I Built an AI-Powered Japanese Trade Law Compliance Checker as a Chrome Extension

Chrome Extension -> POST /api/llm/chat (Vercel) -> Claude API -> Response

// /api/llm/chat/route.ts
export async function POST(request: Request) {
  const { messages, system } = await request.json();

  const response = await fetch('https://api.anthropic.com/v1/messages', {
    method: 'POST',
    headers: {
      'x-api-key': process.env.ANTHROPIC_API_KEY!,
      'anthropic-version': '2023-06-01',
      'content-type': 'application/json',
    },
    body: JSON.stringify({
      model: 'claude-haiku-4-5-20251001',
      max_tokens: 1024,
      system,
      messages,
    }),
  });

  return Response.json(await response.json());
}

function extractContractText(): string {
  // Priority 1: User-selected text
  const selection = window.getSelection()?.toString().trim();
  if (selection && selection.length > 50) return selection;

  // Priority 2: Active textarea / contenteditable
  const activeEl = document.activeElement;
  if (activeEl instanceof HTMLTextAreaElement) return activeEl.value;
  if (activeEl?.getAttribute('contenteditable') === 'true') {
    return activeEl.textContent || '';
  }

  // Priority 3: Main content heuristic
  const candidates = [
    document.querySelector('article'),
    document.querySelector('main'),
    document.querySelector('[role="main"]'),
    document.querySelector('.contract'),
    document.body,
  ].filter(Boolean) as Element[];

  for (const el of candidates) {
    const text = el.textContent?.trim() || '';
    if (text.length > 200) return text.slice(0, 8000);
  }

  return '';
}

const SYSTEM_PROMPT = `You are a Japanese Subcontracting Act compliance expert.

Analyze the provided contract text and identify potential violations or missing required elements.

Output a JSON object with this structure:
{
  "applicable": boolean,
  "violations": [
    {
      "type": string,
      "severity": "high" | "medium" | "low",
      "description": string,
      "relevant_text": string,
      "legal_basis": string
    }
  ],
  "required_clauses_present": {
    "transaction_type": boolean,
    "delivery_date": boolean,
    "inspection_date": boolean,
    "payment_method": boolean,
    "payment_date": boolean,
    "price": boolean
  },
  "summary": string
}

Focus on:
1. Payment terms exceeding 60 days after receipt
2. Promissory note payment requirements
3. Missing mandatory order form fields (Article 3)
4. Price reduction clauses that may violate Article 4
5. Return conditions that may violate Article 4`;

function renderResults(analysis: AnalysisResult): void {
  const container = document.getElementById('results')!;

  if (!analysis.applicable) {
    container.innerHTML = `<div class="notice">この契約書には下請法が適用されない可能性があります。</div>`;
    return;
  }

  const highCount = analysis.violations.filter(v => v.severity === 'high').length;
  const statusClass = highCount > 0 ? 'danger' :
    analysis.violations.length > 0 ? 'warning' : 'safe';

  container.innerHTML = `
    <div class="status ${statusClass}">
      ${highCount > 0 ? `Warning: ${highCount} critical issues` :
        analysis.violations.length > 0 ? `Note: ${analysis.violations.length} issues` :
        'No issues detected'}
    </div>
    ${analysis.violations.map(v => `
      <div class="violation severity-${v.severity}">
        <p>${v.description}</p>
        <code>${v.relevant_text}</code>
        <small>${v.legal_basis}</small>
      </div>
    `).join('')}
    <div class="summary">${analysis.summary}</div>`;
}