fix(openapi3): missing discriminator mapping entry when first union variant causes circular emit

.chronus/changes/fix-openapi3-discriminator-circular-emit-2026-4-13.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Fix missing discriminator mapping entry when the first union variant causes a circular emit, affecting both the OpenAPI 3.0 and 3.2 emitters.

packages/openapi3/src/schema-emitter-3-2.ts

@@ -3,6 +3,7 @@ import {
   AssetEmitter,
   createAssetEmitter,
   ObjectBuilder,
+  Placeholder,
   TypeEmitter,
 } from "@typespec/asset-emitter";
 import { compilerAssert, DiscriminatedUnion, Type } from "@typespec/compiler";
@@ -98,7 +99,13 @@ export class OpenAPI32SchemaEmitter extends OpenAPI31SchemaEmitter {
     for (const [key, model] of variants.entries()) {
       const ref = this.emitter.emitTypeReference(model);
       compilerAssert(ref.kind === "code", "Unexpected ref schema. Should be kind: code");
-      mapping[key] = (ref.value as any).$ref;
+      if (ref.value instanceof Placeholder) {
+        ref.value.onValue((resolvedValue: any) => {
+          mapping[key] = (resolvedValue as any).$ref;
+        });
+      } else {
+        mapping[key] = (ref.value as any).$ref;
+      }
     }
     return mapping;
   }

packages/openapi3/src/schema-emitter.ts

@@ -720,7 +720,13 @@ export class OpenAPI3SchemaEmitterBase<
     for (const [key, model] of variants.entries()) {
       const ref = this.emitter.emitTypeReference(model);
       compilerAssert(ref.kind === "code", "Unexpected ref schema. Should be kind: code");
-      mapping[key] = (ref.value as any).$ref;
+      if (ref.value instanceof Placeholder) {
+        ref.value.onValue((resolvedValue) => {
+          mapping[key] = (resolvedValue as any).$ref;
+        });
+      } else {
+        mapping[key] = (ref.value as any).$ref;
+      }
     }
     return mapping;
   }

packages/openapi3/test/discriminator.test.ts

@@ -345,6 +345,33 @@ worksFor(supportedVersions, ({ checkFor, openApiFor }) => {
     ]);
   });
 
+  it("includes all variants in discriminator mapping when first union variant causes circular emit", async () => {
+    const openApi = await openApiFor(`
+      @discriminator("kind")
+      model Pet { kind: string; }
+
+      model Cat extends Pet { kind: "cat"; }
+      model Dog extends Pet { kind: "dog"; }
+      model Bird extends Pet { kind: "bird"; }
+
+      union PetVariant {
+        cat: Cat,
+        dog: Dog,
+        bird: Bird,
+      }
+
+      op read(): { @body body: PetVariant };
+      `);
+    deepStrictEqual(openApi.components.schemas.Pet.discriminator, {
+      propertyName: "kind",
+      mapping: {
+        cat: "#/components/schemas/Cat",
+        dog: "#/components/schemas/Dog",
+        bird: "#/components/schemas/Bird",
+      },
+    });
+  });
+
   it("discriminator always needs to be marked as required", async () => {
     const openApi = await openApiFor(`
      @discriminator("kind")