{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$id": "https://adcontextprotocol.org/schemas/3.2.0-beta.0/core/agent-webhook-challenge.json",
  "title": "Agent Webhook Challenge",
  "description": "Proof-of-control challenge payload sent by a seller to an agent-level sync_agent_notification_configs.notification_configs[] URL before activating a new or changed active subscriber. Agent-level challenges are valid before any seller account exists, so they bind the seller agent URL, subscriber ID, requested agent-level event types, delivery auth mode, and challenge nonce instead of an account_id. The seller sends this payload as an HTTPS POST after URL normalization and SSRF validation, and before treating the subscriber as active. The challenge POST itself MUST be signed with the seller's RFC 9421 webhook profile key even when the candidate config selects legacy delivery auth; new signers use `adcp_use: \"request-signing\"` and deprecated `webhook-signing` keys remain accepted during the compatibility window. `delivery_auth` describes the future webhook delivery mode, not the challenge's own signing mode.",
  "type": "object",
  "properties": {
    "type": {
      "type": "string",
      "const": "webhook.challenge",
      "description": "Discriminator for endpoint proof-of-control challenges."
    },
    "scope": {
      "type": "string",
      "const": "agent",
      "description": "Discriminator for agent-level endpoint proof challenges."
    },
    "challenge": {
      "type": "string",
      "description": "Opaque, cryptographically random value that the receiver must echo in the response body. Recommended encoding: base64url without padding.",
      "minLength": 32,
      "maxLength": 255,
      "pattern": "^[A-Za-z0-9_.:-]{32,255}$"
    },
    "subscriber_id": {
      "type": "string",
      "description": "Buyer-supplied subscriber identifier from the sync_agent_notification_configs.notification_configs[] entry being challenged.",
      "minLength": 1,
      "maxLength": 64,
      "pattern": "^[A-Za-z0-9_.:-]{1,64}$"
    },
    "seller_agent_url": {
      "type": "string",
      "format": "uri",
      "description": "Exact seller agent URL whose RFC 9421 webhook profile key signs this challenge and that will send subsequent webhooks."
    },
    "delivery_auth": {
      "type": "object",
      "description": "Authentication/signing mode the seller will use for subsequent webhooks delivered to this notification config.",
      "properties": {
        "mode": {
          "type": "string",
          "enum": [
            "rfc9421",
            "Bearer",
            "HMAC-SHA256"
          ],
          "description": "Future webhook delivery authentication mode. `rfc9421` is used when notification_configs[].authentication is absent; Bearer and HMAC-SHA256 are the deprecated legacy modes selected by notification_configs[].authentication."
        },
        "credential_fingerprint": {
          "type": "string",
          "description": "SHA-256 hex fingerprint of the exact legacy credential string supplied in notification_configs[].authentication.credentials. Required when mode is Bearer or HMAC-SHA256; absent for rfc9421.",
          "pattern": "^[a-f0-9]{64}$"
        }
      },
      "required": [
        "mode"
      ],
      "allOf": [
        {
          "if": {
            "properties": {
              "mode": {
                "type": "string",
                "const": "rfc9421"
              }
            },
            "required": [
              "mode"
            ]
          },
          "then": {
            "not": {
              "required": [
                "credential_fingerprint"
              ]
            }
          }
        },
        {
          "if": {
            "properties": {
              "mode": {
                "type": "string",
                "enum": [
                  "Bearer",
                  "HMAC-SHA256"
                ]
              }
            },
            "required": [
              "mode"
            ]
          },
          "then": {
            "required": [
              "credential_fingerprint"
            ]
          }
        }
      ],
      "additionalProperties": false
    },
    "event_types": {
      "type": "array",
      "description": "Normalized agent-level notification types requested by the subscriber at the time of the challenge. Part of the endpoint proof scope; changing event_types[] requires a fresh challenge before the new set can become active. Currently only `capabilities.changed` is valid.",
      "items": {
        "type": "string",
        "enum": [
          "capabilities.changed"
        ]
      },
      "minItems": 1,
      "uniqueItems": true
    }
  },
  "required": [
    "type",
    "scope",
    "challenge",
    "subscriber_id",
    "seller_agent_url",
    "delivery_auth",
    "event_types"
  ],
  "additionalProperties": false,
  "examples": [
    {
      "description": "Agent-level endpoint proof-of-control challenge",
      "data": {
        "type": "webhook.challenge",
        "scope": "agent",
        "challenge": "example-challenge-token-000000000000",
        "subscriber_id": "registry-cache",
        "seller_agent_url": "https://seller.example/adcp",
        "delivery_auth": {
          "mode": "rfc9421"
        },
        "event_types": [
          "capabilities.changed"
        ]
      }
    }
  ]
}