Merge pull request #6421 from howieleung/main

Better doc to avoid 4xx in particular 409

Author: denrea

articles/ai-foundry/agents/concepts/capability-hosts.md

@@ -50,6 +50,14 @@ Capability hosts follow a hierarchy where more specific configurations override
 2. **Account-level capability host** - Provides shared defaults for all projects under the account.
 3. **Project-level capability host** - Overrides account-level and service defaults for that specific project. 
 
+## Understand capability host constraints
+
+When creating capability hosts, be aware of these important constraints to avoid conflicts:
+
+- **One capability host per scope**: Each account and each project can only have one active capability host. Attempting to create a second capability host with a different name at the same scope will result in a 409 conflict.
+
+- **Configuration updates are not supported**: If you need to change configuration, you must delete the existing capability host and recreate it.
+
 
 ## Recommended setup 
 
@@ -79,6 +87,7 @@ PUT https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{
   }
 }
 ```
+
 **Project capability host**
 
 This configuration overrides service defaults and any account-level settings. All agents in this project will use your specified resources:
@@ -134,6 +143,120 @@ DELETE https://management.azure.com/subscriptions/{subscriptionId}/resourceGroup
 DELETE https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.CognitiveServices/accounts/{accountName}/projects/{projectName}/capabilityHosts/{name}?api-version=2025-06-01
 ```
 
+## Troubleshooting
+
+If you're experiencing issues when creating capability hosts, this section provides solutions to the most common problems and errors.
+
+### HTTP 409 Conflict errors
+
+#### Problem: Multiple capability hosts per scope
+
+**Symptoms:** You receive a 409 Conflict error when trying to create a capability host, even though you believe the scope is empty.
+
+**Error message:**
+```json
+{
+  "error": {
+    "code": "Conflict",
+    "message": "There is an existing Capability Host with name: existing-host, provisioning state: Succeeded for workspace: /subscriptions/.../workspaces/my-workspace, cannot create a new Capability Host with name: new-host for the same ClientId."
+  }
+}
+```
+
+**Root cause:** Each account and each project can only have one active capability host. You're trying to create a capability host with a different name when one already exists at the same scope.
+
+**Solution:**
+1. **Check existing capability hosts** - Query the scope to see what already exists
+2. **Use consistent naming** - Ensure you're using the same name across all requests for the same scope
+3. **Review your requirements** - Determine if the existing capability host meets your needs
+
+**Validation steps:**
+```http
+# For account-level capability hosts
+GET https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.CognitiveServices/accounts/{accountName}/capabilityHosts?api-version=2025-06-01
+
+# For project-level capability hosts  
+GET https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.CognitiveServices/accounts/{accountName}/projects/{projectName}/capabilityHosts?api-version=2025-06-01
+```
+
+#### Problem: Concurrent operations in progress
+
+**Symptoms:** You receive a 409 Conflict error indicating that another operation is currently running.
+
+**Error message:**
+```json
+{
+  "error": {
+    "code": "Conflict", 
+    "message": "Create: Capability Host my-host is currently in non creating, retry after its complete: /subscriptions/.../workspaces/my-workspace"
+  }
+}
+```
+
+**Root cause:** You're trying to create a capability host while another operation (update, delete, modify) is in progress at the same scope.
+
+**Solution:**
+1. **Wait for current operation to complete** - Check the status of ongoing operations
+2. **Monitor operation progress** - Use the operations API to track completion
+3. **Implement retry logic** - Add exponential backoff for temporary conflicts
+
+**Operation monitoring:**
+```http
+GET https://management.azure.com/subscriptions/{subscriptionId}/providers/Microsoft.CognitiveServices/locations/{location}/operationResults/{operationId}?api-version=2025-06-01
+```
+
+### Best practices for conflict prevention
+
+#### 1. Pre-request validation
+Always verify the current state before making changes:
+- Query existing capability hosts in the target scope
+- Check for any ongoing operations
+- Understand the current configuration
+
+#### 2. Implement retry logic with exponential backoff
+```csharp
+try 
+{
+    var response = await CreateCapabilityHostAsync(request);
+    return response;
+}
+catch (HttpRequestException ex) when (ex.Message.Contains("409"))
+{
+    if (ex.Message.Contains("existing Capability Host with name"))
+    {
+        // Handle name conflict - check if existing resource is acceptable
+        var existing = await GetExistingCapabilityHostAsync();
+        if (IsAcceptable(existing))
+        {
+            return existing; // Use existing resource
+        }
+        else
+        {
+            throw new InvalidOperationException("Scope already has a capability host with different name");
+        }
+    }
+    else if (ex.Message.Contains("currently in non creating"))
+    {
+        // Handle concurrent operation - implement retry with backoff
+        await Task.Delay(TimeSpan.FromSeconds(30));
+        return await CreateCapabilityHostAsync(request); // Retry once
+    }
+}
+```
+
+#### 3. Understand idempotent behavior
+The system supports idempotent create requests:
+- **Same name + same configuration** → Returns existing resource (200 OK)
+- **Same name + different configuration** → Returns 400 Bad Request  
+- **Different name** → Returns 409 Conflict
+
+#### 4. Configuration change workflow
+Since updates aren't supported, follow this sequence for configuration changes:
+1. Delete the existing capability host
+2. Wait for deletion to complete  
+3. Create a new capability host with the desired configuration
+
+
 ## Next steps
 - Learn more about the [Standard Agent Setup](standard-agent-setup.md) 
 - Get started with [Agent Service](../environment-setup.md)