--- layout: default ---

If-Modified-Since

IANA permanent request RFC 9110 3 spellings

Declared by 52 providers across 711 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 3 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.

If-Modified-SinceIF-MODIFIED-SINCEif-modified-since

The registry

Status	permanent
Reference	RFC 9110, Section 13.1.3: HTTP Semantics

Providers declaring it (52)

agicap aib-group-uk aldermore amazon-s3 autodesk-fusion avito aws-s3 bank-of-ireland-uk bank-of-scotland ccp-games clio cloudflare-r2 co-operative-bank commure coventry-building-society cubefs engineering-platform erply first-direct gb-bank halifax leeds-building-society lloyds-banking-group metro-bank mettle microsoft-azure monument-bank nationwide-building-society nhost open-banking-uk oracle oracle-database paragon-bank phrase principality-building-society puppet-labs recognise-bank salesforce salesforce-experience-cloud santander-uk secure-trust-bank shawbrook-bank skipton-building-society starling-bank sumup tigris-data tsb-bank val-town virgin-money-uk weatherbys-bank xero zoho

Explore

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