[Enhancement] Reject unsupported output formats on the analytics-engine route with a 4xx

plugin/src/main/java/org/opensearch/sql/plugin/SQLPlugin.java

@@ -107,6 +107,7 @@
 import org.opensearch.sql.opensearch.storage.script.CompoundedScriptEngine;
 import org.opensearch.sql.plugin.config.EngineExtensionsHolder;
 import org.opensearch.sql.plugin.config.OpenSearchPluginModule;
+import org.opensearch.sql.plugin.rest.AnalyticsEngineFormatSupport;
 import org.opensearch.sql.plugin.rest.AnalyticsExecutorHolder;
 import org.opensearch.sql.plugin.rest.RestPPLGrammarAction;
 import org.opensearch.sql.plugin.rest.RestPPLQueryAction;
@@ -271,6 +272,13 @@ public void onFailure(Exception e) {
               }
             });
       } else {
+        // Analytics route only emits JSON; reject unsupported formats (e.g. csv) with a 4xx.
+        try {
+          AnalyticsEngineFormatSupport.validateFormat(sqlRequest.format());
+        } catch (Exception e) {
+          RestSqlAction.handleException(channel, e);
+          return true;
+        }
         unifiedQueryHandler.execute(
             sqlRequest.getQuery(),
             QueryType.SQL,

plugin/src/main/java/org/opensearch/sql/plugin/rest/AnalyticsEngineFormatSupport.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.plugin.rest;
+
+import java.util.Locale;
+import org.opensearch.sql.common.error.ErrorCode;
+import org.opensearch.sql.common.error.ErrorReport;
+import org.opensearch.sql.common.error.QueryProcessingStage;
+import org.opensearch.sql.protocol.response.format.Format;
+
+/**
+ * Guards the analytics-engine query route, which only produces JSON. Rejects the alternate output
+ * formats (csv/raw/viz) the route does not implement, instead of silently answering with JSON.
+ */
+public final class AnalyticsEngineFormatSupport {
+
+  private AnalyticsEngineFormatSupport() {}
+
+  /**
+   * Throw an {@link ErrorReport} if the requested output format is unsupported on the analytics
+   * engine. JSON/JDBC (the default) is supported; csv/raw/viz are not.
+   */
+  public static void validateFormat(Format format) {
+    // JDBC (the default) is the JSON contract the analytics route emits.
+    if (format == Format.JDBC) {
+      return;
+    }
+    throw ErrorReport.wrap(
+            new IllegalArgumentException(
+                String.format(
+                    Locale.ROOT,
+                    "response in %s format is not supported on the analytics engine",
+                    format.getFormatName())))
+        .code(ErrorCode.UNSUPPORTED_OPERATION)
+        .stage(QueryProcessingStage.ANALYZING)
+        .location("while selecting the response format for the analytics engine")
+        .suggestion(
+            "The analytics engine only returns JSON. Remove the 'format' parameter, or run this"
+                + " query against a non-analytics index to use the requested format.")
+        .build();
+  }
+}

plugin/src/main/java/org/opensearch/sql/plugin/transport/TransportPPLQueryAction.java

@@ -40,6 +40,7 @@
 import org.opensearch.sql.opensearch.setting.OpenSearchSettings;
 import org.opensearch.sql.plugin.config.EngineExtensionsHolder;
 import org.opensearch.sql.plugin.config.OpenSearchPluginModule;
+import org.opensearch.sql.plugin.rest.AnalyticsEngineFormatSupport;
 import org.opensearch.sql.plugin.rest.AnalyticsExecutorHolder;
 import org.opensearch.sql.plugin.rest.RestUnifiedQueryAction;
 import org.opensearch.sql.ppl.PPLService;
@@ -185,6 +186,13 @@ protected void doExecute(
             task,
             createExplainResponseListener(transformedRequest, clearingListener));
       } else {
+        // Analytics route only emits JSON; reject unsupported formats (e.g. csv) with a 4xx.
+        try {
+          AnalyticsEngineFormatSupport.validateFormat(format(transformedRequest));
+        } catch (Exception e) {
+          clearingListener.onFailure(e);
+          return;
+        }
         unifiedQueryHandler.execute(
             transformedRequest.getRequest(),
             QueryType.PPL,

plugin/src/test/java/org/opensearch/sql/plugin/rest/AnalyticsEngineFormatSupportTest.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.plugin.rest;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+import org.opensearch.sql.common.error.ErrorCode;
+import org.opensearch.sql.common.error.ErrorReport;
+import org.opensearch.sql.protocol.response.format.Format;
+
+/**
+ * Verifies {@link AnalyticsEngineFormatSupport#validateFormat(Format)} accepts the JSON/JDBC
+ * default the analytics route emits and rejects unsupported output formats (csv/raw/viz) with a
+ * structured {@link ErrorReport} so the REST layer returns a clean 4xx instead of silently
+ * returning JSON.
+ */
+public class AnalyticsEngineFormatSupportTest {
+
+  @Test
+  public void jdbcFormatIsAccepted() {
+    // No exception expected — JDBC is the JSON contract the analytics route emits.
+    AnalyticsEngineFormatSupport.validateFormat(Format.JDBC);
+  }
+
+  @Test
+  public void csvFormatIsRejectedAsUnsupportedOperation() {
+    ErrorReport report =
+        assertThrows(() -> AnalyticsEngineFormatSupport.validateFormat(Format.CSV));
+    assertEquals(ErrorCode.UNSUPPORTED_OPERATION, report.getCode());
+    assertTrue(report.getMessage().toLowerCase().contains("csv"));
+    assertTrue(report.getMessage().contains("analytics engine"));
+    assertNotNull(report.getSuggestion());
+  }
+
+  @Test
+  public void rawFormatIsRejected() {
+    ErrorReport report =
+        assertThrows(() -> AnalyticsEngineFormatSupport.validateFormat(Format.RAW));
+    assertEquals(ErrorCode.UNSUPPORTED_OPERATION, report.getCode());
+    assertTrue(report.getMessage().toLowerCase().contains("raw"));
+  }
+
+  @Test
+  public void vizFormatIsRejected() {
+    ErrorReport report =
+        assertThrows(() -> AnalyticsEngineFormatSupport.validateFormat(Format.VIZ));
+    assertEquals(ErrorCode.UNSUPPORTED_OPERATION, report.getCode());
+  }
+
+  private static ErrorReport assertThrows(Runnable runnable) {
+    try {
+      runnable.run();
+    } catch (ErrorReport e) {
+      return e;
+    }
+    throw new AssertionError("Expected ErrorReport to be thrown, but nothing was thrown");
+  }
+}