Build an AI Terraform Cost Preview Bot with Claude API

pip install anthropic infracost-python    # or shell out to the infracost CLI directly

import subprocess
import json
import anthropic
 
client = anthropic.Anthropic()
 
 
def get_cost_estimate(terraform_dir: str) -> dict:
    """Infracost does the actual pricing math — this tool's value is
    explaining the WHY, not recalculating cloud pricing itself."""
    result = subprocess.run(
        ["infracost", "breakdown", "--path", terraform_dir, "--format", "json"],
        capture_output=True, text=True
    )
    return json.loads(result.stdout)
 
 
def get_cost_diff(terraform_dir: str, base_branch: str) -> dict:
    result = subprocess.run(
        ["infracost", "diff", "--path", terraform_dir, "--compare-to", f"infracost-base-{base_branch}.json"],
        capture_output=True, text=True
    )
    return json.loads(result.stdout)

EXPLAIN_PROMPT = """Explain this Terraform infrastructure cost change in
plain English for a PR review comment.
 
Cost diff data:
{cost_diff}
 
Terraform plan resource changes:
{plan_summary}
 
Write a concise explanation that:
1. States the total monthly cost delta clearly upfront
2. Identifies the SPECIFIC resource(s) driving the biggest cost change
   and why (e.g. "instance type upgraded from r6g.large to r6g.2xlarge"
   not just "compute costs increased")
3. Flags if any single change looks like it might be unintentional
   (e.g. a resource count went from 1 to 3 across all AZs when the PR
   description suggests only one new environment was intended)
4. Keep it under 150 words — this goes in a PR comment, not a report
 
Do not just restate numbers — explain what's actually driving them."""
 
 
def generate_cost_explanation(cost_diff: dict, plan_summary: dict) -> str:
    response = client.messages.create(
        model="claude-sonnet-5",
        max_tokens=500,
        messages=[{
            "role": "user",
            "content": EXPLAIN_PROMPT.format(cost_diff=cost_diff, plan_summary=plan_summary)
        }]
    )
    return response.content[0].text

name: Terraform Cost Preview
on:
  pull_request:
    paths: ["**.tf"]
 
jobs:
  cost-preview:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0
 
      - uses: infracost/actions/setup@v3
        with:
          api-key: ${{ secrets.INFRACOST_API_KEY }}
 
      - name: Generate baseline cost
        run: |
          git checkout ${{ github.event.pull_request.base.sha }}
          infracost breakdown --path . --format json --out-file infracost-base.json
 
      - name: Generate cost diff
        run: |
          git checkout ${{ github.event.pull_request.head.sha }}
          infracost diff --path . --compare-to infracost-base.json --format json --out-file cost-diff.json
 
      - name: Generate AI explanation and post comment
        run: python cost_bot.py --diff cost-diff.json --pr ${{ github.event.pull_request.number }}
        env:
          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

## 💰 Infrastructure Cost Preview
 
**Estimated monthly cost change: +$847/month** (current: $2,140 → new: $2,987)
 
The increase is driven almost entirely by one change: the `payments-db`
RDS instance is being upgraded from `db.r6g.large` to `db.r6g.2xlarge`
(+$612/mo), plus a new read replica being added in `us-west-2` (+$235/mo).
 
⚠️ Worth double-checking: the PR description mentions "prep for Black
Friday traffic" but this instance upgrade is permanent, not autoscaled —
if this is meant to be temporary capacity for a traffic event, consider
using RDS's built-in storage autoscaling or a scheduled instance resize
instead of a permanent 2x instance class bump.
 
All other resource changes in this PR are cost-neutral or under $10/mo.