7. A minimal upstream MCP client instead of the SDK client

• Status: accepted
• Date: 2026-09-30

Context

The generic upstream-MCP provider calls registrar MCP servers such as GoDaddy's over Streamable HTTP. Invariant 1 requires that only allowlisted tools can be called, refused before any request is made. Invariant 2 requires every request to go through core/net.

Decision

packages/providers/src/upstream-mcp/client.ts is a small JSON-RPC client that speaks only initialize, notifications/initialized, tools/list, and tools/call.

• callTool checks the allowlist first, before initializing or sending anything. A method outside those four is refused too.
• Requests go through createOriginFetch, restricted to the config URL's origin, over HTTPS only.
• Responses may be application/json or a finite text/event-stream. Both are read by readJsonMessages in core/net, so the no-raw-body lint rule keeps its existing exemptions.
• It negotiates the protocol version, sends mcp-protocol-version afterward, and echoes mcp-session-id if a server sets one. It never opens the standalone GET event stream.

The provider also calls tools/list once and refuses to run unless the check tool exists and declares readOnlyHint: true without destructiveHint: true. Config validation rejects an allowlist containing a tool whose name describes a write (describesWrite). Both checks are defense in depth; the allowlist is the enforcement.

Why not the SDK client

The brief requires the official SDK for Titlesearch's own MCP server. That still holds for step 5. For the client side:

• The SDK's Streamable HTTP client transport may open a long-lived GET event stream. The fixed-origin fetch buffers and caps whole responses, so that stream would hang until the timeout.
• The allowlist check belongs at the single point where requests leave. With the SDK, that point would be a wrapper around a much larger surface: resources, prompts, sampling, and elicitation handlers.
• The client needs only four methods, all covered by tests against recorded GoDaddy traffic.

Revisit this if an upstream needs features beyond stateless request and response.

Consequences

MCP protocol changes that affect these four methods must be tracked by hand. The nightly contract canary catches drift for GoDaddy.