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

-32021

-32021

Error message

modern request is missing {CLIENT_CAPABILITIES_KEY}

What it means

Error "modern request is missing {CLIENT_CAPABILITIES_KEY}" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/13-tools-and-protocols/31-mcp-conformance-versioning-and-operations/code/main.py:204

    if not isinstance(params, dict):
        raise ProtocolViolation("params must be an object", -32602)
    wire_headers = normalized_headers(headers)
    if era == "legacy":
        if body["method"] != "initialize" and wire_headers.get("mcp-protocol-version") == MODERN_VERSION:
            raise ProtocolViolation("modern metadata reached the legacy branch", -32020)
        return
    if era != "modern":
        raise ValueError(f"unknown era: {era}")

    metadata = params.get("_meta")
    if not isinstance(metadata, dict):
        raise ProtocolViolation("modern request is missing params._meta", -32602)
    protocol_version = metadata.get(PROTOCOL_VERSION_KEY)
    capabilities = metadata.get(CLIENT_CAPABILITIES_KEY)
    if not isinstance(protocol_version, str):
        raise ProtocolViolation(f"modern request is missing {PROTOCOL_VERSION_KEY}", -32602)
    if not isinstance(capabilities, dict):
        raise ProtocolViolation(f"modern request is missing {CLIENT_CAPABILITIES_KEY}", -32021)

    mirrored = {
        "mcp-protocol-version": protocol_version,
        "mcp-method": body["method"],
    }
    name = routing_name(body)
    if name is not None:
        mirrored["mcp-name"] = name
    for header, body_value in mirrored.items():
        header_value = wire_headers.get(header)
        if header == "mcp-name" and header_value is not None:
            header_value = decode_header_value(header_value)
        if header_value != body_value:
            raise ProtocolViolation(
                f"{header} does not match the JSON-RPC body",
                -32020,
            )
    if protocol_version != MODERN_VERSION:

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:204 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/16ee77d3306b347f. Report an issue: GitHub.