diff --git a/CHANGELOG.md b/CHANGELOG.md
index 57bfb1d2a0a78..da3dc8a9e5648 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add overload constructor for Translog to accept Channel Factory as a parameter ([#18918](https://github.com/opensearch-project/OpenSearch/pull/18918))
 - Add subdirectory-aware store module with recovery support ([#19132](https://github.com/opensearch-project/OpenSearch/pull/19132))
 - Add a dynamic cluster setting to control the enablement of the merged segment warmer ([#18929](https://github.com/opensearch-project/OpenSearch/pull/18929))
+- Publish transport-grpc-spi exposing QueryBuilderProtoConverter and QueryBuilderProtoConverterRegistry ([#18949](https://github.com/opensearch-project/OpenSearch/pull/18949))
 - Support system generated search pipeline. ([#19128](https://github.com/opensearch-project/OpenSearch/pull/19128))
 
 ### Changed
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index ad4c4bc5be459..d2e182270befb 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -21,6 +21,7 @@ woodstox          = "6.4.0"
 kotlin            = "1.7.10"
 antlr4            = "4.13.1"
 guava             = "33.2.1-jre"
+opensearchprotobufs = "0.6.0"
 protobuf          = "3.25.8"
 jakarta_annotation = "1.3.5"
 google_http_client = "1.44.1"
diff --git a/modules/transport-grpc/build.gradle b/modules/transport-grpc/build.gradle
index 1e51154877d3b..806779a93fb97 100644
--- a/modules/transport-grpc/build.gradle
+++ b/modules/transport-grpc/build.gradle
@@ -21,6 +21,7 @@ testClusters {
 }
 
 dependencies {
+  api project('spi')
   compileOnly "com.google.code.findbugs:jsr305:3.0.2"
   runtimeOnly "com.google.guava:guava:${versions.guava}"
   implementation "com.google.errorprone:error_prone_annotations:2.24.1"
diff --git a/modules/transport-grpc/spi/README.md b/modules/transport-grpc/spi/README.md
new file mode 100644
index 0000000000000..9a3609abbe528
--- /dev/null
+++ b/modules/transport-grpc/spi/README.md
@@ -0,0 +1,277 @@
+# transport-grpc-spi
+
+Service Provider Interface (SPI) for the OpenSearch gRPC transport module. This module provides interfaces and utilities that allow external plugins to extend the gRPC transport functionality.
+
+## Overview
+
+The `transport-grpc-spi` module enables plugin developers to:
+- Implement custom query converters for gRPC transport
+- Extend gRPC protocol buffer handling
+- Register custom query types that can be processed via gRPC
+
+## Key Components
+
+### QueryBuilderProtoConverter
+
+Interface for converting protobuf query messages to OpenSearch QueryBuilder objects.
+
+```java
+public interface QueryBuilderProtoConverter {
+    QueryContainer.QueryContainerCase getHandledQueryCase();
+    QueryBuilder fromProto(QueryContainer queryContainer);
+}
+```
+
+### QueryBuilderProtoConverterRegistry
+
+Interface for accessing the query converter registry. This provides a clean abstraction for plugins that need to convert nested queries without exposing internal implementation details.
+
+## Usage for Plugin Developers
+
+### 1. Add Dependency
+
+Add the SPI dependency to your plugin's `build.gradle`:
+
+```gradle
+dependencies {
+    compileOnly 'org.opensearch.plugin:transport-grpc-spi:${opensearch.version}'
+    compileOnly 'org.opensearch:protobufs:${protobufs.version}'
+}
+```
+
+### 2. Implement Custom Query Converter
+
+```java
+public class MyCustomQueryConverter implements QueryBuilderProtoConverter {
+
+    @Override
+    public QueryContainer.QueryContainerCase getHandledQueryCase() {
+        return QueryContainer.QueryContainerCase.MY_CUSTOM_QUERY;
+    }
+
+    @Override
+    public QueryBuilder fromProto(QueryContainer queryContainer) {
+        // Convert your custom protobuf query to QueryBuilder
+        MyCustomQuery customQuery = queryContainer.getMyCustomQuery();
+        return new MyCustomQueryBuilder(customQuery.getField(), customQuery.getValue());
+    }
+}
+```
+
+### 3. Register Your Converter
+
+In your plugin's main class, return the converter from createComponents:
+
+```java
+public class MyPlugin extends Plugin {
+
+    @Override
+    public Collection<Object> createComponents(Client client, ClusterService clusterService,
+                                             ThreadPool threadPool, ResourceWatcherService resourceWatcherService,
+                                             ScriptService scriptService, NamedXContentRegistry xContentRegistry,
+                                             Environment environment, NodeEnvironment nodeEnvironment,
+                                             NamedWriteableRegistry namedWriteableRegistry,
+                                             IndexNameExpressionResolver indexNameExpressionResolver,
+                                             Supplier<RepositoriesService> repositoriesServiceSupplier) {
+
+        // Return your converter instance - the transport-grpc plugin will discover and register it
+        return Collections.singletonList(new MyCustomQueryConverter());
+    }
+}
+```
+
+**Step 3b: Create SPI Registration File**
+
+Create a file at `src/main/resources/META-INF/services/org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter`:
+
+```
+org.opensearch.mypackage.MyCustomQueryConverter
+```
+
+**Step 3c: Declare Extension in Plugin Descriptor**
+
+In your `plugin-descriptor.properties`, declare that your plugin extends transport-grpc:
+
+```properties
+extended.plugins=transport-grpc
+```
+
+### 4. Accessing the Registry (For Complex Queries)
+
+If your converter needs to handle nested queries (like k-NN's filter clause), you'll need access to the registry to convert other query types. The transport-grpc plugin will inject the registry into your converter.
+
+```java
+public class MyCustomQueryConverter implements QueryBuilderProtoConverter {
+
+    private QueryBuilderProtoConverterRegistry registry;
+
+    @Override
+    public void setRegistry(QueryBuilderProtoConverterRegistry registry) {
+        this.registry = registry;
+    }
+
+    @Override
+    public QueryBuilder fromProto(QueryContainer queryContainer) {
+        MyCustomQuery customQuery = queryContainer.getMyCustomQuery();
+
+        MyCustomQueryBuilder builder = new MyCustomQueryBuilder(
+            customQuery.getField(),
+            customQuery.getValue()
+        );
+
+        // Handle nested queries using the injected registry
+        if (customQuery.hasFilter()) {
+            QueryContainer filterContainer = customQuery.getFilter();
+            QueryBuilder filterQuery = registry.fromProto(filterContainer);
+            builder.filter(filterQuery);
+        }
+
+        return builder;
+    }
+}
+```
+
+**Registry Injection Pattern**
+
+**How k-NN Now Accesses Built-in Converters**:
+
+The gRPC plugin **injects the populated registry** into converters that need it:
+
+```java
+// 1. Converter interface has a default setRegistry method
+public interface QueryBuilderProtoConverter {
+    QueryBuilder fromProto(QueryContainer queryContainer);
+
+    default void setRegistry(QueryBuilderProtoConverterRegistry registry) {
+        // By default, converters don't need a registry
+        // Converters that handle nested queries should override this method
+    }
+}
+
+// 2. GrpcPlugin injects registry into loaded extensions
+for (QueryBuilderProtoConverter converter : queryConverters) {
+    // Inject the populated registry into the converter
+    converter.setRegistry(queryRegistry);
+
+    // Register the converter
+    queryRegistry.registerConverter(converter);
+}
+```
+
+**Registry Access Pattern for Converters with Nested Queries**:
+```java
+public class KNNQueryBuilderProtoConverter implements QueryBuilderProtoConverter {
+
+    private QueryBuilderProtoConverterRegistry registry;
+
+    @Override
+    public void setRegistry(QueryBuilderProtoConverterRegistry registry) {
+        this.registry = registry;
+        // Pass the registry to utility classes that need it
+        KNNQueryBuilderProtoUtils.setRegistry(registry);
+    }
+
+    @Override
+    public QueryBuilder fromProto(QueryContainer queryContainer) {
+        // The utility class can now convert nested queries using the injected registry
+        return KNNQueryBuilderProtoUtils.fromProto(queryContainer.getKnn());
+    }
+}
+```
+
+
+## Testing
+
+### Unit Tests
+
+```bash
+./gradlew :modules:transport-grpc:spi:test
+```
+
+### Testing Your Custom Converter
+
+```java
+@Test
+public void testCustomQueryConverter() {
+    MyCustomQueryConverter converter = new MyCustomQueryConverter();
+
+    // Create test protobuf query
+    QueryContainer queryContainer = QueryContainer.newBuilder()
+        .setMyCustomQuery(MyCustomQuery.newBuilder()
+            .setField("test_field")
+            .setValue("test_value")
+            .build())
+        .build();
+
+    // Convert and verify
+    QueryBuilder result = converter.fromProto(queryContainer);
+    assertThat(result, instanceOf(MyCustomQueryBuilder.class));
+
+    MyCustomQueryBuilder customQuery = (MyCustomQueryBuilder) result;
+    assertEquals("test_field", customQuery.fieldName());
+    assertEquals("test_value", customQuery.value());
+}
+```
+
+## Real-World Example: k-NN Plugin
+See the k-NN plugin https://github.com/opensearch-project/k-NN/pull/2833/files for an example on how to use this SPI, including handling nested queries.
+
+**1. Dependency in build.gradle:**
+```gradle
+compileOnly "org.opensearch.plugin:transport-grpc-spi:${opensearch.version}"
+compileOnly "org.opensearch:protobufs:0.8.0"
+```
+
+**2. Converter Implementation with Registry Access:**
+```java
+public class KNNQueryBuilderProtoConverter implements QueryBuilderProtoConverter {
+
+    private QueryBuilderProtoConverterRegistry registry;
+
+    @Override
+    public void setRegistry(QueryBuilderProtoConverterRegistry registry) {
+        this.registry = registry;
+    }
+
+    @Override
+    public QueryContainer.QueryContainerCase getHandledQueryCase() {
+        return QueryContainer.QueryContainerCase.KNN;
+    }
+
+    @Override
+    public QueryBuilder fromProto(QueryContainer queryContainer) {
+        KnnQuery knnQuery = queryContainer.getKnn();
+
+        KNNQueryBuilder builder = new KNNQueryBuilder(
+            knnQuery.getField(),
+            knnQuery.getVectorList().toArray(new Float[0]),
+            knnQuery.getK()
+        );
+
+        // Handle nested filter query using injected registry
+        if (knnQuery.hasFilter()) {
+            QueryContainer filterContainer = knnQuery.getFilter();
+            QueryBuilder filterQuery = registry.fromProto(filterContainer);
+            builder.filter(filterQuery);
+        }
+
+        return builder;
+    }
+}
+```
+
+**3. Plugin Registration:**
+```java
+// In KNNPlugin.createComponents()
+KNNQueryBuilderProtoConverter knnQueryConverter = new KNNQueryBuilderProtoConverter();
+return ImmutableList.of(knnStats, knnQueryConverter);
+```
+
+**4. SPI File:**
+```
+# src/main/resources/META-INF/services/org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter
+org.opensearch.knn.grpc.proto.request.search.query.KNNQueryBuilderProtoConverter
+```
+
+**Why k-NN needs the registry:**
+The k-NN query's `filter` field is a `QueryContainer` protobuf type that can contain any query type (MatchAll, Term, Terms, etc.). The k-NN converter needs access to the registry to convert these nested queries to their corresponding QueryBuilder objects.
diff --git a/modules/transport-grpc/spi/build.gradle b/modules/transport-grpc/spi/build.gradle
new file mode 100644
index 0000000000000..82ccdd824a696
--- /dev/null
+++ b/modules/transport-grpc/spi/build.gradle
@@ -0,0 +1,22 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+apply plugin: 'opensearch.build'
+apply plugin: 'opensearch.publish'
+
+base {
+  group = 'org.opensearch.plugin'
+  archivesName = 'transport-grpc-spi'
+}
+
+dependencies {
+  api project(":server")
+  api "org.opensearch:protobufs:${versions.opensearchprotobufs}"
+
+  testImplementation project(":test:framework")
+}
diff --git a/modules/transport-grpc/spi/licenses/protobufs-0.6.0.jar.sha1 b/modules/transport-grpc/spi/licenses/protobufs-0.6.0.jar.sha1
new file mode 100644
index 0000000000000..a02b4926d87f5
--- /dev/null
+++ b/modules/transport-grpc/spi/licenses/protobufs-0.6.0.jar.sha1
@@ -0,0 +1 @@
+1675c5085e1376fd1a107b87f7e325944ab5b4dc
\ No newline at end of file
diff --git a/modules/transport-grpc/spi/licenses/protobufs-LICENSE.txt b/modules/transport-grpc/spi/licenses/protobufs-LICENSE.txt
new file mode 100644
index 0000000000000..44cbce8f123a1
--- /dev/null
+++ b/modules/transport-grpc/spi/licenses/protobufs-LICENSE.txt
@@ -0,0 +1,475 @@
+
+                                 Apache License
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+Some code in core/src/java/org/apache/lucene/util/UnicodeUtil.java was
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+
+/*
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+ *
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+ *
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+ */
+
+
+Some code in core/src/java/org/apache/lucene/util/ArrayUtil.java was
+derived from Python 2.4.2 sources available at
+http://www.python.org. Full license is here:
+
+  http://www.python.org/download/releases/2.4.2/license/
+
+Some code in core/src/java/org/apache/lucene/util/UnicodeUtil.java was
+derived from Python 3.1.2 sources available at
+http://www.python.org. Full license is here:
+
+  http://www.python.org/download/releases/3.1.2/license/
+
+Some code in core/src/java/org/apache/lucene/util/automaton was
+derived from Brics automaton sources available at
+www.brics.dk/automaton/. Here is the copyright from those sources:
+
+/*
+ * Copyright (c) 2001-2009 Anders Moeller
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
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+ * 2. Redistributions in binary form must reproduce the above copyright
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+ */
+
+The levenshtein automata tables in core/src/java/org/apache/lucene/util/automaton
+were automatically generated with the moman/finenight FSA package.
+Here is the copyright for those sources:
+
+# Copyright (c) 2010, Jean-Philippe Barrette-LaPierre, <jpb@rrette.com>
+#
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+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+# OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+# HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+# WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+# OTHER DEALINGS IN THE SOFTWARE.
+
+Some code in core/src/java/org/apache/lucene/util/UnicodeUtil.java was
+derived from ICU (http://www.icu-project.org)
+The full license is available here:
+  http://source.icu-project.org/repos/icu/icu/trunk/license.html
+
+/*
+ * Copyright (C) 1999-2010, International Business Machines
+ * Corporation and others.  All Rights Reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * provided that the above copyright notice(s) and this permission notice appear
+ * in all copies of the Software and that both the above copyright notice(s) and
+ * this permission notice appear in supporting documentation.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS.
+ * IN NO EVENT SHALL THE COPYRIGHT HOLDER OR HOLDERS INCLUDED IN THIS NOTICE BE
+ * LIABLE FOR ANY CLAIM, OR ANY SPECIAL INDIRECT OR CONSEQUENTIAL DAMAGES, OR
+ * ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
+ * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * Except as contained in this notice, the name of a copyright holder shall not
+ * be used in advertising or otherwise to promote the sale, use or other
+ * dealings in this Software without prior written authorization of the
+ * copyright holder.
+ */
+
+The following license applies to the Snowball stemmers:
+
+Copyright (c) 2001, Dr Martin Porter
+Copyright (c) 2002, Richard Boulton
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright notice,
+    * this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above copyright
+    * notice, this list of conditions and the following disclaimer in the
+    * documentation and/or other materials provided with the distribution.
+    * Neither the name of the copyright holders nor the names of its contributors
+    * may be used to endorse or promote products derived from this software
+    * without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+The following license applies to the KStemmer:
+
+Copyright © 2003,
+Center for Intelligent Information Retrieval,
+University of Massachusetts, Amherst.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+this list of conditions and the following disclaimer in the documentation
+and/or other materials provided with the distribution.
+
+3. The names "Center for Intelligent Information Retrieval" and
+"University of Massachusetts" must not be used to endorse or promote products
+derived from this software without prior written permission. To obtain
+permission, contact info@ciir.cs.umass.edu.
+
+THIS SOFTWARE IS PROVIDED BY UNIVERSITY OF MASSACHUSETTS AND OTHER CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
+GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGE.
+
+The following license applies to the Morfologik project:
+
+Copyright (c) 2006 Dawid Weiss
+Copyright (c) 2007-2011 Dawid Weiss, Marcin Miłkowski
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright notice,
+    this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above copyright notice,
+    this list of conditions and the following disclaimer in the documentation
+    and/or other materials provided with the distribution.
+
+    * Neither the name of Morfologik nor the names of its contributors
+    may be used to endorse or promote products derived from this software
+    without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+---
+
+The dictionary comes from Morfologik project. Morfologik uses data from
+Polish ispell/myspell dictionary hosted at http://www.sjp.pl/slownik/en/ and
+is licenced on the terms of (inter alia) LGPL and Creative Commons
+ShareAlike. The part-of-speech tags were added in Morfologik project and
+are not found in the data from sjp.pl. The tagset is similar to IPI PAN
+tagset.
+
+---
+
+The following license applies to the Morfeusz project,
+used by org.apache.lucene.analysis.morfologik.
+
+BSD-licensed dictionary of Polish (SGJP)
+http://sgjp.pl/morfeusz/
+
+Copyright © 2011 Zygmunt Saloni, Włodzimierz Gruszczyński,
+             Marcin Woliński, Robert Wołosz
+
+All rights reserved.
+
+Redistribution and  use in  source and binary  forms, with  or without
+modification, are permitted provided that the following conditions are
+met:
+
+1. Redistributions of source code must retain the above copyright
+   notice, this list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the
+   distribution.
+
+THIS SOFTWARE IS PROVIDED BY COPYRIGHT HOLDERS “AS IS” AND ANY EXPRESS
+OR  IMPLIED WARRANTIES,  INCLUDING, BUT  NOT LIMITED  TO,  THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED.  IN NO EVENT  SHALL COPYRIGHT  HOLDERS OR  CONTRIBUTORS BE
+LIABLE FOR  ANY DIRECT,  INDIRECT, INCIDENTAL, SPECIAL,  EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES  (INCLUDING, BUT NOT LIMITED  TO, PROCUREMENT OF
+SUBSTITUTE  GOODS OR  SERVICES;  LOSS  OF USE,  DATA,  OR PROFITS;  OR
+BUSINESS INTERRUPTION) HOWEVER CAUSED  AND ON ANY THEORY OF LIABILITY,
+WHETHER IN  CONTRACT, STRICT LIABILITY, OR  TORT (INCLUDING NEGLIGENCE
+OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
+IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/modules/transport-grpc/spi/licenses/protobufs-NOTICE.txt b/modules/transport-grpc/spi/licenses/protobufs-NOTICE.txt
new file mode 100644
index 0000000000000..44cbce8f123a1
--- /dev/null
+++ b/modules/transport-grpc/spi/licenses/protobufs-NOTICE.txt
@@ -0,0 +1,475 @@
+
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
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+      "License" shall mean the terms and conditions for use, reproduction,
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+      the Work and Derivative Works thereof.
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+      "Contribution" shall mean any work of authorship, including
+      the original version of the Work and any modifications or additions
+      to that Work or Derivative Works thereof, that is intentionally
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+      communication on electronic mailing lists, source code control systems,
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+      designated in writing by the copyright owner as "Not a Contribution."
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+      "Contributor" shall mean Licensor and any individual or Legal Entity
+      on behalf of whom a Contribution has been received by Licensor and
+      subsequently incorporated within the Work.
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+   2. Grant of Copyright License. Subject to the terms and conditions of
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+      by such Contributor that are necessarily infringed by their
+      Contribution(s) alone or by combination of their Contribution(s)
+      with the Work to which such Contribution(s) was submitted. If You
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+      cross-claim or counterclaim in a lawsuit) alleging that the Work
+      or a Contribution incorporated within the Work constitutes direct
+      or contributory patent infringement, then any patent licenses
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+   4. Redistribution. You may reproduce and distribute copies of the
+      Work or Derivative Works thereof in any medium, with or without
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+      meet the following conditions:
+
+      (a) You must give any other recipients of the Work or
+          Derivative Works a copy of this License; and
+
+      (b) You must cause any modified files to carry prominent notices
+          stating that You changed the files; and
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+      any Contribution intentionally submitted for inclusion in the Work
+      by You to the Licensor shall be under the terms and conditions of
+      this License, without any additional terms or conditions.
+      Notwithstanding the above, nothing herein shall supersede or modify
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+   6. Trademarks. This License does not grant permission to use the trade
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+
+
+Some code in core/src/java/org/apache/lucene/util/UnicodeUtil.java was
+derived from unicode conversion examples available at
+http://www.unicode.org/Public/PROGRAMS/CVTUTF.  Here is the copyright
+from those sources:
+
+/*
+ * Copyright 2001-2004 Unicode, Inc.
+ *
+ * Disclaimer
+ *
+ * This source code is provided as is by Unicode, Inc. No claims are
+ * made as to fitness for any particular purpose. No warranties of any
+ * kind are expressed or implied. The recipient agrees to determine
+ * applicability of information provided. If this file has been
+ * purchased on magnetic or optical media from Unicode, Inc., the
+ * sole remedy for any claim will be exchange of defective media
+ * within 90 days of receipt.
+ *
+ * Limitations on Rights to Redistribute This Code
+ *
+ * Unicode, Inc. hereby grants the right to freely use the information
+ * supplied in this file in the creation of products supporting the
+ * Unicode Standard, and to make copies of this file in any form
+ * for internal or external distribution as long as this notice
+ * remains attached.
+ */
+
+
+Some code in core/src/java/org/apache/lucene/util/ArrayUtil.java was
+derived from Python 2.4.2 sources available at
+http://www.python.org. Full license is here:
+
+  http://www.python.org/download/releases/2.4.2/license/
+
+Some code in core/src/java/org/apache/lucene/util/UnicodeUtil.java was
+derived from Python 3.1.2 sources available at
+http://www.python.org. Full license is here:
+
+  http://www.python.org/download/releases/3.1.2/license/
+
+Some code in core/src/java/org/apache/lucene/util/automaton was
+derived from Brics automaton sources available at
+www.brics.dk/automaton/. Here is the copyright from those sources:
+
+/*
+ * Copyright (c) 2001-2009 Anders Moeller
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+The levenshtein automata tables in core/src/java/org/apache/lucene/util/automaton
+were automatically generated with the moman/finenight FSA package.
+Here is the copyright for those sources:
+
+# Copyright (c) 2010, Jean-Philippe Barrette-LaPierre, <jpb@rrette.com>
+#
+# Permission is hereby granted, free of charge, to any person
+# obtaining a copy of this software and associated documentation
+# files (the "Software"), to deal in the Software without
+# restriction, including without limitation the rights to use,
+# copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the
+# Software is furnished to do so, subject to the following
+# conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+# OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+# HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+# WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+# OTHER DEALINGS IN THE SOFTWARE.
+
+Some code in core/src/java/org/apache/lucene/util/UnicodeUtil.java was
+derived from ICU (http://www.icu-project.org)
+The full license is available here:
+  http://source.icu-project.org/repos/icu/icu/trunk/license.html
+
+/*
+ * Copyright (C) 1999-2010, International Business Machines
+ * Corporation and others.  All Rights Reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * provided that the above copyright notice(s) and this permission notice appear
+ * in all copies of the Software and that both the above copyright notice(s) and
+ * this permission notice appear in supporting documentation.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS.
+ * IN NO EVENT SHALL THE COPYRIGHT HOLDER OR HOLDERS INCLUDED IN THIS NOTICE BE
+ * LIABLE FOR ANY CLAIM, OR ANY SPECIAL INDIRECT OR CONSEQUENTIAL DAMAGES, OR
+ * ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
+ * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * Except as contained in this notice, the name of a copyright holder shall not
+ * be used in advertising or otherwise to promote the sale, use or other
+ * dealings in this Software without prior written authorization of the
+ * copyright holder.
+ */
+
+The following license applies to the Snowball stemmers:
+
+Copyright (c) 2001, Dr Martin Porter
+Copyright (c) 2002, Richard Boulton
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright notice,
+    * this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above copyright
+    * notice, this list of conditions and the following disclaimer in the
+    * documentation and/or other materials provided with the distribution.
+    * Neither the name of the copyright holders nor the names of its contributors
+    * may be used to endorse or promote products derived from this software
+    * without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+The following license applies to the KStemmer:
+
+Copyright © 2003,
+Center for Intelligent Information Retrieval,
+University of Massachusetts, Amherst.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+this list of conditions and the following disclaimer in the documentation
+and/or other materials provided with the distribution.
+
+3. The names "Center for Intelligent Information Retrieval" and
+"University of Massachusetts" must not be used to endorse or promote products
+derived from this software without prior written permission. To obtain
+permission, contact info@ciir.cs.umass.edu.
+
+THIS SOFTWARE IS PROVIDED BY UNIVERSITY OF MASSACHUSETTS AND OTHER CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
+GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGE.
+
+The following license applies to the Morfologik project:
+
+Copyright (c) 2006 Dawid Weiss
+Copyright (c) 2007-2011 Dawid Weiss, Marcin Miłkowski
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright notice,
+    this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above copyright notice,
+    this list of conditions and the following disclaimer in the documentation
+    and/or other materials provided with the distribution.
+
+    * Neither the name of Morfologik nor the names of its contributors
+    may be used to endorse or promote products derived from this software
+    without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+---
+
+The dictionary comes from Morfologik project. Morfologik uses data from
+Polish ispell/myspell dictionary hosted at http://www.sjp.pl/slownik/en/ and
+is licenced on the terms of (inter alia) LGPL and Creative Commons
+ShareAlike. The part-of-speech tags were added in Morfologik project and
+are not found in the data from sjp.pl. The tagset is similar to IPI PAN
+tagset.
+
+---
+
+The following license applies to the Morfeusz project,
+used by org.apache.lucene.analysis.morfologik.
+
+BSD-licensed dictionary of Polish (SGJP)
+http://sgjp.pl/morfeusz/
+
+Copyright © 2011 Zygmunt Saloni, Włodzimierz Gruszczyński,
+             Marcin Woliński, Robert Wołosz
+
+All rights reserved.
+
+Redistribution and  use in  source and binary  forms, with  or without
+modification, are permitted provided that the following conditions are
+met:
+
+1. Redistributions of source code must retain the above copyright
+   notice, this list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the
+   distribution.
+
+THIS SOFTWARE IS PROVIDED BY COPYRIGHT HOLDERS “AS IS” AND ANY EXPRESS
+OR  IMPLIED WARRANTIES,  INCLUDING, BUT  NOT LIMITED  TO,  THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED.  IN NO EVENT  SHALL COPYRIGHT  HOLDERS OR  CONTRIBUTORS BE
+LIABLE FOR  ANY DIRECT,  INDIRECT, INCIDENTAL, SPECIAL,  EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES  (INCLUDING, BUT NOT LIMITED  TO, PROCUREMENT OF
+SUBSTITUTE  GOODS OR  SERVICES;  LOSS  OF USE,  DATA,  OR PROFITS;  OR
+BUSINESS INTERRUPTION) HOWEVER CAUSED  AND ON ANY THEORY OF LIABILITY,
+WHETHER IN  CONTRACT, STRICT LIABILITY, OR  TORT (INCLUDING NEGLIGENCE
+OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
+IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverter.java b/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverter.java
similarity index 65%
rename from modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverter.java
rename to modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverter.java
index 43ae8b2d6d97e..8e1d95d216ce9 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverter.java
+++ b/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverter.java
@@ -5,7 +5,7 @@
  * this file be licensed under the Apache-2.0 license or a
  * compatible open source license.
  */
-package org.opensearch.transport.grpc.proto.request.search.query;
+package org.opensearch.transport.grpc.spi;
 
 import org.opensearch.index.query.QueryBuilder;
 import org.opensearch.protobufs.QueryContainer;
@@ -31,4 +31,16 @@ public interface QueryBuilderProtoConverter {
      * @throws IllegalArgumentException if the query cannot be converted
      */
     QueryBuilder fromProto(QueryContainer queryContainer);
+
+    /**
+     * Sets the registry for converting nested queries.
+     * This method is called by the gRPC plugin to inject the populated registry
+     * into converters that need to handle nested query types.
+     *
+     * @param registry The registry containing all available converters
+     */
+    default void setRegistry(QueryBuilderProtoConverterRegistry registry) {
+        // By default, converters don't need a registry
+        // Converters that handle nested queries should override this method
+    }
 }
diff --git a/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverterRegistry.java b/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverterRegistry.java
new file mode 100644
index 0000000000000..60dd167ca488d
--- /dev/null
+++ b/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverterRegistry.java
@@ -0,0 +1,29 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+package org.opensearch.transport.grpc.spi;
+
+import org.opensearch.index.query.QueryBuilder;
+import org.opensearch.protobufs.QueryContainer;
+
+/**
+ * Interface for a registry that can convert protobuf queries to OpenSearch QueryBuilder objects.
+ * This interface provides a clean abstraction for plugins that need to convert nested queries
+ * without exposing the internal implementation details of the registry.
+ */
+public interface QueryBuilderProtoConverterRegistry {
+
+    /**
+     * Converts a protobuf query container to an OpenSearch QueryBuilder.
+     * This method handles the lookup and delegation to the appropriate converter.
+     *
+     * @param queryContainer The protobuf query container to convert
+     * @return The corresponding OpenSearch QueryBuilder
+     * @throws IllegalArgumentException if no converter can handle the query type
+     */
+    QueryBuilder fromProto(QueryContainer queryContainer);
+}
diff --git a/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/package-info.java b/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/package-info.java
new file mode 100644
index 0000000000000..79adf99760520
--- /dev/null
+++ b/modules/transport-grpc/spi/src/main/java/org/opensearch/transport/grpc/spi/package-info.java
@@ -0,0 +1,42 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/**
+ * Service Provider Interface (SPI) for extending gRPC transport query conversion capabilities.
+ * <p>
+ * This package provides a minimal, stable API for implementing custom query converters
+ * that can transform protobuf query messages into OpenSearch QueryBuilder objects. External plugins
+ * can implement the {@link org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter} interface
+ * to add support for custom query types in gRPC requests.
+ * </p>
+ * <p>
+ * The SPI contains only two interfaces:
+ * </p>
+ * <ul>
+ *   <li>{@link org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter} -
+ *       Interface for implementing custom query converters</li>
+ *   <li>{@link org.opensearch.transport.grpc.spi.QueryBuilderProtoConverterRegistry} -
+ *       Interface for accessing the registry to convert nested queries</li>
+ * </ul>
+ * <p>
+ * The SPI mechanism leverages OpenSearch's {@code ExtensiblePlugin} framework. Plugins must:
+ * </p>
+ * <ul>
+ *   <li>Implement {@link org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter}</li>
+ *   <li>Return the converter instance from their plugin's {@code createComponents()} method</li>
+ *   <li>Create a {@code META-INF/services} file listing their converter implementation</li>
+ *   <li>Declare {@code transport-grpc} in their plugin descriptor's {@code extended.plugins} list</li>
+ * </ul>
+ * <p>
+ * For converters that need to handle nested queries (e.g., filter clauses), the registry injection
+ * pattern allows access to built-in converters for standard query types like MatchAll, Term, and Terms.
+ * </p>
+ *
+ * @since 3.2.0
+ */
+package org.opensearch.transport.grpc.spi;
diff --git a/modules/transport-grpc/spi/src/test/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverterTests.java b/modules/transport-grpc/spi/src/test/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverterTests.java
new file mode 100644
index 0000000000000..8a46054f3aa2d
--- /dev/null
+++ b/modules/transport-grpc/spi/src/test/java/org/opensearch/transport/grpc/spi/QueryBuilderProtoConverterTests.java
@@ -0,0 +1,171 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+package org.opensearch.transport.grpc.spi;
+
+import org.opensearch.index.query.QueryBuilder;
+import org.opensearch.index.query.TermQueryBuilder;
+import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.test.OpenSearchTestCase;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.instanceOf;
+
+public class QueryBuilderProtoConverterTests extends OpenSearchTestCase {
+
+    public void testConverterInterface() {
+        TestQueryConverter converter = new TestQueryConverter();
+
+        // Test getHandledQueryCase
+        assertEquals(QueryContainer.QueryContainerCase.TERM, converter.getHandledQueryCase());
+
+        // Test fromProto
+        QueryContainer queryContainer = createMockTermQueryContainer();
+
+        QueryBuilder result = converter.fromProto(queryContainer);
+        assertThat(result, instanceOf(TermQueryBuilder.class));
+
+        TermQueryBuilder termQuery = (TermQueryBuilder) result;
+        assertThat(termQuery.fieldName(), equalTo("test_field"));
+        assertThat(termQuery.value(), equalTo("test_value"));
+    }
+
+    public void testConverterWithInvalidQueryContainer() {
+        TestQueryConverter converter = new TestQueryConverter();
+
+        // Create a query container without a term query
+        QueryContainer queryContainer = createMockRangeQueryContainer();
+
+        IllegalArgumentException exception = expectThrows(IllegalArgumentException.class, () -> converter.fromProto(queryContainer));
+        assertTrue(exception.getMessage().contains("QueryContainer does not contain a Term query"));
+    }
+
+    public void testConverterWithComplexFieldValue() {
+        TestQueryConverter converter = new TestQueryConverter();
+
+        // Test with different field value types
+        QueryContainer queryContainer = createMockNumericTermQueryContainer();
+
+        QueryBuilder result = converter.fromProto(queryContainer);
+        assertThat(result, instanceOf(TermQueryBuilder.class));
+
+        TermQueryBuilder termQuery = (TermQueryBuilder) result;
+        assertThat(termQuery.fieldName(), equalTo("numeric_field"));
+        assertThat(termQuery.value(), equalTo(42.5f));
+    }
+
+    public void testMultipleConverterInstances() {
+        TestQueryConverter converter1 = new TestQueryConverter();
+        TestQueryConverter converter2 = new TestQueryConverter();
+
+        // Both should handle the same query case
+        assertEquals(converter1.getHandledQueryCase(), converter2.getHandledQueryCase());
+
+        // Both should produce equivalent results
+        QueryContainer queryContainer = createMockTermQueryContainer();
+
+        QueryBuilder result1 = converter1.fromProto(queryContainer);
+        QueryBuilder result2 = converter2.fromProto(queryContainer);
+
+        assertThat(result1, instanceOf(TermQueryBuilder.class));
+        assertThat(result2, instanceOf(TermQueryBuilder.class));
+
+        TermQueryBuilder termQuery1 = (TermQueryBuilder) result1;
+        TermQueryBuilder termQuery2 = (TermQueryBuilder) result2;
+
+        assertEquals(termQuery1.fieldName(), termQuery2.fieldName());
+        assertEquals(termQuery1.value(), termQuery2.value());
+    }
+
+    /**
+     * Helper method to create a mock term query container
+     */
+    private QueryContainer createMockTermQueryContainer() {
+        return QueryContainer.newBuilder()
+            .setTerm(
+                org.opensearch.protobufs.TermQuery.newBuilder()
+                    .setField("test_field")
+                    .setValue(org.opensearch.protobufs.FieldValue.newBuilder().setStringValue("test_value").build())
+                    .build()
+            )
+            .build();
+    }
+
+    /**
+     * Helper method to create a mock range query container
+     */
+    private QueryContainer createMockRangeQueryContainer() {
+        return QueryContainer.newBuilder()
+            .setRange(org.opensearch.protobufs.RangeQuery.newBuilder().setField("range_field").build())
+            .build();
+    }
+
+    /**
+     * Helper method to create a mock numeric term query container
+     */
+    private QueryContainer createMockNumericTermQueryContainer() {
+        return QueryContainer.newBuilder()
+            .setTerm(
+                org.opensearch.protobufs.TermQuery.newBuilder()
+                    .setField("numeric_field")
+                    .setValue(
+                        org.opensearch.protobufs.FieldValue.newBuilder()
+                            .setGeneralNumber(org.opensearch.protobufs.GeneralNumber.newBuilder().setFloatValue(42.5f).build())
+                            .build()
+                    )
+                    .build()
+            )
+            .build();
+    }
+
+    /**
+     * Test implementation of QueryBuilderProtoConverter for TERM queries
+     */
+    private static class TestQueryConverter implements QueryBuilderProtoConverter {
+        @Override
+        public QueryContainer.QueryContainerCase getHandledQueryCase() {
+            return QueryContainer.QueryContainerCase.TERM;
+        }
+
+        @Override
+        public QueryBuilder fromProto(QueryContainer queryContainer) {
+            if (!queryContainer.hasTerm()) {
+                throw new IllegalArgumentException("QueryContainer does not contain a Term query");
+            }
+
+            org.opensearch.protobufs.TermQuery termQuery = queryContainer.getTerm();
+            String field = termQuery.getField();
+
+            // Handle different field value types
+            org.opensearch.protobufs.FieldValue fieldValue = termQuery.getValue();
+            Object value;
+
+            if (fieldValue.hasStringValue()) {
+                value = fieldValue.getStringValue();
+            } else if (fieldValue.hasGeneralNumber()) {
+                org.opensearch.protobufs.GeneralNumber number = fieldValue.getGeneralNumber();
+                if (number.hasFloatValue()) {
+                    value = number.getFloatValue();
+                } else if (number.hasDoubleValue()) {
+                    value = number.getDoubleValue();
+                } else if (number.hasInt32Value()) {
+                    value = number.getInt32Value();
+                } else if (number.hasInt64Value()) {
+                    value = number.getInt64Value();
+                } else {
+                    throw new IllegalArgumentException("Unsupported number type in TermQuery");
+                }
+            } else if (fieldValue.hasBoolValue()) {
+                value = fieldValue.getBoolValue();
+            } else {
+                throw new IllegalArgumentException("Unsupported field value type in TermQuery");
+            }
+
+            return new TermQueryBuilder(field, value);
+        }
+    }
+}
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/GrpcPlugin.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/GrpcPlugin.java
index aef65ee3e958a..eb1432715fef2 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/GrpcPlugin.java
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/GrpcPlugin.java
@@ -7,6 +7,8 @@
  */
 package org.opensearch.transport.grpc;
 
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.opensearch.cluster.metadata.IndexNameExpressionResolver;
 import org.opensearch.cluster.service.ClusterService;
 import org.opensearch.common.network.NetworkService;
@@ -29,10 +31,10 @@
 import org.opensearch.transport.AuxTransport;
 import org.opensearch.transport.client.Client;
 import org.opensearch.transport.grpc.proto.request.search.query.AbstractQueryBuilderProtoUtils;
-import org.opensearch.transport.grpc.proto.request.search.query.QueryBuilderProtoConverter;
-import org.opensearch.transport.grpc.proto.request.search.query.QueryBuilderProtoConverterRegistry;
+import org.opensearch.transport.grpc.proto.request.search.query.QueryBuilderProtoConverterRegistryImpl;
 import org.opensearch.transport.grpc.services.DocumentServiceImpl;
 import org.opensearch.transport.grpc.services.SearchServiceImpl;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 import org.opensearch.transport.grpc.ssl.SecureNetty4GrpcServerTransport;
 import org.opensearch.watcher.ResourceWatcherService;
 
@@ -65,9 +67,11 @@
  */
 public final class GrpcPlugin extends Plugin implements NetworkPlugin, ExtensiblePlugin {
 
+    private static final Logger logger = LogManager.getLogger(GrpcPlugin.class);
+
     private Client client;
     private final List<QueryBuilderProtoConverter> queryConverters = new ArrayList<>();
-    private QueryBuilderProtoConverterRegistry queryRegistry;
+    private QueryBuilderProtoConverterRegistryImpl queryRegistry;
     private AbstractQueryBuilderProtoUtils queryUtils;
 
     /**
@@ -85,8 +89,19 @@ public GrpcPlugin() {}
     public void loadExtensions(ExtensiblePlugin.ExtensionLoader loader) {
         // Load query converters from other plugins
         List<QueryBuilderProtoConverter> extensions = loader.loadExtensions(QueryBuilderProtoConverter.class);
-        if (extensions != null) {
-            queryConverters.addAll(extensions);
+        if (extensions != null && !extensions.isEmpty()) {
+            logger.info("Loading {} QueryBuilderProtoConverter extensions from other plugins", extensions.size());
+            for (QueryBuilderProtoConverter converter : extensions) {
+                logger.info(
+                    "Discovered QueryBuilderProtoConverter extension: {} (handles: {})",
+                    converter.getClass().getName(),
+                    converter.getHandledQueryCase()
+                );
+                queryConverters.add(converter);
+            }
+            logger.info("Successfully loaded {} QueryBuilderProtoConverter extensions", extensions.size());
+        } else {
+            logger.info("No QueryBuilderProtoConverter extensions found from other plugins");
         }
     }
 
@@ -262,14 +277,31 @@ public Collection<Object> createComponents(
         this.client = client;
 
         // Create the registry
-        this.queryRegistry = new QueryBuilderProtoConverterRegistry();
+        this.queryRegistry = new QueryBuilderProtoConverterRegistryImpl();
 
         // Create the query utils instance
         this.queryUtils = new AbstractQueryBuilderProtoUtils(queryRegistry);
 
-        // Register external converters
-        for (QueryBuilderProtoConverter converter : queryConverters) {
-            queryRegistry.registerConverter(converter);
+        // Inject registry into external converters and register them
+        if (!queryConverters.isEmpty()) {
+            logger.info("Injecting registry and registering {} external QueryBuilderProtoConverter(s)", queryConverters.size());
+            for (QueryBuilderProtoConverter converter : queryConverters) {
+                logger.info(
+                    "Processing external converter: {} (handles: {})",
+                    converter.getClass().getName(),
+                    converter.getHandledQueryCase()
+                );
+
+                // Inject the populated registry into the converter
+                converter.setRegistry(queryRegistry);
+                logger.info("Injected registry into converter: {}", converter.getClass().getName());
+
+                // Register the converter
+                queryRegistry.registerConverter(converter);
+            }
+            logger.info("Successfully injected registry and registered all {} external converters", queryConverters.size());
+        } else {
+            logger.info("No external QueryBuilderProtoConverter(s) to register");
         }
 
         return super.createComponents(
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/AbstractQueryBuilderProtoUtils.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/AbstractQueryBuilderProtoUtils.java
index e67cc8d86bc5a..64ebf7a38efd1 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/AbstractQueryBuilderProtoUtils.java
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/AbstractQueryBuilderProtoUtils.java
@@ -19,7 +19,7 @@
  */
 public class AbstractQueryBuilderProtoUtils {
 
-    private final QueryBuilderProtoConverterRegistry registry;
+    private final QueryBuilderProtoConverterRegistryImpl registry;
 
     /**
      * Creates a new instance with the specified registry.
@@ -27,7 +27,7 @@ public class AbstractQueryBuilderProtoUtils {
      * @param registry The registry to use for query conversion
      * @throws IllegalArgumentException if registry is null
      */
-    public AbstractQueryBuilderProtoUtils(QueryBuilderProtoConverterRegistry registry) {
+    public AbstractQueryBuilderProtoUtils(QueryBuilderProtoConverterRegistryImpl registry) {
         if (registry == null) {
             throw new IllegalArgumentException("Registry cannot be null");
         }
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchAllQueryBuilderProtoConverter.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchAllQueryBuilderProtoConverter.java
index b00dc1385ad20..89aba4fe68ef3 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchAllQueryBuilderProtoConverter.java
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchAllQueryBuilderProtoConverter.java
@@ -9,6 +9,7 @@
 
 import org.opensearch.index.query.QueryBuilder;
 import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 
 /**
  * Converter for MatchAll queries.
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchNoneQueryBuilderProtoConverter.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchNoneQueryBuilderProtoConverter.java
index 58ddb123d4dd2..1259ad35b7e9f 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchNoneQueryBuilderProtoConverter.java
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/MatchNoneQueryBuilderProtoConverter.java
@@ -9,6 +9,7 @@
 
 import org.opensearch.index.query.QueryBuilder;
 import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 
 /**
  * Converter for MatchNone queries.
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistryImpl.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistryImpl.java
new file mode 100644
index 0000000000000..428594629596b
--- /dev/null
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistryImpl.java
@@ -0,0 +1,74 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+package org.opensearch.transport.grpc.proto.request.search.query;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.opensearch.common.inject.Inject;
+import org.opensearch.common.inject.Singleton;
+import org.opensearch.index.query.QueryBuilder;
+import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverterRegistry;
+
+/**
+ * Registry for QueryBuilderProtoConverter implementations.
+ * This class wraps the SPI registry and adds built-in converters for the transport-grpc module.
+ */
+@Singleton
+public class QueryBuilderProtoConverterRegistryImpl implements QueryBuilderProtoConverterRegistry {
+
+    private static final Logger logger = LogManager.getLogger(QueryBuilderProtoConverterRegistryImpl.class);
+    private final QueryBuilderProtoConverterSpiRegistry delegate;
+
+    /**
+     * Creates a new registry and loads all available converters.
+     */
+    @Inject
+    public QueryBuilderProtoConverterRegistryImpl() {
+        // Create the SPI registry which loads external converters
+        this.delegate = new QueryBuilderProtoConverterSpiRegistry();
+
+        // Register built-in converters for this module
+        registerBuiltInConverters();
+    }
+
+    /**
+     * Registers the built-in converters.
+     * Protected for testing purposes.
+     */
+    protected void registerBuiltInConverters() {
+        // Add built-in converters
+        delegate.registerConverter(new MatchAllQueryBuilderProtoConverter());
+        delegate.registerConverter(new MatchNoneQueryBuilderProtoConverter());
+        delegate.registerConverter(new TermQueryBuilderProtoConverter());
+        delegate.registerConverter(new TermsQueryBuilderProtoConverter());
+
+        logger.info("Registered {} built-in query converters", delegate.size());
+    }
+
+    /**
+     * Converts a protobuf query container to an OpenSearch QueryBuilder.
+     *
+     * @param queryContainer The protobuf query container
+     * @return The corresponding OpenSearch QueryBuilder
+     * @throws IllegalArgumentException if the query cannot be converted
+     */
+    public QueryBuilder fromProto(QueryContainer queryContainer) {
+        return delegate.fromProto(queryContainer);
+    }
+
+    /**
+     * Registers a new converter.
+     *
+     * @param converter The converter to register
+     */
+    public void registerConverter(QueryBuilderProtoConverter converter) {
+        delegate.registerConverter(converter);
+    }
+}
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistry.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterSpiRegistry.java
similarity index 61%
rename from modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistry.java
rename to modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterSpiRegistry.java
index fa30794b6ba67..7b22c4bea04d6 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistry.java
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterSpiRegistry.java
@@ -13,71 +13,29 @@
 import org.opensearch.common.inject.Singleton;
 import org.opensearch.index.query.QueryBuilder;
 import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 
 import java.util.HashMap;
-import java.util.Iterator;
 import java.util.Map;
-import java.util.ServiceLoader;
 
 /**
  * Registry for QueryBuilderProtoConverter implementations.
  * This class discovers and manages all available converters.
  */
 @Singleton
-public class QueryBuilderProtoConverterRegistry {
+public class QueryBuilderProtoConverterSpiRegistry {
 
-    private static final Logger logger = LogManager.getLogger(QueryBuilderProtoConverterRegistry.class);
+    private static final Logger logger = LogManager.getLogger(QueryBuilderProtoConverterSpiRegistry.class);
     private final Map<QueryContainer.QueryContainerCase, QueryBuilderProtoConverter> converterMap = new HashMap<>();
 
     /**
-     * Creates a new registry and loads all available converters.
-     */
+    * Creates a new registry. External converters will be registered
+    * via the OpenSearch ExtensiblePlugin mechanism.
+    */
     @Inject
-    public QueryBuilderProtoConverterRegistry() {
-        // Load built-in converters
-        registerBuiltInConverters();
-
-        // Discover converters from other plugins using Java's ServiceLoader
-        loadExternalConverters();
-    }
-
-    /**
-     * Registers the built-in converters.
-     * Protected for testing purposes.
-     */
-    protected void registerBuiltInConverters() {
-        // Add built-in converters
-        registerConverter(new MatchAllQueryBuilderProtoConverter());
-        registerConverter(new MatchNoneQueryBuilderProtoConverter());
-        registerConverter(new TermQueryBuilderProtoConverter());
-        registerConverter(new TermsQueryBuilderProtoConverter());
-
-        logger.info("Registered {} built-in query converters", converterMap.size());
-    }
-
-    /**
-     * Loads external converters using Java's ServiceLoader mechanism.
-     * Protected for testing purposes.
-     */
-    protected void loadExternalConverters() {
-        ServiceLoader<QueryBuilderProtoConverter> serviceLoader = ServiceLoader.load(QueryBuilderProtoConverter.class);
-        Iterator<QueryBuilderProtoConverter> iterator = serviceLoader.iterator();
-
-        int count = 0;
-        int failedCount = 0;
-        while (iterator.hasNext()) {
-            try {
-                QueryBuilderProtoConverter converter = iterator.next();
-                registerConverter(converter);
-                count++;
-                logger.info("Loaded external query converter for {}: {}", converter.getHandledQueryCase(), converter.getClass().getName());
-            } catch (Exception e) {
-                failedCount++;
-                logger.error("Failed to load external query converter", e);
-            }
-        }
-
-        logger.info("Loaded {} external query converters ({} failed)", count, failedCount);
+    public QueryBuilderProtoConverterSpiRegistry() {
+        // External converters are loaded via OpenSearch's ExtensiblePlugin mechanism
+        // and registered manually via registerConverter() calls
     }
 
     /**
@@ -104,6 +62,15 @@ public QueryBuilder fromProto(QueryContainer queryContainer) {
         throw new IllegalArgumentException("Unsupported query type in container: " + queryContainer + " (case: " + queryCase + ")");
     }
 
+    /**
+     * Gets the number of registered converters.
+     *
+     * @return The number of registered converters
+     */
+    public int size() {
+        return converterMap.size();
+    }
+
     /**
      * Registers a new converter.
      *
@@ -137,6 +104,6 @@ public void registerConverter(QueryBuilderProtoConverter converter) {
             );
         }
 
-        logger.debug("Registered query converter for {}: {}", queryCase, converter.getClass().getName());
+        logger.info("Registered query converter for {}: {}", queryCase, converter.getClass().getName());
     }
 }
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermQueryBuilderProtoConverter.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermQueryBuilderProtoConverter.java
index 19c70df31d80c..c57bbfe69736f 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermQueryBuilderProtoConverter.java
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermQueryBuilderProtoConverter.java
@@ -9,6 +9,7 @@
 
 import org.opensearch.index.query.QueryBuilder;
 import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 
 /**
  * Converter for Term queries.
diff --git a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermsQueryBuilderProtoConverter.java b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermsQueryBuilderProtoConverter.java
index 36b135ffcec63..612970181faf4 100644
--- a/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermsQueryBuilderProtoConverter.java
+++ b/modules/transport-grpc/src/main/java/org/opensearch/transport/grpc/proto/request/search/query/TermsQueryBuilderProtoConverter.java
@@ -9,6 +9,7 @@
 
 import org.opensearch.index.query.QueryBuilder;
 import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 
 /**
  * Converter for Terms queries.
diff --git a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/GrpcPluginTests.java b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/GrpcPluginTests.java
index 7ce74470e0395..7f912935f24fc 100644
--- a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/GrpcPluginTests.java
+++ b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/GrpcPluginTests.java
@@ -20,7 +20,7 @@
 import org.opensearch.threadpool.ThreadPool;
 import org.opensearch.transport.AuxTransport;
 import org.opensearch.transport.client.Client;
-import org.opensearch.transport.grpc.proto.request.search.query.QueryBuilderProtoConverter;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 import org.opensearch.transport.grpc.ssl.SecureNetty4GrpcServerTransport;
 import org.junit.Before;
 
@@ -325,6 +325,13 @@ public void testCreateComponentsWithExternalConverters() {
         assertNotNull("QueryUtils should be initialized after createComponents", newPlugin.getQueryUtils());
 
         // Verify that the external converter was registered by checking it was called
-        Mockito.verify(mockConverter).getHandledQueryCase();
+        // Note: getHandledQueryCase() is called 3 times:
+        // 1. In loadExtensions() for logging
+        // 2. In createComponents() for logging
+        // 3. In QueryBuilderProtoConverterSpiRegistry.registerConverter() for registration
+        Mockito.verify(mockConverter, Mockito.times(3)).getHandledQueryCase();
+
+        // Verify that setRegistry was called to inject the registry
+        Mockito.verify(mockConverter, Mockito.times(1)).setRegistry(Mockito.any());
     }
 }
diff --git a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/EmptyQueryBuilderProtoConverterRegistry.java b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/EmptyQueryBuilderProtoConverterRegistry.java
deleted file mode 100644
index 1b5c20df855f3..0000000000000
--- a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/EmptyQueryBuilderProtoConverterRegistry.java
+++ /dev/null
@@ -1,49 +0,0 @@
-/*
- * SPDX-License-Identifier: Apache-2.0
- *
- * The OpenSearch Contributors require contributions made to
- * this file be licensed under the Apache-2.0 license or a
- * compatible open source license.
- */
-package org.opensearch.transport.grpc.proto.request.search.query;
-
-import org.apache.logging.log4j.LogManager;
-import org.apache.logging.log4j.Logger;
-
-/**
- * A test-specific implementation of QueryBuilderProtoConverterRegistry that starts with no converters.
- * This class is used for testing scenarios where we need a clean registry without any built-in converters.
- */
-public class EmptyQueryBuilderProtoConverterRegistry extends QueryBuilderProtoConverterRegistry {
-
-    private static final Logger logger = LogManager.getLogger(EmptyQueryBuilderProtoConverterRegistry.class);
-
-    /**
-     * Creates a new empty registry with no converters.
-     * This constructor calls the parent constructor but doesn't register any converters.
-     */
-    public EmptyQueryBuilderProtoConverterRegistry() {
-        // The parent constructor will call registerBuiltInConverters() and loadExternalConverters(),
-        // but we'll override those methods to do nothing
-    }
-
-    /**
-     * Override the parent's registerBuiltInConverters method to do nothing.
-     * This ensures no built-in converters are registered.
-     */
-    @Override
-    protected void registerBuiltInConverters() {
-        // Do nothing - we want an empty registry for testing
-        logger.debug("Skipping registration of built-in converters for testing");
-    }
-
-    /**
-     * Override the parent's loadExternalConverters method to do nothing.
-     * This ensures no external converters are loaded.
-     */
-    @Override
-    protected void loadExternalConverters() {
-        // Do nothing - we want an empty registry for testing
-        logger.debug("Skipping loading of external converters for testing");
-    }
-}
diff --git a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistryTests.java b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistryTests.java
index e4f969b973b6d..7b7e6dd9381ed 100644
--- a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistryTests.java
+++ b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterRegistryTests.java
@@ -13,18 +13,19 @@
 import org.opensearch.protobufs.QueryContainer;
 import org.opensearch.protobufs.TermQuery;
 import org.opensearch.test.OpenSearchTestCase;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
 
 /**
  * Test class for QueryBuilderProtoConverterRegistry to verify the map-based optimization.
  */
 public class QueryBuilderProtoConverterRegistryTests extends OpenSearchTestCase {
 
-    private QueryBuilderProtoConverterRegistry registry;
+    private QueryBuilderProtoConverterRegistryImpl registry;
 
     @Override
     public void setUp() throws Exception {
         super.setUp();
-        registry = new QueryBuilderProtoConverterRegistry();
+        registry = new QueryBuilderProtoConverterRegistryImpl();
     }
 
     public void testMatchAllQueryConversion() {
diff --git a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterSpiRegistryTests.java b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterSpiRegistryTests.java
new file mode 100644
index 0000000000000..89277787744e4
--- /dev/null
+++ b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoConverterSpiRegistryTests.java
@@ -0,0 +1,211 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+package org.opensearch.transport.grpc.proto.request.search.query;
+
+import org.opensearch.index.query.QueryBuilder;
+import org.opensearch.index.query.TermQueryBuilder;
+import org.opensearch.protobufs.QueryContainer;
+import org.opensearch.test.OpenSearchTestCase;
+import org.opensearch.transport.grpc.spi.QueryBuilderProtoConverter;
+
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.instanceOf;
+
+public class QueryBuilderProtoConverterSpiRegistryTests extends OpenSearchTestCase {
+
+    private QueryBuilderProtoConverterSpiRegistry registry;
+
+    @Override
+    public void setUp() throws Exception {
+        super.setUp();
+        registry = new QueryBuilderProtoConverterSpiRegistry();
+    }
+
+    public void testEmptyRegistry() {
+        assertEquals(0, registry.size());
+    }
+
+    public void testRegisterConverter() {
+        TestQueryConverter converter = new TestQueryConverter();
+        registry.registerConverter(converter);
+
+        assertEquals(1, registry.size());
+    }
+
+    public void testRegisterNullConverter() {
+        IllegalArgumentException exception = expectThrows(IllegalArgumentException.class, () -> registry.registerConverter(null));
+        assertThat(exception.getMessage(), containsString("Converter cannot be null"));
+    }
+
+    public void testRegisterConverterWithNullQueryCase() {
+        QueryBuilderProtoConverter converter = new QueryBuilderProtoConverter() {
+            @Override
+            public QueryContainer.QueryContainerCase getHandledQueryCase() {
+                return null;
+            }
+
+            @Override
+            public QueryBuilder fromProto(QueryContainer queryContainer) {
+                return null;
+            }
+        };
+
+        IllegalArgumentException exception = expectThrows(IllegalArgumentException.class, () -> registry.registerConverter(converter));
+        assertThat(exception.getMessage(), containsString("Handled query case cannot be null"));
+    }
+
+    public void testRegisterConverterWithNotSetQueryCase() {
+        QueryBuilderProtoConverter converter = new QueryBuilderProtoConverter() {
+            @Override
+            public QueryContainer.QueryContainerCase getHandledQueryCase() {
+                return QueryContainer.QueryContainerCase.QUERYCONTAINER_NOT_SET;
+            }
+
+            @Override
+            public QueryBuilder fromProto(QueryContainer queryContainer) {
+                return null;
+            }
+        };
+
+        IllegalArgumentException exception = expectThrows(IllegalArgumentException.class, () -> registry.registerConverter(converter));
+        assertThat(exception.getMessage(), containsString("Cannot register converter for QUERYCONTAINER_NOT_SET case"));
+    }
+
+    public void testFromProtoWithRegisteredConverter() {
+        TestQueryConverter converter = new TestQueryConverter();
+        registry.registerConverter(converter);
+
+        // Create a mock query container that the test converter can handle
+        QueryContainer queryContainer = createMockTermQueryContainer();
+
+        QueryBuilder result = registry.fromProto(queryContainer);
+        assertThat(result, instanceOf(TermQueryBuilder.class));
+
+        TermQueryBuilder termQuery = (TermQueryBuilder) result;
+        assertThat(termQuery.fieldName(), equalTo("test_field"));
+        assertThat(termQuery.value(), equalTo("test_value"));
+    }
+
+    public void testFromProtoWithNullQueryContainer() {
+        IllegalArgumentException exception = expectThrows(IllegalArgumentException.class, () -> registry.fromProto(null));
+        assertThat(exception.getMessage(), containsString("Query container cannot be null"));
+    }
+
+    public void testFromProtoWithUnregisteredQueryType() {
+        QueryContainer queryContainer = createMockTermQueryContainer();
+
+        IllegalArgumentException exception = expectThrows(IllegalArgumentException.class, () -> registry.fromProto(queryContainer));
+        assertThat(exception.getMessage(), containsString("Unsupported query type in container"));
+        assertThat(exception.getMessage(), containsString("TERM"));
+    }
+
+    public void testReplaceExistingConverter() {
+        TestQueryConverter converter1 = new TestQueryConverter();
+        TestQueryConverter converter2 = new TestQueryConverter();
+
+        registry.registerConverter(converter1);
+        assertEquals(1, registry.size());
+
+        // Register second converter for same query case - should replace
+        registry.registerConverter(converter2);
+        assertEquals(1, registry.size());
+
+        // Verify the second converter is used
+        QueryContainer queryContainer = createMockTermQueryContainer();
+
+        QueryBuilder result = registry.fromProto(queryContainer);
+        assertThat(result, instanceOf(TermQueryBuilder.class));
+    }
+
+    public void testMultipleConvertersForDifferentQueryTypes() {
+        TestQueryConverter termConverter = new TestQueryConverter();
+        TestRangeQueryConverter rangeConverter = new TestRangeQueryConverter();
+
+        registry.registerConverter(termConverter);
+        registry.registerConverter(rangeConverter);
+
+        assertEquals(2, registry.size());
+
+        // Test term query
+        QueryContainer termContainer = createMockTermQueryContainer();
+        QueryBuilder termResult = registry.fromProto(termContainer);
+        assertThat(termResult, instanceOf(TermQueryBuilder.class));
+
+        // Test range query
+        QueryContainer rangeContainer = createMockRangeQueryContainer();
+        QueryBuilder rangeResult = registry.fromProto(rangeContainer);
+        assertNotNull(rangeResult); // TestRangeQueryConverter returns a mock QueryBuilder
+    }
+
+    /**
+     * Helper method to create a mock term query container
+     */
+    private QueryContainer createMockTermQueryContainer() {
+        return QueryContainer.newBuilder()
+            .setTerm(
+                org.opensearch.protobufs.TermQuery.newBuilder()
+                    .setField("test_field")
+                    .setValue(org.opensearch.protobufs.FieldValue.newBuilder().setStringValue("test_value").build())
+                    .build()
+            )
+            .build();
+    }
+
+    /**
+     * Helper method to create a mock range query container
+     */
+    private QueryContainer createMockRangeQueryContainer() {
+        return QueryContainer.newBuilder()
+            .setRange(org.opensearch.protobufs.RangeQuery.newBuilder().setField("range_field").build())
+            .build();
+    }
+
+    /**
+     * Test converter implementation for TERM queries
+     */
+    private static class TestQueryConverter implements QueryBuilderProtoConverter {
+        @Override
+        public QueryContainer.QueryContainerCase getHandledQueryCase() {
+            return QueryContainer.QueryContainerCase.TERM;
+        }
+
+        @Override
+        public QueryBuilder fromProto(QueryContainer queryContainer) {
+            if (!queryContainer.hasTerm()) {
+                throw new IllegalArgumentException("QueryContainer does not contain a Term query");
+            }
+
+            org.opensearch.protobufs.TermQuery termQuery = queryContainer.getTerm();
+            String field = termQuery.getField();
+            String value = termQuery.getValue().getStringValue();
+
+            return new TermQueryBuilder(field, value);
+        }
+    }
+
+    /**
+     * Test converter implementation for RANGE queries
+     */
+    private static class TestRangeQueryConverter implements QueryBuilderProtoConverter {
+        @Override
+        public QueryContainer.QueryContainerCase getHandledQueryCase() {
+            return QueryContainer.QueryContainerCase.RANGE;
+        }
+
+        @Override
+        public QueryBuilder fromProto(QueryContainer queryContainer) {
+            if (!queryContainer.hasRange()) {
+                throw new IllegalArgumentException("QueryContainer does not contain a Range query");
+            }
+
+            // Return a simple mock QueryBuilder for testing
+            return new TermQueryBuilder("mock_field", "mock_value");
+        }
+    }
+}
diff --git a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoTestUtils.java b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoTestUtils.java
index 71a9e24b1d5f6..0d38566db0b97 100644
--- a/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoTestUtils.java
+++ b/modules/transport-grpc/src/test/java/org/opensearch/transport/grpc/proto/request/search/query/QueryBuilderProtoTestUtils.java
@@ -24,7 +24,7 @@ private QueryBuilderProtoTestUtils() {
      */
     public static AbstractQueryBuilderProtoUtils createQueryUtils() {
         // Create a new registry
-        QueryBuilderProtoConverterRegistry registry = new QueryBuilderProtoConverterRegistry();
+        QueryBuilderProtoConverterRegistryImpl registry = new QueryBuilderProtoConverterRegistryImpl();
 
         // Register all built-in converters
         registry.registerConverter(new MatchAllQueryBuilderProtoConverter());