pii-mcp

Pattern-based PII scrubbing for MCP servers (regex + checksums), with an optional NER pass for person names in the Rust backend. Masks emails, IBANs, cards, BICs, MACs, IMEIs, IPs, coordinates, BSNs, US SSNs, German tax IDs, Dutch BTW-ids, Dutch passport/ID numbers, phones, street + house number addresses, Dutch and UK postcodes, and Dutch license plates in tool results. Language packs: en, nl, and opt-in de.

Example

from pii_mcp import scrub_text

scrub_text(
    "Contact ada@example.com. IBAN NL91 ABNA 0417 1643 00. card 4111111111111111"
)
# Contact [EMAIL]. IBAN [IBAN]. card [CREDIT_CARD]

Install

pip install "pii-mcp[fastmcp]"   # FastMCP >= 3.0.0
# or
pip install pii-mcp              # core; platform wheels include Rust acceleration
# or
npm install pii-mcp

FastMCP

from fastmcp import FastMCP
from pii_mcp.fastmcp import PiiScrubMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(PiiScrubMiddleware())  # languages=["en", "nl"] by default
# mcp.add_middleware(PiiScrubMiddleware(languages=["en", "nl", "de"]))

Results only. On scrub failure or oversize, the result is withheld, never forwarded unmasked.

Core

from pii_mcp import scrub_text, scrub_payload

scrub_text("mail ada@example.com")
scrub_payload({"email": "ada@example.com"}, languages=["en"])

Scope

Aligned with AP: wat zijn persoonsgegevens where pattern/checksum detection can reach them. Person names need the opt-in NER build (below). Free-text health data (allergies), photos/audio/video, and unstructured klant-/personeelsnummers stay out of scope.

AP coverage (pattern layer)

Person names (optional NER)

ner=True runs an XLM-R token classifier (Davlan/xlm-roberta-base-ner-hrl, AFL-3.0; EN/NL/DE among its languages) after the pattern detectors and masks person names as [PERSON]. It runs in the Rust core on candle (CPU, fp32) and is off by default: published wheels and the npm package are built without it, and no weights ship with any package.

Build with the ner feature:

maturin develop --release --features ner                  # Python
(cd typescript && npm run build:native -- --features ner) # Node
cargo build -p pii-core --features ner                    # Rust

Download the pinned weights (1.1 GB) and the matching XLM-R tokenizer (MIT) into one directory, and point PII_MCP_NER_MODEL at it:

mkdir -p ner-model && cd ner-model
M=https://huggingface.co/Davlan/xlm-roberta-base-ner-hrl/resolve/253f557bd8249b8515114cfd7f71974fe5fa4d2f
T=https://huggingface.co/FacebookAI/xlm-roberta-base/resolve/e73636d4f797dec63c3081bb6ed5c7b0bb3f2089
curl -fL -O "$M/config.json" -O "$M/model.safetensors" -O "$T/tokenizer.json"
export PII_MCP_NER_MODEL="$PWD"

scrub_text("Mail Ada Lovelace at ada@example.com", ner=True)
# {'text': 'Mail [PERSON] at [EMAIL]', ...}

• Pattern hits are masked first; the NER pass never re-tags a placeholder, so nothing is counted twice.
• PII_MCP_NER_THRESHOLD (default 0.9) is the minimum person probability per token. The default favors precision; lower it (for example 0.5) to mask more names at the cost of more false positives.
• ner=True raises PiiScrubError when the build has no ner feature, the backend is pure Python / TypeScript, or the model directory is missing or invalid. Text is never returned with the NER pass silently skipped.

Cost on macOS arm64 (M4 Pro), scripts/bench_backends.py with PII_MCP_NER_MODEL set:

Inference time grows with input length, at about 0.3 s per KiB here: a 1 MiB tool result takes about 5 minutes. Use ner=True for results of a few KiB, not for bulk text. Model load takes 2.0 s once per process; peak RSS is about 2.4 GB. Inference is fp32 only: candle's XLM-R implementation builds its attention mask in F32, so fp16 weights do not run.

On the eval (PII_MCP_NER_MODEL=<dir> python eval/score.py --ner, EN/NL/DE names in 15 contexts), person scores recall 0.987 and precision 0.931. Most false positives are company names such as Albert Heijn. The address generators only produce the street shapes the patterns support, so the address recall of 1.0 covers those shapes only; precision is measured with the en, nl and de packs against bare street names and company names. Words that end in a street suffix and are followed by a number (Keypad 3, Supermarkt 24) are masked on purpose, since dropping those suffixes leaks real addresses such as Nieuwmarkt 4.

Packages

Rust core (Python)

Published platform wheels ship pii_mcp._native (PyO3 over pii-core). Pip prefers those on supported OS/arch; elsewhere (or with --no-binary) you get the pure-Python hatchling wheel/sdist and the same scrub API.

When _native is importable, scrub_text / scrub_payload use it. Force the Python path with PII_MCP_BACKEND=python; require native with PII_MCP_BACKEND=native.

Local development from a clone (optional):

pip install -e ".[native]"
maturin develop --release --manifest-path crates/pii-mcp-native/Cargo.toml

Performance (Python vs Rust release)

Medians from scripts/bench_backends.py on macOS arm64 / CPython 3.14.7 (release native build; debug builds are not representative):

python scripts/bench_backends.py