feat(history): replay a window of a task's history, with older messages on demand

source/cydo/domain/tasks/model.d

@@ -975,6 +975,25 @@ struct TaskHistoryStartMessage
 	string type = "task_history_start";
 	int tid;
 	int total;
+	int window_start; // first replayed seq; > 0 means older history exists unsent
+	int window_limit; // window applied, in messages; 0 = the whole history
+}
+
+/// Frames a replay of the messages *older* than what the client already holds,
+/// so it can prepend them instead of rebuilding its list.
+struct TaskHistoryPrependStartMessage
+{
+	string type = "task_history_prepend_start";
+	int tid;
+	int window_start; // first seq in this batch; 0 means the task's beginning
+	int before_seq;   // where the batch stops, i.e. the client's current start
+}
+
+struct TaskHistoryPrependEndMessage
+{
+	string type = "task_history_prepend_end";
+	int tid;
+	int window_start;
 }
 
 struct TaskHistoryEndMessage
@@ -1031,6 +1050,14 @@ struct WsMessage
 	@JSONOptional Nullable!uint expected_num_turns;
 	string correlation_id;
 	string tool_use_id;
+	// request_history: 0 lets the server pick from its config for device_class,
+	// >0 is an explicit window, -1 asks for the whole history. the client is not
+	// asked to know the configured window, because it learns that from
+	// server_status, which arrives after the tasks list that triggers the first
+	// history request
+	@JSONOptional int limit;
+	@JSONOptional string device_class; // "mobile" or "desktop"
+	@JSONOptional int before_seq; // request_history_before: older than this
 }
 
 struct TaskCreatedMessage
@@ -1168,6 +1195,8 @@ struct ServerStatusMessage
 	bool auth_enabled;
 	bool dev_mode;
 	string build_id;
+	int history_window_desktop; // initial replay window in messages, 0 = full
+	int history_window_mobile;
 }
 
 struct ScanStatusMessage

source/cydo/runtime/config/package.d

@@ -104,6 +104,12 @@ struct WorkspaceConfig
 	@Optional ProjectDiscoveryConfig project_discovery;
 }
 
+struct HistoryWindowConfig
+{
+	@Optional int desktop;
+	@Optional int mobile;
+}
+
 struct CydoConfig
 {
 	@Key("name") WorkspaceConfig[] workspaces;
@@ -115,6 +121,10 @@ struct CydoConfig
 	@Optional bool dev_mode;
 	@Optional string log_level = "info";
 	@Optional string system_keyword = "SYSTEM";
+	/// How much of a task's history to replay when it is opened, in messages
+	/// (user and assistant bubbles), per device class. Absent or zero replays
+	/// everything, which is the default.
+	@Optional HistoryWindowConfig history_window;
 
 	/// Called by configy during parsing (configy/read.d:650), so a semantic
 	/// error surfaces on the same path as a YAML syntax error.

source/cydo/server/app.d

@@ -1099,6 +1099,8 @@ class App
 			authUser.length > 0 || authPass.length > 0,
 			config.dev_mode,
 			webDistDir,
+			config.history_window.desktop,
+			config.history_window.mobile,
 		).representation));
 		ws.send(Data(buildNoticesList(activeNotices).representation));
 		if (discoveryService.scanInProgress)
@@ -1265,6 +1267,7 @@ class App
 		{
 			case "create_task":       handleCreateTaskMsg(ws, json); break;
 			case "request_history":   handleRequestHistory(ws, json); break;
+			case "request_history_before": handleRequestHistoryBefore(ws, json); break;
 			case "message":           handleUserMessage(json); break;
 			case "resume":            handleResumeMsg(json); break;
 			case "interrupt":         handleInterruptMsg(json); break;
@@ -1460,7 +1463,30 @@ class App
 
 	private void handleRequestHistory(WebSocketAdapter ws, WsMessage json)
 	{
-		historyPipeline.handleRequestHistory(ws, json.tid);
+		historyPipeline.handleRequestHistory(ws, json.tid,
+			resolveHistoryLimit(json.limit, json.device_class));
+	}
+
+	private void handleRequestHistoryBefore(WebSocketAdapter ws, WsMessage json)
+	{
+		historyPipeline.handleRequestHistoryBefore(ws, json.tid, json.before_seq,
+			resolveHistoryLimit(json.limit, json.device_class));
+	}
+
+	/// Turn a client's request into an actual window size.
+	///
+	/// The server owns the numbers because it is the only side that always
+	/// knows them: a client that has not yet processed server_status would
+	/// otherwise ask for everything, which on a long task means replaying tens
+	/// of thousands of events.
+	private int resolveHistoryLimit(int requested, string deviceClass)
+	{
+		if (requested != 0)
+			return requested < 0 ? 0 : requested; // negative asks for it all
+		auto window = deviceClass == "mobile"
+			? config.history_window.mobile
+			: config.history_window.desktop;
+		return window > 0 ? window : 0;
 	}
 
 	private void sendHistoryReplaySupplementalState(WebSocketAdapter ws, int tid)
@@ -3395,6 +3421,8 @@ class App
 			authUser.length > 0 || authPass.length > 0,
 			config.dev_mode,
 			webDistDir,
+			config.history_window.desktop,
+			config.history_window.mobile,
 		));
 		infof("Config reloaded successfully");
 		discoveryService.endScan();

source/cydo/web/snapshots.d

@@ -90,13 +90,16 @@ string readBuildId(string webDistDir)
 	return m[1].idup;
 }
 
-string buildServerStatus(bool authEnabled, bool devMode, string webDistDir)
+string buildServerStatus(bool authEnabled, bool devMode, string webDistDir,
+	int historyWindowDesktop = 0, int historyWindowMobile = 0)
 {
 	return toJson(ServerStatusMessage(
 		"server_status",
 		authEnabled,
 		devMode,
 		readBuildId(webDistDir),
+		historyWindowDesktop,
+		historyWindowMobile,
 	));
 }