Fix OpenAPI import: preserve description and decorators for anyOf/oneOf with null (#8514)

The OpenAPI 3.1 import tool was losing description and decorator
information when converting schemas with `anyOf`/`oneOf` containing a
definition + `null` type. This is a common pattern in OpenAPI 3.1 as an
equivalent to `nullable: true` from OpenAPI 3.0.

**Before this fix:**
```yaml
# OpenAPI 3.1 schema
ReasoningEffort:
  anyOf:
    - type: string
      enum: [minimal, low, medium, high]
      default: medium
      description: "Constrains effort on reasoning for reasoning models..."
    - type: 'null'
```

Generated incomplete TypeSpec:
```typescript
union ReasoningEffort {
  "minimal" | "low" | "medium" | "high",
  null,
}
```

**After this fix:**
```typescript
/**
 * Constrains effort on reasoning for reasoning models...
 */
union ReasoningEffort {
  "minimal" | "low" | "medium" | "high" = "medium",
  null,
}
```

## Changes Made

1. **Enhanced union metadata extraction**: Added logic to identify and
extract descriptions and decorators from meaningful (non-null) members
in `anyOf`/`oneOf` scenarios.

2. **Support for type arrays**: Fixed handling of OpenAPI 3.1 type
arrays like `["string", "null"]` to properly preserve constraints and
descriptions.

3. **Improved type identification**: Updated `getTypeSpecKind` to
correctly identify type arrays containing "null" as unions rather than
scalars.

## Test Coverage

Added comprehensive test suite covering:
- `anyOf` with definition + null
- `oneOf` with constraints + null  
- Type arrays like `["integer", "null"]`
- Reference types with null
- Edge cases and backward compatibility

All existing tests continue to pass (1609 tests).

Fixes #8513.

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---------

Signed-off-by: Vincent Biret <vibiret@microsoft.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: baywet <7905502+baywet@users.noreply.github.com>
Co-authored-by: Vincent Biret <vibiret@microsoft.com>
Co-authored-by: Timothee Guerin <tiguerin@microsoft.com>

Author: 3 people

.chronus/changes/copilot-fix-8513-2025-8-23-9-38-32.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+[converter] anyOf/oneOf type + type:null gets imported properly and maintains decorators, documentation,...

packages/openapi3/src/cli/actions/convert/transforms/transform-component-schemas.ts

@@ -1,9 +1,9 @@
 import { printIdentifier } from "@typespec/compiler";
-import { OpenAPI3Schema, Refable } from "../../../../types.js";
+import { OpenAPI3Schema, OpenAPISchema3_1, Refable } from "../../../../types.js";
 import {
   TypeSpecDataTypes,
+  TypeSpecDecorator,
   TypeSpecEnum,
-  TypeSpecModel,
   TypeSpecModelProperty,
   TypeSpecUnion,
 } from "../interfaces.js";
@@ -18,7 +18,7 @@ import { getScopeAndName } from "../utils/get-scope-and-name.js";
  * @param schemas
  * @returns
  */
-export function transformComponentSchemas(context: Context, models: TypeSpecModel[]): void {
+export function transformComponentSchemas(context: Context, models: TypeSpecDataTypes[]): void {
   const schemas = context.openApi3Doc.components?.schemas;
   if (!schemas) return;
 
@@ -115,17 +115,64 @@ export function transformComponentSchemas(context: Context, models: TypeSpecMode
   }
 
   function populateUnion(types: TypeSpecDataTypes[], name: string, schema: OpenAPI3Schema): void {
+    // Extract description and decorators from meaningful union members
+    const unionMetadata = extractUnionMetadata(schema);
+
     const union: TypeSpecUnion = {
       kind: "union",
       ...getScopeAndName(name),
-      decorators: getDecoratorsForSchema(schema),
-      doc: schema.description,
+      decorators: [...getDecoratorsForSchema(schema), ...unionMetadata.decorators],
+      doc: schema.description ?? unionMetadata.description,
       schema,
     };
 
     types.push(union);
   }
 
+  /**
+   * Extracts meaningful description and decorators from union members.
+   * Handles anyOf/oneOf with null, and type arrays like ["string", "null"].
+   */
+  function extractUnionMetadata(schema: OpenAPI3Schema | OpenAPISchema3_1): {
+    description?: string;
+    decorators: TypeSpecDecorator[];
+  } {
+    // Handle anyOf/oneOf scenarios
+    const unionMembers = schema.anyOf || schema.oneOf;
+    if (unionMembers) {
+      const meaningfulMembers = unionMembers.filter((member) => {
+        if ("$ref" in member) return true; // Redundant, but help the type system understand whether it's a ref or not, and give us access to the type property
+        return member.type !== "null"; // Non-null types are meaningful
+      });
+
+      // If we have exactly one meaningful member and at least one null, extract from the meaningful one
+      if (meaningfulMembers.length === 1 && unionMembers.length > meaningfulMembers.length) {
+        const meaningfulMember = meaningfulMembers[0];
+        if (!("$ref" in meaningfulMember)) {
+          return {
+            description: meaningfulMember.description,
+            decorators: getDecoratorsForSchema(meaningfulMember),
+          };
+        }
+      }
+    }
+
+    // Handle type array scenarios like type: ["string", "null"]
+    if (Array.isArray(schema.type)) {
+      const nonNullTypes = schema.type.filter((t) => t !== "null");
+      // If we have exactly one non-null type, this is essentially a nullable version of that type
+      // The schema itself should contain the relevant constraints/description for the non-null type
+      if (nonNullTypes.length === 1) {
+        // Create a schema without the null type to extract decorators for the non-null part
+        const nonNullSchema = { ...schema, type: nonNullTypes[0] };
+        return { decorators: getDecoratorsForSchema(nonNullSchema) };
+        // The description should already be on the main schema, so we don't override it here
+      }
+    }
+
+    return { decorators: [] };
+  }
+
   function populateScalar(types: TypeSpecDataTypes[], name: string, schema: OpenAPI3Schema): void {
     types.push({
       kind: "scalar",
@@ -225,7 +272,13 @@ function getTypeSpecKind(schema: OpenAPI3Schema): TypeSpecDataTypes["kind"] {
 
   if (schema.enum && schema.type === "string" && !schema.nullable) {
     return "enum";
-  } else if (schema.anyOf || schema.oneOf || schema.enum || schema.nullable) {
+  } else if (
+    schema.anyOf ||
+    schema.oneOf ||
+    schema.enum ||
+    schema.nullable ||
+    (Array.isArray(schema.type) && schema.type.includes("null"))
+  ) {
     return "union";
   } else if (
     schema.type === "object" ||

packages/openapi3/src/cli/actions/convert/utils/decorators.ts

@@ -1,6 +1,12 @@
 import { printIdentifier } from "@typespec/compiler";
 import { ExtensionKey } from "@typespec/openapi";
-import { Extensions, OpenAPI3Parameter, OpenAPI3Schema, Refable } from "../../../../types.js";
+import {
+  Extensions,
+  OpenAPI3Parameter,
+  OpenAPI3Schema,
+  OpenAPISchema3_1,
+  Refable,
+} from "../../../../types.js";
 import { TSValue, TypeSpecDecorator } from "../interfaces.js";
 
 const validLocations = ["header", "query", "path"];
@@ -142,7 +148,9 @@ function getHeaderArgs({ explode }: OpenAPI3Parameter): TSValue | undefined {
   return undefined;
 }
 
-export function getDecoratorsForSchema(schema: Refable<OpenAPI3Schema>): TypeSpecDecorator[] {
+export function getDecoratorsForSchema(
+  schema: Refable<OpenAPI3Schema | OpenAPISchema3_1>,
+): TypeSpecDecorator[] {
   const decorators: TypeSpecDecorator[] = [];
 
   if ("$ref" in schema) {
@@ -196,11 +204,11 @@ function createTSValue(value: string): TSValue {
   return { __kind: "value", value };
 }
 
-function getOneOfSchemaDecorators(schema: OpenAPI3Schema): TypeSpecDecorator[] {
+function getOneOfSchemaDecorators(schema: OpenAPI3Schema | OpenAPISchema3_1): TypeSpecDecorator[] {
   return [{ name: "oneOf", args: [] }];
 }
 
-function getArraySchemaDecorators(schema: OpenAPI3Schema) {
+function getArraySchemaDecorators(schema: OpenAPI3Schema | OpenAPISchema3_1) {
   const decorators: TypeSpecDecorator[] = [];
 
   if (typeof schema.minItems === "number") {
@@ -214,7 +222,7 @@ function getArraySchemaDecorators(schema: OpenAPI3Schema) {
   return decorators;
 }
 
-function getNumberSchemaDecorators(schema: OpenAPI3Schema) {
+function getNumberSchemaDecorators(schema: OpenAPI3Schema | OpenAPISchema3_1) {
   const decorators: TypeSpecDecorator[] = [];
 
   if (typeof schema.minimum === "number") {
@@ -246,7 +254,7 @@ const knownStringFormats = new Set([
   "uri",
 ]);
 
-function getStringSchemaDecorators(schema: OpenAPI3Schema) {
+function getStringSchemaDecorators(schema: OpenAPI3Schema | OpenAPISchema3_1) {
   const decorators: TypeSpecDecorator[] = [];
 
   if (typeof schema.minLength === "number") {

packages/openapi3/test/tsp-openapi3/union-anyof-with-null.test.ts

@@ -0,0 +1,181 @@
+import OpenAPIParser from "@apidevtools/swagger-parser";
+import { OpenAPI } from "openapi-types";
+import { beforeAll, describe, expect, it } from "vitest";
+import { generateDataType } from "../../src/cli/actions/convert/generators/generate-model.js";
+import { TypeSpecUnion } from "../../src/cli/actions/convert/interfaces.js";
+import { transformComponentSchemas } from "../../src/cli/actions/convert/transforms/transform-component-schemas.js";
+import { createContext } from "../../src/cli/actions/convert/utils/context.js";
+import { OpenAPI3Document } from "../../src/types.js";
+
+describe("tsp-openapi: union anyOf with null", () => {
+  let parser: OpenAPIParser;
+  let doc: OpenAPI.Document<{}>;
+
+  beforeAll(async () => {
+    parser = new OpenAPIParser();
+    doc = await parser.bundle({
+      openapi: "3.1.0",
+      info: { title: "Test", version: "1.0.0" },
+      paths: {},
+      components: {
+        schemas: {
+          ReasoningEffort: {
+            anyOf: [
+              {
+                type: "string",
+                enum: ["minimal", "low", "medium", "high"],
+                default: "medium",
+                description: `Constrains effort on reasoning for
+[reasoning models](https://platform.openai.com/docs/guides/reasoning).
+Currently supported values are \`minimal\`, \`low\`, \`medium\`, and \`high\`. Reducing
+reasoning effort can result in faster responses and fewer tokens used
+on reasoning in a response.`,
+              },
+              {
+                type: "null",
+              },
+            ],
+          },
+          NullableNumberWithConstraints: {
+            oneOf: [
+              {
+                type: "number",
+                minimum: 0,
+                maximum: 100,
+                description: "A percentage value between 0 and 100",
+              },
+              {
+                type: "null",
+              },
+            ],
+          },
+          ModelOrNull: {
+            anyOf: [
+              {
+                $ref: "#/components/schemas/SomeModel",
+              },
+              {
+                type: "null",
+              },
+            ],
+          },
+          SomeModel: {
+            type: "object",
+            properties: {
+              id: { type: "string" },
+            },
+          },
+        },
+      },
+    });
+  });
+
+  it("generates proper TypeSpec code with description and null", () => {
+    const context = createContext(parser, doc as OpenAPI3Document);
+    const types: TypeSpecUnion[] = [];
+    transformComponentSchemas(context, types);
+
+    const union = types.find(
+      (t) => t.name === "ReasoningEffort" && t.kind === "union",
+    ) as TypeSpecUnion;
+    expect(union).toBeDefined();
+    expect(union.doc).toBeTruthy();
+
+    // Generate the actual TypeSpec code
+    const generatedCode = generateDataType(union, context);
+
+    // Verify the generated code contains all expected elements
+    expect(generatedCode).toContain("/**");
+    expect(generatedCode).toContain("Constrains effort on reasoning");
+    expect(generatedCode).toContain("*/");
+    expect(generatedCode).toContain("union ReasoningEffort {");
+    expect(generatedCode).toContain('"minimal"');
+    expect(generatedCode).toContain('"low"');
+    expect(generatedCode).toContain('"medium"');
+    expect(generatedCode).toContain('"high"');
+    expect(generatedCode).toContain("null");
+    expect(generatedCode).toContain("}");
+  });
+
+  it("preserves description from oneOf members with constraints when one is null", () => {
+    const context = createContext(parser, doc as OpenAPI3Document);
+    const types: TypeSpecUnion[] = [];
+    transformComponentSchemas(context, types);
+
+    const union = types.find(
+      (t) => t.name === "NullableNumberWithConstraints" && t.kind === "union",
+    ) as TypeSpecUnion;
+    expect(union).toBeDefined();
+    expect(union.doc).toBe("A percentage value between 0 and 100");
+
+    // Check that decorators from the number schema are preserved
+    expect(union.decorators).toBeDefined();
+    const hasMinConstraint = union.decorators.some((d) => d.name === "minValue");
+    const hasMaxConstraint = union.decorators.some((d) => d.name === "maxValue");
+    expect(hasMinConstraint).toBe(true);
+    expect(hasMaxConstraint).toBe(true);
+  });
+
+  it("handles reference + null anyOf correctly", () => {
+    const context = createContext(parser, doc as OpenAPI3Document);
+    const types: TypeSpecUnion[] = [];
+    transformComponentSchemas(context, types);
+
+    const union = types.find(
+      (t) => t.name === "ModelOrNull" && t.kind === "union",
+    ) as TypeSpecUnion;
+    expect(union).toBeDefined();
+    // For reference + null, there's no description on the ref itself, so doc should be undefined
+    expect(union.doc).toBeUndefined();
+
+    const generatedCode = generateDataType(union, context);
+    expect(generatedCode).toContain("SomeModel");
+    expect(generatedCode).toContain("null");
+  });
+
+  it("handles oneOf with null type array properly", async () => {
+    const docWithTypeArray = await parser.bundle({
+      openapi: "3.1.0",
+      info: { title: "Test", version: "1.0.0" },
+      paths: {},
+      components: {
+        schemas: {
+          NullableString: {
+            type: ["string", "null"],
+            enum: ["value1", "value2", null],
+            default: "value1",
+            description: "A nullable string with enum values",
+          },
+          NullableInteger: {
+            type: ["integer", "null"],
+            minimum: 1,
+            maximum: 10,
+            description: "A nullable integer between 1 and 10",
+          },
+        },
+      },
+    });
+
+    const context = createContext(parser, docWithTypeArray as OpenAPI3Document);
+    const types: TypeSpecUnion[] = [];
+    transformComponentSchemas(context, types);
+
+    const stringUnion = types.find(
+      (t) => t.name === "NullableString" && t.kind === "union",
+    ) as TypeSpecUnion;
+    expect(stringUnion).toBeDefined();
+    expect(stringUnion.doc).toBe("A nullable string with enum values");
+
+    const integerUnion = types.find(
+      (t) => t.name === "NullableInteger" && t.kind === "union",
+    ) as TypeSpecUnion;
+    expect(integerUnion).toBeDefined();
+    expect(integerUnion.doc).toBe("A nullable integer between 1 and 10");
+
+    // Check that constraints are preserved for the integer union
+    const hasMinConstraint = integerUnion.decorators.some((d) => d.name === "minValue");
+    const hasMaxConstraint = integerUnion.decorators.some((d) => d.name === "maxValue");
+    expect(hasMinConstraint).toBe(true);
+    expect(hasMaxConstraint).toBe(true);
+  });
+});