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If-Modified-Since

IANA permanent request RFC 9110 3 spellings

Declared by 60 providers across 1136 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 (60)

acronis agicap aib-group-uk aldermore amazon-s3 autodesk-fusion avito bank-of-ireland-uk bank-of-scotland barclays btc-war-live-market-data-api ccp-games cdisc clio cloudflare-r2 co-operative-bank commure coventry-building-society cubefs eclipse engineering-platform erply first-direct gb-bank halifax leeds-building-society lloyds-banking-group mediamath metro-bank mettle microsoft-azure microsoft-azure-batch monument-bank nationwide-building-society nedap nhost open-banking-uk optum oracle oracle-database paragon-bank phrase principality-building-society 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 zoho-crm

Explore

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

Work with this as data

Every header here is available over the APIs.io API and to AI agents over MCP. HTTP Headers is not yet its own endpoint on the v1 API. Reach it through catalog search and the tag graph, or the MCP server.

MCP server

One button, every client — Claude, Cursor, VS Code and the rest.

https://apis.io/mcp

Tools for http headers

3 MCP tools reach this

apis_io_searchSTART HERE — APIs, providers and tags for one query, each with its total.
resolveTurn a domain, URL or GitHub org into the provider it belongs to.
find_cohortsEvery scored population of providers in the catalog.

All 92 tools →

Call it yourself

curl for this page

Search the catalog

curl "https://apis.io/api/v1/search?q=if-modified-since&limit=10"

Everything under a tag

curl "https://apis.io/api/v1/tags/if-modified-since"

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