# I measured what it costs an AI agent to read my Obsidian vault — a derived index made it ~150x cheaper

> Source: <https://dev.to/oroborolabs/i-measured-what-it-costs-an-ai-agent-to-read-my-obsidian-vault-a-derived-index-made-it-150x-34ca>
> Published: 2026-08-29 12:07:54+00:00

I keep a plain Obsidian vault — about 1.4 MB of markdown across notes, projects and an inbox — and I let coding agents (the Claude Code / Cursor kind) work inside it: searching, summarizing, drafting. The first thing I measured changed how I set all of this up: **an agent that reads the whole vault isn't being thorough. It's drowning.**

Every file it opens costs context. Read enough files and the actual question gets buried under files that were never relevant. Three boring fixes, each measured on my own setup:

A small script walks the vault and writes one line per note: title, what it's about, size, last change. That map is ~9 KB standing in for ~1.4 MB — the same answer to "what exists here and where," for about **150× less context**. It's generated, never hand-maintained: I rerun the script after heavy edits, because a stale map is worse than none — it asserts confidently things that stopped being true.

Measured cost per question on my setup:

Most questions die on the second rung and never pay the 35 KB. The specific tools don't matter; the point is that "just read everything" is the most expensive possible answer to most questions.

Reading is free; writing is gated. A plain `AGENTS.md`

at the vault root states it: the index is the front door, the agent may read anything, and it writes nothing without asking — with folder ownership explicit (my notes vs. a staging area it may use). This is the piece that stops an assistant from quietly reorganizing your knowledge base overnight.

Part 1 is an afternoon of Python (walk the folder, pull first heading + gist + size + modified date, one line per note). Part 2 done well is the fiddly one — ranking by term rarity and returning section trails took real iteration to stop returning junk. Part 3 is one text file.

**Full disclosure:** I ended up packaging all of this — the vault starter (notes/people/projects/inbox with example notes teaching each folder's pattern), the two tools, the rules file, a 10-minute tutorial and a short ebook on the method, including a what-NOT-to-do section — as a US$15 one-time template. It's my product; the link is below. The three ideas above work fine with nothing but your own scripts — build your own index and you're 90% there.

[Second Brain Starter — US$15 one-time](https://oroborolabs.gumroad.com/l/ikkpfm)

Happy to answer questions about the measurement method, or about where this approach breaks.

*Cross-posted from Oroboro Labs — this post is the canonical version; the original carries the full method notes.*