Here's an example of a barebones gall agent that just starts a thread:

thread-starter.hoon

/+  default-agent, dbug
=*  card  card:agent:gall
%-  agent:dbug
^-  agent:gall
|_  =bowl:gall
+*  this      .
    def   ~(. (default-agent this %|) bowl)
::
++  on-init  on-init:def
++  on-save  on-save:def
++  on-load  on-load:def
++  on-poke
  |=  [=mark =vase]
  ^-  (quip card _this)
  ?+    mark  (on-poke:def mark vase)
      %noun
    ?+    q.vase  (on-poke:def mark vase)
        (pair term term)
      =/  tid  `@ta`(cat 3 'thread_' (scot %uv (sham eny.bowl)))
      =/  ta-now  `@ta`(scot %da now.bowl)
      =/  start-args  [~ `tid byk.bowl(r da+now.bowl) p.q.vase !>(q.q.vase)]
      :_  this
      :~
        [%pass /thread/[ta-now] %agent [our.bowl %spider] %poke %spider-start !>(start-args)]
      ==
    ==
  ==
++  on-watch  on-watch:def
++  on-leave  on-leave:def
++  on-peek   on-peek:def
++  on-agent
   |=  [=wire =sign:agent:gall]
   ^-  (quip card _this)
   ?+    -.wire  (on-agent:def wire sign)
       %thread
     ?+    -.sign  (on-agent:def wire sign)
         %poke-ack
       ?~  p.sign
         %-  (slog leaf+"Thread started successfully" ~)
         `this
       %-  (slog leaf+"Thread failed to start" u.p.sign)
       `this
     ==
   ==
++  on-arvo   on-arvo:def
++  on-fail   on-fail:def
--

And here's a minimal thread to test it with:

test-thread.hoon

/-  spider
=,  strand=strand:spider
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
|=  strand-input:strand
?+    q.arg  [~ %fail %not-foo ~]
    %foo
  [~ %done arg]
==

Save them as /app/thread-starter.hoon and /ted/test-thread.hoon respectively in the %base desk, |commit %base, and start the app with |rein %base [& %thread-starter].

Now you can poke it with a pair of thread name and argument like:

:thread-starter [%test-thread %foo]

You should see Thread started successfully.

Now try poking it with [%fake-thread %foo], you should see something like:

Thread failed to start
/app/spider/hoon:<[355 5].[355 60]>
[%no-file-for-thread %fake-thread]
/app/spider/hoon:<[354 5].[355 60]>
/app/spider/hoon:<[353 3].[359 19]>
/app/spider/hoon:<[350 3].[359 19]>
/app/spider/hoon:<[346 3].[359 19]>
/app/spider/hoon:<[343 3].[359 19]>
/app/spider/hoon:<[341 3].[359 19]>
/app/spider/hoon:<[340 3].[359 19]>
/app/spider/hoon:<[336 3].[359 19]>
/app/spider/hoon:<[335 3].[359 19]>
/app/spider/hoon:<[202 24].[202 68]>
/app/spider/hoon:<[200 7].[207 9]>
/app/spider/hoon:<[199 5].[208 17]>
/app/spider/hoon:<[197 5].[208 17]>
/app/spider/hoon:<[196 5].[208 17]>
/sys/vane/gall/hoon:<[1.370 9].[1.370 37]>

Analysis

We can ignore the input logic, here's the important part:

=/  tid  `@ta`(cat 3 'thread_' (scot %uv (sham eny.bowl)))
=/  ta-now  `@ta`(scot %da now.bowl)
=/  start-args  [~ `tid byk.bowl(r da+now.bowl) p.q.vase !>(q.q.vase)]
:_  this
:~
  [%pass /thread/[ta-now] %agent [our.bowl %spider] %poke %spider-start !>(start-args)]
==

You can generate a tid any way you like, just make sure it's unique. Here we just use the hash of some entropy prefixed with thread_.

Then it's just a poke to %spider with the mark %spider-start and a vase containing start-args. Spider will then respond with a %poke-ack with a (unit tang) which will be ~ if it started successfully or else contain an error and a traceback if it failed. Here we test for this and print the result:

++  on-agent
   |=  [=wire =sign:agent:gall]
   ^-  (quip card _this)
   ?+    -.wire  (on-agent:def wire sign)
       %thread
     ?+    -.sign  (on-agent:def wire sign)
         %poke-ack
       ?~  p.sign
         %-  (slog leaf+"Thread started successfully" ~)
         `this
       %-  (slog leaf+"Thread failed to start" u.p.sign)
       `this
     ==
   ==