# Routing and capabilities (/concepts/routing)



## Candidate order [#candidate-order]

Automatic selection tries explicit fallback providers first, then other configured managed or BYOK
providers in their configured order.

An explicit provider is primary. Listed fallback providers follow in order. Candidates must be unique.

OpenMetal does not currently calculate cheapest, fastest, reliable, or balanced rankings.

## Source support [#source-support]

Portable environments work across all current providers. OCI images work with Blaxel, Daytona,
Modal, Northflank, and Vercel. Provider templates work with CodeSandbox, E2B, Freestyle, and
Runloop.

Recognized portable environments are `metal/base`, `metal/node`, and `metal/python`.

## Runtime support [#runtime-support]

CodeSandbox and Northflank adapters currently expose lifecycle only, without process execution or
file access, so automatic routing never selects them.

Current public process streaming works with Cloudflare, E2B, Modal, and Vercel. Request
`features.process.ordered_output` to route only to them. Process cancellation is not confirmed for
those adapters.

Filesystem read and write support is broader. List, delete, append, and parent creation vary by
provider; request them under `features.filesystem` when you depend on them.

Portable leased HTTP endpoints are currently available through Blaxel and Daytona.

Reliable pause and resume are available through CodeSandbox, E2B, Freestyle, Northflank, and
Runloop. Daytona pause reporting is inconsistent.

## Hard requirements [#hard-requirements]

Source, resources, `features`, `network`, and `regions` are hard requirements. Before calling a
candidate, OpenMetal removes it if its declared capabilities cannot satisfy every requirement,
records each unmet requirement on the provider attempt, and moves to the next candidate. After
provisioning, it checks runtime requirements again against the capabilities discovered on the
sandbox and destroys a sandbox that falls short.

When no candidate qualifies, the create operation fails with `no_eligible_provider`.
`error.details.attempts` lists each provider with its `unmet_requirements`, for example
`["isolation", "network.allow_domains"]`.

Automatic routing adds a portable baseline: process execution plus file read and write. Set
`features.process.execute`, `features.filesystem.read`, or `features.filesystem.write` to `false`
to opt out. Explicit providers only need what you request.

| Requirement             | Current support                                                                                                                                                                         |
| ----------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `features.isolation`    | Provider-reported microVM: Blaxel, CodeSandbox, E2B, Runloop, Vercel. VM: Freestyle, Prime Intellect. Container: Cloudflare, Daytona, Modal. Northflank is unknown and never qualifies. |
| `features.process`      | See runtime support below.                                                                                                                                                              |
| `features.filesystem`   | See runtime support below.                                                                                                                                                              |
| `features.public_ports` | Blaxel only.                                                                                                                                                                            |
| `features.pause_resume` | Providers with reliable pause and resume.                                                                                                                                               |
| `features.pty`          | Never satisfied. Interactive PTY sessions are not part of the API.                                                                                                                      |
| `network` restrictions  | Never satisfied yet. No adapter enforces outbound restrictions, so requests that restrict egress fail closed.                                                                           |
| `regions`               | Never satisfied yet. No adapter declares every region it may place a sandbox in.                                                                                                        |

<Callout type="warn" title="Provider-reported isolation">
  Isolation claims come from provider documentation. OpenMetal has not independently verified them.
</Callout>

## Fallback safety [#fallback-safety]

Capacity, provider unavailable, known absent timeout, and provider-declined unsupported requests can move to the next candidate. Unknown provider outcomes enter `reconciling` first so OpenMetal does not create duplicate live capacity.

See [route across providers](/guides/routing-and-providers) for a complete request.