# Extension routing

Version 0.1 — SAND-WP-0006. Select backend when a profile lists multiple extensions.

## Profile route block

```yaml
extension: ext.compose-ssh   # default / explicit fallback
route:
  strategy: prefer-self-hosted
  extensions:
    - ext.compose-ssh
    - ext.saas-stub
  max_cost_per_hour_usd: 1.0
```

## Strategies

| Strategy | Behavior |
|----------|----------|
| `explicit` | Use `profile.extension` (default when no route) |
| `prefer-self-hosted` | First self-hosted candidate with resolvable host; else SaaS |
| `lowest-cost` | Self-hosted if available; else cheapest `estimate_cost` |
| `lowest-latency` | Self-hosted if available; else last candidate (v0) |

## Testing SaaS fallback

```bash
# Skip self-hosted when host unavailable:
unset SANDBOXER_HOST
SANDBOXER_FORCE_SAAS=1 sandboxer create --profile profile.burst-sandbox

# Or use metered-only profile:
sandboxer create --profile profile.saas-stub
```

## Reference profiles

| Profile | Route |
|---------|-------|
| `profile.burst-sandbox` | compose-ssh → saas-stub fallback |
| `profile.saas-stub` | explicit `ext.saas-stub` |

Resolver: `sandboxer.routing.resolver.resolve_extension`.