karatelabs/karate · error · RuntimeException

readAsStream() needs at least one argument

Error message

readAsStream() needs at least one argument

What it means

karate.readAsStream(path) reads a file relative to the current resource context and returns it as an InputStream for streaming large files. The method requires exactly one path argument; Karate throws this error immediately when it is called with zero arguments, before attempting any file resolution.

Solutions

  1. Pass the file path as the first argument: karate.readAsStream('data/large.csv')
  2. If wrapping in a function, forward the parameter: function read(p) { return karate.readAsStream(p); }
  3. Use karate.read(path) instead if you want the whole file content in memory rather than a stream
  4. Check the calling code for an undefined variable silently swallowing the argument

Example fix

// before
var stream = karate.readAsStream(); // fails: no argument

// after
var stream = karate.readAsStream('classpath:data/large.csv');
Defensive patterns

Strategy: type-guard

Validate before calling

// JS in scenario
if (path == null || path === undefined || path === '') {
  throw new Error('readAsStream requires a non-empty path argument');
}
var stream = karate.readAsStream(path);

Type guard

function hasPathArg(args) {
  return Array.isArray(args) && args.length > 0 && args[0] != null && ('' + args[0]) !== '';
}

Try / catch

try {
  var stream = karate.readAsStream(path);
} catch (e) {
  if (('' + e).indexOf('needs at least one argument') !== -1) {
    karate.log('readAsStream called without a path');
  }
  throw e;
}

Prevention

When it happens

Trigger: Invoking karate.readAsStream() with no arguments — e.g. a typo'd variable means the intended path argument is missing (karate.readAsStream()), or the call was forwarded from a wrapper function that dropped its arguments.

Common situations: Refactoring a helper like function read(p) { return karate.readAsStream(); } and forgetting to thread the parameter through; calling readAsStream conditionally with an undefined JS variable which some setups coerce to a zero-arg call; confusing readAsStream() with karate.read() used without parens elsewhere.

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

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/ec30ee970eb25175. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/core/KarateJs.java:1080

     * lookup as well as {@code .status}, {@code .body}, {@code .headers} and friends.
     * Returns null before any request has been made.
     */
    private Object getResponse() {
        if (mockHandler != null) {
            // In mock context, 'response' is a variable in the engine being constructed
            return engine.get("response");
        }
        return prevResponse;
    }

    /**
     * karate.readAsStream(path) - Read file as InputStream.
     * Useful for streaming large files without loading into memory.
     */
    private JavaInvokable readAsStream() {
        return args -> {
            if (args.length == 0) {
                throw new RuntimeException("readAsStream() needs at least one argument");
            }
            String path = args[0] + "";
            Resource resource = getCurrentResource().resolve(path);
            try {
                return resource.getStream();
            } catch (Exception e) {
                throw new RuntimeException("Failed to open stream for: " + path, e);
            }
        };
    }

    /**
     * karate.render(template) - Render HTML template (similar to doc).
     * Returns the rendered HTML string without sending to doc consumer.
     */
    @SuppressWarnings("unchecked")
    private JavaInvokable render() {
        return args -> {

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