lda-wf/docs/fastmcp_resource_link_issue_lda_tries.md

title

title
Namespace transform does not rewrite ResourceLink URIs returned from proxied tools

summary

With a Namespace transform on a proxy, listed resources are namespaced correctly. However, ResourceLink.uri values returned from a proxied tools/call result keep the upstream URI instead of the proxied URI.

Tested on FastMCP 3.3.0.

minimal example

from fastmcp.server import create_proxy
from fastmcp.server.transforms import Namespace

proxy = create_proxy(upstream_client, name="Proxy-everything")
proxy.add_transform(Namespace("everything"))

An upstream resource looking like this:

demo://resource/dynamic/text/2

will be exposed correctly as:

demo://everything/resource/dynamic/text/2

But if an upstream tool returns:

mcp.types.ResourceLink(
    type="resource_link",
    name="dynamic-text",
    uri="demo://resource/dynamic/text/2",
)

the proxied tool result still contains:

demo://resource/dynamic/text/2

Observed Behavior

resources/list:
  upstream: demo://resource/dynamic/text/2
  proxied:  demo://everything/resource/dynamic/text/2

resources/read:
  demo://everything/resource/dynamic/text/2
  -> works

tools/call result content:
  ResourceLink.uri == demo://resource/dynamic/text/2
  -> raw upstream URI leaks through unchanged

expected behavior

ResourceLink content returned by proxied tools should expose the same proxied URI as resources/list and resources/read.

scope of this issue

This is specifically about mcp.types.ResourceLink content inside tools/call results. Normal resources/list, resource_templates/list, and resources/read already behave correctly through the proxy.

Session-scoped resources returned by some tools may have additional lifecycle constraints beyond URI rewriting. This issue is about ordinary ResourceLink URI projection only.