From 67bc972e7c0e9093797e41148541abce7869ffa9 Mon Sep 17 00:00:00 2001
From: Max Isbey <224885523+maxisbey@users.noreply.github.com>
Date: Thu, 1 Oct 2026 12:57:01 +0000
Subject: [PATCH] Make the deeply nested body test independent of stack size

The test posted a well-formed 100,000-deep JSON array and asserted
PARSE_ERROR, which only holds where json.loads hits its recursion guard.
On CPython 3.14 with a large enough C stack the body parses, the server
correctly answers INVALID_REQUEST, and the assertion fails.

Leave the body unterminated so it is unparseable on every interpreter:
RecursionError where the guard trips, JSONDecodeError where the scanner
reaches the end of input. The server maps both to 400 + PARSE_ERROR.

Fixes #3146
---
 tests/server/test_streamable_http_modern.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tests/server/test_streamable_http_modern.py b/tests/server/test_streamable_http_modern.py
index 11e775840f..2558872545 100644
--- a/tests/server/test_streamable_http_modern.py
+++ b/tests/server/test_streamable_http_modern.py
@@ -1139,8 +1139,9 @@ async def broken_list(ctx: ServerRequestContext, params: PaginatedRequestParams
 
 
 async def test_modern_post_with_deeply_nested_body_is_parse_error_not_a_crash() -> None:
-    """Deep nesting makes json.loads raise RecursionError; still an unparseable body: 400 + PARSE_ERROR."""
-    body = b"[" * 100_000 + b"]" * 100_000
+    """Unterminated deep nesting makes json.loads raise RecursionError or, where the stack is deep enough
+    to reach the end of input, JSONDecodeError; an unparseable body either way: 400 + PARSE_ERROR."""
+    body = b"[" * 100_000
     async with _asgi_client(_x_mcp_server()) as http:
         response = await http.post("/mcp", content=body, headers={"content-type": "application/json"})
     assert response.status_code == 400