mcp

MCP compatibility fixtures

This directory is the language-neutral compatibility surface for the AsDecided MCP server. It is intentionally separate from the RAC artifact conformance examples: RAC describes the corpus; these vectors describe the wire contract of an MCP implementation serving that corpus.

The authoritative manifest is conformance/vectors.json. It is a small, versioned JSON document with no implementation-specific paths, timestamps, or model calls. A vector contains:

• transport: stdio or http;
• request: one JSON-RPC request object;
• http (HTTP vectors only): request headers, path, optional allow-list, and an optional declared content_length used to exercise framing policy;
• expect: either exact legacy frames or schema-constrained assertions.

Assertions use JSON Pointer. equals pins a scalar/object value, exists requires a field to be present, and absent requires it not to be present. Current-protocol vectors use assertions so an implementation can choose its own package version and audit path. The legacy vector set is explicitly byte-oriented: frozen 2025 behavior is represented by exact_frames and must not be silently normalized into current semantics.

The vectors cover the MCP 2026-07-28 discovery and metadata contract, current result typing and cache hints, JSON-RPC envelope validity, notifications and IDs, legacy initialization, HTTP version-carrier agreement, Origin policy, structured errors, and tool-schema object roots. They are protocol fixtures, not a Python oracle: a future implementation reads the same JSON and runs its own transport adapter.

The format is deliberately additive. New vectors MUST have stable IDs and MUST avoid asserting incidental fields such as timestamps, temporary paths, or implementation package versions. Changes to a frozen legacy frame require a reviewed compatibility decision; changes to current semantic assertions are documented in CHANGELOG.md.

The current vectors target the official MCP 2026-07-28 specification and AsDecided's declared dual-era compatibility policy. They do not replace the official specification; they make the subset AsDecided ships executable and portable across implementations.