> ## Documentation Index
> Fetch the complete documentation index at: https://docs.geometry.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Why deterministic

> LLMs guess dates. GEOMETRY doesn't.

# Why deterministic

Ask a plain LLM what moon phase governed a date, or what was retrograde, and you get a confident guess — often different the next time you ask. GEOMETRY replaces the guess with a **lookup**: sky anchors computed offline with the standard **Swiss Ephemeris** stack, stored once on the date spine, returned identically on every call.

|                   |                                                                         |
| ----------------- | ----------------------------------------------------------------------- |
| **Without tools** | Captured model runs on the same question can hedge, disagree, or invent |
| **With GEOMETRY** | Same tool call → same JSON bytes (within the published spine)           |

Reproduce it yourself:

1. Ask any chatbot: *What was the moon phase on 1985-03-15, and which planets were retrograde that day?*
2. Then ask the same via MCP after connecting `https://mcp.geometry.app/mcp`

Caption: same question, same model — the only difference is the data layer.

Demo / marketing surfaces use Tool 2 ([`get_card_for_birthdate`](/tools/get-card-for-birthdate)) for this proof — not Tool 1.