diff --git a/gateway/platforms/api_server.py b/gateway/platforms/api_server.py
index a029596849d..5ba09d67492 100644
--- a/gateway/platforms/api_server.py
+++ b/gateway/platforms/api_server.py
@@ -669,7 +669,16 @@ if AIOHTTP_AVAILABLE:
                         return web.json_response(_openai_error("Request body too large.", code="body_too_large"), status=413)
                 except ValueError:
                     return web.json_response(_openai_error("Invalid Content-Length header.", code="invalid_content_length"), status=400)
-        return await handler(request)
+        try:
+            return await handler(request)
+        except web.HTTPRequestEntityTooLarge:
+            # aiohttp's client_max_size tripped mid-read (chunked bodies carry
+            # no Content-Length) — return a proper 413 instead of letting the
+            # handler's broad JSON except turn it into 400 "Invalid JSON".
+            return web.json_response(
+                _openai_error("Request body too large.", code="body_too_large"),
+                status=413,
+            )
 else:
     body_limit_middleware = None  # type: ignore[assignment]
 
diff --git a/gateway/platforms/webhook.py b/gateway/platforms/webhook.py
index 9fb72b6ec81..c9acf61b32c 100644
--- a/gateway/platforms/webhook.py
+++ b/gateway/platforms/webhook.py
@@ -191,7 +191,10 @@ class WebhookAdapter(BasePlatformAdapter):
                         f"real target (telegram, discord, slack, github_comment, etc.)."
                     )
 
-        app = web.Application()
+        # client_max_size makes aiohttp enforce the cap on every read path,
+        # including Transfer-Encoding: chunked bodies that carry no
+        # Content-Length and would otherwise bypass the header check below.
+        app = web.Application(client_max_size=self._max_body_bytes)
         app.router.add_get("/health", self._handle_health)
         app.router.add_post("/webhooks/{route_name}", self._handle_webhook)
         # Multi-profile multiplexing: a /p/<profile>/webhooks/<route> prefix
@@ -479,9 +482,21 @@ class WebhookAdapter(BasePlatformAdapter):
         # Read body (must be done before any validation)
         try:
             raw_body = await request.read()
+        except web.HTTPRequestEntityTooLarge:
+            # aiohttp's client_max_size tripped — chunked or lying
+            # Content-Length. Same 413 as the header check above.
+            return web.json_response(
+                {"error": "Payload too large"}, status=413
+            )
         except Exception as e:
             logger.error("[webhook] Failed to read body: %s", e)
             return web.json_response({"error": "Bad request"}, status=400)
+        if len(raw_body) > self._max_body_bytes:
+            # Defense in depth: enforce the cap on the actual bytes read even
+            # if the server-level limit was bypassed or misconfigured.
+            return web.json_response(
+                {"error": "Payload too large"}, status=413
+            )
 
         # Validate HMAC signature FIRST (skip only for the explicit local-test
         # INSECURE_NO_AUTH mode). Missing/empty secrets must fail closed here,
diff --git a/tests/gateway/test_webhook_adapter.py b/tests/gateway/test_webhook_adapter.py
index 9e5340042ee..3c5c59a2001 100644
--- a/tests/gateway/test_webhook_adapter.py
+++ b/tests/gateway/test_webhook_adapter.py
@@ -69,7 +69,8 @@ def _make_adapter(routes=None, **kwargs):
 
 def _create_app(adapter: WebhookAdapter) -> web.Application:
     """Build the aiohttp Application from the adapter (without starting a full server)."""
-    app = web.Application()
+    # Mirror connect(): client_max_size enforces the cap on chunked bodies.
+    app = web.Application(client_max_size=adapter._max_body_bytes)
     app.router.add_get("/health", adapter._handle_health)
     app.router.add_post("/webhooks/{route_name}", adapter._handle_webhook)
     return app
@@ -741,6 +742,29 @@ class TestBodySize:
             )
             assert resp.status == 413
 
+    @pytest.mark.asyncio
+    async def test_chunked_oversized_payload_rejected(self):
+        """Chunked request bodies (no Content-Length) over the limit return 413."""
+        routes = {"big": {"secret": _INSECURE_NO_AUTH, "prompt": "test"}}
+        adapter = _make_adapter(routes=routes, max_body_bytes=100)
+        adapter.handle_message = AsyncMock()
+
+        async def _chunked_body():
+            payload = json.dumps({"data": "x" * 500}).encode("utf-8")
+            for i in range(0, len(payload), 64):
+                yield payload[i : i + 64]
+                await asyncio.sleep(0)
+
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            resp = await cli.post(
+                "/webhooks/big",
+                data=_chunked_body(),
+                headers={"Content-Type": "application/json"},
+            )
+            assert resp.status == 413
+            adapter.handle_message.assert_not_awaited()
+
 
 # ===================================================================
 # INSECURE_NO_AUTH