Bug: VPC Lattice v2 parser model rejects real events because headers and query strings are lists #8773

Expected Behaviour

VpcLatticeV2Model and VpcLatticeV2Envelope parse the events VPC Lattice sends to a Lambda target group that uses the V2 event structure.

Current Behaviour

With the V2 structure, VPC Lattice sends every header and query string value as a list, even when there's only one value. The VPC Lattice docs show "headers": {"header-key": ["header-value", ...]} and "queryStringParameters": {"key": ["value", ...]}, and the multi-value sections on that page say the same.

VpcLatticeV2Model types both fields as dict[str, str], so every real V2 event fails with a ValidationError, and VpcLatticeV2Envelope can't parse any of them.

The repo already has a real V2 event in tests/events/vpcLatticeV2EventWithQueryString.json. It came from a live target group in aws-powertools/powertools-lambda-typescript#5764 and was added in #8572, and it fails to parse. The parser tests pass because they use vpcLatticeV2Event.json, which has plain string values. #8572 listed this as a follow-up, and the TypeScript parser fixed the same schema in aws-powertools/powertools-lambda-typescript#5773.

Code snippet

from aws_lambda_powertools.utilities.parser.models import VpcLatticeV2Model

# Shortened from tests/events/vpcLatticeV2EventWithQueryString.json
event = {
    "version": "2.0",
    "path": "/echo?QS1=value1&QS1=value2&single=one",
    "method": "POST",
    "headers": {
        "content-type": ["application/json"],
        "header1": ["value1", "value2"],
    },
    "queryStringParameters": {"QS1": ["value1", "value2"], "single": ["one"]},
    "body": '{"hello":"world"}',
    "requestContext": {
        "serviceNetworkArn": "arn:aws:vpc-lattice:eu-west-1:123456789012:servicenetwork/sn-0123456789abcdef0",
        "serviceArn": "arn:aws:vpc-lattice:eu-west-1:123456789012:service/svc-0123456789abcdef0",
        "targetGroupArn": "arn:aws:vpc-lattice:eu-west-1:123456789012:targetgroup/tg-0123456789abcdef0",
        "identity": {"sourceVpcArn": "arn:aws:ec2:eu-west-1:123456789012:vpc/vpc-0123456789abcdef0"},
        "region": "eu-west-1",
        "timeEpoch": "1790935900260914",
    },
}

VpcLatticeV2Model(**event)

pydantic_core._pydantic_core.ValidationError: 4 validation errors for VpcLatticeV2Model
headers.content-type
  Input should be a valid string [type=string_type, input_value=['application/json'], input_type=list]
headers.header1
  Input should be a valid string [type=string_type, input_value=['value1', 'value2'], input_type=list]
queryStringParameters.QS1
  Input should be a valid string [type=string_type, input_value=['value1', 'value2'], input_type=list]
queryStringParameters.single
  Input should be a valid string [type=string_type, input_value=['one'], input_type=list]

Possible Solution

Type headers and queryStringParameters as dict[str, list[str]], like multiValueHeaders in the API Gateway model.

Steps to Reproduce

Run the snippet above. Parsing tests/events/vpcLatticeV2EventWithQueryString.json with VpcLatticeV2Model, or with parse(..., envelope=envelopes.VpcLatticeV2Envelope), fails the same way.

Powertools for AWS Lambda (Python) version

latest

AWS Lambda function runtime

3.12

Packaging format used

PyPi