"""Invariants for ``tools/mcp_tool_errors._format_connect_error`` on malformed exception chains.

``__cause__``/``__context__`` can form a cycle (the same OAuth error re-raised on the SSE fallback,
a raised-and-caught pair) and stdio failures can nest deeper than the recursion limit; either used
to turn ``hermes mcp test`` into a RecursionError that hid the real connect error (#111952, #111997).
"""
import sys

from tools.mcp_tool_errors import _format_connect_error


def test_format_connect_error_reports_real_messages_on_cyclic_chain():
    """A two-node ``__cause__``/``__context__`` cycle renders every distinct message, once, in chain order."""
    first = RuntimeError("first failure")
    second = RuntimeError("second failure")
    first.__cause__ = second
    second.__context__ = first

    assert _format_connect_error(first) == "first failure; second failure"


def test_format_connect_error_finds_missing_executable_through_deep_cyclic_chain():
    """A missing stdio binary wrapped deeper than the recursion limit, with the chain looping back to the top,
    is still reported as the missing executable rather than as a RecursionError."""
    missing = FileNotFoundError(2, "No such file or directory", "/opt/homebrew/bin/removed-mcp-server")
    current = missing
    for _ in range(sys.getrecursionlimit() + 10):
        wrapper = RuntimeError("stdio startup failed")
        wrapper.__cause__ = current
        current = wrapper
    missing.__context__ = current

    assert _format_connect_error(current) == "missing executable '/opt/homebrew/bin/removed-mcp-server'"