Preview Infrastructure Changes with cdk diff Before Deploying (2026)

cdk diff is the closest thing you’ll get to a "dry run" for your AWS CDK deployments, but its real power lies in showing you exactly what AWS CloudFormation will change, not just what your CDK code thinks will change.

Let’s see it in action. Imagine you have a simple CDK app with an S3 bucket.

# app.py
from aws_cdk import App, Stack
from constructs import Construct
from aws_cdk import aws_s3 as s3

class MyS3Stack(Stack):
    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        s3.Bucket(self, "MyBucket",
            bucket_name="my-unique-cdk-demo-bucket-12345", # Note: bucket names must be globally unique
            removal_policy=s3.RemovalPolicy.DESTROY,
            auto_delete_objects=True
        )

app = App()
MyS3Stack(app, "MyS3Stack")
app.synth()

Now, let’s deploy it.

# First, bootstrap if you haven't already
cdk bootstrap aws://ACCOUNT-NUMBER/REGION

# Synthesize and deploy
cdk deploy

After deployment, you’ll see output indicating resources were created. Now, let’s say you want to add versioning to your bucket. You modify app.py:

# app.py (modified)
from aws_cdk import App, Stack
from constructs import Construct
from aws_cdk import aws_s3 as s3

class MyS3Stack(Stack):
    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        s3.Bucket(self, "MyBucket",
            bucket_name="my-unique-cdk-demo-bucket-12345",
            versioned=True, # Added versioning
            removal_policy=s3.RemovalPolicy.DESTROY,
            auto_delete_objects=True
        )

app = App()
MyS3Stack(app, "MyS3Stack")
app.synth()

Before you run cdk deploy again, you run cdk diff.

cdk diff

The output will look something like this:

# ... other output ...
IAM Statement Changes
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----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------* **Lack of Permissions:** The IAM user or role running the CDK command doesn't have the necessary permissions to perform actions like `cloudformation:CreateStack`, `cloudformation:UpdateStack`, `s3:CreateBucket`, etc.
    * **Diagnosis:** Check the IAM policy attached to the user/role running `cdk deploy`. You can use the AWS CLI to list attached policies: `aws iam list-attached-user-policies --user-name YOUR_USER_NAME` or `aws iam list-attached-role-policies --role-name YOUR_ROLE_NAME`. Then, inspect the policy document for the required CloudFormation and resource-specific permissions.
    * **Fix:** Add the necessary permissions to the IAM policy. For example, to allow stack creation and updates:
        ```json
        {
            "Effect": "Allow",
            "Action": [
                "cloudformation:CreateStack",
                "cloudformation:UpdateStack",
                "cloudformation:DeleteStack",
                "cloudformation:DescribeStacks",
                "cloudformation:DescribeStackEvents",
                "cloudformation:CreateChangeSet",
                "cloudformation:ExecuteChangeSet",
                "cloudformation:DescribeChangeSet",
                "cloudformation:DeleteChangeSet"
            ],
            "Resource": "*"
        }
        ```
        And for S3 bucket creation:
        ```json
        {
            "Effect": "Allow",
            "Action": [
                "s3:CreateBucket",
                "s3:DeleteBucket",
                "s3:PutBucketVersioning",
                "s3:GetBucketVersioning"
            ],
            "Resource": "arn:aws:s3:::my-unique-cdk-demo-bucket-12345"
        }
        ```
        This grants the explicit permissions needed for CloudFormation to manage the stack and for S3 to create/configure the bucket.

*   **`cdk.json` Configuration Errors:** The `cdk.json` file might contain incorrect settings for the CDK CLI, such as an invalid `app` command or missing context.
    *   **Diagnosis:** Examine your `cdk.json` file. Ensure the `app` property correctly points to your synthesized CloudFormation template (e.g., `"app": "python app.py"`).
    *   **Fix:** Correct the `app` command. For instance, if you were using a different language or build process, ensure it's accurately reflected. If context is missing, add it:
        ```json
        {
          "app": "python app.py",
          "context": {
            "aws-cdk:enableDiffNoFail": "true"
          }
        }
        ```
        This ensures the CLI knows how to invoke your application code.

*   **CloudFormation Stack Policy Conflicts:** If a CloudFormation stack policy is in place, it might be preventing updates to specific resources.
    *   **Diagnosis:** Navigate to the CloudFormation console, select your stack, and check the "Stack policy" tab. Review the JSON policy to see if it restricts updates to the S3 bucket or other resources.
    *   **Fix:** Modify the stack policy to allow the necessary updates. For example, to allow updates to all resources:
        ```json
        {
          "Statement": [
            {
              "Effect": "Allow",
              "Action": "Update:Modify",
              "Principal": "*",
              "Resource": "*"
            }
          ]
        }
        ```
        This explicitly permits modifications to all resources within the stack.

*   **Underlying AWS Service Quotas or Limits:** While less common for a simple S3 bucket, you might hit AWS service quotas (e.g., maximum number of buckets per account in a region).
    *   **Diagnosis:** Check the AWS Service Quotas console for S3 in your region. Look for any limits that might have been reached.
    *   **Fix:** Request a quota increase through the AWS Service Quotas console or contact AWS Support. This addresses the hard limit imposed by AWS on resource creation.

*   **Incorrect Stack Name or Region:** Deploying to a different stack name or region than intended can lead to `cdk diff` not showing expected changes, or `cdk deploy` failing because the stack doesn't exist or is in an unexpected state.
    *   **Diagnosis:** Verify the stack name and region specified in your `cdk.json` or passed via command-line arguments (e.g., `--stack-name MyStack --region us-east-1`).
    *   **Fix:** Ensure consistency. For example, if your `cdk.json` implicitly uses a default stack name and region, but you're trying to deploy with explicit arguments, make sure they match. If you want to deploy to a new stack, use a new name: `cdk deploy MyNewStack --stack-name MyNewStack --region us-west-2`.

*   **Outdated CDK CLI or Dependencies:** An older version of the AWS CDK CLI or its dependencies might not correctly interpret new constructs or features.
    *   **Diagnosis:** Check your installed CDK version with `cdk --version`. Compare it to the latest stable release.
    *   **Fix:** Upgrade your CDK CLI: `npm install -g aws-cdk` (or `yarn global add aws-cdk`). Also, ensure your project's `package.json` has up-to-date CDK library versions and run `npm install` or `yarn install`. This brings in the latest code generation and diffing logic.

After fixing the versioning issue, running `cdk deploy` will now successfully update your S3 bucket to be versioned. The next error you'll likely encounter is a `ResourceNotFoundException` if you attempt to deploy to a region where you haven't run `cdk bootstrap`.