HTTP Message Signatures (RFC 9421)

A cryptographic signature scheme for HTTP messages, used by the emerging web-bot-auth profile to let agents authenticate themselves to origins. Provider support for verifying or surfacing RFC 9421 signatures is a forward-looking agent-readiness signal.

Documentation

Work with this as data

Every API here is available over the APIs.io API and to AI agents over MCP.

MCP server

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

https://apis.io/mcp

Tools for apis

7 MCP tools reach this
  • find_apisBrowse and filter every API in the catalog.
  • get_api_artifactsOne API's artifacts, grouped by type.
  • get_openapiThe primary OpenAPI for this API.
  • find_similar_apisAPIs that look like this one.
  • 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
This API
curl "https://apis.io/api/v1/apis/rfc-9421-http-message-signatures"
All apis
curl "https://apis.io/api/v1/apis?limit=25"

Discovery needs no key. Ratings and market analysis are Pro.

Get an API key

Free tier, no email required.

A second provider on the same verified email joins the account you already have.

API entry from apis.yml

apis.yml Raw ↑
aid: agent-readiness:rfc-9421-http-message-signatures
name: HTTP Message Signatures (RFC 9421)
tags:
- Identity
- Bot Auth
- Agent Readiness
- Standards
humanURL: https://www.rfc-editor.org/rfc/rfc9421.html
baseURL: https://www.rfc-editor.org/rfc/rfc9421.html
properties:
- url: https://www.rfc-editor.org/rfc/rfc9421.html
  type: Documentation
description: A cryptographic signature scheme for HTTP messages, used by the emerging web-bot-auth profile
  to let agents authenticate themselves to origins. Provider support for verifying or surfacing RFC 9421
  signatures is a forward-looking agent-readiness signal.