Does Azure Front Door insert a separator when appending a response header?

Definition: No. In Azure Front Door Standard and Premium, the response-header Append action joins the configured value directly to an existing header value by string concatenation. Microsoft documents that the action adds no delimiter.

Key facts

• Azure Front Door uses the modify response header action for headers in responses before they are returned to clients, according to Microsoft's rule set actions reference.
• With Append, an existing response-header value receives the configured value through string concatenation, and Microsoft says no delimiters are added.
• The response-header action has a header name and a header value. The value is the text to append or overwrite.
• Microsoft's REST API model represents the response action as ModifyResponseHeader with headerAction, headerName, and value parameters.
• The Azure PowerShell documentation shows Append with a header name and value in New-AzFrontDoorHeaderActionObject.

What does Append do to an existing response header?

Microsoft's rule set actions reference defines the response-header action as a change made before the response reaches the client. Its Append operator adds the specified header when it is absent. When the header already exists, Front Door concatenates the configured value with the existing value and inserts no comma, space, or other delimiter.

The same reference distinguishes Overwrite, which replaces the existing value, and Delete, which removes the response header. These are separate operators in the same action, so the selected operator determines whether the existing value is retained, replaced, or removed.

Where is the value configured?

The response-header action's Header name field identifies the header to modify. Its Header value field contains the value to append or overwrite. Because the documented operation adds no delimiter, a separator required by the header's format must be included in the configured value itself.

The Azure REST API describes the same action as ModifyResponseHeader. Its response action object points to HeaderActionParameters; those parameters expose headerAction, headerName, and value. The HeaderAction enumeration contains Append, Overwrite, and Delete.

For PowerShell, Microsoft's example uses the Append action with X-Content-Type-Options and the value nosniff:

$headerActions = New-AzFrontDoorHeaderActionObject -HeaderActionType "Append" -HeaderName "X-Content-Type-Options" -Value "nosniff"

Which response headers can the rule change?

The rule actions reference says that some Azure Front Door reserved headers cannot be modified by rules engine actions, including the request-header and response-header actions. It also excludes headers prefixed with x-ec and x-fd, alongside its listed reserved names. The separator behavior applies to an eligible response header that the rule can modify.

Related terms

• Which response headers cannot be modified by Azure Front Door rules?
• How can Azure Front Door ignore selected tracking parameters in its cache key?
• How can Azure Front Door preserve the rest of a path during a rewrite?

FAQ

Does Append replace the existing response-header value?

No. Microsoft defines Append as adding the configured value to the existing value. Overwrite replaces the existing value, while Delete removes the header.

How do I add a comma or space between values?

Put the required separator in the configured header value. Front Door's documented Append operation supplies the value through string concatenation and adds no delimiter automatically.

Sources

1. Microsoft Learn, "Rule set actions - Azure Front Door", 4 June 2025
2. Microsoft Learn, "Rules - Update - REST API (Azure Front Door Service)", 28 September 2025
3. Microsoft Learn, "New-AzFrontDoorRulesEngineActionObject (Az.FrontDoor)", 6 October 2026

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