cdk-preflight

Catch deploy-time CloudFormation failures at cdk synth time.

Some CloudFormation constraints are not expressed in resource provider schemas — they live only in documentation, in service API validation, or across multiple properties. Templates that violate them pass cdk synth, pass CloudFormation pre-deployment validation, and then fail minutes into a deployment, burning a rollback cycle.

cdk-preflight is a curated Rego rule pack for exactly those constraints, evaluated with the CloudFormation validation engine that ships inside aws-cdk-lib (>= 2.267.0). By default a violation fails cdk synth — a template that is known to fail at deploy time never leaves your machine.

The pack aims at every deploy-time failure that no existing CDK mechanism already catches — nothing narrower. Every bundled rule is backed by a fail/pass template pair, and the failure has been reproduced against real AWS (or is explicitly marked doc-only). Rules that the built-in validation engine already covers are deliberately not duplicated — a test suite enforces this.

Quick start

npm i -D cdk-preflight
npx cdk-preflight init   # inserts Preflight.apply(app) into your CDK app

or add one line yourself:

import { Preflight } from 'cdk-preflight';

const app = new App();
Preflight.apply(app);

On violation, cdk synth fails with one error per finding, including the construct trace:

ERROR idle_timeout.timeout_seconds is 5000 but must be between 1 and 4000 seconds (cdk-preflight)
   MyStack/Alb/Resource (Alb16C2F182) aws-cdk-lib.aws_elasticloadbalancingv2.CfnLoadBalancer

Synthesis finished with errors

Observe-only mode

To roll the rules out gradually, start with enforce: false: findings then surface as synth warnings through the CDK built-in validator, with construct traces and per-finding acknowledgement:

Preflight.apply(app, { enforce: false });

WARNING idle_timeout.timeout_seconds is 5000 but must be between 1 and 4000 seconds (CloudFormation Validate)
   MyStack/Alb (Alb) aws-cdk-lib.aws_elasticloadbalancingv2.CfnLoadBalancer
   Acknowledge with 'CloudFormation-Validate::pf-elbv2-lb-idle-timeout-range'

To opt out of a single rule, pass its id in exclude. In observe-only mode, individual findings can also be suppressed with the CDK acknowledge mechanism shown in the warning text.

Bundled rules

See docs/rules.md for the generated rule table. Highlights:

• ELBv2: idle_timeout / deregistration_delay / slow_start attribute ranges (stringly-typed Key/Value attributes are invisible to schema validation)
• IAM: managed (6,144 chars) and inline (role/group/user) policy document size limits
• CloudFront: MinTTL <= DefaultTTL <= MaxTTL ordering, ACM certificates must live in us-east-1
• Step Functions: Next/Default/Choices must reference defined states (a dangling StartAt is already caught by the engine's built-in E3601)
• EC2: security group TCP/UDP port ranges and FromPort <= ToPort

For AI agents

To add cdk-preflight to a CDK app:

1. npm i -D cdk-preflight
2. npx cdk-preflight init (finds the entry point via cdk.json and inserts Preflight.apply(app); use --dry-run to preview)
3. Run cdk synth. Violations fail the synth with a validation report; each finding carries a rule id, the actual value, and a suggested fix
4. To fix a finding, prefer correcting the resource. To opt out instead: exclude: ['<rule-id>'] disables one rule, enforce: false switches to warn-only reporting

The machine-readable findings are written to cdk.out/validation-report.json when synthesizing with -c @aws-cdk/core:validationReportJson=true.

Report missed deploy errors

cdk-preflight only knows the failure modes it has benched. When a deploy fails on a constraint the pack should have caught, that report is how new rules get added — but at that moment cdk-preflight is out of the loop (it passed the synth), so the instruction has to already sit in your agent's context. Paste this into your repository's AGENTS.md (or CLAUDE.md):

## cdk-preflight: report missed deploy errors

If `cdk deploy` fails even though `cdk synth` (with cdk-preflight enabled) passed,
and the failure is a service-side validation rejection — CREATE_FAILED/UPDATE_FAILED
with a 400-class message such as "Invalid request provided: ..." — cdk-preflight is
missing a rule. Report it:

1. Qualify first. NOT reportable: AccessDenied / permissions, service quotas,
   throttling, name conflicts with existing resources, stabilization timeouts.
2. Redact account-specific data (account IDs -> 123456789012, real ARNs/domains
   -> placeholders).
3. Search existing issues: https://github.com/badmintoncryer/cdk-preflight/issues
4. With your user's approval, open a "Missed deploy error" issue with the verbatim
   error message, the resource type, a minimal template snippet, and your
   aws-cdk-lib / cdk-preflight versions.

Scope and rule lifecycle

A constraint belongs in the pack when violating it makes a real deployment fail and no layer that sees the same synthesized template already blocks it. There is no further "is this worth a rule" question — if the gap is real, it gets a rule.

CDK L2 construct validation is deliberately not one of those layers. CfnXxx usage, escape hatches, addPropertyOverride, cloudformation-include and migrated templates all bypass L2, so an L2 guard covering the same mistake neither disqualifies a rule nor retires one.

That makes growth the normal state, and it has a consequence worth knowing before you upgrade: new rules land in minor releases, so a minor upgrade can newly fail a cdk synth that passed yesterday. That is intended, not a regression. If you need a frozen rule set, pin the version; to drop a single rule, exclude: ['<rule-id>']; to see everything without failing the build, enforce: false.

Rules move the other way too. Once the validation engine bundled in aws-cdk-lib (or CloudFormation's own pre-deploy validation) starts blocking a constraint, the rule is deleted rather than kept as a duplicate — staying on an older aws-cdk-lib and an older cdk-preflight keeps the old behavior.

How it works

Preflight.apply() evaluates the rules with the cloudformation-validate Rust/WASM engine that ships inside aws-cdk-lib — no extra binaries, no network access at synth time. In the default enforce mode the engine is invoked through a dedicated CDK validation plugin so that violations fail synthesis; with enforce: false the rules are instead injected into the CDK built-in CloudFormationValidatePlugin and reported as warnings.

Constraints that can be expressed in schemas or generic engine rules also make good upstream PRs to that engine, but nothing here waits on one — the upstream release cycle is deliberately slower than this pack's. Each rule's meta.yaml tracks its upstream status so that retirement stays bookkeeping.

Requirements

• aws-cdk-lib >= 2.267.0 (the first release that bundles the built-in CloudFormation validator)

Contributing

Rule authoring, the verification gates (including real-deploy reproduction), and the test layout are documented in AGENTS.md — written for AI coding agents and humans alike.

License

Apache-2.0