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

modern support was not proven; strict policy forbids fallbac

Error message

modern support was not proven; strict policy forbids fallback

What it means

Error "modern support was not proven; strict policy forbids fallback" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/13-tools-and-protocols/31-mcp-conformance-versioning-and-operations/code/main.py:340

    ):
        raise ProtocolViolation("task result pollIntervalMs must be a non-negative integer")


def select_era(
    observation: dict[str, Any],
    policy: str,
    legacy_allowed: bool = False,
    legacy_evidence: dict[str, Any] | None = None,
) -> str:
    if policy not in {"strict", "fallback"}:
        raise ValueError("policy must be strict or fallback")
    kind = observation.get("kind")
    if kind == "discover_success":
        return "modern"
    if kind == "jsonrpc_error" and observation.get("code") in MODERN_ERROR_CODES:
        return "modern"
    if policy == "strict":
        raise ProtocolViolation("modern support was not proven; strict policy forbids fallback")
    if kind not in {"empty", "timeout", "connection_closed", "unrecognized"}:
        raise ProtocolViolation("probe outcome is not safe evidence for fallback")
    if not legacy_allowed:
        raise ProtocolViolation("legacy fallback is not allowlisted for this endpoint")
    if not isinstance(legacy_evidence, dict):
        raise ProtocolViolation("inconclusive modern probe is not positive legacy evidence")
    if (
        legacy_evidence.get("kind") != "initialize_success"
        or legacy_evidence.get("protocolVersion") != LEGACY_VERSION
    ):
        raise ProtocolViolation("legacy probe did not prove the configured legacy era")
    return "legacy"


def notification_http_outcome(request: dict[str, Any]) -> tuple[int, None]:
    if "id" in request:
        raise ValueError("request has an id and is not a notification")
    return 202, None

View on GitHub (pinned to 39ea8a1c6d)

When it happens

Trigger: Thrown at phases/13-tools-and-protocols/31-mcp-conformance-versioning-and-operations/code/main.py:340 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/20d4cad5933bc62a. Report an issue: GitHub.