You do not need a live LLM to reproduce most bugs in an AI chat interface.

These failures are ordinary UI-state problems:

the first token arrives but the spinner remains;

a tool finishes but its card still says Running;

Stop aborts the request, yet a buffered token appears afterward;

retry duplicates the partial answer;

an error removes text the user already received.

Make the stream injectable, then test those event sequences deterministically.

Define an application-level event contract

type ChatEvent = | { type : ' token ' ; messageId : string ; value : string } | { type : ' tool.started ' ; id : string ; name : string } | { type : ' tool.completed ' ; id : string ; summary : string } | { type : ' completed ' ; messageId : string } | { type : ' failed ' ; messageId : string ; message : string }

type ChatStream = ( prompt : string , signal : AbortSignal , ) => AsyncIterable < ChatEvent >

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The React component receives a ChatStream .…