HTTP 405 Error: Method Not Allowed

The HTTP 405 "Method Not Allowed" error indicates that the HTTP method used in the request (GET, POST, PUT, DELETE, etc.) is not supported by the targeted resource. The server understands the request but refuses to execute it with this method.

This error is common during REST API development when route configuration is incorrect, or when a client uses the wrong method to interact with an endpoint. The server should include an "Allow" header listing the accepted methods.

For API monitoring, 405 generally signals a configuration problem rather than an outage. However, a sudden behavior change (an endpoint that accepted POST now returning 405) merits investigation.

Main causes of 405 errors

The 405 error can come from several sources. Here are the most common:

• Wrong HTTP method: Using POST on an endpoint that only accepts GET, or sending DELETE when only PUT is allowed.
• Routing configuration: The framework or web server has no route defined for this URL + method combination.
• HTML form: The form's method attribute doesn't match what the server expects (e.g., GET instead of POST).
• Server restriction: The web server (Apache, Nginx) may block certain methods globally or for specific directories.

HTTP methods explained

Understanding HTTP methods is essential for resolving 405 errors:

• GET: Retrieves a resource. Should not modify server state. Can be cached.
• POST: Creates a new resource or sends data. The request body contains the data.
• PUT / PATCH: PUT replaces a resource completely, PATCH modifies it partially. Both are idempotent.
• DELETE: Deletes a resource. Should be idempotent (deleting twice = same result).

Resolving 405 errors

Based on the identified cause, here are the solutions to apply:

• Check documentation: Consult the API documentation to know which methods are accepted by each endpoint.
• Examine Allow header: The 405 response should contain an Allow header listing accepted methods (e.g., "Allow: GET, POST").
• Fix client code: Modify your code to use the correct method. In fetch() or axios, check the method parameter.
• Configure server: If you control the server, add the missing method to routes or server configuration.

Configuration examples

Here are examples for properly handling HTTP methods:

// JavaScript - Specify method
fetch("/api/users/123", {
    method: "DELETE"  // or GET, POST, PUT, PATCH
});

// PHP Laravel - Define routes
Route::get("/users", [UserController::class, "index"]);
Route::post("/users", [UserController::class, "store"]);
Route::put("/users/{id}", [UserController::class, "update"]);
Route::delete("/users/{id}", [UserController::class, "destroy"]);

// Express.js - Multiple routes
app.route("/users/:id")
    .get(getUser)
    .put(updateUser)
    .delete(deleteUser);

Explicitly define accepted methods for each endpoint. Use REST conventions: GET to read, POST to create, PUT/PATCH to update, DELETE to remove.

Monitoring HTTP methods

MoniTao allows testing different HTTP methods:

• Configure method: Select GET, POST, PUT, or DELETE in the monitor configuration depending on what you're testing.
• Test endpoints: Create separate monitors for different methods on the same endpoint if needed.
• Detect changes: A sudden 405 on a working endpoint signals a configuration change to investigate.
• Check Allow header: In check details, verify the Allow header lists expected methods.

Frequently asked questions about HTTP 405

Conclusion

The HTTP 405 Method Not Allowed error indicates an incompatibility between the method used and those accepted by the endpoint. Good API documentation and consistent REST conventions prevent most of these errors.

MoniTao lets you configure the HTTP method for each monitor, ensuring your REST endpoints respond correctly according to expected semantics. An unexpected 405 signals a configuration change to investigate immediately.

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