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46. Host-side API server design for sandboxed agents

Date: 2026-06-02

Status​

Accepted

Context​

ADR 0024 introduced the api_servers harness field as planned but not implemented. ADR 0017/ADR 0025 established the host-side REST server as Tier 3 of the credential delivery model — for cases where providers (Tier 2) cannot handle, originally scoped to credentials in request bodies and response transformation. Practice revealed additional cases beyond provider reach: long-running operations exceeding MCP timeouts, operations the sandbox deliberately blocks (container builds, see NVIDIA/OpenShell#113), and multi-step atomic operations.

The host-side-api-server experiment (fullsend-ai/experiments#28) validated the end-to-end flow with two servers (Go container builder, Python repo provisioner), testing lifecycle management, API discoverability, L7 policy tuning, per-run auth, and file transfer. This ADR records design decisions informed by that experiment and by subsequent review discussion on the implementation model.

Options​

API discoverability​

Three approaches were tested. /tools.json (structured tool-use schema) was the most token-efficient under full access (92k tokens vs 107k for OpenAPI, 100k for baked instructions). Both discovery-based methods fail under restricted policies where the endpoint is blocked; baked instructions succeed (84k) but can drift from the actual API. OpenAPI's verbose structure adds context tokens without proportional benefit for LLM agents.

Per-run authentication​

UUID bearer token via provider placeholders: simple, proven, no key management. The proxy resolves the placeholder — the real token never enters the sandbox. No claims or expiry.

Short-lived JWTs with claims: per-operation authorization and audit trail, but adds signing key management and JWT library dependencies in every server. The L7 policy already restricts reachable endpoints, making JWT claims a redundant second layer.

File transfer between server and sandbox​

openshell sandbox upload/download from the server: works today, validated by experiment, handles real-time exchange during request handling. Couples server to OpenShell CLI.

Shared host mount: transparent POSIX access, no CLI coupling. Depends on OpenShell mount support that is not yet universally available (NVIDIA/OpenShell#1509).

HTTP multipart via the API: fully portable, but large files through the L7 proxy add overhead.

Implementation model​

Language-agnostic process contract: each server is an independent binary in any language. The runner manages it via CLI flags (--port, --token, --bind-address) and expects /healthz, /tools.json, bearer auth, and clean SIGTERM shutdown. The experiment validated this with Go and Python servers. Maximally flexible, but every server re-implements boilerplate (CLI parsing, health endpoints, auth middleware, security hardening).

Go interface: fullsend-maintained servers implement an internal Go interface, compiled to a single binary. Simplifies deployment (one binary, no runtime dependencies) and enables stub-based testing. The process contract remains the runner's enforcement boundary — the interface is an internal pattern, not a runner requirement. Provides a seam for adapting deployment topology across workflow platforms (sidecars, pods).

Decision​

Adopt the host-side API server design with the following implementation model, policy model, and security requirements. The host-side-api-server experiment validated the end-to-end flow; the implementation model was refined during review.

Process contract. Every compiled API server binary accepts --port, --token, and --bind-address flags; serves GET /healthz (unauthenticated) and GET /tools.json (structured tool-use schema) for agent discovery; validates bearer tokens on all other endpoints; and shuts down cleanly on SIGTERM. Servers write logs to stderr; the runner collects and bundles logs from all API servers so they are available for inspection after the run completes. The runner starts servers after pre-script, health-checks before sandbox creation, and tears down after sandbox destruction. If a server crashes mid-run, the run fails.

Implementation model. The runner enforces the process contract above — any binary that satisfies it is a valid API server. Internally, fullsend-maintained servers use a Go interface as the recommended pattern: each server provides HTTP handlers and a /tools.json schema, and the main() wires the implementation to the process contract (CLI flags, health endpoints, auth middleware, signal handling). Harnesses reference a compiled Go binary, the workflow runner downloads it to the host, and fullsend run manages its lifecycle. The interface makes servers testable via stub implementations and provides the seam for adapting deployment topology across workflow platforms (e.g., sidecars or separate pods on Kubernetes/OpenShift). Implementors who need more control can write their own main() against the process contract directly.

Network policy via composable provider profiles. Each API server ships atomic capability profiles — one per logical group of endpoints (e.g., builder-build, builder-push, builder-read). Harnesses list which profiles to attach. Composition is additive per OpenShell's provider-backed policy composition (NVIDIA/OpenShell#1037). Different agent roles compose different capability sets for the same server. Requires OpenShell >= v0.0.37 and #776.

Per-run auth: UUID bearer token via OpenShell provider placeholders. JWTs are a future enhancement when per-operation claims become necessary.

File transfer: openshell sandbox upload/download from the server during request handling. Shared host mount (NVIDIA/OpenShell#1509) will be evaluated as an alternative when available.

Bind address: servers default to 127.0.0.1, runner overrides to 0.0.0.0 for sandboxed agents. NVIDIA/OpenShell#1633 (supervisor-proxied host-local endpoints) would eliminate this requirement.

Security hardening: timing-safe token comparison, 1 MB request body limits, rate limiting on unauthenticated endpoints, credential scrubbing in error messages, bounded in-memory state.

Consequences​

  • The api_servers harness field (ADR 0024) will gain a providers sub-field and defined runtime behavior. The initial implementation targets Go servers behind an internal interface; the process contract keeps the door open for other languages.
  • Implementing this design requires #776 (provider-backed policy composition) as a prerequisite.
  • Servers are coupled to the OpenShell CLI for file transfer until shared host mounts are universally available.
  • Servers must bind to 0.0.0.0 on shared hosts, widening the attack surface until NVIDIA/OpenShell#1633 ships.
  • API servers (Tier 3) are now clearly scoped to cases providers cannot handle: long-running operations, sandbox capability gaps, credentials in request bodies, response transformation, and multi-step atomic operations.
  • Fullsend-maintained servers follow a Go interface pattern (testable, platform-portable). The process contract remains the enforcement boundary — servers in other languages or with custom main() implementations are valid as long as they satisfy it.