Capability declaration

x-ms-has-immutability-policy

`x-ms-has-immutability-policy` appears on header objects. Its value is a structured object. Used by a single provider across 1 OpenAPI document. It sits in the `x-ms-` namespace, so its meaning is defined by ms tooling rather than by OpenAPI. Consistent with capability declaration — this is inferred from where the key appears and what it carries, not from any published definition.

Capability declaration x-ms- namespace derived description HTTP header, not an extension

How this description was produced. It is assembled from what was measured in the corpus — where this key appears in a document, what shape its value takes, the values observed, and how many providers use it. It is not taken from a published definition, because for most extensions none exists. Read it as evidence, not as a specification. If you own this extension and want it described properly, tell us.

This is an HTTP header name, not a vendor extension. It appears under a headers: map, which is the same syntactic position an extension key occupies — but OpenAPI models headers natively and this is not extending anything. It is listed here because it turns up when you inventory the x- namespace, and leaving it out would hide why the namespace looks bigger than it is.

1 occurrences

1 documents

1 providers

vendor-named

Where it appears

Location in the document	Occurrences
header	1

What its value looks like

Value shape	Occurrences
object	1

Providers publishing it

All 1.

microsoft-azure

Why this is not in OpenAPI

Extensions exist because a provider needed something the specification would not carry. Most of what they hold is not the API contract at all — it is operational metadata about the contract: documentation, lifecycle, policy, provenance, and now agents. That metadata is usually better placed alongside the contract, in an Overlay or an APIs.json index, than crammed inside it. See everything else doing the capability declaration job.