diff --git a/surfsense_backend/tests/unit/services/streaming/test_sse_envelope.py b/surfsense_backend/tests/unit/services/streaming/test_sse_envelope.py
new file mode 100644
index 000000000..511e4575a
--- /dev/null
+++ b/surfsense_backend/tests/unit/services/streaming/test_sse_envelope.py
@@ -0,0 +1,51 @@
+"""Pin the exact SSE wire bytes the FE parser depends on."""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from app.services.streaming.envelope import (
+    format_done,
+    format_sse,
+    get_response_headers,
+)
+
+pytestmark = pytest.mark.unit
+
+
+class TestFormatSse:
+    def test_dict_payload_is_json_serialised(self) -> None:
+        frame = format_sse({"type": "start", "messageId": "msg_1"})
+        assert frame.startswith("data: ")
+        assert frame.endswith("\n\n")
+        body = frame[len("data: ") : -2]
+        assert json.loads(body) == {"type": "start", "messageId": "msg_1"}
+
+    def test_string_payload_is_emitted_verbatim(self) -> None:
+        frame = format_sse('{"already":"json"}')
+        assert frame == 'data: {"already":"json"}\n\n'
+
+    def test_nested_payload_round_trips(self) -> None:
+        payload = {
+            "type": "data-action-log",
+            "data": {"id": 7, "tool_name": "ls", "reversible": False},
+        }
+        frame = format_sse(payload)
+        body = frame.removeprefix("data: ").removesuffix("\n\n")
+        assert json.loads(body) == payload
+
+
+class TestFormatDone:
+    def test_done_marker_is_literal(self) -> None:
+        assert format_done() == "data: [DONE]\n\n"
+
+
+class TestResponseHeaders:
+    def test_headers_pin_ai_sdk_v1_protocol(self) -> None:
+        headers = get_response_headers()
+        assert headers["Content-Type"] == "text/event-stream"
+        assert headers["Cache-Control"] == "no-cache"
+        assert headers["Connection"] == "keep-alive"
+        assert headers["x-vercel-ai-ui-message-stream"] == "v1"