501 - Not Implemented
HTTP 501 Not Implemented means the server does not support the functionality required to fulfill this request.
Last reviewed: February 18, 2026|Editorial standard: source-backed technical guidance
What Does Not Implemented Mean?
The requested HTTP functionality is not implemented by this server path, so retrying unchanged requests will not succeed until capability mismatch is resolved.
Common Causes
- -OpenAPI spec lists endpoint, but deployed service binary omits handler implementation for this region.
- -Proxy forwards WebDAV method such as PROPFIND to backend that supports only standard REST verbs.
- -Feature toggle exposes route path while operation class is excluded at build time in current artifact.
How to Fix Not Implemented
- 1Verify requested method/feature is actually supported by this server and API version.
- 2Route calls to a backend/service version that implements the required capability.
- 3Update client behavior to supported methods or protocol features when implementation is intentionally absent.
Step-by-Step Diagnosis for Not Implemented
- 1Capture exact method, protocol feature, and route that triggered 501.
- 2Confirm whether the origin recognizes and supports that functionality for any target resource.
- 3Inspect routing/service discovery to ensure requests are not hitting an implementation-incomplete backend.
- 4Retest against documented supported capability matrix and corrected route/version selection.
Capability Matrix and Method Support Audit
- -Check supported methods per endpoint (example: client sends
PATCH, service only implementsGETandPUT). - -Validate protocol extension support (example: legacy backend does not implement required HTTP extension behavior).
Routing and Version Compatibility Checks
- -Trace request routing to confirm calls do not land on partial deployments (example: canary pool missing feature flag for new method handler).
- -Compare client SDK version assumptions with deployed API capabilities (example: SDK expects v3 endpoint features, server still on v2).
Implementation Examples
curl -i -X GET https://api.example.com/v1/resource
# Response:
# HTTP/1.1 501 Not Implemented
# {"error":"Not Implemented","code":"501"}const response = await fetch('https://api.example.com/v1/resource', {
method: 'GET',
headers: { 'Accept': 'application/json' }
});
if (response.status === 501) {
console.error('Handle 501 Not Implemented');
}import requests
response = requests.get(
'https://api.example.com/v1/resource',
headers={'Accept': 'application/json'}
)
if response.status_code == 501:
print('Handle 501 Not Implemented')How to Verify the Fix
- -Repeat requests using supported method/feature paths and confirm 501 is eliminated.
- -Validate routing sends traffic to nodes that implement required capability.
- -Confirm unsupported capability requests are either translated gracefully or rejected with clear guidance.
How to Prevent Recurrence
- -Publish explicit capability matrix by endpoint, method, and API version.
- -Gate client SDK releases against server capability conformance tests.
- -Block partial rollouts where routing can expose unimplemented handlers in production.
Pro Tip
- -derive compatibility tests directly from OpenAPI/contract diffs so unsupported methods fail in CI before deployment.
Decision Support
Compare Guide
429 Too Many Requests vs 503 Service Unavailable
Use 429 for caller-specific throttling and 503 for service-wide outages, so retry behavior, escalation paths, and incident ownership stay correct.
Compare Guide
500 Internal Server Error vs 502 Bad Gateway: Root Cause
Debug 500 vs 502 faster: use 500 for origin failures and 502 for invalid upstream responses at gateways, then route incidents to the right team.
Playbook
API Timeout Playbook (502 / 504 / DEADLINE_EXCEEDED)
Use this playbook to separate invalid upstream responses from upstream wait expiration and deadline exhaustion, and apply timeout budgets, safe retries, and circuit-breaker controls safely.
Playbook
Availability and Dependency Playbook (500 / 503 / ServiceUnavailable)
Use this playbook to separate origin-side 500 failures from temporary 503 dependency or capacity outages, then apply safe retry and escalation paths.
Official References
Provider Context
This guidance is specific to HTTP services. Always validate implementation details against official provider documentation before deploying to production.