diff --git a/packages/shared/src/mcpResponsePayloads.test.ts b/packages/shared/src/mcpResponsePayloads.test.ts
new file mode 100644
index 0000000000000..9a3737ab93f53
--- /dev/null
+++ b/packages/shared/src/mcpResponsePayloads.test.ts
@@ -0,0 +1,71 @@
+import { expect, it } from "@effect/vitest";
+import * as Effect from "effect/Effect";
+import * as Stream from "effect/Stream";
+
+import { responsePayloads } from "./mcpResponsePayloads.ts";
+
+it.effect("reads SSE line endings across byte-chunk boundaries without splitting CRLF", () =>
+  Effect.gen(function* () {
+    for (const separator of [
+      "\n\n",
+      "\r\r",
+      "\r\n\r\n",
+      "\r\n\n",
+      "\n\r",
+      "\n\r\n",
+      "\r\r\n",
+      "\r\n\r",
+    ]) {
+      for (const dataLineSeparator of ["\n", "\r", "\r\n"]) {
+        const bytes = new TextEncoder().encode(
+          `: keep-alive${separator}data: {"id":1,${dataLineSeparator}data: "result":{"text":"café😀"}}${separator}` +
+            'data: {"id":2,"result":{}}\n\ndata: {"id":3}',
+        );
+        for (let cut = 0; cut <= bytes.length; cut += 1) {
+          const response = new Response(
+            new ReadableStream<Uint8Array>({
+              start(controller) {
+                controller.enqueue(bytes.subarray(0, cut));
+                controller.enqueue(bytes.subarray(cut));
+                controller.close();
+              },
+            }),
+            { headers: { "content-type": "text/event-stream" } },
+          );
+          const payloads = yield* Stream.runCollect(
+            responsePayloads(response, (cause) => ({
+              _tag: "ResponsePayloadError" as const,
+              cause,
+              separator,
+              dataLineSeparator,
+              cut,
+            })),
+          );
+          expect(
+            Array.from(payloads),
+            JSON.stringify({ separator, dataLineSeparator, cut }),
+          ).toEqual([
+            { id: 1, result: { text: "café😀" } },
+            { id: 2, result: {} },
+          ]);
+        }
+      }
+      const response = new Response(
+        new ReadableStream<Uint8Array>({
+          start(controller) {
+            controller.enqueue(new TextEncoder().encode(`data: {"id":4}${separator}`));
+          },
+        }),
+        { headers: { "content-type": "text/event-stream" } },
+      );
+      const payloads = yield* Stream.runCollect(
+        responsePayloads(response, (cause) => ({
+          _tag: "ResponsePayloadError" as const,
+          cause,
+          separator,
+        })).pipe(Stream.take(1)),
+      );
+      expect(Array.from(payloads)).toEqual([{ id: 4 }]);
+    }
+  }),
+);
diff --git a/packages/shared/src/mcpResponsePayloads.ts b/packages/shared/src/mcpResponsePayloads.ts
index fe5a89153d6e0..0d40e8ba7fd33 100644
--- a/packages/shared/src/mcpResponsePayloads.ts
+++ b/packages/shared/src/mcpResponsePayloads.ts
@@ -2,23 +2,25 @@
 import * as Effect from "effect/Effect";
 import * as Stream from "effect/Stream";
 
+const SSE_EVENT_SEPARATOR = /(?:\r\n|\r(?!\n)|\n){2}/u;
+
 async function* sseDataLines(response: Response): AsyncGenerator<string> {
   if (response.body === null) return;
   const decoder = new TextDecoder();
   let buffered = "";
   for await (const chunk of response.body) {
     buffered += decoder.decode(chunk as Uint8Array, { stream: true });
-    let separatorIndex = buffered.search(/\n\n|\r\n\r\n/u);
-    while (separatorIndex !== -1) {
-      const rawEvent = buffered.slice(0, separatorIndex);
-      buffered = buffered.slice(separatorIndex).replace(/^(?:\r?\n){2}/u, "");
+    let separator = SSE_EVENT_SEPARATOR.exec(buffered);
+    while (separator !== null) {
+      const rawEvent = buffered.slice(0, separator.index);
+      buffered = buffered.slice(separator.index + separator[0].length);
       const data = rawEvent
-        .split(/\r?\n/u)
+        .split(/\r\n|\r|\n/u)
         .filter((line) => line.startsWith("data:"))
         .map((line) => line.slice("data:".length).trimStart())
         .join("\n");
       if (data.length > 0) yield data;
-      separatorIndex = buffered.search(/\n\n|\r\n\r\n/u);
+      separator = SSE_EVENT_SEPARATOR.exec(buffered);
     }
   }
 }