510 - Not Extended
HTTP 510 Not Extended is an obsolete status code from an experimental extension model and should not be emitted by modern HTTP APIs.
Last reviewed: February 9, 2026|Editorial standard: source-backed technical guidance
What Does Not Extended Mean?
An outdated extension mechanism is being enforced in the request path, causing modern clients to fail against nonstandard protocol expectations.
Common Causes
- -Endpoint requires declared HTTP extension token per contract, but client omits mandatory extension header.
- -Gateway expects feature extension parameter for experimental route and rejects requests missing that declaration.
- -Integration refactor removed required extension metadata, so origin cannot honor requested capability set.
How to Fix Not Extended
- 1Identify component emitting 510 and replace extension-dependent behavior with modern standards-compliant semantics.
- 2Map failure to appropriate current status code (for example 400/401/403/426/501) based on actual condition.
- 3Retest clients against updated contract and remove legacy extension requirements from docs/SDKs.
Step-by-Step Diagnosis for Not Extended
- 1Capture request/response pair and identify extension token or policy path driving 510 emission.
- 2Trace middleware/proxy/plugin chain for obsolete extension negotiation checks.
- 3Validate current API contract against runtime behavior to confirm 510 is unintended legacy behavior.
- 4Retest after replacing extension path with standards-compliant handling.
Legacy Extension Path Identification
- -Inspect old extension frameworks or modules (example: deprecated policy plugin still requires extension declaration headers).
- -Confirm whether 510 is intentional or accidental fallback from legacy code path.
Contract Modernization and Compatibility Checks
- -Map legacy extension requirement to supported modern equivalent (example: required capability now enforced via versioned endpoint instead of extension token).
- -Verify generated SDKs and docs no longer mention obsolete extension negotiation steps.
Implementation Examples
curl -i -X GET https://api.example.com/v1/resource
# Response:
# HTTP/1.1 510 Not Extended
# {"error":"Not Extended","code":"510"}const response = await fetch('https://api.example.com/v1/resource', {
method: 'GET',
headers: { 'Accept': 'application/json' }
});
if (response.status === 510) {
console.error('Handle 510 Not Extended');
}import requests
response = requests.get(
'https://api.example.com/v1/resource',
headers={'Accept': 'application/json'}
)
if response.status_code == 510:
print('Handle 510 Not Extended')How to Verify the Fix
- -Repeat affected requests and confirm 510 is replaced by expected modern status semantics.
- -Validate client compatibility across supported runtimes and SDK versions.
- -Monitor for residual 510 emissions and legacy extension header usage in traffic.
How to Prevent Recurrence
- -Ban obsolete status codes/extension mechanisms in API linting and design reviews.
- -Version and deprecate legacy protocol behaviors through explicit migration plans.
- -Add CI checks that reject non-allowlisted status codes per endpoint.
Pro Tip
- -curate a protocol-debt inventory and tie deprecation milestones to deployment gates so legacy extension code cannot silently persist.
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.