Investigate @orpc/mcp McpRegistrar (or Hono MCP adapter) in every-plugin to replace the hand-rolled MCP bridge #673

Context

host/src/services/mcp.ts hand-rolls MCP tool generation today:

• Generates an OpenAPI spec from the oRPC router (@orpc/openapi), walks paths, and hand-builds each tool's inputSchema from operation parameters + request-body properties — the exact seam that broke in Host /api/mcp silently fails to mount when any operation input has a $ref (e.g. z.json()) #670 (a $ref in any input kills Ajv compilation; fixed stopgap in PR fix(host): self-contained MCP tool schemas; skip a failing tool instead of the whole mount #671).
• Executes each tool call by constructing a Request and round-tripping through the OpenAPI handler, flattening every response body to a single text content block.
• Carries auth context via a module-level AsyncLocalStorage bridge (mcpRequestStorage) into tool calls.
• Skips any operation with an SSE response entirely — streaming procedures are invisible to MCP clients.

Proposed direction

@orpc/mcp ships McpRegistrar, which registers every router procedure directly as an MCP tool through the same standard handler as the RPC/OpenAPI handlers:

• Schema-derived tool inputs (no manual $ref/$defs surgery — the Host /api/mcp silently fails to mount when any operation input has a $ref (e.g. z.json()) #670 bug class disappears), with per-tool overrides via mcp.tool metadata and procedure filter.
• Proper error mapping: ORPCError → MCP tool execution errors (model-readable JSON).
• Structured content outputs; Smart Coercion plugin for Date/BigInt inputs.
• Per-request context via register(server, { context: async ctx => ... }) — likely replaces the AsyncLocalStorage bridge.
• Interceptors/plugin reuse (timeout, logging; CORS-compression-style HTTP plugins are no-ops, Rethrow unsupported).

Alternative to evaluate: a Hono MCP adapter for the transport side (transport mounting is currently the SDK's WebStandardStreamableHTTPServerTransport wired by hand).

Spike questions

1. Placement — should this live in every-plugin (plugin runtime consumes/hosts it) instead of host-internal code, so plugin RPC surfaces become MCP tools generically?
2. Which plugin surfaces — per-plugin MCP (/api/rpc/{plugin}/... → plugin MCP) vs. the composed base API router as today.
3. SSE/streaming procedures — currently forcibly skipped; does McpRegistrar AsyncIteratorObject progress mapping give us these for free?
4. Auth flow — map the current buildPluginContext + x-api-key/session middleware into the registrar's context function-of-request; confirm per-request context is evaluated per tool call, not per server.
5. Version/gate — @orpc/mcp@beta, MCP TS SDK v2 (@modelcontextprotocol/server); check compatibility with the current @orpc pin and the Effect dev toolchain additions.
6. Teardown — closeMcpServer() / mcpTransport lifecycle must survive the swap (host reload paths call it).

Notes

• PR fix(host): self-contained MCP tool schemas; skip a failing tool instead of the whole mount #671 merges as the stopgap (self-contained tool schemas + per-tool skip). This ticket is the systemic fix that should obsolete the hand-built schema machinery.
• Deferred from Host /api/mcp silently fails to mount when any operation input has a $ref (e.g. z.json()) #670: only advertise /api/mcp in /.well-known/mcp.json after a successful mount — fold that decision in here (or as an immediate follow-up if the swap lands later).