{"id":48279360,"title":"Show HN: PrismCat – Local transparent proxy and debugging console for LLM APIs","points":2,"user":"etgpao","time":1779801086,"time_ago":"4 hours ago","type":"link","content":"

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:\nIt uses subdomain routing to achieve transparent proxying. For example, if you add an upstream named `openai`, PrismCat exposes `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:\n- Fully local-first: written in Go, stores data in SQLite, never sends your keys or prompts to a third party.\n- Interactive Playground: Tweak and replay failed requests in the UI without re-running your scripts.\n- SSE capture: Captures and reconstructs SSE chunks so you can inspect the raw flow.\n- 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!","url":"https://github.com/paopaoandlingyia/PrismCat","domain":"github.com","comments":[{"id":48279425,"level":0,"user":"alexwwang","time":1779801379,"time_ago":"4 hours ago","content":"

I guess it is useful for industrial scenario. Nice work. I have stared it.","comments":[]},{"id":48279389,"level":0,"user":"seacen","time":1779801229,"time_ago":"4 hours ago","content":"

sounds cool","comments":[]}],"comments_count":2}