PrismCat – LLM API Debugging & Monitoring Proxy

Developers can't see what LLM SDKs like LangChain silently inject into prompts, making debugging streaming errors and function-calling issues extremely difficult. PrismCat is a single-binary, self-hosted transparent proxy that logs all LLM API traffic with a debugging UI. Target users: AI engineers and LLM application developers.

The explosion of LangChain, LlamaIndex, and similar abstraction layers has created a genuine debugging black box problem — developers shipping LLM apps in 2024-2025 routinely have no idea what token payload actually hits the API wire, and that gap is only widening as agentic frameworks add more prompt injection layers. LangSmith is the closest incumbent, but it's tightly coupled to the LangChain ecosystem and cloud-hosted, which makes it a non-starter for teams with data residency requirements or those using other frameworks. The $1k-5k/mo band is realistic but tight — this is a tool developers want free or near-free until it proves indispensable, so conversion from self-hosted to a paid tier requires a compelling reason like team collaboration features or alerting, otherwise it stays a glorified `mitmproxy` wrapper. The biggest risk is that LLM providers and framework authors solve observability natively, the way AWS eventually absorbed most CloudWatch-adjacent startups, leaving a narrow window before the pain point disappears.