How I test AI agent frontends without calling the API once

How I test AI agent frontends without calling the API once Testing AI agent applications is broken. Not the model calls — those you can mock. What nobody knows how to test is the streaming layer: the event sequence your frontend actually receives, the state transitions that happen across a multi-turn agent loop, the subtle timing between a tool_use and its tool_result. Most teams either skip this entirely or write flaky integration tests that hit the real API on every CI run. There's a better way, and it comes from a realization that took us longer to arrive at than it should have. A .jsonl recording is just a test fixture in disguise. Once you see it that way, your production streams become a regression test suite you're building automatically, whether you meant to or not. The problem with testing streaming frontends Consider what you're actually testing when you write a test for an AI agent UI. You care about: Does isStreaming flip to false after done? Does activeTools clear when too