events: EventPattern should validate against empty arrays at synth time

[vertti]

Describe the bug

When an EventPattern property (such as detailType, source, account, etc.) is set to an empty array [], CDK synthesizes the template without any error. However, EventBridge rejects empty arrays in event patterns, causing a CloudFormation deployment failure:

Resource handler returned message: "Event pattern is not valid. Reason: Empty arrays are not allowed
at [Source: (String)"{"detail-type":[],"source":["example.source"],"account":["123456789012"]}";
line: 1, column: 18] (Service: EventBridge, Status Code: 400, ...)

CDK should validate that no EventPattern field is set to an empty array during synthesis, providing a clear error message before deployment.

Expected Behavior

cdk synth should throw a validation error when any EventPattern field is an empty array, e.g.:

Error: Invalid event pattern: 'detailType' must be a non-empty array

Current Behavior

CDK synthesizes the template successfully. The error is only discovered at deploy time when CloudFormation calls the EventBridge PutRule API.

Reproduction Steps

import * as events from 'aws-cdk-lib/aws-events';

const rule = new events.Rule(this, 'Rule', {
  eventPattern: {
    source: ['my.source'],
    detailType: [],   // <-- empty array, should be caught at synth time
  },
});

Running cdk synth succeeds. Running cdk deploy fails with the EventBridge validation error above.

Additional Information/Context

• The Match class already validates for non-empty collections (e.g., Match.anythingBut() throws if given an empty list), so there's precedent for this kind of validation in the same module.
• The fix would be in renderEventPattern() in packages/aws-cdk-lib/aws-events/lib/util.ts — adding a check that no array value is empty before rendering the pattern.
• All array-typed fields on the EventPattern interface should be validated: account, detailType, detail, id, region, resources, source, time, version.

CDK CLI Version

2.1104.0

Framework Version

aws-cdk-lib 2.236.0

Node.js Version

24.13.0

OS

macOS (Darwin 25.2.0)

Language

TypeScript

Language Version

5.9.3

Other information

No response

[pahud]

Hi @vertti,

This is a good catch — and a really well-written issue. The pointer to renderEventPattern() and the Match.anythingBut() precedent makes this straightforward to reason about.

Problem at a Glance:

┌──────────────────┐     ┌───────────────────────────────────┐
│  EventPattern    │────▶│  renderEventPattern()             │
│  detailType: []  │     │  copies values with no validation │
└──────────────────┘     └──────────────┬────────────────────┘
                                        │
                                        ▼
                                ┌───────────────┐
                                │ CFN Template   │
                                │ detail-type:[] │
                                └───────┬───────┘
                                        │
                                        ▼
                                ┌───────────────┐
                                │ ❌ EventBridge │
                                │ rejects empty  │
                                │ arrays at API  │
                                └───────────────┘

It appears the fix belongs in renderEventPattern() — adding a check that rejects empty arrays before they make it into the synthesized template. As you noted, Match.anythingBut() already does this kind of validation, so there's a clear pattern to follow.

If you're interested in submitting a PR for this, it would be a welcome contribution. The change itself is small — a guard clause in the for loop of renderEventPattern() that throws when an array value has length 0.

[pahud]

Here we go! #36911

Happy weekend!