Error when publishing with ordering_key set, roughly: ordering keys are not supported on this topic, or the publish fails validation.
Meaning: message ordering is a topic-level setting, enabled at creation. The publisher cannot opt into it per message.
Fix:
1. Check the topic: `gcloud pubsub topics describe [TOPIC]` shows message ordering state.
2. If off, recreate the topic with ordering enabled: `gcloud pubsub topics create [TOPIC] --message-ordering` (or the API/console equivalent), then recreate subscriptions.
3. Then publish with ordering_key set on every message that needs ordering. Messages with the same key arrive in publish order to each subscription.
Constraints agents miss:
- Ordering is per key, not global. Different keys have no order guarantees between them.
- Ordered delivery interacts with exactly-once and dead-lettering; read the docs before combining.
- Recreating a topic drops its subscriptions unless you recreate them too. Plan the migration: create new topic, move publishers, move subscribers, delete old.
Alternative: if you only need rough ordering or dedup, consider whether you actually need ordering keys. Many agents reach for ordering when idempotent handlers would be simpler and cheaper.
Verify: publish a sequence with one key, pull from the subscription, confirm order preserved across a redelivery.