tests/client/test_client.py: 14 failures on Windows — AttributeError: '...Result' object has no attribute '_meta'

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

On a clean checkout of main, tests/client/test_client.py fails with 14 errors of the form AttributeError: 'CallToolResult' object has no attribute '_meta'. Did you mean: 'meta'?

The snapshots pass the wire name (snapshot(CallToolResult(_meta={...}))), but inline_snapshot's generic_call_adapter.argument() does a plain getattr(value, '_meta') and the pydantic model exposes the field as meta. The data is there — the repr shows meta={'io.modelcontextprotocol/serverInfo': ...}.

I'd expect the test suite to pass on a fresh clone with locked dependencies.

Ruled out: unlocked dependency resolution (reproduced with --frozen and --no-sync), Python version (3.12.14 and 3.14.2 both fail), and a non-ASCII checkout path (reproduces under a plain ASCII path too).

I only have Windows to test on, so I don't know whether this reproduces on Linux or macOS. If CI is green, the difference is presumably platform-specific and I'm happy to run whatever would help narrow it down.

┃ File ┃ Function ┃ Function Line ┃ Error Line ┃ Error ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━┩
│ tests\client\test_client.py │ test_client_with_simple_server │ 133 │ │ │
│ tests\client\test_client.py │ test_client_list_tools │ 151 │ │ │
│ tests\client\test_client.py │ test_client_call_tool │ 179 │ │ │
│ tests\client\test_client.py │ test_read_resource │ 191 │ │ │
│ tests\client\test_client.py │ test_get_prompt │ 272 │ │ │
│ tests\client\test_client.py │ test_client_list_resources_wi… │ 339 │ │ │
│ tests\client\test_client.py │ test_client_list_resource_tem… │ 358 │ │ │
│ tests\client\test_client.py │ test_list_prompts │ 369 │ │ │
│ tests\client\test_client.py │ test_complete_with_prompt_ref… │ 387 │ │ │
│ tests\client\test_client.py │ test_call_tool_auto_loop_disp… │ 768 │ 797 │ AttributeError │
│ tests\client\test_client.py │ test_call_tool_auto_loop_disp… │ 810 │ 846 │ AttributeError │
│ tests\client\test_client.py │ test_call_tool_auto_loop_disp… │ 857 │ 881 │ AttributeError │
│ tests\client\test_client.py │ test_get_prompt_auto_loop_res… │ 945 │ 968 │ AttributeError │
│ tests\client\test_client.py │ test_read_resource_auto_loop_… │ 976 │ 999 │ AttributeError

Example Code

┃  File                         ┃  Function                        ┃  Function Line  ┃  Error Line  ┃  Error           ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━┩
│  tests\client\test_client.py  │  test_client_with_simple_server  │  133            │              │                  │
│  tests\client\test_client.py  │  test_client_list_tools          │  151            │              │                  │
│  tests\client\test_client.py  │  test_client_call_tool           │  179            │              │                  │
│  tests\client\test_client.py  │  test_read_resource              │  191            │              │                  │
│  tests\client\test_client.py  │  test_get_prompt                 │  272            │              │                  │
│  tests\client\test_client.py  │  test_client_list_resources_wi…  │  339            │              │                  │
│  tests\client\test_client.py  │  test_client_list_resource_tem…  │  358            │              │                  │
│  tests\client\test_client.py  │  test_list_prompts               │  369            │              │                  │
│  tests\client\test_client.py  │  test_complete_with_prompt_ref…  │  387            │              │                  │
│  tests\client\test_client.py  │  test_call_tool_auto_loop_disp…  │  768            │  797         │  AttributeError  │
│  tests\client\test_client.py  │  test_call_tool_auto_loop_disp…  │  810            │  846         │  AttributeError  │
│  tests\client\test_client.py  │  test_call_tool_auto_loop_disp…  │  857            │  881         │  AttributeError  │
│  tests\client\test_client.py  │  test_get_prompt_auto_loop_res…  │  945            │  968         │  AttributeError  │
│  tests\client\test_client.py  │  test_read_resource_auto_loop_…  │  976            │  999         │  AttributeError

git clone https://github.com/modelcontextprotocol/python-sdk.git
cd python-sdk
uv sync --frozen --all-extras --dev
uv run --no-sync pytest tests/client/test_client.py -q

# → 14 failed, 33 passed

Python & MCP Python SDK

Python 3.12.14 (also reproduced on 3.14.2)
mcp 2.2.1.dev1+6affe5c0 (source checkout of main, commit 6affe5c0)
inline_snapshot 0.28.0, pydantic 2.12.5 (both from uv.lock)
uv 0.12.16
Windows 11, PowerShell

まず、固定された依存関係を使用して Windows 上で uv run --no-sync pytest tests/client/test_client.py -q を実行します。tests/client/test_client.py 内の失敗するケースを読み、そこにある CallToolResult の snapshot 引数を、報告されている meta/_meta の動作と比較します。14 件の失敗が解消され、期待される結果を弱めることなくクライアントのテストファイルがパスすれば完了です。