Refresh #24 (MCP 2026 wire alignment): body is partly outdated and has no implementation #78

Type: edit #24 rather than open a new issue.
Depends on: #22, #23

Summary

[Issue #24 — Align 2026 wire: drop ping, add -32021, discover _meta, MUST headers](#24)
has an open PR (#37) with the same head commit as #36 (e458282) and contains none of the #24 work. Checked on main, two of the issue's rows no longer apply, while the rest still do.

Still true on main

• ping is answered on the auto and modern eras, including a header-only ping with an unparseable body.
• JsonRpcErrorCode has −32020 and −32022 but not −32021 (Missing Required Client Capability).
• server/discover _meta contains only io.modelcontextprotocol/cacheHint; serverInfo sits in the body.

No longer true

• A normal tools/call on the 2026 wire already returns resultType: "complete" and _meta["io.modelcontextprotocol/serverInfo"].
• Mcp-Method and Mcp-Name are still enforced (HTTP 400, −32020) after the sidecar is removed. With [mcp-2026] Remove JSON-RPC sidecar; one /mcp owner #34 merged the check comes from the official mcp SDK.

Acceptance criteria for the refreshed issue

• ping → −32601 on auto and modern; still answered on legacy.
• −32021 defined and returned for a task-augmented call when tasks are unavailable (see [mcp-2026] Hide tasks on 2026 wire until tasks/update exists #25).
• serverInfo carried in discover _meta.
• Tests are HTTP-level (TestClient), not sidecar unit tests.