diff --git a/plugins/platforms/sms/adapter.py b/plugins/platforms/sms/adapter.py
index e40ec0ac39a..1646b1b5956 100644
--- a/plugins/platforms/sms/adapter.py
+++ b/plugins/platforms/sms/adapter.py
@@ -42,6 +42,7 @@ TWILIO_API_BASE = "https://api.twilio.com/2010-04-01/Accounts"
 MAX_SMS_LENGTH = 1600  # ~10 SMS segments
 DEFAULT_WEBHOOK_PORT = 8080
 DEFAULT_WEBHOOK_HOST = "127.0.0.1"
+_TWILIO_WEBHOOK_MAX_BODY_BYTES = 65_536  # 64 KiB — Twilio payloads are small
 
 
 def check_sms_requirements() -> bool:
@@ -293,7 +294,20 @@ class SmsAdapter(BasePlatformAdapter):
         from aiohttp import web
 
         try:
+            content_length = request.content_length
+            if content_length is not None and content_length > _TWILIO_WEBHOOK_MAX_BODY_BYTES:
+                return web.Response(
+                    text='<?xml version="1.0" encoding="UTF-8"?><Response></Response>',
+                    content_type="application/xml",
+                    status=413,
+                )
             raw = await request.read()
+            if len(raw) > _TWILIO_WEBHOOK_MAX_BODY_BYTES:
+                return web.Response(
+                    text='<?xml version="1.0" encoding="UTF-8"?><Response></Response>',
+                    content_type="application/xml",
+                    status=413,
+                )
             # Twilio sends form-encoded data, not JSON
             form = urllib.parse.parse_qs(raw.decode("utf-8"), keep_blank_values=True)
         except Exception as e:
diff --git a/tests/gateway/test_sms.py b/tests/gateway/test_sms.py
index 85a9501f06a..bfb70289b75 100644
--- a/tests/gateway/test_sms.py
+++ b/tests/gateway/test_sms.py
@@ -459,10 +459,11 @@ class TestWebhookSignatureEnforcement:
         adapter._message_handler = AsyncMock()
         return adapter
 
-    def _mock_request(self, body, headers=None):
+    def _mock_request(self, body, headers=None, content_length=None):
         request = MagicMock()
         request.read = AsyncMock(return_value=body)
         request.headers = headers or {}
+        request.content_length = content_length
         return request
 
     @pytest.mark.asyncio
@@ -536,3 +537,27 @@ class TestWebhookSignatureEnforcement:
         request = self._mock_request(body, headers={"X-Twilio-Signature": sig})
         resp = await adapter._handle_webhook(request)
         assert resp.status == 200
+
+    @pytest.mark.asyncio
+    async def test_webhook_rejects_oversized_body_via_content_length(self):
+        """POST with Content-Length exceeding 64 KiB returns 413 before reading."""
+        adapter = self._make_adapter(webhook_url="")
+        body = b"From=%2B15551234567&To=%2B15550001111&Body=hello&MessageSid=SM123"
+        request = self._mock_request(body, content_length=65_537)
+        resp = await adapter._handle_webhook(request)
+        assert resp.status == 413
+        # request.read must NOT have been called — we bailed on Content-Length
+        request.read.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_webhook_rejects_oversized_body_via_read_length(self):
+        """POST whose actual read size exceeds 64 KiB returns 413.
+
+        Covers the case where Content-Length is absent (chunked transfer) but
+        the body still exceeds the cap.
+        """
+        adapter = self._make_adapter(webhook_url="")
+        oversized = b"x" * 65_537
+        request = self._mock_request(oversized, content_length=None)
+        resp = await adapter._handle_webhook(request)
+        assert resp.status == 413