Do not allow project_routing as a query param

server/src/main/java/org/elasticsearch/action/search/RestOpenPointInTimeAction.java

@@ -55,7 +55,6 @@ public RestChannelConsumer prepareRequest(final RestRequest request, final NodeC
         final OpenPointInTimeRequest openRequest = new OpenPointInTimeRequest(indices);
         final boolean crossProjectEnabled = crossProjectModeDecider.crossProjectEnabled();
         if (crossProjectEnabled) {
-            openRequest.projectRouting(request.param("project_routing", null));
             openRequest.indicesOptions(IndicesOptions.fromRequest(request, OpenPointInTimeRequest.DEFAULT_CPS_INDICES_OPTIONS));
         } else {
             openRequest.indicesOptions(IndicesOptions.fromRequest(request, OpenPointInTimeRequest.DEFAULT_INDICES_OPTIONS));

server/src/main/java/org/elasticsearch/rest/action/RestFieldCapabilitiesAction.java

@@ -62,7 +62,6 @@ public RestChannelConsumer prepareRequest(final RestRequest request, final NodeC
 
         final boolean crossProjectEnabled = crossProjectModeDecider.crossProjectEnabled();
         if (crossProjectEnabled) {
-            fieldRequest.projectRouting(request.param("project_routing", null));
             // Setting includeResolvedTo to always include index resolution data structure in the linked project responses,
             // in order to allow the coordinating node to call CrossProjectIndexResolutionValidator#validate
             fieldRequest.includeResolvedTo(true);

server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestResolveIndexAction.java

@@ -9,9 +9,11 @@
 
 package org.elasticsearch.rest.action.admin.indices;
 
+import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.admin.indices.resolve.ResolveIndexAction;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.client.internal.node.NodeClient;
+import org.elasticsearch.common.ParsingException;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.IndexMode;
@@ -20,22 +22,26 @@
 import org.elasticsearch.rest.Scope;
 import org.elasticsearch.rest.ServerlessScope;
 import org.elasticsearch.rest.action.RestToXContentListener;
+import org.elasticsearch.search.crossproject.CrossProjectModeDecider;
+import org.elasticsearch.xcontent.XContentParser;
 
 import java.io.IOException;
 import java.util.Arrays;
 import java.util.EnumSet;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicReference;
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
+import static org.elasticsearch.rest.RestRequest.Method.POST;
 
 @ServerlessScope(Scope.PUBLIC)
 public class RestResolveIndexAction extends BaseRestHandler {
     private static final Set<String> CAPABILITIES = Set.of("mode_filter");
-    private final Settings settings;
+    private final CrossProjectModeDecider crossProjectModeDecider;
 
     public RestResolveIndexAction(Settings settings) {
-        this.settings = settings;
+        this.crossProjectModeDecider = new CrossProjectModeDecider(settings);
     }
 
     @Override
@@ -45,7 +51,7 @@ public String getName() {
 
     @Override
     public List<Route> routes() {
-        return List.of(new Route(GET, "/_resolve/index/{name}"));
+        return List.of(new Route(GET, "/_resolve/index/{name}"), new Route(POST, "/_resolve/index/{name}"));
     }
 
     @Override
@@ -57,17 +63,28 @@ public Set<String> supportedCapabilities() {
     protected BaseRestHandler.RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) throws IOException {
         String[] indices = Strings.splitStringByCommaToArray(request.param("name"));
         String modeParam = request.param("mode");
-        final boolean crossProjectEnabled = settings != null && settings.getAsBoolean("serverless.cross_project.enabled", false);
-        String projectRouting = null;
-        if (crossProjectEnabled) {
-            projectRouting = request.param("project_routing");
-        }
+
+        final boolean crossProjectEnabled = crossProjectModeDecider.crossProjectEnabled();
+        AtomicReference<String> projectRouting = new AtomicReference<>();
         IndicesOptions indicesOptions = IndicesOptions.fromRequest(request, ResolveIndexAction.Request.DEFAULT_INDICES_OPTIONS);
+
         if (crossProjectEnabled) {
+            request.withContentOrSourceParamParserOrNull(parser -> {
+                try {
+                    // If parser is null, there's no request body. projectRouting will then yield `null`.
+                    if (parser != null) {
+                        projectRouting.set(parseProjectRouting(parser));
+                    }
+                } catch (Exception e) {
+                    throw new ElasticsearchException("Couldn't parse request body", e);
+                }
+            });
+
             indicesOptions = IndicesOptions.builder(indicesOptions)
                 .crossProjectModeOptions(new IndicesOptions.CrossProjectModeOptions(true))
                 .build();
         }
+
         ResolveIndexAction.Request resolveRequest = new ResolveIndexAction.Request(
             indices,
             indicesOptions,
@@ -76,8 +93,44 @@ protected BaseRestHandler.RestChannelConsumer prepareRequest(RestRequest request
                 : Arrays.stream(modeParam.split(","))
                     .map(IndexMode::fromString)
                     .collect(() -> EnumSet.noneOf(IndexMode.class), EnumSet::add, EnumSet::addAll),
-            projectRouting
+            projectRouting.get()
         );
         return channel -> client.admin().indices().resolveIndex(resolveRequest, new RestToXContentListener<>(channel));
     }
+
+    private static String parseProjectRouting(XContentParser parser) throws ParsingException {
+        try {
+            XContentParser.Token first = parser.nextToken();
+            if (first == null) {
+                return null;
+            }
+
+            if (first != XContentParser.Token.START_OBJECT) {
+                throw new ParsingException(
+                    parser.getTokenLocation(),
+                    "Expected [" + XContentParser.Token.START_OBJECT + "] but found [" + first + "]",
+                    parser.getTokenLocation()
+                );
+            }
+
+            String projectRouting = null;
+            for (XContentParser.Token token = parser.nextToken(); token != XContentParser.Token.END_OBJECT; token = parser.nextToken()) {
+                if (token == XContentParser.Token.FIELD_NAME) {
+                    String currentName = parser.currentName();
+                    if ("project_routing".equals(currentName)) {
+                        parser.nextToken();
+                        projectRouting = parser.text();
+                    } else {
+                        throw new ParsingException(parser.getTokenLocation(), "request does not support [" + parser.currentName() + "]");
+                    }
+                }
+            }
+
+            return projectRouting;
+        } catch (ParsingException e) {
+            throw e;
+        } catch (Exception e) {
+            throw new ParsingException(parser == null ? null : parser.getTokenLocation(), "Failed to parse", e);
+        }
+    }
 }

server/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java

@@ -108,9 +108,6 @@ public RestChannelConsumer prepareRequest(final RestRequest request, final NodeC
         request.param("min_compatible_shard_node");
 
         final boolean crossProjectEnabled = crossProjectModeDecider.crossProjectEnabled();
-        if (crossProjectEnabled) {
-            searchRequest.setProjectRouting(request.param("project_routing"));
-        }
 
         /*
          * We have to pull out the call to `source().size(size)` because

x-pack/plugin/async-search/src/main/java/org/elasticsearch/xpack/search/RestSubmitAsyncSearchAction.java

@@ -72,7 +72,6 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
         boolean crossProjectEnabled = crossProjectModeDecider.crossProjectEnabled();
         if (crossProjectEnabled) {
             submit.getSearchRequest().setCcsMinimizeRoundtrips(true);
-            submit.getSearchRequest().setProjectRouting(request.param("project_routing"));
         }
 
         IntConsumer setSize = size -> submit.getSearchRequest().source().size(size);

x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/plugin/RestEqlSearchAction.java

@@ -69,7 +69,6 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
                 indicesOptions = IndicesOptions.builder(indicesOptions)
                     .crossProjectModeOptions(new IndicesOptions.CrossProjectModeOptions(true))
                     .build();
-                eqlRequest.projectRouting(request.param("project_routing"));
             }
             eqlRequest.indicesOptions(indicesOptions);
             if (request.hasParam("wait_for_completion_timeout")) {

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/plugin/RestSqlQueryAction.java

@@ -56,11 +56,6 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
             sqlRequest = SqlQueryRequest.fromXContent(parser);
         }
 
-        String routingParam = request.param("project_routing");
-        if (routingParam != null) {
-            // takes precedence on the parameter in the body
-            sqlRequest.projectRouting(routingParam);
-        }
         if (sqlRequest.projectRouting() != null && crossProjectModeDecider.crossProjectEnabled() == false) {
             throw new InvalidArgumentException("[project_routing] is only allowed when cross-project search is enabled");
         }

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/plugin/RestSqlTranslateAction.java

@@ -49,11 +49,6 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
         try (XContentParser parser = request.contentOrSourceParamParser()) {
             sqlRequest = SqlTranslateRequest.fromXContent(parser);
         }
-        String routingParam = request.param("project_routing");
-        if (routingParam != null) {
-            // takes precedence on the parameter in the body
-            sqlRequest.projectRouting(routingParam);
-        }
         if (sqlRequest.projectRouting() != null && crossProjectModeDecider.crossProjectEnabled() == false) {
             throw new InvalidArgumentException("[project_routing] is only allowed when cross-project search is enabled");
         }