diff --git a/CHANGELOG.md b/CHANGELOG.md
index c6951ef4..624b0043 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Added
 
 - Added the ability to set timeout for Opensearch and Elasticsearch clients by setting the environmental variable `ES_TIMEOUT` [#408](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/408)
+- Added `collection-search#filter` conformance class to CollectionSearchExtension to enable compatibility with stac-auth-proxy collection filtering  [#411](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/411)
 
 ### Changed
 
diff --git a/stac_fastapi/elasticsearch/stac_fastapi/elasticsearch/app.py b/stac_fastapi/elasticsearch/stac_fastapi/elasticsearch/app.py
index 7e145072..c348822f 100644
--- a/stac_fastapi/elasticsearch/stac_fastapi/elasticsearch/app.py
+++ b/stac_fastapi/elasticsearch/stac_fastapi/elasticsearch/app.py
@@ -31,6 +31,7 @@
 )
 from stac_fastapi.extensions.core import (
     AggregationExtension,
+    CollectionSearchExtension,
     FilterExtension,
     FreeTextExtension,
     SortExtension,
@@ -60,6 +61,14 @@
     FilterConformanceClasses.ADVANCED_COMPARISON_OPERATORS
 )
 
+# Adding collection search extension for compatibility with stac-auth-proxy
+# (https://github.com/developmentseed/stac-auth-proxy)
+# The extension is not fully implemented yet but is required for collection filtering support
+collection_search_extension = CollectionSearchExtension()
+collection_search_extension.conformance_classes.append(
+    "https://api.stacspec.org/v1.0.0-rc.1/collection-search#filter"
+)
+
 aggregation_extension = AggregationExtension(
     client=EsAsyncBaseAggregationClient(
         database=database_logic, session=session, settings=settings
@@ -75,6 +84,7 @@
     TokenPaginationExtension(),
     filter_extension,
     FreeTextExtension(),
+    collection_search_extension,
 ]
 
 if TRANSACTIONS_EXTENSIONS:
diff --git a/stac_fastapi/opensearch/stac_fastapi/opensearch/app.py b/stac_fastapi/opensearch/stac_fastapi/opensearch/app.py
index c047014a..b31281fa 100644
--- a/stac_fastapi/opensearch/stac_fastapi/opensearch/app.py
+++ b/stac_fastapi/opensearch/stac_fastapi/opensearch/app.py
@@ -25,6 +25,7 @@
 from stac_fastapi.core.utilities import get_bool_env
 from stac_fastapi.extensions.core import (
     AggregationExtension,
+    CollectionSearchExtension,
     FilterExtension,
     FreeTextExtension,
     SortExtension,
@@ -60,6 +61,14 @@
     FilterConformanceClasses.ADVANCED_COMPARISON_OPERATORS
 )
 
+# Adding collection search extension for compatibility with stac-auth-proxy
+# (https://github.com/developmentseed/stac-auth-proxy)
+# The extension is not fully implemented yet but is required for collection filtering support
+collection_search_extension = CollectionSearchExtension()
+collection_search_extension.conformance_classes.append(
+    "https://api.stacspec.org/v1.0.0-rc.1/collection-search#filter"
+)
+
 aggregation_extension = AggregationExtension(
     client=EsAsyncBaseAggregationClient(
         database=database_logic, session=session, settings=settings
@@ -75,6 +84,7 @@
     TokenPaginationExtension(),
     filter_extension,
     FreeTextExtension(),
+    collection_search_extension,
 ]