OpenAI Realtime API vs. AssemblyAI Voice Agent API

Don't pay 4× more for worse speech accuracy. Get production-ready voice agents with the industry's best ASR and a developer experience that ships same-day:

$4.50/hr flat vs. ~$18/hr per-token billing
94.07% word accuracy
One WebSocket with no SDK required
Session resumption, live mid-call updates, and BAA available

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At a glance: OpenAI Realtime API vs. AssemblyAI's Voice Agent API

Model

AssemblyAI Voice Agent API

OpenAI Realtime API

Price

$4.50/hr flat

~$18/hr (per-token)

ASR model

Universal-3.5 Pro Realtime

GPT-4o multimodal

Word accuracy

94.07%

93.13%

Missed entity rate (names, emails, phones)

16.7%

23.33%

End-to-end latency

~1 second (~150ms P50 STT)

~1 second

Turn detection

Speech-aware VAD (semantic + neural)

Basic VAD

Mid-session updates

Prompt + voice + tools + VAD

Prompt + tools only

Session resumption

30-second reconnect window

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Tool calling behavior

Speaks naturally while waiting

Goes silent

Certifications

BAA-eligible, SOC 2 Type 2, ISO 27001

SOC 2

What AssemblyAI's Voice Agent API gives you that OpenAI doesn't

Industry-leading accuracy

Universal-3.5 Pro Realtime: 16.7% missed entity rate vs. 23.3% for OpenAI, so your agent captures the emails, phone numbers, and account IDs it needs to act.

$4.50/hr flat pricing

OpenAI charges per audio input and output token. AssemblyAI charges one flat rate covering STT, LLM reasoning, and TTS. At 5,000 hrs/month, that's $60,500/month in savings.

Speech-aware turn detection

Built into Universal-3.5 Pro Realtime, not bolted on. Distinguishes a thoughtful pause from a conversation ending. Doesn't cut you off mid-thought, doesn't leave dead air.

Session resumption

WebSocket drops on mobile or phone networks? Reconnect within 30 seconds—context preserved. OpenAI doesn't support this. Critical for any phone-based agent.

Natural voice generation

Purpose-built TTS voices tuned for conversation, not narration. Prosody, pacing, and intonation built for real-time dialogue.

Live mid-conversation updates

Change the system prompt, swap voices, add/remove tools, adjust VAD settings—all via JSON without dropping the connection. More comprehensive than OpenAI's partial implementation.

One dedicated pipeline

Universal-3.5 Pro Realtime for STT, a separate LLM for reasoning, purpose-built TTS for voice generation. Each model optimized for its specific job.

One WebSocket

No SDK required, works with any WebSocket client. Integrates natively with Claude Code, Cursor, and other AI coding tools. Most developers ship a working agent the same day.

Start building

Get your free API key and ship a working voice agent the same day—no credit card required.

Ready to ship a production voice agent?

Get higher ASR accuracy, speech-aware turn detection, and flat $4.50/hr pricing—most developers ship a working agent the same day.

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Investments in STT improvements always pay for themselves, since it is such a critical building block of the voice pipeline.

Lindsay Liu, Co-Founder & CEO at Super.

Playground

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Frequently asked questions

: AssemblyAI's Voice Agent API uses a dedicated pipeline—Universal-3.5 Pro Realtime for speech-to-text, a separate LLM for reasoning, and purpose-built TTS for voice—versus OpenAI's single multimodal model. The result is higher word accuracy (94.07% vs 93.13%), a lower missed entity rate (16.7% vs 23.33%), speech-aware turn detection, and session resumption, all at a flat $4.50/hr instead of per-token billing.
: OpenAI's Realtime API charges per audio input and output token, which compounds unpredictably as usage grows. AssemblyAI charges one flat $4.50/hr rate covering STT, LLM reasoning, and TTS. At 5,000 hours per month, that works out to roughly $60,500/month in savings.
: No. If a WebSocket drops on a mobile or phone network, the OpenAI Realtime API does not preserve the session. AssemblyAI lets you reconnect within a 30-second window with context preserved—critical for any phone-based agent.
: AssemblyAI is considered a business associate under HIPAA and offers a standard Business Associate Addendum (BAA) for customers processing protected health information (PHI). To execute a BAA, contact our sales team.
: Most developers ship a working agent the same day. The Voice Agent API needs no SDK, works with any WebSocket client, and integrates natively with Claude Code, Cursor, and other AI coding tools.