MockOpenAI

A local mock server for OpenAI-compatible APIs — deterministic responses and per-request failure simulation for any Ruby application.

MockOpenAI lets you test any Ruby app that calls an LLM without hitting real APIs. It works with Rails, Sinatra, CLI tools, background jobs, or plain Ruby scripts.

Features

• Drop-in replacement for /v1/chat/completions
• Unified rule engine for success + failure behavior
• Per-request matching (exact, substring, regex)
• Simulate real LLM failures, including:
  • Timeouts
  • Rate limits
  • Malformed JSON
  • Internal server errors
  • Truncated streaming
• RSpec metadata API (works with Rails, Sinatra, or plain Ruby)
• Stateless local mock server for end-to-end and system tests
• File-based shared state (no client wrapping, no monkey-patching)
• Deterministic tests for CI/CD
• Zero token costs
• Fast, reliable CI