Every vendor in this cluster gives something away before it charges, but the free tiers are not comparable. The bill also climbs in different currencies — units, spans, requests, traces or scored outputs — so the free tier's ceiling and the paid step's unit both matter. Langfuse Hobby is the most generous on features: all platform features, 50K units a month, 30 days of data access and 2 users, no credit card; Core at $29/mo is the cheapest paid step (100K units included, then $8/100K, 90-day retention, unlimited users), Pro at $199/mo adds three-year retention plus SOC2 and ISO27001, Enterprise at $2,499/mo adds audit logs and uptime SLA. Braintrust Starter is free forever with unlimited users, projects, datasets, playgrounds and experiments, but stops at 10K scores a month and jumps straight to $249/mo on Pro. Arize AX Free covers 25K spans, 1 GB of ingestion and 15 days of retention with unlimited users and evals — and ships Phoenix, a free open-source platform you can run entirely locally with no account. Helicone Hobby covers 10,000 requests with 7-day retention and is the fastest to install (request-level gateway, one line in front of your LLM calls); Pro at $79/mo removes the seat cap, Team at $799/mo adds SOC-2, HIPAA and 5 organizations. LangSmith Developer is free at 5K base traces but only for a single seat; Plus at $39/seat/mo (10K base traces) then metered at $1.00 per LangChain Standard Unit. If you want the widest free feature set with a real self-hosted option, pick Langfuse; if you are running evals and need unlimited seats before you pay, Braintrust's free tier is the only one that does; if your stack is already on LangChain, LangSmith's Developer seat is the natural first stop. Prices read from the vendor pages on 2026-10-03.
Free Hobby gives 50K units a month, 30 days of data access, 2 users and every platform feature, with no card required. Core is $29/mo for 100K units, 90-day access and unlimited users, and self-hosting is a documented option at every tier.
Free tier: Yes
Paid from: $29/mo (Core) (Usage is billed in units (100K units included on paid tiers, then $8 per additional 100K, lower at volume); a separate self-hosted pricing page exists for on-prem deployments)
Langfuse is the open-source-first LLM observability platform: free Hobby with 50K units a month, Core at $29/mo for 100K units and 90-day retention, Pro at $199/mo with three-year retention, Enterprise at $2,499/mo.
AX Free gives 25K spans, 1 GB of ingestion, 15-day retention and unlimited users and evaluations; AX Pro is $50/mo for 50K spans and 10 GB. Phoenix, the open-source platform, is free to self-host locally with no account.
Free tier: Yes
Paid from: $50/mo (AX Pro) (Billed on signals, spans, ingestion volume and retention rather than seats - users and evals are unlimited on every tier; self-hosted deployment is Enterprise-only, while the open-source Phoenix platform is free and local)
Arize AX is the cheapest paid step in the cluster at $50/mo, with a $0 Free tier (25K spans, 1 GB, 15-day retention) and a free open-source sibling in Arize Phoenix for fully local tracing.
Starter is $0 with unlimited users, projects, datasets, playgrounds and experiments, but only 10K scores a month and 14-day retention. The next step is Pro at $249/mo - the largest jump in the cluster.
Free tier: Yes
Paid from: $249/mo (Pro) (Scored outputs are the billing unit (LLM-as-a-judge, autoevals, custom scorers) alongside processed data and model credits; retention beyond 30 days is billed at $0.50/GB/mo up to 180 days on Pro)
Braintrust is the observability and eval platform with the most explicit score pricing in the cluster: Starter is $0 with 10K scores a month, Pro jumps to $249/mo with 50K scores plus cheaper per-1K rates.
Free Hobby covers 10,000 requests with 1 GB, one seat and 7-day retention, plus a pricing calculator so overage is easy to forecast. Pro is $79/mo with a 7-day free trial.
Free tier: Yes · 7-day free trial on Pro and Team
Paid from: $79/mo (Pro) (Requests are the billing unit and usage-based pricing applies on top of every paid tier; storage beyond 1 GB is usage-billed, and Helicone announced it has joined Mintlify)
Helicone is the request-level observability and gateway layer priced per request: Hobby free with 10,000 requests a month, Pro at $79/mo, Team at $799/mo, and usage-based pricing on top.
Developer is $0 with 5K base traces a month and 14-day retention, capped at a single seat. Plus is $39/seat with 10K traces, then $1.00 per LSU - so adding read-only viewers costs real money.
Free tier: Yes
Paid from: $39/seat/mo (Plus) (Trace volume is metered in LangChain Standard Units (LSU) at $1.00/LSU on top of the seat fee; base traces carry 14-day retention, extended traces 180 days, and retention auto-upgrades can raise your bill)
LangSmith is the observability and evaluation platform from the LangChain team: $0/seat with 5K base traces a month, Plus at $39/seat with 10K, then pay-as-you-go at $1.00 per LangChain Standard Unit.
Bottom line: Langfuse is the free-tier winner on volume and the cheapest paid step at $29/mo, and it is also the only one here that documents self-hosting as a supported option at every tier. Arize AX matches it on the things never billed - unlimited users and unlimited evals - and adds a genuinely free open-source sibling. Braintrust and Helicone are free but narrower, and LangSmith's free tier is the most constrained because it is also the only seat-priced product in the set.
Yes - four of the five have a standing free tier with no trial clock: Langfuse (50K units, 2 users), Braintrust (10K scores, unlimited users), Arize AX (25K spans, 1 GB) and Helicone (10,000 requests). LangSmith Developer is free but limited to one seat.
All of them state that sign-up needs no credit card, including Langfuse Hobby, which says so explicitly on its pricing page. LangSmith signs up through smith.langchain.com with Google, GitHub or email.
Langfuse Core at $29/mo, then Arize AX Pro at $50/mo, then Helicone Pro at $79/mo. LangSmith Plus is $39 per seat. Braintrust Pro is $249/mo, the priciest entry in the cluster.
Arize AX Free is the most generous in volume terms for production - 25K spans and 10 GB on Pro is real capacity - but at 10 signals a month on Free it is a monitoring tool, not a full observability program. For fully local work with no account, Arize Phoenix is the free open-source option.