diff --git a/.changeset/openapi-vendor-json-content-type.md b/.changeset/openapi-vendor-json-content-type.md
new file mode 100644
index 0000000000..24c4da544e
--- /dev/null
+++ b/.changeset/openapi-vendor-json-content-type.md
@@ -0,0 +1,5 @@
+---
+"@executor-js/plugin-openapi": patch
+---
+
+Preserve vendor +json Content-Type on OpenAPI object request bodies.
diff --git a/packages/plugins/openapi/src/sdk/invoke.ts b/packages/plugins/openapi/src/sdk/invoke.ts
index 148b4a052a..2f9bd0f6f5 100644
--- a/packages/plugins/openapi/src/sdk/invoke.ts
+++ b/packages/plugins/openapi/src/sdk/invoke.ts
@@ -838,12 +838,8 @@ const applyRequestBody = (
   });
 
   if (isJsonContentType(contentType)) {
-    // Pre-serialized JSON strings pass through with the declared media
-    // type preserved (important for `application/vnd.foo+json` etc.).
-    if (typeof bodyValue === "string") {
-      return sent(HttpClientRequest.bodyText(request, bodyValue, contentType));
-    }
-    return sent(HttpClientRequest.bodyJsonUnsafe(request, bodyValue));
+    const text = typeof bodyValue === "string" ? bodyValue : JSON.stringify(bodyValue);
+    return sent(HttpClientRequest.bodyText(request, text, contentType));
   }
 
   if (isFormUrlEncoded(contentType)) {
diff --git a/packages/plugins/openapi/src/sdk/non-json-body.test.ts b/packages/plugins/openapi/src/sdk/non-json-body.test.ts
index 87deeaba98..d8619e13fe 100644
--- a/packages/plugins/openapi/src/sdk/non-json-body.test.ts
+++ b/packages/plugins/openapi/src/sdk/non-json-body.test.ts
@@ -447,6 +447,36 @@ describe("OpenAPI non-JSON request body dispatch", () => {
     }),
   );
 
+  it.effect("application/vnd.api+json: object body keeps the declared content type", () =>
+    Effect.gen(function* () {
+      const { server, captured } = yield* startEchoServer({
+        name: "createNote",
+        path: "/notes",
+        payload: JsonNameObject,
+        transformSpec: replaceRequestBodyContent("/notes", "post", {
+          "application/vnd.api+json": {
+            schema: {
+              type: "object",
+              properties: {
+                name: { type: "string" },
+              },
+            },
+          },
+        }),
+      });
+
+      const executor = yield* createExecutor(makeTestConfig({ plugins: testPlugins() }));
+      const conn = yield* addOpenApiTestConnection(executor, server, { slug: "notes-jsonapi" });
+
+      yield* executor.execute(conn.address("body.createNote"), {
+        body: { name: "Acme" },
+      });
+
+      expect(captured.contentType).toBe("application/vnd.api+json");
+      expect(decodeJsonNameBody(captured.body.toString("utf8"))).toEqual({ name: "Acme" });
+    }),
+  );
+
   it.effect("multipart/form-data: only encoder-supported file shapes are advertised", () =>
     Effect.gen(function* () {
       const { server } = yield* startEchoServer({