Kustomize Resource Validation

Kustomize Resource Validation -- Auto-trigger Skill

Automatically activates when any kustomization.yaml file is edited.

Activation

Trigger: Any file named kustomization.yaml inside a Kustomize module directory (any directory containing base/ and overlays/ structure) is created or edited.

Scope: This applies to both base and overlay kustomization.yaml files.

Instructions

After the user edits a kustomization.yaml, perform these validations in order. Use the file system tools (Glob, Read) to verify references -- do not guess.

1. Resource Reference Validation

For every entry in the resources: list, verify the target exists:

• Directory references (e.g., ../../base): Resolve the relative path from the kustomization.yaml location and confirm the directory exists and contains its own kustomization.yaml.
• File references (e.g., gitlab-runner-secret.yaml): Confirm the file exists in the same directory as the kustomization.yaml.

Report missing references as FAIL.

2. Patch Reference Validation

For every entry in the patches: list that uses a path: key, verify:

• The referenced YAML file exists relative to the kustomization.yaml directory.
• The file contains valid Kubernetes resource structure (must have apiVersion, kind, and metadata fields).

Report missing or malformed patches as FAIL.

3. Orphaned File Detection

List all .yaml files in the same directory as the kustomization.yaml, then check whether each file is referenced as a resources entry or a patches path entry.

Exclude from orphan detection:

• kustomization.yaml itself
• Files inside the env/ subdirectory (used by secretGenerator)
• Files matching *.env

Report unreferenced files as WARN with the message:

WARNING: <filename> exists in <directory>/ but is not referenced in kustomization.yaml. It may be orphaned.

4. Cross-Environment Consistency

This check only applies when the edited file is in an overlay directory (e.g., overlays/dev/kustomization.yaml).

Discover sibling environments by listing subdirectories under the overlays/ parent directory. Compare the resources: and patches: lists across all discovered sibling environments. Report differences as WARN -- not FAIL, since environment-specific resources are sometimes intentional.

Example warning format:

WARNING: overlays/dev/kustomization.yaml includes patch airflow-sa-patch.yaml which is absent from overlays/stg/ and overlays/prd/. Verify this is intentional.

5. Build Validation

Determine the Kustomize module from the file path (the nearest ancestor directory containing both base/ and overlays/).

• If the file is in an overlay directory: Run kustomize build for that specific environment only.
• If the file is in a base directory: Discover all environments under overlays/ and run kustomize build for each.

kustomize build <module>/overlays/<env>

Report build failures as FAIL with the error output.

6. SecretGenerator / ConfigMapGenerator Check

If secretGenerator or configMapGenerator entries exist in the edited file, verify:

1. All referenced envs file paths exist relative to the kustomization.yaml directory.
2. All referenced files paths exist relative to the kustomization.yaml directory.
3. Each generator entry specifies a namespace.
4. If the repo convention is to use generatorOptions.disableNameSuffixHash: true, check it is set. If missing, warn:

WARNING: generatorOptions.disableNameSuffixHash is not set to true. Consider setting disableNameSuffixHash: true at the top level of kustomization.yaml to maintain consistent resource names.

Kustomize module conventions to detect:

• If the module uses namePrefix in overlays, verify namePrefix is set and matches the environment directory name (e.g., dev-, stg-, prd-).
• If sibling overlays do not use namePrefix, do not flag its absence.

Output Format

Always show the validation summary table, regardless of whether issues are found.

Kustomize Validation: <path/to/kustomization.yaml>

| # | Check                  | Status | Details                              |
|---|------------------------|--------|--------------------------------------|
| 1 | Resources              | PASS   | 5 resources, 0 missing               |
| 2 | Patches                | PASS   | 3 patches, 0 missing                 |
| 3 | Orphaned Files         | WARN   | 1 orphaned: unused-config.yaml       |
| 4 | Cross-Env Consistency  | OK     | Consistent across all environments   |
| 5 | Build (<env>)          | PASS   | Build succeeded                      |
| 6 | Generators             | PASS   | 2 secretGenerators, all refs valid   |

If there are errors or warnings, list them after the table:

Errors:
- [#1 Resources] File `missing-secret.yaml` not found in overlays/dev/

Warnings:
- [#3 Orphaned] `unused-config.yaml` is not referenced in kustomization.yaml
- [#4 Cross-Env] Patch `extra-patch.yaml` exists only in dev overlay

If all checks pass with no warnings, end with: "All checks passed. No issues found."

Check 2a: Patch-Resource Match Validation

For each entry in the patches: list, verify that the patch targets a resource that actually exists in the build:

1. Read the patch file and extract its apiVersion, kind, and metadata.name
2. Read all resources in the resources: list and extract their apiVersion, kind, and metadata.name
3. If the patch targets a resource not found in the resources list, report as WARN:

   WARNING: Patch <patch-file> targets <kind>/<name> but no matching resource found in resources:. The patch may apply to a resource from a base directory.

4. For strategic merge patches (no op field), verify the patch structure matches the target resource's structure
5. For JSON patches (contains op: add/remove/replace), verify the path exists in the target resource

This check reduces the risk of patches silently doing nothing because their target was renamed or removed.

Error Handling

Discovery Failures

• kustomization.yaml is empty or invalid YAML: Report as FAIL with the parse error. Skip all subsequent checks.
• Base directory referenced but doesn't exist: Report as FAIL in Check 1 and skip Check 5 (build will fail anyway).
• Overlay directory with no sibling environments: Skip Check 4 (cross-env consistency). Only one environment exists.

Tool Failures

• kustomize not installed: Skip Check 5 (build validation). Report as SKIP with install command: brew install kustomize. All other checks (1-4, 6) work with file reading only.
• kustomize build fails: Report the error output as FAIL in Check 5. Continue with Check 6.
• kustomize build times out (> 30s): Kill the process, report as FAIL with timeout message.

Edge Cases

• Deeply nested base references (base referencing another base): Follow the chain up to 3 levels deep. Beyond that, warn about complexity.
• Remote resources (resources: [https://...]): Skip existence check, note as INFO that remote resources can't be validated offline.
• Helm chart generators: If helmCharts: section exists, note it as INFO — these are validated by Helm, not Kustomize.

Report Persistence

After all checks complete, write the full validation report to the repository's docs/reports/ directory.

File path

docs/reports/kustomize-validation-<YYYY-MM-DD>.md

• Use today's date in YYYY-MM-DD format.
• Create docs/reports/ if it does not exist (use mkdir -p).
• If a report file for today already exists, overwrite it.

File content

The written report must include:

1. A header with repository name, date, branch (git rev-parse --abbrev-ref HEAD), and trigger file path.
2. The same validation summary table(s) shown in the terminal output.
3. Any Errors / Warnings sections, or "All checks passed." if clean.

Example header

# Kustomize Validation Report

**Repository:** <repo-root-directory-name>
**Date:** <YYYY-MM-DD>
**Branch:** <current-branch>
**Triggered by:** <path/to/kustomization.yaml>

Error handling

• If the write fails (e.g., permission error), print a warning to the terminal but do not fail the pipeline.

Dry-Run Support

This skill is read-only by default — validation doesn't modify files. All checks use Glob/Read to verify references. The only command that has side effects is kustomize build, which writes to stdout only. The Report Persistence step is the sole exception — it writes one Markdown file to docs/reports/.

Rollback Strategy

No rollback needed — this skill only reads and reports. The only file written is the Markdown report under docs/reports/, which can be safely deleted.