Handle client disconnect for large payloads (#701)

Catch `ClientDisconnect` in FastAPI worker to return HTTP 499 for large
payloads and reduce noisy error logs.

---
Linear Issue:
[TOO-189](https://linear.app/arcadedev/issue/TOO-189/catch-clientdisconnect-and-return-499-for-large-payloads)

<a
href="https://cursor.com/background-agent?bcId=bc-f777f89b-d2bc-4c0c-bcb1-b76fcf601e05"><picture><source
media="(prefers-color-scheme: dark)"
srcset="https://cursor.com/open-in-cursor-dark.svg"><source
media="(prefers-color-scheme: light)"
srcset="https://cursor.com/open-in-cursor-light.svg"><img alt="Open in
Cursor"
src="https://cursor.com/open-in-cursor.svg"></picture></a>&nbsp;<a
href="https://cursor.com/agents?id=bc-f777f89b-d2bc-4c0c-bcb1-b76fcf601e05"><picture><source
media="(prefers-color-scheme: dark)"
srcset="https://cursor.com/open-in-web-dark.svg"><source
media="(prefers-color-scheme: light)"
srcset="https://cursor.com/open-in-web-light.svg"><img alt="Open in Web"
src="https://cursor.com/open-in-web.svg"></picture></a>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Eric Gustin <eric@arcade.dev>

libs/arcade-serve/arcade_serve/fastapi/worker.py

@@ -4,6 +4,8 @@ from typing import Any, Callable
 from fastapi import Depends, FastAPI, Request
 from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
 from opentelemetry.metrics import Meter
+from starlette.requests import ClientDisconnect
+from starlette.responses import Response
 from starlette.routing import Mount
 
 from arcade_serve.core.base import (
@@ -96,7 +98,13 @@ class FastAPIRouter(Router):
             if use_auth_for_route
             else None,
         ) -> Any:
-            body_str = await request.body()
+            try:
+                body_str = await request.body()
+            except ClientDisconnect:
+                # Client disconnected while reading request body (often due to large payloads)
+                # Return HTTP 499 (Client Closed Request)
+                return Response(status_code=499)
+
             body_json = json.loads(body_str) if body_str else {}
             request_data = RequestData(
                 path=request.url.path,

libs/arcade-serve/pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "arcade-serve"
-version = "3.1.3"
+version = "3.1.4"
 description = "Arcade Serve - Serving infrastructure for Arcade tools and workers"
 readme = "README.md"
 license = {text = "MIT"}

libs/tests/worker/test_worker_fastapi.py

@@ -1,4 +1,5 @@
 from typing import Annotated
+from unittest.mock import AsyncMock, patch
 
 import pytest
 from arcade_core.schema import ToolCallRequest, ToolContext, ToolReference
@@ -6,6 +7,7 @@ from arcade_serve.fastapi.worker import FastAPIWorker
 from arcade_tdk import tool
 from fastapi import FastAPI
 from fastapi.testclient import TestClient
+from starlette.requests import ClientDisconnect
 
 
 @tool()
@@ -164,3 +166,15 @@ def test_call_tool_route_tool_not_found(client_no_auth, call_tool_payload):
         # Ideally, this might be a 404 or 400, but BaseWorker.call_tool raises ValueError
         # which isn't automatically mapped to a 4xx by FastAPI unless handled explicitly.
         # TODO fix this.
+
+
+def test_client_disconnect_returns_499(client_no_auth, call_tool_payload):
+    """Test that ClientDisconnect during body read returns HTTP 499."""
+    # Mock request.body() to raise ClientDisconnect
+    with patch("starlette.requests.Request.body", new_callable=AsyncMock) as mock_body:
+        mock_body.side_effect = ClientDisconnect()
+
+        response = client_no_auth.post("/worker/tools/invoke", json=call_tool_payload)
+
+        # Verify that we get a 499 status code
+        assert response.status_code == 499