How to Test TypeScript AI Agents Without Making Real Model API Calls

Developers building AI agents in TypeScript often skip proper testing, relying on live runs that can miss critical failures like runaway loops or malformed tool responses. A lightweight interface called ModelClient can be used to inject a scripted fake client during tests, replacing real API calls entirely. The scripted client replays predefined responses in sequence, records all outgoing requests, and clamps to its last step to simulate infinite-loop scenarios safely. This approach allows developers to assert on turn limits, budget checks, and tool result pairing before any broken request reaches the API. The technique keeps production code unchanged while making edge-case agent behaviour fully testable in isolation.
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