CEL cheat sheet

In scope

What an expression can see.

Every message-driven block evaluates against the same activation. These are the variables in scope:

Variable	Type	What it is
`body`	JSON value	The decoded message body — usually a map, sometimes a list/scalar. Rewritten by `set-payload`.
`vars`	map	Per-message variables. Read here; write with `set-variable`. Sources seed it (see below).
`env`	map	Resolved values of the flow's declared `env:` entries — e.g. `env.HTTP_PORT`. Only declared keys exist.
`eventID`	string	Stable unique id for this message.
`correlationID`	string	Caller-supplied id for grouping related messages (optional).

From the source

What sources put in `vars`.

From	Variable	Example
http source	`vars.method`	`vars.method == "POST"`
http source	`vars.query` (always a map)	`has(vars.query.currency)`
http path `/orders/{id}`	`vars.<param>`	`vars.id`
http captured header	`vars["X-Header"]`	`vars["X-Tenant"]`
cron source payload	`now`	`string(now)`
error path	`vars.error`	`vars.error.message` · `.flow` · `.block`

Copy-paste

Recipes.

Every snippet below is lifted from a working sample in samples/.

Build a string · coerce types

"hello, " + body.name + "!"
"processing " + string(size(body.orders)) + " orders"

Safe access with a default

has(vars.query.currency) ? vars.query.currency : "USD"

Route on method or value (switch / if)

vars.method == "POST"
body.amount >= 1000.0
size(body.orders) > 0

Build a JSON object (set-payload)

{"orderId": vars.id, "currency": vars.currency, "status": "found"}

Reach into nested fields & dashed headers

body.current.temperature_2m
vars["X-Tenant"]

Read environment variables

"listening on " + env.HTTP_PORT

Iterate a list (foreach)

items: body.orders        # the list to split
as: order                 # each item -> vars.order
vars.order.amount >= vars.threshold

Recover from an error (error path)

{"error": vars.error.message,
 "failedBlock": vars.error.block,
 "flow": vars.error.flow}

Gotchas worth knowing. JSON numbers are always double (float) — wrap with int(...) when you need a whole number. Use has(map.key) before reading a key that might be missing, or the expression errors. Only env: entries you declare are in scope — an undeclared env.X fails at build time. Expressions never mutate state: read vars/body here, and change them with set-variable / set-payload. Use bracket syntax (vars["X-Tenant"]) for keys with dashes.

CEL cheat sheet

What an expression can see.

What sources put in vars.

Recipes.

What sources put in `vars`.