These two solve different problems, and only one is taking new customers. Helicone has been in maintenance mode since March 2026 and new cloud sign-ups are disabled; Fiddler is under active development, with tracing, online evaluation and real-time guardrails. For a new project Fiddler is usually the practical choice, and Helicone suits teams already running it while they plan a move.[1][6]
Fiddler is an actively developed commercial platform for monitoring, evaluating and guarding ML, LLM and agent systems; Helicone is an open-source LLM request logger and gateway that has been in maintenance mode since March 2026, with new cloud sign-ups disabled. On 19 October 2026 Helicone said it is no longer part of Mintlify, with no new owner named. For a new deployment Fiddler is usually the practical choice; existing Helicone users should plan a migration.
| Decision factor | Fiddler | Helicone |
|---|---|---|
| Status | Independent and actively developed; $30M Series C in January 2026 | Maintenance mode since March 2026; no longer part of Mintlify as of 19 October 2026, no new owner named |
| New accounts | Self-serve Developer plan; Enterprise through sales | Cloud sign-ups disabled since March 2026 |
| Licence | Proprietary platform; Python client is Apache-2.0 | Apache-2.0; separate Rust gateway is GPL-3.0 |
| Evaluation | Experiments, online evaluator rules, LLM judges and code evaluators | Datasets and posted scores; by its own docs not an evaluation framework |
| Agent tracing | Hierarchical traces across agents, tools and LLM calls | Partial: header-based session grouping |
| Gateway | None of its own; integrates with Kong, AgentGateway, LiteLLM and Portkey | Yes: OpenAI-compatible gateway for 100+ providers |
| Run-time checks | Partial: real-time content scores; inline blocking through a third-party gateway | Partial: prompt-injection and jailbreak blocking, OpenAI models only |
| Self-hosting | Enterprise: VPC, on-premises or SageMaker | Free via Docker or Kubernetes |
| Capability | Fiddler | Helicone | Prefactor |
|---|---|---|---|
| Observe | |||
| LLM call tracing | ✓ | ✓ | ✓ |
| Multi-step agent tracing (tool calls, nested spans) | ✓ | Partial | ✓ |
| Cost tracking | Partial | ✓ | ✓ |
| OpenTelemetry support | ✓ | Partial | ✓ |
| Evaluate | |||
| Offline evals (datasets, experiments) | ✓ | Partial | ✓ |
| Online evals on production traffic | ✓ | Partial | ✓ |
| LLM-as-a-judge scoring | ✓ | Partial | ✓ |
| Code-based evaluators | ✓ | — | ✓ |
| Human review and annotation | Partial | Partial | ✓ |
| Prompt versioning and playground | — | ✓ | — |
| Act on findings | |||
| Runtime controls on live agent actions | Partial | Partial | ✓ |
| Gateway or proxy in the request path | — | ✓ | — |
| Licence and deployment | |||
| Open source | Partial | ✓ | Partial |
| Self-hosting | ✓ | ✓ | Partial |
Mintlify announced its acquisition of Helicone on 3 March 2026 and said the product would move to maintenance only: patches, bug fixes and support for new models, with help for customers moving to other platforms. Cloud sign-ups have shown "Sign ups are disabled" since the same day, and the feature changelog has no entries after November 2025. On 19 October 2026 Helicone's site began stating that it is no longer part of Mintlify, with details to follow and no new owner named.
In practice that settles most new decisions. A team starting today cannot open a Helicone Cloud account on any plan, and a self-hosted install would run on code that receives fixes rather than features. Fiddler raised a $30M Series C in January 2026 and has kept shipping releases since, including a hosted judge model in September and Portkey integration in October. For a new deployment Fiddler, or another actively developed tool, is the practical choice.
Helicone is a request logger and gateway first. Its OpenAI-compatible gateway routes calls to more than 100 providers with failover, caching and rate limits, and it logs cost per request and per session. It also has prompt versioning with environments and a playground. Its documentation says plainly that it does not run evaluations for you, Python evaluators were turned off in October 2026, and agent runs are grouped by session headers rather than a span model.
Fiddler is an evaluation and monitoring platform. It traces agents hierarchically, runs experiments with 13 pre-built evaluators, scores production spans with evaluator rules, and monitors classic ML models for drift and fairness. Its guardrails score content in real time, with inline blocking when it sits behind a third-party gateway. It has no gateway of its own and no prompt versioning.
Cost tracking is one area where Helicone is stronger. It records exact cost per request through its gateway, rolls cost up by session and custom property, carries pricing for more than 300 models and sends alerts at graduated thresholds. Fiddler tracks token usage across agent interactions and stores monetary cost only when the trace source emits cost attributes, as LiteLLM does. If per-request spend reporting is what you use Helicone for most, check that the replacement covers it before you switch. Helicone also had a four-day observability outage in May 2026 when its AWS account was locked; the proxy stayed up and queued logs were processed afterwards.
If you run Helicone today, separate its three jobs and replace each one. Logging: Fiddler accepts OTLP traces and offers native Python SDKs for LangGraph, LangChain, Strands Agents and Google ADK, so instrumentation moves to OpenTelemetry. Routing: if your calls go through Helicone's proxy, point them at the providers directly or at a gateway Fiddler integrates with, such as LiteLLM or Portkey. Prompts: Fiddler has no prompt versioning, so prompts kept in Helicone need another home.
Self-hosted Helicone is Apache-2.0 and keeps working, so there is no forced cut-over date. Plan the move while the code still receives fixes, and run both in parallel for a period so you can compare costs and traces before switching off the old path. Note that Helicone's own prompt-injection blocking works with OpenAI models only, so check what replaces it.
As of 20 October 2026, Helicone lists Hobby free with 10K requests, Pro at $79 a month and Team at $799 a month, but new cloud accounts cannot be opened on any of them. Fiddler's Developer plan charges $0.002 per trace and is hosted only; its free plan covers guardrails only and is reached through a demo request, and VPC or on-premises deployment is Enterprise. See Fiddler's pricing and Helicone's pricing.
We sell the layer this section describes. Read it with that in mind.
Stated plainly, since Prefactor publishes this page: Prefactor is an agent evaluation platform that scores each step of a run and acts on what it finds through runtime policies on tool calls and data access, approval holds for risky actions, and a kill switch for a run, an agent or a team. For teams leaving Helicone it is one of several actively developed options. Fiddler's guardrails check content rather than tool actions, and Helicone's blocking is limited to prompt injection on OpenAI models. Prefactor has no prompt playground or prompt registry, no gateway, and its platform is hosted, so it does not replace Helicone's routing or prompt features. See the full comparison of 15 platforms.
Reviewed against public product and documentation pages on October 20, 2026. If a vendor has changed a feature, product name, or positioning since then, send a correction and we will update it. Numbered source links in the page body point to the ordered sources below.
Prefactor scores each step of every agent run and can hold or stop an action when a score says it should not happen. Start on the free tier: 5,000 spans a month, no credit card required.
Prefactor helps teams observe, evaluate, and improve their AI agents in production, across every framework and provider.