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Retrieval Rails

Retrieval rails execute during RAG (Retrieval-Augmented Generation) pipelines to validate retrieved documents, filter chunks, and fact-check bot responses against the knowledge base.

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When Retrieval Rails Execute

Retrieval rails run at two key points in RAG workflows: 
Query → Retrieve Docs → Retrieval Rails → LLM → Fact Check → Response
                        ↓                        ↓
                   Filter/Validate         Verify Against Context
Retrieval rails ensure only trusted, relevant, and safe content is used as context for LLM responses.

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Built-in Retrieval Rails

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Fact Checking with AlignScore

Verifies bot responses against retrieved evidence using information alignment scoring.

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Configuration

rails:
  config:
    fact_checking:
      fallback_to_self_check: true
      parameters:
        endpoint: "http://localhost:5000/alignscore"

  output:
    flows:
      - alignscore check facts

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Action Implementation

From nemoguardrails/library/factchecking/align_score/actions.py:41: 
@action(output_mapping=alignscore_check_facts_mapping)
async def alignscore_check_facts(
    llm_task_manager: LLMTaskManager,
    context: Optional[dict] = None,
    llm: Optional[BaseLLM] = None,
    config: Optional[RailsConfig] = None,
    **kwargs,
):
    """Checks the facts for the bot response using an information alignment score."""
    fact_checking_config = llm_task_manager.config.rails.config.fact_checking
    fallback_to_self_check = fact_checking_config.fallback_to_self_check

    alignscore_api_url = fact_checking_config.parameters.get("endpoint")

    evidence = context.get("relevant_chunks", [])
    response = context.get("bot_message")

    alignscore = await alignscore_request(alignscore_api_url, evidence, response)
    if alignscore is None:
        log.warning("AlignScore endpoint not set up properly. Falling back to the ask_llm approach for fact-checking.")
        if fallback_to_self_check:
            return await self_check_facts(llm_task_manager, context, llm, config)
        else:
            return 1.0  # Assume OK if we can't verify
    else:
        return alignscore
Output mapping: 
def alignscore_check_facts_mapping(result: float) -> bool:
    """Returns True (block) if score is below 0.5"""
    THRESHOLD = 0.5
    return result < THRESHOLD

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Self Check Facts

Uses the main LLM to verify responses against retrieved evidence.

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Configuration

rails:
  output:
    flows:
      - self check facts

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Action Implementation

From nemoguardrails/library/self_check/facts/actions.py:43: 
@action(output_mapping=mapping_self_check_facts)
async def self_check_facts(
    llm_task_manager: LLMTaskManager,
    context: Optional[dict] = None,
    llm: Optional[BaseLLM] = None,
    config: Optional[RailsConfig] = None,
    **kwargs,
):
    """Checks the facts for the bot response by appropriately prompting the base llm."""
    _MAX_TOKENS = 3
    evidence = context.get("relevant_chunks", [])
    response = context.get("bot_message")

    if not evidence:
        # If there is no evidence, we always return true
        return True
        
    task = Task.SELF_CHECK_FACTS
    prompt = llm_task_manager.render_task_prompt(
        task=task,
        context={
            "evidence": evidence,
            "response": response,
        },
    )
    stop = llm_task_manager.get_stop_tokens(task=task)
    max_tokens = llm_task_manager.get_max_tokens(task=task)
    max_tokens = max_tokens or _MAX_TOKENS

    llm_call_info_var.set(LLMCallInfo(task=task.value))

    response = await llm_call(
        llm,
        prompt,
        stop=stop,
        llm_params={"temperature": config.lowest_temperature, "max_tokens": max_tokens},
    )

    if llm_task_manager.has_output_parser(task):
        result = llm_task_manager.parse_task_output(task, output=response)
    else:
        result = llm_task_manager.parse_task_output(task, output=response, forced_output_parser="is_content_safe")

    is_not_safe = result[0]
    result = float(not is_not_safe)
    return result
Mapping function: 
def mapping_self_check_facts(result: float) -> bool:
    """Returns True (block) if score is below 0.5"""
    THRESHOLD = 0.5
    return result < THRESHOLD

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AutoAlign Fact Checking

Integration with AutoAlign for advanced fact verification.

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Configuration

From examples/configs/autoalign/autoalign_factcheck_config/config.yml: 
rails:
  config:
    autoalign:
      parameters:
        fact_check_endpoint: "https://<AUTOALIGN_ENDPOINT>/content_moderation"
        multi_language: False
      output:
        guardrails_config:
          {
            "fact_checker": {
              "mode": "DETECT",
              "knowledge_base": [
                {
                  "add_block_domains": [],
                  "documents": [],
                  "knowledgeType": "web",
                  "num_urls": 3,
                  "search_engine": "Google",
                  "static_knowledge_source_type": ""
                }
              ],
              "content_processor": {
                "max_tokens_per_chunk": 100,
                "max_chunks_per_source": 3,
                "use_all_chunks": false,
                "name": "Semantic Similarity",
                "filter_method": {
                  "name": "Match Threshold",
                  "threshold": 0.5
                },
                "content_filtering": true,
                "content_filtering_threshold": 0.6,
                "factcheck_max_text": false,
                "max_input_text": 150
              },
              "mitigation_with_evidence": false
            },
          }
  output:
    flows:
      - autoalign factcheck output

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Sensitive Data Detection in Retrieval

Filter PII from retrieved documents before using as context.

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Configuration

rails:
  config:
    sensitive_data_detection:
      retrieval:
        entities:
          - PERSON
          - EMAIL_ADDRESS
          - PHONE_NUMBER
          - SSN
        score_threshold: 0.5

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Usage in Flows

define flow sanitize retrieved chunks
  user ask question
  $relevant_chunks = execute retrieve_relevant_chunks
  
  # Filter sensitive data from chunks
  for $chunk in $relevant_chunks
    $has_sensitive = execute detect_sensitive_data(source="retrieval", text=$chunk)
    if $has_sensitive
      $chunk = execute mask_sensitive_data(source="retrieval", text=$chunk)
  
  bot provide answer

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Usage Examples

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Conditional Fact Checking

Only check facts for specific topics: From examples/configs/autoalign/autoalign_groundness_config/rails/factcheck.co: 
define user ask about pluto
  "What is pluto?"
  "How many moons does pluto have?"
  "Is pluto a planet?"

define flow answer pluto question
  user ask about pluto
  # For pluto questions, we activate the fact checking.
  $check_facts = True
  bot provide pluto answer

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Custom Accuracy Thresholds

From examples/configs/rag/fact_checking/rails/factcheck.co: 
define flow answer report question
  user ask about report
  $check_facts = True
  bot provide report answer

define subflow check facts
  """Add the ability to flag potentially inaccurate responses.

  Flag potentially inaccurate responses when the confidence is between 0.4 and 0.6.
  """
  if $check_facts == True
    $check_facts = False

    $accuracy = execute check_facts
    
    # Hard block if accuracy is very low
    if $accuracy < 0.4
      if $config.enable_rails_exceptions
        create event FactCheckLowAccuracyRailException(message="Fact check triggered. The accuracy of the response is below 0.4.")
      else
        bot inform answer unknown
      stop

    # Warning if accuracy is medium
    if $accuracy < 0.6
      $bot_message_potentially_inaccurate = True

define flow flag potentially inaccurate response
  bot ...

  if $bot_message_potentially_inaccurate
    $bot_message_potentially_inaccurate = False
    if $config.enable_rails_exceptions
      create event PotentiallyInaccurateResponseRailException(message="Potentially inaccurate response detected.")
    else
      bot inform answer potentially inaccurate
    stop

define bot inform answer potentially inaccurate
  "Attention: the answer above is potentially inaccurate."
This example shows:
  • Block responses with accuracy < 0.4
  • Warn for accuracy between 0.4 and 0.6
  • Allow for accuracy ≥ 0.6

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Multi-Stage Retrieval Validation

define flow validated rag
  user ask question
  
  # Stage 1: Retrieve documents
  $documents = execute retrieve_documents
  
  # Stage 2: Filter sensitive data
  for $doc in $documents
    $has_pii = execute detect_sensitive_data(source="retrieval", text=$doc)
    if $has_pii
      $documents = remove($documents, $doc)
  
  # Stage 3: Rank and select top chunks
  $relevant_chunks = execute rank_and_select($documents, top_k=5)
  
  # Stage 4: Generate response
  bot provide answer
  
  # Stage 5: Fact check against chunks
  $accuracy = execute self_check_facts
  if $accuracy < 0.5
    bot inform cannot verify answer
    stop

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Advanced Configurations

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Custom Fact Checking Prompts

Override the default fact-checking task prompt: 
prompts:
  - task: self_check_facts
    content: |
      You are given some evidence and a response.
      
      Evidence:
      {{ evidence }}
      
      Response:
      {{ response }}
      
      Is the response fully supported by the evidence?
      Answer with 'yes' if supported, 'no' if not.

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Chunk-Level Validation

define flow validate each chunk
  user ask question
  $chunks = execute retrieve_chunks
  
  $validated_chunks = []
  for $chunk in $chunks
    # Check chunk quality
    $relevance = execute compute_relevance($chunk, $user_message)
    $has_sensitive = execute detect_sensitive_data(source="retrieval", text=$chunk)
    
    if $relevance > 0.7 and not $has_sensitive
      $validated_chunks = append($validated_chunks, $chunk)
  
  $relevant_chunks = $validated_chunks
  bot provide answer

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Fallback Strategies

define flow fact checked answer with fallback
  user ask question
  $relevant_chunks = execute retrieve_relevant_chunks
  
  if not $relevant_chunks
    bot inform no information available
    stop
  
  bot provide answer
  
  $accuracy = execute alignscore_check_facts
  
  if $accuracy < 0.5
    # Try self-check as fallback
    $accuracy = execute self_check_facts
    
    if $accuracy < 0.5
      bot provide conservative answer
      bot suggest human expert

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Best Practices

  1. Always validate retrieved chunks - Don’t assume knowledge base quality
  2. Use appropriate thresholds - Tune accuracy thresholds based on domain risk
  3. Provide fallbacks - Have graceful degradation when fact checks fail
  4. Filter sensitive data early - Remove PII before using chunks as context
  5. Cache fact check results - Avoid redundant checks for similar responses
  6. Monitor accuracy scores - Track distribution to tune thresholds

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Performance Considerations

Rail TypeLatency ImpactAccuracy
AlignScoreMedium (~200-500ms)High
Self Check FactsMedium (~200-500ms)Medium-High
AutoAlignHigh (~1-2s)Very High
Sensitive Data DetectionLow (~50-100ms)High
Fact-checking adds latency to every RAG response. Consider using it selectively for high-stakes queries.

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Troubleshooting

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No Evidence Available

define flow handle missing evidence
  user ask question
  $relevant_chunks = execute retrieve_relevant_chunks
  
  if not $relevant_chunks
    bot inform no evidence
    bot suggest alternative
    stop

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AlignScore Endpoint Issues

rails:
  config:
    fact_checking:
      fallback_to_self_check: true  # Enable automatic fallback
      parameters:
        endpoint: "http://localhost:5000/alignscore"
        timeout: 10  # seconds
From the implementation: 
if alignscore is None:
    log.warning("AlignScore endpoint not set up properly. Falling back to the ask_llm approach for fact-checking.")
    if fallback_to_self_check:
        return await self_check_facts(llm_task_manager, context, llm, config)

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See Also

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