[Enh]: Support MCP protocol revision 2026-07-28 (stateless transport, server/discover, cacheable lists)

Comienza con la especificación de MCP del 2026-07-28 y el análisis de brechas de #3429; revisa también el issue de packaging relacionado, #3658. Este issue solicita una hoja de ruta o una decisión explícita en lugar de una implementación, por lo que estará terminado cuando la posición de soporte de DAB quede registrada con cualquier milestone o label aplicable.

Summary

DAB's MCP server advertises and implements 2025-06-18 as a fixed default
(McpProtocolDefaults.DEFAULT_PROTOCOL_VERSION). Two spec revisions have shipped since:
2025-11-25 and 2026-07-28. #3429 analysed the gap to 2025-11-25 and was closed without
a follow-up item, and there is currently no issue tracking 2026-07-28 at all.

This issue asks for a decision and, if applicable, a roadmap entry — not necessarily an
immediate implementation.

Why this revision is different

2026-07-28 is not additive; it restructures the transport and lifecycle:

• Protocol-level sessions and the Mcp-Session-Id header are removed.
• The initialize / notifications/initialized handshake is removed. Every request
  carries its protocol version and client capabilities in _meta.
• A new server/discover RPC is mandatory for advertising versions, capabilities and identity.
• The GET stream endpoint and resources/subscribe are replaced by subscriptions/listen.
• SSE resumability (Last-Event-ID) is removed.
• MCP-Protocol-Version, Mcp-Method and Mcp-Name headers become required, with
  server-side header/body validation (-32020 HeaderMismatch).
• All results carry a resultType; server-initiated requests are replaced by the
  Multi Round-Trip Requests (MRTR) pattern.
• ping, logging/setLevel are removed; Roots, Sampling and Logging are deprecated.

Because the handshake itself is gone, the compatibility burden shifts entirely to clients.
The spec defines a probe-and-fall-back path, but a server that only speaks 2025-06-18
depends on every client retaining that path indefinitely. #3429 documented this failure mode
in practice (a client disconnecting on version downgrade over stdio).

What DAB would gain beyond conformance

Several of the changes map onto capabilities DAB users already ask for:

• Statelessness. With no protocol session, DAB instances become trivially scalable behind
  a load balancer, and a restart no longer invalidates in-flight clients. It also removes the
  Mcp-Session-Id friction that the MCP Inspector guidance currently works around.
• ttlMs / cacheScope on tools/list, resources/list, resources/read. DAB already
  has an entity cache with TTL; surfacing it at the protocol level would let clients stop
  re-fetching. This matters at scale: our production configuration has 315 entities and
  describe_entities responses are substantial.
• Deterministic tool ordering (a SHOULD in this revision) improves client-side caching and
  LLM prompt-cache hit rates. Should be trivial, since DAB generates the tool surface.
• Mcp-Name header. Lets reverse proxies route, rate-limit and audit per tool without
  parsing the request body. Today an operator behind nginx sees only POST /mcp and cannot
  tell which entity was read — a real gap for auditing.
• Tasks extension (io.modelcontextprotocol/tasks) is a natural fit for
  aggregate_records over large tables.

Deployment context

Read-only ERP database exposed through DAB 2.0.9, HTTP transport behind an nginx reverse
proxy, 315 entities, consumed by MCP clients. Nothing is broken today — clients still
negotiate 2025-06-18 successfully over HTTP. The concern is that continued operation
depends on client-side backwards compatibility that operators do not control, while DAB has
no stated position.

Ask

1. Is support for 2026-07-28 on the roadmap?
2. If yes, could it be tracked with a milestone/label the way other MCP work is (2.x, 2.3)?
3. If no — please say so explicitly. A clear "DAB stays on 2025-06-18 for now" is genuinely
   actionable for operators, who can then plan a translating layer instead of waiting.
   (Related: #3658, packaging Microsoft.DataApiBuilder.Mcp as a NuGet SDK, would make such
   a layer considerably cheaper to build.)

References

• Changelog for the revision: https://modelcontextprotocol.io/specification/2026-07-28/changelog
• Streamable HTTP transport: https://modelcontextprotocol.io/specification/2026-07-28/basic/transports/streamable-http
• #3429 — gap analysis for 2025-11-25 (closed)
• #3658 — Microsoft.DataApiBuilder.Mcp NuGet package