Organising the list must not move the session to "today".
-    let organise_only = body.title.is_none() && body.model.is_none() && body.agent.is_none() && body.clean_room.is_none() && body.permission_mode.is_none();
+    let organise_only = body.title.is_none() && body.model.is_none() && body.agent.is_none() && body.clean_room.is_none() && body.permission_mode.is_none() && body.instructions.is_none();
+    if body.instructions.as_ref().is_some_and(|i| i.len() > 20_000) {
+        return Err(ApiError(StatusCode::BAD_REQUEST, "instructions too long (max 20,000 characters)".into()));
+    }
     if let Some(m) = &body.model {
         app.registry().resolve(m)?;
     }
@@ -824,6 +830,9 @@ async fn update_session(Extension(user): Extension<User>, State(app): State<AppS
         if let Some(p) = body.permission_mode {
             s.permission_mode = p;
         }
+        if let Some(i) = body.instructions {
+            s.instructions = i.trim().to_string();
+        }
     })?;
     Ok(Json(json!(s)))
 }