## 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.

### last_period_amount 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/pst_v73_F92KSTQKCQvvSz75wQ