diff --git a/sdks/typescript-agent-sdk/tests/sse-reconnect.test.ts b/sdks/typescript-agent-sdk/tests/sse-reconnect.test.ts
new file mode 100644
index 00000000000..07c55db5024
--- /dev/null
+++ b/sdks/typescript-agent-sdk/tests/sse-reconnect.test.ts
@@ -0,0 +1,99 @@
+/**
+ * Validation #4 — SSE auto-reconnect.
+ *
+ * Mid-stream, the mock proxy kills the underlying socket. The SDK should
+ * reconnect with `?starting_seq=N` transparently and not lose or duplicate
+ * events.
+ */
+
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { Agent } from "../src/index.js";
+import { MockProxy } from "./mock-proxy.js";
+
+describe("SSE auto-reconnect", () => {
+  let proxy: MockProxy;
+  let baseUrl: string;
+
+  beforeEach(async () => {
+    proxy = new MockProxy({ killSseAfterMs: 60 });
+    baseUrl = await proxy.start();
+  });
+
+  afterEach(async () => {
+    await proxy.stop();
+  });
+
+  it("transparently reconnects mid-stream and preserves event order without loss or duplication", async () => {
+    const agent = await Agent.create({
+      apiKey: "test-key",
+      baseUrl,
+      name: "resumer",
+      model: { id: "test-model" },
+    });
+    const session = await agent.createSession();
+    const run = await session.send("kick off");
+
+    // Emit two events before the mock kills the socket, then continue.
+    const driver = (async () => {
+      // Wait for the SSE connection to open.
+      await new Promise((r) => setTimeout(r, 20));
+      proxy.emit(run.id, "delta", { text: "alpha" });
+      proxy.emit(run.id, "delta", { text: "beta" });
+      // Kill happens at ~60ms after first connect.
+      await new Promise((r) => setTimeout(r, 200));
+      proxy.emit(run.id, "delta", { text: "gamma" });
+      proxy.complete(run.id, "alphabetagamma");
+    })();
+
+    const seenSeqs: number[] = [];
+    const seenTexts: string[] = [];
+    for await (const ev of run.stream()) {
+      seenSeqs.push(ev.seq);
+      if (ev.type === "delta") {
+        seenTexts.push((ev.data as { text: string }).text);
+      }
+    }
+    await driver;
+
+    // Each seq seen exactly once.
+    expect(new Set(seenSeqs).size).toBe(seenSeqs.length);
+    // Strictly increasing — no out-of-order replays leaked through.
+    for (let i = 1; i < seenSeqs.length; i++) {
+      expect(seenSeqs[i]).toBeGreaterThan(seenSeqs[i - 1]);
+    }
+    // All three deltas present (none lost).
+    expect(seenTexts).toEqual(["alpha", "beta", "gamma"]);
+  });
+
+  it("respects an explicit startingSeq on first connect", async () => {
+    // Disable mid-stream kill so we test the resume-from-N path only.
+    const proxy2 = new MockProxy();
+    const url2 = await proxy2.start();
+    try {
+      const agent = await Agent.create({
+        apiKey: "test-key",
+        baseUrl: url2,
+        name: "resume-explicit",
+        model: { id: "test-model" },
+      });
+      const session = await agent.createSession();
+      const run = await session.send("go");
+
+      // Pre-populate events so they exist when stream() opens.
+      proxy2.emit(run.id, "delta", { text: "0" });
+      proxy2.emit(run.id, "delta", { text: "1" });
+      proxy2.emit(run.id, "delta", { text: "2" });
+      proxy2.complete(run.id, "012");
+
+      const seqs: number[] = [];
+      for await (const ev of run.stream({ startingSeq: 2 })) {
+        seqs.push(ev.seq);
+      }
+      // Should not see 0 or 1 — only seq >= 2 (delta) and the done event (seq 3).
+      expect(seqs.every((s) => s >= 2)).toBe(true);
+      expect(seqs).toContain(2);
+    } finally {
+      await proxy2.stop();
+    }
+  });
+});