Multi-value nullable unions emit an object variant for an explicit null-only branch #414

Regression

OpenAI's 3.1 schema replaces a legacy nullable: true on a two-value union with a third type: null branch. AutoSDK generates a catch-all object as an additional CLR union variant instead of preserving the nullable union over the two non-null variants.

Seen in tryAGI/OpenAI#416 (head eaefd98ae2c87a719658472ab5a671921f849ae9), for RealtimeSessionCreateRequestGA.Tracing, its Realtime projection, CLI command and serializer contexts. All 11 changed files were inspected. The PR has no package/workflow changes; normal build CI succeeds, so compilation alone does not establish nullability/schema fidelity. The existing two-variant collapse from #390 does not cover this shape.

Minimal reproduction

openapi: 3.1.0
info: {title: Null Union Reproduction, version: 1.0.0}
paths: {}
components:
  schemas:
    Request:
      type: object
      properties:
        tracing:
          oneOf:
            - type: string
              enum: [auto]
            - type: object
              properties:
                workflow_name: {type: string}
            - type: 'null'

Generate with autosdk generate openapi.yaml --namespace NullUnionRepro --output Generated.

Confirmed with CLI 0.35.0-preview.2.67+83957f817c7b8e3fb764c3809111911ac810d84b; the current organization regeneration also emits the same shape in OpenAI #416. The current generator's IsNullableUnion helper requires exactly two variants.

Observed generated property:

OneOf<RequestTracingEnum?, RequestTracingEnum2, object>? Tracing

Expected: a nullable union of the enum and the typed configuration, with no catch-all object branch introduced by the null-only schema. The additional generic argument also changes the public request/method signature for an equivalent wire schema.

Required fix and validation

• Exclude exact null-only branches from multi-value oneOf and anyOf CLR variant lists while retaining outer nullability.
• Preserve actual unconstrained object alternatives, named/reference unions, branch naming/order and multiple concrete-value variants; do not collapse a true multi-value union to its first branch.
• Cover legacy nullable versus explicit null equivalence, two and three-plus concrete variants, enum/object/reference choices, null-only edge cases, round trips and invalid primitive inputs.
• Validate AutoSDK units/snapshots and compiled serializer/NativeAOT behavior, regenerate OpenAI from its spec inputs, and wait for Publish/trimming before calling the regression fixed. Generated files must not be edited by hand.

Default reproduction is local and key-free; no provider call is necessary.

Compiled local runtime evidence

The generated minimal reproduction compiles and runs on .NET10 with its source-generated serializer context and no external packages. Observations:

{"tracing":true} => ArgumentOutOfRangeException
{"tracing":"unexpected"} => introduced_object=True
{"tracing":null} => introduced_object=False
{"tracing":"auto"} => introduced_object=False
{"tracing":{"workflow_name":"test"}} => introduced_object=False

The invalid string taking the newly introduced object branch confirms the schema-widening behavior. The boolean exception is a separate existing enum-converter fallback/error-shape concern, not a successful invalid-input round trip. No provider request was made.

OpenAI#416 was merged by the existing auto-merge workflow as 5bb64663a6d446210fce811628d5e29ca2cfd76f at2026-10-04T03:26:48Z while an attempted draft conversion raced it. The draft conversion command succeeded, but the final PR state is merged; this is not a prevention guarantee. PR Test succeeded; Publish is monitored separately. This issue remains open until the generator fix and regenerated SDK validation are complete.