Status Codes Overview

#️⃣ What Are Status Codes?

Status codes in FastAPI are HTTP response codes that indicate whether a request was successful or failed — and explain the reason.

They are globally standardized by the HTTP specification and used by all web systems.

Whenever a client (browser, mobile app, frontend, or API consumer) sends a request, the server responds with:

• A status code

• A response body (optional)

• Response headers

👉 The status code is the primary signal that tells the client how to interpret the result.

⚙️ Why Status Codes Matter

Status codes play a crucial role in API communication:

• ✅ Indicate success or failure

• 🔎 Explain what happened

• 🛠️ Help with debugging and monitoring

• 🔄 Enable proper client-side error handling

Without them, clients would have no consistent way to know whether a request worked.

📊 Status Code Categories

HTTP status codes are grouped into five series, ranging from 100 to 599.

Each series communicates a specific class of outcome.

🔹 1xx — Informational (Request Received)

These indicate the request was received and processing is ongoing.

Examples:

• 100 Continue — Client may continue sending the request

• 101 Switching Protocols — Server switching protocols (e.g., to WebSocket)

• 102 Processing — Still processing

• 103 Early Hints — Preliminary headers sent

👉 Rarely used directly in REST APIs.

🔹 2xx — Success (Request Completed)

These indicate the request was successfully processed.

Common examples:

• 200 OK — Standard success (typically GET)

• 201 Created — Resource successfully created (typically POST)

• 202 Accepted — Request accepted for processing later

• 204 No Content — Success with no response body (PUT/DELETE)

👉 In most APIs, 200, 201, and 204 are used most frequently.

🔹 3xx — Redirection (Further Action Needed)

These indicate that the client must take additional steps.

Examples:

• 301 Moved Permanently

• 302 Found

• 303 See Other

• 304 Not Modified — Common with caching

• 307 Temporary Redirect

• 308 Permanent Redirect

👉 Less common in APIs, except for caching scenarios.

🔹 4xx — Client Errors (Invalid Request)

These occur when the problem is caused by the client.

Examples:

• 400 Bad Request — Invalid input

• 401 Unauthorized — Authentication required

• 403 Forbidden — Access denied

• 404 Not Found — Resource does not exist

• 405 Method Not Allowed

• 409 Conflict

• 415 Unsupported Media Type

• 422 Unprocessable Entity — Validation error (very common in FastAPI)

• 429 Too Many Requests — Rate limiting

👉 Indicates the client must fix the request.

🔹 5xx — Server Errors (Server Failure)

These indicate a failure on the server side.

Examples:

• 500 Internal Server Error — Unexpected server failure

• 501 Not Implemented

• 502 Bad Gateway

• 503 Service Unavailable

• 504 Gateway Timeout

👉 Critical for diagnosing production issues.

📝 Focus Areas in This Course

For practical API development, we will mainly work with:

• ✅ 2xx — Success responses

• ✅ 3xx — Redirection (when relevant)

• ✅ 5xx — Server errors

As you build more advanced features (authentication, validation, rate limiting), important 4xx codes such as 400, 401, 403, 404, and 422 will also become essential.