diff --git a/src/agents/provider-transport-fetch.test.ts b/src/agents/provider-transport-fetch.test.ts
index 8e6e98c18734..5582b79b11fd 100644
--- a/src/agents/provider-transport-fetch.test.ts
+++ b/src/agents/provider-transport-fetch.test.ts
@@ -1369,6 +1369,42 @@ describe("buildGuardedModelFetch", () => {
     expect(refreshTimeout).toHaveBeenCalledTimes(2);
   });
 
+  it("handles a valid large SSE event split before its boundary", async () => {
+    const payload = { text: "x".repeat(70 * 1024) };
+    const encoder = new TextEncoder();
+    fetchWithSsrFGuardMock.mockResolvedValue({
+      response: new Response(
+        new ReadableStream({
+          start(controller) {
+            controller.enqueue(encoder.encode(`data: ${JSON.stringify(payload)}`));
+            controller.enqueue(encoder.encode("\n\n"));
+            controller.close();
+          },
+        }),
+        { headers: { "content-type": "text/event-stream" } },
+      ),
+      finalUrl: "https://openrouter.ai/api/v1/chat/completions",
+      release: vi.fn(async () => undefined),
+    });
+    const model = {
+      id: "gpt-5.4",
+      provider: "openrouter",
+      api: "openai-completions",
+      baseUrl: "https://openrouter.ai/api/v1",
+    } as unknown as Model<"openai-completions">;
+
+    const response = await buildGuardedModelFetch(model)(
+      "https://openrouter.ai/api/v1/chat/completions",
+      { method: "POST" },
+    );
+    const items = [];
+    for await (const item of Stream.fromSSEResponse(response, new AbortController())) {
+      items.push(item);
+    }
+
+    expect(items).toEqual([payload]);
+  });
+
   it("errors on oversized SSE body without event boundary in sanitizer", async () => {
     const oversized = "x".repeat(16 * 1024 * 1024 + 1024);
     const encoder = new TextEncoder();
diff --git a/src/agents/provider-transport-fetch.ts b/src/agents/provider-transport-fetch.ts
index 75cfcde76428..2ac178fefb27 100644
--- a/src/agents/provider-transport-fetch.ts
+++ b/src/agents/provider-transport-fetch.ts
@@ -51,12 +51,8 @@ const log = createSubsystemLogger("provider-transport-fetch");
  *  without Content-Length. */
 const SSE_SYNTHESIZE_JSON_MAX_BYTES = 16 * 1024 * 1024;
 
-/** Max bytes for the internal SSE sanitization buffer between event boundaries.
- *  A single legitimate event (e.g. a large reasoning summary on the chatgpt-responses
- *  API) can far exceed 64 KiB, so bound this at the same 16 MiB ceiling as the
- *  JSON-synthesis path: only a genuinely boundary-less (hostile/broken) stream trips
- *  the guard, not a real large event. */
-const SSE_SANITIZE_BUFFER_MAX_BYTES = 16 * 1024 * 1024;
+/** Max decoded characters buffered while waiting for the next SSE event boundary. */
+const SSE_SANITIZE_BUFFER_MAX_CHARS = 16 * 1024 * 1024;
 
 const BLOCKED_EXACT_ORIGIN_TRUST_HOSTNAME_LABELS = new Set(["instance-data"]);
 const PLAIN_DECIMAL_NUMBER_RE = /^\d+(?:\.\d+)?$/;
@@ -179,9 +175,9 @@ function sanitizeOpenAISdkSseResponse(
     for (;;) {
       const boundary = findSseEventBoundary(buffer);
       if (!boundary) {
-        if (buffer.length > SSE_SANITIZE_BUFFER_MAX_BYTES) {
+        if (buffer.length > SSE_SANITIZE_BUFFER_MAX_CHARS) {
           throw new Error(
-            `SSE response exceeded max buffer size (${SSE_SANITIZE_BUFFER_MAX_BYTES} bytes) without event boundary`,
+            `SSE response exceeded max buffer size (${SSE_SANITIZE_BUFFER_MAX_CHARS} chars) without event boundary`,
           );
         }
         return enqueued;