debugging-api-endpoints-and-async-pitfalls

name	debugging_api_endpoints_and_async_pitfalls
description	How to debug 500 API errors by cross-referencing API specifications and detecting missing await in asynchronous token interpolation.
source	auto-skill
extracted_at	2026-07-14T07:09:28.356Z

Debugging API Endpoints and Async Pitfalls in Flutter/Dart Clients

When working with Flutter API client integration, HTTP requests can fail with 500 Bad Response or 404 Not Found errors due to incorrect endpoint paths or improperly resolved headers.

1. Verifying API Endpoints via Local Specifications

Before assuming a server-side bug or rewriting code:

1. Search the workspace (using tools like grep) for any markdown documentation (e.g., FLUTTER_MODELS.md, API.md, or OpenApi specifications) describing server routes.
2. Check the endpoint structure. Frequently, paths are structured differently in the client than in the backend:
   • Example: /books/category/$categoryId instead of /categories/$categoryId/books.
   • Example: /books/read/$id instead of /books/$id/read.
3. Align client paths strictly with the documented routes.

2. Preventing Async Interpolation Pitfalls in Authorization Headers

When injecting authorization tokens dynamically into headers using string interpolation:

• The Issue: Forgetting to await a Future<String?> token getter inside an interpolated string:

  // BUG: interpolates to "Bearer Instance of 'Future<String?>'"
  'Authorization': 'Bearer ${SecureStorageService().getAuthToken()}',
  

• The Fix: Explicitly use await inside the interpolation:

  'Authorization': 'Bearer ${await SecureStorageService().getAuthToken()}',
  

• Always review security or storage service integrations to ensure async methods are properly awaited before placing their output in HTTP headers.

3. Resolving Route Parameter Type Mismatches in Backend Controllers

When passing parameters from client URL paths (e.g. /api/saved-books/:bookId) to databases expecting specific numeric types (e.g. MySQL Int with Prisma):

• The Issue: Express route parameters (destructured from req.params) are strings by default. Passing raw string parameters (e.g. "1") to ORMs like Prisma expecting Int fields will result in database type validation failures and return 500 Internal Server Error.
• The Fix: Explicitly convert parameter strings to integers (e.g., using parseInt(param, 10) in JavaScript/TypeScript) before sending them to database query/model helpers.