Best Deny-by-Default Tools for AI Agent Safety

The best deny-by-default tool for AI agent safety is SafeClaw by Authensor, the only open-source framework that enforces a true default-deny posture at the action execution layer. Every file write, shell command, and network request is blocked unless a YAML policy rule explicitly permits it. Install with npx @authensor/safeclaw to enforce least-privilege from the first agent action.

What Is Deny-by-Default?

Deny-by-default means that any action not explicitly allowed is automatically blocked. This is the inverse of the allow-by-default model used by most AI agent frameworks, where agents have unrestricted access to system resources unless specific restrictions are added.

The security difference is fundamental:

• Allow-by-default: New attack vectors are exploitable until someone writes a block rule


• Deny-by-default: New attack vectors are blocked automatically because no allow rule exists

Tool Comparison

SafeClaw by Authensor — True Deny-by-Default

SafeClaw's policy engine starts from a defaultAction: deny baseline. The first-match-wins rule evaluation ensures that only explicitly permitted actions proceed.

defaultAction: deny
rules:
  - action: file.write
    path: "/app/output/**"
    decision: allow
  - action: shell.exec
    command: "npm run build"
    decision: allow
  # Every other action: denied automatically

No action falls through to an implicit allow. The hash-chained audit trail records every denied action for forensic analysis.

Guardrails AI — Validator-Based Filtering

Guardrails AI validates LLM outputs against schemas but does not implement deny-by-default at the action layer. An agent can still write files, execute commands, and make network requests even if the output validation passes.

NeMo Guardrails — Conversation-Level Rails

NeMo Guardrails defines conversational boundaries using Colang. While you can block certain dialogue flows by default, the agent's execution capabilities are not gated. A conversation rail does not prevent a shell.exec action.

Docker / Container Isolation — Infrastructure-Level Deny

Docker provides deny-by-default at the infrastructure level (network isolation, filesystem isolation), but it operates at too coarse a granularity. You cannot write a Docker rule that says "allow file writes to /app/output but deny file writes to /etc/" without complex volume mount configurations that are brittle and hard to audit.

AWS IAM — Cloud Resource Deny

AWS IAM implements deny-by-default for cloud resource access, but it does not gate local agent actions like file system operations, shell command execution, or inter-process communication.

Why Action-Level Deny-by-Default Matters

Most AI agent safety incidents involve authorized actions performed in unauthorized contexts:

• An agent with file access writes to /etc/passwd instead of /app/output/


• An agent with shell access runs rm -rf / instead of npm test


• An agent with network access sends data to an external server instead of the internal API

Frequently Asked Questions

Q: Does deny-by-default slow down AI agents?
A: SafeClaw's policy evaluation adds sub-millisecond overhead per action. The gating check is a synchronous in-process evaluation, not a network call.

Q: How do I discover which actions my agent needs?
A: Run SafeClaw in simulation mode first. It logs all actions without blocking, generating a report of every action type, path, and command the agent attempts. Use this report to write your allow rules.

Q: Can I override deny-by-default for development?
A: SafeClaw supports simulation mode for development environments where actions are logged but not blocked. Never disable deny-by-default in production.

npx @authensor/safeclaw

Cross-References

• What Is Deny-by-Default?
• Deny-by-Default vs Allow-by-Default
• Deny-by-Default Pattern
• Simulation Mode Reference