---
title: MARKLESS_CALLBACK_PROP_ARITY_UNSUPPORTED
description: "Callback props accept at most one parameter: a Markless compiler error."
sidebar: { hidden: true }
search: { keywords: ["CALLBACK_PROP_ARITY_UNSUPPORTED","callback prop arity unsupported","markless error"] }
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---

**Error** from the compiler, while it builds your `.tsrx` files.

## What happened

Markless prints the message, the reason, and a suggestion:

```text
MARKLESS_CALLBACK_PROP_ARITY_UNSUPPORTED: Callback prop `<propName>` is unsupported because <value>. Lazy callback symbols accept zero parameters or one simple identifier, object pattern, or array pattern without top-level defaults or rest.
The callback transport carries one callback value across the lazy-symbol boundary. Multiple parameters, defaults, and rest bindings do not have supported parameter semantics when the symbol runs.
Pass a single object instead — change `(kind, payload) => ...` to `({ kind, payload }) => ...` and invoke the callback with one object value; remove parameter defaults and rest bindings.
https://markless.dev/errors/MARKLESS_CALLBACK_PROP_ARITY_UNSUPPORTED
```

Words in angle brackets change from case to case.

## How to fix it

Start with the suggestion in the message above.

This page has no guide yet. [Help write it](/contributing/improve-these-docs).

## Where it comes from

`@markless/compiler` raises this code. Source:

- [packages/compiler/src/passes/semantic-graph/diagnostics.ts](https://github.com/compiled-run/markless/blob/main/packages/compiler/src/passes/semantic-graph/diagnostics.ts)

Read [how to read a Markless error](/tooling/diagnostics), or go back to [all error codes](/errors).