Werk #20022: REST API: reject an empty match_on list when creating or editing a rule

Component	REST API
Title	REST API: reject an empty match_on list when creating or editing a rule
Date	Jul 21, 2026
Level	Trivial Change
Class	Bug Fix
Compatibility	Incompatible - Manual interaction might be required
Checkmk versions & editions	3.0.0b1 Not yet released Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Cloud, Checkmk Ultimate MT 2.5.0p10 Not yet released Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Cloud, Checkmk Ultimate MT 2.4.0p35 Not yet released Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Cloud, Checkmk Ultimate MT

The rule editor has always refused to save a rule with an empty "Explicit hosts" or "Service" condition ("Please add at least one host."/"Please add at least one service pattern."). The REST API's rule create/update endpoints did not enforce the same rule: a client could submit {"match_on": [], "operator": ...} for a host_name or service_description condition without any error.

An empty match_on list is a valid, deliberate condition meaning "matches no host/service", but a rule in this state is effectively dead - it can never apply - which is why the rule editor blocks it.

The REST API's rule create/update endpoints (POST/PUT .../objects/rule/...) now reject an empty match_on list with a 400 error, matching the rule editor's own validation. If your automation has been sending an empty match_on list - intentionally or not - it will now receive this error instead of a silent success.

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