ADR-0055: Questions about the core's host are answered by the core

• Status: Accepted
• Date: 2026-09-03
• Deciders: jka
• Decision: D55

Context

The remote file browser opened at / when it had nothing better to start from. That is correct for a POSIX core and wrong for a Windows core, where the root is C:/ and / is not even an absolute path. Using the GUI's own root would be wrong the other way round: a Windows GUI on a Linux core would offer C:/.

Decision

session.hello reports fsroot, the root of the core's filesystem. The GUI records it for each attachment and reads it again after reconnecting, since the new core may be a different host. Further host facts can join it in the greeting when needed.

General rule: if the answer depends on the core's host, ask the core.

Consequences

• The field is additive and does not bump the protocol version. An older core omits it and the client keeps its default; an older client ignores it. The forward-compatibility rule of ADR-0019 applies to the protocol itself.
• The GUI still judges whether a path is absolute by its shape (a leading / or a drive prefix), not with Tcl's file pathtype or file normalize, which apply the client's rules. fsroot says where to start; the shape test says what an absolute path looks like.
• Later decisions follow the same rule: stale-buffer detection (ADR-0094) and TLS trust (ADR-0109) are decided on the core's host.