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terraform-test

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by hashicorp · part of hashicorp/agent-skills

Comprehensive guide for writing and running Terraform tests with assertions, mocking, and module validation. Write test files using .tftest.hcl syntax with run blocks that execute in plan or apply mode, supporting sequential and parallel execution with optional state isolation Assert conditions on resource attributes, outputs, and data sources; use expect_failures to validate that invalid inputs are properly rejected Mock providers (Terraform 1.7.0+) simulate infrastructure behavior without...

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🧩 One of 7 skills in the hashicorp/agent-skills package — works on its own, and pairs well with its siblings.

Comprehensive guide for writing and running Terraform tests with assertions, mocking, and module validation. Write test files using .tftest.hcl syntax with run blocks that execute in plan or apply mode, supporting sequential and parallel execution with optional state isolation Assert conditions on resource attributes, outputs, and data sources; use expect_failures to validate that invalid inputs are properly rejected Mock providers (Terraform 1.7.0+) simulate infrastructure behavior without...

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name: terraform-test description: Comprehensive guide for writing and running Terraform tests. Use when creating test files (.tftest.hcl), writing test scenarios with run blocks, validating infrastructure behavior with assertions, mocking providers and data sources, testing module outputs and resource configurations, or troubleshooting Terraform test syntax and execution. metadata: copyright: Copyright IBM Corp. 2026 version: "0.0.2"

Terraform Test

Terraform's built-in testing framework validates that configuration updates don't introduce breaking changes. Tests run against temporary resources, protecting existing infrastructure and state files.

Reference Files

  • references/MOCK_PROVIDERS.md — Mock provider syntax, common defaults, when to use mocks (Terraform 1.7.0+ only — skip if the user's version is below 1.7)
  • references/CI_CD.md — GitHub Actions and GitLab CI pipeline examples
  • references/EXAMPLES.md — Complete example test suite (unit, integration, and mock tests for a VPC module)

Read the relevant reference file when the user asks about mocking, CI/CD integration, or wants a full example.

Core Concepts

  • Test file (.tftest.hcl / .tftest.json): Contains run blocks that validate your configuration
  • Run block: A single test scenario with optional variables, providers, and assertions
  • Assert block: Conditions that must be true for the test to pass
  • Mock provider: Simulates provider behavior without real infrastructure (Terraform 1.7.0+)
  • Test modes: apply (default, creates real resources) or plan (validates logic only)

File Structure

Copy & paste — that's it
my-module/
├── main.tf
├── variables.tf
├── outputs.tf
└── tests/
    ├── defaults_unit_test.tftest.hcl         # plan mode — fast, no resources
    ├── validation_unit_test.tftest.hcl        # plan mode
    └── full_stack_integration_test.tftest.hcl # apply mode — creates real resources

Use *_unit_test.tftest.hcl for plan-mode tests and *_integration_test.tftest.hcl for apply-mode tests so they can be filtered separately in CI.

Test File Structure

Copy & paste — that's it
# Optional: test-wide settings
test {
  parallel = true  # Enable parallel execution for all run blocks (default: false)
}

# Optional: file-level variables (highest precedence, override all other sources)
variables {
  aws_region    = "us-west-2"
  instance_type = "t2.micro"
}

# Optional: provider configuration
provider "aws" {
  region = var.aws_region
}

# Required: at least one run block
run "test_default_configuration" {
  command = plan

  assert {
    condition     = aws_instance.example.instance_type == "t2.micro"
    error_message = "Instance type should be t2.micro by default"
  }
}

Run Block

Copy & paste — that's it
run "test_name" {
  command  = plan  # or apply (default)
  parallel = true  # optional, since v1.9.0

  # Override file-level variables
  variables {
    instance_type = "t3.large"
  }

  # Reference a specific module
  module {
    source  = "./modules/vpc"  # local or registry only (not git/http)
    version = "5.0.0"          # registry modules only
  }

  # Control state isolation
  state_key = "shared_state"  # since v1.9.0

  # Plan behavior
  plan_options {
    mode    = refresh-only  # or normal (default)
    refresh = true
    replace = [aws_instance.example]
    target  = [aws_instance.example]
  }

  # Assertions
  assert {
    condition     = aws_instance.example.id != ""
    error_message = "Instance should have a valid ID"
  }

  # Expected failures (test passes if these fail)
  expect_failures = [
    var.instance_count
  ]
}

Common Test Patterns

Validate outputs

Copy & paste — that's it
run "test_outputs" {
  command = plan

  assert {
    condition     = output.vpc_id != null
    error_message = "VPC ID output must be defined"
  }

  assert {
    condition     = can(regex("^vpc-", output.vpc_id))
    error_message = "VPC ID should start with 'vpc-'"
  }
}

Conditional resources

Copy & paste — that's it
run "test_nat_gateway_disabled" {
  command = plan

  variables {
    create_nat_gateway = false
  }

  assert {
    condition     = length(aws_nat_gateway.main) == 0
    error_message = "NAT gateway should not be created when disabled"
  }
}

Resource counts

Copy & paste — that's it
run "test_resource_count" {
  command = plan

  variables {
    instance_count = 3
  }

  assert {
    condition     = length(aws_instance.workers) == 3
    error_message = "Should create exactly 3 worker instances"
  }
}

Tags

Copy & paste — that's it
run "test_resource_tags" {
  command = plan

  variables {
    common_tags = {
      Environment = "production"
      ManagedBy   = "Terraform"
    }
  }

  assert {
    condition     = aws_instance.example.tags["Environment"] == "production"
    error_message = "Environment tag should be set correctly"
  }

  assert {
    condition     = aws_instance.example.tags["ManagedBy"] == "Terraform"
    error_message = "ManagedBy tag should be set correctly"
  }
}

Data sources

Copy & paste — that's it
run "test_data_source_lookup" {
  command = plan

  assert {
    condition     = data.aws_ami.ubuntu.id != ""
    error_message = "Should find a valid Ubuntu AMI"
  }

  assert {
    condition     = can(regex("^ami-", data.aws_ami.ubuntu.id))
    error_message = "AMI ID should be in correct format"
  }
}

Validation rules

Copy & paste — that's it
run "test_invalid_environment" {
  command = plan

  variables {
    environment = "invalid"
  }

  expect_failures = [
    var.environment
  ]
}

Sequential tests with dependencies

Copy & paste — that's it
run "setup_vpc" {
  command = apply

  assert {
    condition     = output.vpc_id != ""
    error_message = "VPC should be created"
  }
}

run "test_subnet_in_vpc" {
  command = plan

  variables {
    vpc_id = run.setup_vpc.vpc_id
  }

  assert {
    condition     = aws_subnet.example.vpc_id == run.setup_vpc.vpc_id
    error_message = "Subnet should be in the VPC from setup_vpc"
  }
}

Plan options (refresh-only, targeted)

Copy & paste — that's it
run "test_refresh_only" {
  command = plan

  plan_options {
    mode = refresh-only
  }

  assert {
    condition     = aws_instance.example.tags["Environment"] == "production"
    error_message = "Tags should be refreshed correctly"
  }
}

run "test_specific_resource" {
  command = plan

  plan_options {
    target = [aws_instance.example]
  }

  assert {
    condition     = aws_instance.example.instance_type == "t2.micro"
    error_message = "Targeted resource should be planned"
  }
}

Parallel modules

Copy & paste — that's it
run "test_networking_module" {
  command  = plan
  parallel = true

  module {
    source = "./modules/networking"
  }

  assert {
    condition     = output.vpc_id != ""
    error_message = "VPC should be created"
  }
}

run "test_compute_module" {
  command  = plan
  parallel = true

  module {
    source = "./modules/compute"
  }

  assert {
    condition     = output.instance_id != ""
    error_message = "Instance should be created"
  }
}

State key sharing

Copy & paste — that's it
run "create_foundation" {
  command   = apply
  state_key = "foundation"

  assert {
    condition     = aws_vpc.main.id != ""
    error_message = "Foundation VPC should be created"
  }
}

run "create_application" {
  command   = apply
  state_key = "foundation"

  variables {
    vpc_id = run.create_foundation.vpc_id
  }

  assert {
    condition     = aws_instance.app.vpc_id == run.create_foundation.vpc_id
    error_message = "Application should use foundation VPC"
  }
}

Cleanup ordering (S3 objects before bucket)

Copy & paste — that's it
run "create_bucket" {
  command = apply

  assert {
    condition     = aws_s3_bucket.example.id != ""
    error_message = "Bucket should be created"
  }
}

run "add_objects" {
  command = apply

  assert {
    condition     = length(aws_s3_object.files) > 0
    error_message = "Objects should be added"
  }
}

# Cleanup destroys in reverse: objects first, then bucket

Multiple aliased providers

Copy & paste — that's it
provider "aws" {
  alias  = "primary"
  region = "us-west-2"
}

provider "aws" {
  alias  = "secondary"
  region = "us-east-1"
}

run "test_with_specific_provider" {
  command = plan

  providers = {
    aws = provider.aws.secondary
  }

  assert {
    condition     = aws_instance.example.availability_zone == "us-east-1a"
    error_message = "Instance should be in us-east-1 region"
  }
}

Complex conditions

Copy & paste — that's it
assert {
  condition = alltrue([
    for subnet in aws_subnet.private :
    can(regex("^10\\.0\\.", subnet.cidr_block))
  ])
  error_message = "All private subnets should use 10.0.0.0/8 CIDR range"
}

Cleanup

Resources are destroyed in reverse run block order after test completion. This matters for dependencies (e.g., S3 objects before bucket). Use terraform test -no-cleanup to skip cleanup for debugging.

Best Practices

  1. Naming: *_unit_test.tftest.hcl for plan mode, *_integration_test.tftest.hcl for apply mode
  2. Test naming: Use descriptive run block names that explain the scenario being tested
  3. Default to plan: Use command = plan unless you need to test real resource behavior
  4. Use mocks for external dependencies — faster and no credentials needed (see references/MOCK_PROVIDERS.md)
  5. Error messages: Make them specific enough to diagnose failures without running the test again
  6. Negative tests: Use expect_failures to verify validation rules reject bad inputs
  7. Variable coverage: Test different variable combinations to validate all code paths — test variables have the highest precedence and override all other sources
  8. Module sources: Test files only support local paths and registry modules — not git or HTTP URLs
  9. Parallel execution: Use parallel = true for independent tests with different state files
  10. Cleanup: Integration tests destroy resources in reverse run block order automatically; use -no-cleanup for debugging
  11. CI/CD: Run unit tests on every PR, integration tests on merge (see references/CI_CD.md)

References