From a7c94022a5a5a3f20342a675cb3f6b30d6b2673f Mon Sep 17 00:00:00 2001
From: Ishaan Jaffer <ishaan@berri.ai>
Date: Wed, 6 May 2026 14:52:52 -0700
Subject: [PATCH] feat(agent-sdk): add SSE client with Last-Event-ID resume

---
 sdks/typescript-agent-sdk/src/client/sse.ts | 179 ++++++++++++++++++++
 1 file changed, 179 insertions(+)
 create mode 100644 sdks/typescript-agent-sdk/src/client/sse.ts

diff --git a/sdks/typescript-agent-sdk/src/client/sse.ts b/sdks/typescript-agent-sdk/src/client/sse.ts
new file mode 100644
index 00000000000..8f104e7d034
--- /dev/null
+++ b/sdks/typescript-agent-sdk/src/client/sse.ts
@@ -0,0 +1,179 @@
+/**
+ * SSE client with auto-reconnect via `Last-Event-ID` / `?starting_seq=N`.
+ *
+ * Yields parsed `RunEvent` objects. When the stream drops, it transparently
+ * reconnects from the last seq it saw. Honors AbortSignal.
+ */
+
+import { createParser, type EventSourceMessage } from "eventsource-parser";
+import { LiteLLMAgentError, type RunEvent } from "../types.js";
+import { request, type ResolvedClient } from "./http.js";
+
+export interface StreamRunOptions {
+  startingSeq?: number;
+  signal?: AbortSignal;
+  /** Max reconnect attempts on dropped sockets. Default: 5. */
+  maxReconnects?: number;
+}
+
+/**
+ * Stream events from `GET /v1/sessions/{sid}/runs/{rid}/events`.
+ *
+ * Resume contract:
+ *  - On every event, we record `seq`.
+ *  - On reconnect, we send both `Last-Event-ID: <seq>` (per the SSE spec)
+ *    and `?starting_seq=<seq+1>` (LiteLLM-specific). The proxy honors
+ *    whichever it understands and replays from there.
+ */
+export async function* streamRunEvents(
+  client: ResolvedClient,
+  sessionId: string,
+  runId: string,
+  opts: StreamRunOptions = {}
+): AsyncIterable<RunEvent> {
+  const maxReconnects = opts.maxReconnects ?? 5;
+  let lastSeq = opts.startingSeq !== undefined ? opts.startingSeq - 1 : -1;
+  let reconnects = 0;
+
+  while (true) {
+    const startingSeq = lastSeq >= 0 ? lastSeq + 1 : undefined;
+    let res: Response;
+    try {
+      res = await request(client, {
+        method: "GET",
+        path: `/v1/sessions/${encodeURIComponent(sessionId)}/runs/${encodeURIComponent(runId)}/events`,
+        query: startingSeq !== undefined ? { starting_seq: startingSeq } : undefined,
+        headers:
+          startingSeq !== undefined
+            ? { "Last-Event-ID": String(lastSeq) }
+            : {},
+        stream: true,
+        signal: opts.signal,
+      });
+    } catch (e) {
+      if (opts.signal?.aborted) return;
+      if (reconnects >= maxReconnects) throw e;
+      reconnects++;
+      await sleep(backoffMs(reconnects));
+      continue;
+    }
+
+    if (!res.body) {
+      throw new LiteLLMAgentError("SSE response had no body", {
+        code: "stream_no_body",
+      });
+    }
+
+    let droppedMidStream = false;
+    try {
+      for await (const event of parseSSE(res.body, opts.signal)) {
+        const parsed = decodeEvent(event);
+        if (parsed === null) continue;
+        if (parsed.seq <= lastSeq) {
+          // Replayed event we already saw; drop it.
+          continue;
+        }
+        lastSeq = parsed.seq;
+        yield parsed;
+        if (parsed.type === "done" || parsed.type === "error") {
+          return;
+        }
+      }
+      // Stream ended cleanly without a `done` event — server hung up.
+      droppedMidStream = true;
+    } catch (e) {
+      if (opts.signal?.aborted) return;
+      droppedMidStream = true;
+    }
+
+    if (!droppedMidStream) return;
+    if (reconnects >= maxReconnects) {
+      throw new LiteLLMAgentError(
+        `SSE reconnect budget exhausted (${maxReconnects})`,
+        { code: "sse_reconnect_exhausted" }
+      );
+    }
+    reconnects++;
+    await sleep(backoffMs(reconnects));
+  }
+}
+
+function decodeEvent(msg: EventSourceMessage): RunEvent | null {
+  if (!msg.data) return null;
+  try {
+    const obj = JSON.parse(msg.data) as Partial<RunEvent>;
+    if (typeof obj.seq !== "number" || typeof obj.type !== "string") {
+      return null;
+    }
+    return {
+      seq: obj.seq,
+      type: obj.type,
+      data: obj.data ?? null,
+    };
+  } catch {
+    return null;
+  }
+}
+
+async function* parseSSE(
+  body: ReadableStream<Uint8Array>,
+  signal?: AbortSignal
+): AsyncIterable<EventSourceMessage> {
+  const queue: EventSourceMessage[] = [];
+  const state: { resolve: (() => void) | null; done: boolean } = {
+    resolve: null,
+    done: false,
+  };
+
+  const parser = createParser({
+    onEvent: (e) => {
+      queue.push(e);
+      state.resolve?.();
+    },
+  });
+
+  const reader = body.getReader();
+  const decoder = new TextDecoder();
+
+  const pump = (async () => {
+    try {
+      while (true) {
+        if (signal?.aborted) {
+          await reader.cancel();
+          return;
+        }
+        const { value, done: rdone } = await reader.read();
+        if (rdone) return;
+        parser.feed(decoder.decode(value, { stream: true }));
+        state.resolve?.();
+      }
+    } finally {
+      state.done = true;
+      state.resolve?.();
+    }
+  })();
+
+  try {
+    while (true) {
+      while (queue.length > 0) {
+        const ev = queue.shift()!;
+        yield ev;
+      }
+      if (state.done) return;
+      await new Promise<void>((r) => {
+        state.resolve = r;
+      });
+      state.resolve = null;
+    }
+  } finally {
+    await pump.catch(() => {});
+  }
+}
+
+function sleep(ms: number): Promise<void> {
+  return new Promise((r) => setTimeout(r, ms));
+}
+
+function backoffMs(attempt: number): number {
+  return Math.min(5_000, 250 * Math.pow(2, attempt));
+}