fix(openapi3): avoid duplicating parameter @extension on schema

.chronus/changes/openapi3-fix-extension-dup-on-parameters-2026-6-8-0-0-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Fix `@extension` being duplicated on both the parameter object and its `schema` in OpenAPI output. Parameter extensions are now emitted only on the parameter object.

packages/openapi3/src/openapi.ts

@@ -1632,6 +1632,15 @@ function createOAPIEmitter(
     // Description is already provided in the parameter itself.
     delete schema.description;
 
+    const extensions = getExtensions(program, param);
+    if (extensions && !("$ref" in schema)) {
+      for (const key of extensions.keys()) {
+        if (key in schema) {
+          delete schema[key];
+        }
+      }
+    }
+
     const oaiParam: OpenAPI3ParameterBase = {
       required: !param.optional,
       description: getDoc(program, param),

packages/openapi3/test/extensions.test.ts

@@ -46,6 +46,33 @@ describe("target", () => {
     strictEqual(oapi.components.parameters.PetId["x-parameter-extension"], "foobaz");
   });
 
+  it("does not duplicate parameter extensions on parameter schemas", async () => {
+    const oapi = await openApiFor(
+      `
+      @route("/{id}") @get op get(
+        @extension("x-my-ext", "path") @path id: string,
+        @extension("x-my-ext", "query") @query filter: string,
+      ): void;
+      `,
+    );
+
+    const parameters = oapi.paths["/{id}"].get.parameters as Array<{
+      name: string;
+      schema: Record<string, unknown>;
+      [key: string]: unknown;
+    }>;
+    const pathParam = parameters.find((x) => x.name === "id");
+    const queryParam = parameters.find((x) => x.name === "filter");
+
+    ok(pathParam);
+    ok(queryParam);
+
+    expect(pathParam["x-my-ext"]).toEqual("path");
+    expect(pathParam.schema["x-my-ext"]).toBeUndefined();
+    expect(queryParam["x-my-ext"]).toEqual("query");
+    expect(queryParam.schema["x-my-ext"]).toBeUndefined();
+  });
+
   it("adds at root of document when on namespace", async () => {
     const oapi = await openApiFor(
       `