> ## Documentation Index > Fetch the complete documentation index at: https://docs.barndoor.ai/llms.txt > Use this file to discover all available pages before exploring further. # Masking & Partial Redaction > Replace a detected value with a placeholder, or keep the first or last few characters readable and mask the rest — credit-card style. The **Mask** action replaces a detected value in place. By default the whole value becomes a typed placeholder such as ``. With a **mask format**, the value keeps its shape: the first or last few characters stay readable and everything else is replaced with a mask character, the way a receipt shows `XXXX XXXX XXXX 1234`. Use masking when an agent or a downstream tool still needs to *recognise* a value without *seeing* it — matching a support ticket to an account by its last four digits, telling two commit SHAs apart by their prefix, or keeping a record id searchable. Masking is irreversible. If you need to restore the original value later, use **Tokenize** instead. If nothing downstream needs the value at all, **Redact** or **Block** is the safer choice. ## Mask versus Redact and Tokenize | Action | Output | Reversible | Value stays recognisable | | ---------------------- | -------------------------- | ------------------- | ------------------------ | | **Redact** | `<…:REDACTED>` placeholder | No | No | | **Mask** (no format) | `` | No | No | | **Mask** (keep last 4) | `XXX-XX-6789` | No | Yes | | **Tokenize** | `` reversible token | Yes, via detokenize | No | Mask is available on every profile type that reports it under **Supported Actions** on the Protection Profiles dashboard, and on both MCP policies and LLM policies. ## Choose Mask in a policy Masking is configured per policy, in the **Profiles & Action** step of the MCP policy wizard or the LLM policy wizard. 1. Create or edit a policy ([MCP Policies](/how-tos/data-control-center/mcp-policies)). 2. In **Profiles & Action**, select the protection profiles to run. 3. Set **Action** to **Mask**. The **Mask format** panel appears under the action. MCP policy wizard on the Profiles & Action step with Mask selected 4. Choose a **Mask format** (see below), then save the policy. The format applies to every value the policy's profiles detect. To mask different types differently — for example last-4 for SSNs and first-8 for order ids — create two policies with single-type profiles and order them by priority. ## Mask formats ### Full placeholder The default. Every detected value becomes ``, where `TYPE` is the detection type (`SSN`, `CREDIT_CARD`, and so on). This is the same behaviour as Redact with a different label, and it is what existing Mask policies do. ### Keep suffix Keeps the **last N characters** readable and masks the rest. This is the card and SSN convention: the tail is enough to match a record, and the head is what makes the value sensitive. Mask format set to Keep suffix, keeping the last 4 characters | Setting | Meaning | | -------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------- | | **Keep last (characters)** | How many characters at the end stay readable, 1 to 16. Separators count. | | **Mask character** | The character written over the hidden part. Default `X`. One character; not a digit, not a space, and not `<`, `>`, `"` or `\`. | | **Keep separators** | On by default. Dashes, spaces and dots inside the masked region are left in place so `111-22-6789` becomes `XXX-XX-6789` rather than `XXXXXX6789`. | The **Preview** line shows the format applied to a sample card number, exactly as the runtime would render it. ### Keep prefix Keeps the **first N characters** readable and masks the rest. Use it for identifiers whose leading part is the meaningful one: a commit SHA, an order or record id with a fixed prefix, an API key's vendor prefix. Mask format set to Keep prefix, keeping the first 8 characters with * as the mask character The settings are the same as Keep suffix, with **Keep first (characters)** in place of Keep last. ## Examples With **Keep separators** on and `X` as the mask character: | Detected value | Format | Result | | ------------------------------------------ | ------------- | --------------------------------------------- | | `111-22-6789` (SSN) | Keep suffix 4 | `XXX-XX-6789` | | `111-22-6789` (SSN) | Keep prefix 7 | `111-22-XXXX` | | `4111 1111 1111 1234` (card) | Keep suffix 4 | `XXXX XXXX XXXX 1234` | | `9f86d081884c7d659a2f…` (40-character SHA) | Keep prefix 8 | `9f86d081XXXXXXXX…` | | `ACCT-Zoë-00917` (custom type) | Keep prefix 8 | `ACCT-Zoë-XXXXX` | | `555-0142` (phone) | Keep prefix 8 | `` — see the reveal floor below | Characters are counted the way a person reads them, not as bytes: an accented letter or an emoji is one character. The masked value is emitted bare, not wrapped in `<…>`. That keeps the residue usable in a follow-up tool call, and re-scanning already-masked output with the same policy leaves it unchanged. ## What is always fully masked Two safeguards stop a partial format from revealing more than intended. Both fall back to the **full placeholder**, never to passing the value through. ### The reveal floor A partial format is only applied when enough of the value is actually hidden: **at least 4 characters, and at least a third of the value's letters and digits**. Otherwise the whole value becomes ``. This lets the standard forms through (last-4 of a 9-digit SSN, first-6 and last-4 of a 16-digit card) and rejects the abuses (keep 12 of a 16-digit card, keep 8 of a 10-digit phone number). A format tuned for 40-character SHAs cannot leak most of a short phone number the same profile happens to catch. ### Free-text and payload-level types Some detection types have no meaningful prefix or suffix to keep. Values of these types are always fully masked, whatever format the policy sets: * **Names, places and addresses** — `PERSON`, `ORGANIZATION`, `LOCATION`, `ADDRESS` * **Emails and dates of birth** — `EMAIL`, `DATE_OF_BIRTH` * **Passwords** — every character is secret; there is no conventional visible tail * **Payload-level verdicts** — prompt injection, guardrail interventions, code-execution and obfuscation findings When a selected profile emits one of these types, the Mask format panel lists it under the settings so you know which of the policy's detections will get the placeholder instead. Mask format panel noting that EMAIL, LOCATION and PERSON are always fully masked Everything pattern-shaped is eligible for partial masking: government and financial identifiers, phone numbers, every secret and API-key type, and all [custom detection types](/how-tos/data-control-center/protection-profiles#create-a-custom-detection-type). ## Custom patterns are full-match A custom regex detection type always masks the **whole** match. A pattern such as `^(.{8}).*$` does not keep the first eight characters — capture groups are ignored, and the pattern matches the entire line, so the entire line is masked. To keep a prefix, write the pattern to match the identifier itself and set the policy's mask format to **Keep prefix**. ## Verify a mask policy * **Test Workbench** — go to **Data Control Center → Test**, paste a sample payload, and run the policy. The output shows the masked form the runtime will produce. * **Dry run** — turn on **Dry Run** on the policy to record detections without rewriting anything, then check **Detection Activity** to see what the policy would mask. * **Audit** — each masked finding's audit record carries how many characters were left readable and whether the reveal floor forced the full placeholder. The revealed text itself is never recorded. ## API The same setting is exposed on the enforcement-policy API as an optional `mask_format` object, valid only when `action` is `POLICY_ACTION_MASK`: ```json theme={null} { "action": "POLICY_ACTION_MASK", "mask_format": { "keep": "SUFFIX", "keep_chars": 4, "mask_char": "X", "preserve_separators": true } } ``` Omit it, or send `null`, for the full placeholder. Sending a format with any other action is rejected. The detection-type catalog reports `partial_mask_eligible` for each type. The Terraform provider's `enforcement_policy` resource does not yet expose `mask_format`. Configure it through the console or the API until that binding ships.