mgajewskik/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.

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

| Failure category | Symptoms | Primary references | |------------------|----------|--------------------| | Identity churn | Resource addresses shift after refactor, count index churn, missing moved blocks | Code Patterns: count vs for_each, Code Patterns: moved blocks, Code Patterns: LLM mistakes | | Secret exposure | Secrets in defaults, state, logs, CI artifacts | Security & Compliance, Code Patterns: write-only, State Management | | Blast radius | Oversized stacks, shared prod/non-prod state, unsafe applies | State Management, Module Patterns | | CI drift | Local plan ≠ CI plan, apply without reviewed artifact, unpinned versions | CI/CD Workflows, Code Patterns: versions | | Compliance gaps | Missing policy stage, no approval model, no evidence retention | Security & Compliance, CI/CD Workflows | | Testing blind spots | Plan-only validation of computed values, set-type indexing, mock/real confusion | Testing Frameworks | | State corruption / recovery | Stuck lock, backend migration, drift reconciliation | State Management | | Provider upgrade risk | Breaking-change provider bump, unpinned modules | Code Patterns: versions, Module Patterns | | Provider lifecycle | Removing a provider with resources still in state, orphaned resources, removed block usage | State Management: Provider Removal |

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

| Type | When to Use | Scope | |------|-------------|-------| | Resource module | Single logical group of connected resources | VPC + subnets, SG + rules | | Infrastructure module | Collection of resource modules for a purpose | Multiple resource modules in one region/account | | Composition | Complete infrastructure | Spans multiple regions/accounts |

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?

| Situation | Approach | Tools | Cost | |-----------|----------|-------|------| | Quick syntax check | Static analysis | validate, fmt | Free | | Pre-commit validation | Static + lint | validate, tflint, trivy, checkov | Free | | Terraform 1.6+, simple logic | Native test framework | terraform test | Free-Low | | Pre-1.6, or Go expertise | Integration testing | Terratest | Low-Med | | Security/compliance focus | Policy as code | OPA, Sentinel | Free | | Cost-sensitive workflow | Mock providers (1.7+) | Native tests + mocks | Free | | Multi-cloud, complex | Full integration | Terratest + real infra | Med-High |

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

| Scenario | Use | Why | |----------|-----|-----| | Boolean condition (create / don't) | count = condition ? 1 : 0 | Optional singleton toggle | | Items may be reordered or removed | for_each = toset(list) | Stable resource addresses | | Reference by key | for_each = map | Named access | | Multiple named resources | for_each | Better identity stability |

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.

Security & Compliance

Essential checks:

trivy config .
checkov -d .

Don't: store secrets in variables or .tfvars, use default VPC, skip encryption, open security groups to 0.0.0.0/0, use inline ingress/egress blocks in aws_security_group.

Do: source secrets from AWS Secrets Manager / Parameter Store or use write_only arguments on 1.11+, create dedicated VPCs, enforce encryption at rest and TLS, least-privilege SGs, use separate aws_vpc_security_group_{ingress,egress}_rule resources (AWS provider v5+).

Marking a variable sensitive = true masks display only — the value still lives in state. Use write_only / *_wo on 1.11+, or keep secret material out of Terraform entirely via runtime lookups.

See Security & Compliance for trivy/checkov pipelines, state-file hardening, compliance mappings, and the LLM-mistake checklist.

State Management

Never use local state in teams or production. Remote backends provide automatic locking, encryption, versioning, audit logging, and safe collaboration.

Minimum Viable Backend (AWS S3, 1.10+)

terraform {
  backend "s3" {
    bucket        = "my-terraform-state"
    key           = "prod/vpc/terraform.tfstate"
    region        = "us-east-1"
    encrypt       = true
    use_lockfile  = true   # Native S3 locking, 1.10+
  }
}

On Terraform < 1.10, use dynamodb_table = "terraform-state-lock" instead of use_lockfile. Azure Storage, GCS, and Terraform Cloud all offer built-in locking — see the State Management reference for syntax.

State Organization

| Pattern | Use When | Example Path | |---------|----------|--------------| | Per environment | Different teams per env | prod/terraform.tfstate, staging/... | | Per component | Independent lifecycles | prod/vpc/, prod/eks/, prod/rds/ | | Hybrid (recommended) | Both benefits | prod/networking/, prod/compute/, staging/networking/ |

Split state when: different teams, different update cadences, or >500 resources. Combine when: tightly coupled resources, <100 resources, same lifecycle.

See State Management for locking, migration, multi-team isolation, disaster recovery, and the LLM-mistake checklist.

Version Management

| Component | Strategy | Example | |-----------|----------|---------| | Terraform runtime | Pin minor | required_version = "~> 1.9" | | Providers | Pin major | version = "~> 5.0" | | Modules (prod) | Pin exact | version = "5.1.2" | | Modules (dev) | Allow patch | version = "~> 5.1" |

Commit .terraform.lock.hcl intentionally. Keep provider/runtime upgrades in a separate PR from functional changes. See Code Patterns: Version Management for constraint syntax and upgrade workflow.

Modern Terraform Features (1.0+)

| Feature | Min version | Common use | |---------|-------------|------------| | try() | 0.13+ | Safe fallbacks, replaces element(concat()) | | nullable = false | 1.1+ | Prevent null silently overriding defaults | | moved blocks | 1.1+ | Refactor without destroy/recreate | | optional() with defaults | 1.3+ | Typed object attributes | | import blocks | 1.5+ | Declarative imports, reviewable in VCS | | check blocks | 1.5+ | Runtime assertions | | Native terraform test | 1.6+ | Built-in test framework | | Mock providers | 1.7+ | Cost-free unit testing | | removed blocks | 1.7+ | Declarative resource removal | | Provider-defined functions | 1.8+ | Provider-specific transformations (requires provider to declare functions) | | Cross-variable validation | 1.9+ | Reference other var.* in validation blocks | | write_only arguments | 1.11+ | Secrets never stored in state | | S3 native lock-file | 1.10+ | State locking without DynamoDB |

Before emitting a feature, verify the runtime floor. See Code Patterns: Feature Guard Table for the full table with common LLM error patterns per feature.

Runtime-Specific Guidance

• Terraform 1.0-1.5 / OpenTofu 1.0-1.5: Terratest for integration, static analysis + plan validation only (no native tests).
• 1.6+: native terraform test / tofu test available — migrate simple unit tests, keep Terratest for complex integration.
• 1.7+: mock providers cut test cost — mock for unit tests, real runs for final integration.
• 1.10+: S3 native lock-file (use_lockfile) is the correct default for new configurations — DynamoDB locking is no longer required.
• 1.11+: write_only arguments for secret handling keep credentials out of state.
• Terraform vs OpenTofu: both supported. For licensing, governance, and feature delta, see Quick Reference: Terraform vs OpenTofu.

Reference Files

Progressive disclosure — essentials here, depth on demand:

• Testing Frameworks — static analysis, native tests, Terratest, mock providers
• Module Patterns — structure, variable/output contracts, terraform_remote_state rules, release checklist
• CI/CD Workflows — GitHub Actions, GitLab CI, Atlantis, cost control
• Security & Compliance — trivy/checkov, secrets handling, compliance mappings
• State Management — backends, locking, migration, multi-team, recovery
• Code Patterns — block ordering, count/for_each deep dive, modern features, version management, locals
• Quick Reference — command cheat sheets, flowcharts, troubleshooting

License

Apache License 2.0. See LICENSE for full terms.

Copyright © 2026 Anton Babenko