OpenTax by Invaro

OpenTax by Invaro is a deterministic tax computation engine built to run underneath AI agents, assistants, and automation workflows. It suits tax and accounting firms, developers, and advisers who need repeatable calculations with rules they can inspect. Its free, no-expiry evaluation is an unusually accessible way to assess that approach, though coverage across 29 states may limit which cases it can handle.

Overview

OpenTax is designed to calculate from explicit rules rather than fill gaps with plausible estimates. Tax rules carry statutory citations and effective dates, and computations can return a Merkle-rooted proof for offline verification. When its rule corpus cannot resolve a case, the engine declines to answer and gives a reason. That boundary is valuable in professional workflows where an unsupported result could mislead a client, but it also means users need a process for cases outside the engine’s coverage.

A deterministic compiler converts W-2 and 1099 boxes and birth dates into facts the engine can process. The maker says request facts are computed in memory and not stored, while usage records retain limited metadata rather than request arguments. The hosted MCP endpoint does not store or log tax facts. The CLI and MCP package send an anonymous usage ping that can be disabled, a distinction worth considering when choosing between hosted and local use.

Key features

Rules that can be inspected and checked

Statutory citations and effective dates make the rule basis visible to reviewers. Merkle-rooted proofs provide a way to verify computations offline. Together, these features suit firms that need repeatable calculations and an evidence trail; they are less compelling for someone seeking only a guided consumer filing experience.

Broad return types, with state limits

OpenTax supports complete line sets for individual Form 1040, corporate Form 1120, and fiduciary Form 1041 returns. This range makes it relevant to firms serving individuals, businesses, and fiduciary clients. The maker says state coverage spans 29 states and is growing, so firms with clients across many jurisdictions should check whether their work falls within that reach.

Agent and local interfaces

The hosted service offers REST, MCP, and OpenAPI interfaces. Its MCP connector is aimed at assistants, IDEs, agents, and automation platforms, with examples including Claude, ChatGPT, Cursor, n8n, Zapier, and Make. For teams that want deployment control, an installable package combines an MCP server, CLI, and HTTP host, and the HTTP server can be self-hosted. This flexibility makes OpenTax a stronger fit for technical teams building tax workflows than for users who want a standalone, guided filing product.

Explicit handling of unsupported cases

When the corpus cannot answer, OpenTax refuses with a reason instead of estimating. That is a sound safeguard for advice and preparation workflows, but it places more responsibility on the firm to recognize when a case needs manual work or another tool.

Pricing

Evaluation

The Evaluation plan costs nothing and has no expiry. Anonymous use is capped at 600 requests per hour; using a key raises the allowance to 6,000 requests per hour. That makes it practical to assess the engine over time and to connect a prototype, while the hourly limits matter for high-volume production workloads. The published plan is an evaluation, so teams should confirm that its allowance and terms fit before relying on it at scale.

The engine is offered under AGPL-3.0, with a commercial license also available. Teams should account for the license that fits their intended use when adopting or distributing it.

Platforms

OpenTax is available through API and web interfaces, with support for Linux, macOS, and Windows. Self-hosting is also supported through its installable package and HTTP server. That spread gives technical teams options to integrate it into existing systems or keep the service within their own environment.

Who it's for

OpenTax is aimed at tax and accounting firms, freelancers and S-corp clients, advisory engagements, and financial planners. Its strongest case is for professionals who want calculations that can be traced to cited rules, checked offline, or embedded beneath an agent or automation workflow. It is a weaker fit for taxpayers looking for a complete, guided filing journey, and for practices whose cases fall outside its state or rule coverage.

Pros and cons

• Pros: Citations, effective dates, and offline-verifiable proofs make the basis for computations reviewable.
• Pros: Support for Forms 1040, 1120, and 1041 serves individual, corporate, and fiduciary work.
• Pros: REST, MCP, OpenAPI, and local deployment options accommodate different integration and hosting needs.
• Pros: The free evaluation has no expiry, with a higher request allowance when using a key.
• Cons: Coverage across 29 states can leave firms needing another path for some state cases.
• Cons: Refusal on unsupported cases protects against estimates but can interrupt workflows that expect an answer for every return.
• Cons: The CLI and MCP package send an anonymous usage ping unless it is disabled.

Alternatives

TelosTax is a free, open-source tax preparation app for tax year 2025, with Android and iOS apps as well as Linux, macOS, web, and Windows support. Choose it when you want tax preparation software across those platforms rather than an engine designed to power agents and integrations.

H&R Block Online offers a free online plan for simple tax situations and Form 1040, billed per state filed. It is a better match for eligible taxpayers seeking online filing than for teams embedding a computation engine.

Quicko has a free Basic plan for total income up to 12 lac, with one app connection and support for salary, house property, and IFOS. It is an alternative for readers whose needs fit those plan limits.

Taxfix offers a Basic individual annual plan at €39.99 a year, with self-prepared individual returns free until submission. Choose it when an individual return workflow matches your needs.

TaxAct provides free federal filing for eligible standard returns, with state filing additional. It suits eligible filers looking for a filing product rather than a rules engine.

TurboTax offers a free edition for simple Form 1040 returns. It is an option for taxpayers whose return fits that eligibility.

BharatTax AI has a free plan with 5 AI credits.

Filabl has a free Basic plan covering invoicing, quotes, expense tracking, contact management, basic profit and loss reports, and email support.

Verdict

OpenTax is a strong choice for firms and technical teams that need traceable tax computations beneath assistants, agents, or their own systems. Its cited rules, offline proofs, multiple return types, and no-expiry evaluation are clear advantages. Look elsewhere if you need a consumer filing experience, broader state coverage, or a tool that can resolve cases beyond its supported rule corpus.