fix(server): keep upstream resource annotations and _meta when proxying - #2469

Why

ResourceDefinition carries annotations and _meta (resources.ts:28 and :36), and both survive local registration. The proxy mount copies only three fields (mcp-proxy.ts:405):

host.registerResource(
  {
    name,
    uri: proxiedResourceUri(plan.namespace, upstreamUri),
    ...(resource.title !== undefined && { title: resource.title }),
    ...(resource.description !== undefined && { description: resource.description }),
    ...(resource.mimeType !== undefined && { mimeType: resource.mimeType }),
  },
  ...

ProxyResource does not even declare the other two, so they are dropped at the introspection boundary. The moment a server is composed with use(), its upstream resources lose their client hints and their extension metadata from resources/list, with nothing reported.

Driving mountProxyConnection with an upstream resource that has both, the mounted definition comes back as { name: 'docs_guide', uri } and nothing else.

Why this one is a straight passthrough

Both fields are documented on the resource descriptor and neither has a framework-derived counterpart. That is not true of a tool's _meta, where the server computes _meta.ui.* from view and visibility and those derived values take precedence over colliding entries. There is no such derivation for resources, so there is nothing to collide with and no precedence question to answer.

Prompts need no equivalent change: PromptDefinition has no _meta.

The tool case, deliberately not in this PR

Proxied tools drop _meta the same way, and ProxyTool does not declare it either. I left that out because it is a policy question rather than a bug fix: forwarding upstream _meta verbatim would let an upstream set framework-owned _meta.ui.* keys, including visibility, on a tool this server re-exposes. That wants your call on whether to pass it through, or to forward everything except the framework-owned namespace. Happy to send that separately once you say which.

Test

Added to tests/proxy.test.ts, which already exercises mountProxyConnection with a fake host. On the unfixed source it fails with expected { name: 'docs_guide', …(1) } to match object { annotations: { …(2) }, …(1) }.

Validation

From libraries/typescript/packages/server: vitest run — 476 passed, 44 files.
tsc --noEmit, eslint, prettier --check — clean.
Monorepo preflight — 0 failing packages.

Summary by cubic

Fixes proxied resources losing their annotations and _meta fields when composed through use().

• ProxyResource now declares and forwards these fields from upstream resources.
• Tools' _meta is intentionally left unchanged; forwarding upstream _meta.ui.* would conflict with server-computed values.

^{Written for commit 94131b9. Summary will update on new commits.}