Corrective RAG (CRAG)
Build RAG systems that validate retrieval quality and self-correct when needed.
When to Use
- Need high-accuracy, grounded responses
- Want to detect and handle retrieval failures
- Combining internal knowledge with web search fallback
- Building production RAG with quality guarantees
CRAG Architecture
┌─────────────────────────────────────────────────────────┐
│ User Query │
└─────────────────────────┬───────────────────────────────┘
│
▼
┌─────────────────────┐
│ Initial Retrieval │
└──────────┬──────────┘
│
▼
┌─────────────────────┐
│ Relevance Grader │
│ (CORRECT/INCORRECT/│
│ AMBIGUOUS) │
└──────────┬──────────┘
│
┌────────────────┼────────────────┐
│ │ │
CORRECT AMBIGUOUS INCORRECT
│ │ │
▼ ▼ ▼
┌──────────┐ ┌──────────────┐ ┌──────────┐
│ Use │ │ Use + Search │ │ Web │
│ As-Is │ │ Fallback │ │ Search │
└────┬─────┘ └──────┬───────┘ └────┬─────┘
│ │ │
└────────────────┼───────────────┘
│
▼
┌─────────────────────┐
│ Knowledge Refiner │
│ (Extract key info) │
└──────────┬──────────┘
│
▼
┌─────────────────────┐
│ Generate Answer │
└─────────────────────┘Implementation
1. Relevance Grader
from langchain_openai import ChatOpenAI
from langchain.prompts import ChatPromptTemplate
from pydantic import BaseModel, Field
class RelevanceGrade(BaseModel):
"""Grade for document relevance."""
grade: str = Field(description="CORRECT, INCORRECT, or AMBIGUOUS")
confidence: float = Field(description="Confidence score 0-1")
reasoning: str = Field(description="Brief explanation")
GRADER_PROMPT = """You are a relevance grader. Assess if the document is relevant to the question.
Question: {question}
Document: {document}
Grade as:
- CORRECT: Document directly answers or contains information for the question
- AMBIGUOUS: Document is somewhat related but may not fully answer
- INCORRECT: Document is not relevant to the question
Return JSON with grade, confidence (0-1), and brief reasoning."""
def grade_document(question: str, document: str) -> RelevanceGrade:
llm = ChatOpenAI(model="gpt-4", temperature=0)
prompt = ChatPromptTemplate.from_template(GRADER_PROMPT)
chain = prompt | llm.with_structured_output(RelevanceGrade)
return chain.invoke({"question": question, "document": document})
def grade_all_documents(question: str, documents: list) -> dict:
"""Grade all documents and categorize."""
results = {"correct": [], "ambiguous": [], "incorrect": []}
for doc in documents:
grade = grade_document(question, doc.page_content)
results[grade.grade.lower()].append({
"document": doc,
"confidence": grade.confidence,
"reasoning": grade.reasoning
})
return results2. Web Search Fallback
from langchain_community.tools import TavilySearchResults
def web_search_fallback(query: str, num_results: int = 5) -> list:
"""Search web when retrieval fails."""
search = TavilySearchResults(max_results=num_results)
results = search.invoke(query)
# Convert to document format
docs = []
for result in results:
docs.append(Document(
page_content=result["content"],
metadata={
"source": result["url"],
"title": result.get("title", ""),
"type": "web_search"
}
))
return docs3. Knowledge Refiner
REFINER_PROMPT = """Extract only the information relevant to answering the question.
Question: {question}
Document:
{document}
Extract the key facts, numbers, and statements that help answer the question.
Remove irrelevant information. If nothing is relevant, return "NO_RELEVANT_INFO".
Extracted information:"""
def refine_knowledge(question: str, documents: list) -> str:
"""Extract relevant info from documents."""
llm = ChatOpenAI(model="gpt-4", temperature=0)
refined_parts = []
for doc in documents:
prompt = REFINER_PROMPT.format(
question=question,
document=doc.page_content
)
result = llm.invoke(prompt).content
if "NO_RELEVANT_INFO" not in result:
refined_parts.append(result)
return "\n\n".join(refined_parts)4. Full CRAG Pipeline
from langgraph.graph import StateGraph, END
from typing import TypedDict, List
class CRAGState(TypedDict):
question: str
documents: List
graded_docs: dict
refined_knowledge: str
web_results: List
final_answer: str
retrieval_quality: str
def retrieve(state: CRAGState) -> CRAGState:
"""Initial retrieval."""
docs = retriever.invoke(state["question"])
return {"documents": docs}
def grade_documents(state: CRAGState) -> CRAGState:
"""Grade retrieved documents."""
graded = grade_all_documents(state["question"], state["documents"])
# Determine overall quality
if len(graded["correct"]) >= 2:
quality = "CORRECT"
elif len(graded["correct"]) + len(graded["ambiguous"]) >= 2:
quality = "AMBIGUOUS"
else:
quality = "INCORRECT"
return {"graded_docs": graded, "retrieval_quality": quality}
def route_by_quality(state: CRAGState) -> str:
"""Route based on retrieval quality."""
return state["retrieval_quality"].lower()
def use_retrieved(state: CRAGState) -> CRAGState:
"""Use correctly retrieved docs."""
correct_docs = [d["document"] for d in state["graded_docs"]["correct"]]
refined = refine_knowledge(state["question"], correct_docs)
return {"refined_knowledge": refined}
def search_and_combine(state: CRAGState) -> CRAGState:
"""Use retrieved + web search."""
# Use what we have
usable_docs = (
[d["document"] for d in state["graded_docs"]["correct"]] +
[d["document"] for d in state["graded_docs"]["ambiguous"]]
)
# Add web search
web_docs = web_search_fallback(state["question"])
all_docs = usable_docs + web_docs
refined = refine_knowledge(state["question"], all_docs)
return {"refined_knowledge": refined, "web_results": web_docs}
def web_search_only(state: CRAGState) -> CRAGState:
"""Fallback to web search."""
web_docs = web_search_fallback(state["question"])
refined = refine_knowledge(state["question"], web_docs)
return {"refined_knowledge": refined, "web_results": web_docs}
def generate_answer(state: CRAGState) -> CRAGState:
"""Generate final answer from refined knowledge."""
llm = ChatOpenAI(model="gpt-4")
prompt = f"""Answer the question based only on the provided knowledge.
If the knowledge is insufficient, say so.
Question: {state['question']}
Knowledge:
{state['refined_knowledge']}
Answer:"""
answer = llm.invoke(prompt).content
return {"final_answer": answer}
# Build the graph
workflow = StateGraph(CRAGState)
workflow.add_node("retrieve", retrieve)
workflow.add_node("grade", grade_documents)
workflow.add_node("use_retrieved", use_retrieved)
workflow.add_node("search_and_combine", search_and_combine)
workflow.add_node("web_search", web_search_only)
workflow.add_node("generate", generate_answer)
workflow.set_entry_point("retrieve")
workflow.add_edge("retrieve", "grade")
workflow.add_conditional_edges(
"grade",
route_by_quality,
{
"correct": "use_retrieved",
"ambiguous": "search_and_combine",
"incorrect": "web_search"
}
)
workflow.add_edge("use_retrieved", "generate")
workflow.add_edge("search_and_combine", "generate")
workflow.add_edge("web_search", "generate")
workflow.add_edge("generate", END)
crag = workflow.compile()
# Run
result = crag.invoke({"question": "What is the latest LangChain version?"})Evaluation Metrics
def evaluate_crag(test_cases: list, crag_pipeline) -> dict:
"""Evaluate CRAG performance."""
metrics = {
"correct_retrievals": 0,
"fallback_triggered": 0,
"answer_quality": [],
"grounding_score": []
}
for case in test_cases:
result = crag_pipeline.invoke({"question": case["question"]})
# Track retrieval quality
if result["retrieval_quality"] == "CORRECT":
metrics["correct_retrievals"] += 1
else:
metrics["fallback_triggered"] += 1
# Grade answer quality
quality = grade_answer(
case["question"],
result["final_answer"],
case.get("expected_answer")
)
metrics["answer_quality"].append(quality)
# Check grounding
grounding = check_grounding(
result["final_answer"],
result["refined_knowledge"]
)
metrics["grounding_score"].append(grounding)
return {
"retrieval_success_rate": metrics["correct_retrievals"] / len(test_cases),
"fallback_rate": metrics["fallback_triggered"] / len(test_cases),
"avg_answer_quality": sum(metrics["answer_quality"]) / len(test_cases),
"avg_grounding": sum(metrics["grounding_score"]) / len(test_cases)
}Best Practices
- Calibrate grader thresholds - test on labeled data
- Log all decisions - track when/why fallback triggers
- Rate limit web search - prevent abuse and cost overruns
- Cache web results - same queries shouldn't re-search
- Monitor grading accuracy - grader can also make mistakes
- Set confidence thresholds - don't use ambiguous docs below 0.5