- OpenTax is a deterministic, open-source tax computation engine that produces identical returns for identical facts, with every line traced to its legal statute.
- It is the only open-source tax engine on the market, licensed under AGPL-3.0, and scored 96% exact returns on TaxCalcBench, the highest ever recorded.
- The engine amplifies any AI model (e.g., Claude, ChatGPT) by handling math and rules deterministically, while the model handles reading and writing.
- Key features include: cited rules from the U.S. Code, Merkle-rooted proofs for verifiability, deterministic document-to-fact compilation, and loud refusal for out-of-scope queries.
- It supports federal and state taxes (1040, 1120, 1041) across 29 states, and can review returns, compute quarterly estimates, model entity elections, find tax cliffs, and handle law changes (e.g., OBBBA).
- The engine can be used via an MCP server, CLI, or TypeScript library, and is free to try at https://opentax.invaro.ai/mcp.