Arthur vs HiddenLayer
Both are guardrails & safety tools. Here is how they actually differ on price, billing model and deployment.
Arthur
Open-sourced its real-time evaluation engine and, unusually for this segment, publishes actual prices. Free tier has unlimited seats; Premium is $60/mo. The meter is use cases rather than volume, which is worth understanding.
HiddenLayer
One of the last independent AI security vendors, and the only one here with a serious security research record - 48+ CVEs disclosed in ML frameworks. Still Series A while every direct competitor was bought by a security giant.
| Arthur | HiddenLayer | |
|---|---|---|
| Category | Guardrails & Safety | Guardrails & Safety |
| Our rating | 4/5 | 4/5 |
| Starting price | $60/mo | Not published |
| Billing meter | use-case | No usage metering |
| Free plan | Yes | No |
| Free self-hosting | Yes, free | No or paid tier only |
| Best for | Teams that want open-source guardrails running inside their own stack with the option of an inexpensive managed tier, and who value published pricing and unlimited seats. | Enterprises and government buyers who need model supply chain security, AI asset discovery and airgapped operation, rather than application-layer output filtering. |
Our verdict on Arthur
Arthur is the most transparent vendor in a segment that has almost none. It publishes actual prices - Free at zero with unlimited seats and four use cases, Premium at $60 a month for a hundred - while Lakera, Fiddler, HiddenLayer, CalypsoAI and Prompt Security all require a sales conversation before you can learn anything about cost. It also open-sourced the Arthur Engine, a real-time evaluation and guardrails service covering PII and sensitive data leakage, hallucination, prompt injection and toxicity, which runs locally inside your own stack rather than shipping prompts to a third party. That combination of published pricing, permissive licensing and in-environment execution is genuinely rare here. Two things to understand. The meter is use cases rather than requests or volume, and what constitutes a use case is not obvious from the pricing page, so establish that before budgeting. And Arthur Shield, the enterprise LLM firewall, is a separate product with custom pricing, so the transparent prices do not cover the whole guardrails story.
Full Arthur review →Our verdict on HiddenLayer
HiddenLayer is the most technically credible independent left in AI security, and it solves a different problem from most of this category. Where Guardrails AI validates output and Lakera detects prompt injection, HiddenLayer secures models and the pipeline around them - AI discovery, supply chain security, runtime defence and attack simulation, all model-agnostic and agentless. Supply chain in particular is genuinely under-covered, because a tampered model artefact bypasses every runtime guardrail you have deployed. The credibility signal that matters is research output rather than marketing - 48+ CVEs disclosed in ML frameworks and 25+ granted patents is substance, and very little in this segment can point to comparable work. The tension is capital. It remains Series A at roughly $56M while Protect AI went to Palo Alto, Robust Intelligence to Cisco, Lakera to Check Point and CalypsoAI to F5. Independence is a real virtue in a consolidated market, but competing against acquirers' balance sheets on that base is hard, and it makes HiddenLayer a conspicuous target.
Full HiddenLayer review →Frequently Asked Questions
What is the main difference between Arthur and HiddenLayer?
Arthur: Teams that want open-source guardrails running inside their own stack with the option of an inexpensive managed tier, and who value published pricing and unlimited seats. HiddenLayer: Enterprises and government buyers who need model supply chain security, AI asset discovery and airgapped operation, rather than application-layer output filtering. Both sit in Guardrails & Safety, so the decision usually comes down to billing model and deployment rather than raw capability.
Which is cheaper, Arthur or HiddenLayer?
It depends entirely on your workload shape, because they meter differently - Arthur bills on use-case and HiddenLayer bills on no usage metering. Published starting prices are $60/mo and Not published respectively, but those numbers are not comparable until you apply them to the same traffic. Use our cost calculator to model both against your own request volume and span count.
Can I self-host Arthur or HiddenLayer?
Arthur: Yes, free. HiddenLayer: No or paid tier only. Free self-hosting means no licence fee, not no cost - you still own the infrastructure, upgrades and on-call.