pi-agent-config/docs/pi-runtime-notes.md

# Pi Runtime Notes

> Verified against **pi 0.84.3** on Debian 13 / node v22.23.2, 2026-08-27.
> Every claim below is either quoted from `dist/` source or reproduced by the
> probe harness in [`evidence/probe-harness/`](evidence/probe-harness/).
> Results: [`evidence/2026-08-27-isolation-probe.md`](evidence/2026-08-27-isolation-probe.md).

This file records behaviour that the official docs either do not state or state
only in passing, but which determines whether a dedicated Pi agent works at all.
Re-verify after every `pi update`.

---

## 1. The skills section requires an active `read` tool

`dist/core/system-prompt.js`:

```javascript
const hasRead = tools.includes("read");
// Append skills section (only if read tool is available)
if (hasRead && skills.length > 0) {
    prompt += formatSkillsForPrompt(skills);
}
```

`tools` is the **active** tool-name list, passed as `selectedTools` from
`agent-session.js:_rebuildSystemPrompt(this.getActiveToolNames())`.

**Consequence:** `--no-tools` silently removes the entire skills block.
Any `SKILL.md` is then unreachable, and `--skill <path>` becomes a no-op.

Probe evidence (`--no-tools` vs `--no-builtin-tools`):

| Flags | `available_skills` in prompt | prompt length |
|---|---|---|
| `--no-tools` | **false** | 1859 |
| `--no-builtin-tools` | true | 3413 |

This was a live defect in the Curator scenario: it ran `--no-tools --skill …`
for its whole lifetime, so its 64-line media policy never reached the model.

## 2. Skill injection is progressive, not full text

`formatSkillsForPrompt` emits only `<name>`, `<description>` and `<location>`
per skill, plus the instruction "Use the read tool to load a skill's file when
the task matches its description."

So the body of a `SKILL.md` costs nothing up front but **is only ever read via
the `read` tool**. A skill body is unreachable without a working `read`.

Corollary: if you override `read` with a path-restricted implementation, the
allowlist must include the skill directory or skill bodies cannot be loaded.

## 3. `--skill` does not double-load an auto-discovered skill

`dist/core/skills.js:loadSkills()` dedupes twice — by canonical realpath, and
by skill `name`. Defaults are added first, so on a name collision the
**auto-discovered** skill wins and a collision diagnostic is emitted.
Same file via symlink is skipped silently.

## 4. Tool gating semantics

| Flag | registry filter | initial active set | extension tools |
|---|---|---|---|
| *(none)* | none | `settings.defaultTools ?? [read,bash,edit,write]` | all activated |
| `--tools a,b` | **strict allowlist across every source** | `[a,b]` | must be named to exist |
| `--no-tools` | empty allowlist | `[]` | **also killed** |
| `--no-builtin-tools` | none | `[]` | **all activated** |
| `--exclude-tools x` | removes `x` | filtered | applies to all sources |

From `dist/core/agent-session.js`:

```javascript
const isAllowedTool = (name) =>
  (!allowedToolNames || allowedToolNames.has(name)) && !excludedToolNames?.has(name);
```

Under `--no-builtin-tools` the built-ins stay **registered but inactive**, so an
extension can re-enable them with `pi.setActiveTools([...])`. Under `--no-tools`
they were never registered and cannot be revived.

Probed: `--no-builtin-tools` + one extension →
`ACTIVE_TOOLS=["probe_plain_schema","read"]`, while `bash/edit/write/grep/find/ls`
remain listed as `[builtin]` and inactive.

## 5. `registerTool` accepts a plain JSON Schema object — verified

Declared type is TypeBox `TSchema`, but a raw JSON Schema object works:

```typescript
pi.registerTool({
  name: "probe_plain_schema",
  parameters: { type: "object", properties: { q: { type: "string" } }, required: ["q"] } as any,
  ...
});
```

Probe result: the tool appears as `probe_plain_schema[cli]` and is active.

**Why this matters:** a backend can serve its tool definitions as JSON Schema
over HTTP and a generic bridge extension can register them at runtime, so the
schema has exactly one source of truth. See `shared/extensions/`.

Note `--tools` is a *registry-level* allowlist, so dynamically registered tools
must either be named in `--tools` or you must use `--no-builtin-tools`
(no allowlist) and let the extension govern the active set.

## 6. An extension can override a built-in tool by name — verified

Registering `name: "read"` replaces the built-in in the registry: the probe
showed `read[cli]` instead of `read[builtin]` while other built-ins kept
`[builtin]`. This is the supported way to give an agent a path-restricted
`read` while still satisfying the requirement in §1.

`memo-guard.ts` in the memo-inbox scenario has used this in production.

## 7. Only tools with `promptSnippet` appear in the human-readable tool list

`dist/core/system-prompt.js`: "A tool appears in Available tools only when the
caller provides a one-line snippet."

The tool *schemas* are always sent through the provider's tool-calling API, so
an omitted `promptSnippet` does not break invocation — but the model loses the
prose overview. Always set `promptSnippet`, and `promptGuidelines` where useful.

## 8. `.pi/SYSTEM.md` replaces the default prompt; the replacement branch omits tools and guidelines

Default prompt (`system-prompt.js:82`) begins:

> You are an expert coding assistant operating inside pi, a coding agent harness.
> You help users by reading files, executing commands, editing code, and writing new files.

and ends with a block of **absolute paths to pi's own README / docs / examples**
plus "read the docs and examples, and follow .md cross-references before
implementing".

For a non-coding agent this is not merely noise: it is a ready-made escalation
path for prompt injection.

The `customPrompt` branch (`system-prompt.js:13-33`) concatenates only:

```
SYSTEM.md → APPEND_SYSTEM.md → <project_context>(AGENTS.md) → skills section → cwd
```

It does **not** include `toolsList` or `Guidelines`. A replacement `SYSTEM.md`
must therefore carry its own tool overview and output discipline.

`.pi/SYSTEM.md` lives under `.pi/`, so it requires **project trust** — pass
`--approve` in non-interactive modes.

Probe evidence:

| Combination | coding-assistant text | pi-docs block | SYSTEM.md applied | prompt length |
|---|---|---|---|---|
| isolation flags only | true | true | false | 2619 |
| `+ .pi/SYSTEM.md + --approve` | **false** | **false** | **true** | **960** |

## 9. Context files are loaded from every parent directory

Load order: `~/.pi/agent/AGENTS.md`, then each parent directory walking up from
cwd, then cwd. `AGENTS.override.md` replaces `AGENTS.md`/`CLAUDE.md` **for that
directory only**; other directories still layer normally.

So a stray `~/AGENTS.md` or `~/pi-workspaces/AGENTS.md` silently contaminates
every scenario rooted below it. Neither existed on this host as of 2026-08-27,
but nothing prevents one from appearing. Use `-nc` plus an explicit
`SYSTEM.md`, or an `AGENTS.override.md` in the workspace, to make the
personality deterministic. Probed: `-nc` removes the AGENTS.md marker.

## 10. Project trust gates `.pi/`, and CLI `-e` bypasses it

Trust is required when the directory contains `.pi/settings.json`,
`.pi/{extensions,skills,prompts,themes}`, `.pi/SYSTEM.md`,
`.pi/APPEND_SYSTEM.md`, or a project `.agents/skills`. A bare `.pi` does not
count.

Always loaded regardless of trust: `AGENTS.override.md`, `AGENTS.md`,
`CLAUDE.md`, user/global extensions, and **CLI `-e` extensions** — the last one
is deliberate so they can handle the `project_trust` event.

Non-interactive modes (`-p`, `--mode json`, `--mode rpc`) never prompt; they use
`defaultProjectTrust` (`ask` default / `always` / `never`) unless `--approve` /
`--no-approve` overrides for the run.

**Design consequence:** loading a scenario extension with an absolute
`-e /path/to/ext.ts` avoids the trust question entirely, which is the most
robust option for a systemd-managed gateway.

## 11. User-global resources leak into every scenario

Probed with no isolation flags, from an unrelated workspace, the skills list was:

```
PROBE_SKILLNAMES=["find-skills","modsearch","summarize"]
```

Those come from `~/.agents/skills/`. `find-skills` in particular instructs the
agent to discover and install further skills — an unwanted capability surface
for a narrow-purpose agent. `~/.pi/agent/extensions/*.ts` leak the same way.

With `--no-skills --skill <abs>` the list becomes exactly `["probe-skill"]`.

## 12. No output-schema flag; use a terminating tool with constrained sampling

There is no `--output-schema` / `--response-format`. Structured output is
achieved with a tool whose `parameters` is the desired schema:

```typescript
{
  name: "emit_result",
  parameters: <schema>,
  constrainedSampling: { type: "json_schema", strict: "prefer" },
  async execute(_id, params) {
    return { content: [...], details: params, terminate: true };
  }
}
```

`ConstrainedSamplingConfig`:

```typescript
| { type: "json_schema"; strict: "prefer" | "require" }
| { type: "grammar"; variants: Partial<Record<"openai_lark" | "openai_regex", string>> }
```

`strict: "prefer"` falls back gracefully; `"require"` fails the request if the
provider cannot honour it. `terminate: true` ends the turn only when *every*
finalized tool result in the batch sets it.

This replaces regex-scraping JSON out of prose.

## 13. Tools must truncate their own output

Built-in caps are **50 KB** and **2000 lines**. Helpers exported for this:
`truncateHead`, `truncateTail`, `truncateLine`, `formatSize`,
`DEFAULT_MAX_BYTES`, `DEFAULT_MAX_LINES`.

Errors: **throw** from `execute` to set `isError: true`. Returning an
error-shaped object does not mark the call failed.

`AgentToolResult.content` goes to the model; `details` is persisted in the
session but **not** sent to the model.

## 14. Per-call gating hook

```typescript
pi.on("tool_call", async (event) => {
  if (!ALLOWED.includes(event.toolName)) {
    return { block: true, reason: "…" };
  }
});
```

`ToolCallEventResult = { block?, reason?, terminate? }`. To rewrite arguments,
mutate `event.input` in place. Later handlers observe earlier mutations and no
re-validation happens afterwards.

Combined with `pi.setActiveTools()` on `session_start` and
`resources_discover`, this gives two independent layers of capability control
that survive a mistake in the CLI flags.

## 15. RPC mode is strict JSONL and is meant to be long-lived

`--mode rpc`: commands as JSON objects on stdin, one per line; responses and
events as JSON lines on stdout.

> Split records on `\n` only. Accept optional `\r\n` by stripping a trailing
> `\r`. Node `readline` is **not** protocol-compliant because it also splits on
> `U+2028`/`U+2029`, which are valid inside JSON strings.

33 commands, including `prompt`, `steer`, `follow_up`, `abort`, `new_session`,
`get_state`, `set_model`, `set_thinking_level`, `compact`,
`set_auto_compaction`, `get_session_stats`, `switch_session`, `fork`,
`get_entries`.

`message_update` carries **deltas only** — no cumulative `message`, no
`partial`. Treat `message_end.message` as authoritative.

There is **no** command to inject a tool result from the host. Host-side
capabilities must be exposed as an extension tool that calls back out.

Probing `get_state` alone starts the agent, fires `session_start`, and exits
without ever contacting the model — a zero-cost way to inspect the resolved
system prompt and tool set.

## 16. Usage and cost are recorded per assistant message

Every `AssistantMessage` carries:

```typescript
Usage { input, output, cacheRead, cacheWrite, totalTokens,
        cost: { input, output, cacheRead, cacheWrite, total } }
```

`ToolResultMessage`, `CompactionEntry` and `BranchSummaryEntry` carry an
optional `usage` for nested LLM work. Live equivalent: RPC
`get_session_stats`, which also returns
`contextUsage { tokens, contextWindow, percent }`.

Observability therefore needs no extra instrumentation — read the session JSONL
or subscribe to `message_update`.

## 17. Sessions never rotate or expire

`--session-id <id>` is an idempotent "use this project session, create if
missing" primitive (mutually exclusive with `--session`, `--continue`,
`--resume`; validated against `^[A-Za-z0-9](?:[A-Za-z0-9._-]*[A-Za-z0-9])?$`).

There is no built-in TTL, rotation, size cap or pruning. Files grow
monotonically until deleted by hand.

## 18. Auto-compaction rarely triggers on large-context models

Condition: `contextTokens > contextWindow - reserveTokens`.

```json
{ "compaction": { "enabled": true, "reserveTokens": 16384, "keepRecentTokens": 20000 } }
```

With a 1,050,000-token context window the threshold is ~1.03 M tokens, so
latency and cost degrade for a very long time before compaction ever fires.
A gateway must implement its own rotation policy — see
[`gateway-patterns.md`](gateway-patterns.md).

## 19. `allowed-tools` in SKILL.md frontmatter is not enforced

`dist/core/skills.js:loadSkillFromFile` consumes only `name`, `description` and
`disable-model-invocation`. `allowed-tools` is documented as experimental and is
**not read** in 0.84.3.

The memo-inbox scenario declares `allowed-tools:` in its `SKILL.md`; the real
enforcement is the `ALLOWED_TOOLS` array plus `setActiveTools` and the
`tool_call` hook inside `memo-guard.ts`. Do not rely on the frontmatter field
for security.

## 20. There is no sandbox

`docs/security.md`: "Pi does not include a built-in sandbox. Built-in tools can
read files, write files, edit files, and run shell commands with the permissions
of the pi process. Extensions are TypeScript modules that run with the same
permissions."

A path-restricted `read` override is an application-level boundary, not a kernel
one. Real isolation requires bubblewrap / container / micro-VM; pi ships
`examples/extensions/sandbox/` (bubblewrap, `.pi/sandbox.json`) and
`examples/extensions/gondolin/` (micro-VM) as starting points.

---

## Re-verification

```bash
cd docs/evidence/probe-harness
./collect-evidence.sh > ../$(date -u +%Y-%m-%d)-isolation-probe.md
```

The harness costs zero model tokens: it sends a single `get_state` over RPC and
reads the resolved system prompt from `session_start`.