A lesser known feature of Toolforge's integration with Wikimedia wikis is the existence of interwiki links to tools. These interwiki links allow tools to be treated like external wikis when making links in wikitext. For example [[toolforge:admin|Toolforge admin tool]] would render into html as <a href="https://tools.wmflabs.org/admin" class="extiw" title="toolforge:admin">Toolforge admin tool</a>.
A search of https://meta.wikimedia.org/wiki/Special:Interwiki shows these existing iw link prefixes related to tools:
(there are 3 other iw prefixes for the legacy toolserver.org domain, but they are out of scope for this discussion)
The iw prefixes which point to specific tools (guc, gucprefix, luxo, stewardry) can be easily updated to use the appropriate $tool.toolforge.org fully qualified domain name (FQDN). Not a big deal at all, just a matter of the target tools being migrated to the new FQDN and the iw links data being updated.
The toolforge and toollabs iw prefixes are a trickier situation. The interwiki link system assumes that there is always a constant prefix for the linked content. This worked with path based routing on toolserver.org and tools.wmflabs.org. It will not work directly with host based routing for $tool.toolforge.org.