LangSmith vs Langfuse vs Arize: Real LLM Observability Cost in 2026
The short answer
LangSmith prices usage in LangChain Storage Units at $1.00 each (its pricing page). Third-party trackers report that a base trace costs 0.005 LSU, or $5.00 per 1,000 traces; LangSmith's public pricing page does not confirm that per-trace rate, so this article treats it as a reported, derived assumption (older $0.50 and $2.50 per 1,000 rates are no longer used). Langfuse charges $29 a month for Core and graduated per-unit overage that falls from $8 to $6 per 100,000 units, with unlimited users. Arize AX Pro is $50 a month for 50,000 spans; its overage is reported at two different rates 80 times apart, so its high-volume cost cannot be stated as one figure. For the same three workloads, LangSmith would cost $167, $2,762 and $50,535 a month under the reported 0.005-LSU-per-trace assumption, against Langfuse Core at $34, $311 and $7,531. In workload B, and under that same assumption, LangSmith is cheaper than Langfuse Core only if a trace generates more than about 82 billable units; that threshold is specific to each scenario and plan.
How each platform charges
| LangSmith | Langfuse | Arize | |
|---|---|---|---|
| Free | Developer: 1 seat, 5,000 base traces/month | Hobby: 50,000 units, 2 users | AX Free: 25,000 spans, 1 GB, 15-day retention |
| Paid | Plus $39 per seat/month, 10,000 base traces | Core $29, 100,000 units, unlimited users; Pro $199 with the same units and 3-year data access | AX Pro $50: 50,000 spans, 10 GB, 30-day retention, unlimited users |
| Unit | Trace: 0.005 LSU ($5.00 per 1,000) and extended-retention upgrade 0.0025 LSU ($2.50 per 1,000), both reported and derived, not confirmed on LangSmith's pricing page | Unit = trace, observation or score | Span, plus ingested GB |
| Overage | $5.00 per 1,000 base traces (reported, derived from 0.005 LSU) | $8 per 100,000 units (100,000–1M), $7 (1M–10M), $6.50 (10M–50M), $6 (50M+) | $3 per GB above 10 GB; span overage reported at $0.0008 each (RFP.wiki, Langfuse) or $10 per million (UsagePricing, DevTune); one review describes AX Pro as hard-capped |
| Retention | 14 days base; extended traces up to 180 days for new traces from 14 Sep 2026 | 30 / 90 days / 3 years by plan | 15 / 30 days |
| Other meters | Deployment, Engine, Fleet and sandbox use LangChain Compute Units at $1.50 each; excluded here | None | None |
| Self-hosting | Enterprise only | Free, MIT license | Phoenix free |
Langfuse was reported acquired by ClickHouse in January 2026.
Workloads
Assumptions, illustrative: 8 billable units or spans per trace (12 for the agent workload); 5 KB per span (20 KB sensitivity).
| A: early product | B: growing production | C: high-volume agent | |
|---|---|---|---|
| Seats | 3 | 8 | 15 |
| Traces per month | 20,000 | 500,000 | 10,000,000 |
| Units or spans | 160,000 | 4,000,000 | 120,000,000 |
Monthly cost
| A | B | C | |
|---|---|---|---|
| LangSmith Plus (0.005 LSU per trace, reported) | 3 × $39 + 10,000 × $0.005 = $167 | 8 × $39 + 490,000 × $0.005 = $2,762 | 15 × $39 + 9,990,000 × $0.005 = $50,535 |
| Langfuse Core | $29 + 60,000 ÷ 100,000 × $8 = $33.80 | $29 + $72 + $210 = $311 | $29 + $72 + $630 + $2,600 + $4,200 = $7,531 |
| Arize AX Pro, span overage $0.0008 | $50 + $88 = $138 | $50 + $3,160 + $30 = $3,240 | $50 + $95,960 + $1,770 = $97,780 |
| Arize AX Pro, span overage $10 per million | $50 + $1.10 = $51.10 | $50 + $39.50 + $30 = $119.50 | $50 + $1,199.50 + $1,770 = $3,019.50 |
| Arize, 20 KB spans ($0.0008 / $10 per million) | $138 / $51.10 | $3,420 / $299.50 | $103,180 / $8,419.50 |
Langfuse bands for C: 100,000–1M at $8 (900,000 units = $72); 1M–10M at $7 ($630); 10M–50M at $6.50 ($2,600); 50M–120M at $6 ($4,200). Per trace at C: LangSmith $0.0051 (under the reported 0.005-LSU assumption), Langfuse $0.0008.
Evaluator and model costs are separate
Observability tools record traces; evaluator calls run on model APIs. As an illustration, a judge call of 1,500 input and 200 output tokens at an assumed $0.15 and $0.60 per million tokens costs $0.000345; scoring 10% of B's traces adds $17.25 a month and scoring every trace in C adds $3,450. The application's own model traffic is a separate bill.
Crossover
Each crossover holds only for its scenario. In workload B, LangSmith Plus costs $2,762 (assuming the reported 0.005 LSU per trace), and Langfuse Core reaches $2,762 at about 41.2 million units, or 82.5 units per trace; Langfuse Pro ($199 base) reaches it at about 38.6 million units, or 77.3 units per trace. At 39.0 million units, Core costs $2,616, so a threshold near 39 million describes Pro, not Core. In workload C, Core reaches LangSmith's $50,535 at about 836.7 million units (83.7 per trace). The thresholds differ because seat counts, trace volumes, graduated bands and plan fees differ, and they should not be applied to other workloads. Seats matter on LangSmith: ten Plus seats cost $390 before any traces.
What the advertised price misses
LangSmith seats scale linearly; Arize bills payload separately from spans, and its overage reading determines whether workload C is $3,020 or $97,780; retention is priced (extended traces add 0.0025 LSU each); self-hosting has no license fee but needs infrastructure.
Sources
LangSmith: langchain.com/pricing; trace rates as reported by ZenML, Sep 2026 and UsagePricing, 29 Jul 2026. Langfuse: langfuse.com/pricing (plans, 100,000 included units and graduated bands), Langfuse's comparison page, ZenML. Arize: arize.com/pricing; overage from Langfuse's comparison, 24 Jul 2026, RFP.wiki, 15 Jun 2026 and UsagePricing, 9 Jun 2026.