## TL;DR

Run circleci diagnostic to confirm the CLI version and token state. Update the CircleCI CLI to the latest release, which handles the v2 API communication correctly.

## Steps

1. Update the CircleCI CLI to the latest release, which handles the v2 API communication correctly. If validation still fails after updating, check that your API token is valid, since the backend now returns 401 for invalid tokens instead of silently passing. Run circleci diagnostic to confirm the CLI version and token state.

## When to use

Use this skill when you run into "CircleCI CLI: config validate fails with failed to decode body".

## When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

## Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.