rohitg00/ai-engineering-from-scratch · error · ValueError

at least one modern and legacy version must be configured

Error message

at least one modern and legacy version must be configured

What it means

Error "at least one modern and legacy version must be configured" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/13-tools-and-protocols/08-building-an-mcp-client/code/main.py:301

class MergedTool:
    canonical_name: str
    peer_name: str
    local_name: str
    description: str


class MultiServerClient:
    def __init__(
        self,
        *,
        supported_modern: tuple[str, ...] = (PROTOCOL_VERSION,),
        supported_legacy: tuple[str, ...] = (LEGACY_VERSION,),
        probe_version: str | None = None,
        discovery_timeout_ms: int = 1_000,
        legacy_probe_timeout_ms: int = 1_000,
    ) -> None:
        if not supported_modern or not supported_legacy:
            raise ValueError("at least one modern and legacy version must be configured")
        if discovery_timeout_ms <= 0 or legacy_probe_timeout_ms <= 0:
            raise ValueError("probe timeouts must be positive")
        self.supported_modern = supported_modern
        self.supported_legacy = supported_legacy
        self.probe_version = probe_version or supported_modern[0]
        self.discovery_timeout_ms = discovery_timeout_ms
        self.legacy_probe_timeout_ms = legacy_probe_timeout_ms
        self.client_capabilities = CLIENT_CAPABILITIES.copy()
        self.peers: dict[str, Peer] = {}
        self.registry: dict[str, MergedTool] = {}
        self._next_request_id = 1

    def _new_id(self) -> int:
        request_id = self._next_request_id
        self._next_request_id += 1
        return request_id

    def add_server(

View on GitHub (pinned to 39ea8a1c6d)

When it happens

Trigger: Thrown at phases/13-tools-and-protocols/08-building-an-mcp-client/code/main.py:301 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rohitg00/ai-engineering-from-scratch@39ea8a1c6d (2026-08-26). Data as JSON: /api/errors/8722f9bfc338194c. Report an issue: GitHub.