Both run experiments, score production traffic and offer content guardrails. Fiddler fits enterprises that want one vendor for ML model monitoring and agent observability, with low-latency guardrails; Opik fits teams that want an open-source platform for agent tracing, evals and prompt work. Neither offers approval holds on agent actions.[1][4]
The difference between Fiddler and Opik is scope and licence: Fiddler is a commercial platform that monitors classic ML models, LLM applications and agents in one place, with real-time content guardrails and VPC or on-premises deployment on Enterprise. Opik, from Comet, is an Apache-2.0 tracing and evaluation platform for LLM applications and agents, free to self-host, with prompt versioning and a prompt optimiser that Fiddler does not offer.
| Decision factor | Fiddler | Opik |
|---|---|---|
| Ownership | Independent; $30M Series C in January 2026, $100M raised in total | Comet ML, independent; reports $70M in total funding |
| Scope | Classic ML models, LLM applications and agents | LLM applications and agents |
| Licence | Proprietary platform; Python client is Apache-2.0 | Apache-2.0 for the core platform and SDKs |
| Self-hosting | VPC or on-premises on Enterprise | Free; all features except user administration |
| Guardrails | Real-time content scoring; inline blocking through a third-party gateway | Blocking call your code makes; fails closed |
| Prompt tooling | None documented | Prompt Library, Agent Playground, Prompt Optimizer |
| Human review | Span annotations in public preview; no review queue | Annotation queues |
| Paid pricing | Developer: $0.002 per trace | Pro Cloud: $19 a month for 100k spans |
| Capability | Fiddler | Opik | Prefactor |
|---|---|---|---|
| Observe | |||
| LLM call tracing | ✓ | ✓ | ✓ |
| Multi-step agent tracing (tool calls, nested spans) | ✓ | ✓ | ✓ |
| Cost tracking | Partial | ✓ | ✓ |
| OpenTelemetry support | ✓ | ✓ | ✓ |
| Evaluate | |||
| Offline evals (datasets, experiments) | ✓ | ✓ | ✓ |
| Online evals on production traffic | ✓ | ✓ | ✓ |
| LLM-as-a-judge scoring | ✓ | ✓ | ✓ |
| Code-based evaluators | ✓ | ✓ | ✓ |
| Human review and annotation | Partial | ✓ | ✓ |
| Prompt versioning and playground | — | ✓ | — |
| Act on findings | |||
| Runtime controls on live agent actions | Partial | Partial | ✓ |
| Gateway or proxy in the request path | — | Partial | — |
| Licence and deployment | |||
| Open source | Partial | ✓ | Partial |
| Self-hosting | ✓ | ✓ | Partial |
Fiddler covers traditional ML monitoring, including drift, performance, explainability and fairness, alongside LLM and agent observability. Since early 2026 it has positioned itself as a control plane for AI agents, and it raised a $30M Series C in January 2026, bringing its total funding to $100M. For an organisation that already monitors tabular or classic models, one vendor for both may matter more than any single feature.
Opik is built for LLM applications and agents, and does not document classic model monitoring. Its strength is breadth of instrumentation: integrations from LangGraph and CrewAI to OpenAI Agents, Google ADK, the Claude Agent SDK and dozens more, and SDKs in Python, TypeScript, Java and .NET. Fiddler's native agent SDKs (LangGraph, LangChain V1, Strands Agents and Google ADK) are Python only; other languages go through raw OpenTelemetry or the REST API.
Fiddler Guardrails score content in real time, covering harmful content across 11 dimensions, faithfulness, PII and PHI, and secrets, using Fiddler's own Centor Models. Your application decides what to block. Inline blocking or redaction without code changes needs a third-party AI gateway such as Kong, AgentGateway, LiteLLM or Portkey; Portkey support with inline redaction arrived in October 2026. Fiddler documents no human approval step or tool-call policy for agent actions.
Opik's guardrails run as a blocking validate() call in your code that fails closed and raises an exception, covering PII, topics, prompt injection, judge checks and custom classifiers. PII and topic checks are English only, and self-hosted guards need a separate guardrails server, with a GPU recommended. Like Fiddler, Opik has no approval or hold workflow. The practical difference is placement: Fiddler can sit in a gateway you already run, while Opik's checks live in application code.
Both run experiments against datasets and score live traffic. Fiddler's Evaluator Rules apply evaluators to new production spans that match filters, with backfill and downsampling to limit judge cost, using 13 pre-built evaluators, custom judges on Fiddler-hosted models and Python functions wrapped as evaluators. Opik's online rules score sampled traces, spans and threads with more than 30 built-in metrics, custom judges and custom Python metrics.
The gaps are on Fiddler's side here. Its human review is span-level annotations, in public preview since August 2026, with no review queue, while Opik has annotation queues that can fill automatically from feedback-score conditions. Fiddler documents no prompt versioning or playground; its Prompt Specs are templates for judge evaluators. Opik has a Prompt Library with environment-tagged versions, an Agent Playground and a Prompt Optimizer with six algorithms.
Both accept OpenTelemetry. Fiddler takes OTLP directly, maps the GenAI semantic conventions, offers its own OTel SDK and can ingest OTLP files from S3. Opik accepts OTLP over HTTP only. On cost, Fiddler stores monetary cost only when the trace source emits cost attributes, such as LiteLLM's, and an API for judge spend is in private preview. Opik tracks token usage and model cost on traces and lets you cap judge spend per evaluation.
As of 20 October 2026, Fiddler's free plan covers guardrails only and is reached through a demo request. The Developer plan is $0.002 per trace on SaaS and adds observability, experiments, custom evaluators, RBAC and SSO. VPC and on-premises deployment sit on Enterprise, which is quote-based, and Fiddler can also run inside your own AWS account as an Amazon SageMaker Partner AI App.
Opik's Free Cloud covers 25k spans a month with 60-day retention, Pro Cloud is $19 a month for 100k spans and then $5 per 100k, and self-hosting is free. Fiddler bills per trace and Opik per span, and a trace can hold many spans, so compare on your own traffic: at 100,000 traces a month, Fiddler's Developer rate comes to $200. See Fiddler's pricing and Opik'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 can act on a failing score through runtime policies on tool calls and data access, approval holds and a kill switch for a run, an agent or a team. Fiddler and Opik both offer content guardrails, but neither documents approval holds or tool-call policies. Prefactor has no prompt playground or prompt registry, does not monitor classic ML models, and its platform is hosted. 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.