diff --git a/open-sse/config/constants.ts b/open-sse/config/constants.ts
index 462dcd481c..6faf2df056 100644
--- a/open-sse/config/constants.ts
+++ b/open-sse/config/constants.ts
@@ -17,6 +17,11 @@ export const FETCH_TIMEOUT_MS = upstreamTimeouts.fetchTimeoutMs;
 // idle for this duration. Override with STREAM_IDLE_TIMEOUT_MS env var.
 export const STREAM_IDLE_TIMEOUT_MS = upstreamTimeouts.streamIdleTimeoutMs;
 
+// Timeout for the first useful SSE event. Keep this much shorter than the
+// post-start idle timeout so slow-thinking models can keep streaming after the
+// first token, while dead 200 OK streams fail fast enough for combo fallback.
+export const STREAM_READINESS_TIMEOUT_MS = upstreamTimeouts.streamReadinessTimeoutMs;
+
 // Timeout for reading the full response body after headers arrive (ms).
 // Prevents indefinite hangs when the upstream sends headers but stalls on the body.
 // Defaults to FETCH_TIMEOUT_MS. Override with FETCH_BODY_TIMEOUT_MS env var.
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index bc5310a4ea..0a2c7b9dad 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -10,6 +10,7 @@ import {
 } from "../utils/stream.ts";
 import { ensureStreamReadiness } from "../utils/streamReadiness.ts";
 import { createStreamController, pipeWithDisconnect } from "../utils/streamHandler.ts";
+import { createSseHeartbeatTransform } from "../utils/sseHeartbeat.ts";
 import { addBufferToUsage, filterUsageForFormat, estimateUsage } from "../utils/usageTracking.ts";
 import { refreshWithRetry } from "../services/tokenRefresh.ts";
 import { createRequestLogger } from "../utils/requestLogger.ts";
@@ -32,6 +33,7 @@ import {
   HTTP_STATUS,
   PROVIDER_MAX_TOKENS,
   STREAM_IDLE_TIMEOUT_MS,
+  STREAM_READINESS_TIMEOUT_MS,
 } from "../config/constants.ts";
 import {
   classifyProviderError,
@@ -3941,7 +3943,7 @@ export async function handleChatCore({
 
   // Streaming response
   const streamReadiness = await ensureStreamReadiness(providerResponse, {
-    timeoutMs: STREAM_IDLE_TIMEOUT_MS,
+    timeoutMs: STREAM_READINESS_TIMEOUT_MS,
     provider,
     model,
     log,
@@ -3990,8 +3992,9 @@ export async function handleChatCore({
 
   const responseHeaders = {
     "Content-Type": "text/event-stream",
-    "Cache-Control": "no-cache",
+    "Cache-Control": "no-cache, no-transform",
     Connection: "keep-alive",
+    "X-Accel-Buffering": "no",
     [OMNIROUTE_RESPONSE_HEADERS.cache]: "MISS",
     ...buildOmniRouteResponseMetaHeaders({
       provider,
@@ -4236,6 +4239,9 @@ export async function handleChatCore({
   } else {
     finalStream = pipeWithDisconnect(providerResponse, transformStream, streamController);
   }
+  finalStream = finalStream.pipeThrough(
+    createSseHeartbeatTransform({ signal: streamController.signal })
+  );
 
   return {
     success: true,
diff --git a/open-sse/utils/sseHeartbeat.ts b/open-sse/utils/sseHeartbeat.ts
index 1ceb6820b3..db8d0ff624 100644
--- a/open-sse/utils/sseHeartbeat.ts
+++ b/open-sse/utils/sseHeartbeat.ts
@@ -1,4 +1,4 @@
-const DEFAULT_INTERVAL_MS = 15_000;
+export const DEFAULT_SSE_HEARTBEAT_INTERVAL_MS = 15_000;
 
 type SseHeartbeatTransformOptions = {
   intervalMs?: number;
@@ -6,7 +6,7 @@ type SseHeartbeatTransformOptions = {
 };
 
 export function createSseHeartbeatTransform({
-  intervalMs = DEFAULT_INTERVAL_MS,
+  intervalMs = DEFAULT_SSE_HEARTBEAT_INTERVAL_MS,
   signal,
 }: SseHeartbeatTransformOptions = {}) {
   let intervalId: ReturnType<typeof setInterval> | undefined;
diff --git a/open-sse/utils/streamHandler.ts b/open-sse/utils/streamHandler.ts
index 89d695c6fb..2bfbcef68e 100644
--- a/open-sse/utils/streamHandler.ts
+++ b/open-sse/utils/streamHandler.ts
@@ -3,6 +3,7 @@ import { trackPendingRequest } from "@/lib/usageDb";
 // Stream handler with disconnect detection - shared for all providers
 
 const PENDING_REQUEST_CLEARED_MARKER = "__omniroutePendingRequestCleared";
+const DISCONNECT_ABORT_DELAY_MS = 2_000;
 
 type StreamDisconnectEvent = {
   reason: string;
@@ -97,7 +98,7 @@ export function createStreamController({
       // Delay abort to allow cleanup
       abortTimeout = setTimeout(() => {
         abortController.abort();
-      }, 500);
+      }, DISCONNECT_ABORT_DELAY_MS);
 
       onDisconnect?.({ reason, duration: Date.now() - startTime });
     },
@@ -201,7 +202,9 @@ export function createDisconnectAwareStream(transformStream, streamController) {
     cancel(reason) {
       streamController.handleDisconnect(reason || "cancelled");
       reader.cancel();
-      writer.abort();
+      setTimeout(() => {
+        writer.abort();
+      }, DISCONNECT_ABORT_DELAY_MS).unref?.();
     },
   });
 }
diff --git a/src/shared/utils/runtimeTimeouts.ts b/src/shared/utils/runtimeTimeouts.ts
index b2794a7745..672466a1c2 100644
--- a/src/shared/utils/runtimeTimeouts.ts
+++ b/src/shared/utils/runtimeTimeouts.ts
@@ -8,6 +8,7 @@ type ReadTimeoutOptions = {
 
 export const DEFAULT_FETCH_TIMEOUT_MS = 600_000;
 export const DEFAULT_STREAM_IDLE_TIMEOUT_MS = 600_000;
+export const DEFAULT_STREAM_READINESS_TIMEOUT_MS = 30_000;
 export const DEFAULT_FETCH_CONNECT_TIMEOUT_MS = 30_000;
 export const DEFAULT_FETCH_KEEPALIVE_TIMEOUT_MS = 4_000;
 export const DEFAULT_API_BRIDGE_PROXY_TIMEOUT_MS = 30_000;
@@ -24,6 +25,7 @@ function hasEnvValue(env: EnvSource, name: string): boolean {
 export type UpstreamTimeoutConfig = {
   fetchTimeoutMs: number;
   streamIdleTimeoutMs: number;
+  streamReadinessTimeoutMs: number;
   fetchHeadersTimeoutMs: number;
   fetchBodyTimeoutMs: number;
   fetchConnectTimeoutMs: number;
@@ -89,10 +91,20 @@ export function getUpstreamTimeoutConfig(
       logger,
     }
   );
+  const streamReadinessTimeoutMs = readTimeoutMs(
+    env,
+    "STREAM_READINESS_TIMEOUT_MS",
+    DEFAULT_STREAM_READINESS_TIMEOUT_MS,
+    {
+      allowZero: true,
+      logger,
+    }
+  );
 
   return {
     fetchTimeoutMs,
     streamIdleTimeoutMs,
+    streamReadinessTimeoutMs,
     fetchHeadersTimeoutMs: readTimeoutMs(env, "FETCH_HEADERS_TIMEOUT_MS", fetchTimeoutMs, {
       allowZero: true,
       logger,