terraform plan fails on a GCP budget: amount validation error - specified_amount and last_period_amount are both set

TL;DR

A GCP budget can use exactly one amount type: either a fixed specified_amount or a last_period_amount that tracks the previous period. You set both, and the provider refuses. Remove one of the two blocks and the plan will validate.

Use this when

• terraform plan fails on google_billing_budget with an amount validation error.
• The budget_amount block contains both specified_amount and last_period_amount.
• A module you consume recently added one of the two blocks.

Not for this skill when

• The plan fails on billing account permissions. That is an IAM problem.
• The budget applies but alerts misbehave. That is a threshold or filter problem.

Steps

1. Open the google_billing_budget resource and find the amount block. Verify: both specified_amount and last_period_amount are present.
2. Decide which one you want. Fixed number: keep specified_amount with a units value like "1000". Track-last-period: keep last_period_amount alone. Verify: only one block remains.
3. Run terraform validate then terraform plan. Verify: the amount validation error is gone.
4. If a shared module sets one of them, override or fork the module rather than fighting it per resource. Verify: the plan shows a single amount block in the rendered config.
5. Apply and confirm the budget in the GCP console shows the intended amount type. Verify: the console's budget detail matches your choice.

lastperiodamount gotcha

With last_period_amount, the budget amount changes every period, which can surprise alerting. Verify: your alert thresholds make sense against a moving amount.

Compatibility: Terraform Google provider 4.x and later, google_billing_budget resource.

Why it happens

The provider mirrors the API's one-of constraint: the budget amount is a union type. HCL lets you write both blocks without a syntax error, so the mistake surfaces only at validation time.

Edge cases / pitfalls

• units inside specified_amount is a string like "500", not a number. A bare number can fail a different validation.
• Imported budgets sometimes render with both blocks in state. Clean the config, not the state.
• The same one-of rule applies in the raw REST API, so this is not a provider quirk.
• Currency follows the billing account; you do not set it on the budget.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstv73F92KSTQKCQvvSz75wQ