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}

首先阅读 src/mcp/shared/session.py 和 src/mcp/client/streamable_http.py 中的任务组处理逻辑，然后检查 issue 中提到的其他 create_task_group() 位置。复现一个伴随已取消的兄弟任务的连接失败，并验证真实异常会在受影响的调用点之间传播，且不会产生取消噪声。