TTS API comparison 2026
Cartesia Sonic 3.5 vs OpenAI GPT-4o mini TTS Cartesia vs OpenAI · text-to-speech for voice agents
Cartesia Sonic 3.5 ranks 8th of 112 with 58.5/100; OpenAI GPT-4o mini TTS ranks 34th with 14.4/100: a gap of 44.1 points on the same public rules, built on Coval, Speko and Artificial Analysis.
Data checked on September 30, 2026 · Coval 30-day window ending October 1, 2026 · updated after each benchmark capture
Side-by-side benchmark
| Measure | Cartesia Sonic 3.5 | OpenAI GPT-4o mini TTS |
|---|---|---|
| Overall score | 58.5/100 | 14.4/100 |
| Rank | 8th of 112 | 34th of 112 |
| Real-time speed /60 | 37.1 | 1.3 |
| Voice quality /20 | 15.8 | 1 |
| Accuracy /15 | 4 | 10.4 |
| Price /5 | 1.6 | 1.7 |
| P50 / median time-to-first-audio Coval, 30-day window | 280 ms | 567.4 ms |
| P99 time-to-first-audio (slowest turns) Coval, 30-day window | 421.8 ms | 5896.6 ms |
| Latency consistency (standard deviation) Coval, 30-day window | 45.6 ms | 1575.6 ms |
| P50 time-to-first-audio, US East Speko | 121 ms | 691 ms |
| Accuracy: word error rate (WER) Coval, 30-day window | 5.8% | 4.7% |
| Pronunciation robustness (0 to 1) Speko | 0.2 | 0.4 |
| Speech Arena Elo (blind listener preference) Artificial Analysis | 1187 | not measured |
| Naturalness (Elo) Speko | 1574 | 1424 |
| Price per 1M characters Artificial Analysis | 49 USD | not measured |
Choose Cartesia Sonic 3.5 if…
- → Ranks 8th of 112 in the overall benchmark, against 34th for OpenAI GPT-4o mini TTS
- → Better P50 / median time-to-first-audio: 280 ms vs 567.4 ms (Coval, 30-day window)
- → Better P99 time-to-first-audio (slowest turns): 421.8 ms vs 5896.6 ms (Coval, 30-day window)
- → Better latency consistency (standard deviation): 45.6 ms vs 1575.6 ms (Coval, 30-day window)
- → Better P50 time-to-first-audio, US East: 121 ms vs 691 ms (Speko)
- → Better naturalness (Elo): 1574 vs 1424 (Speko)
Choose OpenAI GPT-4o mini TTS if…
- → Better accuracy: word error rate (WER): 4.7% vs 5.8% (Coval, 30-day window)
- → Better pronunciation robustness (0 to 1): 0.4 vs 0.2 (Speko)
Frequently asked questions
- What is the difference between Cartesia Sonic 3.5 and OpenAI GPT-4o mini TTS?
- In the 2026 TTS API benchmark, Cartesia Sonic 3.5 ranks 8th of 112 with 58.5/100 and OpenAI GPT-4o mini TTS ranks 34th with 14.4/100, a gap of 44.1 points on the same rules.
- Which is faster for real-time voice agents, Cartesia Sonic 3.5 or OpenAI GPT-4o mini TTS?
- On Coval's 30-day window ending October 1, 2026, the P50 time-to-first-audio is 280 ms for Cartesia Sonic 3.5 and 567.4 ms for OpenAI GPT-4o mini TTS; the P99 is 421.8 ms and 5896.6 ms.
- On which criteria does OpenAI GPT-4o mini TTS beat Cartesia Sonic 3.5?
- OpenAI GPT-4o mini TTS scores higher on Accuracy (10.4 vs 4 out of 15), Price (1.7 vs 1.6 out of 5).
- How are these two text-to-speech APIs scored?
- Both are scored by the same public rules: 4 criteria, 100 points (Real-time speed 60, Voice quality 20, Accuracy 15, Price 5), computed from three independent benchmarks: Coval, Speko and Artificial Analysis. A measurement a benchmark does not publish counts zero, and the model page says why.
- Can these numbers be checked?
- Yes. Each model page lists every measurement with its source and date, and links to the benchmark page it comes from. The rules are on the methodology page.