CEL reference

Swarm evaluates all guard checks, rule conditions, filter conditions, and on_complete branch conditions with CEL. CEL is strongly typed and non-Turing-complete. For the conceptual intro see Expressions.

Context variables

Variable	Holds	Available in
`entity`	Declared entity fields, `current_state`, `gates`	All conditions
`payload`	The current event’s payload fields	All conditions
`policy`	Flow policy merged with root policy	All conditions
`fan_out`	Fan-out metadata, e.g. `fan_out.count`	`data_accumulation` expressions
`accumulated`	The received items from the current `accumulate` step	`on_complete`, `filter`

Declared entity fields are always readable at a type-valid value; an undeclared field reference is a contract error caught at boot.

Where each variable is available

The five condition sites run at different points in the handler pipeline, so accumulated and fan_out are not usable everywhere. Referencing one where it is not available is a boot error.

Site	`entity`	`payload`	`policy`	`accumulated`	`fan_out`
`guard.check`	yes	yes	yes	no	no
`rules.condition`	yes	yes	yes	no	no
`on_complete.condition`	yes	yes	yes	yes	no
`filter.condition`	yes	yes	yes	yes	no
`data_accumulation.expression`	yes	yes	yes	no	yes

accumulated is only readable after accumulation completes (on_complete, filter), so it is absent in guard (runs before accumulation) and in rules (an alternative to on_complete). fan_out.count is populated after the fan-out step and is read only in data_accumulation.expression; in on_complete, read the entity field you wrote it to instead.

Operators

Comparison: ==, !=, <, <=, >, >=
Logical: &&, ||, !
Arithmetic: +, -, *, /
Membership: in, contains
String: startsWith, endsWith, matches
List: size, exists, all, filter, map

The prefix rule

Every condition carries a context prefix: payload., entity., policy., accumulated., or fan_out.. A bare identifier is a dialect violation. See Dialect rules.

Examples

entity.retry_count <= policy.max_retries
payload.category == "billing"
entity.kill_reason != null
accumulated.size() >= policy.required_dimensions
accumulated.filter(d, d.score >= policy.threshold).size() > 0

Cross-entity count

query_entities(<field> <op> <value>).count returns how many flow-owned entities match a single comparison, for use inside a condition. The operator is one of ==, !=, >=, <=, >, <, and the value is a literal or a resolved reference (payload.*, event.*):

query_entities(request_id == payload.request_id).count == 1

This is for cardinality assertions, most often idempotency: accept the event only when exactly one entity already carries its request_id. The predicate is a single comparison, not a compound expression, and only .count is exposed.

Presence

has(entity.field) is true for declared entity fields (redundant under universal presence, useful for dynamic maps). Comparing against null works; timestamp and uuid fields read null when unwritten, while most others read their non-null type default.

Contract reference

Language reference

Operations reference

Context variables

Where each variable is available

Operators

The prefix rule

Examples

Cross-entity count

Presence

​Context variables

​Where each variable is available

​Operators

​The prefix rule

​Examples

​Cross-entity count

​Presence

Context variables

Where each variable is available

Operators

The prefix rule

Examples

Cross-entity count

Presence