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Conditional Requests & Concurrency

ETag

IANA permanent response RFC 9110 reached by regulation 4 spellings

Declared by 125 providers across 7568 published specification files in the APIs.io network.

What this header is for, and how to use it →

A count here is providers whose published contract declares this header — not providers who send it. Response headers set at the edge rarely appear in an OpenAPI at all, so a low number can measure documentation practice rather than deployment.

Spelled 4 ways

HTTP field names are case-insensitive (RFC 9110 §5.1), so every spelling below is the same header on the wire. A contract is not the wire: generated clients key off the string, and a developer reading two of these sees two different headers.

ETagetagEtageTag

Reached by regulation

Basis: mandated. The law, or a technical standard the law makes binding, names this header. Observable at the edge, without credentials.

onc-health-it-certification

Carried by the fhir standard.

The registry

Statuspermanent
ReferenceRFC 9110, Section 8.8.3: HTTP Semantics

Providers declaring it (125)

aib-group-uk aimlapi akamai aldermore amazon-cloudfront amazon-s3 an-api-of-ice-and-fire apache-iceberg apache-ozone azure-storage-account azure-storage-accounts bank-of-ireland-uk bank-of-scotland bitpowr-technologies-inc brightcove carnegie-mellon-university ccp-games cdr-energy chargy cint cloudflare-r2 co-operative-bank commure companies-house coventry-building-society cubefs cybersource cyclecalcs dapr deakin dedaluslabs devcycle elastic-path electric-sql equinix erasmus-university-rotterdam etsi eve-online fhir figshare filebase first-direct galileo-technologies gb-bank glide-apps halifax hbku hku humanitec ilert indykite influxdata influxdb interhyp jp-morgan-chase keboola kings-college-london lakefs leeds-building-society lloyds-banking-group loughborough macquarie mastercard metro-bank mettle microsoft-azure microsoft-azure-devops miro monash monument-bank nationwide-building-society newstore nhost open-banking-uk openfeature oracle oracle-database orion palo-alto-networks paragon-bank planomy-tax-data principality-building-society pypi recognise-bank rmit salesforce santander-uk secure-trust-bank shawbrook-bank shyft sigstore six-group skipton-building-society smarty soracom spp starling-bank stockholm subspace symphony synack tago-io tsb-bank university-of-adelaide university-of-amsterdam university-of-auckland university-of-canterbury university-of-rochester university-of-sheffield university-of-sussex university-of-the-witwatersrand uon uphold upwind upwind-security viglink virgin-money-uk weatherbys-bank wikidata wikimedia wikipedia wikisource worldcat wso2 zoominfo

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All headers Conditional Requests & Concurrency Guidance on headers.apievangelist.com