Terraform Skill

Terraform Skill for Claude

Diagnose-first guidance for Terraform and OpenTofu. Core file is a workflow; depth lives in references loaded on demand.

Response Contract

Every Terraform/OpenTofu response must include:

1. Assumptions & version floor — runtime (terraform or tofu), exact version, providers, state backend, execution path (local/CI/Cloud/Atlantis), environment criticality. State assumptions explicitly if the user did not provide them.
2. Risk category addressed — one or more of: identity churn, secret exposure, blast radius, CI drift, compliance gaps, state corruption, provider upgrade risk, testing blind spots.
3. Chosen remediation & tradeoffs — what was chosen, what was traded off, why.
4. Validation plan — exact commands (fmt -check, validate, plan -out, policy check) tailored to runtime and risk tier.
5. Rollback notes — for any destructive or state-mutating change: how to undo, what evidence to keep.

Never recommend direct production apply without a reviewed plan artifact and approval.

Never run terraform destroy (targeted or full) without first running terraform plan -destroy and showing the user every resource that will be deleted — including implicit dependents pulled in via locals or for_each. Get explicit confirmation before proceeding. Never use -auto-approve on destroy.

Workflow

1. Capture execution context — runtime+version, provider(s), backend, execution path, environment criticality.
2. Diagnose failure mode(s) using the routing table below. If intent spans categories, load both references.
3. Load only the matching reference file(s) — do not preload depth the task does not need.
4. Propose fix with risk controls — why this addresses the mode, what could still go wrong, guardrails (tests/approvals/rollback).
5. Generate artifacts — HCL, migration blocks (moved, import), CI changes, policy rules.
6. Validate before finalizing — run validation commands tailored to risk tier.
7. Emit the Response Contract at the end.

Diagnose Before You Generate

When to Use This Skill

Activate when: creating or reviewing Terraform/OpenTofu configurations or modules, setting up or debugging tests, structuring multi-environment deployments, implementing IaC CI/CD, choosing module patterns or state organization, configuring or migrating remote state backends.

Don't use for: basic HCL syntax questions Claude already knows, provider API reference (link to docs), cloud-platform questions unrelated to Terraform/OpenTofu.

Core Principles

Module Hierarchy

Flow: resource → resource module → infrastructure module → composition.

Directory Layout

environments/   # prod/ staging/ dev/  — per-env configurations
modules/        # networking/ compute/ data/ — reusable modules
examples/       # minimal/ complete/ — docs + integration fixtures

Separate environments from modules. Use examples/ as both documentation and test fixtures. Keep modules small and single-responsibility.

See Module Patterns for architecture principles, naming conventions, variable/output contracts.

Naming Conventions (summary)

• Descriptive resource names (aws_instance.web_server, not aws_instance.main)
• Reserve this for genuine singleton resources only
• Prefix variables with context (vpc_cidr_block, not cidr)
• Standard files: main.tf, variables.tf, outputs.tf, versions.tf

See Module Patterns: Variable Naming and Code Patterns: Block Ordering for examples.

Block Ordering (summary)

Resource blocks: count/for_each first → arguments → tags → depends_on → lifecycle. Variable blocks: description → type → default → validation → nullable → sensitive.

See Code Patterns: Block Ordering & Structure for the full rules and examples.

Testing Strategy

Decision Matrix: Which Testing Approach?

Native Test Rules (1.6+)

Before writing test code: validate resource schemas via Terraform MCP so assertions target real attributes.

• command = plan — fast, for input-derived values only
• command = apply — required for computed values (ARNs, generated names) and set-type nested blocks
• Set-type blocks cannot be indexed with [0] — use for expressions or materialize via command = apply
• Common set types: S3 encryption rules, lifecycle transitions, IAM policy statements

See Testing Frameworks for static-analysis pipelines, native-test patterns, Terratest integration, mock providers, and the full LLM-mistake checklist.

Count vs For_Each — Quick Rule

Never use list index as long-lived identity — removing a middle element reshuffles every address after it. For the decision matrix, safe migration playbook, moved block patterns, and known-at-plan failure cases, see Code Patterns: count vs for_each.

Locals for Dependency Management

Using try() in a local to prefer a conditional resource's attribute over its parent is a specialized but high-value pattern — it forces correct deletion order without explicit depends_on. Common use: VPC + secondary CIDR associations + subnets.

See Code Patterns: Locals for Dependency Management for the full pattern and worked example.

Module Development

Standard layout:

my-module/
├── README.md       # Usage documentation
├── main.tf         # Primary resources
├── variables.tf    # Typed inputs with descriptions
├── outputs.tf      # Output values
├── versions.tf     # required_version + required_providers
├── examples/
│   ├── minimal/
│   └── complete/
└── tests/
    └── module_test.tftest.hcl   # or Go for Terratest

Variable contracts: always description, always explicit type, use validation for complex constraints, use sensitive = true for secrets, prefer optional() with typed defaults (1.3+) over untyped map(any).

Output contracts: always description, mark sensitive outputs, expose stable subsets (not whole provider objects).

See Module Patterns for the full contract patterns, module release checklist, and LLM-mistake checklist.

CI/CD

Pipeline stages: validate → test → plan → apply (with environment protection).

Cost control: mock providers on PR validation, real-cloud integration only on main or scheduled, tag test resources, auto-cleanup.

Drift prevention: pin runtime and providers, commit .terraform.lock.hcl, apply the reviewed plan artifact from the plan stage (do not re-run plan inside the apply job), run policy/security stage on every path to apply.

See CI/CD Workflows for GitHub Actions, GitLab CI, and Atlantis templates plus the LLM-mistake checklist.