@ramarivera/pi-model-guardrails

A safety extension for the Pi coding agent. It does two things:

1. Deterministic destructive-command guard. A native-TS port of the rule engine from destructive_command_guard (DCG) — ~200 regex rules across 10 packs (git, filesystem, system, package managers, containers, kubernetes, infrastructure, remote, platform, ci/cd) that block catastrophic shell commands (rm -rf ~, git reset --hard, kubectl delete --all, docker compose down -v, …) before they run.
2. Inviolable-constraint policy + a deviation state machine. A project declares constraints it must never violate. When a deviation is detected, the session arms a state machine that routes every subsequent tool call through a non-deterministic LLM grader until the model has provably steered back on track — and won't silently backslide.

The deterministic guard is the hard floor: it cannot be downgraded by config or talked around by the model. The policy + grader layers sit on top to catch the softer "the model is drifting off the rails" failure mode that pattern rules alone can't see.

How it works

Every bash tool call flows through one pipeline:

command
  │
  ▼
┌──────────────────────────────────────────────────────────┐
│ 1. Engine (deterministic)                                 │
│    normalize → candidate packs → regex match              │
│    + Tier-3 indirection resolution (x=rm; $x -rf ~)       │
│    → strictest decision wins                               │
└──────────────────────────────────────────────────────────┘
  │  EngineDecision (allow / warn / deny + severity)
  ▼
┌──────────────────────────────────────────────────────────┐
│ 2. Policy engine                                          │
│    Critical / inviolable floor (config CANNOT downgrade)  │
│    > per-rule override > allowlist > defaultMode > engine  │
└──────────────────────────────────────────────────────────┘
  │  Verdict
  ▼
┌──────────────────────────────────────────────────────────┐
│ 3. Deviation state machine                                │
│    COMPLIANT → WATCH → GATED → RECOVERING → HALTED         │
└──────────────────────────────────────────────────────────┘
  │  if armed & call looks clean → "gate-required"
  ▼
┌──────────────────────────────────────────────────────────┐
│ 4. LLM grader (only while armed)                          │
│    grades the call vs the active constraints + recent      │
│    actions; a clean grade advances recovery, a dirty       │
│    grade holds the gate. Fails TOWARD the gate.            │
└──────────────────────────────────────────────────────────┘

In a clean (COMPLIANT) session the grader never runs — calls are adjudicated by the deterministic engine alone, with effectively zero overhead. The grader only wakes up once the session is armed.

The deviation state machine

State	Meaning
COMPLIANT	Normal. Deterministic guard only; no grading.
WATCH	A soft signal fired. Mutating calls start getting graded.
GATED	A deviation is confirmed. Every call is graded before it runs.
RECOVERING	The model is producing clean graded calls; counting consecutive clean calls toward recovery.
HALTED	An inviolable / critical constraint was hit. Terminal for the model — no tool call can clear it.

Properties that hold by construction:

• Strictest-wins merge — parallel/competing signals never soften the state.
• Consecutive (not cumulative) recovery — one dirty call resets the recovery streak.
• Grade-immunity of the hard floor — a clean LLM grade can never rescue a deterministically-denied command.
• Fail-toward-gate — grader timeout, crash, or malformed output holds the gate rather than waving the call through.
• Resume-safe — state is persisted to a session entry and rehydrated on resume/fork; a stateEpoch defends against a fork "resetting" an armed session.

Recovering from HALTED

HALTED is deliberately terminal for the model: an inviolable-constraint breach cannot be cleared by any tool call, graded or not. The escape hatch is a human:

/guardrails-clear-halt

A human types this slash command in the Pi TUI. It shows what was violated and requires an interactive y/n confirmation before clearing the halt back to COMPLIANT. It is gated on an interactive UI (it refuses to clear blind in RPC/print mode), and the model cannot reach it — slash commands originate in the human input editor, never in the model's tool-call stream.

Tier-3 indirection resolution

The regex packs match on literal command text, so one level of shell indirection can hide a sink:

x=rm;  $x -rf ~                 # variable indirection
alias d=rm;  d -rf ~            # alias expansion

The engine resolves these structurally (a bounded, quote-aware pure-TS pass) and re-runs the same rules on the expansion, so an aliased rm -rf is caught by the very rule that catches a bare rm -rf. It is fail-open to the regex engine — on any parsing surprise it simply defers, so it can only add detection, never remove the baseline. Interprocedural function-wrapper dataflow and eval/base64 decoding are out of scope by design.

This is the "native structural analysis" layer, implemented in pure TS rather than via @ast-grep/napi: ast-grep's napi binding has no built-in Bash grammar, and a per-platform native grammar binary that can fail to load is the wrong dependency for a guard that must always load.

Configuration

Config is layered: a global file is merged with a per-project file (project wins). Both are optional — with no config you get the deterministic guard with sensible defaults.

• Global: ~/.pi/agent/guardrails.json (or $PI_CODING_AGENT_DIR/guardrails.json, or $PI_MODEL_GUARDRAILS_CONFIG)
• Project: <cwd>/.pi/guardrails.json (JSONC — comments allowed)

{
  // Pin which constraints can NEVER be downgraded by config or allowlist.
  "policy": {
    "defaultMode": "warn",        // deny | warn | log | allow
    "inviolable": ["no-history-rewrite"],
    "rules": {                    // override a specific engine rule's mode ("<pack.id>:<rule-name>")
      "core.git:reset-hard": "deny"
    },
    "constraints": [
      {
        "id": "no-history-rewrite",
        "title": "Never rewrite shared git history",
        "statement": "Do not force-push or hard-reset shared branches.",
        "severity": "inviolable",          // inviolable | critical | high | medium | low
        "allowlistable": false,
        "appliesWhen": "branch is shared",
        "requiredBehavior": "Ask the human to rebase locally instead.",
        "detect": { "ruleIds": ["core.git:reset-hard"], "regex": "push\\s+--force" }
      }
    ],
    "allowlist": [
      // ttl is an ABSOLUTE expiry as a Unix epoch in MILLISECONDS (the entry is
      // inert once Date.now() >= ttl); omit it for no expiry.
      { "rule": "core.filesystem:rm-rf-general", "reason": "scratch dir cleanup", "ttl": 1893456000000, "paths": ["/tmp/build"] }
    ]
  },

  // The degraded-mode grader. Defaults shown; secrets come from env, never inline.
  "grader": {
    "enabled": true,
    "model": "gemini-3.5-flash",
    "fallbackModel": null,
    "apiKeyEnv": "GEMINI_API_KEY",   // read process.env[...] — do NOT inline keys
    "baseUrlEnv": null,
    "timeoutMs": 8000,
    "maxTokens": 512,
    "maxRetries": 1,
    "temperature": 0.1,
    "cache": true
  },

  // Deviation state-machine tuning (defaults are sensible).
  "machine": {
    "watchCleanStreak": 2,
    "gatedCleanStreak": 3,
    "recoveringWatermark": 1,
    "cooldownTurns": 2,
    "haltRequiresHumanAck": true
  },

  // Optional model gating + telemetry.
  "modelWhitelist": [],
  "modelBlacklist": [],
  "observability": { "enabled": true, "logFile": ".pi/model-guardrails/events.jsonl" }
}

Notes:

• The grader is on by default. If it is enabled but no model/key can be resolved, the gate fails closed (armed sessions block gated calls) and the extension warns loudly at session start.
• apiKeyEnv / baseUrlEnv name environment variables; the key/URL are read from process.env at load. Never put a secret in the file.
• allowlistable: false on a constraint forbids ever allowlisting around it. Wildcard allowlist entries require riskAcknowledged: true.

Install

pi install npm:@ramarivera/pi-model-guardrails

Source map

Path	Responsibility
src/engine/	Deterministic DCG-port engine: normalize, matcher, evaluate, rm-parser, indirection, registry, packs/*
src/policy/	Inviolable-constraint policy engine (resolvePolicy)
src/state/	Deviation state machine (transition, clearHalt)
src/grade.ts	LLM grader: prompt, timeout, retry, cache, fail-toward-gate
src/guard.ts	Pure composition: engine → policy → state machine
src/extension.ts	Pi wiring: event handlers, state persistence, steering injection, /guardrails-clear-halt
src/config.ts	Layered JSONC config loader

Local development

This checkout is live-enabled for Pi through .pi/extensions/model-guardrails/index.ts, which imports src/index.ts → src/extension.ts. Tests, the package entrypoint, and manual Pi runs all load the same symbol, so behavior does not drift.

npm install
npm run check       # biome + tsc
npm test            # unit suite
npm run test:e2e    # loads through the live .pi shim
npm run pack:dry-run

Publishing

Publishing uses GitHub Actions trusted publishing (.github/workflows/publish.yml); no NPM_TOKEN required. Configure npm trusted publishing for ramarivera/pi-model-guardrails before the first publish.

Credits

The deterministic rule engine is a faithful native-TS port of destructive_command_guard by Jeffrey Emanuel. The policy engine, deviation state machine, LLM grading gate, and Tier-3 indirection resolver are additions specific to this extension.