Refactoring Agent: AI-Powered Code Improvement and Technical Debt Reduction

Why Automated Refactoring Matters

Technical debt accumulates silently. Functions grow too long, classes take on too many responsibilities, and duplicated code spreads across modules. Developers know these problems exist but rarely have dedicated time to fix them. A refactoring agent identifies code smells, proposes targeted improvements, applies them, and verifies that all tests still pass.

The critical requirement is safety: every refactoring must preserve existing behavior. The agent must prove that its changes do not break anything.

Detecting Code Smells

The agent starts with static analysis to identify candidates for refactoring.

import ast
import os
from dataclasses import dataclass
from openai import OpenAI

client = OpenAI()

@dataclass
class CodeSmell:
    file_path: str
    function_name: str
    smell_type: str
    severity: str
    description: str
    source_code: str

class RefactoringAgent:
    def __init__(self, project_dir: str, model: str = "gpt-4o"):
        self.project_dir = project_dir
        self.model = model
        self.smell_thresholds = {
            "long_function": 50,
            "too_many_params": 5,
            "deep_nesting": 4,
            "duplicate_blocks": 3,
        }

    def detect_smells(self) -> list[CodeSmell]:
        smells = []
        for root, _, files in os.walk(self.project_dir):
            for fname in files:
                if not fname.endswith(".py") or fname.startswith("test_"):
                    continue
                path = os.path.join(root, fname)
                with open(path) as f:
                    source = f.read()
                tree = ast.parse(source)
                smells.extend(self._analyze_file(tree, source, path))
        smells.sort(key=lambda s: (
            {"high": 0, "medium": 1, "low": 2}[s.severity]
        ))
        return smells

    def _analyze_file(
        self, tree: ast.Module, source: str, path: str
    ) -> list[CodeSmell]:
        smells = []
        for node in ast.walk(tree):
            if not isinstance(node, ast.FunctionDef):
                continue
            func_source = ast.get_source_segment(source, node) or ""
            line_count = len(func_source.split("\n"))
            param_count = len(node.args.args)
            nesting = self._max_nesting(node)

            if line_count > self.smell_thresholds["long_function"]:
                smells.append(CodeSmell(
                    file_path=path, function_name=node.name,
                    smell_type="long_function", severity="medium",
                    description=f"Function is {line_count} lines long",
                    source_code=func_source,
                ))
            if param_count > self.smell_thresholds["too_many_params"]:
                smells.append(CodeSmell(
                    file_path=path, function_name=node.name,
                    smell_type="too_many_params", severity="medium",
                    description=f"Function has {param_count} parameters",
                    source_code=func_source,
                ))
            if nesting > self.smell_thresholds["deep_nesting"]:
                smells.append(CodeSmell(
                    file_path=path, function_name=node.name,
                    smell_type="deep_nesting", severity="high",
                    description=f"Nesting depth of {nesting}",
                    source_code=func_source,
                ))
        return smells

Generating Refactoring Plans

For each code smell, the agent produces a specific refactoring plan with before and after code.

@dataclass
class RefactoringPlan:
    smell: CodeSmell
    pattern: str
    original_code: str
    refactored_code: str
    explanation: str

def plan_refactoring(self, smell: CodeSmell) -> RefactoringPlan:
    response = client.chat.completions.create(
        model=self.model,
        messages=[
            {"role": "system", "content": """You are a refactoring expert.
Propose a refactoring for the given code smell.

Rules:
- Preserve ALL existing behavior exactly
- Use standard refactoring patterns (Extract Method,
  Introduce Parameter Object, Replace Nested Conditional
  with Guard Clauses, etc.)
- Keep the public interface unchanged
- Name extracted functions clearly

Return JSON with:
- "pattern": name of the refactoring pattern applied
- "refactored_code": the improved code
- "explanation": why this refactoring improves the code"""},
            {"role": "user", "content": (
                f"Smell: {smell.smell_type} - {smell.description}\n"
                f"Function: {smell.function_name}\n"
                f"Code:\n{smell.source_code}"
            )},
        ],
        temperature=0.2,
        response_format={"type": "json_object"},
    )

    import json
    data = json.loads(response.choices[0].message.content)
    return RefactoringPlan(
        smell=smell,
        pattern=data["pattern"],
        original_code=smell.source_code,
        refactored_code=data["refactored_code"],
        explanation=data["explanation"],
    )

Applying and Validating Changes

Safety is ensured by running the full test suite before and after each refactoring.

import subprocess

def apply_refactoring(self, plan: RefactoringPlan) -> dict:
    with open(plan.smell.file_path) as f:
        original_file = f.read()

    if plan.original_code not in original_file:
        return {"success": False, "reason": "Original code not found"}

    baseline = subprocess.run(
        ["python", "-m", "pytest", "-q", "--tb=line"],
        capture_output=True, text=True, cwd=self.project_dir,
    )
    baseline_passed = baseline.returncode == 0

    refactored_file = original_file.replace(
        plan.original_code, plan.refactored_code, 1
    )

    try:
        with open(plan.smell.file_path, "w") as f:
            f.write(refactored_file)

        after = subprocess.run(
            ["python", "-m", "pytest", "-q", "--tb=line"],
            capture_output=True, text=True, cwd=self.project_dir,
        )

        if after.returncode == 0:
            return {
                "success": True,
                "pattern": plan.pattern,
                "explanation": plan.explanation,
            }
        else:
            with open(plan.smell.file_path, "w") as f:
                f.write(original_file)
            return {
                "success": False,
                "reason": f"Tests failed after refactoring: {after.stdout[-500:]}",
            }
    except Exception as e:
        with open(plan.smell.file_path, "w") as f:
            f.write(original_file)
        return {"success": False, "reason": str(e)}

The pattern is clear: save the original, apply the change, run tests, and revert if anything fails. This guarantees your codebase is never left in a broken state.

FAQ

How does the agent decide which refactorings to apply first?

Code smells are sorted by severity. Deep nesting is high severity because it directly impacts readability and bug risk. Long functions are medium. The agent processes the highest-severity smells first, which means the most impactful improvements happen first within your time budget.

Can the refactoring agent handle changes that span multiple files?

Yes, but multi-file refactorings are riskier. For cross-file changes like extracting a shared utility function, the agent generates changes for all affected files and applies them atomically. If any test fails after the combined change, all files are reverted together.

What if there are no tests for the code being refactored?

The agent should generate tests first using a test generation pipeline, then run the refactoring. Without tests, there is no way to verify behavior preservation. The agent flags codebases with low coverage and recommends adding tests before attempting refactoring.

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