Setting up Infracost plan policies with custom data input

When using Infracost with Spacelift plan policies to monitor cost changes, you need to properly configure both the data collection and policy structure to access cost information.

Required Setup

To use Infracost data in your plan policies, you need:

1. Set the INFRACOST_API_KEY environment variable

2. Add the infracost label to your stack

3. Add a pre-plan hook with the following commands:

   infracost breakdown --path . --out-file infracost.custom.spacelift.json --format json

The pre-plan hook is essential for generating the custom data that your policy can access, even though it may not be explicitly mentioned in the Infracost documentation.

Correct Policy Structure

When writing your plan policy to access Infracost data, make sure to include custom in the data path. The correct structure is:

input.third_party_metadata.custom.infracost.projects[0].breakdown.totalMonthlyCost

Here's an example policy snippet that warns when monthly costs exceed a threshold:

warn[sprintf("monthly cost greater than $%d ($%.2f)", [threshold, monthly_cost])] {
  threshold := 100
  monthly_cost := to_number(input.third_party_metadata.custom.infracost.projects[0].breakdown.totalMonthlyCost)
  monthly_cost > threshold
}

Testing Your Policy

Use the policy workbench to test your policy. Add sample := true to your policy to see the actual data structure and verify the correct path to your Infracost data.

Policy Behavior with PR Comments

When using deny rules that fail the run, Infracost breakdown information will not appear in GitHub PR comments since the API call is made after a successful plan phase. If you want to see both the cost breakdown and policy warnings in your PR, use warn instead of deny in your policy rules.

Common Error Messages

You may see error messages like "terraform JSON plan is missing the 'format_version' property" in your logs. This is typically a harmless warning and doesn't prevent the policy from working correctly as long as you have the proper setup described above.