I shipped a push-to-talk agent onto AssemblyAI's Sync Speech-to-Text API the week it landed. One HTTP POST, transcript in the same response, no job ID. Then security asked which region the bytes hit. The sample used sync.assemblyai.com. That host is global nearest-region routing, not a privacy default.
If you're hunting for an AssemblyAI Sync API alternative, you probably like that DX and still need audio under your control. I compared the wider products in Privocio vs AssemblyAI. This is the short-clip case: one request, and whether the hop off your network is acceptable.
Sync STT is the new default for short audio
A sync speech to text API returns a finished transcript in the same HTTP response as the upload. No polling, no webhook, no socket session. AssemblyAI launched that shape on July 14, 2026 for clips from 80 ms to 2 minutes (40 MB max), on Universal-3.5 Pro.
Their Sync vs async post puts median latency around 134 ms at p50 and prices Sync at $0.45/hr, same as Universal-3.5 Pro Realtime. Async pre-recorded is $0.21/hr. You pay more than batch because a person or an agent is waiting.
I use this path for dictation, IVR utterances, push-to-talk, and agent turns where we already own VAD. If you need overlapping partials and barge-in, stay on a socket. I wrote that budget in streaming speech-to-text over WebSockets.
The hidden cost of 134 ms
134 ms is a strong number. It's also a marketing number for round-trip transcription, not a privacy number. The audio still leaves your VPC or laptop and lands on a vendor GPU.
A support bot can fire dozens of short utterances per session. Each one is a copy of a customer's voice on someone else's accelerator, even if the JSON comes back fast. Our privacy policy is the opposite default: we don't train on your audio.
I don't have a published Privocio p50 that "beats" 134 ms, and I won't invent one. Measure latency with your codec and region. Once the LLM is in the loop, STT is rarely the only delay. The question that stalls deals is whether the bytes are allowed to leave.
Same geography argument in data residency for speech-to-text. Sync made the hop cheaper in latency, not smaller in trust.
EU residency is opt-in on AssemblyAI Sync
AssemblyAI documents three Sync hosts in Cloud endpoints and data residency:
sync.assemblyai.comis the default. It routes to the nearest region for speed.sync.us.assemblyai.compins US residency (us-west-2, us-east-1).sync.eu.assemblyai.compins EU residency (eu-north-1).
Nearest-region routing is a latency choice. It's not a GDPR choice. If a teammate copies the global host from a quickstart, EU audio can leave the Union until someone notices. I've watched that in code review: the SDK worked, staging was in Frankfurt, production used the default hostname because nobody changed it.
Opt-in residency is better than none. It's still a footgun. A private HTTP transcription API shouldn't require a special hostname to stop being global. I treat "remember to swap the host" as a bug waiting for a Friday deploy. Legal path: speech-to-text and GDPR.
What Sync-shaped private transcription should look like
I want the same developer motion as Sync: POST a short clip, read text, go. OpenAI-shaped POST /v1/audio/transcriptions counts. The private version has to cover two-minute clips without a job queue, keep audio on infrastructure you can name, and price agent volume as a line item instead of $0.45 times every turn.
Privocio Go and Pro take up to 15 minutes per request, so Sync's short-clip niche fits. Raw, Clean, and Agent output are in the API docs. Enterprise adds self-host. I walk the HTTP call in the Python speech-to-text API guide.
I'm not asking you to recreate Universal-3.5 Pro. I'm asking whether "POST audio, get text" has to mean "POST audio to a public cloud you don't operate."
AssemblyAI Sync vs private one-shot HTTP
| Dimension | AssemblyAI Sync | Privocio private HTTP |
|---|---|---|
| How you get text | One POST to /transcribe, no polling | One POST to /v1/audio/transcriptions (or /v1/transcriptions) |
| Latency class | ~134 ms p50 (vendor published) | Not a published p50 race; measure on your clips |
| Pricing | $0.45/hr metered | $19 / 400 hr / 4 weeks on Go |
| Where audio runs | AssemblyAI cloud, region follows host | Hosted-private, or Enterprise self-host |
| Default residency | Global nearest region | No training on your data; pin the deploy for EU/US needs |
| EU / US pin | Opt-in sync.eu / sync.us hosts | Contract and deploy path, not a forgotten hostname |
| Clip limits | 80 ms to 2 min, 40 MB | Go/Pro: 15 min, 100 MB |
| Best fit | Universal-3.5 Pro short clips if off-network audio is allowed | Sovereignty, flat cost, agent-ready output |
At 400 hours of Sync audio you're looking at about $180. Privocio Go is $19 for those 400 hours each 4-week cycle. Roughly 9.5x cheaper at that allowance. AssemblyAI still wins if you specifically need Universal-3.5 Pro and you already pin sync.eu or sync.us in every environment.
Keep the Sync DX, cut the hop
Most Sync call sites I see are one function: buffer the utterance, POST, read .text. You don't need a new architecture. You need a new base URL and a rule that audio can't hit the global vendor host.
If you already use the OpenAI Python SDK, Privocio is a base_url change:
from openai import OpenAI
client = OpenAI(
api_key="YOUR_PRIVOCIO_KEY",
base_url="https://api.privocio.com/v1",
)
with open("utterance.wav", "rb") as audio:
result = client.audio.transcriptions.create(
model="whisper-1",
file=audio,
language="en",
)
print(result.text)
Same idea in Node: set baseURL to https://api.privocio.com/v1. Raw HTTP is in the docs.
Then check the obvious: dictation still returns in one response, agent turns still POST a finished clip, and staging isn't secretly still on sync.assemblyai.com. If you needed partials, don't fake them with Sync. Use streaming. Try a clip on the transcribe tool before you rip out production keys.
When to stay on AssemblyAI Sync
Stay when Universal-3.5 Pro on short clips is the requirement, off-network processing is already approved, and you've pinned sync.eu or sync.us in Terraform, not a wiki. Their async API still has diarization and Speech Understanding that Sync doesn't. If you need those, Sync was never the whole product.
Switch when audio sovereignty is the deal, default-global routing makes GDPR reviewers nervous, or agent-scale hours make $0.45/hr a line item. Also switch when you want Agent-mode transcripts for an LLM, as in speech-to-text for AI agents.
I've left teams on AssemblyAI Sync. I've also failed a security review because the default host was still in the Helm chart.
Frequently asked questions
What is an AssemblyAI Sync API alternative?
A private one-shot HTTP transcription API with the same shape as AssemblyAI Sync: POST a short clip, read the transcript in that response. Privocio does this on /v1/audio/transcriptions with OpenAI SDK compatibility, flat 4-week pricing, and a no-training rule.
Does AssemblyAI Sync keep audio in the EU by default?
No. The default host sync.assemblyai.com routes to the nearest region for latency. EU residency requires sync.eu.assemblyai.com. US residency requires sync.us.assemblyai.com. I treat the global host as unsafe for EU production until the pin is in code.
How does AssemblyAI Sync pricing compare to Privocio?
Sync is $0.45/hr of audio. At 400 hours that's about $180. Privocio Go is $19 for 400 hours per 4-week cycle on the pricing page. AssemblyAI's number buys Universal-3.5 Pro and a published 134 ms p50.
Can I keep one-shot HTTP transcription without WebSockets?
Yes. That's the point of both Sync and Privocio's HTTP routes. Use a WebSocket or SSE stream only when you need partials or barge-in. For push-to-talk and VAD-segmented agent turns, one POST is enough.
When should I stay on AssemblyAI Sync?
When you need Universal-3.5 Pro, off-network processing is approved, and every environment uses the residency host you think it uses. If any of those is false, look at a private HTTP path first.
Conclusion: Keep Sync DX, drop the vendor hop
Sync made short-form STT feel like a normal JSON API, and that DX is worth copying. The part I won't copy is default-global audio routing and a metered $0.45/hr bill on every agent turn. If you're building voice agents that already own VAD, keep the one-shot POST and put the bytes on infrastructure you can audit.
Start with the pricing table or a clip on transcribe. For LeMUR and the rest of the product, use Privocio vs AssemblyAI. For the privacy frame, the private speech-to-text API guide is the hub.