Beyond the Toolbox

Most people use the Toolbox for TLA+. But you can also use vscode or the command line.

The Command Line

The main CLI file is tla2tools.jar. You can download it directly from this link. You can also find it in the base directory of your toolbox installation.

Tlatools has four subtools: TLC, the pluscal translator, a latex pdf generator (Tla2Tex), and a parser (SANY). I’ll leave documenting the latter two tools to Lamport and focus on the translator and model checker.

The Pluscal Translator

To translate a PlusCal algorithm in a file, write

$ java -cp tla2tools.jar pcal.trans file.tla

This will:

  1. Translate the text in file to TLA+

  2. Write file.old as a backup

  3. Write file.cfg as a configuration file, overwriting it if it already exists.

To prevent (3), add -nocfg as a flag before file.tla. There is no way to prevent the translator from writing file.old. I have a shell watcher that finds and deletes them.

You can read about all of the other translator options here, page 67-69, or by running java -cp tla2tools.jar pcal.trans -h.


To run TLC on a model, write:

$ java -jar tla2tools.jar -config configfile.cfg specfile.tla

If -config isn’t provided, TLC will by default look for specfile.cfg. This is the file that holds all of the configuration options for the model run.

TLC has other flags, but if you can’t write a config format that’s all moot, so let’s talk about that.

Config Format

The model checking config language is a special DSL for using TLC from the command line. It’s what the toolbox abstracts away on the backend.

All config files need a SPECIFICATION {spec} line, where Spec is whatever action encompasses your initial and next states. By convention, this should be called Spec, but this isn’t required— useful if you want different configs to test different variations of your system.

Invariants you want to check must be prefixed with INVARIANT, temporal properties with PROPERTY. Both can have commas, eg INVARIANT TypeInvariant, IsSafe is a valid line. Unlike in the toolbox, you cannot make expressions invariants— they must be named operators.


Why does that work in the toolbox? When you make an expression an invariant, the Toolbox makes a separate MC.tla file, adds that expression as an operator, and then model checks MC.tla with the new operator as an invariant. It does a similar thing to get around restrictions on constant expressions, below.

Constants are written as CONSTANT name = value. Values can be simple values or sets of simple values, but not functions or expressions. To set a model value instead of an ordinary assignment, write CONSTANT name = name instead. To make a set of model values, write name = {a, b, c}, where a, b, c are identifiers (not strings).


Since the config files can’t use imports and negative numbers are technically an Integers import, you can’t set a negative number as a constant.

A basic config file might look like this:



  Const1 = {"a", "b", "c"}
  Const2 = Const2
  Const3 = {c1, c2, c3}

Config files may also have CONSTRAINT, ACTION-CONSTRAINT, VIEW, which work equivalently to their Toolbox options. You can also disable deadlock checking in the cfg with CHECK_DEADLOCK FALSE.

Finally, we have ALIAS. This lets us effectively simulate the Error Trace Explorer on the command line. Let’s say we have the following spec:

---- MODULE aliases ----
EXTENDS Integers

Init ==
  x = 0

Next == x' = x + 1
Inv == x < 10
Spec == Init /\ [][Next]_x

Alias ==
  [x |-> x,
   nextx |-> x',
   incx |-> x + 1]

If we add ALIAS Alias to our config file, then the error trace will show the values of x, nextx, and incx in the error output.


The alias replaces the standard error output. If you don’t include some variables in the alias, then they won’t show on the error output either.

TLC Options

Now that we know how to run a config file, let’s get back to the TLC options. You can read all of them with java -jar tla2tools.jar -help (not -h), or by reading them here (pages 9-11). Most of them are self-explanatory or equivalent to toolbox options. See the Toolbox topic for more information on how to use them. The main things of note are:


Will continue model checking even after a violation is found. Every single invariant violation will be dumped as output.


Don’t pass this in as a flag in the toolbox, or it will think it’s an error:

An error has occurred. See error log for more details.
assertion failed: Two traces are provided. Unexpected. This is a bug
-dump file

Writes all of the states that TLC reached to file in no particular order. If you want to know how the states connect to each other, instead write

-dump dot file

This outputs a graphviz graph file instead. Nodes are states, labelled with their variable assignments. TLC will not append the file extension to the filename; you’ll have to add that yourself.


If your spec includes a liveness property, TLC will also write file_liveness. This is an internal representation and can be ignored.

You can also write -dump dot,colorize file to color the edges based on the actions they involve and -dump dot,actionlabels to label the edges with the corresponding action. Both can be used together.

-metadir dir

Instead of storing the seen statespace in the same directory as the spec, TLC will instead store it in dir. I find this useful when scripting against the CLI, as I can store the state space in a temporary directory for easier cleanup.

-workers num/auto

Specifies the number of worker threads to use for model checking. This is very important. Without this, the CLI defaults to a single worker. Pass in auto to use as many workers as you have cores.


Newer versions of TLA+ save an error file whenever it finds a property error. This flag disables writing the file.

-fpmem num

What percentage of the system memory to earmark for model checking, expressed as a decimal. Defaults to 0.25 (1/4 the memory).