AssemblyAI

What it does

AssemblyAI is a speech-to-text API built for developers rather than end users — instead of a transcript editor or export screen, it provides a REST and streaming API that teams call from their own applications. Its core transcription models are paired with optional add-ons like speaker diarization, sentiment analysis, and PII redaction, so a team pays only for the specific audio-intelligence features their product needs.

This makes it a natural fit for engineering teams building transcription, call analysis, or voice-agent features into their own product, rather than for someone who just wants to transcribe a personal recording — for that, one of the consumer-facing tools on this page is a simpler starting point.

Ideal users

• Engineering teams building transcription, voice, or audio-intelligence features into their own product
• Startups needing a pay-as-you-go speech-to-text API without seat licenses or subscriptions
• Developers building voice agents or real-time transcription features that need low-latency streaming
• Teams that need add-on audio intelligence — speaker labels, sentiment, PII redaction — via API rather than a UI

Who should avoid it

• You want a finished transcription app with an editor and export options — Sonix, Trint, or Descript fit better
• You need guaranteed human-verified accuracy for legal or medical documents — Rev is built for that
• You have no engineering resources to integrate an API — a consumer-facing tool is a better fit
• You need a fully free, self-hosted option with no per-minute cost — Whisper is the better starting point

Key features

• Pay-as-you-go batch and real-time streaming transcription API, no subscription or seat licenses
• Universal-2 and Universal-3.5 Pro speech models with published accuracy benchmarks
• Add-on audio-intelligence models: speaker diarization, sentiment analysis, PII redaction, and more
• Voice Agent API bundling speech recognition, LLM reasoning, and text-to-speech for conversational apps
• SDKs and documentation for common languages and frameworks
• $50 in free credits on signup, no credit card required to start

Pricing

Paid — $0.15/hour (Universal-2, pay-as-you-go)

As of mid-2026, per AssemblyAI's official pricing page: pre-recorded transcription starts at $0.15/hour (Universal-2) and $0.21/hour (Universal-3.5 Pro); real-time streaming is similarly priced from $0.15/hour, with a Universal-3.5 Pro Realtime tier around $0.45/hour. The Voice Agent API (combining STT, LLM reasoning, and TTS) is billed at $0.075/minute, about $4.50/hour, all-inclusive. Add-ons stack on top of the base rate per hour of audio: speaker diarization adds $0.02 (standard) to $0.065 (experimental) for pre-recorded audio or $0.12 for streaming, sentiment analysis adds $0.02, PII text redaction adds $0.08, PII audio redaction adds $0.05, entity detection adds $0.08, translation adds $0.06, and medical mode, topic detection, and content moderation each add $0.15. In-region (US/EU) processing carries roughly a 10% premium over global routing. New accounts get $50 in free credits.

Typical workflows

• A developer integrates AssemblyAI's API into a customer-support platform to transcribe calls, tag sentiment, and redact PII automatically before storing call records.
• A startup builds a voice-agent product using AssemblyAI's Voice Agent API to handle speech recognition and response generation in one integrated pipeline rather than assembling separate STT, LLM, and TTS services.

Integrations

• REST API and streaming WebSocket API
• SDKs for major programming languages
• LLM Gateway for combining transcripts with downstream language-model processing

Privacy & security notes

AssemblyAI is SOC 2 Type II certified and PCI-DSS 4.0 Level 1 compliant, encrypts data at rest (AES-128/256) and in transit (TLS 1.2+), and lets customers choose US or EU data processing and storage, self-serve, via its Dublin, Ireland facility for EU. On the standard asynchronous API, audio files are deleted within 24-48 hours and transcripts begin deletion at 72 hours by default (configurable via a Time-To-Live setting down to 1 hour, or retained indefinitely if no TTL is set); the Streaming API offers zero data retention for customers who opt out of the model-training program. Training opt-out, TTL, and BAA requests can all be managed from the dashboard's Data Controls page.