RequestConfiguration instances share the same HeadersCollection instance by default

The default for RequestConfiguration.headers is an instance of HeadersCollection. This is created at class definition time, meaning that all instances of RequestConfiguration share the same instance of HeadersCollection by default. This leads to conflict when different instances of RequestConfiguration add/remove headers.

kiota-python/packages/abstractions/kiota_abstractions/base_request_configuration.py

Lines 16 to 22 in 806c946

    
           @dataclass 
        
           class RequestConfiguration(Generic[QueryParameters]): 
        
               """ 
        
               Configuration for the request such as headers, query parameters, and middleware options. 
        
               """ 
        
               # Request headers 
        
               headers: HeadersCollection = HeadersCollection()

Instead, a factory should be used so that a new instance is created:

from dataclasses import dataclass, field
from typing import Generic, Optional, TypeVar

QueryParameters = TypeVar("QueryParameters")

@dataclass
class RequestConfiguration(Generic[QueryParameters]):
    headers: HeadersCollection = field(default_factory=HeadersCollection)
    options: Optional[list[RequestOption]] = None
    query_parameters: Optional[QueryParameters] = None

The same/similar issue also affects HeadersInspectionHandlerOption init param defaults.

kiota-python/packages/http/httpx/kiota_http/middleware/options/headers_inspection_handler_option.py

Lines 10 to 20 in 806c946

    
           class HeadersInspectionHandlerOption(RequestOption): 
        
               """Config options for the HeaderInspectionHandler""" 
        
               HEADERS_INSPECTION_HANDLER_OPTION_KEY = "HeadersInspectionHandlerOption" 
        
               def __init__( 
        
                   self, 
        
                   inspect_request_headers: bool = True, 
        
                   inspect_response_headers: bool = True, 
        
                   request_headers: HeadersCollection = HeadersCollection(), 
        
                   response_headers: HeadersCollection = HeadersCollection(),

Same here. Every RequestConfiguration() that omits headers= mutates one shared collection, and in an async service that talks to many mailboxes a Prefer added for one call rides along on the others. Reproduces on 1.10.1 and the line is unchanged in 1.12.0 and on main:

microsoft-kiota-abstractions 1.10.1
two configs share one HeadersCollection: True
header added on a is visible on b: {'outlook.body-content-type=text'}

Script: https://gist.github.com/HardMax71/515894a3c37374424ee77c5f3f5a1aa9

The field(default_factory=HeadersCollection) fix in the description is the whole change, plus the same for HeadersInspectionHandlerOption. Happy to send that PR with a test if nobody has it in flight.

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RequestConfiguration instances share the same HeadersCollection instance by default #507

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	@dataclass
	class RequestConfiguration(Generic[QueryParameters]):
	"""
	Configuration for the request such as headers, query parameters, and middleware options.
	"""
	# Request headers
	headers: HeadersCollection = HeadersCollection()

	class HeadersInspectionHandlerOption(RequestOption):
	"""Config options for the HeaderInspectionHandler"""

	HEADERS_INSPECTION_HANDLER_OPTION_KEY = "HeadersInspectionHandlerOption"

	def __init__(
	self,
	inspect_request_headers: bool = True,
	inspect_response_headers: bool = True,
	request_headers: HeadersCollection = HeadersCollection(),
	response_headers: HeadersCollection = HeadersCollection(),

RequestConfiguration instances share the same HeadersCollection instance by default #507

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HardMax71 commented on Sep 3, 2026

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