BoundaryML/baml · error · anyhow::Error

Template string '{}' expects {} arguments, but {} were provi

Error message

Template string '{}' expects {} arguments, but {} were provided

What it means

When rendering a BAML template_string at runtime, `render_template` compares the number of supplied arguments with the template's declared inputs. Any mismatch (too few or too many) bails with this error naming the template, the expected count, and the actual count.

Source

Thrown at engine/baml-lib/baml-core/src/ir/ir_helpers/mod.rs:2041

            item_type(&ir, &TypeIR::recursive_type_alias("JsonValue")).expect("should be Some");
        let mut expected = TypeIR::recursive_type_alias("JsonValue");
        expected.meta_mut().streaming_behavior.needed = true;
        assert_eq!(ret, expected, "{ret} != {expected}");
    }
}

/// Implementation of TemplateStringRenderer for IntermediateRepr.
/// This allows template_string calls to be resolved during test argument evaluation.
impl TemplateStringRenderer for IntermediateRepr {
    fn render_template(&self, name: &str, args: &[serde_json::Value]) -> Result<String> {
        // Find the template string definition
        let template = self.find_template_string(name)?;
        let template_content = template.template();
        let template_params = template.inputs();

        // Validate argument count
        if args.len() != template_params.len() {
            anyhow::bail!(
                "Template string '{}' expects {} arguments, but {} were provided",
                name,
                template_params.len(),
                args.len()
            );
        }

        // Build the arguments map for minijinja
        let mut args_map = serde_json::Map::new();
        for (param, arg) in template_params.iter().zip(args.iter()) {
            args_map.insert(param.name.clone(), arg.clone());
        }

        // Collect all template_strings as Jinja macro definitions, then chain
        // the target template at the end. This matches how the prompt renderer
        // in jinja-runtime injects macros.
        let full_template = self
            .walk_template_strings()

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Pass exactly as many arguments as the template_string declares, in declaration order
2. Update stale call sites after changing the template_string signature in .baml
3. Compare the counts in the error message (expects N, provided M) to find what to add or remove
4. If parameters became optional, still pass all declared inputs (pass null) since this renderer requires exact arity

Example fix

// before (.baml)
template_string Greet(name: string) #"Hello {{name}}"#
// render_template("Greet", [])  // 0 of 1 args

// after (caller)
render_template("Greet", [json!("alice")])

function assertTemplateArity(templateInputs: number, args: unknown[]): void {
  if (args.length !== templateInputs) {
    throw new Error(`Template expects ${templateInputs} arguments, got ${args.length}`);
  }
}

try {
  const out = render_template(name, args);
} catch (e) {
  if (String(e).includes('expects') && String(e).includes('arguments')) {
    console.error('Align call-site args with the template_string inputs in .baml');
  }
  throw e;
}

When it happens

Trigger: Calling TemplateStringRenderer::render_template(name, args) (used for template_string calls during test argument evaluation) with args.len() != number of inputs declared on the `template_string Name(a, b, ...)` in .baml.

Common situations: Adding a parameter to a template_string without updating call sites; passing a single object of args where positional values are expected; generated test code referencing an older template signature.

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

• Missing required parameter: {param_name}
• Failed to parse template '{}': {}
• --json-args must be a JSON object, got: {json}
• {0}
• interned member `{name}` cannot be another member's child

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/1d23a0839c2f1485. Report an issue: GitHub.