BoundaryML/baml · error · anyhow::Error

Missing required parameter: {param_name}

Error message

Missing required parameter: {param_name}

What it means

BAML's `check_function_params` validates that every non-optional parameter of a BAML function is present in the supplied argument map. When a parameter with no default and not declared optional (`T?`) is missing, the error 'Missing required parameter: <name>' is collected into the scope and returned as an anyhow error.

Source

Thrown at engine/baml-lib/baml-core/src/ir/ir_helpers/mod.rs:976

        for (param_name, param_type) in function_params {
            scope.push(param_name.to_string());
            if let Some(param_value) = params.get(param_name.as_str()) {
                if let Ok(baml_arg) =
                    coerce_settings.coerce_arg(self, param_type, param_value, &mut scope)
                {
                    baml_arg_map.insert(param_name.to_string(), baml_arg);
                }
            } else {
                // Check if the parameter is optional.
                if !param_type.is_optional() {
                    scope.push_error(format!("Missing required parameter: {param_name}"));
                }
            }
            scope.pop(false);
        }

        if scope.has_errors() {
            Err(anyhow::anyhow!(scope))
        } else {
            Ok(baml_arg_map)
        }
    }

    fn get_dummy_args(
        &self,
        indent: usize,
        allow_multiline: bool,
        params: &BamlMap<String, TypeIR>,
    ) -> String {
        params
            .iter()
            .map(|(param_name, param_type)| get_dummy_field(self, indent, param_name, param_type))
            .collect::<Vec<_>>()
            .join("\n")
    }
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Pass a value for the parameter named in the error when invoking the function
2. Declare the parameter optional in .baml (`param: string?` or with a default) if it is legitimately omittable
3. Regenerate the client/SDK after .baml signature changes so new required params are visible
4. Audit call sites for conditionally-built argument objects and ensure all required keys are always set

Example fix

// before (.baml)
function Greet(user_name: string) { ... }
// baml_client.call("Greet", {})  // missing user_name

// after (caller)
baml_client.call("Greet", { "user_name": "alice" })

function assertRequiredParams(params: Record<string, unknown>, required: string[]): void {
  for (const name of required) {
    if (!(name in params) || params[name] === undefined) {
      throw new Error(`Missing required parameter: ${name}`);
    }
  }
}
// call before baml_client.call(fnName, params)

try {
  const res = await baml_client.call('FnName', params);
} catch (e) {
  if (String(e).includes('Missing required parameter:')) {
    const missing = String(e).match(/Missing required parameter: (\w+)/)?.[1];
    console.error(`Provide a value (or make optional in .baml) for: ${missing}`);
  }
  throw e;
}

When it happens

Trigger: Calling a BAML function (via test runner, client, or runtime arg evaluation) with a params map that omits a parameter declared as required (non-optional) in the .baml function signature.

Common situations: Forgetting to pass a newly-added function parameter after editing the .baml file; client SDK generated before the parameter was added; conditionally omitting an argument in code thinking it was optional; null vs absent confusion for optional params.

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

• Template string '{}' expects {} arguments, but {} were provi
• missing parameter: {name}
• dependency name `{name}` is reserved
• --json-args must be a JSON object, got: {json}
• {0}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/11e47f57781a7ffe. Report an issue: GitHub.