---
id: SCH-39
family: GR-SCHEMA
level: extended
profiles: [storage-api, editor-client]
posture: normative-target
added: "1.0"
changed: "1.0"
source: https://flowdrop.io/spec/rules/gr-schema/sch-39
specification: FlowDrop Workflow Specification 1.0-draft
licence: CC BY 4.0
---

# SCH-39 — Compatibility is asymmetric, and the sink needs rules both ways

*GR-SCHEMA (Part I) · level: extended · profiles: storage-api, editor-client · added in 1.0*

A lane with no rules accepts only its own, which is why the sink (a lane worn by outputs as well as inputs) once refused every wire drawn into it.

## The rule

> **Normative.** This is the rule.
>
> 1. Compatibility maps an output lane to the set of input lanes it may enter.
>
> 2. It is seeded by exact match and widened only by explicit rules, so a lane with no rules accepts nothing but itself.
>
> 3. The data sink `mixed` is worn by outputs as well as inputs and so needs rules in both directions; making it an alias of the control sink does not serve, because an alias copies the outgoing set only.
>
> 4. `tool` is excluded from every sink rule in both directions.
>
> 5. A rule that widens states its reason where it is declared, since the served `from`/`to` pair cannot carry one.
>
> 6. The stakes are authoring-time: a wrong rule costs an author a wire they cannot draw, or lets them draw one nothing will reject.

## Related rules

- Names: SCH-38, SCH-43
- Referenced by: SCH-38, SCH-43

---

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.