Werk #19725: MQTT: Fix missing services and UNKNOWN state for zero values

Component	Checks & agents
Title	MQTT: Fix missing services and UNKNOWN state for zero values
Date	Aug 28, 2026
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	3.0.0b1 Not yet released Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Cloud, Checkmk Ultimate MT 2.5.0p13 Not yet released Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Cloud, Checkmk Ultimate MT 2.4.0p37 Not yet released Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Cloud, Checkmk Ultimate MT

Previously, the service MQTT Clients went UNKNOWN with "Item not found in monitoring data". This happened if the broker reported 0 for clients/active, clients/connected and clients/total. An idle broker without connected clients reports exactly these values.

The same defect also hid the service during service discovery. If the broker had no connected clients, Checkmk did not discover an MQTT Clients service at all.

The services MQTT Broker and MQTT Messages had the same defect. They went UNKNOWN if the broker reported 0 for every value that they monitor.

Now 0 is a regular value. The services report OK and show the zero counts, and Checkmk discovers them.

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