diff --git a/gateway/run.py b/gateway/run.py
index a246372da4d..6f5fa65a57f 100644
--- a/gateway/run.py
+++ b/gateway/run.py
@@ -67,6 +67,7 @@ _AGENT_CACHE_MAX_SIZE = 128
 _AGENT_CACHE_IDLE_TTL_SECS = 3600.0  # evict agents idle for >1h
 _PLATFORM_CONNECT_TIMEOUT_SECS_DEFAULT = 30.0
 _ADAPTER_DISCONNECT_TIMEOUT_SECS_DEFAULT = 5.0
+_GATEWAY_PROXY_SSE_BUFFER_MAX_CHARS = 16 * 1024 * 1024
 _TELEGRAM_COMMAND_MENTION_RE = re.compile(r"(?<![\w:/])/([A-Za-z0-9][A-Za-z0-9_-]*)")
 
 _TELEGRAM_NOISY_STATUS_RE = re.compile(
@@ -16219,6 +16220,10 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
                                                 _stream_consumer.on_delta(content)
                                 except json.JSONDecodeError:
                                     pass
+                        if len(buffer) > _GATEWAY_PROXY_SSE_BUFFER_MAX_CHARS:
+                            raise ValueError(
+                                "Proxy SSE stream exceeded max buffer size without a line boundary"
+                            )
 
         except asyncio.CancelledError:
             raise
diff --git a/tests/gateway/test_proxy_mode.py b/tests/gateway/test_proxy_mode.py
index 0c7fa80a0ba..be98f7eb9ac 100644
--- a/tests/gateway/test_proxy_mode.py
+++ b/tests/gateway/test_proxy_mode.py
@@ -334,6 +334,32 @@ class TestRunAgentViaProxy:
 
         assert "Proxy connection error" in result["final_response"]
 
+    @pytest.mark.asyncio
+    async def test_rejects_proxy_sse_without_line_boundary_after_buffer_cap(self, monkeypatch):
+        monkeypatch.setenv("GATEWAY_PROXY_URL", "http://host:8642")
+        monkeypatch.delenv("GATEWAY_PROXY_KEY", raising=False)
+        monkeypatch.setattr("gateway.run._GATEWAY_PROXY_SSE_BUFFER_MAX_CHARS", 16)
+        runner = _make_runner()
+        source = _make_source()
+
+        resp = _FakeSSEResponse(status=200, sse_chunks=[b"data: ", b"x" * 20])
+        session = _FakeSession(resp)
+
+        with patch("gateway.run._load_gateway_config", return_value={}):
+            with _patch_aiohttp(session):
+                with patch("aiohttp.ClientTimeout"):
+                    result = await runner._run_agent_via_proxy(
+                        message="hi",
+                        context_prompt="",
+                        history=[],
+                        source=source,
+                        session_id="test",
+                    )
+
+        assert "Proxy connection error" in result["final_response"]
+        assert "exceeded max buffer size" in result["final_response"]
+        assert result["api_calls"] == 0
+
     @pytest.mark.asyncio
     async def test_skips_tool_messages_in_history(self, monkeypatch):
         monkeypatch.setenv("GATEWAY_PROXY_URL", "http://host:8642")