docs: proposal for new MCP Release candidate ( 28-07-2026 )

[Hritik003]

Description

What's changing in the spec:

• Sessions (Mcp-Session-Id) and initialize/ping/logging/setLevel are removed.
• Every request now carries protocol version, client info, and capabilities in _meta fields.
• server/discover replaces initialize for capability discovery; subscriptions/listen replaces the GET SSE stream.
• Multi Round-Trip Requests (MRTR) with InputRequiredResult replace server-to-client requests (sampling/elicitation/roots).
• New required headers: Mcp-Method, Mcp-Name, MCP-Protocol-Version.

Approach:

• The gateway becomes dual-era: it detects whether each client/backend speaks legacy or modern protocol and handles both.
• For the modern↔modern path, routing no longer needs sessions — the route comes from the x-ai-eg-mcp-route header (already on every request), the target backend from Mcp-Name/name-prefix encoding (already exists), and per-backend capabilities from a lazy server/discover cache.
• A 2×2 compatibility matrix (legacy/modern × client/backend) is defined for all four cells.

Immediate scope (Phase 0 + Phase 1):

• Phase 0: Foundation types, constants, era detection, error codes — zero behavior change.
• Phase 1: Full modern↔modern stateless path (Cell 2) while legacy↔legacy (Cell 1) remains completely unchanged. Covers server/discover, stateless routing, _meta/header injection, MRTR passthrough, subscriptions/listen, and rejection of removed methods.

Deferred (Phases 2–4):

• Phase 2: Cross-era translation (modern client ↔ legacy backend and vice versa) — deferred until mixed-era deployments become common.
• Phase 3: Proxy-level result caching, deterministic ordering, minor spec items.
• Phase 4: Auth hardening, deprecation annotations, docs/examples.

Closes: #2323