localstack · bentsku · Feb 6, 2026 · Feb 4, 2026 · Feb 4, 2026
diff --git a/localstack-core/localstack/services/apigateway/models.py b/localstack-core/localstack/services/apigateway/models.py
@@ -1,7 +1,5 @@
 from typing import Any
 
-from requests.structures import CaseInsensitiveDict
-
 from localstack.aws.api.apigateway import (
     Account,
     Authorizer,
@@ -109,8 +107,7 @@ def __init__(
 
 class ApiGatewayStore(BaseStore):
     # maps (API id) -> RestApiContainer
-    # TODO: remove CaseInsensitiveDict, and lower the value of the ID when getting it from the tags
-    rest_apis: dict[str, RestApiContainer] = LocalAttribute(default=CaseInsensitiveDict)
+    rest_apis: dict[str, RestApiContainer] = LocalAttribute(default=dict)
 
     # account details
     _account: Account = LocalAttribute(default=dict)

diff --git a/localstack-core/localstack/services/apigateway/patches.py b/localstack-core/localstack/services/apigateway/patches.py
@@ -4,8 +4,8 @@
 
 from moto.apigateway import models as apigateway_models
 from moto.apigateway.exceptions import (
+    BadRequestException,
     DeploymentNotFoundException,
-    RestAPINotFound,
     StageStillActive,
 )
 from moto.apigateway.responses import APIGatewayResponse
@@ -169,19 +169,19 @@ def create_rest_api(fn, self, *args, tags=None, **kwargs):
         """
         tags = tags or {}
         result = fn(self, *args, tags=tags, **kwargs)
-        # TODO: lower the custom_id when getting it from the tags, as AWS is case insensitive
+
         if custom_id := tags.get(TAG_KEY_CUSTOM_ID):
             self.apis.pop(result.id)
             result.id = custom_id
             self.apis[custom_id] = result
-        return result
 
-    @patch(apigateway_models.APIGatewayBackend.get_rest_api, pass_target=False)
-    def get_rest_api(self, function_id):
-        for key in self.apis.keys():
-            if key.lower() == function_id.lower():
-                return self.apis[key]
-        raise RestAPINotFound()
+        if not (result.id.islower() or result.id.isnumeric()):
+            self.apis.pop(result.id)
+            raise BadRequestException(
+                f"The RestApiId '{result.id}' cannot contain uppercase characters"
+            )
+
+        return result
 
     @patch(apigateway_models.RestAPI.delete_deployment, pass_target=False)
     def patch_delete_deployment(self, deployment_id: str) -> apigateway_models.Deployment:

diff --git a/tests/aws/services/apigateway/test_apigateway_basic.py b/tests/aws/services/apigateway/test_apigateway_basic.py
@@ -142,7 +142,7 @@ def test_create_rest_api_with_custom_id(self, create_rest_apigw, url_function, a
         if not is_next_gen_api() and url_function == localstack_path_based_url:
             pytest.skip("This URL type is not supported in the legacy implementation")
         apigw_name = f"gw-{short_uid()}"
-        test_id = "testId123"
+        test_id = "test-id123"
         api_id, name, _ = create_rest_apigw(name=apigw_name, tags={TAG_KEY_CUSTOM_ID: test_id})
         assert test_id == api_id
         assert apigw_name == name
@@ -160,6 +160,20 @@ def test_create_rest_api_with_custom_id(self, create_rest_apigw, url_function, a
         assert response.ok
         assert response._content == b'{"echo": "foobar", "response": "mocked"}'
 
+    @markers.aws.only_localstack
+    # This is not a possible feature on aws.
+    def test_create_rest_api_with_invalid_custom_id(self, create_rest_apigw, aws_client):
+        apigw_name = f"gw-{short_uid()}"
+        test_id = "testId123"
+        with pytest.raises(ClientError) as exc:
+            create_rest_apigw(name=apigw_name, tags={TAG_KEY_CUSTOM_ID: test_id})
+
+        assert exc.value.response["Error"]["Code"] == "BadRequestException"
+        assert (
+            exc.value.response["Error"]["Message"]
+            == f"The RestApiId '{test_id}' cannot contain uppercase characters"
+        )
+
     @markers.aws.validated
     def test_update_rest_api_deployment(self, create_rest_apigw, aws_client, snapshot):
         snapshot.add_transformer(snapshot.transform.key_value("id"))
@@ -1505,10 +1519,11 @@ class TestTagging:
     def test_tag_api(self, create_rest_apigw, aws_client, account_id, region_name):
         api_name = f"api-{short_uid()}"
         tags = {"foo": "bar"}
+        custom_id = "c0stiom1d"
 
         # add resource tags
-        api_id, _, _ = create_rest_apigw(name=api_name, tags={TAG_KEY_CUSTOM_ID: "c0stIOm1d"})
-        assert api_id == "c0stIOm1d"
+        api_id, _, _ = create_rest_apigw(name=api_name, tags={TAG_KEY_CUSTOM_ID: custom_id})
+        assert api_id == custom_id
 
         api_arn = arns.apigateway_restapi_arn(api_id, account_id, region_name)
         aws_client.apigateway.tag_resource(resourceArn=api_arn, tags=tags)

diff --git a/tests/aws/services/apigateway/test_apigateway_custom_ids.py b/tests/aws/services/apigateway/test_apigateway_custom_ids.py
@@ -1,3 +1,5 @@
+import pytest
+from botocore.exceptions import ClientError
 from moto.apigateway.utils import (
     ApigwApiKeyIdentifier,
     ApigwResourceIdentifier,
@@ -7,7 +9,7 @@
 from localstack.testing.pytest import markers
 from localstack.utils.strings import long_uid, short_uid
 
-API_ID = "ApiId"
+API_ID = "api-id"
 ROOT_RESOURCE_ID = "RootId"
 PET_1_RESOURCE_ID = "Pet1Id"
 PET_2_RESOURCE_ID = "Pet2Id"
@@ -60,3 +62,23 @@ def test_apigateway_custom_ids(
     assert pet_resource_1["id"] == PET_1_RESOURCE_ID
     assert pet_resource_2["id"] == PET_2_RESOURCE_ID
     assert api_key["id"] == API_KEY_ID
+
+
+@markers.aws.only_localstack
+@markers.requires_in_process
+def test_apigateway_invalid_rest_api_custom_id(
+    aws_client, set_resource_custom_id, create_rest_apigw, account_id, region_name, cleanups
+):
+    rest_api_name = f"apigw-{short_uid()}"
+    bad_api_id = "UpperCaseApi"
+
+    set_resource_custom_id(
+        ApigwRestApiIdentifier(account_id, region_name, rest_api_name), bad_api_id
+    )
+    with pytest.raises(ClientError) as exc:
+        create_rest_apigw(name=rest_api_name)
+    assert exc.value.response["Error"]["Code"] == "BadRequestException"
+    assert (
+        exc.value.response["Error"]["Message"]
+        == f"The RestApiId '{bad_api_id}' cannot contain uppercase characters"
+    )