---
id: SG-5
family: RT-SG
level: extended
profiles: [runtime]
posture: normative-target
added: "1.0"
changed: "1.0"
source: https://flowdrop.io/spec/rules/rt-sg/sg-5
specification: FlowDrop Workflow Specification 1.0-draft
licence: CC BY 4.0
---

# SG-5 — loop_back is a value on a ForEach node and a bare signal everywhere else

*RT-SG (Part II) · level: extended · profiles: runtime · added in 1.0*

## The rule

> **Normative.** This is the rule.
>
> 1. On a ForEach node the reserved `loop_back` input carries the round's item result, standing in for an explicit `item_result` input wherever that is absent.
>
> 2. On every other node type the reserved `loop_back` input is a re-entry signal only: the value is delivered on the port and read by nobody, since a node with no iteration state has nothing to fold it into.
>
> 3. A node type that wants the value declares its own port for it, which is exactly what suppresses the reserved injection.


## What it means

`loop_back` is not one signal with two names. On a ForEach node it carries
data: whatever arrives on it stands in for the round's `item_result` when
that port is not otherwise filled, so a loop body that only wires its result
back to `loop_back` still folds it in. On every other node type the same
port carries nothing readable — a value delivered there is read by nobody,
because a node with no iteration state has nothing to fold a result into. A
node type that wants the value declares its own port under that name, and
that declaration is exactly what stops the reserved, dataless injection.

## Example

A ForEach node mid-loop, resumed with a value on `loop_back` instead of on
`item_result`:

```json title="The re-entry input"
{"loop_back": "loopback_result"}
```

```json title="What the next round reports" verdict="advanced"
{"current_item": "b"}
```

The loop advances exactly as it would have if the same value had arrived on
`item_result` — `loop_back` supplied the round's result, not a re-entry
signal with no payload.

## Related rules

- Names: SG-4
- Referenced by: SG-4

---

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.