API reference

§1. Overview

Base URL https://www.taxcitation.com. Every JSON response has the same envelope:

{ "data": …, "meta": { "requestId": "req_…", "apiVersion": "2026-09-26", "latencyMs": 30,
  "creditsConsumed": 2, "costDollars": { "total": 0.02, "credits": 0.02, "inference": 0 } } }

Errors return {"error": "…"} with status 400, 401, 404, 429 or 500. Failed calls cost nothing.

Identify your client with a header. Any stable ID will do; quota is counted per client.

X-CPA-Copilot-Client: my-app-prod

§2. Access

Federal law is public. California is included in the public tier. Washington, New York and New York City need an API key:

Authorization: Bearer tlk_…

X-Api-Key: tlk_… also works. Get a key from Get API key: finish Stripe Checkout and the key is shown once. Create, name and revoke up to ten keys, and see usage and cost per call, on your account page. Every paid key reads federal, California, Washington, New York and New York City law.

Credits

One credit is $0.01. Each successful call reports what it cost in meta.creditsConsumed; Stripe invoices monthly for what you used, and new accounts start with $5 of credit.

The MCP tools cost the same as their REST equivalents.

Grants

Keys issued by hand can carry narrower grants per jurisdiction and use-case corpus: individual, business, passthrough, payroll, sales-excise, property, real-estate, estate-trust, retirement, wealth-advisory, exempt-orgs, international and procedure. Every endpoint and MCP tool reads only what the key grants; a request can narrow that, never widen it.

§3. Cite IDs

Stable, URL-safe and lower-case, in the form jurisdiction/code/section.

§4. The section object

§5. Resolve

GET/v1/resolve

Exhibit 5.1Request

curl "https://www.taxcitation.com/v1/resolve?cite=R%26TC%20%C2%A7%2019132(a)(1)"

Exhibit 5.2Response

{
  "data": {
    "input": "R&TC § 19132(a)(1)",
    "resolved": true,
    "citeId": "ca/rtc/19132",
    "pinpoint": "(a)(1)",
    "pinpointFound": true,
    "section": {
      "citeId": "ca/rtc/19132",
      "citation": "Cal. Rev. & Tax. Code § 19132",
      "citationShort": "R&TC § 19132",
      "jurisdiction": "US-CA",
      "sourceType": "statute",
      "heading": "Penalties and Additions to Tax",
      "status": "in_force",
      "hierarchy": [
        { "num": "2", "type": "division", "heading": "OTHER TAXES" },
        { "num": "10.2", "type": "part", "heading": "ADMINISTRATION OF FRANCHISE AND INCOME TAX LAWS" },
        { "num": "4", "type": "chapter", "heading": "Payments and Assessments" },
        { "num": "7", "type": "article", "heading": "Penalties and Additions to Tax" }
      ],
      "sourceUrl": "https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC&sectionNum=19132."
    }
  },
  "meta": { "apiVersion": "2026-09-23", "latencyMs": 30 }
}

POST/v1/resolve

Up to 50 citations at once; data is an array of resolutions in the same order.

curl -X POST https://www.taxcitation.com/v1/resolve \
  -H "content-type: application/json" \
  -d '{"citations": ["IRC § 162(a)", "RTC 17041", "Pub. 501 (2024)"]}'

Accepted formats

26 U.S.C. § 162(a) · 26 USC 162 · IRC § 162 · I.R.C. §162 · Internal Revenue Code section 162 · Treas. Reg. § 1.401(k)-1 · Rev. Rul. 2024-1 · PLR 202637005 · Cal. Rev. & Tax. Code § 17041 · R&TC § 17041 · RTC 17041 · Revenue and Taxation Code section 17041 · Cal. Const. art. XIII, § 36 · RCW 82.87.040 · WAC 458-20-101 · N.Y. Tax Law § 601 · 20 NYCRR § 112.3 · N.Y.C. Admin. Code § 11-1701 · Pub. 501 (2024) · Publication 17 2024 · Instructions for Form 1040-SS (2024) · Form 1099-MISC (2024) · cite IDs · bare “section 17041”.

§6. Sections

POST/v1/contents

The text of up to 20 sections, by id or by citation: {"ids": ["us/irc/162", "IRC § 6651(a)(2)", "R&TC 19132"], "text": {"maxCharacters": 20000}, "highlights": {"query": "…"}}. Results have the search result shape plus text; a citation with a subsection returns just that subsection (with pinpoint). Ids you cannot read are listed in data.errors and cost nothing. format: "claude" works here too.

GET/v1/sections/{citeId}

Fetch a section by cite ID. text=0 leaves out the text.

Exhibit 6.1Request

curl "https://www.taxcitation.com/v1/sections/us/irc/162?text=0"

Exhibit 6.2Response

{
  "data": {
    "citeId": "us/irc/162",
    "citation": "26 U.S.C. § 162",
    "citationShort": "IRC § 162",
    "jurisdiction": "US",
    "sourceType": "statute",
    "heading": "Trade or business expenses",
    "status": "in_force",
    "hierarchy": [
      { "num": "A", "type": "subtitle", "heading": "Income Taxes" },
      { "num": "1", "type": "chapter", "heading": "NORMAL TAXES AND SURTAXES" },
      { "num": "B", "type": "subchapter", "heading": "Computation of Taxable Income" },
      { "num": "VI", "type": "part", "heading": "ITEMIZED DEDUCTIONS FOR INDIVIDUALS AND CORPORATIONS" }
    ],
    "sourceUrl": "https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title26-section162&num=0&edition=prelim"
  },
  "meta": { "apiVersion": "2026-09-23", "latencyMs": 1 }
}

§7. Search

POST/v1/search

Search returns sources, not answers: ranked sections with verbatim highlights, for your model to reason over and cite. Nothing in a result is generated.

Exhibit 7.1Request

curl https://www.taxcitation.com/v1/search \
  -H "Authorization: Bearer $TAXCITATION_KEY" \
  -H "Content-Type: application/json" \
  -d '{"query": "late payment penalty per month",
       "objective": "What percentage per month is added when tax shown on a return is paid late?",
       "numResults": 5,
       "filters": {"jurisdictions": ["US"], "sourceTypes": ["statute"], "taxYear": 2025}}'

Exhibit 7.2Response, first result

{
  "id": "us/irc/6651",
  "citation": "26 U.S.C. § 6651",
  "citationShort": "IRC § 6651",
  "title": "Failure to file tax return or to pay tax",
  "url": "https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title26-section6651…",
  "jurisdiction": "US",
  "sourceType": "statute",
  "authority": { "kind": "statute", "substantialAuthority": true },
  "status": "in_force",
  "path": ["Subtitle F — Procedure and Administration", "Chapter 68 — Additions to the Tax…", "…"],
  "rank": 1,
  "highlights": [{
    "pinpoint": "(a)(2)",
    "text": "…there shall be added to the amount shown as tax on such return 0.5 percent of the amount of such tax if the failure is for not more…",
    "score": 1
  }]
}

data.results holds the results; data.warnings says how they were scoped (a jurisdiction your key does not include, a defaulted tax year). authority.kind is statute, regulation, administrative, judicial, procedural (the IRM) or informational (forms and publications); for federal documents, substantialAuthority says whether the document type counts as authority under Treas. Reg. § 1.6662-4(d)(3)(iii). GET /v1/search?q=…&objective=…&limit=…&jurisdiction=…&sourceType=…&taxYear=…&text=1&format=claude takes the same options as query parameters.

§8. Verify

POST/v1/verify

Send claims with their citations, or free text containing citations. Optional asOf.

{"claims": [{"claim": "…", "citation": "…", "quote": "…"}]}
{"text": "…"}

Exhibit 8.1Request

curl -X POST https://www.taxcitation.com/v1/verify \
  -H "content-type: application/json" -H "X-CPA-Copilot-Client: my-app-prod" \
  -d '{"claims": [{"claim": "California imposes a penalty for failure to pay tax.", "citation": "R&TC § 19132(a)(1)"}]}'

Exhibit 8.2Response

{
  "data": {
    "results": [
      {
        "claim": "California imposes a penalty for failure to pay tax.",
        "citation": "R&TC § 19132(a)(1)",
        "verdict": "supported",
        "needsReview": false,
        "citeId": "ca/rtc/19132",
        "resolvedCitation": "Cal. Rev. & Tax. Code § 19132",
        "status": "in_force",
        "pinpointFound": true,
        "sourceUrl": "https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC&sectionNum=19132.",
        "notes": [],
        "confidence": 1
      }
    ],
    "summary": { "total": 1, "supported": 1, "contradicted": 0, "unsupported": 0, "unresolved": 0, "needsReview": 0 }
  },
  "meta": { "apiVersion": "2026-09-23", "latencyMs": 213 }
}

Answer

POST/v1/answer

{"question": "…", "jurisdiction": "US,CA"} returns answer, claims (each with claim, citeId, citation, pinpoint, quote and verification), sources, checks and timingsMs.

§9. MCP

Streamable HTTP at https://www.taxcitation.com/mcp. The same five tools, with the same grants: send your key in the same Authorization header.

claude mcp add --transport http tax-law https://www.taxcitation.com/mcp \
  --header "Authorization: Bearer tlk_…"

Cursor, Claude Desktop, VS Code and other clients take the same URL and header:

{ "mcpServers": { "tax-law": {
    "url": "https://www.taxcitation.com/mcp",
    "headers": { "Authorization": "Bearer tlk_…" } } } }

Without a header the server serves the free tier. A green connection proves little: ask it to resolve IRC § 162 and check you get the section back.

§10. How citations are checked

1. Resolve against the index: does the section exist, what is its status, and is the pinpoint present?
2. Quote: verbatim for statutes; at least 85% word coverage for IRS PDF text.
3. Classify: a TypeSafe Jev choice over supports, contradicts or says nothing, with claims batched per section. Verdicts at 0.8 confidence or above are accepted automatically.

§11. Sources and freshness