bug: EC2 ModifyLaunchTemplate fails by name and mutates the default during DryRun

[AndreKurait]

Is there an existing issue for this?

I searched the existing issues, pull requests, and organization discussions for ModifyLaunchTemplate, modify_launch_template, and EC2 DryRun. I found the historical implementation PR and account-scope fix, but no report tracking these two cases.

Current Behavior

On the stock image below, ModifyLaunchTemplate has two reproducible problems:

1. A request containing LaunchTemplateName and DefaultVersion, with no LaunchTemplateId, returns HTTP 500 / InternalError with message 'LaunchTemplateId'. This happens with and without DryRun=True.
2. A valid ID-based request with DryRun=True returns HTTP 200 and changes the template's default version. A subsequent DescribeLaunchTemplates confirms the change.

An ordinary ID-based modification succeeds and returns the correct nested LaunchTemplate identity.

Expected Behavior

The API accepts either a launch-template name or ID. A valid dry run should return DryRunOperation and leave the default version unchanged.

AWS API reference: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_ModifyLaunchTemplate.html

How are you starting LocalStack?

With docker run, using the stock Pro image, an auth token supplied through the environment, and EC2 enabled. No extensions or source patches were loaded for the reproduction.

Steps To Reproduce

Run the following against the stock LocalStack endpoint. It uses dummy emulator credentials and removes the template afterward.

import os
import uuid

import boto3
from botocore.config import Config
from botocore.exceptions import ClientError

client = boto3.client(
    "ec2",
    endpoint_url=os.environ.get("LOCALSTACK_ENDPOINT", "http://localhost:4566"),
    region_name="eu-west-1",
    aws_access_key_id="123456789012",
    aws_secret_access_key="test",
    config=Config(retries={"max_attempts": 0}),
)
name = "modify-default-" + uuid.uuid4().hex
template_id = client.create_launch_template(
    LaunchTemplateName=name,
    LaunchTemplateData={"InstanceType": "m5.large"},
)["LaunchTemplate"]["LaunchTemplateId"]

try:
    client.create_launch_template_version(
        LaunchTemplateId=template_id,
        LaunchTemplateData={"InstanceType": "m5.xlarge"},
    )
    for request in (
        dict(LaunchTemplateId=template_id, DefaultVersion="2", DryRun=True),
        dict(LaunchTemplateName=name, DefaultVersion="2", DryRun=True),
        dict(LaunchTemplateId=template_id, DefaultVersion="2"),
        dict(LaunchTemplateName=name, DefaultVersion="1"),
    ):
        before = client.describe_launch_templates(
            LaunchTemplateIds=[template_id]
        )["LaunchTemplates"][0]["DefaultVersionNumber"]
        try:
            response = client.modify_launch_template(**request)
            outcome = response["ResponseMetadata"]["HTTPStatusCode"]
        except ClientError as error:
            outcome = error.response["Error"]
        after = client.describe_launch_templates(
            LaunchTemplateIds=[template_id]
        )["LaunchTemplates"][0]["DefaultVersionNumber"]
        print(request, outcome, "default:", before, "->", after)
finally:
    client.delete_launch_template(LaunchTemplateId=template_id)
    client.close()

Observed outcomes, in order:

Request	Result	Default before → after
ID, version 2, DryRun	HTTP 200	1 → 2
Name, version 2, DryRun	HTTP 500, InternalError: 'LaunchTemplateId'	2 → 2
ID, version 2	HTTP 200	2 → 2
Name, version 1	HTTP 500, InternalError: 'LaunchTemplateId'	2 → 2

Environment

• Linux, amd64
• LocalStack version: 2026.11.0.dev102
• Image: localstack/localstack-pro@sha256:4f410f246973e722521f3ee27ec08c07827d9683f233a28fe8858a6591ac5511
• LocalStack build date: 2026-10-16
• LocalStack build git hash: d30ab68e

[ryan-berke]

Hi @AndreKurait, thanks for raising this. I have reproduced this issue on my side, so I will escalate to my engineers for a fix. Once a fix is ready, I will let you know as soon as possible!

[ryan-berke]

Hi @AndreKurait, thanks again for this report. We have issued a fix for this in the dev image, so please pull this image and run your tests again. Please let me know if you have any additional issues.