fix(sessions): name relay handoffs deterministically

src/server/sessions/sessionNameGenerator.ts

@@ -5,6 +5,13 @@ const SESSION_NAME_TIMEOUT_MS = 10_000;
 const SESSION_NAME_MAX_INPUT_CHARS = 4_000;
 const SESSION_NAME_MAX_LENGTH = 60;
 const FALLBACK_SESSION_NAME_MAX_WORDS = 6;
+const RELAY_HANDOFF_FIRST_LINE = /^Relay\s+"([^"\n]+)"\s+leg\s+(\d+)\s+begins now\.?\s*(?:\n|$)/;
+
+export function deterministicSessionName(firstMessage: unknown): string | undefined {
+  if (typeof firstMessage !== "string") return undefined;
+
+  return relayHandoffSessionName(firstMessage.trimStart());
+}
 
 export async function generateShortSessionName<TApi extends Api>(streamFn: StreamFn, model: Model<TApi>, firstMessage: string): Promise<string | undefined> {
   const stream = await streamFn(
@@ -59,6 +66,31 @@ export function cleanSessionName(value: string): string | undefined {
   return title === "" ? undefined : title;
 }
 
+function relayHandoffSessionName(firstMessage: string): string | undefined {
+  const match = RELAY_HANDOFF_FIRST_LINE.exec(firstMessage);
+  if (match === null) return undefined;
+
+  const relayName = match[1]?.replace(/\s+/g, " ").trim();
+  const legNumber = match[2];
+  if (relayName === undefined || relayName === "" || legNumber === undefined) return undefined;
+
+  return cleanSessionName(formatRelaySessionName(relayName, legNumber));
+}
+
+function formatRelaySessionName(relayName: string, legNumber: string): string {
+  const prefix = "Relay ";
+  const suffix = ` leg ${legNumber}`;
+  const maxRelayNameLength = Math.max(1, SESSION_NAME_MAX_LENGTH - prefix.length - suffix.length);
+  const displayedRelayName = truncateRelayName(relayName, maxRelayNameLength);
+  return `${prefix}${displayedRelayName}${suffix}`;
+}
+
+function truncateRelayName(relayName: string, maxLength: number): string {
+  if (relayName.length <= maxLength) return relayName;
+  const truncated = relayName.slice(0, maxLength).replace(/[\s._-]+$/g, "").trim();
+  return truncated === "" ? relayName.slice(0, maxLength).trim() : truncated;
+}
+
 function textFromAssistant(message: AssistantMessage): string {
   return message.content
     .filter((part) => part.type === "text")