You can’t get a cryptographic guarantee, but you can look at three concrete signals:

Incentives.

    ElevenLabs has VC-scale funding and a “own the whole voice stack” narrative to justify it. Moving up into agents grows their TAM. Async is tiny, margin-driven infra. If they jump into end-user agents they’d start competing with (and losing) their own customers. Wrong game theory.

Portability.

    Their SDK is just a thin SSML/gRPC wrapper. It swaps out for Coqui / Azure / local models by changing an ENV var and rebuilding a Docker image. No proprietary DSL, no special voice-clone formats. Easy exits keep vendors honest.

Track record & roadmap.

    ElevenLabs is already shipping agent UIs. Async’s public roadmap stops at “more voices + lower latency.” If they suddenly pivot up-stack, the diff will be obvious months in advance and you can unplug.

TL;DR: nobody’s bullet-proof, but Async’s incentives and architecture make rug-pulling a lot less rational—and a lot easier to escape from—than betting everything on ElevenLabs. Keep an abstraction layer and always have a second TTS plugged in; problem solved.