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ONYST RVG for AI assistants

ONYST offers the fee engine of its German legal fee calculator (RVG, the German Lawyers' Remuneration Act) as an MCP server (Model Context Protocol). With it, an AI assistant calculates RVG fees, determines values in dispute and creates invoices and calculation sheets. Every amount comes from ONYST, not from the language model.

The result is a calculation, not legal advice. Whether a fee applies in the individual case and whether a discretionary rate is appropriate remains the responsibility of the lawyer.

Address

The server speaks Streamable HTTP at https://api.onyst.legal/mcp

ChatGPT uses an address of its own with the same tools: https://api.onyst.legal/mcp/chatgpt

Two ways to sign in

Access key. It is part of the ONYST RVG plan. The firm's administrator creates it in the fee calculator under Firm settings → API / MCP, and the assistant sends it in the x-api-key header. This is the way for programs without a browser and for connectors you set up yourself. Ready-made configurations for Claude Code, Claude Desktop, VS Code and Cursor are on the same page after signing in.

Sign in with an ONYST account. The assistant sends you to the ONYST sign-in page once (OAuth 2.1 with PKCE); without an account you create one there. After that the assistant works with your permissions, and no key is copied. This way is intended for the connector directories of Anthropic and OpenAI; the listing is in preparation. Until then, connect Claude with the access key.

Connecting each assistant

  • Claude Code: one command with the key; it sets up the server for all projects.
  • Claude Desktop: an entry in the configuration file that starts the mcp-remote bridge (Node.js required); then quit Claude Desktop completely and start it again.
  • claude.ai: a custom connector with the server address, no authentication and the request header x-api-key. Skip the suggestion to sign in: a connector you create yourself lacks the credentials that signing in with an account needs.
  • VS Code and Cursor: an entry with address and key; VS Code asks for the key on first start and stores it encrypted.
  • ChatGPT: not yet. ChatGPT accepts no key for servers you add yourself, so ONYST joins through the ChatGPT app directory. The listing is in preparation.

Claude Desktop, claude.ai and VS Code also show the results as a view below the answer (MCP Apps): calculation, invoice draft, document.

The twelve tools

The names are those under which the server lists the tools.

Calculation

With both ways of signing in.

  • rvg_calculate Calculate RVG fees — Fees, expenses and VAT for one matter, each item with its VV number, rate and calculation.
  • rvg_calculate_combined Calculate several matters — Several matters together, with credits between them and one total.
  • rvg_determine_value Determine the value in dispute — The value in dispute from the type of case and its facts, with its legal basis.
  • rvg_fee_catalog Look up the fee catalogue — Fee numbers, rates and required inputs from the fee schedule, instead of quoting them from memory.

Documents

Require the firm's ONYST RVG plan; without it they decline and name the way to get it.

  • rvg_preview_invoice Invoice preview — The invoice as a watermarked draft in the firm's layout. A draft assigns no number and does not appear in the invoice history.
  • rvg_create_invoice Create invoice — The invoice as PDF or e-invoice (ZUGFeRD), via a download link.
  • rvg_create_calculation_sheet Create calculation sheet — The calculation sheet as PDF, via a download link.

Address book

Only after signing in with an ONYST account; the assistant asks before every change.

  • rvg_recipients_search Search recipients — Find invoice recipients in the person's address book instead of asking for the address.
  • rvg_recipient_save Save recipient — Add a recipient or change single fields of an entry.
  • rvg_recipient_delete Delete recipient — Delete an entry from the address book.

Firm details

Only after signing in with an ONYST account; the assistant asks before every change.

  • rvg_firm_profile_get Read firm details — Sender, tax number, bank details, payment terms and the next invoice number from the settings.
  • rvg_firm_profile_update Update firm details — Change single firm details. Number range, logo and letterhead stay in the settings.

Limits and quotas

  • The ONYST RVG plan includes 10,000 requests and 150 completed invoices per calendar month. Beyond that, each further request costs €0.03 net and each further invoice €0.24 net. Requests from the REST API and MCP count together.
  • Not counted are rejected calls, calls with incomplete input, calculations that return no result because of faulty input, and errors on ONYST's side.
  • At most 120 requests per minute and key and 300 per minute and firm.
  • Invoice and calculation sheet arrive as a download link. It is valid for seven days and opens up to ten times.
  • All prices plus statutory VAT.

Ending a connection

  • Signed in with an account: in the fee calculator, open the profile menu → Connected assistants and disconnect. From then on ONYST answers no request from this assistant, not even after a new sign-in in the background; on all servers within 30 seconds at the latest. It stays disconnected until you allow it again. Remove the connector in the assistant as well.
  • Access key: revoke it under Firm settings → API / MCP.
  • Download link of an invoice: withdraw it in the invoice history. The invoice itself stays there.

Data

The server processes the input of the tool calls to calculate and create documents, and returns the results to the assistant you connected. Privacy policy (German)

Support

Questions and incidents: info@onyst.legal · Contact form (German)