Show HN: PrismCat – Local transparent proxy and debugging console for LLM APIs

A developer released PrismCat, a self-hosted transparent proxy and debugging console for LLM APIs that logs all requests and responses without sending data to third parties. The tool uses subdomain routing to capture traffic from SDKs like LangChain and OpenAI wrappers, allowing users to inspect streaming data and replay failed requests through an interactive UI.

Hi HN, I built PrismCat because I got tired of not knowing what LLM SDKs like LangChain or various OpenAI/Anthropic wrappers were silently injecting into my prompts behind the scenes. It's often hard to debug why a streaming request froze, or why a function-calling agent returned malformed JSON, without diving deep into SDK code. PrismCat is a single-binary, self-hosted proxy and UI designed to run silently 24/7 as an LLM "black box" logger. How it works: It uses subdomain routing to achieve transparent proxying. For example, if you add an upstream named openai , PrismCat exposes http://openai.localhost:8080 http://openai.localhost:8080 . Modern operating systems and browsers resolve .localhost to 127.0.0.1 automatically, so you don't even need to edit your /etc/hosts file. You just change one line in your code—the base url —and everything gets captured transparently including Server-Sent Events/streaming . Key features: - Fully local-first: written in Go, stores data in SQLite, never sends your keys or prompts to a third party. - Interactive Playground: Tweak and replay failed requests in the UI without re-running your scripts. - SSE capture: Captures and reconstructs SSE chunks so you can inspect the raw flow. - Request Override Opt-in : Intercept and rewrite outgoing requests/headers without code changes. I'd love to hear your feedback on the implementation especially the subdomain routing approach and what features you'd like to see next Comments URL: https://news.ycombinator.com/item?id=48279360 https://news.ycombinator.com/item?id=48279360 Points: 1 Comments: 2