Elasticsearch Version

8.x, 9.x

Installed Plugins

No response

Java Version

bundled

OS Version

Any

Problem Description

For Elasticsearch APIs that take an index expression, some are cross-cluster enabled (e.g., _search) and some are not (_mapping).

For the endpoints that are not cross-cluster enabled, when they receive a qualified index expression (e.g., clusterA:myindex), an error is returned:

GET blogs,remote1:blogs/_mappings

// response:
    "type": "illegal_argument_exception",
    "reason": "Cross-cluster calls are not supported in this context but remote indices were requested: [remote1:blogs]"

However, when ignore_unavailable is set to true, this error message is no longer displayed and instead the API will process any local indices that were present:

GET blogs,remote1:blogs/_mappings?ignore_unavailable=true

// response gives the blogs mappings of the local cluster only
{
  "blogs": {
    "mappings": {
      "properties": {
        "authors": {
          "properties": {
            "company": {
              "type": "text",

This is incorrect behavior. The ignore_unavailable setting should not be controlling whether this error message is returned. ignore_unavailable should control how to behave with respect to whether concrete indices are present or not on a cluster, but for endpoints that are not cross-cluster enabled at all, requesting a remote cluster is always a violation of the contract of that endpoint and thus should always result in an error.

Steps to Reproduce

See Problem Description section above.

Logs (if relevant)

No response