(aws-bedrock-agentcore-alpha): Runtime defaults idleRuntimeSessionTimeout to 60s instead of 900s

[nakedible-p]

Describe the bug

When I create an AgentCore Runtime using @aws-cdk/aws-bedrock-agentcore-alpha and I do not specify lifecycleConfiguration, the synthesized CloudFormation template sets LifecycleConfiguration.IdleRuntimeSessionTimeout to 60 seconds.

This makes AgentCore runtime sessions terminate after 60s idle, which is inconsistent with the AgentCore service defaults and the CDK docs.

Expected Behavior

If lifecycleConfiguration is omitted, CDK should either:

1. not render LifecycleConfiguration at all (let AgentCore apply its defaults), or
2. render the documented defaults: idleRuntimeSessionTimeout = 900 seconds (15 minutes) and maxLifetime = 28800 seconds (8 hours).

Current Behavior

CDK renders:

"LifecycleConfiguration": {
  "IdleRuntimeSessionTimeout": 60,
  "MaxLifetime": 28800
}

Result: runtime sessions terminate after 60 seconds of idleness, causing frequent cold starts and breaking warm caching.

Reproduction Steps

1. Use aws-cdk-lib@2.232.1 and @aws-cdk/aws-bedrock-agentcore-alpha@2.232.1-alpha.0
2. Create a runtime without lifecycleConfiguration:

import * as agentcore from '@aws-cdk/aws-bedrock-agentcore-alpha';
import * as ecr from 'aws-cdk-lib/aws-ecr';

const repo = ecr.Repository.fromRepositoryName(this, 'Repo', 'my-agent-repo');

new agentcore.Runtime(this, 'Runtime', {
  runtimeName: 'test_runtime',
  agentRuntimeArtifact: agentcore.AgentRuntimeArtifact.fromEcrRepository(repo, 'v1'),
  // lifecycleConfiguration intentionally omitted
});

3. Run npx cdk synth
4. Inspect the synthesized template: it sets IdleRuntimeSessionTimeout to 60.

Notes / References

• AgentCore docs say idleRuntimeSessionTimeout defaults to 900 seconds if not provided: https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-lifecycle-settings.html
• CDK API docs for LifecycleConfiguration.idleRuntimeSessionTimeout also state default 900 seconds: https://docs.aws.amazon.com/cdk/api/v2/docs/%40aws-cdk_aws-bedrock-agentcore-alpha.LifecycleConfiguration.html

Suggested fix

Change the construct default from the minimum allowed value (60s) to the documented default (900s), or omit the property when the user doesn’t specify it.

[mazyu36]

There are two issues:

1. The default value in the JSDoc is set to 900 seconds, but this doesn't match the actual implementation.
   https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-bedrock-agentcore-alpha/lib/runtime/types.ts#L48

2. In the implementation, the default is explicitly set to 60 seconds. However, since this is already the service's default value, there's no need to set it. When we deployed without setting this value, it defaulted to 900 seconds anyway.
   https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-bedrock-agentcore-alpha/lib/runtime/runtime.ts#L285

aws bedrock-agentcore-control get-agent-runtime --agent-runtime-id integ_test_runtime-T2a7DV4Iwj --region us-east-1


{
  "ResponseMetadata": {
// omit
  "lifecycleConfiguration": {
    "idleRuntimeSessionTimeout": 900,
    "maxLifetime": 28800
  },

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