AssemblyAI Universal 3.5 Pro vs GPT-4o Transcribe
AssemblyAI Universal 3.5 Pro is ahead of GPT-4o Transcribe on the Pipecat Dataset, the Ocular Dataset and final-text delay.
AssemblyAI Universal 3.5 Pro has a newer release. See AssemblyAI Universal 3.6 Pro vs GPT-4o Transcribe.
Last updated Methodology by Dileep Chagam
Accuracy vs speed
Each dot is one model. Bottom-left is best: fewer errors and a faster final transcript.
Which to pick
AssemblyAI Universal 3.5 Pro is the better pick over GPT-4o Transcribe on every measure that separates them: accuracy on clean, scripted audio (1.97 points lower), accuracy on real phone conversations (8.22 points lower) and quick turn-taking once the caller stops (472ms sooner).
The four metrics, side by side
- AssemblyAI Universal 3.5 Pro
- 1.93%2nd
- GPT-4o Transcribe
- 3.90%14th
AssemblyAI Universal 3.5 Pro, 1.97 points lower
- AssemblyAI Universal 3.5 Pro
- 4.23%10th
- GPT-4o Transcribe
- 12.45%17th
AssemblyAI Universal 3.5 Pro, 8.22 points lower
- AssemblyAI Universal 3.5 Pro
- 489ms2nd
- GPT-4o Transcribe
- No text until the speaker stops
Not comparable
- AssemblyAI Universal 3.5 Pro
- 180ms9th
- GPT-4o Transcribe
- 652ms17th
AssemblyAI Universal 3.5 Pro, 472ms sooner
Lower is better on every metric. The small figure is each model's place in the whole field. What each metric measures.
- WER · Word error rate
- Share of words the model got wrong: replaced, missing or added. Lower is better.
- Pipecat Dataset
- 1,000 short public clips from Pipecat's STT benchmark dataset.
- Ocular Dataset
- 8 recordings from 4 real phone conversations, licensed from Ocular.
- TTFT · Time to first text
- How long after the audio starts streaming until the first words appear.
- TTFS · Final-text delay
- How long after the caller stops until the full transcript arrives. A voice agent waits for this before it replies.
First text and final text, at three percentiles
AssemblyAI Universal 3.5 Pro shows first text after 1.08s and final text after 692ms, and GPT-4o Transcribe shows first text after 40.63s and final text after 1.97s, on a bad turn (P95) across the same streamed turns.
| Measure | Model | P50 | P90 | P95 |
|---|---|---|---|---|
| Time to first text | AssemblyAI Universal 3.5 Pro | 489ms | 966ms | 1.08s |
| GPT-4o Transcribe | No text until the speaker stops | |||
| Final-text delay | AssemblyAI Universal 3.5 Pro | 180ms | 532ms | 692ms |
| GPT-4o Transcribe | 652ms | 1.73s | 1.97s | |
- P50 / P90 / P95 · Typical, slow and slowest turns
- Half of turns are faster than P50. Only 1 in 10 is slower than P90, and 1 in 20 slower than P95.
How each ranks among all 17 models
Place among the 17 models on each metric. Lower is better on all four.
| Metric | Universal 3.5 Pro | 4o Transcribe |
|---|---|---|
| Pipecat Dataset WER | 1.93%2nd of 17 | 3.90%14th of 17 |
| Ocular Dataset WER | 4.23%10th of 17 | 12.45%17th of 17 |
| Time to first text | 489ms2nd of 15 | Not comparable |
| Final-text delay | 180ms9th of 17 | 652ms17th of 17 |
How much audio each model was tested on
| Model | Pipecat clips | Ocular recordings | Turns timed | Failed |
|---|---|---|---|---|
| AssemblyAI Universal 3.5 Pro | 1,000 | 8 | 205 | 1 |
| GPT-4o Transcribe | 1,000 | 8 | 206 | 0 |
Only clips with usable text count, so the two totals can differ. How WER is scored.
Related comparisons
Both providers, every model: AssemblyAI vs OpenAI
- AssemblyAI Universal 3.5 Pro vs AssemblyAI Universal 3.6 Pro
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- GPT-4o Transcribe vs GPT-4o Mini Transcribe
- GPT-4o Transcribe vs Deepgram Flux Multilingual
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- GPT-4o Transcribe vs Speechmatics Linden
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- GPT-4o Transcribe vs GPT Realtime Whisper
- GPT-4o Transcribe vs Reson8
- GPT-4o Transcribe vs Google Chirp 3
- GPT-4o Transcribe vs AssemblyAI Universal 3.6 Pro
- GPT-4o Transcribe vs Gradium
Frequently asked questions
Which is more accurate, AssemblyAI Universal 3.5 Pro or GPT-4o Transcribe?
AssemblyAI Universal 3.5 Pro is more accurate on both datasets. On the 1,000 public Pipecat clips, AssemblyAI Universal 3.5 Pro scores 1.93% and GPT-4o Transcribe 3.90%, so AssemblyAI Universal 3.5 Pro makes fewer errors by 1.97 points. On the eight licensed Ocular recordings, AssemblyAI Universal 3.5 Pro scores 4.23% and GPT-4o Transcribe 12.45%, so AssemblyAI Universal 3.5 Pro makes fewer errors by 8.22 points. Word error rate counts incorrect, missing, and extra words, so lower is better.
Is AssemblyAI Universal 3.5 Pro faster than GPT-4o Transcribe?
Yes. AssemblyAI Universal 3.5 Pro finishes the transcript 472ms sooner. AssemblyAI Universal 3.5 Pro has the final transcript 180ms after the speaker stops and GPT-4o Transcribe 652ms, at the median. Both are measured on the same 206 streamed turns.
Is AssemblyAI Universal 3.5 Pro or GPT-4o Transcribe better for voice agents?
AssemblyAI Universal 3.5 Pro is the stronger pick for voice agents: it makes fewer errors on real conversations and finishes the transcript 472ms sooner. A voice agent cannot reply until the final transcript arrives, and the Ocular recordings are real conversations.
Do AssemblyAI Universal 3.5 Pro and GPT-4o Transcribe stream text while the speaker is talking?
Only AssemblyAI Universal 3.5 Pro. GPT-4o Transcribe returns no text until the speaker stops, which matters if your agent reacts to partial transcripts.