Policy Design

Design Goal

An LLM safety policy should be easy to inspect, easy to test, and easy to extend. llmshieldr therefore uses explicit rule objects rather than a hidden classifier as its default layer.

A policy is a list with these fields:

shieldr_policy
    name             policy identifier stored in reports
    rules            list of shieldr_rule objects
    thresholds       redact_at and block_at numeric cutoffs
    rate_guard       optional shieldr_rate_guard environment
    trusted_sources  optional allowlist used by scan_context()
    controls         secure_chat() block/refuse/escalate/drop behavior

Each rule is similarly explicit:

shieldr_rule
    id             stable rule identifier
    pattern        regex pattern, or NULL
    fn             R predicate function, or NULL
    owasp          OWASP LLM category
    severity       low, medium, high, or critical
    action         allow, redact, or block
    description    human-readable explanation

Exactly one of pattern or fn must be supplied. This keeps each rule deterministic and makes redaction spans possible for regex rules.

Source Model

The built-in rules are sourced from a small number of security and governance concerns that recur across LLM applications:

• OWASP GenAI / LLM Top 10: https://genai.owasp.org/llm-top-10/
• Prompt-injection patterns: direct override language, hidden instructions, role confusion, and system-prompt extraction attempts
• NLP intent signals: token and stem patterns for override, secret exposure, harmful intent, and unusually dense directive language. Trigger seed groups are expanded with stems at runtime.
• Sensitive information patterns: email, phone, SSN, account numbers, medical record identifiers, subject IDs, API keys, bearer tokens, AWS keys, and credential-bearing connection strings
• Agentic risk patterns: model claims that it sent, deleted, granted, executed, notified, traded, or otherwise acted outside the chat boundary
• Output handling patterns: unsafe shell or SQL snippets, code execution markers, system-prompt structure, and high-confidence medical or financial claims
• RAG-specific signals: untrusted source metadata, anomalous chunk length, and high density of instruction words inside retrieved text
• Resource-exhaustion controls: request and token windows managed by rate_guard(), with optional strict pre-call reservation and local file-locking for shared guards
• Optional scanner controls: invisible Unicode format characters, encoded payloads, URL host policies, token limits, simple language allowlists, and topic bans
• Runtime surfaces: conversation scanning, tool-call scanning, tool-output scanning, and streaming output scanning with rolling context

These sources are intentionally conservative. They are meant to catch common failure modes in R workflows before prompts, retrieved context, or model output cross a trust boundary.

Built-In Policy Construction

Every built-in policy is assembled in policy().

enterprise_default
    prompt injection rules
    PII and secret rules
    system prompt extraction rules
    excessive agency rules

pharma_gxp
    enterprise_default
    MRN and clinical subject identifiers
    diagnosis and treatment claim language
    code-safety checks
    stricter thresholds: redact_at = 0.3, block_at = 0.6

finance_strict
    enterprise_default
    account number rules
    financial advice and guaranteed-return language
    autonomous investment-action language
    rate_guard(max_tokens = 100000)

education_safe
    enterprise_default
    minor-related PII patterns
    academic-integrity bypass language

open_research
    injection rules
    secret rules
    higher thresholds: redact_at = 0.8, block_at = 0.95

comprehensive
    enterprise_default
    pharma_gxp additions
    finance_strict additions
    education_safe additions
    code-safety checks
    rate_guard(max_tokens = 100000)
    moderate thresholds: redact_at = 0.4, block_at = 0.7

custom
    no rules
    default thresholds

baseline
    alias for enterprise_default

The policy names reflect intended operating posture, not legal compliance. For example, pharma_gxp adds useful GxP-style controls, but it does not by itself make a workflow compliant with any regulation.

The comprehensive policy is broad rather than maximally strict. Use explicit threshold overrides when you want pharma-tier thresholds:

policy(
  "comprehensive",
  overrides = list(thresholds = list(redact_at = 0.3, block_at = 0.6))
)
#> llmshieldr policy
#> name: comprehensive
#> rules: 23
#> redact_at: 0.3
#> block_at: 0.6

Check Modes

Scanners can run different layers depending on the workflow:

• checks = "rules" runs the policy’s deterministic rules. Built-in policies include regex rules and the NLP intent rule.
• checks = "nlp" runs only NLP intent rules. This is the local token/stem path that uses tokenizers and SnowballC when installed.
• checks = "llm" runs only the supplied reviewer, such as ollama_reviewer() or your own reviewer function.
• checks = "both" runs policy rules and the supplied reviewer.

rule_id              stable reviewer rule identifier
owasp                OWASP category such as llm01
severity             low, medium, high, or critical
description          human-readable explanation
confidence           optional number from 0 to 1
evidence             optional short evidence string
recommended_action   optional allow, redact, or block
span                 optional start/end offsets for redaction

Scoring

Each finding contributes a numeric severity weight.

The scanner sums contributions and caps the total at 1.0. Findings are deduplicated before scoring. Overlapping span findings from the same source, OWASP category, and action count as the strongest single piece of evidence instead of stacking together. Synthetic context findings are scored separately and capped, so source or anomaly signals cannot by themselves dominate a report.

findings:
    medium email finding      0.3
    high secret finding       0.6

risk_score = min(0.3 + 0.6, 1.0) = 0.9

The score is deliberately coarse. It is not a probability. It is a deterministic severity index that helps a policy decide whether to allow, redact, or block.

Action Resolution

Thresholds define how much risk the policy tolerates.

default thresholds:
    redact_at = 0.40
    block_at  = 0.75

Action resolution is conservative:

if any finding is critical:
    block
else if any finding's rule action is block:
    block
else if risk_score > block_at:
    block
else if any finding's rule action is redact:
    redact
else if risk_score >= redact_at:
    redact
else:
    allow

This means a policy can block either because of accumulated score, a critical finding, or a single rule that explicitly asks to block. A single high-severity redaction finding does not block solely because its score equals a threshold.

Redaction

Regex rules create match spans. When the resolved action is redact, or when a redacting rule contributes to a report, matched spans are replaced with [REDACTED].

input:
    Contact neel@example.com about the ticket.

output:
    Contact [REDACTED] about the ticket.

Function rules may not know exact character spans. They can still contribute findings and affect the action, but span redaction depends on span data.

Use redaction_strategy() to change the replacement operator:

scan_prompt(
  "Contact neel@example.com.",
  redaction = redaction_strategy("mask")
)
#> llmshieldr report
#> action: redact
#> risk_score: 0.300
#> findings: 1

scan_prompt(
  "Contact neel@example.com.",
  redaction = redaction_strategy("hash")
)
#> llmshieldr report
#> action: redact
#> risk_score: 0.300
#> findings: 1

Hash redaction is deterministic for the same matched text. It can support linkage in audits, but it is not anonymization.

Optional Scanners

Optional scanners run beside policy rules and return normal findings.

scanners <- scanner_options(
  max_tokens = 500,
  blocked_topics = c("unreleased earnings"),
  allowed_url_hosts = c("example.com", "docs.example.com")
)

scan_prompt(
  "Email neel@example.com about unreleased earnings.",
  scanners = scanners
)
#> llmshieldr report
#> action: block
#> risk_score: 0.900
#> findings: 2

Default scanner options record invisible Unicode format characters and inspect encoded payloads by decoding candidate URL-encoded and base64-like text, then running policy rules over the decoded text. URL host policies, token limits, language allowlists, and topic bans are enabled only when configured.

Policy Controls

Scanner reports resolve to allow, redact, or block. secure_chat() then uses policy controls to decide what to do with blocked surfaces.

controlled <- policy(
  "enterprise_default",
  overrides = list(
    controls = policy_controls(
      on_prompt_block = "refuse",
      on_context_block = "drop",
      on_output_block = "escalate",
      refusal_message = "Please rephrase the request."
    )
  )
)

on_context_block = "drop" is the default because retrieved context is an untrusted auxiliary input. Other context options are keep_redacted, block, refuse, and escalate.

Context Anomaly Scores

scan_context() adds RAG-specific findings before returning row-aligned reports. It calculates:

• robust z-score of character length
• robust z-score of instruction-word density

Instruction density counts these words per 100 tokens:

ignore, forget, override, instead, disregard

Rows above anomaly_threshold, default 2.5, receive high-severity OWASP LLM08 findings. If source_col is supplied and the policy has trusted_sources, rows from outside that allowlist receive a medium-severity OWASP LLM08 finding.

These RAG-specific findings are marked as synthetic. Their combined contribution is capped at 0.3 per row before being added to normal rule findings.

When context is passed through secure_chat(), included rows are assembled with explicit context row labels, source labels when available, and separator lines. This keeps retrieved text visually distinct from the user prompt and preserves a row-level path back to audit data.

Risk Summary

secure_chat() returns risk_summary, a named numeric vector keyed by OWASP category.

llm01 = prompt injection and instruction override findings
llm02 = sensitive information and secret findings
llm06 = excessive agency findings
llm08 = retrieved-context trust and anomaly findings
llm09 = misinformation and unsupported claim findings
llm10 = resource exhaustion and rate-guard findings

Each category is capped at 1.0. The value is useful for dashboards and audit summaries because it shows which risk category dominated a run.

Rate Guard Semantics

rate_guard() now checks projected usage before counters are incremented. secure_chat() reserves one request before the chat call. With strict = TRUE, it also reserves an estimated prompt token count before the call and records only the positive post-call delta after output scanning. If the chat call or output scan fails, the pre-call reservation is rolled back. concurrent = TRUE wraps each guard operation in a local file lock through the optional filelock package.

Extending a Policy

Start with the closest built-in policy, add rules, then inspect the resulting inventory.

guardrails <- policy()

guardrails <- add_rule(
  guardrails,
  id = "llm02.ticket_id",
  pattern = "\\bTICKET-[0-9]{6}\\b",
  owasp = "llm02",
  severity = "medium",
  action = "redact",
  description = "Internal support ticket identifier."
)

list_rules(guardrails)
#>                                id owasp severity action has_pattern has_fn
#> 1           llm01.injection.basic llm01 critical  block        TRUE  FALSE
#> 2        llm01.injection.indirect llm01 critical  block        TRUE  FALSE
#> 3                llm01.nlp.intent llm01     high  block       FALSE   TRUE
#> 4                 llm02.pii.email llm02   medium redact        TRUE  FALSE
#> 5                 llm02.pii.phone llm02   medium redact        TRUE  FALSE
#> 6                   llm02.pii.ssn llm02     high redact        TRUE  FALSE
#> 7             llm02.phi.condition llm02     high redact        TRUE  FALSE
#> 8            llm02.secret.api_key llm02     high redact        TRUE  FALSE
#> 9             llm02.secret.bearer llm02     high redact        TRUE  FALSE
#> 10               llm02.secret.aws llm02     high redact        TRUE  FALSE
#> 11          llm02.secret.password llm02     high redact        TRUE  FALSE
#> 12 llm02.secret.connection_string llm02     high redact        TRUE  FALSE
#> 13 llm07.system_prompt.extraction llm07 critical  block        TRUE  FALSE
#> 14          llm06.agency.language llm06 critical  block        TRUE  FALSE
#> 15                llm02.ticket_id llm02   medium redact        TRUE  FALSE

When a policy becomes important to production, keep its custom rules in package or application code, test them with representative prompts, and write audit logs for real workflow runs.