feat: adaptive keepalive threshold for web-session providers (#3397)

Integrated into release/v3.8.16

src/app/api/v1/responses/route.ts

@@ -3,6 +3,7 @@ import { withEarlyStreamKeepalive } from "@omniroute/open-sse/utils/earlyStreamK
 import { resolveResponsesApiModel } from "@/app/api/internal/codex-responses-ws/modelResolution";
 import { getModelInfo } from "@/sse/services/model";
 import { getComboByName } from "@/lib/db/combos";
+import { resolveKeepaliveThreshold } from "@omniroute/open-sse/utils/keepaliveThreshold";
 
 // NOTE: We do NOT call initTranslators() here — the translator registry is
 // bootstrapped at module level inside open-sse/translator/index.ts when it
@@ -68,7 +69,19 @@ export async function POST(request) {
   const resolved = await withCodexPreferredModel(request);
   const accept = String(request.headers?.get?.("accept") || "").toLowerCase();
   if (accept.includes("text/event-stream")) {
-    return await withEarlyStreamKeepalive(handleChat(resolved), { signal: request.signal });
+    // Adaptive threshold: web-session and anonymous-fallback providers are slower
+    // to produce the first byte, so use a longer keepalive threshold (15s vs 2s).
+    let model;
+    try {
+      const body = await resolved.clone().json().catch(() => null);
+      model = body?.model;
+    } catch {
+    }
+    const thresholdMs = resolveKeepaliveThreshold(model);
+    return await withEarlyStreamKeepalive(handleChat(resolved), {
+      signal: request.signal,
+      thresholdMs,
+    });
   }
   return await handleChat(resolved);
 }