From eca4a32d87db8da2a1b2aedc0729dff26303369d Mon Sep 17 00:00:00 2001
From: pacocartones <pacocartones@users.noreply.github.com>
Date: Mon, 24 Aug 2026 07:59:08 +0000
Subject: [PATCH] fix(fetch): do not end SSE stream on comment lines

parseSseLine treated a `: ping` keep-alive as done: true, which breaks out of
the line loop in streamSse. When a server or proxy flushes that comment in the
same chunk as the data events that follow, those events are left in the buffer
and the trailing-buffer fallback cannot parse them, so the rest of the model
response is silently dropped.

Per the SSE spec any line starting with a colon is a comment and must be
ignored, so return done: false for it and keep draining the buffer.
---
 packages/fetch/src/stream.test.ts | 26 ++++++++++++++++++++++++++
 packages/fetch/src/stream.ts      |  5 +++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/packages/fetch/src/stream.test.ts b/packages/fetch/src/stream.test.ts
index 084002ac54a..c340780b17b 100644
--- a/packages/fetch/src/stream.test.ts
+++ b/packages/fetch/src/stream.test.ts
@@ -54,6 +54,32 @@ describe("streamSse", () => {
     expect(results).toEqual([{ foo: "bar" }, { baz: 42 }]);
   });
 
+  it("ignores SSE comment lines that share a chunk with data events", async () => {
+    // Servers and proxies send SSE comments (e.g. `: ping`) as keep-alives.
+    // When one arrives in the same chunk as the following data events, those
+    // events must still be yielded rather than silently dropped.
+    const stream = new Readable({
+      read() {
+        this.push(
+          ': ping\n\ndata: {"foo": "bar"}\n\ndata: {"baz": 42}\n\ndata: [DONE]\n\n',
+        );
+        this.push(null); // End of stream
+      },
+    }) as any;
+    const response = {
+      status: 200,
+      body: stream,
+      text: async () => "",
+    } as unknown as Response;
+
+    const results = [];
+    for await (const data of streamSse(response)) {
+      results.push(data);
+    }
+
+    expect(results).toEqual([{ foo: "bar" }, { baz: 42 }]);
+  });
+
   it("throws on malformed JSON", async () => {
     const sseLines = ['data: {"foo": "bar"', "data:[DONE]"];
     const response = createMockResponse(sseLines);
diff --git a/packages/fetch/src/stream.ts b/packages/fetch/src/stream.ts
index f73d61dbafc..29f79ff682f 100644
--- a/packages/fetch/src/stream.ts
+++ b/packages/fetch/src/stream.ts
@@ -115,8 +115,9 @@ function parseSseLine(line: string): { done: boolean; data: any } {
   if (line.startsWith("data:")) {
     return { done: false, data: parseDataLine(line) };
   }
-  if (line.startsWith(": ping")) {
-    return { done: true, data: undefined };
+  if (line.startsWith(":")) {
+    // Comment line (e.g. the `: ping` keep-alive), ignored per the SSE spec
+    return { done: false, data: undefined };
   }
   return { done: false, data: undefined };
 }