AssemblyAI

What AssemblyAI actually meters

Three different clocks, and the one you get depends on the endpoint rather than the plan.

Pre-recorded meters the duration of the file you submit, pro-rated to the exact second, and failed transcripts aren't charged. Multichannel bills per channel, so a one hour stereo file costs two hours.

Streaming meters the WebSocket session, open to close. Idle time counts. AssemblyAI's own FAQ states that a connection held open for 60 minutes carrying 30 minutes of audio bills 60 minutes, and that sessions left open auto-close at three hours and bill the full three. Concurrent sessions accumulate in parallel, so one call dual-streamed under two session IDs for five minutes bills ten.

Voice Agent meters connected conversation time, billed per second, and the $4.50 covers speech to text, the proprietary agent LLM, text to speech, turn detection and hosting with no per-layer add-on.

Then there's the part the rate table buries. Add-ons bill additively per hour on top of the model. Speaker diarization adds $0.02 async and $0.12 streaming, sentiment $0.02, entity detection $0.08, PII text redaction $0.08 async and $0.12 streaming, topic detection $0.15, medical mode $0.15. A Universal-3.5 Pro job with diarization, sentiment, PII redaction and topic detection costs $0.21 + $0.02 + $0.02 + $0.08 + $0.15 = $0.48 an hour. The add-ons add $0.27, which is 129% on top of the base rate.

How credits work

New accounts get $50 in credits with no card, and the credits don't expire.

They cover pre-recorded, streaming, Voice Agent, Speech Understanding and Guardrails. LLM Gateway is excluded and bills from your balance on the first request.

The pricing FAQ sizes that $50 at "up to 185 hours of pre-recorded transcription and up to 333 hours of streaming". The streaming figure checks out: $50 ÷ $0.15 = 333 hours. The pre-recorded one doesn't match any current model. At Universal-3.5 Pro's $0.21 it buys 238 hours, and at Universal-2's $0.15 it buys 333. It resolves only at $0.27 an hour, the retired SLAM-1 rate from January 2026.

After the credits, you deposit funds and draw them down, and auto-pay recharges the card at a threshold you set. SSO costs $199 per connection per month against the same balance, and adding one forces your auto-pay threshold up to $199 and locks auto-pay on.

What happens when you hit the limit

The balance hits $0 and the API stops, returning Your current account balance is negative. Please top up to continue using the API.

Auto-pay is the whole defence. Without it, a runaway streaming session drains the balance and the next request fails. AssemblyAI names improperly closed streaming sessions as the most common cause of negative balances in its own docs, which is a candid thing to publish.

Concurrency throttles rather than blocks. Free accounts open five new streams a minute, pay-as-you-go starts at 100, and the ceiling rises 10% whenever you use 70% of it, with no stated maximum. Enterprise accounts switch to end-of-month invoicing, which removes the $0 cliff.