How to fix Error: Check block assertion failed in Terraform

TerraformINTERMEDIATEMEDIUMUpdated Dec 31, 2025· 112 views

A Terraform check block assertion failed during plan or apply. This non-blocking validation error occurs when a custom assert condition in your check block evaluates to false, indicating your infrastructure doesn't meet expected criteria.

Quick fix

Fixes Error: Check block assertion failed

check "health_validation" {
  data "http" "endpoint" {
    url = "https://api.example.com/health"
  }

  assert {
    condition     = data.http.endpoint.status_code == 200
    error_message = "API endpoint returned status code: ${data.http.endpoint.status_code}"
  }
}
  1. Review the assertion condition and error message
  2. Verify the resource or data source state
  3. Fix the underlying resource configuration
  4. Handle check blocks that depend on external services
terraform — error
check "health_validation" {
  data "http" "endpoint" {
    url = "https://api.example.com/health"
  }
  assert {
    condition     = data.http.endpoint.status_code == 200
    error_message = "API endpoint returned status code: ${data.http.endpoint.status_code}"
  }
}
Error: Check block assertion failed

What this error means

In Terraform 1.5+, check blocks allow you to validate your infrastructure against custom conditions outside the normal resource lifecycle. Each check block contains one or more assert blocks that evaluate a condition expression. When a check block's assertion fails, Terraform evaluates the condition to false and displays a warning with the associated error_message. Unlike preconditions and postconditions, check block failures do not block Terraform from continuing execution—they are informational validations that help you verify your infrastructure meets expected standards. The "Check block assertion failed" error means one or more of your custom validation conditions evaluated to false, typically indicating a resource doesn't have an expected property, attribute value, or connectivity state.

Typical symptoms

  • Check block assertion failed warning appears in terraform plan output
  • Custom validation error message displays with details about the failed condition
  • Infrastructure continues to deploy despite check block failure
  • Error references a specific resource or data source that failed validation
  • Multiple check blocks may fail if several assertions evaluate to false

Common causes

  1. Resource attribute or property doesn't match the expected value in the assert condition
  2. External service or endpoint referenced in the check is unavailable or returns unexpected response
  3. Data source returns different data than the assertion expects
  4. Check block uses dynamic values that weren't available until apply phase
  5. Resource configuration changed and no longer matches the validation criteria

How to fix "Error: Check block assertion failed"

1Review the assertion condition and error message

Examine the check block in your Terraform configuration and identify which assert block failed. The error message will specify the exact check name and the condition that evaluated to false.

hcl
check "health_validation" {
  data "http" "endpoint" {
    url = "https://api.example.com/health"
  }

  assert {
    condition     = data.http.endpoint.status_code == 200
    error_message = "API endpoint returned status code: ${data.http.endpoint.status_code}"
  }
}

The error_message tells you exactly why the assertion failed. Use this information to understand what needs to be fixed.

2Verify the resource or data source state

Run terraform plan and check the actual values of the resource or data source being validated in the check block. Compare the actual values to what the assertion expects.

bash
terraform plan -out=plan.tfplan

Look at the plan output for the resource referenced in the check block. Verify:
- Does the resource have the expected attributes?
- Are the attribute values what the assertion expects?
- Is the data source returning the expected values?

3Fix the underlying resource configuration

Update your Terraform configuration to ensure the resource or data source will satisfy the assertion condition. This is the actual fix—the check is only reporting the problem.

For example, if a check asserts that an AWS load balancer has deletion protection enabled:

hcl
resource "aws_lb" "app" {
  name               = "application-lb"
  load_balancer_type = "application"
  enable_deletion_protection = true  # Add this to satisfy the check
}

Or for a health check that expects a 200 response:

hcl
check "api_health" {
  data "http" "health" {
    url = "https://api.example.com/health"
  }

  assert {
    condition     = data.http.health.status_code == 200
    error_message = "API health check failed: status ${data.http.health.status_code}"
  }
}

Ensure your actual API is running and returns a 200 status code.

4Handle check blocks that depend on external services

If your check block uses nested data sources to validate external endpoints or services, ensure those services are accessible and return expected values before running terraform apply.

bash
# Test the health endpoint manually
curl -v https://api.example.com/health

# Verify DNS resolution
nslookup api.example.com

# Check network connectivity from your Terraform environment
terraform refresh  # Refresh data sources

Wait for the external service to be ready if it's being created by the same Terraform configuration, since check blocks run as the final step.

5Run terraform apply to deploy changes

After fixing the underlying resource configuration, run terraform plan again to verify the assertion now passes, then apply the changes.

bash
terraform plan
terraform apply

The check block will run as the last step. If the assertion still fails, the error will be displayed as a warning, but Terraform will continue. Review the warning and adjust the resource configuration further if needed.

Advanced notes

Check blocks execute as the final step of terraform plan and apply operations, after all resources have been planned or provisioned. This means check blocks can use nested data sources to validate infrastructure state after apply.

Unlike preconditions and postconditions, check block failures are non-blocking—they don't prevent Terraform from continuing execution. This makes them ideal for optional validations and infrastructure health checks.

Check blocks are only available in Terraform 1.5.0 and later. If you need this feature, ensure your Terraform version is >= 1.5.0 by specifying the required version constraint in your configuration.

For critical validations that must block Terraform execution, use preconditions on resources or postconditions instead of check blocks.

Frequently asked questions

What does "Error: Check block assertion failed" mean?
5+, check blocks allow you to validate your infrastructure against custom conditions outside the normal resource lifecycle. Each check block contains one or more assert blocks that evaluate a condition expression. When a check block's assertion fails, Terraform evaluates the condition to false and displays a warning with the associated error_message.
How do I fix "Error: Check block assertion failed"?
A Terraform check block assertion failed during plan or apply. This non-blocking validation error occurs when a custom assert condition in your check block evaluates to false, indicating your infrastructure doesn't meet expected criteria.
What causes "Error: Check block assertion failed"?
Resource attribute or property doesn't match the expected value in the assert condition External service or endpoint referenced in the check is unavailable or returns unexpected response Data source returns different data than the assertion expects
Is the "Error: Check block assertion failed" error serious?
It is a medium-severity error — more of a blocker than a risk. The steps above resolve it.

Official resources & further reading