[flutter_svg] SvgNetworkLoader parses non-2xx error pages as SVG, hiding the real failure #195162

What package does this bug report belong to?

flutter_svg

What target platforms are you seeing this bug on?

Android, iOS

Have you already upgraded your packages?

Yes

Dependency versions

flutter_svg 2.3.1 (also present on current main of flutter/packages). No other dependencies are involved — the bug is in SvgNetworkLoader itself.

Steps to reproduce

1. Point SvgPicture.network (or SvgNetworkLoader) at a URL whose server answers with a non-2xx status and an HTML body, e.g. a 404 or a 503 error page.
2. Let the picture load.

Expected results

The load fails with an error that names the HTTP status and URL, so the cause is visible (e.g. an http.ClientException).

Actual results

SvgNetworkLoader.prepareMessage returns response.bodyBytes without checking response.statusCode, so the HTML error page is handed to the SVG parser and fails as StateError: Bad state: Invalid SVG data. The actual 404/503 is lost.

We ran into this in production: during a short backend 503 outage, a request got the server's error page, and our crash reporter showed only "Invalid SVG data".

Code sample

// Any URL that returns 404 with an HTML body.
final bytes = await const SvgNetworkLoader('https://example.com/missing.svg')
    .prepareMessage(null);
// bytes is the 404 page; parsing it later throws
// StateError: Bad state: Invalid SVG data

Screenshots or Videos

No response

Logs

StateError: Bad state: Invalid SVG data
  parser.dart: SvgParser._parseTree
  parser.dart: SvgParser.parse
  vector_graphics_compiler.dart: parse
  vector_graphics_compiler.dart: encodeSvg
  loaders.dart: SvgLoader._load.<fn>.<fn>

Flutter Doctor output

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.44.8, on macOS 27.0.1 26A434 darwin-arm64, locale en-US)
[!] Android toolchain - develop for Android devices (Android SDK version 36.1.0)
[!] Xcode - develop for iOS and macOS (Xcode 27.0)
[✓] Chrome - develop for the web
[✓] Connected device (3 available)
[✓] Network resources