Output

debug is the only way to produce observable output in Vertex. It is an expression: it evaluates its argument, prints it, and returns the value. It can appear anywhere an expression is valid.

debug "hello world"
debug 42
debug factorial(10)

Because debug returns its argument, you can drop it into the middle of an expression to inspect an intermediate value without rearranging the code:

let total = debug compute_base() + compute_tax()   // prints base, then adds tax

Stringification

debug converts its argument to a string before printing, following these rules:

Type	Representation
`Int`, `Long`	Decimal digits, no leading zeros; negatives with `-`
`Double`	Shortest decimal form that round-trips
`Decimal`	Exact decimal form, scale preserved
`Bool`	`true` or `false`
`String`	The raw characters, no surrounding quotes
`Option<T>`	`Some(value)` or `None`
`Result<T, E>`	`Ok(value)` or `Error(value)`
`List<T>`	`[elem1, elem2, ...]`
`Map<K, V>`	`{k: v, ...}` (iteration order)
Sum types	`Variant(field1, field2)` or just `Variant` for unit

Output streams

In JIT mode, debug writes to stdout. Every other diagnostic the compiler or JIT produces (warnings, DML notices, test output headers) goes to stderr. A program that prints via debug and a runner that reads stdout see each other cleanly.

In Salesforce mode, debug translates to Apex System.debug(...). Output appears in the Apex debug log, not in your terminal. vertex run --sf filters the log and prints only the lines that correspond to Vertex’s debug calls.

`W001`: the debug-in-source warning

The checker emits a W001 warning wherever debug appears in your code:

warning[W001]: `debug` in source
  at billing.vtx:18:5
    |
 18 |     debug "in compute_total"
    |     ^^^^^ debug output is a dev tool; remove before shipping

debug is a development aid, not a logging API. W001 is the reminder to remove it before committing. The warning is suppressed for scripts executed via vertex run (where debug is the point) but not in vertex build.

Why not `print` or `println`?

There is one way to produce output. No print, no println, no System.out.*. debug is the one way, and the W001 warning means you see a reminder every time you leave one in.