BoundaryML/baml · error

env.get expects exactly one argument

Error message

env.get expects exactly one argument

What it means

The built-in `env.get(name)` function in the BAML interpreter requires exactly one argument. Calling it with zero or multiple arguments bails with `env.get expects exactly one argument` before any evaluation of the arguments occurs.

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:1768

                } else {
                    let cell = lookup_cell(scopes, name)
                        .with_context(|| format!("unbound variable `{}` at {:?}", name, meta.0))?;
                    EvalValue::Reference(cell)
                }
            }
            Expr::Function(arity, body, meta) => {
                EvalValue::Function(*arity, body.clone(), meta.clone())
            }
            Expr::Call {
                func,
                type_args,
                args,
                meta: _,
            } => {
                if let Expr::Var(func_name, _) = func.as_ref() {
                    if func_name == "env.get" {
                        if args.len() != 1 {
                            bail!("env.get expects exactly one argument");
                        }

                        let key_val = expect_value(
                            evaluate_expr(
                                &args[0],
                                scopes,
                                thir,
                                run_llm_function,
                                watch_handler,
                                function_name,
                            )
                            .await?,
                        )?;

                        let key = match key_val {
                            BamlValueWithMeta::String(value, _) => value,
                            _ => bail!("env.get argument must be a string"),
                        };

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Call `env.get` with exactly one string argument
2. Split multiple lookups into separate `env.get` calls, one per variable
3. Check generated/templated BAML for empty or duplicated arguments
4. Ensure the argument is not accidentally spread from a list

Example fix

// before
let v = env.get("A", "B");
// after
let a = env.get("A");
let b = env.get("B");

// BAML: pass exactly one string argument
let v = env.get("MY_VAR");

// host-side
match result {
    Err(e) if e.to_string().contains("env.get expects exactly one argument") => {
        // fix call arity in the BAML source
    }
    other => other?,
}

When it happens

Trigger: `env.get()` with no arguments, or `env.get("A", "B")` with two or more, inside interpreted BAML code reached via `evaluate_expr_with_context`.

Common situations: Refactoring code that reads several env vars and merging the calls incorrectly; templating that splices in an empty argument list; copy-paste from languages where extra args are ignored.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• env.get argument must be a string
• baml.media.image.from_url expects 1 argument, got {} at {:?}
• baml.fetch_as expects 1 argument (url), got {} at {:?}
• environment context missing
• environment context corrupted

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/6ae8a4471b38b197. Report an issue: GitHub.