The model sees tokens.
Not your data.
Synonym swaps sensitive data for safe tokens before it reaches an AI model, then restores it exactly when the answer comes back.
Thank you for your order. Please settle the invoice to GB57 NWBK 6016 1317 9336 77<IBAN_1>. A copy has been sent to emma.walker@northwind.co.uk<EMAIL_1>.
Kind regards
You see the real names. The model never does.
Highlighted values become tokens before the message leaves your browser — and come back as themselves.
<PERSON_1> <EMAIL_1> <IBAN_1> — and answered as usualChoose a data type to try:
Name and phone number recognized in free text.
Your input
System output
One trip out, one trip back.
Watch a message go out and come back. Real values never cross the line.
You hold the only key to your data.
Flip the switch and watch what survives the end of a session.
Who holds what
Reversible, in full hover
Restored completely or not at all.
Temporary by design hover
In memory, for one session, then wiped.
Survives rephrasing hover
Reworded or translated — the token still maps back.
Four ways in. One shared mapping.
Pick a door to see what actually leaves your machine.
One gateway. Four ways to work.
The web app, the browser extension, the MCP server and the API — the same session and token map behind each.
public/img/product/app.pngWeb app
Paste text or upload a CSV, XLSX or PDF. Synonym recognises the sensitive columns, tokenises the rest with detectors and shows you what is happening at every step.
Open the app
public/img/product/reveal.pngHidden by default. Revealed only to you.
The live preview shows exactly what a model would see: tokens where the names, companies, IBANs and card numbers were. Move the lens over any token and the original appears inside it — or flip to Revealed. The real values never leave your side.
Try it with your own file
public/img/product/extension.pngBrowser extension
Works inside ChatGPT, Claude and Gemini. Point it at your gateway once, and every prompt is masked before it leaves the tab — the reply is restored where you read it.
How the extension worksIt checks before it hides.
A number is only masked if it really is an identifier — verified the way a registry or bank would. Look-alikes stay readable.
<COMPANY_ID_1>Slovak and Czech company IDs — the same check the business register uses.
Built for the fields that show up in your work.
Nine industries where a document names a real person, company or account.
Legal
Client names, case numbers, contract terms.
Finance
IBANs, account numbers, company IDs.
Healthcare
Diagnoses, insurance IDs, birth numbers.
HR & recruiting
Passports, birth numbers, home addresses.
Insurance
Insured IDs, claims, payout accounts.
Public sector
Applicants, permits, citizen records.
Customer support
Tickets, IPs, cards, contact details.
Accounting & tax
Invoices, VAT numbers, supplier accounts.
Real estate
Leases, owners, property addresses.
Be one of our first five customers.
We're looking for five companies to work with closely. You use Synonym on real work, we build what you need — and you don't pay while we do it.
TelluswhatyoudowithChatGPT,ClaudeorGemini.
We'llmakeSynonymfitit.
- No cost during the pilot, and a locked-in price afterwards
- We only ask for one short call every two weeks and honest feedback
- Good fit: law firms, accountants, clinics, HR teams, insurers, public offices — anywhere in the EU
Where certification stands today.
GDPR by design. The rest is in progress, and we say so.
GDPR
Key stays with you, storage is temporary, no permanent history.
SOC 2 Type II
Audit scope being finalized; controls mapped to the architecture.
ISO 27001
ISMS documentation in draft.
HIPAA / BAA
Processing agreement for healthcare organizations and partners.
Straight answers.
The questions people ask before they paste anything into Synonym.
What is Synonym?
Synonym is a reversible pseudonymization gateway for large language models. It finds personal and sensitive identifiers in your text or files, replaces them with tokens such as <PERSON_1> or <IBAN_1> before anything reaches ChatGPT, Claude, Gemini or an internal model, and puts the real values back into the answer — only on your side.
How is this different from anonymization?
Anonymization destroys the link to the person. Synonym keeps it, but only for you: the token map lives in memory for one session and is wiped when the session ends, so the model never sees real data and you still get a usable, restored reply.
Which data does it detect?
Checksum- and format-validated identifiers — birth numbers, IBANs, company and VAT IDs, bank accounts, cards, passports, VINs, IMEIs, court file numbers, cadastral numbers and more — plus opt-in health fields. Language-model detection of names, organisations and places is rolling out to paid plans.
Does Synonym work with Slovak and Czech data?
Yes. It was built for the identifiers Slovak and Czech regulators care about — rodné číslo, IČO, DIČ, IČ DPH, Czech bank account formats — with English identifiers alongside.
Is my data stored?
No. Content is processed in memory, the token map exists only for the length of a session (up to 4 hours on Free, 24 hours on paid plans) and is never written to a database. Synonym is hosted in the EU and never uses your content to train models.
How do I use it?
Four ways: paste text or upload CSV, XLSX or PDF in the app; run Synonym as an MCP server in Claude Desktop or Cursor; add the browser extension to ChatGPT, Gemini or Copilot; or call the REST API — mask on the way out, unmask on the way back.
Keep using AI. Stop sending the data.
Set up in minutes, or talk to us about deploying Synonym across your company.