This implements MCP Events (draft SEP-3415), an MCP extension for subscribing to events. An mcp destination is one MCP Events webhook subscription: a callback URL that receives one topic's events, wrapped in the MCP Events envelope and signed with Standard Webhooks. Your MCP server creates it on behalf of an agent and refreshes or deletes it as the agent subscribes, renews and unsubscribes.

Creating an MCP Destination

curl 'https://api.outpost.hookdeck.com/2026-09-01/tenants/<TENANT_ID>/destinations' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer <API_KEY>' \
--data '{
  "id": "sub_12345",
  "type": "mcp",
  "topics": ["order.created"],
  "config": {
    "url": "https://agent.example.com/mcp-events/abc123",
    "principal": "user-123"
  },
  "credentials": {
    "secret": "whsec_MfKQ9r8GKYqrTwjUPD8ILPZIo2LaLaSw"
  },
  "expires_at": "2030-01-01T00:00:00Z"
}'

Before the destination is stored, Outpost sends a verification challenge to the callback URL. If the callback fails it, including with a non-2xx reply, the request returns 422 with the reason as the validation type: challenge_failed, connection_refused, timeout, tls_error, https_required or address_not_allowed. If Outpost hits a verification limit, the request returns 429 with a Retry-After header and data.code of rate_limited. If verification can't run, it returns 503 with data.code of verification_unavailable. A callback that resolves to a non-public address returns config.url must resolve to a public address. A callback that was already verified for the same tenant, principal and URL is not challenged again for EXPERIMENTAL_MCP_VERIFICATION_TTL_SECONDS.

Verification is limited to 10 challenges per tenant and principal per minute, and 120 failed challenges per tenant and callback host per minute. Subscriptions without a principal share the tenant's budget.

Configuration

Field	Type	Required	Description
id	string	No	Subscription ID, sent as X-MCP-Subscription-Id. 1 to 128 characters of A-Z a-z 0-9 _ -.
topics	string[]	Yes	Exactly one topic. * is not accepted.
config.url	string	Yes	Callback URL. https, in normalized form (lowercase host, no default port, no credentials); http only with EXPERIMENTAL_MCP_ALLOW_INSECURE_CALLBACKS, for addresses in DESTINATIONS_PRIVATE_ADDRESS_ALLOWLIST, for development. Must resolve to a public address, unless allowed by DESTINATIONS_PRIVATE_ADDRESS_ALLOWLIST.
config.principal	string	No	The subject the subscription belongs to, up to 512 bytes. Verification is cached per tenant, principal and URL.
config.arguments	string	No	Canonical JSON object, up to 4 KiB. Stored with the destination.
credentials.secret	string	Yes	whsec_ followed by base64 of 24 to 64 bytes.
expires_at	string	No	RFC 3339 time in the future. From then on the subscription matches no events and receives no deliveries. null on update clears it.

config.url, config.principal, config.arguments and topics cannot change after creation. To rotate the secret, update credentials.secret: the old secret keeps signing for 24 hours alongside the new one.

Delivery

Each event is sent as an HTTP POST:

POST /mcp-events/abc123 HTTP/1.1
Content-Type: application/json
webhook-id: msg_x1yQ3o8mT5vZk2cJ0aNwXg
webhook-timestamp: 1790000000
webhook-signature: v1,K5oZfzN95Z9UVu1EsfQmfVNQhnkZ2/d+...
X-MCP-Subscription-Id: sub_12345

{"eventId":"evt_123","name":"order.created","timestamp":"2026-10-27T12:00:00Z","data":{"order_id":"ord_42"},"cursor":null}

• cursor is always null: there is no replay, and Outpost doesn't send gap or terminated envelopes.
• webhook-id is random, set when the event is matched to the subscription, and the same on every automatic retry. Receivers should dedupe retries on webhook-id. A dedupe on eventId must be scoped to the subscription (X-MCP-Subscription-Id): one event is delivered once per matching subscription.
• webhook-timestamp and webhook-signature are computed again on every attempt. During a secret rotation, the signature header carries one signature per valid secret.
• Redirects are never followed.
• Deliveries and verification challenges connect directly, checking each address at connection time, or through the DESTINATIONS_PROXY_URL forward proxy when it is set. With a proxy, connections go to the proxy and aren't checked: the proxy resolves the callback host and must refuse private addresses itself. The check when a subscription is created or refreshed still runs. HTTP_PROXY/HTTPS_PROXY don't apply.

Retries

A 2xx response is a success. Any other response, a network error or a refused address is a failed attempt and is retried 1, 3, 5 and 5 minutes after the previous attempt: up to 5 attempts within 14 minutes of the first (RETRY_SCHEDULE and MAX_RETRY_LIMIT don't apply to mcp). A 3xx is a failure with its status as the attempt code.

These are not retried, and don't count toward auto-disabling the subscription: a 410 or 413 response, and an event whose envelope exceeds 256 KiB (attempt code payload_too_large, no request sent).