square/retrofit · error · IllegalStateException

Response must be parameterized as Response<Foo> or Response<

Error message

Response must be parameterized as Response<Foo> or Response<? extends Foo>

What it means

Thrown by RxJavaCallAdapterFactory when the outer reactive type is parameterized but its argument is raw `Response`. The factory needs `Response<T>` to extract the body type T; raw Response fails the check at RxJavaCallAdapterFactory.java:127. The same rule applies whether the outer type is Observable or Single.

Source

Thrown at retrofit-adapters/rxjava/src/main/java/retrofit2/adapter/rxjava/RxJavaCallAdapterFactory.java:128

    boolean isBody = false;
    Type responseType;
    if (!(returnType instanceof ParameterizedType)) {
      String name = isSingle ? "Single" : "Observable";
      throw new IllegalStateException(
          name
              + " return type must be parameterized"
              + " as "
              + name
              + "<Foo> or "
              + name
              + "<? extends Foo>");
    }

    Type observableType = getParameterUpperBound(0, (ParameterizedType) returnType);
    Class<?> rawObservableType = getRawType(observableType);
    if (rawObservableType == Response.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException(
            "Response must be parameterized" + " as Response<Foo> or Response<? extends Foo>");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
    } else if (rawObservableType == Result.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException(
            "Result must be parameterized" + " as Result<Foo> or Result<? extends Foo>");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
      isResult = true;
    } else {
      responseType = observableType;
      isBody = true;
    }

    return new RxJavaCallAdapter(
        responseType, scheduler, isAsync, isResult, isBody, isSingle, false);
  }

View on GitHub (pinned to d0b112dad0)

Solutions

1. Parameterize Response: `Observable<Response<User>> getUser();`.
2. Or drop the wrapper: `Observable<User> getUser();`.
3. If you want both body and error handling in one stream, use `Observable<Result<User>>`.

Example fix

// before
@GET("users/{id}")
Observable<Response> getUser(@Path("id") String id);

// after
@GET("users/{id}")
Observable<Response<User>> getUser(@Path("id") String id);

for (Method m : MyService.class.getDeclaredMethods()) {
    Type rt = m.getGenericReturnType();
    if (rt instanceof ParameterizedType
            && ((ParameterizedType) rt).getActualTypeArguments()[0] == retrofit2.Response.class) {
        throw new IllegalStateException(m + " uses raw Response inside an RxJava type.");
    }
}

// Fail fast in a wiring test:
try {
    retrofit.create(MyService.class).getUser("1");
} catch (IllegalStateException e) {
    if (e.getMessage().contains("Response must be parameterized")) {
        throw new IllegalStateException("Fix raw Response<> inside RxJava return in MyService.", e);
    }
    throw e;
}

When it happens

Trigger: A service method returns `Observable<Response>` or `Single<Response>` (raw Response) rather than `Observable<Response<User>>`.

Common situations: Switching from a body return to a Response-wrapped return and forgetting the inner generic; or autocompleting Response without its type argument.

Related errors

• Response must be parameterized as Response<Foo> or Response<
• Response must be parameterized as Response<Foo> or Response<
• ${name} return type must be parameterized as ${name}<Foo> or
• Result must be parameterized as Result<Foo> or Result<? exte
• Response must be parameterized as Response<Foo> or Response<

AI-assisted analysis of square/retrofit@d0b112dad0 (2026-08-04). Data as JSON: /data/errors/1f768b71dd03200c.json. Report an issue: GitHub.