Arthur vs Fiddler AI
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.
Fiddler AI
Guardrails powered by purpose-built models that run inside your own VPC rather than calling an external API. That architecture removes the per-check inference cost that every judge-based guardrail quietly adds to your model provider bill.
| Arthur | Fiddler AI | |
|---|---|---|
| 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. | Regulated enterprises that need guardrails running inside their own environment, particularly those already running predictive ML alongside LLM systems, and who can work with enterprise procurement. |
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 Fiddler AI
Fiddler has made one architectural decision that sets it apart and it is the right one. Its guardrails run on purpose-built models - branded Fiddler Centor Models, previously Trust Models - that execute entirely within your own cloud or VPC, covering Safety, PII and Faithfulness. Almost every competitor implements guardrails by calling an LLM to judge each request, which means every check is a billable inference call landing on your foundation model provider's invoice, plus latency in the request path. Fiddler calls this the Trust Tax, and while that is marketing language the underlying criticism is accurate and it is the same cost trap we have flagged on NeMo Guardrails and on LLM-as-judge evaluation generally. Solving it with small purpose-trained models running in your environment is a genuinely better design, and it improves the privacy posture at the same time. Two caveats temper it. Nothing is published on pricing. And the VPC deployment that constitutes the main selling point starts at Enterprise, with the Developer tier running on Fiddler's cloud only - so the entry tier does not give you the thing that makes the product interesting.
Full Fiddler AI review →Frequently Asked Questions
What is the main difference between Arthur and Fiddler AI?
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. Fiddler AI: Regulated enterprises that need guardrails running inside their own environment, particularly those already running predictive ML alongside LLM systems, and who can work with enterprise procurement. 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 Fiddler AI?
It depends entirely on your workload shape, because they meter differently - Arthur bills on use-case and Fiddler AI 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 Fiddler AI?
Arthur: Yes, free. Fiddler AI: No or paid tier only. Free self-hosting means no licence fee, not no cost - you still own the infrastructure, upgrades and on-call.