---
title: MARKLESS_COMPONENT_PROP_EXPRESSION_UNSUPPORTED
description: "A prop expression that reads state needs a route the child can follow: a Markless compiler error."
sidebar: { hidden: true }
search: { keywords: ["COMPONENT_PROP_EXPRESSION_UNSUPPORTED","component prop expression unsupported","markless error"] }
pagination: false
---

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

## What happened

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

```text
MARKLESS_COMPONENT_PROP_EXPRESSION_UNSUPPORTED: Prop `<propName>` on `<<childComponentName>>` is written as `<source>`, which reads state but is not an expression this compiler can route.
A child reads a prop through the graph node the parent named. An expression with no node behind it can only be seeded once, from whatever the value happened to be at render - so the child would show a stale value forever and nothing would say so.
Pass a value the compiler can follow: a plain read (`group.value`), or an expression built only from reads, operators and method calls on those reads (`group.value.includes(item.value)`). If the value needs a function of your own, put that function inside a `computed()` and pass the computed.
https://markless.dev/errors/MARKLESS_COMPONENT_PROP_EXPRESSION_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).