docs: add creative agent setup verification

docs/creative/implementing-creative-agents.mdx

@@ -428,13 +428,64 @@ When updating format definitions:
 Before launching your creative agent:
 
 - [ ] MCP and/or A2A endpoints are accessible
+- [ ] `get_adcp_capabilities` returns `creative.supported_formats` with at least one entry
 - [ ] All format_ids use proper namespacing (`domain:id`)
 - [ ] Domain in format_id matches your `agent_url` domain
 - [ ] `list_creative_formats` returns all your formats
 - [ ] `preview_creative` validates manifests and generates previews
 - [ ] Format definitions include complete asset requirements
 - [ ] Documentation available for your custom formats
 
+To verify a remote creative agent is reachable and advertising its format surface, call `get_adcp_capabilities` on the agent endpoint and confirm that the response includes a non-empty `creative.supported_formats` array. Each entry follows the [canonical format declaration](/docs/creative/canonical-formats) shape.
+
+<Accordion title="Verify an MCP creative agent">
+
+```bash
+export AGENT_URL="https://your-creative-agent.example.com/mcp"
+
+curl -X POST "$AGENT_URL" \
+  -H "Content-Type: application/json" \
+  -H "Accept: application/json, text/event-stream" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "tools/call",
+    "params": {
+      "name": "get_adcp_capabilities",
+      "arguments": {
+        "adcp_version": "3.1",
+        "adcp_major_version": 3,
+        "protocols": ["creative"]
+      }
+    }
+  }'
+```
+
+Read the AdCP payload from the MCP result and verify that it contains a creative capability block like this:
+
+```json
+{
+  "creative": {
+    "supported_formats": [
+      {
+        "capability_id": "display_banner",
+        "format": {
+          "format_kind": "image",
+          "params": {
+            "width": 300,
+            "height": 250
+          }
+        }
+      }
+    ]
+  }
+}
+```
+
+An unreachable endpoint, an authorization error, a missing `creative` block, or an empty `supported_formats` array means the agent is not ready for creative-format discovery. If the endpoint requires authentication, include the credential configured for that agent.
+
+</Accordion>
+
 ## Integration patterns
 
 ### Pattern 1: creative agency