Verifiable US tax computation backed by cited federal and state tax rules for individuals and businesses.
Computes federal individual income tax, business entity tax, estate and trust tax, and state returns (15 states) from a machine-checkable corpus of tax rules. Returns tax amounts with full derivation proofs and citations so results can be verified offline.
Try asking Muse: "Calculate my 2025 federal income tax from my W-2 wages, investment income, and dependent children."
Source: Found in the official MCP Registry (io.github.Invaro/opentax) · First listed September 24, 2026
Each bar is one check, every 15 minutes. Green means it answered. Last checked 1 h ago.
What Muse can see: It does not ask you to sign in, so it cannot see your accounts. It only sees what Muse sends it from your request.
Before it acts: Read what Muse plans to do before you approve it, and remove the app from Muse when you stop using it.
musedirectory.ai is not part of Meta. More about how Muse handles your information
Not in Muse's Connectors list yet, but Muse can still use it. Copy the request below and paste it into Muse. Muse asks before it shares anything with the app's site. Meta does not review apps used this way, so only use ones you trust. We tested this in the Muse app on September 24, 2026: Muse used an app's link directly this way and returned a live answer.
https://opentax.invaro.ai/mcpNot in Muse's Connectors list yet. Muse can still use it: its page gives you a request to paste into Muse.
OpenTax is a remote MCP server, so any assistant that can add a custom connector by link can use it. Same link, same tools.
In ChatGPT, open Settings, then Connectors (called Apps & Connectors on some accounts).
Under Advanced settings, switch on Developer mode. That is what lets you add a connector by link.
Choose Create, give it a name, paste the link as the MCP server URL, and save. If it needs a sign-in, ChatGPT asks for it here.
Start a chat, open the tools menu, pick the connector, and ask. ChatGPT asks before it runs a tool the first time.
Custom connectors need a paid ChatGPT plan, and on Team, Enterprise and Edu accounts only an admin can add them. Our "Working" label means the server answered us; it cannot tell whether your plan lets you add it. All connectors that work in ChatGPT
In Claude, open Settings, then Connectors.
Choose Add custom connector, give it a name, and paste the link.
If it needs a sign-in, Claude sends you to it. Then turn the connector on in a chat from the tools menu and ask.
Custom connectors are available on paid Claude plans; on Team and Enterprise accounts an admin may need to add them. Our "Working" label means the server answered us; it cannot tell whether your plan lets you add it. All connectors that work in Claude
https://opentax.invaro.ai/mcpResponse time at the last check: 264ms. Worked in 99.6% of checks over 30 days.
Screened, no issues found
Screening looks for known threats and hidden instructions at the time of the check, and runs again weekly and whenever the tool list changes. It cannot see the server's code, so only connect what you need and review what Muse asks to do. How screening works.
calculate_taxCompute US federal INDIVIDUAL income tax (or balance due if withholding is given) from a content-addressed corpus of cited rules. NEVER estimate tax yourself — call this, and report ONLY numbers returned by oracle calls made with the real facts (never hand-check or approximate a calculate_business_taxCompute US federal BUSINESS-ENTITY tax from the same cited corpus: check-the-box entity classification, Form 1120 corporate income tax (§ 179/168(k)/174A/163(j)/DRD/NOL, § 250, GBC/FTC/BEAT), S-corp entity taxes, corporate estimates, the § 4501 buyback excise, AET and PHC taxes. calculate_fiduciary_taxCompute US federal income tax for an ESTATE or TRUST (Form 1041): the § 1(e) compressed brackets and § 642(b) exemption. Input is taxable income before the exemption, after the §§ 651/661 distribution deduction. Retained capital gains refuse loudly (§ 1(h) trust breakpoints not mdetermine_dependentDetermine whether ONE candidate person is the taxpayer's § 152 dependent — qualifying child or qualifying relative, including multiple-support agreements and the divorced-parents release — as a proof-backed yes/no with citations. Feed the result into calculate_tax's credits groupcompute_state_returnCompose a STATE return's printed-form line set deterministically (2025 IL-1040 / VA 760 / CA 540 / NY IT-201 / PA-40 / NJ-1040 / OH IT 1040 / NC D-400 / GA 500 / MD 502 / MO-1040 / WI Form 1 / MN M1 / SC1040 / AL Form 40 / OR-40 / OK Form 511 / CT-1040 / KS K-40 / AR AR1000F / NMcompute_returnCompute the COMPLETE Form 1040 bottom-line set in one call — the lines that determine the return (1a, 9, 10, 11, 12e, 15, 16, 17 AMT, 19, 20, 22, 23, 24, 25d, 27a, 28, 32, 33, 34/37), each whole-dollar rounded by the engine, plus the modeled Schedule 3 credits (schedule3) and, whcompute_form_2210Form 2210 — the estimated tax underpayment penalty (26 U.S.C. § 6654) for tax years 2025-2027, composed from the printed form: Part I (the required annual payment: 90% of this year's tax, or 100% / 110% of last year's — pass priorYearTax and priorYearAGI in facts, or noPriorYearRverify_tax_claimVerify a claimed tax amount (yours, a user's, or another tool's) against the law. Returns verdict 'verified' or 'refuted' with the correct value. Use this as a self-check before presenting any tax number. Put asOf (and target, if any) INSIDE the facts object — e.g. facts: {..., "list_input_factsDiscover every input the tax corpus understands: id, type, whether required, and its documented default. Call this if unsure what information to collect from the user.explain_ruleGet a tax rule in full: statutory citation with its verbatim excerpt, validity window, parameters, the formula and applicability guard (the rule IR — rules are data), the rules and facts it depends on, and its content hash. Use to quote and explain the actual law behind an answerfind_tax_cliffsFind exact dollar amounts where one more cent of an input costs MORE than a cent of tax (marginal rate over 100%) — e.g. the EITC investment-income kill switch, CTC phase-out steps. Every probe is a real evaluation.compare_filing_statusesCompute the answer under every filing status for the same facts — e.g. to answer 'should we file jointly or separately?'. Statuses that need more facts report their error instead of guessing.lookup_tax_parameterLook up the current-law dollar amounts behind a question ('standard deduction', 'CTC phase-out threshold', 'tips deduction cap') with their statutory citations and validity windows. Use this to fact-check ANY tax number before stating it — your training data likely predates the Osearch_tax_rulesFull-text search over the encoded tax-law corpus ('kiddie tax', 'NIIT threshold', 'california renters credit'). Returns matching rules: id, title, statutory citation, effective window, and a verbatim excerpt of the law text. A hit means the engine computes this; zero hits means iverify_factFact-check a claimed dollar amount about tax law ('the 2026 MFJ standard deduction is $32,200', 'CTC is $2,000 per child') against the corpus. Returns verified / refuted (with the correct value and citation) / unknown. Never states a verdict it cannot ground.is_tipped_occupationDetermine whether a job is on the Treasury Tipped Occupation list (Treas. Reg. § 1.224-1, final Apr 2026) for the § 224 'no tax on tips' deduction. Fuzzy-matches the job name; returns the official listing (name, TTC code, category) or a definitive 'not listed'.https://opentax.invaro.ai/mcpNot in Muse's Connectors list yet, but Muse can still use it. Paste this into Muse: "Use OpenTax to help me. It is a free service with an MCP server at https://opentax.invaro.ai/mcp. It does not need an API key. Ask me before you share anything with it." Muse asks before it shares anything with the app's site. Meta does not review apps used this way, so only use ones you trust. We tested this in the Muse app on September 24, 2026: Muse used an app's link directly this way and returned a live answer.
At the last check (November 13, 2026, 13:45 UTC) the endpoint was working, answering in 264ms. Over the last 7 days it answered 100% of health checks. It is checked every 15 minutes.
It was screened on September 24, 2026 with the result "screened, no issues found". Screening checks the domain against threat feeds and reads the tools for hidden instructions and requests for passwords or card numbers. It cannot see the server's code, so grant only the access you need.
It exposes 16 tools, including calculate_tax, calculate_business_tax, calculate_fiduciary_tax, determine_dependent. For example, you could ask Muse: "Calculate my 2025 federal income tax from my W-2 wages, investment income, and dependent children."
This listing was added from public sources (Found in the official MCP Registry (io.github.Invaro/opentax)). If you build OpenTax, claim it to correct the details and get your badge.
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