Using conn.c


Prior to 2022, the only way to interact with a running ship from Earth was via HTTP requests sent to the %lens agent. In addition to %lens, there was a Python script helper named herb which would automatically format HTTP requests for %lens based on user inputs. However, there were several pain points for using %lens and herb. %lens, herb, and the difficulties of using them are documented more fully here.

Unrelated to the above, inconveniences around writing boilerplate code to interact with the %spider agent was hampering adoption of threads outside of the Arvo kernel.


Starting in 2022, tools for solving the above issues began to appear (though work on them began in 2021). In order, they are:

  • conn.c
  • The Khan vane
  • The urbit eval utility
  • -eval and -khan-eval
  • The click thin client

Together, these tools are the building blocks for performing any action on a running ship (poke, scry, or command) from Earth, and receiving back programmatically usable output.


conn.c is a driver in Vere. It is a part of the "King" (a.k.a. "Urth") process. It exposes a Unix domain socket at /path/to/pier/.urb/conn.sock for sending/receiving data from external processes.

Input to conn.c must be a newt-encoded jammed noun that fits mold [request-id command arguments], where:

  • request-id is a client-supplied atomic identifier with type @. It exists entirely for the benefit of the client, allowing responses to be matched to requests.
  • command is one of:
    • %peek
    • %peel
    • %ovum
    • %fyrd
    • %urth

These commands cover all possible cases for the following 2x2 matrix:

arvo%ovum, %fyrd%peek

For a valid command, the output from conn.c is a newt-encoded jammed noun with type [request-id output], where:

  • request-id matches the input request-id
  • output depends on the command

For an invalid command, the output from conn.c is a newt-encoded jammed noun with type [0 %bail error-code error-string]. However, conn.c makes no guarantees that:

  • It will be able to sufficiently recover from the error to guarantee this output
  • It will produce a meaningful error code and message


The argument to an %ovum command is a raw kernel move which is injected directly into the Arvo event loop. This is a very powerful - and potentially dangerous - tool. For example, if a ship somehow got into a state where Clay was no longer working properly (meaning new files could not be compiled to fix the state of the kernel), the source code for a new, working Clay could be directly injected into the ship using an %ovum.

The output of an %ovum command is:

  • [%news %done] if the move completed successfully
  • [%news %drop] if the move was dropped
  • [%bail goof] if an error occurred


%fyrd is a direct shortcut to the Khan vane. The Khan vane coordinates and manages threads, and is described in further detail below. The arguments to a %fyrd command are (in order):

  1. The name of the desk in which the thread lives (e.g. %base) or beak for the thread (e.g. [%zod %base %10])
  2. The name of the thread (e.g. %hi)
  3. Mark to which the output should be cast (e.g. %tape)
  4. Mark for how to interpret the input argument to thread (e.g. %ship)
  5. Input argument to thread (e.g. ~zod)

The output of a %fyrd command is [%avow (each page goof)], the value of each depending on whether the thread succeeded or not.


The argument to the %urth command is a subcommand for the action to perform. Currently, the only valid commands are %pack and %meld.

%urth will return %& if given a valid command as input, otherwise it will return [0 %bail 0xfffffff9 %urth-bad]. No other output is emitted.


The %peek command is used to perform a namespace read request (a.k.a. scry) using Arvo's external peek interface (arm +22 in arvo.hoon). The argument to %peek is the nom input to +peek in arvo.hoon (lyc is auto-filled as [~ ~], i.e. "request from self"). That is to say that the argument to %peek must have type:

$+ each path
$% [%once vis=view syd=desk tyl=spur]
[%beam vis=view bem=beam]

Practically speaking, this means that the input will look like one of these three examples:

[%& p=path]
[%| p=[%once vis=view syd=desk tyl=spur]]
[%| p=[%beam vis=view bem=beam]]


  • path is a [view beam], with the view passed in as a coin
  • view is the vane code for the scry, as well as an optional care, possibly appended to the vane (e.g. %j, %gx, etc.)
  • beam is a [beak spur]
  • desk is used to auto-generate a beak: [our desk now]
  • spur is the scry endpoint for the agent or vane

The output of a %peek command is [%peek (unit (unit scry-output))], where ~ means that the scry endpoint is invalid, and [~ ~] means that the scry resolved to nothing.

See here for more information on scrying.


%peel attempts to emulate a scry-like namespace, like the one used by Arvo and accessed by %peek. The argument to %peel should be a path. Valid paths result in a non-null unit containing the result of the scry. Invalid paths result in null (i.e. ~). The valid paths and the data they return are:

/help (unit (list path)) Supported %peel paths
/live (unit ~) Pier health check; succeeds if pier is running
/khan (unit ~) Khan health check; succeeds if Khan vane is running
/info (unit mass) Pier info as a mass
/v (unit @t) Returns version of the Vere binary as a cord
/who (unit @) Returns the Azimuth identity of the ship as an atom

Note that the pier info above is returned as a mass report, i.e. type (pair cord (each * (list mass))). This is not the same as the |mass memory report. /mass is meant to be a valid %peel path which returns the |mass memory report, but it is currently unimplemented.


The Khan vane is a command / response interface for running threads. Khan was introduced to make running threads a kernel-level feature, as simple as poking an agent or setting a timer. Threads allow users to run arbitrarily complex code on their ships in the same way that bash allows them to do so on Linux.

Khan's API exposes three thread requests:

  • %fard: Kernel thread requests
  • %fyrd: External thread requests
  • %lard: "Inline" thread requests

"Kernel" above doesn't mean that this interface is hidden or protected from userspace agents; thread requests by userspace agents should almost certainly use %fard. It just means that %fard thread requests are expected to originate from within the kernel or a userspace agent. Specifically:

  • %fard commands take the thread input argument as a cage
  • The data in the vase of the cage is a unit (as expected by %spider)
  • The output is also a cage (see below for more information)

%fyrd thread requests, on the other hand, perform some extra services that are useful when running threads from the dojo or via conn.c. Specifically:

  • %fyrd commands take the thread input as a raw noun
  • Khan performs mark conversion on both the input and output for %fyrd requests
  • Khan automatically lifts the converted input into a unit

"Inline" threads are a particularly specialized Khan thread request where the thread has already been compiled and is passed as a part of the input.

Khan requests expect the following input:

  • %fard: p=[=bear name=term args=cage]
  • %fyrd: p=[=bear name=term args=(pair mark page)]
  • %lard: [=bear =shed]


  • bear is a desk or a beak; if bear is a desk, then the it will be converted to a beak using our and now as default values
  • shed is a pre-computed chain of strands that produce a vase (the canonical thread)

All three produce the same output if an error occured while running the thread: [vow %| goof], where vow is %arow for %fard and %lard, and %avow for %fyrd.

If the thread succeeded, %fard and %lard produce [%arow %& %noun vase]. %fyrd produces [%avow %& mark noun], where mark is the output mark and noun is the output as a raw noun after mark conversion.

See here for more information about threads.

urbit eval

eval is a utility command in the Urbit binary. Originally, it was introduced to evaluate snippets of Hoon code using the binary to emulate Arvo from the associated ivory pill. This allowed it to run any Hoon code fragments that used kernel and STL functions (e.g. anything in hoon.hoon, arvo.hoon, lull.hoon, and zuse.hoon). Notably, this did not (and does not) evaluate any Hoon fragments that require pier state (e.g. scries, our, now, etc.).


$ echo '(add 2 2)' | ./urbit eval
loom: mapped 2048MB
lite: arvo formula 2a2274c9
lite: core 4bb376f0
lite: final state 4bb376f0
eval (run):

The result (i.e. 4) is printed to stdout. If the command had failed to compile, the stack trace would have been printed to stdout instead. All other messages are printed to stderr.


eval was extended with several options that make it useful for processing Hoon nouns as input to or output from conn.c:

  • -j, --jam: output result as a jammed noun
  • -c, --cue: read input as a jammed noun
  • -n, --newt: write output / read input as a newt-encoded jammed noun, when paired with -j or -c respectively
  • -k: treat the input as the jammed noun input of a %fyrd request to conn.c; if the result is a goof, pretty-print it to stderr instead of returning it

-eval and -khan-eval

Two threads that evaluate arbitrary Hoon were added to the suite of threads included with Arvo: ted/eval.hoon and ted/khan-eval.hoon.

Both threads take the same input: Hoon code as a cord and an optional (list path). The optional (list path) is a list of Clay file dependencies which need to be included for the Hoon to be evaluated (i.e. if the Hoon code includes libraries or types defined outside of the kernel). Each path can be a beam (i.e. [beak spur]) or just a spur, in which case the default beak (i.e. [our %base now]) will be prepended.

ted/eval.hoon expects the input to be a Hoon expression. It's very similar to urbit eval, except that it has access to ship state: now, our, vane & agent state, etc.

ted/khan-eval.hoon expects the input to be a thread. It attempts to compile the thread using the dependencies (if any) and then sends it to Khan as a %lard thread request.

Both threads return regular thread output, i.e. a vase.


  • -eval '(add 2 2)'
  • -eval '(my-add 2 2)' [/lib/my-add/hoon ~]
    • Where my-add is defined in lib/my-add.hoon in %base
  • -eval '(my-add 2 2)' [/(scot %p our)/my-desk/(scot %da now)/lib/my-add/hoon ~]
    • Where my-add is defined in lib/my-add.hoon in %my-desk
  • -khan-eval '=/ m (strand ,vase) ;< ~ bind:m (poke [~zod %hood] %helm-hi !>(\'\')) (pure:m !>(\'success\'))'


click is a bash thin client which auto-formats -eval and -khan-eval thread calls via %fyrd requests to conn.c and coordinates chaining together the appropriate commands to execute those requests on a running ship.

Using click, a call like:

echo $'[0 %fyrd %base %khan-eval %noun %ted-eval \'=/ m (strand ,vase) ;< ~ bind:m (poke [~zod %hood] %helm-hi !>(\\\'\\\')) (pure:m !>(\\\'success\\\'))\']' |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/zod/.urb/conn.sock |
/path/to/urbit eval -cn

instead looks like:

/path/to/click -k /path/to/zod $'=/ m (strand ,vase) ;< ~ bind:m (poke [~zod %hood] %helm-hi !>(\\\'\\\')) (pure:m !>(\\\'success\\\'))'

or even more conveniently:

/path/to/click -k -i threads/poke.hoon /path/to/zod
click [options] <path-to-pier> <hoon> [<dependencies> ...]
click [options] -i <path-to-file> <path-to-pier> [<dependencies> ...]
click [-o|-p] -e -i <path-to-file> <path-to-pier>
Thin client for interacting with running Urbit ship via conn.c
-e Execute jammed Hoon
-h Show usage info
-i <path-to-file> Read input from file
-j Jam only
-k Execute command using "khan-eval" thread
-o <path-to-file> Output to file
-p Filter failure stack traces from result and pretty-print them to stderr
-x Jam to hex

Using these tools

Below are examples of how to execute common commands on a running ship from Earth.


Blocked by issues; not currently doable in a way that returns the results as data.


echo "[0 %urth %pack]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
echo "[0 %ovum %d /test %pack ~]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< ~ bind:m (flog [%pack ~]) (pure:m !>(\\\'success\\\'))'


echo "[0 %urth %meld]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
echo "[0 %ovum %d /test %meld ~]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< ~ bind:m (flog [%meld ~]) (pure:m !>(\\\'success\\\'))'


|ota ~bus

echo "[0 %ovum [%g /test [%deal [~zod ~zod] %hood %raw-poke %kiln-install %base ~bus %kids]]]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< ~ bind:m (poke [our %hood] %kiln-install !>([%base ~bus %kids])) (pure:m !>(\\\'success\\\'))'

|ota %disable

echo "[0 %ovum [%g /test [%deal [~zod ~zod] %hood %raw-poke %kiln-install %base ~zod %base]]]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< ~ bind:m (poke [our %hood] %kiln-install !>([%base our %base])) (pure:m !>(\\\'success\\\'))'

|ota ~bus %desk

echo "[0 %ovum [%g /test [%deal [~zod ~zod] %hood %raw-poke %kiln-install %base ~zod %desk]]]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< ~ bind:m (poke [our %hood] %kiln-install !>([%base ~bus %desk])) (pure:m !>(\\\'success\\\'))'


|install ~sampel-palnet %desk

echo "[0 %ovum [%g /test [%deal [~zod ~zod] %hood %raw-poke %kiln-install %desk ~sampel-palnet %desk]]]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< ~ bind:m (poke [our %hood] %kiln-install !>([%desk ~sampel-palnet %desk])) (pure:m !>(\\\'success\\\'))'

|install ~sampel-palnet %desk, =local %my-desk

echo "[0 %ovum [%g /test [%deal [~zod ~zod] %hood %raw-poke %kiln-install %my-desk ~sampel-palnet %desk]]]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< ~ bind:m (poke [our %hood] %kiln-install !>([%my-desk ~sampel-palnet %desk])) (pure:m !>(\\\'success\\\'))'


/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< code=@p bind:m (scry @p /j/code/(scot %p our)) (pure:m !>((crip (slag 1 (scow %p code)))))'


+vats %base %kids

/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< now=@da bind:m get-time (pure:m !>((crip ~(ram re [%rose [~ ~ ~] (report-vats our now [%base %kids ~] %$ |)]))))' \

+vats, =filt %exists

/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< now=@da bind:m get-time (pure:m !>((crip ~(ram re [%rose [~ ~ ~] (report-vats our now ~ %exists |)]))))' \

+vats %base, =verb &, =filt %running

/path/to/click -kp /path/to/pier/zod \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< now=@da bind:m get-time (pure:m !>((crip ~(ram re [%rose [~ ~ ~] (report-vats our now [%base ~] %exists &)]))))' \

Additional Notes

Alternative click calls

Any example above that uses click has two additional options that have been omitted for brevity, since the actual code for the call would be identical in each example:

  1. Custom -thread in %desk:
echo "[0 %fyrd %desk %thread %noun %noun ~]" |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -cn
  1. Pass inline thread to click from file:
/path/to/bin/click -k -i path/to/thread.hoon /path/to/pier/zod

Undocked ships

click assumes that the ship at the given pier is docked (i.e. that /path/to/pier/.run exists). If for whatever reason the running ship is undocked, it's still possible to work around this assumption using the click-format helper script. For example, the call for +vats becomes:

/path/to/click-format -k \
$'=/ m (strand ,vase) ;< our=@p bind:m get-our ;< now=@da bind:m get-time (pure:m !>((crip ~(ram re [%rose [~ ~ ~] (report-vats our now)]))))' \
'/sur/hood/hoon' |
/path/to/urbit eval -jn |
nc -U -W 1 /path/to/pier/zod/.urb/conn.sock |
/path/to/urbit eval -ckn

Issues and Future Work

Currently, there are a number of minor issues and one major issue impacting interactions between Earth and Mars.

The minor issues are:

  • conn.c's simulated namespace for %peel
    • Unprincipled namespace simulation for no reason other than consistency with Arvo scry
  • /mass path for conn.c %peel not implemented
  • No mass mark in Arvo, so attempting to scry for |mass with %peek crashes the ship

The major issue is the lack of "thick" clients which are able to consume the newt-encoded jammed nouns emitted by conn.c as input. Though not officially codified yet, it makes sense for newt-encoded jammed nouns to be the narrow waist of Urbit, and recent design decisions appear to be heading in this direction. Unfortunately, the narrow waist of bash is text, and it's not always easy or useful to convert nouns to text (particularly stack traces).

There exist already two external noun libraries, in Rust and Haskell (link to Haskell lib coming soon). Adding more, while not trivial, is not difficult. The proliferation of noun representation libraries in other languages would open many doors with regards to the support, hosting, and application opportunities available (the ever-fabled "Quake over Urbit").