fix: harden resume and SSE replay

backend/src/routes/sessions.ts

@@ -8,6 +8,18 @@ import type { ReadinessManager } from "../runtime/readiness-manager.js";
 
 const BOOTSTRAP_FAILURE_MESSAGE =
   "Session startup failed. Check configuration and connectivity, then Resume the session.";
+const READINESS_FAILURE_MESSAGE =
+  "Session services are not ready. Check configuration and connectivity, then try again.";
+
+function eventCursor(...values: unknown[]): number {
+  let cursor = 0;
+  for (const value of values.flatMap((item) => Array.isArray(item) ? item : [item])) {
+    if (typeof value !== "string" || !/^\d+$/.test(value)) continue;
+    const parsed = Number(value);
+    if (Number.isSafeInteger(parsed)) cursor = Math.max(cursor, parsed);
+  }
+  return cursor;
+}
 
 export function sessionRoutes(
   app: FastifyInstance,
@@ -57,7 +69,7 @@ export function sessionRoutes(
     const b = req.body as { question: string; name?: string };
     const s = d.getSettings();
     const ensure = await d.readiness.ensure(s.workspace ?? "");
-    if (!ensure.ok) return reply.code(503).send({ error: ensure.error ?? "Ollama/embeddings non disponibili" });
+    if (!ensure.ok) return reply.code(503).send({ error: READINESS_FAILURE_MESSAGE });
     // Settings (global) supply workspace/provider/model/thinking. The new-question
     // form sends only the question text. `workspace` selects the tht `-c <config>`.
     const { id } = await d.tht.sessionNew({
@@ -115,7 +127,7 @@ export function sessionRoutes(
     }
     const settings = d.getSettings();
     const ensure = await d.readiness.ensure(settings.workspace ?? "");
-    if (!ensure.ok) return reply.code(503).send({ error: ensure.error ?? "Ollama/embeddings non disponibili" });
+    if (!ensure.ok) return reply.code(503).send({ error: READINESS_FAILURE_MESSAGE });
     const saved = manifest as { provider?: string; model?: string; thinking?: string } | null;
     const options = {
       provider: saved?.provider,
@@ -131,7 +143,7 @@ export function sessionRoutes(
     bindRuntime(id, rt);
     info(id, "Resuming session");
     bootstrap(id, rt, d.mgr.configure(rt, options), null, () => d.mgr.start(id, rt, options));
-    return reply.code(200).send({ id });
+    return reply.code(200).send({ id, alreadyActive: false });
   });
   app.post("/sessions/:id/close", async (req) => {
     const id = (req.params as { id: string }).id;
@@ -146,6 +158,10 @@ export function sessionRoutes(
   app.get("/sessions/:id/events", (req, reply) => {
     const id = (req.params as any).id;
     const rt = d.mgr.get(id);
+    const afterId = eventCursor(
+      req.headers["last-event-id"],
+      (req.query as { lastEventId?: unknown }).lastEventId,
+    );
     // Add CORS headers manually: reply.raw.writeHead bypasses Fastify's onSend hooks
     // (where @fastify/cors injects headers), so we must set them explicitly here.
     const origin = (req.headers.origin as string | undefined) ?? "*";
@@ -160,8 +176,12 @@ export function sessionRoutes(
     // Send the handshake immediately. Without this, Node waits for the first event body and
     // proxies/clients cannot establish an idle SSE subscription or inspect its headers.
     reply.raw.flushHeaders();
-    const send = (event: string, data: object) => reply.raw.write(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`);
-    const off = d.hub.subscribe(id, send, rt?.bridge.pendingWidget() ?? null);
+    const send = (event: string, data: object, eventId: number) =>
+      reply.raw.write(`id: ${eventId}\nevent: ${event}\ndata: ${JSON.stringify(data)}\n\n`);
+    const off = d.hub.subscribe(id, send, {
+      afterId,
+      pending: rt?.bridge.pendingWidget() ?? null,
+    });
     req.raw.on("close", off);
   });
   app.post("/sessions/:id/rename", async (req, reply) => {