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Fabro supports $variable placeholders that let you parameterize workflows without editing the DOT file.

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Run config variables

Define variables in the [vars] section of a run config TOML file:
run.toml
version = 1
goal = "Run tests for $repo_name"
graph = "check.fabro"

[vars]
repo_name = "fabro"
repo_url = "https://github.com/fabro-sh/fabro"
language = "rust"
These variables are expanded into the DOT source before the graph is parsed. You can use $variable anywhere in the DOT file — goals, prompts, labels, scripts, or any other attribute:
check.fabro
digraph Check {
    graph [goal="Run tests for $repo_name"]

    start [shape=Mdiamond, label="Start"]
    exit  [shape=Msquare, label="Exit"]

    clone [label="Clone", shape=parallelogram, script="git clone $repo_url repo"]
    test  [label="Test", prompt="Run the $language test suite in the repo/ directory."]

    start -> clone -> test -> exit
}
When launched with fabro run run.toml, Fabro replaces $repo_name, $repo_url, and $language with their values before parsing the graph.

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Undefined variables

If a $variable in the DOT file has no matching entry in [vars], Fabro raises an error. This catches typos early — a misspelled $langauge fails immediately rather than passing a literal $langauge to the LLM.

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Escaping $

To include a literal $ in the output, write $$: 
test [prompt="The env var is $$HOME"]
This produces The env var is $HOME without treating $HOME as a variable reference. A bare $ not followed by an identifier character (e.g. costs $5) does not need escaping.

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The $goal variable

Inside agent and prompt node prompts, Fabro automatically expands $goal to the workflow’s goal attribute. This happens at runtime, after graph parsing:
example.fabro
digraph Example {
    graph [goal="Implement the login feature"]

    plan [label="Plan", prompt="Create a plan for: $goal"]
}
The plan node’s prompt becomes "Create a plan for: Implement the login feature".

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Variable merging

When using server-level run defaults alongside a run config TOML, variables are merged. Task config vars override default vars when keys collide:
SourcePriority
Run config TOML [vars]Highest — wins on collision
Server defaults [vars]Lowest — provides fallback values