ContextLens — py-spy/pprof but for what's inside your LLM prompt

In multi-turn agent loops, the full context re-sends on every API call. A tool result added at turn 3 gets billed again at turns 4, 5, 6, 7... forever. Most of it is never read again. Standard observability tools tell you the total token count. They never tell you what's in there or how much of it is waste . That's what ContextLens fixes. What it does ContextLens is a diagnostic profiler for LLM agent context windows. It: Decomposes the context window into regions: system prompt, tool schemas, tool results, retrieved chunks, user messages, assistant messages Tracks which blocks get re-billed across turns using SHA-256 content hashing Runs 5 waste detectors and ranks findings by dollar cost Prints a concrete one-line fix for each finding Renders an interactive D3 treemap report as a self-contained HTML file No API key required. Works offline on saved traces. The five detectors Detector What it finds Duplicate Same block re-sent verbatim across multiple turns Near-Duplicate >85% Jaccard similarity between distinct blocks Stale Tool Result Tool output never referenced by a later assistant message Unused Tool Schema Tool defined every turn but never called Redundant Retrieval Retrieved chunk with <15% overlap with model output ---Run the built-in demo (simulates a 30-turn agent loop, no API key needed): python -c "import contextlens; contextlens.demo()" python examples/demo.py Live capture — Anthropic import anthropic import contextlens as cl client = anthropic.Anthropic() with cl.capture_anthropic(client, model="claude-3-5-sonnet-20241022") as collector: for turn in range(20): client.messages.create( model="claude-3-5-sonnet-20241022", max_tokens=1024, system="You are a helpful assistant.", messages=build_messages(turn), ) report = cl.analyze_trace(collector.build_trace()) print(f"Recoverable waste: {report.recoverable_tokens:,} tokens (${report.recoverable_cost_usd:.4f})") Live capture — OpenAI import openai import contextlens as cl client = openai.OpenAI() with cl.ca