ExceptionGroup wrapping obscures real errors from task groups

Summary

When SDK internal anyio task group tasks fail, the real error is wrapped in a BaseExceptionGroup alongside Cancelled exceptions from cancelled sibling tasks. This makes error classification extremely difficult for callers — they cannot reliably determine the root cause of a failure.

There are 16 create_task_group() usages across the SDK with no except* syntax or ExceptionGroup unwrapping anywhere.

Expected Behavior

• Callers should receive the original exception, not an ExceptionGroup wrapping it alongside cancellation noise
• When a task group has one real failure and N cancelled siblings, only the real failure should propagate

Current Behavior

• A single connection failure produces a BaseExceptionGroup containing the real error plus multiple Cancelled exceptions
• Callers must manually unwrap exception groups to find the root cause
• This affects ~80+ call sites across the SDK where task groups are used

Affected Code

• src/mcp/shared/session.py (session task groups)
• src/mcp/client/streamable_http.py (transport task groups)
• All other create_task_group() sites

Related

• #1742 — broader typed error classes design
• #1830 — CancelledError indistinguishable from external cancellation (closed but related)

_{AI Disclaimer}

まず src/mcp/shared/session.py と src/mcp/client/streamable_http.py のタスクグループ処理を読み、次に issue で言及されている create_task_group() の他の箇所を調べます。キャンセルされた兄弟タスクを伴う接続失敗を再現し、実際の例外がキャンセル由来のノイズなしに影響を受ける呼び出し箇所へ伝播することを確認します。