The headers default to adding "Dart3.0" to the user-agent, resulting in some image sources detecting it as Dart's user agent and returning it directly to Forbidden

The user agent Dart/3.0 (dart:io) (in your case) is set in dart:io which this package uses under-the-hood when running on platforms that have dart:io (so macOS/Windows/Linux/Android/iOS as opposed to a browser). When running in the browser, it will use the browser's user agent by default (also, some browsers prevent setting the user agent - my understanding is that User-Agent used to be a blocked/protected header but isn't anymore) so it might not be feasible to add this as a general option (as it would then be expected to work consistently on both dart:io platforms and the web).

With dart:io there wouldn't be a 'default user agent' that this package or the dart:io library can inherit like there is with a browser (because the HTTP client stack is Dart's own but also because native platforms don't often set their own user agent), so there's nothing else to get a default from. On the other hand, not sending a user agent at all is very often problematic. For example, anti-DDOS or firewall services like CloudFlare generally block requests without a user agent (albeit depending on the settings for the site), so in dart:io the user agent Dart/<version> (dart:io) is set as a reasonable default (but some services might not recognize this user agent or flag it as suspicious due to relatively small usage compared to ordinary web browsers).

A simple workaround (if you're not targeting Browsers - i.e., Flutter for Web) is to set one explicitly - that is; just create the IOClient with a custom user agent (e.g., one from this list of most common user agents) and pass it into a dart:http client. Like so:

import 'package:http/http.dart';

// Add these imports - but note they won't work on browsers (i.e., Flutter for Web)
import 'dart:io';
import 'package:http/io_client.dart';

/// Create a package:http Client with a custom [userAgent].
/// Alternatively, set [userAgent] to null to not send any user agent.
Client createCustomClient({
  // User agent for Chrome on Windows - should avoid any issues.
  final String? userAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) "
      "AppleWebKit/537.36 (KHTML, like Gecko) "
      "Chrome/114.0.0.0 "
      "Safari/537.36",
}) {
  // Initialize a (lower-level) HttpClient from dart:io with a custom user agent.
  // (Don't use this directly).
  final innerClient = HttpClient()
    ..userAgent = userAgent;

  // Pass the lower-level client into an IOClient object (from the http package).
  return IOClient(innerClient);
}

Future<void> main() async {
  // Place the following where you initialize your client in your code.

  final client = createCustomClient();

  // You can now use `client`. For example:

  // echo.hoppscotch.io is a service that echoes back the request headers
  // and body as a JSON response.
  final response = await client.get(Uri.parse('https://echo.hoppscotch.io'));

  // The headers.user-agent property should match the value passed into
  // `createCustomClient`.
  // (Or it'll be the default in `createCustomClient`).
  print(response.body);
}

If you instead use get, post, etc., as opposed to calling them on a specific Client instance, you can use runWithClient. Wrap as much of your code (probably whatever you call in your main method) with a call to runWithClient and use the createCustomClient provided above.

If you're using Flutter, you can just wrap your call to runApp like so:

// Paste the `createCustomClient` function from above.

void main() {
  runWithClient(() {
      runApp(const MyApp());
  }, () => createCustomClient());
}

Then, calls to get, post, etc., - even when not called explicitly on a client - will use your custom user agent.

If you need to support Dart/Flutter on the Web too, that's still doable but you'll need to use a conditional import - probably by placing createCustomClient in an external file and making one for web, using this one for dart:io and a stub one, and then conditionally importing the one relevant to the current platform. (Feel free to ask for further details).

dart-lang / http

The headers default to adding "Dart3.0" to the user-agent, resulting in some image sources detecting it as Dart's user agent and returning it directly to Forbidden #990