# Isolation Baseline for a Dedicated Pi Agent

> Applies to every scenario in `scenarios/`. Mechanisms verified against
> pi 0.84.3 — see [`pi-runtime-notes.md`](pi-runtime-notes.md) for the source
> quotes and [`evidence/`](evidence/) for reproduction.

A "dedicated agent" is not achieved by narrowing what the model is *told*. It is
achieved by narrowing what the process *loads* and what it *can call*. Pi offers
four independent layers. Use all four; each one covers a different failure mode.

---

## The four layers

```
┌──────────────────────────────────────────────────────────────┐
│ L1 Loading      what enters the process at all               │
│   --no-extensions -e / --no-skills --skill /                 │
│   --no-prompt-templates / --no-themes / -nc / project trust  │
├──────────────────────────────────────────────────────────────┤
│ L2 Personality  who the agent is                             │
│   .pi/SYSTEM.md (replace) · .pi/APPEND_SYSTEM.md · -nc        │
│   AGENTS.md only if parent-dir layering is acceptable          │
├──────────────────────────────────────────────────────────────┤
│ L3 Capability   which tools exist and are active             │
│   --no-builtin-tools · --tools · --exclude-tools              │
│   pi.setActiveTools() · same-name registerTool override       │
├──────────────────────────────────────────────────────────────┤
│ L4 Invocation   per-call authorization                       │
│   pi.on("tool_call") -> { block: true, reason }               │
└──────────────────────────────────────────────────────────────┘
```

Layers 3 and 4 must be enforced **inside the extension**, not only on the
command line, so that a flag mistake in a systemd unit cannot silently widen the
agent's reach.

---

## Baseline flag set

```bash
pi --mode rpc \
   --session-id   "<scenario>-<subject>" \
   --session-dir  "<dedicated session dir>" \
   --no-builtin-tools \
   --no-extensions        -e     "<abs>/scenario-tools.ts" \
   --no-skills            --skill "<abs>/.pi/skills/<name>" \
   --no-prompt-templates \
   --no-themes \
   --approve \
   --provider "<provider>" --model "<model>" --thinking "<level>" \
   --name "<display name>"
```

Launched with:

| Property | Value | Reason |
|---|---|---|
| `cwd` | the scenario workspace | bounds `AGENTS.md` and `.pi` discovery |
| `env` | explicit minimal allowlist | keeps backend API keys out of the node process |
| `start_new_session=True` | yes | lets the parent kill the whole process group |
| timeout handling | `killpg`, not `kill` | pi is a node CLI and may leave children |

### Why each flag

| Flag | Failure mode it prevents |
|---|---|
| `--no-extensions` + `-e <abs>` | `~/.pi/agent/extensions/*.ts` (currently `herdr-agent-state.ts`, `pi-memo-trust.ts`) being injected into an unrelated agent. The absolute `-e` path also loads before project trust resolves, so the extension works without trusting the directory. |
| `--no-skills` + `--skill <abs>` | `~/.agents/skills/*` leaking in. Probed leak: `["find-skills","modsearch","summarize"]` — `find-skills` actively instructs the agent to install more skills. |
| `--no-prompt-templates` | `~/.pi/agent/prompts/*.md` becoming callable slash commands. |
| `--no-themes` | pointless discovery I/O and a source of run-to-run variation. |
| `--no-builtin-tools` | `bash`/`edit`/`write`/`grep`/`find`/`ls` being active. Chosen over `--tools` because it imposes no registry-level allowlist, so an extension may register tools dynamically. Built-ins stay registered-but-inactive; the extension governs the active set. |
| `--approve` | required to load `.pi/SYSTEM.md` and `.pi/settings.json`. Combined with `--no-skills` it does **not** re-open skill auto-discovery. |
| `--session-id` + `--session-dir` | sessions colliding with the user's interactive history. |

### Do not use `--no-tools`

It disables extension tools as well, which removes `read`, which removes the
skills block from the system prompt (see `pi-runtime-notes.md` §1). Any
`SKILL.md` then becomes dead weight. This was a real, long-lived defect in the
Curator scenario.

---

## Required extension responsibilities

A scenario extension must do all five:

```typescript
const ALLOWED_TOOLS = ["read", /* … scenario tools … */];

export default function scenarioGuard(pi: ExtensionAPI) {
  // 1. path-restricted `read` override — shadows the built-in by name
  pi.registerTool({ name: "read", /* allowlist-checked implementation */ });

  // 2. scenario tools, each with promptSnippet so they appear in the prose list
  pi.registerTool({ name: "…", promptSnippet: "…", promptGuidelines: [ "…" ] });

  // 3. declarative active set, reasserted after resource discovery
  const restrict = () => pi.setActiveTools(ALLOWED_TOOLS);
  pi.on("session_start", async () => restrict());
  pi.on("resources_discover", async () => restrict());

  // 4. per-call gate — defence in depth against layer-3 mistakes
  pi.on("tool_call", async (event) => {
    if (!ALLOWED_TOOLS.includes(event.toolName)) {
      return { block: true, reason: `<scenario> blocks tool: ${event.toolName}` };
    }
  });

  // 5. every tool truncates its own output (50 KB / 2000 line caps)
}
```

The `read` override must still permit the skill directory, otherwise skill
bodies cannot be loaded (progressive disclosure, `pi-runtime-notes.md` §2).

---

## Measured effect

Same workspace, same extension, only flags differ:

| Configuration | system prompt | active tools | skills visible | coding-assistant persona | pi-docs paths |
|---|---:|---|---|---|---|
| `--no-tools` (Curator, as found) | 1859 | *(none)* | **no** | yes | yes |
| `--no-builtin-tools`, no isolation | 3413 | own + `read` | 3 foreign | yes | yes |
| `+ --no-skills --skill` | 2619 | own + `read` | **1, correct** | yes | yes |
| `+ .pi/SYSTEM.md --approve` | **960** | own + `read` | 1, correct | **no** | **no** |

Full baseline is 72 % smaller than the unisolated default *and* strictly more
capable, because the skill is finally reachable.

---

## Residual risk

1. **No kernel boundary.** Pi ships no sandbox; extensions run with the process's
   full permissions. The `read` override is application-level. A future step is
   bubblewrap (`examples/extensions/sandbox/`) or a container.
2. **`env` discipline is the only secret boundary.** Without an explicit `env=`
   allowlist the node process inherits every `*_API_KEY` in the unit file.
3. **Parent-directory context files.** Nothing prevents a future
   `~/AGENTS.md` from layering into every scenario, and `AGENTS.override.md`
   does **not** prevent it -- it only shadows its own directory (verified: a
   marker in `/tmp/AGENTS.md` still reached the prompt). The only effective
   switch is `-nc`, which means durable role text must live in
   `.pi/APPEND_SYSTEM.md` rather than `AGENTS.md`.
4. **Workspace should be read-only to the service.** `--approve` trusts
   everything project-local, so a writable `.pi/` is a code-execution path.
   Enforce with systemd `ReadOnlyPaths=`.
5. **Prompt injection is not solved by any of this.** Untrusted text must be
   delimited and labelled as data, and any write must be gated by deterministic
   code outside the model.

---

## Per-scenario conformance

| Scenario | L1 loading | L2 personality | L3 capability | L4 invocation | `env` minimal |
|---|---|---|---|---|---|
| `curator` | target | target | target | target | target |
| `memo-inbox` | **gap** | **gap** | ok (`setActiveTools`) | ok (`tool_call`) | **gap** |
| `pi-grok` | not assessed | not assessed | not assessed | not assessed | not assessed |

memo-inbox already implements L3 and L4 correctly and is the reference for those
layers; its L1/L2/`env` gaps are tracked as a follow-up and must not be changed
in the same pass that migrates its configuration into this repository.

Check conformance with:

```bash
scripts/pi-diff.sh <scenario>
```