AI Engineering from Scratch（英文原版）

Given a list of MCP server transports, produce a client harness that prefers MCP 2026-07-28 and isolates legacy compatibility.

Produce:

1. Peer configuration. Map a stable server name to a pinned command or endpoint, arguments, environment allowlist, authorization context, transport kind, and an explicit allow_legacy flag that defaults to false.
2. Modern request builder. Stamp protocol version, current client capabilities, and recommended client identity into every params._meta immediately before serialization.
3. stdio era probe. Send server/discover first. Accept a valid DiscoverResult, retry -32022 at a mutually supported modern version, and treat -32020 plus -32021 as correctable modern errors.
4. Legacy compatibility probe. Treat an unrecognized error, timeout, connection close, or empty response as ambiguous. Send one deadline-bound initialize only when that exact peer has allow_legacy: true. Select legacy only after a correlated JSON-RPC success containing a configured legacy revision, object capabilities, and non-empty server identity. Otherwise fail closed.
5. Tool cache. Honor ttlMs and cacheScope in the negotiated authorization context. Treat absent legacy resultType as "complete".
6. Namespace merge. Sort peers and tools. Prefix or reject collisions. Forbid silent overwrite.
7. Router. Map canonical tool names to peer plus local name, create a new request id, send the era-correct request, and verify the response id.
8. Recovery. On transport loss, fail in-flight work, restart or reconnect, repeat discovery and lists, re-open subscriptions, and retry only operations allowed by the safety policy.

Hard rejects:

• Sending modern requests without current _meta.
• Falling back to initialization after a recognized modern error.
• Sending initialize to a peer that is not explicitly allowlisted for legacy compatibility.
• Treating a timeout, connection close, empty response, unrecognized error, malformed result, or unsupported revision as proof of legacy behavior.
• Treating a process, connection, or Mcp-Session-Id as modern protocol state.
• Sharing a private cached list across authorization contexts.
• Silently overwriting a duplicate tool name.
• Accepting a modern success without resultType.

Refusal rules:

• Refuse to spawn a command outside a pinned allowlist.
• Refuse to route a tool when the owner is ambiguous.
• Refuse to retry a non-idempotent call automatically without an application idempotency key or user decision.

Output a complete Python harness, at least six conformance tests, and a startup report listing peer, selected era, selected version, cache scope, and canonical tool names.