diff --git a/apify-api/openapi/components/parameters/runAndBuildParameters.yaml b/apify-api/openapi/components/parameters/runAndBuildParameters.yaml
index 32bf01c9a3..6aadf96ffb 100644
--- a/apify-api/openapi/components/parameters/runAndBuildParameters.yaml
+++ b/apify-api/openapi/components/parameters/runAndBuildParameters.yaml
@@ -179,13 +179,19 @@ webhooks:
   in: query
   description: |
     Specifies optional webhooks associated with the Actor run, which can be used to receive a notification
-    e.g. when the Actor finished or failed. The value is a Base64-encoded
-    JSON array of objects defining the webhooks. For more information, see
+    e.g. when the Actor finished or failed. The value is a Base64-encoded JSON array whose items follow
+    the WebhookRepresentation schema. For more information, see
     [Webhooks documentation](https://docs.apify.com/platform/integrations/webhooks).
   style: form
   explode: true
   schema:
     type: string
+    contentEncoding: base64
+    contentMediaType: application/json
+    contentSchema:
+      type: array
+      items:
+        $ref: ../schemas/webhooks/WebhookRepresentation.yaml
     example: dGhpcyBpcyBqdXN0IGV4YW1wbGUK...
 
 waitForFinishRun:
diff --git a/apify-api/openapi/components/schemas/webhooks/WebhookRepresentation.yaml b/apify-api/openapi/components/schemas/webhooks/WebhookRepresentation.yaml
new file mode 100644
index 0000000000..d0a7c90c07
--- /dev/null
+++ b/apify-api/openapi/components/schemas/webhooks/WebhookRepresentation.yaml
@@ -0,0 +1,28 @@
+title: WebhookRepresentation
+description: |
+  Minimal representation of an ad-hoc webhook attached to a single Actor run or build via the
+  `webhooks` query parameter. The query parameter value is a Base64-encoded JSON array whose
+  items match this schema. Persistent webhook fields (e.g. `condition`) are not used here.
+required:
+  - eventTypes
+  - requestUrl
+type: object
+properties:
+  eventTypes:
+    type: array
+    items:
+      $ref: ./WebhookEventType.yaml
+    examples:
+      - [ACTOR.RUN.SUCCEEDED]
+  requestUrl:
+    type: string
+    description: The URL to which the webhook sends its payload.
+    examples: ["http://example.com/"]
+  payloadTemplate:
+    type: [string, "null"]
+    description: Optional template for the JSON payload sent by the webhook.
+    examples: ['{\n "userId": {{userId}}...']
+  headersTemplate:
+    type: [string, "null"]
+    description: Optional template for the HTTP headers sent by the webhook.
+    examples: ['{\n "Authorization": "Bearer ..."}']