Support MCP protocol version 2025-11-25 (v4)

[guillempuche]

What is the problem this feature would solve?

Re-filing #6157 against v4 — that issue was auto-closed by the v3 feature-freeze sweep, and reopening it is rejected by GitHub (422), so this is a fresh copy with the ask narrowed to where v4 actually stands.

McpServer in v4 main now routes through a pluggable protocol registry: multiple adapters, per-session protocol selection, and mcp-protocol-version header negotiation. But McpProtocol.ts still exports exactly one adapter:

export const v2025_06_18: ProtocolAdapter = Internal.make({
  protocolVersion: "2025-06-18",
  clientRpcs: McpSchema.ClientRpcs,
  clientNotificationRpcs: McpSchema.ClientNotificationRpcs,
  serverRequestRpcs: McpSchema.ServerRequestRpcs,
  serverNotificationRpcs: McpSchema.ServerNotificationRpcs
})

export type ProtocolVersion = ProtocolAdapter["protocolVersion"]

So a server can't advertise 2025-11-25, and none of that spec's additions are reachable.

The concrete ask: add a McpProtocol.v2025_11_25 adapter alongside the existing one, widen ProtocolVersion to the union, and add the McpSchema shapes it needs.

From the 2025-11-25 changelog, the pieces that matter most for a server implementation:

• Tasks (experimental) — durable async requests with polling and deferred result retrieval (SEP-1686)
• Icons for tools, resources, resource templates, and prompts (SEP-973)
• Input validation errors as tool execution errors, so the model can self-correct instead of the request failing at the protocol level (SEP-1303)
• Tool calling in sampling via tools and toolChoice (SEP-1577)
• Elicitation enhancements — default values (SEP-1034), titled/multi-select enums (SEP-1330), URL mode (SEP-1036)
• Protocol-level — payloads decoupled from RPC method definitions (SEP-1319), JSON Schema 2020-12 as the default dialect (SEP-1613)

The OAuth changes (SEP-991, SEP-1046, SEP-985, SEP-835, OIDC Discovery) are a larger, separable chunk — happy for those to be their own issue if that's cleaner.

How this helps my use case

I run a CRM exposed entirely over MCP — ~89 tools across several toolkits, plus resources and prompts.

• Tasks — my company-research tools run for minutes against external providers. Today the client has to hold the request open; I've had to build my own poll-a-handle convention on top because the protocol has no primitive for it. Tasks is exactly that primitive.
• Input validation as tool errors — handlers currently turn bad input into protocol-level failures, which reads to the model as a crash rather than "you passed a bad id, try again". SEP-1303 makes the retry loop work.
• Icons — with that many tools grouped into toolkits, icons are a real navigability win in clients.
• Elicitation defaults and titled enums — my create/update tools have many optional fields; basic elicitation works on 2025-06-18 but is clumsy without defaults.

What alternatives have you considered?

Staying on 2025-06-18 — clients negotiate down, so nothing is broken today. But as clients adopt 2025-11-25, Effect servers can't advertise the new capabilities, and workarounds like a hand-rolled polling convention for long-running tools stay necessary.

The registry refactor already landed the hard part; the remaining work is a second adapter plus schema additions. Happy to take a first pass at it if that direction is welcome.