---
id: CFG-20
family: GR-CFG
level: extended
profiles: [storage-api, runtime]
posture: normative-target
added: "1.0"
changed: "1.0"
source: https://flowdrop.io/spec/rules/gr-cfg/cfg-20
specification: FlowDrop Workflow Specification 1.0
licence: CC BY 4.0
---

# CFG-20 — A parameter's when_missing flag is fail unless the node type says skip

*GR-CFG (Part I) · level: extended · profiles: storage-api, runtime · added in 1.0*

Most required parameters fail their node when nothing resolves. A few, such as a trigger node's event, mean "this node has nothing to do" when nothing resolves. The flag lets a node type say which, and keeps the default every existing node type has.

## The rule

> **Normative.** This is the rule.
>
> 1. A parameter's `when_missing` flag says what a missing value means: `fail` or `skip`.
>
> 2. An implementation MUST treat a value it does not recognise as `fail`.
>
> 3. The flag MUST be read from the node type's own parameter configuration, as the gate flags are (SCH-2).
>
> 4. Where the node type records no value, an implementation MUST treat the flag as `fail`.
>
> 5. The flag MUST have no effect on a parameter that is not required.


## What it means

`when_missing` sits beside `required`, and only means something together with it.
`required` says the node cannot run without a value. `when_missing` says what that
absence means: `fail` (the node is broken, CFG-9) or `skip` (the node has nothing to
do, CFG-21). A node type that never mentions the flag gets `fail`, so every node type
that exists today behaves exactly as before.

The flag belongs to the node type, like the gate flags, and not to the author's node
configuration: whether "nothing arrived" is an error is a property of what the node
does.

## Example

```json title="A trigger node type's event parameter" verdict="skip"
{ "event": { "connectable": true, "required": true, "when_missing": "skip" } }
```

```json title="A required parameter that says nothing about when_missing" verdict="fail"
{ "prompt": { "connectable": true, "required": true } }
```

## Why

Recorded under [OPEN-21](https://flowdrop.io/spec/rulings/open-21), [OPEN-22](https://flowdrop.io/spec/rulings/open-22), [OPEN-25](https://flowdrop.io/spec/rulings/open-25).

## Fixtures

Test cases whose outcome this rule decides (fixtures/README.md defines each operation):

- A required skip parameter nothing supplies skips its node (`resolve-parameters`): fixtures/presence/cfg-skip-when-absent.json

## Related rules

- Names: SCH-2, CFG-1, CFG-9, CFG-21
- Referenced by: SCH-47, CFG-21

---

Rule identifiers are permanent and are never renumbered. This specification carries
no implementation status: each implementation publishes its own standing against
these rules. Licensed CC BY 4.0.