# Compatibility

What counts as a breaking change, how versions protect you and the change classes the changelog uses.

Within a version, Kelly makes only changes that don't break a client built to these docs; anything else ships as a new dated version.

## What doesn't break you

- New operations, topics, fields, enum values, error codes and headers. Ignore what you don't know: every enum is open, and Wire clients ignore unknown `op` and `type` values and unknown members.
- Rewording `detail`, descriptions and other prose.

## What would

Removing or renaming a field, operation or code, changing a field's type or meaning, making an optional input required, or narrowing the values an input accepts. These ship only in a new version, sent as `Api-Version`. The versions served today are `2026-10-05`.

## Change classes

Every [changelog](/changelog) line carries one class: **Additive**, **Contract** (can break a client), **Agent behaviour** (changes what an AI client's model reads), **Operational** or **Commercial** (prices, limits, access).

## Notice

No version has been retired yet, so no notice period has been used. How a version is retired is on the [deprecation](/policies/deprecation) page.

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Last updated October 16, 2026

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