What the stack defends, where the trust boundaries sit, what the operator must provide, and what it deliberately does not defend. Read this before exposing any part of a deployment beyond a single trusted host.

  agent workload (untrusted code)
      │ syscalls
  guest kernel + virtio drivers
      │
══ VM boundary (KVM) ═══════════════ the isolation unit
      │ vsock (none lane) / nft-locked NIC (egress lane)
  host: sandboxd + cocoon            trusted
      │ HTTP control/data plane, bearer tokens
  clients: SDK / MCP / operators     trusted per token scope

• The VM boundary is the security boundary. Everything inside the guest — the workload, the guest kernel, and silkd itself — is untrusted by the host. silkd is a convenience daemon, not a defense: a compromised guest can lie to its own client about its own state, but gains nothing toward the host, siblings, or the network beyond its lanes.
• What a compromised guest can reach. On the none lane: nothing but vsock — the relay back to its own client and the guarded-egress proxy. On the egress lane: the same vsock paths plus a NIC whose every packet except IPv4 broadcast DHCP is dropped by an nftables lock in the host root netns (egress); the lock is fail-closed and applied before the claim is handed out. On both lanes the proxy refuses loopback, private, link-local (cloud metadata), CGN, and the IPv4-embedding IPv6 ranges, so an allow-listed name that resolves or rebinds to an internal address cannot reach the host or a sibling.
• What never enters the guest. Egress credentials: the proxy injects the secret host-side, so prompt injection can exfiltrate at most the proxy's answers, and every credentialed call is journaled. Git auth tokens travel as in-memory headers, never guest disk. Secrets come from the host environment, never the config file.
• Sandbox identity. Every clone is reseeded (entropy, machine-id), so branches and forks never share an identity with their source.

These are hard constraints, not suggestions; the guarantees above assume all of them.

1. The control/data plane runs on a trusted private network. sandboxd serves plain HTTP; api_token, tenant tokens, and per-sandbox tokens are cleartext bearers on the wire, and on a cluster the SDK dials every node it is redirected to. Keep nodes and clients inside a VPC, WireGuard mesh, or equivalent. The only surface designed to face a browser is the preview listener, and it belongs behind a TLS-terminating proxy (deploy). Never expose listen publicly.
2. One sandboxd per host. The restart sweep owns the whole sandbox_egress_* nftables namespace; a second daemon would clear the first's locks.
3. The egress bridge shares no broadcast domain with untrusted listeners. The DHCP lock exception is matched by header shape, so a packet in that shape can reach the local L2 segment.
4. Set mesh.cluster_key unless the gossip network is itself trusted, and open the memberlist port node-to-node only.
5. Treat the checkpoint store like backups. A checkpoint embeds full guest memory — including any secret the workload held at capture. Restrict store access; use S3 server-side encryption or an encrypted mount where that matters.
6. No custom NAT64 prefix routed on a sandboxd host — the SSRF guard cannot see through an operator-specific translator prefix (egress).

Three credentials, in descending scope (API reference): the root api_token (operator surfaces, full access), tenant tokens (resource-creating verbs, everything stamped and quota'd per tenant), and per-sandbox tokens (that sandbox only — holding a handle amplifies to nothing node-level). Two capability tokens ride on top: preview URLs are HMAC-signed, expire with the claim's lease, and die with the sandbox (no revocation list to leak); a checkpoint id is the unguessable capability to branch it. On a cluster, deleting a checkpoint does not revoke that capability fleet-wide the instant it runs: the delete is best-effort — broadcast to every peer the node currently sees — so a peer that is offline or partitioned at that moment keeps its own replica branchable until checkpoint_ttl_hours ages it out (placement lifecycle). Tenants are isolated at the API layer — listings filter, deletes answer 404 rather than confirming existence, and operator surfaces answer tenants 403.

The volume catalog is an operator-owned data boundary. A volume name and its access list must mean the same thing fleet-wide, although membership is node-local. An empty entry tenants list permits every authenticated scope; a nonempty list permits only those configured tenants, while the root token always has access. Config load rejects an access-list name that is not a configured tenant. Claim lookup returns byte-identical errors for an unknown and a forbidden name, and catalog discovery filters before replying, so a tenant cannot enumerate restricted entries by probing. Gossip carries names only. Neither gossip, discovery, persisted claims, usage events, nor the sandbox index exposes host image paths or access lists.

Read-only is integrity protection for the shared image, not confidentiality. Mounting a dataset into an egress-lane sandbox gives that sandbox an export path to every destination its tenant egress policy permits; review the volume access list and egress policy together. With directio=off, concurrent readers share the host page cache, which improves reuse but lets one tenant's large scan evict another's cached pages — the same holds for a writer: a large rw write can evict cached pages backing another volume's readers exactly like a large scan would. directio=on is the per-volume mitigation; v1 has no per-tenant cache quota or accounting. Operators must keep a read-only entry's attached image immutable and publish a new name/path for new content.

A writable entry (writable: true) adds a channel the read-only model does not have: whichever tenant is permitted to claim it rw changes what every other permitted tenant reads next, and any of them can be the writer in turn — a multi-tenant access list on a writable entry is a bidirectional channel between those tenants, not just a shared read. Recommend a writable entry's tenants name exactly one tenant — an empty list permits every authenticated scope, which for a writable entry means every tenant can write to every other tenant's next read. A dataset that genuinely needs multiple writers needs an out-of-band process for who writes when, which the catalog ACL does not provide.

Facts to plan around, stated so the boundary is honest:

• The VMM processes are not additionally sandboxed. cloud-hypervisor runs as an ordinary host process under cocoon; a VMM escape lands on the host with the VMM's privileges (upstream: cocoonstack/cocoon#83). Compensate with dedicated sandbox nodes and a minimal host.
• HTTPS interception is HTTP/1.1 only and breaks certificate-pinning clients — scope intercept rules to hosts you control (egress).
• The audit and usage journals are local JSONL with size rotation — no tamper-evidence. Ship them off-node if integrity against a host compromise matters (at which point the journals are the least of it).
• Rate limiting is capacity-based only (max_claims, per-tenant caps answer 429). The API assumes callers inside the trust boundary; front it with your own limiter if semi-trusted automation can reach it.
• A dead node's sandboxes die with it — memory state is node-local by design (clusters); durability is the checkpoint store's job, not the node's.