What are you generating using Kiota, clients or plugins?

API Client/SDK

In what context or format are you using Kiota?

Windows executable

Client library/SDK language

TypeScript

Describe the bug

This is to follow up the comment below:

Happening in typescript.

• OpenAPI Spec OneOf: https://github.com/Software-Hardware-Integration-Lab/OpenAPI/blob/8e8fabe8651c268258946ca1bc4c4788d2019880/specs/SHIELD.json#L1022

Behavior:
Types are generated correctly; execution time deserialization puts the Managed Privilege Device properties into additional properties instead of using the privileged device as the resultant type and putting them on the root object.
(The second item/type in the OneOf list is never deserializes, only the first time. If an object of the second type is received, its properties are put into additional properties instead of the root object as expected.)

Originally posted by @elliot-huffman in #2720

Issue Summary

Problem Area: Microsoft Kiota OpenAPI client code generator
Component: Deserialization logic for schemas using oneOf
Spec Reference: SHIELD.json#L1022 (https://github.com/Software-Hardware-Integration-Lab/OpenAPI/blob/8e8fabe8651c268258946ca1bc4c4788d2019880/specs/SHIELD.json#L1022)

Description

When generating TypeScript clients using Kiota from an OpenAPI schema that includes both direct properties and a oneOf construct, the deserialization behavior is incorrect.

The generated types appear correct.

However, at runtime, deserialization only works for the first type listed in the oneOf array.

If an object matching the second type is received, its properties are incorrectly placed into the additionalProperties map rather than being deserialized into the expected root object.

As a result, the schema-level properties are not present in the deserialized object when the second oneOf type is used.

We had opened a Microsoft Support ticket about this.
The support team routed us to the issue board here.
Support Service request number for your reference: 2509050040003673.

Expected behavior

The deserializer should correctly identify and instantiate any of the types listed in the oneOf array.
Schema-level properties should be preserved and placed on the root object regardless of which oneOf type is matched.

Actual Behavior
Only the first oneOf type is deserialized correctly.
Other types are ignored, and their properties are treated as additionalProperties.

How to reproduce

1. Use Kiota to generate a TypeScript client from the provided OpenAPI spec.
2. Send or receive an object matching the second type in the oneOf list.
3. Observe that the deserialized object does not match the expected structure.

Open API description file

https://github.com/Software-Hardware-Integration-Lab/OpenAPI/blob/8e8fabe8651c268258946ca1bc4c4788d2019880/specs/SHIELD.json#L1022

Kiota Version

1.0.0-preview.97

Latest Kiota version known to work for scenario above?(Not required)

No response

Known Workarounds

No response

Configuration

No response

Debug output

</details>


### Other information

_No response_