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A handler continues the flow by emitting the next event. An emitted event’s payload is producer-complete: the handler must populate every field the event declares, through emit.fields. Nothing is copied automatically from the triggering event, and there are no defaults. Each entry in fields is a CEL expression, usually reading the entity you just wrote or the incoming payload:
Two pieces of sugar reduce the restating; both are shorthand that expands into the explicit map before anything runs — that expanded map is what swarm verify and the runtime actually see:
  • emit.from: entity (or payload — exactly one namespace, no lists) auto-fills every required declared payload field whose name matches a source field. Optional fields are never auto-filled; explicit fields: entries win over from:. So if ticket.assigned declares category (required) and note (optional), from: entity fills category from entity.category and leaves note unset.
  • Inside fields:, a bare namespace value means the same-named source field: category: entity lowers to category: entity.category. Dotted CEL stays the form for renames and computed values.
The bare string form, emit: ticket.assigned, emits with an empty payload. swarm verify accepts it, so the empty payload only shows up at runtime when a subscriber receives {}. Use the bare form only for events that declare no fields; use fields the moment an event carries data.

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Routing is not the producer’s job

An emit does not choose who receives it. The producer says what happened; the receiving side decides who hears it:
  • Inside a flow — delivery comes from subscriptions (subscribes_to on nodes, subscriptions on agents). An internal emit needs nothing beyond event and fields.
  • Across flows — the event leaves through a declared output pin, and the receiving flow decides which instance gets it. See Composing flows for pins, connect edges, and resolution.
  • Replies — reply correlation is declared on the receiving pin, not chosen per-emit by the producer.
A handler’s top-level emit is valid only when the handler has a single emit site. If the handler also branches with rules or on_complete, the emit moves onto the branch; a handler-level emit alongside rules is a boot error. See Branching.
Older Swarm versions accepted emit.target and emit.broadcast — producer-side recipient and cardinality selection. Both are retired and now fail at load with a RETIRED-EMIT-ROUTING error naming the replacement. If you are migrating: a target: sender reply becomes receiver-owned reply correlation on the pin; a broadcast: true fan-out becomes either ordinary subscriptions (every subscriber in the flow already receives an internal event) or explicit connect edges per receiving flow.