SKM-P03 · Portability
`context: fork` / `context: background` is a Claude Code-only field
Runs the skill as a forked subagent — a Claude Code mechanism with no equivalent elsewhere.
What the rule is
`context: fork` (and `context: background`) makes the skill run as a forked subagent — a Claude Code-specific mechanism with no equivalent in the other four runtimes.
This is one of the fields that degrades quietly rather than breaking anything: in a runtime that doesn't recognise it, the skill still runs, just inline rather than as a subagent. The behavioural difference — isolation, parallelism — is simply lost.
Where it comes from
- Agent Skills compatibilityRuntime documentation
- Agent skills not workingDiagnosis
Which runtimes enforce it
| Runtime | Effect | Note |
|---|---|---|
| Claude Code | Enforced (home runtime) | The field's home runtime — the skill runs as a forked subagent. |
| Codex | Ignored | Ignored — the skill runs inline rather than as a subagent. |
| Cursor | Ignored | Ignored — the skill runs inline rather than as a subagent. |
| OpenClaw | Ignored | Ignored — the skill runs inline rather than as a subagent. |
| Hermes Agent | Ignored | Ignored — the skill runs inline rather than as a subagent. |
How to fix it
- If the skill is shared across runtimes, treat `context: fork` as a Claude Code-only enhancement, not a requirement the skill's instructions depend on. Write the body so it still makes sense running inline.
Elsewhere
- SKILL.md, the formatThe full frontmatter field table this rule is drawn from.
- Agent Skills Not WorkingThe diagnostic reference this rule’s portability and failure evidence comes from.
- All rulesEvery check the validator runs, grouped by class.