MQTT Explorer

What you actually use it for

• Confirming that a publisher is actually publishing, and to which topics, before you debug the consumer.
• Reading the JSON payload a publisher emits, including whether DataSetMetaData and StatusCodes arrive as you expect.
• Checking a topic hierarchy generated from OPC UA browse paths, including how reserved characters were escaped.
• Diagnosing a Unified Namespace layout by browsing what is really on the broker rather than what the design document says.
• Watching retained messages and last-will behaviour when the publisher's connection state matters.
• Verifying that a subscriber's topic filter matches the topics that actually exist.

Walkthrough

1. Connect to the broker with credentials and TLS as configuredEnter host, port, credentials and TLS settings. If the broker rejects you, resolve that before assuming a publishing problem: a client that cannot connect and a publisher that is not publishing look identical from the consumer's side.
2. Subscribe broadly, then narrowStart with a wildcard subscription to see the whole topic tree, then narrow to the branch you care about. Broad subscriptions on a busy production broker are expensive, so narrow quickly.
3. Read a payload and check its encodingOpen a message and inspect the JSON. If the publisher uses non-reversible or compact JSON, type information is lost and cannot be round-tripped. Also note that a Good StatusCode is encoded as zero and omitted, so an absent status field means Good, not missing.
4. Map topics back to the OPC UA address spaceWhere the publisher derives topics from browse paths, the topic hierarchy mirrors the address space from the root folder down, with MQTT reserved characters escaped. Reading the topic tree therefore tells you how the model was traversed.
5. Check cadence, not just presenceWatch the message rate against the publishing interval you configured. A topic that exists but updates once a minute when you expected once a second is a subscription tuning problem upstream in OPC UA, not an MQTT problem.
6. Do not publish into a production broker while exploringThe tool can publish as well as subscribe. On a live Unified Namespace, an accidental publish to a machine topic can be consumed as real data. Connect with a subscribe-only credential where the broker supports it.

What it will not do

• It is an MQTT tool with no OPC UA knowledge: it will show you a UADP binary payload as bytes and will not decode it.
• No OPC UA semantics at all, so it cannot tell you what a value means, only what was published.
• Desktop-oriented, so it is awkward for automated checks or for a headless gateway.
• Broad wildcard subscriptions can be heavy on a busy production broker.
• It can publish, which makes it a hazard on a live broker if used carelessly.

Alternatives

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.