Identifying Lambda runtime version changes

The runtime version number and ARN are logged in the INIT_START log line, which Lambda emits to CloudWatch Logs each time that it creates a new execution environment. Because the execution environment uses the same runtime version for all function invocations, Lambda emits the INIT_START log line only when Lambda executes the init phase. Lambda doesn't emit this log line for each function invocation. Lambda emits the log line to CloudWatch Logs, but it is not visible in the console.

INIT_START Runtime Version: python:3.13.v14    Runtime Version ARN: arn:aws:lambda:eu-south-1::runtime:7b620fc2e66107a1046b140b9d320295811af3ad5d4c6a011fad1fa65127e9e6I

Rather than working directly with the logs, you can use Amazon CloudWatch Contributor Insights to identify transitions between runtime versions. The following rule counts the distinct runtime versions from each INIT_START log line. To use the rule, replace the example log group name /aws/lambda/* with the appropriate prefix for your function or group of functions.

{
  "Schema": {
    "Name": "CloudWatchLogRule",
    "Version": 1
  },
  "AggregateOn": "Count",
  "Contribution": {
    "Filters": [
      {
        "Match": "eventType",
        "In": [
          "INIT_START"
        ]
      }
    ],
    "Keys": [
      "runtimeVersion",
      "runtimeVersionArn"
    ]
  },
  "LogFormat": "CLF",
  "LogGroupNames": [
    "/aws/lambda/*"
  ],
  "Fields": {
    "1": "eventType",
    "4": "runtimeVersion",
    "8": "runtimeVersionArn"
  }
}

The following CloudWatch Contributor Insights report shows an example of a runtime version transition as captured by the preceding rule. The orange line shows execution environment initialization for the earlier runtime version (python:3.13.v12), and the blue line shows execution environment initialization for the new runtime version (python:3.13.v14).