Debugging a 405 Method Not Allowed Error in a Mobile API

The Problem

While developing a mobile application, a specific API endpoint for fetching user profile data suddenly started failing. The mobile client was receiving a 405 Method Not Allowed HTTP status code, and the data refused to load.

Context

The application relies on a Spring Boot backend exposing RESTful endpoints. The mobile client (built in Flutter) communicates with this API using the http package. The endpoint in question was /api/v1/users/profile.

What I Tried

At first, I assumed the authentication token was invalid or the endpoint URL was misspelled.

1. I checked the JWT token — it was valid.
2. I verified the URL string in the Dart code — it perfectly matched the backend controller.
3. I checked the server logs, expecting to see a NullPointerException or a 400 Bad Request. Instead, Spring Security was just quietly rejecting the request at the filter level.

What Failed

I spent an hour looking at the backend logic, assuming the database query was failing or the routing was broken. None of this was the issue because the request wasn’t even reaching my controller method.

What Worked & Technical Explanation

The breakthrough came when I bypassed the mobile app entirely and used Postman to test the endpoint.

When I sent a GET request in Postman: 200 OK. When I looked closely at my Flutter network layer:

// The bug
final response = await http.post(
  Uri.parse('https://api.example.com/api/v1/users/profile'),
  headers: {'Authorization': 'Bearer $token'},
);

The issue was glaringly simple: I was using http.post for an endpoint that the backend explicitly defined as a @GetMapping.

A 405 Method Not Allowed means exactly what it says: the server exists, the route exists, but the HTTP verb (GET, POST, PUT, DELETE) you used is not supported for that specific route. It is fundamentally different from a 404 Not Found (route doesn’t exist) or a 400 Bad Request (payload is wrong).

I corrected the mobile client to use http.get:

// The fix
final response = await http.get(
  Uri.parse('https://api.example.com/api/v1/users/profile'),
  headers: {'Authorization': 'Bearer $token'},
);

Lessons Learned

1. Read the HTTP Status Code literally. Don’t assume a 405 is a generic crash. It has a highly specific meaning defined by the HTTP protocol.
2. Isolate the client from the server. When an API fails in a mobile app, test the exact same request in curl or Postman immediately. This tells you instantly if the bug is in the client code (Flutter) or the server code (Spring Boot).

What I Would Do Differently

I will implement a centralized API client class in Flutter with strict typed methods (fetchProfile(), updateProfile()) rather than writing raw http.get or http.post calls scattered throughout the UI code. This reduces the surface area for simple typo bugs like using the wrong HTTP verb.