# Analysis — Durable Agent Runtime

> Part of [[durable-agent-runtime/index|Durable Agent Runtime]]
> Harness/loop analysis moved to [[harness-engineering/index|Harness Engineering]] (2026-08-11).

## The primitive families

| Family | Example | Compute primitive | Where state lives | Cold start | Hibernation |
| --- | --- | --- | --- | --- | --- |
| Durable objects | celld (Deno) | V8 isolate running Worker bundles | Per-object SQLite, replicated to S3 | Fast (isolate) | Idle cells hibernate |
| Actors | rivet | Lightweight process (Node/Bun/Deno) | In-memory + SQLite/BYO DB | ~20ms | Actors hibernate when idle |
| VM sandboxes | hypeman, E2B | Firecracker/Cloud Hypervisor/QEMU microVM | VM disk (snapshot for standby) | ~30s VM / sub-30ms from snapshot | standby = snapshot to disk |
| Durable harness library | pi-durable (2026) | Node process or Cloudflare Durable Object | Immutable transcript + documents + task checkpoints in SQLite/JSONL/DO storage | Reopen + reconcile (ms) | Task sleeps end the invocation; host alarm wakes (onWake on main, not in 1.3.0) |
| Collaborative substrate | Radicle COBs | Signed, CRDT-like Git commit DAGs | Git objects (replicated, self-certifying) | Instant (local git) | Repos persist; nodes sync on demand |

(Harness libraries — nanocodex, Codex, pi — moved to [[harness-engineering/index|Harness Engineering]].)

The industry convergence: **compute is disposable, state is durable elsewhere.**

- Amp orbs: ephemeral E2B sandbox + durable thread on Amp Server (PostgreSQL). Sandbox destroyed on archive; conversation survives.
- celld: nodes are replaceable; the S3 bucket is the durable source of truth. Object = its own SQLite DB → sharding and blast-radius containment by construction, no consensus.
- rivet: actor state in-memory for 0ms reads, persisted for durability; runs indefinitely, sleeps when idle.
- hypeman: standby = snapshot VM to disk, restore in milliseconds — VM-level hibernation.

## The two hard problems everyone solves

1. **Cold start latency** — the gap between "VM ~30s" and "actor ~20ms" is closed with snapshots (E2B, hypeman standby, celld/rivet hibernation) or pre-provisioned pools. This is why orbs can feel local despite being remote.
2. **State checkpointing & recovery** — thread JSONL (Amp), SQLite replication (celld), actor state persistence (rivet), VM disk snapshots (hypeman). Everything is replayable; nothing is lost on compute failure.

## Where pi-durable fits (2026-10-20)

`@earendil-works/pi-durable` is not a new substrate; it is a **durable harness library** that runs on an existing one. It commits the loop's own state — immutable transcript entries, typed documents, per-phase task checkpoints, idempotent submissions — and resumes on reopen. The substrate can be a Node process with SQLite/JSONL or a Cloudflare Durable Object; tools execute through a pluggable `ExecutionEnv`, so a VM sandbox (hypeman/E2B) can sit underneath without the harness knowing.

Its semantics sharpen the "compute disposable, state durable" thesis: the harness process is disposable too, but **phases are at-least-once**, not exactly-once. External effects repeat on rerun unless memoized or idempotent; tools declare `replay: "safe"` to rerun, otherwise the model gets an `interrupted` result. Only submission admission is exactly-once (`requestId`). That is a useful, honest contract to compare against Temporal-style durable execution and against our own kanban/session design. Full write-up: [[durable-agent-runtime/pi-durable|pi-durable]].

## What this means for hermes-vm (our stack)

Already have (the "durable compute host" layer):
- Persistent remote dev VM (hermes-vm), systemd services, kanban DB, session DBs, herdr panes, tailscale.
- herdr = the "shared tmux + durable agent loop" half of an orb, already built.

Genuine gaps (the "environment contract" layer):
- Per-repo `.agents/setup`-style bootstrap + AGENTS.md per repo — cheap, ~90% of the reliability win.
- Idempotent dev-server script + ports metadata file + `/__dev`-style auth bypass for our apps.
- Event-driven wake: Forgejo webhooks → kanban task (self-hosted equivalent of Amp's event-driven orbs).
- Formalize tailscale serve as per-repo portals.

Not worth building on a single box:
- A distributed durable-object layer (celld) or actor platform (rivet) — solves multi-node, multi-region, scale-to-zero billing problems we don't have. One VM = one durable host.
- An E2B-style per-thread VM fleet — same reason. NixOS already gives cheaper isolation: `nix develop`, rootless podman (already enabled via `modules/dev/containers.nix`), worktrees, NixOS containers/systemd-nspawn if stronger isolation is ever needed.

Interesting but optional:
- **hypeman** — if we ever want real per-task VM isolation with snapshot/standby on our own box, hypeman is the lean OSS option (multi-hypervisor, OCI images, ms restore). Watch it; don't adopt yet.

## Verdict

Build the **environment contracts** (setup hooks, AGENTS.md, dev-server skills, event wake) on the existing durable host. Do not build the sandbox platform or a distributed state layer. herdr + systemd + kanban + tailscale already provide the durability; the missing piece is making the environment "assume an agent and tell it where the light switches are."

If we ever build a long-running, multi-client agent *service* (as opposed to cron-driven one-shot tasks), pi-durable is the closest reference implementation: one SQLite-backed Session per service, tools on `NodeExecutionEnv`, views for clients. For the Rust-core direction, treat its spec as the design input and reimplement only the commit/task/ownership semantics we actually need.