Packer legacy JSON template deprecation in CI

By Daniel Zoghalchali·Latchkey

Packer deprecated the legacy JSON template format in favor of HCL2. Newer Packer prints a deprecation notice for .json templates and can fail if the JSON uses features no longer supported.

What this error means

packer build on a .json template prints a deprecation warning, or fails on a legacy field, recommending you migrate to HCL2 with packer hcl2_upgrade.

Warning: Deprecated: the JSON template format is deprecated.
Use HCL2 instead. To upgrade this template run: packer hcl2_upgrade template.json

Common causes

The pipeline still builds a JSON template

The .json format is deprecated, so newer Packer warns on every run and may reject removed fields.

A JSON-only feature has no HCL equivalent path

Some legacy constructs are unsupported in current Packer, causing a hard failure rather than a warning.

How to fix it

Upgrade the template to HCL2

1. Run packer hcl2_upgrade on the JSON template.
2. Review the generated .pkr.hcl and adjust variables and sources.
3. Point the CI build at the HCL2 template.

packer hcl2_upgrade template.json
packer validate template.pkr.hcl

Pin an older Packer only as a stopgap

If you cannot migrate immediately, pin a Packer version that still accepts the JSON template, but plan the HCL2 move.

How to prevent it

• Migrate JSON templates to HCL2 with packer hcl2_upgrade.
• Run packer validate on the HCL2 output in CI.
• Track Packer release notes for removed legacy features.

Frequently asked questions

Related guides

References

• Terraform documentation
• GitHub Actions documentation