policyshield 0.5.0

Declarative firewall for AI agent tool calls

🛡️ PolicyShield

Declarative firewall for AI agent tool calls.

Write rules in YAML → PolicyShield enforces them at runtime → get a full audit trail.

LLM calls web_fetch(url="...?email=[e-mail]")
      │
      ▼
  PolicyShield intercepts
      │
      ├─ PII detected → REDACT → tool runs with masked args
      ├─ Destructive cmd → BLOCK → tool never executes
      └─ Sensitive action → APPROVE → human reviews first

---

Installation

pip install policyshield

Or from source:

git clone https://github.com/mishabar410/PolicyShield.git
cd PolicyShield
pip install -e ".[dev]"

---

Quick Start (Standalone)

Step 1. Create a rules file rules.yaml:

shield_name: my-agent
version: 1
rules:
  - id: no-delete
    when:
      tool: delete_file
    then: block
    message: "File deletion is not allowed."

  - id: redact-pii
    when:
      tool: [web_fetch, send_message]
    then: redact
    message: "PII redacted before sending."

Step 2. Use in Python:

from policyshield.shield import ShieldEngine

engine = ShieldEngine("rules.yaml")

# This will be blocked:
result = engine.check("delete_file", {"path": "/data"})
print(result.verdict)  # Verdict.BLOCK
print(result.message)  # "File deletion is not allowed."

# This will redact PII from args:
result = engine.check("send_message", {"text": "Email me at [e-mail]"})
print(result.verdict)  # Verdict.REDACT
print(result.modified_args)  # {"text": "Email me at [EMAIL]"}

Step 3. Validate your rules:

policyshield validate rules.yaml
policyshield lint rules.yaml

Or scaffold a full project:

policyshield init --preset security --no-interactive

---

Using with Nanobot

PolicyShield integrates with nanobot to enforce policies on all tool calls your agent makes.

Step 1. Install PolicyShield alongside nanobot

# In your nanobot project:
pip install policyshield

Step 2. Create rules for your agent

Create policies/rules.yaml in your project root:

shield_name: my-nanobot
version: 1
rules:
  # Block dangerous shell commands
  - id: block-rm-rf
    when:
      tool: exec
      args_match:
        command: { contains: "rm -rf" }
    then: block
    message: "Destructive shell commands are not allowed."

  # Redact PII from any outgoing messages
  - id: redact-pii-messages
    when:
      tool: send_message
    then: redact

  # Block all file deletion
  - id: block-delete
    when:
      tool: delete_file
    then: block
    message: "File deletion is disabled."

Step 3. Run nanobot through PolicyShield

The simplest way — just prefix your usual command:

policyshield nanobot --rules policies/rules.yaml agent -m "Hello!"
policyshield nanobot --rules policies/rules.yaml gateway

Or if you create AgentLoop in your own Python code:

from nanobot.agent.loop import AgentLoop
from policyshield.integrations.nanobot import shield_agent_loop

loop = AgentLoop(bus=bus, provider=provider, workspace=workspace)
shield_agent_loop(loop, rules_path="policies/rules.yaml")  # ← one line

That's it. Every tool call your agent makes will now pass through PolicyShield. Blocked tools return an error message to the LLM, which replans automatically.

What happens under the hood

shield_agent_loop() monkey-patches your existing loop instance (no nanobot source changes needed):

1. Wraps the ToolRegistry — every execute() call is checked against your rules
2. Filters blocked tools from LLM context — the LLM never sees tools it can't use
3. Injects constraints into the system prompt — the LLM knows what's forbidden
4. Scans tool results for PII — post-call audit and tainting
5. Tracks sessions — rate limits work per-conversation

Optional: standalone mode (no AgentLoop)

You can also use PolicyShield with nanobot's ToolRegistry directly, without AgentLoop:

from policyshield.integrations.nanobot.installer import install_shield

# Create a shielded registry
registry = install_shield(rules_path="policies/rules.yaml")

# Register your tools
registry.register_func("echo", lambda message="": f"Echo: {message}")
registry.register_func("delete_file", lambda path="": f"Deleted {path}")

# This works:
result = await registry.execute("echo", {"message": "hello"})
# → "Echo: hello"

# This is blocked:
result = await registry.execute("delete_file", {"path": "/etc/passwd"})
# → "🛡️ BLOCKED: File deletion is disabled."

Configuration options

shield_agent_loop(
    loop,
    rules_path="policies/rules.yaml",  # Required. Path to YAML rules
    mode="ENFORCE",       # ENFORCE (default) | AUDIT (log only) | DISABLED
    fail_open=True,       # True (default): shield errors don't block tools
)

See the full nanobot integration guide for approval flows, custom PII patterns, rate limiting, and more.

---

Rules DSL

rules:
  # Block by tool name
  - id: no-destructive-shell
    when:
      tool: exec
      args_match:
        command: { regex: "rm\\s+-rf|mkfs|dd\\s+if=" }
    then: block
    severity: critical

  # Block multiple tools at once
  - id: no-external-pii
    when:
      tool: [web_fetch, web_search, send_email]
    then: redact

  # Human approval required
  - id: approve-file-delete
    when:
      tool: delete_file
    then: approve
    approval_strategy: per_rule

# Rate limiting
rate_limits:
  - tool: web_fetch
    max_calls: 10
    window_seconds: 60
    per_session: true

# Custom PII patterns
pii_patterns:
  - name: EMPLOYEE_ID
    pattern: "EMP-\\d{6}"

Built-in PII detection: EMAIL, PHONE, CREDIT_CARD, SSN, IBAN, IP, PASSPORT, DOB + custom patterns.

---

Features

Category	What you get
YAML DSL	Declarative rules with regex, glob, exact match, session conditions
Verdicts	ALLOW · BLOCK · REDACT · APPROVE (human-in-the-loop)
PII Detection	EMAIL, PHONE, CREDIT_CARD, SSN, IBAN, IP, PASSPORT, DOB + custom patterns
Async Engine	Full async/await support for FastAPI, aiohttp, async agents
Approval Flow	InMemory, CLI, Telegram, and Webhook backends with caching strategies
Rate Limiting	Sliding-window per tool/session, configurable in YAML
Hot Reload	File-watcher auto-reloads rules on change
Input Sanitizer	Normalize args, block prompt injection patterns
OpenTelemetry	OTLP export to Jaeger/Grafana (spans + metrics)
Trace & Audit	JSONL log, stats, violations, CSV/HTML export
Rule Testing	YAML test cases for policies (policyshield test)
Rule Linter	Static analysis: duplicates, broad patterns, missing messages, conflicts

---

Other Integrations

LangChain

from policyshield.integrations.langchain import PolicyShieldTool, shield_all_tools

safe_tool = PolicyShieldTool(wrapped_tool=my_tool, engine=engine)
safe_tools = shield_all_tools([tool1, tool2], engine)

CrewAI

from policyshield.integrations.crewai import shield_crewai_tools

safe_tools = shield_crewai_tools([tool1, tool2], engine)

---

CLI

policyshield validate ./policies/          # Validate rules
policyshield lint ./policies/rules.yaml    # Static analysis (6 checks)
policyshield test ./policies/              # Run YAML test cases

policyshield trace show ./traces/trace.jsonl
policyshield trace violations ./traces/trace.jsonl
policyshield trace stats ./traces/trace.jsonl --format json
policyshield trace export ./traces/trace.jsonl -f html

# Run nanobot with PolicyShield enforcement
policyshield nanobot --rules rules.yaml agent -m "Hello!"
policyshield nanobot --rules rules.yaml gateway

# Initialize a new project
policyshield init --preset security --no-interactive

---

Docker

# Validate rules
docker compose run policyshield validate policies/

# Lint rules
docker compose run lint

# Run tests
docker compose run test

---

Examples

Example	Description
nanobot_shield_example.py	Nanobot standalone — run this to see PolicyShield in action
nanobot_shield_agentloop.py	AgentLoop configuration reference
nanobot_rules.yaml	Example policy rules for nanobot
langchain_demo.py	LangChain tool wrapping
async_demo.py	Async engine usage
policies/	Production-ready rule sets (security, compliance, full)

---

Development

git clone https://github.com/mishabar410/PolicyShield.git
cd PolicyShield
python -m venv .venv && source .venv/bin/activate
pip install -e ".[dev,langchain]"

pytest tests/ -v                 # 570 tests
ruff check policyshield/ tests/  # Lint
ruff format --check policyshield/ tests/  # Format check

📖 Documentation: mishabar410.github.io/PolicyShield

---

Roadmap

Version	Status
v0.1	✅ Core: YAML DSL, verdicts, PII, trace, CLI
v0.2	✅ Linter, hot reload, rate limiter, approval flow, LangChain
v0.3	✅ Async engine, CrewAI, OTel, webhooks, rule testing, policy diff
v0.4	✅ Nanobot: monkey-patch, CLI wrapper, session propagation, PII scan
v0.5	✅ DX: PyPI publish, docs site, GitHub Action, Docker, CLI init
v1.0	📋 Stable API, dashboard UI, performance benchmarks

See ROADMAP.md for the full roadmap including v0.6–v1.0 and future ideas.

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License

MIT