Tax Numbers in ChatGPT
US tax figures, IRS-sourced. The MCP server is https://tax.openkrill.app/mcp. No key. The steps below are only for ChatGPT. A call to get_tax_parameters was checked against that server on 2026-10-01.
What this server answers
Get the US federal tax numbers right, with the IRS document each one comes from.
Ask "what are the 2026 tax brackets for a married couple?", "what is the standard deduction for head of household?", "what was the IRS mileage rate on 15 March?", "when is the next estimated tax payment due?" or "roughly how much federal tax on 90,000 dollars of wages and 30,000 dollars of freelance income?".
Tax Numbers holds the federal income tax figures for tax years 2025 and 2026: brackets, standard deduction, long-term capital gains thresholds, the Social Security wage base and self-employment tax, 401(k) and IRA limits, the standard mileage rates, and the filing and estimated-payment deadlines. Every figure comes with the Revenue Procedure, notice or IRS page it was read from, and every answer says when the table was last verified. A year is added only after all of its figures have been checked.
It can also work out a simple federal estimate: standard deduction only, with self-employment tax, long-term capital gains and the Additional Medicare Tax. Every estimate lists its assumptions and what it leaves out (credits, state tax, alternative minimum tax, itemized deductions and more), and says it is an estimate and not tax advice.
It does not fill in or file returns, give advice on what to do, or cover state taxes. Your figures are used for the one calculation and are not stored; the only lasting record is a daily count of calls per tool.
What it can do
- Looks up federal tax brackets, standard deduction and capital gains thresholds with IRS citations
- Gives the Social Security wage base and self-employment tax rules
- Gives 401(k) and IRA contribution limits
- Finds the IRS standard mileage rate for any date
- Lists filing and estimated-payment deadlines for a tax year
- Estimates federal tax with stated assumptions and exclusions
Tools
get_tax_parameters, Look up US federal tax figures. Use this when the user asks for a published US federal tax figure, for example "what are the 2026 tax brackets for married filing jointly", "standard deduction for head of household in 2025", "long-term capital gains thresholds", "Social Security wage base", "401(k) contribution limit". Pass the tax year (2025 and 2026), the topic (brackets, standard_deduction, mileage, retirement_limits, capital_gains, self_employment or all) and optionally the filing status (single, mfj, mfs, hoh). Returns rows with the figure and the IRS document and section it was read from, the year's status (current, or amended when a later law or notice changed a figure, with amended_by), and when the table was last verified. It does not give advice and does not cover state taxes.get_mileage_rate, Look up the IRS mileage rate. Use this when the user asks for the IRS standard mileage rate on a day, for example "what was the business mileage rate on 15 March 2026", "how much per mile can I deduct for charity driving", "medical mileage rate in 2025". Pass the date (YYYY-MM-DD); the rate in force that day is returned in cents per mile for business, medical and charity use, with the period it covers, the IRS notice that announced it and when it was last verified. Rates are available from 1 January 2022. It does not add up trips or compute a deduction.tax_deadlines, List US federal tax deadlines. Use this when the user asks when a US federal individual tax date falls, for example "when is the 2025 return due", "when is the extension deadline", "when is the next estimated tax payment due". Pass the tax year (the year the income was earned) and optionally as_of (today, YYYY-MM-DD) to get the next deadline. Returns the four estimated payment dates, the filing date and the extended filing date, each with whether the IRS publishes it or it follows the standing rule, and its source. Weekend and holiday moves are already applied. It does not cover state deadlines or disaster-relief postponements.estimate_federal_tax, Estimate US federal income tax. Use this when the user wants a rough US federal income tax figure, for example "how much federal tax on 90,000 dollars of wages", "estimate my tax with 40,000 dollars of freelance income", "what is my marginal rate". Pass the tax year, filing_status (single, mfj, mfs, hoh) and wages in dollars, and optionally self_employment_income, other_ordinary_income, long_term_capital_gains and adjustments. It uses the standard deduction and adds self-employment tax (with the Social Security wage base and the half-deduction), the Additional Medicare Tax and long-term capital gains rates. Returns taxable income, ordinary tax, capital gains tax, self-employment tax, total, marginal and effective rate, the assumptions made and a not_included list (credits, state tax, alternative minimum tax, itemized deductions and more). It is an estimate, not tax advice, and it is not a return.
Add Tax Numbers in ChatGPT
ChatGPT does not read a local config file. A custom MCP server is added in the product, and only after developer mode is on. These steps were checked against OpenAI's plugin connect guide and the ChatGPT MCP guide on 2026-10-01. Developer mode can be hidden by a workspace policy, so a Team or Enterprise admin may have to allow it first.
- Open ChatGPT settings, then Security and login, and turn on Developer mode.
- Open ChatGPT plugins and select the plus button.
- Enter a name such as "Tax Numbers" and a one-line description of what you want it to answer.
- Under Connection, choose the public endpoint and paste
https://tax.openkrill.app/mcp. Include the/mcppath. Do not add a tunnel id: this server is already public. - Create the connection and review the tools ChatGPT lists. You should see
get_tax_parameters,get_mileage_rate,tax_deadlines,estimate_federal_tax. The feedback tools on the same server are optional and not required for the lookup. - Start a new conversation, add the connection from the tools menu, and ask: What is the 2026 standard deduction for a single filer?
No API key and no OAuth client id are required. The server answers without a login. Calls are limited per network and per day, and a limit comes back as a tool error rather than a partial answer. If the tool list is stale after a server change, open the connection and select Refresh, then start a new conversation.
The ChatGPT desktop app, Codex CLI and IDE extension share a different config. In the desktop app, open Settings, then MCP servers, choose Add server, select Streamable HTTP, and paste the same URL. In ~/.codex/config.toml the equivalent is a table named mcp_servers.tax with url = "https://tax.openkrill.app/mcp". ChatGPT on the web does not read that file. A published directory listing is also a different path: this page is the developer-mode connection, not a claim that Tax Numbers is already in the ChatGPT directory.
Treat the server as untrusted until you have read a few answers. A remote MCP server sees the arguments the model sends. Tax Numbers is read-only and documents what it sends onward on the privacy page.
A call checked on 2026-10-01
The 2026 single-filer standard deduction is a published IRS figure in the plugin's table. It is not tax advice. The request below was posted to https://tax.openkrill.app/mcp as tools/call. HTTP 200. Source: the Tax Numbers server, read 2026-10-01.
{
"method": "tools/call",
"params": {
"name": "get_tax_parameters",
"arguments": {
"year": 2026,
"topic": "standard_deduction",
"filing_status": "single"
}
}
}- status. current
- year. 2026
- rows. 2 returned
- source document. Rev. Proc. 2025-32 (2026 inflation adjustments)
- source date. 2025-10-09
Repeat the call yourself if you need a newer reading. Cached answers expire. A rate limit is not a result: wait and try again. Nothing in the call is a ranking, a filing, or advice.