---
title: MARKLESS_PROTOCOL_VERSION_MISMATCH
description: "Unsupported resumability protocol version: a Markless runtime error."
sidebar: { hidden: true }
search: { keywords: ["PROTOCOL_VERSION_MISMATCH","protocol version mismatch","markless error"] }
pagination: false
---

**Error** from the runtime, while a page renders or runs.

## What happened

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

```text
MARKLESS_PROTOCOL_VERSION_MISMATCH: Unsupported <type> protocol version <actualVersion>.
The <type> payload must use a supported protocol version.
Serve a <type> payload generated by this runtime.
https://markless.dev/errors/MARKLESS_PROTOCOL_VERSION_MISMATCH
```

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/serializer` raises this code. Source:

- [packages/serializer/src/protocol-client.ts](https://github.com/compiled-run/markless/blob/main/packages/serializer/src/protocol-client.ts)
- [packages/serializer/src/protocol-validation.ts](https://github.com/compiled-run/markless/blob/main/packages/serializer/src/protocol-validation.ts)

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