/common-agent-guardrails

name: common-agent-guardrails
description: Define deterministic guardrails for agent tool calls — protected paths, test-file locks during bug fixes, post-edit formatters, production approval gates, secret deny rules. Use when writing or reviewing a hook policy, or moving an always-do-X rule out of prose into enforcement.
metadata:
  triggers:
    files:
      - "guardrails.yaml"
      - "hooks.json"
      - ".claude/settings.json"
      - "**/hooks/*.py"
      - "**/hooks/*.js"
    keywords:
      - guardrail
      - protected path
      - block edit
      - approval gate
      - hook policy
      - pretooluse

Agent Guardrail Standard

**Priority: P1 (HIGH)**

A rule that must always hold belongs in a hook, not in a prompt. Prompts persuade; hooks decide.

1. What Belongs in a Guardrail

• **Absolute compliance only**: encode a rule here when a single violation is unacceptable, not when it is merely preferred.
• **Deterministic check**: the decision must be computable from tool name, arguments, and repo state — never from model judgment.
• **Paired with a skill**: the skill teaches the rule, the guardrail enforces it. Ship both or the rule decays.

2. Required Guardrail Classes

| Class | Trigger | Decision | | --- | --- | --- | | Protected paths | edit to generated, vendored, or frozen files | block | | Secret deny | edit or read of `.env*`, `.ssh/`, `credentials*.json\|yaml`, identity files | block | | Test lock | test-file edit while a bug-fix workflow is active | block | | Formatter | after any accepted edit | run, then report | | Production action | deploy, migration, or destructive command against production | ask named owner | | Scope fence | writes outside the declared task scope | ask |

3. Decision Semantics

• **Exit 0**: allow, stay silent. Reserve stdout for the formatter class.
• **Exit 2**: block. Print the rule violated and the sanctioned alternative on stderr, never a generic denial.
• **Any other exit**: ask a human. Use it when the check is inconclusive, not as a soft block.
• **Fail closed on production classes**: if the guardrail cannot evaluate, treat production actions as blocked.

4. Test Lock During Bug Fixes

• Write the failing test first; the agent proves the bug reproduces before touching production code.
• Lock every test path for the duration of the fix so the proof cannot be weakened into passing.
• Unlock only when the fix is verified, or when the operator states the test itself is the defect.

5. Placement and Ownership

• **Team level**: repository-tracked config, reviewed in pull requests like code.
• **Organization level**: managed settings the project cannot override; use for secret deny and production gates.
• **Log every decision**: timestamp, rule, tool, target, and outcome. An unlogged block is unauditable.
• **Version the policy**: a guardrail change is a control change and needs the same review as the control it enforces.

Anti-Patterns

• **No prompt-only rules**: If it must always hold, enforce it in a hook.
• **No broad path globs**: Name the protected directories, not the whole repo.
• **No silent blocks**: Print the rule and the allowed alternative.
• **No agent self-approval**: A human role authorizes production actions.
• **No guardrail without a skill**: Pair enforcement with the rule that explains it.
• **No unlogged decisions**: Record every block and ask.

Red Flags

• **Stop if "just this once, disable the hook"**: Re-run with the sanctioned path or escalate to the policy owner.
• **Stop if the fix edits the failing test**: Restore the test and fix the production code.
• **Stop if a production command runs with no named authorizer**: Block and request sign-off.

References

• [Guardrail Policy Template](references/guardrails-template.yaml)
• [Runtime Adapters](references/runtime-adapters.md)

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• exit 2 blocks
• protected paths
• test lock
• approval gate
• fail closed
• log the block decision