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For a browser-based OpenAI Realtime voice session, we don’t relay the live audio stream through Vercel by default. We let the browser connect to OpenAI directly over WebRTC, after a small application-server endpoint issues a short-lived client secret. The standard OpenAI API key stays on the server.
This is a design choice, not a claim that Vercel cannot handle WebSockets: Vercel announced WebSocket support in public beta on June 22, 2026. The direct WebRTC approach follows OpenAI’s documented browser pattern and avoids adding a relay hop with its own runtime and connection-management requirements.
How the browser-to-Realtime flow works
- The browser requests a session credential. It calls an endpoint on the application’s server, which can authenticate the user and decide whether to allow a session.
- The server mints an ephemeral client secret. Using its protected standard OpenAI API key, the server requests a short-lived credential for the Realtime session.
- The server returns only the ephemeral secret. It does not send the standard API key to the browser.
- The browser connects to OpenAI over WebRTC. The browser creates a peer connection and uses the ephemeral secret to establish its Realtime session directly.
- The application server handles work that belongs to the application. Authentication, authorization, session configuration, and business or tool logic can remain server-side without relaying every live audio frame.
OpenAI also documents a unified WebRTC setup in which the browser sends an SDP offer to a developer-controlled server and that server creates the Realtime call. Choose that initialization flow when the backend needs more control at setup; it is distinct from continuously proxying the live audio connection. OpenAI’s WebRTC guide describes the available setup patterns.
Why use WebRTC for a browser client?
OpenAI distinguishes between client location and transport. Its Realtime WebSockets documentation describes WebSocket as a good fit for server-to-server Realtime applications, while saying WebRTC is more robust in most cases for browser or mobile clients. That guidance supports using WebRTC for a browser voice session rather than making the browser’s audio stream pass through an application-server WebSocket relay.
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A relay adds another service hop and brings the relay’s runtime, connection duration, scaling, and reconnect behavior into the path. That is an architectural trade-off, not a measured latency result: the cited official documentation does not establish a numerical latency, reliability, or cost advantage for direct WebRTC over a proxy.
What changed in Vercel’s WebSocket support?
Vercel announced public-beta WebSocket support for Vercel Functions on June 22, 2026. Its changelog announcement says WebSockets run on Fluid compute and follow the same Function limits and pricing model; Active CPU billing applies while messages are processed, rather than while a connection is idle.
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There is a documentation conflict: Vercel’s general Function limits page still says Functions do not support acting as a WebSocket server. For the current beta model, the newer, specific announcement and WebSocket Knowledge Base guidance are more directly applicable. Check Vercel’s current feature status and your project’s eligibility before designing a deployment around the beta.
Connection lifetime and deployment limits
Under Vercel’s documented beta model, an established WebSocket connection is pinned to a Function for that Function’s maximum duration. Later connections are not guaranteed to reach the same Function. The Vercel Functions Limits page, last updated December 18, 2025, lists Fluid compute maximum durations of 300 seconds (5 minutes) on Hobby and 800 seconds (13 minutes) on Pro and Enterprise for the listed Node.js and Python runtimes. These are deployment limits, not measured audio-session performance; actual settings depend on plan, runtime, and project configuration. Confirm the current limits for your deployment.
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Reconnects and state
Design clients to reconnect rather than assuming one Function will own every future connection. Do not treat a Function’s in-memory state as durable or shared across connections. Vercel’s WebSocket guidance recommends Redis for durable state that must persist across connections.
When a WebSocket relay is the better fit
Use a server-side WebSocket connection when the server itself is the trusted Realtime client, or when the application has a concrete requirement to inspect or transform every event or to keep ownership of the Realtime connection on the server. OpenAI presents WebSocket as a good fit for server-to-server Realtime work; this is a different client placement from a browser connecting directly.
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- Browser or mobile client: Prefer the documented ephemeral-secret and direct-WebRTC pattern in most cases.
- Trusted server process: Consider WebSocket when the server is the Realtime client or must process the event stream.
- Backend control during setup: Consider OpenAI’s unified WebRTC initialization pattern without turning it into a continuous audio relay.
- Vercel-hosted relay: Account for public-beta status, Function duration, reconnection, cross-connection state, and the applicable plan and runtime limits.
Keep the credential boundary clear
The standard OpenAI API key belongs on the server. A browser can receive the short-lived ephemeral client secret needed to establish its session, not the standard key. OpenAI documents the ephemeral-secret approach in its Realtime WebRTC guide and its Realtime client secrets guide.
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