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Use 401 Unauthorized when valid authentication credentials for the requested resource are missing or invalid, and include a WWW-Authenticate challenge. Use 403 Forbidden when the server understands the request but refuses it, often because the authenticated caller lacks permission. Use 404 Not Found when the resource has no current representation—or when policy deliberately conceals a forbidden resource’s existence.
What each status code tells the client
These meanings come from RFC 9110, HTTP Semantics, published by the IETF in June 2022. The codes describe different conditions; they are not interchangeable labels for “something went wrong.”
| Status | Server’s meaning | Typical API condition |
|---|---|---|
| 401 Unauthorized | The request has not been applied because valid authentication credentials for the target resource are absent or lacking. | No credentials, invalid credentials, or credentials incomplete for the authentication scheme. |
| 403 Forbidden | The server understands the request but refuses to fulfill it. | The caller is authenticated but lacks permission, or another policy blocks the operation. |
| 404 Not Found | The target resource has no current representation—or the server chooses not to disclose that a forbidden resource exists. | The resource is absent, or its existence is intentionally concealed. |
401: challenge the client to authenticate
A 401 is not a generic permission-denied response. It indicates an authentication problem: the client needs to supply valid credentials for the target resource. RFC 9110 requires a server generating a 401 to send at least one applicable WWW-Authenticate challenge. For example, an API using a bearer-token scheme should return a challenge appropriate to that scheme, rather than an unexplained 401.
A 401 response without WWW-Authenticate fails that HTTP requirement. Check both the status and header when implementing or debugging authentication.
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403: refuse an understood request
A 403 means the server understood the request and will not fulfill it. The caller may have valid credentials that are insufficient for the operation, but the refusal does not have to be credential-related. Do not interpret a 403 as proof that the client is unauthenticated.
Because a 403 is a refusal, clients generally should not retry the identical request with the same credentials and expect a different result. The API may document a remedy, such as requesting a different permission, but clients should follow the service’s documented contract.
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404: absence or deliberate concealment
Use 404 when the target resource has no current representation. RFC 9110 also permits an origin server to use 404 to hide the current existence of a forbidden resource. That is an information-disclosure policy: it can prevent a caller from distinguishing a protected resource from a nonexistent one. It does not remove the need to enforce access controls.
A 404 alone does not establish whether a resource never existed, is temporarily unavailable, or has been removed permanently. If the server knows the condition is likely permanent, RFC 9110 says 410 Gone is preferable to 404.
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Choose a response with this decision rule
Evaluate authentication first, then the authorization decision, then whether the resource exists and whether its existence may be disclosed.
- Are valid credentials for this resource missing or invalid? Return 401 and include an applicable
WWW-Authenticatechallenge. - Are credentials valid, but the request is refused? Return 403 when the server understands the request but denies the operation.
- Is there no current representation? Return 404, unless the server knows the absence is likely permanent—in that case, use 410.
- Would confirming a forbidden resource’s existence disclose information? A deliberate 404 may conceal it, as RFC 9110 allows.
Common mistakes that break API behavior
- Using 403 for missing credentials. If the client must authenticate, use 401 with the required challenge. Reserve 403 for a refusal of an understood request.
- Sending 401 without a challenge. A server-generated 401 must include at least one applicable
WWW-Authenticatechallenge. - Describing every 403 as an authentication failure. Credentials may be valid but insufficient, and the refusal may have another cause.
- Claiming every 404 means “never existed.” The response may conceal a forbidden resource and does not distinguish temporary from permanent absence.
- Making clients depend on undocumented behavior. Clients that assume only a fixed set of undocumented statuses or response details can break as an API evolves.
- Discarding application-specific error details. The HTTP status may not identify the precise service-level cause. For Amazon S3, AWS specifically says its service error code is more informative than the HTTP status and recommends using that code for handling and reporting errors.
Document status, headers, and error bodies as one contract
An API response contract should specify more than a three-digit status. It should define the expected status, relevant headers, and response-body model together. Amazon API Gateway’s method-response documentation describes method responses in terms of expected status codes, headers, and body models.
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For each error your API exposes, document the HTTP status, a stable machine-readable application error code, a human-readable message, and any retry guidance the service promises. Amazon Prime API guidance describes structured error code and message fields, cautions clients against relying on undocumented response details, and notes that additional status codes may be supported in the future.
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These are service-specific examples, not universal AWS response rules. In particular, AWS’s advice to use a service error code for detailed handling applies to its S3 error contract; other APIs should follow their own documented fields. Clients should handle documented responses and avoid treating an unrecognized status as impossible.
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Quick reference for API authors
| Request condition | Response | Key detail |
|---|---|---|
| Credentials are absent, invalid, or incomplete for the authentication scheme | 401 Unauthorized | Include an applicable WWW-Authenticate challenge. |
| The request is understood, but policy refuses the operation | 403 Forbidden | May reflect insufficient credentials or another reason for refusal. |
| The target has no current representation | 404 Not Found | Does not establish whether absence is temporary or permanent. |
| The resource is forbidden and its existence should not be revealed | 404 Not Found | Use as a deliberate concealment policy, not instead of access control. |
| The server knows the resource is likely permanently gone | 410 Gone | RFC 9110 prefers 410 over 404 in this case. |
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