It looks like fossil is relying on some sh(1) or bash(1) specific semantic when it comes to syncing via ssh. Maybe using something like:

<verbatim>/bin/sh -c 'exec /path/to/fossil'</verbatim>

or threads?  Here's what's currently happening with tcsh:

<verbatim>% fs remote-url 'ssh://usr@remote-host.example.com:4321/src/.fossils/proj.fossil?fossil=/usr/local/bin/fossil'
password for usr: 
ssh://usr:*@remote-url.example.com:4321/src/.fossils/proj.fossil?fossil=/usr/local/bin/fossil
% fs pull
Server:    ssh://usr@remote-host.example.com:4321/src/.fossils/proj.fossil?fossil=/usr/local/bin/fossil
ssh -p 22 -t -e none -p 4321 usr@remote-url.example.com
Pseudo-terminal will not be allocated because stdin is not a terminal.
fossil: ssh connection failed: [Warning: no access to tty (Bad file descriptor).
Thus no job control in this shell.]
Killed by signal 2.
Exit 1
% fs set ssh-command 'ssh -p 4321 -T -e none'
% fs pull
Server:    ssh://usr@remote-host.example.com:4321/src/.fossils/proj.fossil?fossil=/usr/local/bin/fossil
ssh -p 22 -T -e none -p 4321 usr@remote-url.example.com
fossil: ssh connection failed: [Warning: no access to tty (Bad file descriptor).]
Killed by signal 2.
Exit 1
% fs set ssh-command
ssh-command          (local)  ssh -p 4321 -T -e none
% fs sync
Server:    ssh://remote-host.example.com/src/.fossils/project.fossil
ssh -p 22 -T -e none -p 4321 remote-host.example.com
fossil: ssh connection failed: [Warning: no access to tty (Bad file descriptor).
Thus no job control in this shell.]
Killed by signal 2.
Exit 1</verbatim>

Both client and server are running the same version of fossil.  When I change the shell to sh, I get the following output:

<verbatim>% fs sync
Server:    ssh://usr@remote-host.example.com:4321/src/.fossils/proj.fossil
ssh -p 22 -T -e none -p 4321 remote-host.example.com
                Bytes      Cards  Artifacts     Deltas
Sent:            4005         85          0          0
fossil: server says: 501 Not Implemented
Total network traffic: 2341 bytes sent, 0 bytes received
Closing SSH tunnel: Killed by signal 2.</verbatim>

When I sync via http over an ssh tunnel, things work just fine:

<verbatim>% fs remote-url http://usr@127.0.0.1:9099/
password for usr:
http://usr@127.0.0.1:9099/
% fs sync
Server:    http://usr@127.0.0.1:9099/
                Bytes      Cards  Artifacts     Deltas
Sent:            4005         85          0          0
Received:        3820         83          0          0
Total network traffic: 2368 bytes sent, 1275 bytes received</verbatim>