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Spanlens is an MIT-licensed LLM observability platform that gives you full visibility into every request your app makes to OpenAI, Anthropic, or Gemini. It works as a drop-in replacement for the provider SDK, so you swap one import and start seeing data immediately. No agents to run, no infrastructure to wire up.
It's built for teams shipping LLM-powered products who need to answer real questions fast: why did the bill spike, which agent step is slow, did the new prompt actually improve quality?
Core capabilities:
Spanlens supports OpenAI, Anthropic, Google Gemini, Mistral, Azure, Bedrock, and Vertex, plus framework integrations for LangChain, LlamaIndex, Vercel AI SDK, and LangGraph. It also ingests OpenTelemetry spans over OTLP/HTTP.
For teams that can't send prompt data to a third party, it's fully self-hostable. Prompts and completions stay inside your own network. Data can be exported as JSON, CSV, or Parquet, or streamed to S3 or BigQuery.
If you're evaluating [LangSmith alternatives](/alternatives/langsmith), Spanlens covers similar ground with a flat monthly pricing model rather than per-seat fees, and a free self-hosted option that has no usage cap.
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Every Sunday we deconstruct one proprietary app and pick the best open source alternatives worth switching to.
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