Pseudonym MCP server

pseudonym-mcp

Local pseudonymisation tools for LLM workflows — replace detected PII with opaque tokens before you hand text to a cloud LLM, then restore those tokens afterward.

Expose MCP tools (masktext and unmasktext) that your client or agent can call as an explicit privacy step. The server detects PII locally, replaces it with opaque tokens, and keeps the token mapping in memory for later restoration.

It is a defense-in-depth measure, not a compliance silver bullet. Read the Limitations and GDPR & AI Compliance sections before assuming this stack does more than it does.

What you get

• 46 recognizers across twelve locales, all of them live by default: Poland (PESEL, NIP, REGON, dowód, paszport, księga wieczysta), the EU (IBAN with mod-97, VAT for all 27 member states, plus DE, IT, ES, FR, NL, CZ/SK, SE, FI and UK national IDs) and the US (SSN, ITIN, EIN, ABA routing, cards). 24 of them verify a real check digit. --lang narrows that set when you want fewer false positives — see Locale coverage is fail-closed. Heuristic language detection (detectLanguage()) infers the language from text content — --lang remains the authoritative override.
• Hybrid NER engine: Regex for structured PII (SSN, credit cards, IBAN, email, phone) + local Ollama LLM for unstructured entities (names, organisations).
• Local-detection architecture: Detection and substitution happen on your machine when the MCP tool is called. The cloud LLM call still happens (that's the point) — but it can see tokens instead of detected PII when your workflow uses the masked output.
• Session-keyed mapping store: Tokens like [PERSON:1] map back to originals in an isolated, per-request session. Multiple round-trips preserve token coherence.
• Unmask workflow support: masktext returns autounmask for clients that want to honor that preference, but this server does not intercept arbitrary LLM responses automatically.
• Flexible engines: Run regex only (no Ollama required), llm only, or hybrid (default).
• Strict validation: SSN area-number validation, credit card Luhn checksum, PESEL checksum — all configurable.
• Graceful degradation: If Ollama is unavailable, the regex phase still runs and no exception is thrown.
• MCP-native: Works with Claude Code, Claude Desktop, Cursor — any MCP-compatible client.

❌ Without / ✅ With

❌ Without pseudonym-mcp:

• Prompt: "John Smith, SSN 123-45-6789, card 4111 1111 1111 1111" → sent verbatim to the LLM provider
• Every name, ID number, and credit card in your prompt is processed and potentially logged by the provider
• A breach at the provider's end exposes those values in cleartext
• Sending personal data to a non-EU LLM provider without further safeguards raises GDPR Article 44 questions you'll need to answer

✅ With pseudonym-mcp used before the cloud call:

Shortened. The full README is on GitHub.