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.

Context: GitHub issue CircleCI-Public/circleci-cli#904 (closed): circleci config validate fails with failed to decode body: invalid character I. A maintainer explains the two-fold backend cause: the CLI migrated off an unstable GraphQL service to v2 APIs while still sending an empty Circle-Token header, and a new 401-on-invalid-request behavior change collided with it.

## Matched source
Source: Published skill
Original query: "CircleCI CLI: config validate fails with failed to decode body"
Key terms: body, circleci, config, decode, failed, fails, validate