.Net: [Python] KernelJsonSchemaBuilder maps None to JSON "object" in multi-member unions

[michaelkillgta]

Description

KernelJsonSchemaBuilder maps NoneType to JSON type "object" in multi-member unions. A type like int | str | None becomes anyOf: [integer, string, object] instead of including "null".

This breaks function-calling / agent tool schemas for pydantic model parameters: models may emit {} where null/omit was intended.

Distinct from open #14443 (Ge constraint objects in descriptions).

Environment

• semantic-kernel==1.44.1 (Python)
• Python 3.13

Minimal repro

import json
from typing import Annotated
from pydantic import BaseModel, Field
from semantic_kernel.connectors.ai.function_calling_utils import (
    kernel_function_metadata_to_function_call_format,
)
from semantic_kernel.functions import KernelPlugin, kernel_function
from semantic_kernel.schema.kernel_json_schema_builder import KernelJsonSchemaBuilder

direct = KernelJsonSchemaBuilder.build(int | str | None)
print(json.dumps(direct))
# {"anyOf": [{"type": "integer"}, {"type": "string"}, {"type": "object"}]}
# null missing; object stand-in for None

class Payload(BaseModel):
    value: int | str | None = Field(default=None, description="flexible")

@kernel_function(name="handle")
def handle(payload: Annotated[Payload, "payload"]) -> str:
    """handle payload"""
    return str(payload)

fn = KernelPlugin(name="P", functions=[handle])["handle"]
fmt = kernel_function_metadata_to_function_call_format(fn.metadata)
print(json.dumps(fmt["function"]["parameters"]["properties"]["payload"]["properties"]["value"]))
# anyOf includes type object for None

Expected

None in unions should map to JSON Schema "null" (or an equivalent nullable form), not "object".

Actual

anyOf includes {"type": "object"} and no "null".

Root cause (pointer)

semantic_kernel/schema/kernel_json_schema_builder.py — TYPE_MAPPING / get_json_schema default "object"; handle_complex_type Union branch only special-cases 2-arg Optional, not multi-member unions containing None.

Suggested fix direction

Map type(None) → {"type": "null"} in TYPE_MAPPING/get_json_schema, and/or strip None from multi-unions into a proper nullable / anyOf+null form.

[minahilatiqdev]

Hi uma-prasad (@michaelkillgta), thanks for the clear repro and root cause pointer — makes this easy to pick up.

I'd like to work on this if it's still available. From what I can see, TYPE_MAPPING in kernel_json_schema_builder.py doesn't have an entry for NoneType, so it falls back to "object" in get_json_schema. The handle_complex_type union handling also only special-cases the 2-arg Optional case, not unions with more than two members that include None.

My plan is to add a proper mapping for type(None) → {"type": "null"} and update the union handling so multi-member unions correctly include "null" instead of falling back to "object". I'll also add test cases covering both the direct KernelJsonSchemaBuilder.build() call and the function-calling format path you referenced.

Let me know if this approach sounds right, or if there's a preferred direction (e.g., always emitting anyOf with null vs. some other nullable representation).

[aniketkrs]

Taking this on — opening a PR with the fix shortly.