Beacon: A Java Mock Server for LLM App Testing with Fault Injection
LazyTie3857 · reddit · 2026-07-30
A developer introduced Beacon, an open-source Java mock server tailored for testing LLM applications.
Since traditional HTTP mocks fall short for non-deterministic LLMs, Beacon focuses on simulating real-world LLM behaviors. Key features include:
- Compatibility with official OpenAI, Anthropic, and Gemini Java SDKs
- Prompt-based mocking instead of HTTP route matching
- Fault injection for common failure modes like empty responses, invalid/truncated JSON, and Markdown-wrapped JSON
The tool aims to help developers test their app's resilience against imperfect LLM outputs without spending tokens.
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