retrieval-lens

A black-box flight recorder for RAG retrieval inside MCP agents.

retrieval-lens is an MCP server that logs every retrieval step your RAG agent makes — what chunks were retrieved, their scores, sources, and rankings — so you can audit, replay, and diff retrieval runs after the fact.

The Problem

When a RAG agent gives a wrong answer, you need to know: did retrieval fail, or did generation fail? Right now there's no easy way to answer that. Your observability tool shows you the LLM call. It doesn't show you which chunks the model saw before it answered, what scores they had, or how retrieval changed between yesterday and today.

retrieval-lens fixes that. Every retrieval run is logged. Nothing is hidden.

Demo

When your RAG agent gives a wrong answer, ask retrieval-lens what it saw:

await mcp.call("retrieval_diff", {
  run_id_a: "support-bot-before-embedding-refresh",
  run_id_b: "support-bot-after-embedding-refresh",
  match_by: "source"
});

See docs/demo-diff.png for real output from Claude Code.

MCP Tools

Tool	What it does
`retrieval_observe`	Log a retrieval run — query, chunks, scores, sources, rankings
`retrieval_query`	Replay what the model saw before a specific answer
`retrieval_diff`	Compare two retrieval runs — what changed, what score drifted
`retrieval_stats`	Aggregate score distributions, top sources, runs over time

Quickstart

Run retrieval-lens directly with npx:

npx retrieval-lens

Add retrieval-lens to Claude Code with one command:

claude mcp add retrieval-lens npx retrieval-lens

Then call retrieval_observe after every retrieval step in your RAG pipeline:

await mcp.call("retrieval_observe", {
  run_id: crypto.randomUUID(),
  query: "what is the refund policy?",
  chunks: [
    { content: "Refunds are processed within 5 days...", score: 0.91, source: "policy.md", rank: 1 },
    { content: "Contact support for refund requests...", score: 0.74, source: "faq.md", rank: 2 }
  ],
  pipeline_tag: "support-bot"
});

Adapters

LangChain

See examples/langchain-adapter.ts

LlamaIndex

See examples/llamaindex-adapter.ts

Why not LangSmith / Langfuse?

Those are full observability platforms. retrieval-lens is surgical:

Local-first — SQLite, zero signup, no data leaves your machine
MCP-native — one config line, works in any MCP client
Retrieval-only — focused on the layer where most RAG failures actually happen

Status

🚧 Active development. Harness-first build using harness engineering principles.

F05 — scaffold
F01 — retrieval_observe
F02 — retrieval_query
F03 — retrieval_diff
F04 — retrieval_stats

License

MIT

Retrieval Lens

README