GitLab MR Review Agent

Problem context

At AlfaStrakhovanie, our data engineering team processes 50+ merge requests per week across Spark jobs, dbt models, and Airflow DAGs. Senior engineers were spending 4–6 hours/week on reviews that were repetitive: checking naming conventions, validating SQL query patterns, verifying that new models referenced the correct data contracts.

The goal: automate the repetitive 60% of review feedback so senior engineers can focus on architecture and logic.

Architecture

GitLab Webhook (MR event)
        │
        ▼
FastAPI service (webhook handler)
        │
        ├── fetch MR diff via GitLab API
        ├── chunk diff by file/hunk
        │
        ▼
Qdrant retrieval (RAG)
        │
        ├── query: Confluence data contracts
        ├── query: internal coding standards docs
        ├── query: similar past MR reviews
        │
        ▼
Claude API (claude-3-5-sonnet)
        │
        ├── system prompt: role + company context
        ├── retrieved context (top-k chunks)
        ├── MR diff
        │
        ▼
Structured JSON output (per-file comments)
        │
        ▼
GitLab API → post inline comments on MR

Trade-offs explored

RAG vs. fine-tuning: Fine-tuning was ruled out — our codebase evolves faster than any fine-tuning cycle. RAG over live Confluence gives current context without retraining.

Claude vs. GPT-4o: Tested both. Claude produced more actionable feedback with fewer false positives on Spark job patterns. Lower hallucination rate on domain-specific SQL.

Chunk size for diffs: Hunks of ~150 lines performed better than full files. Large diffs (500+ lines) caused context dilution and worse recall.

Implementation highlights

# Simplified retrieval step
async def retrieve_context(diff_chunk: str, k: int = 8) -> list[str]:
    embedding = await embed(diff_chunk)
 
    results = await qdrant_client.search(
        collection_name="internal_docs",
        query_vector=embedding,
        limit=k,
        with_payload=True,
    )
 
    return [hit.payload["text"] for hit in results]

# Review generation
async def generate_review(diff: str, context: list[str]) -> ReviewOutput:
    response = await anthropic.messages.create(
        model="claude-3-5-sonnet-20241022",
        max_tokens=2048,
        system=REVIEW_SYSTEM_PROMPT,
        messages=[
            {
                "role": "user",
                "content": f"<context>\n{chr(10).join(context)}\n</context>\n\n<diff>\n{diff}\n</diff>",
            }
        ],
    )
    return parse_review(response.content[0].text)

Results

Review turnaround: reduced from avg. 18h → 11h (−40%)
False positive rate: ~8% (manually validated over 200 reviews)
Adoption: 3 teams actively using it; 2 more onboarding
Cost: ~$0.04 per MR review (Claude API). Pays back in minutes of senior engineer time saved.

What I'd do differently

Chunk by semantic boundary, not line count. Splitting mid-function degraded context quality. A tree-sitter parser for Python/SQL would be better.
Add confidence scores. Some comments are high-confidence (naming violations), others speculative (performance concerns). Surface that to reviewers.
Human-in-the-loop for critical paths. Production pipeline MRs should require explicit human sign-off even if the agent approves.