Bug: VPCLatticeV2Resolver with enable_validation fails on every request body (AttributeError: 'list' object has no attribute 'strip') #8776

Expected Behaviour

With enable_validation=True, a VPCLatticeV2Resolver route that takes a body parameter (Body(), Form() or a Pydantic model) should parse and validate the request body, the same as the other resolvers do.

Current Behaviour

VPC Lattice V2 sends every header value as a list, for example "content-type": ["application/json"]. OpenAPIRequestValidationMiddleware reads Content-Type as a string in two places:

• _get_body calls app.current_event.headers.get("content-type", "").strip()
• handler passes the same value to _is_form_content_type, which also calls .strip()

On a real V2 event both get a list, so every request to a route with a body parameter fails with AttributeError: 'list' object has no attribute 'strip' before it reaches the handler.

Header, query and cookie parameters, CORS and compression already handle list values. Only these two Content-Type reads don't.

Code snippet

from typing import Annotated

from pydantic import BaseModel

from aws_lambda_powertools.event_handler import VPCLatticeV2Resolver
from aws_lambda_powertools.event_handler.openapi.params import Body
from tests.functional.utils import load_event

# Event captured from a live VPC Lattice V2 target group
event = load_event("vpcLatticeV2EventWithQueryString.json")
print("content-type as VPC Lattice V2 sends it:", event["headers"]["content-type"])


class Item(BaseModel):
    hello: str


app = VPCLatticeV2Resolver(enable_validation=True)


@app.post("/echo")
def echo(item: Annotated[Item, Body()]):
    return item.model_dump()


print(app.resolve(event, {}))
# AttributeError: 'list' object has no attribute 'strip'
#   openapi_validation.py:144  content_type = app.current_event.headers.get("content-type", "").strip()

Possible Solution

Read Content-Type through a small helper that joins a list value into a string, and use it in both places. The other resolvers always pass a string there, so nothing changes for them. I have a fix with tests ready and can open a PR once this is triaged.

Steps to Reproduce

1. Check out develop and run the snippet above from the repo root.
2. The fixture tests/events/vpcLatticeV2EventWithQueryString.json is the event captured from a live V2 target group in Bug: VpcLatticeV2Schema rejects every real V2 event powertools-lambda-typescript#5764.
3. A Form() body fails the same way, at the _is_form_content_type call.

Did you run the reproduction yourself?

Yes, I ran it and saw the failure

Powertools for AWS Lambda (Python) version

3.38.0

AWS Lambda function runtime

3.12

Packaging format used

PyPi

Debugging logs

No response