promptscrub

Strip PII from LLM prompts. Rehydrate it in responses. Your users see real data. Your LLM provider never does.

Drop-in Faraday middleware for OpenAI, Anthropic, Gemini — and any LLM library built on Faraday (RubyLLM, langchainrb, llm.rb).

Your App                   PromptScrub                LLM API
   │                           │                          │
   │  "SSN is 123-45-6789"     │                          │
   │──────────────────────────►│  redact                  │
   │                           │  "SSN is <SSN_001>"      │
   │                           │─────────────────────────►│
   │                           │                          │ generate
   │                           │◄─────────────────────────│
   │                           │  "Your SSN <SSN_001>..."  │
   │                           │  rehydrate               │
   │  "Your SSN 123-45-6789..."│                          │
   │◄──────────────────────────│                          │

No infra to deploy. No gateway to operate. Just middleware.

# What leaves your app (sent to the LLM):
"Summarize the support ticket for SSN <SSN_001>, card <CARD_001>, email <EMAIL_001>"

# What comes back from the LLM (raw):
"The ticket for SSN <SSN_001> shows a duplicate charge on card <CARD_001>. Contact <EMAIL_001>."

# What your application receives (after rehydration):
"The ticket for SSN 234-56-7890 shows a duplicate charge on card 4532015112830366. Contact john.smith@hospital.com."

Your code never changes. Your LLM provider never sees real data.

gem "promptscrub"

require "faraday"
require "promptscrub"

conn = Faraday.new("https://api.openai.com") do |f|
  f.use PromptScrub::Middleware
  f.request :json
  f.response :json
  f.adapter Faraday.default_adapter
end

# PII is stripped before the request leaves your app.
# Tokens are rehydrated in the response. Transparent to your code.
response = conn.post("/v1/chat/completions", {
  model: "gpt-4o",
  messages: [{ role: "user", content: "Summarize claim for SSN 234-56-7890, card 4532015112830366" }]
})

RubyLLM.configure do |c|
  c.faraday do |f|
    f.use PromptScrub::Middleware
  end
end

Same value always maps to the same token within a request — so alice@corp.com appearing twice becomes <EMAIL_001> twice.

PromptScrub.configure do |config|
  # Add a custom detector
  config.add_detector(:zip, /\b\d{5}(-\d{4})?\b/)

  # Opt out of a built-in
  config.disable_detector(:phone)

  # Redact only outbound (skip rehydration)
  config.scrub_response = false
end

For streaming responses where your app processes chunks directly, use StreamRehydrator to wrap your callback:

vault      = PromptScrub::Vault.new
redactor   = PromptScrub::Redactor.new(vault, PromptScrub.configuration.detectors)
rehydrator = PromptScrub::StreamRehydrator.new(vault) do |clean_chunk|
  print clean_chunk  # user sees real values
end

# Before streaming request:
redacted_prompt = redactor.scrub(user_prompt)

# For each SSE chunk received:
rehydrator.call(raw_chunk)

# After stream ends:
rehydrator.flush

StreamRehydrator buffers partial tokens at chunk boundaries (e.g. <EMAIL_ split across two chunks) and flushes them correctly when the token completes.

1. Redact — on every outgoing request, Redactor scans the body string with all registered detectors and replaces matches with <TYPE_NNN> tokens. Each unique value gets a stable token stored in a per-request Vault.
2. Send — the redacted body hits the LLM API. The model never sees real PII.
3. Rehydrate — on the response, Rehydrator scans for token patterns and substitutes original values from the vault. Your application code receives the real data.

The vault is in-memory and scoped to a single request — no persistence, no shared state between requests.

• Tokens are not encrypted. The vault lives in your process memory for the duration of a request.
• Detection is regex-based. It will catch well-formed PII; obfuscated or unusual formats may slip through.
• For high-assurance use cases (HIPAA, PCI-DSS), add custom detectors for your specific data patterns and review false-negative rates in your domain.
• promptscrub is client-side middleware. It does not replace network-level controls or data governance policies.

I built this myself — which means it works great for the cases I thought of, and probably has rough edges for the ones I didn't. If you hit something weird, open an issue. I read them all and respond fast.

Want to fix something or add a feature? Send a PR. No CLA, no process overhead, no committee review. If the tests pass and the change makes sense, it's getting merged. I'm one person and I genuinely appreciate the help — you can take this further than I can alone.

Not sure where to start? Look for good first issue labels, or just open an issue and ask.

git clone https://github.com/jibranusman95/promptscrub
cd promptscrub
bundle install
bundle exec rspec    # all green? you're good to go
bundle exec rubocop  # no new offenses

See CONTRIBUTING.md for full guidelines.

Everyone who's made this better:

Small, sharp Ruby gems built to the same standard — 100% test coverage, zero dependencies beyond what's needed.

MIT — see LICENSE.