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fix: make deeply-nested-body test platform-independent (fixes #3146) #3147

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fix: make deeply-nested-body test platform-independent (fixes #3146) #3147

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29 changes: 27 additions & 2 deletions tests/server/test_streamable_http_modern.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1008,12 +1008,37 @@ async def broken_list(ctx: ServerRequestContext, params: PaginatedRequestParams
assert "Mcp-Param header validation skipped: the tools/list listing failed" in caplog.text


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."""
async def test_modern_post_with_deeply_nested_body_is_rejected_not_a_crash() -> None:
"""A deeply nested body is rejected with a 400, never a crash.

Which JSON-RPC code it gets is platform-dependent: since CPython 3.12 the C
json scanner guards recursion by actual C-stack headroom, so depending on
the thread's stack size the body either fails to parse (RecursionError ->
PARSE_ERROR) or parses into a giant list that then fails request
validation (-> INVALID_REQUEST). Both are correct rejections; the
deterministic PARSE_ERROR mapping is covered by the monkeypatch test below.
"""
body = b"[" * 100_000 + 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
assert response.json()["error"]["code"] in (PARSE_ERROR, INVALID_REQUEST)


async def test_modern_post_recursion_error_during_parse_is_parse_error(monkeypatch: pytest.MonkeyPatch) -> None:
"""RecursionError raised while parsing the body maps to 400 + PARSE_ERROR (deterministically)."""
real_loads = json.loads
body = b"[" * 50 + b"]" * 50

def _raise_for_request_body(s: Any, *args: Any, **kwargs: Any) -> Any:
if s == body: # `json` here is the stdlib module; only fail the request-body parse.
raise RecursionError("maximum recursion depth exceeded")
return real_loads(s, *args, **kwargs)

monkeypatch.setattr("mcp.server._streamable_http_modern.json.loads", _raise_for_request_body)
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
assert response.json()["error"]["code"] == PARSE_ERROR


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