BoundaryML/baml · error

watch codegen error: undefined variable: {variable}

Error message

watch codegen error: undefined variable: {variable}

What it means

A panic during codegen of a Watch statement: the watched variable must be present in the compiler's locals map to get a slot index. The panic fires when the variable named in the watch statement has no corresponding local.

Source

Thrown at engine/baml-compiler/src/codegen.rs:967

                    self.emit(Instruction::Jump(loop_start - self.next_insn_index()));

                    for loc in break_locs {
                        self.patch_jump(loc);
                    }
                }
            },
            thir::Statement::Assert { condition, .. } => {
                self.compile_expression(condition);
                self.emit(Instruction::Assert);
            }
            thir::Statement::WatchOptions {
                variable,
                channel,
                when,
                ..
            } => {
                let Some(local_index) = self.locals.get(variable).copied() else {
                    panic!("watch codegen error: undefined variable: {variable}");
                };

                self.emit_string_literal(channel.as_ref().unwrap_or(variable).as_str()); // This adds LoadConst

                match when.as_ref() {
                    Some(WatchWhen::Manual) => {
                        self.emit_string_literal("manual");
                    }

                    Some(WatchWhen::Never) => {
                        self.emit_string_literal("never");
                    }

                    Some(WatchWhen::Auto) => {
                        // No action needed.
                    }

                    Some(WatchWhen::FunctionName(fn_name)) => {

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Ensure the watched variable is declared before the watch statement in the same scope
2. Fix the variable name spelling in the watch statement
3. Remove the stale watch statement if the variable no longer exists

Example fix

// before
watch counter via "ui"   // 'counter' not declared in scope
// after
let counter = 0
watch counter via "ui"

fn validate_watch_var(locals: &[&str], var: &str) -> Result<(), String> {
    if !locals.contains(&var) {
        return Err(format!("watched variable '{}' is not declared in scope", var));
    }
    Ok(())
}

When it happens

Trigger: Watching a variable that was never declared, is out of scope at the watch point, or whose name is misspelled in the watch statement.

Common situations: Typo'd variable names in watch directives; watching a variable declared in a different block/scope; declarations removed by refactoring while watch statements remained.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• undefined function: {name}
• undefined field: {class_name}.{field}
• array access should be either map or array.
• watch options codegen: undefined function: {fn_name}
• sys_op callee must resolve to a statically-known global func

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/ed111f7321288829. Report an issue: GitHub.