diff --git a/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb b/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb
index eac218050..3843513a4 100644
--- a/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb
+++ b/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb
@@ -8,6 +8,7 @@
 
 require "elastic_graph/constants"
 require "elastic_graph/datastore_core/index_definition"
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
 require "elastic_graph/support/hash_util"
 require "stringio"
 require_relative "implementation_shared_examples"
@@ -16,6 +17,12 @@ module ElasticGraph
   class DatastoreCore
     module IndexDefinition
       RSpec.describe RolloverIndexTemplate, :uses_datastore, :builds_indexer do
+        # Documents indexed by these specs are validated against the JSON schemas, so the schemas
+        # defined in this file all include the JSON ingestion schema definition extension.
+        def build_datastore_core(**options, &block)
+          super(schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension], **options, &block)
+        end
+
         # Use different index names than any other tests use, because most tests expect a specific index
         # configuration (based on `config/schema.graphql`) and we do not want to mess with it here.
         let(:index_prefix) { unique_index_name }
diff --git a/elasticgraph-graphql/spec/acceptance/elasticgraph_graphql_acceptance_support.rb b/elasticgraph-graphql/spec/acceptance/elasticgraph_graphql_acceptance_support.rb
index 9956e95e9..e7b3c5304 100644
--- a/elasticgraph-graphql/spec/acceptance/elasticgraph_graphql_acceptance_support.rb
+++ b/elasticgraph-graphql/spec/acceptance/elasticgraph_graphql_acceptance_support.rb
@@ -7,6 +7,7 @@
 # frozen_string_literal: true
 
 require "elastic_graph/graphql/datastore_search_router"
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
 require "elastic_graph/schema_definition/schema_elements/type_namer"
 require "elastic_graph/spec_support/builds_admin"
 require "graphql"
@@ -91,6 +92,9 @@ def self.with_both_casing_forms(&block)
             schema_element_name_form: :camelCase,
             derived_type_name_formats: derived_type_name_formats,
             enum_value_overrides_by_type: enum_value_overrides_by_type,
+            # The camelCase schema definition below is derived from the repository's main test schema,
+            # which uses the JSON ingestion schema definition DSL, so it requires this extension.
+            schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension],
             schema_definition: ->(schema) do
               # standard:disable Security/Eval -- it's ok here in a test.
               schema.as_active_instance { eval(camel_case_schema_def) }
diff --git a/elasticgraph-graphql/spec/integration/elastic_graph/graphql/datastore_query/misc_spec.rb b/elasticgraph-graphql/spec/integration/elastic_graph/graphql/datastore_query/misc_spec.rb
index 90ccfe347..e44d8d0e5 100644
--- a/elasticgraph-graphql/spec/integration/elastic_graph/graphql/datastore_query/misc_spec.rb
+++ b/elasticgraph-graphql/spec/integration/elastic_graph/graphql/datastore_query/misc_spec.rb
@@ -9,6 +9,7 @@
 require_relative "datastore_query_integration_support"
 require "elastic_graph/errors"
 require "elastic_graph/graphql/datastore_search_router"
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
 
 module ElasticGraph
   class GraphQL
@@ -85,7 +86,7 @@ def msearch(body:, **args, &block)
 
       context "when the indexed type has nested `default_sort_fields`" do
         let(:graphql) do
-          build_graphql(schema_definition: lambda do |schema|
+          build_graphql(schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension], schema_definition: lambda do |schema|
             schema.object_type "Money" do |t|
               t.field "currency", "String!"
               t.field "amount_cents", "Int!"
diff --git a/elasticgraph-graphql/spec/integration/elastic_graph/graphql/resolvers/nested_relationships_spec.rb b/elasticgraph-graphql/spec/integration/elastic_graph/graphql/resolvers/nested_relationships_spec.rb
index a09583209..2657de370 100644
--- a/elasticgraph-graphql/spec/integration/elastic_graph/graphql/resolvers/nested_relationships_spec.rb
+++ b/elasticgraph-graphql/spec/integration/elastic_graph/graphql/resolvers/nested_relationships_spec.rb
@@ -7,6 +7,7 @@
 # frozen_string_literal: true
 
 require "elastic_graph/graphql/resolvers/nested_relationships"
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
 
 module ElasticGraph
   class GraphQL
@@ -16,7 +17,7 @@ module Resolvers
         # is implemented via a filter on `id` (the search routing field)
         context "when the field being resolved is a relay connection field", :expect_search_routing do
           let(:graphql) do
-            build_graphql(schema_definition: lambda do |schema|
+            build_graphql(schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension], schema_definition: lambda do |schema|
               schema.object_type "Component" do |t|
                 t.field "id", "ID!"
                 t.field "name", "String!"
@@ -62,7 +63,7 @@ module Resolvers
 
         describe "a relates_to_many/relates_to_one bidirectional relationship with an array foreign key from the one to the many" do
           let(:graphql) do
-            build_graphql(schema_definition: lambda do |schema|
+            build_graphql(schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension], schema_definition: lambda do |schema|
               schema.object_type "Money" do |t|
                 t.field "currency", "String!"
                 t.field "amount_cents", "Int"
@@ -241,7 +242,7 @@ module Resolvers
 
         describe "a relates_to_many/relates_to_one bidirectional relationship with a scalar foreign key from the many to the one" do
           let(:graphql) do
-            build_graphql(schema_definition: lambda do |schema|
+            build_graphql(schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension], schema_definition: lambda do |schema|
               schema.object_type "ElectricalPart" do |t|
                 t.field "id", "ID!"
                 t.field "name", "String!"
@@ -381,7 +382,7 @@ module Resolvers
 
         describe "a relates_to_many/relates_to_many bidirectional relationship with an array foreign key from a many to a many" do
           let(:graphql) do
-            build_graphql(schema_definition: lambda do |schema|
+            build_graphql(schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension], schema_definition: lambda do |schema|
               schema.object_type "Component" do |t|
                 t.field "id", "ID!"
                 t.field "name", "String!"
@@ -573,7 +574,7 @@ module Resolvers
 
         describe "a relates_to_one/relates_to_one bidirectional relationship with a scalar foreign key from a one to a one" do
           let(:graphql) do
-            build_graphql(schema_definition: lambda do |schema|
+            build_graphql(schema_definition_extension_modules: [JSONIngestion::SchemaDefinition::APIExtension], schema_definition: lambda do |schema|
               schema.object_type "Manufacturer" do |t|
                 t.field "id", "ID!"
                 t.field "name", "String"
diff --git a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_executor_spec.rb b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_executor_spec.rb
index 7cf193e1d..ec1e962a7 100644
--- a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_executor_spec.rb
+++ b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_executor_spec.rb
@@ -447,7 +447,6 @@ def resolve(field:, object:, args:, context:)
 
               schema.scalar_type "Operands" do |t|
                 t.mapping type: nil
-                t.json_schema type: "null"
               end
 
               schema.object_type "Widget" do |t|
diff --git a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/resolvers/get_record_field_value_spec.rb b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/resolvers/get_record_field_value_spec.rb
index 89479c1f8..10c32c891 100644
--- a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/resolvers/get_record_field_value_spec.rb
+++ b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/resolvers/get_record_field_value_spec.rb
@@ -19,7 +19,6 @@ module Resolvers
           self.schema_artifacts = generate_schema_artifacts do |schema|
             schema.scalar_type "MyInt" do |t|
               t.mapping type: "integer"
-              t.json_schema type: "integer"
             end
 
             schema.object_type "PersonIdentifiers" do |t|
diff --git a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/scalar_coercion_adapters/no_op_spec.rb b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/scalar_coercion_adapters/no_op_spec.rb
index cb84c3ff4..73bb09df8 100644
--- a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/scalar_coercion_adapters/no_op_spec.rb
+++ b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/scalar_coercion_adapters/no_op_spec.rb
@@ -14,7 +14,6 @@ module ScalarCoercionAdapters
       RSpec.describe "NoOp" do
         include_context("scalar coercion adapter support", "SomeCustomScalar", schema_definition: ->(schema) do
           schema.scalar_type "SomeCustomScalar" do |t|
-            t.json_schema type: "null"
             t.mapping type: nil
           end
 
diff --git a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb
index 48dc10135..90f21fdb8 100644
--- a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb
+++ b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb
@@ -955,7 +955,6 @@ def search_index_definitions_from(type_name: "TheType")
             schema = define_schema do |s|
               s.scalar_type "CustomScalar" do |t|
                 t.mapping type: "keyword"
-                t.json_schema type: "string"
               end
             end
 
@@ -967,7 +966,6 @@ def search_index_definitions_from(type_name: "TheType")
             schema = define_schema do |s|
               s.scalar_type "CustomScalar" do |t|
                 t.mapping type: "keyword"
-                t.json_schema type: "string"
                 t.grouping_missing_value_placeholder "MISSING"
               end
             end
@@ -980,7 +978,6 @@ def search_index_definitions_from(type_name: "TheType")
             schema = define_schema do |s|
               s.scalar_type "CustomScalar" do |t|
                 t.mapping type: "keyword"
-                t.json_schema type: "string"
                 t.grouping_missing_value_placeholder nil
               end
             end
@@ -1007,7 +1004,6 @@ def search_index_definitions_from(type_name: "TheType")
                 t.field "name", "String"
                 t.field "count", "Int"
                 t.field "price", "Float"
-                t.field "big_number", "JsonSafeLong"
                 t.index "things"
               end
             end
@@ -1016,7 +1012,6 @@ def search_index_definitions_from(type_name: "TheType")
             expect(schema.type_named("String").grouping_missing_value_placeholder).to eq MISSING_STRING_PLACEHOLDER_VALUE
             expect(schema.type_named("Int").grouping_missing_value_placeholder).to eq MISSING_NUMERIC_PLACEHOLDER
             expect(schema.type_named("Float").grouping_missing_value_placeholder).to eq MISSING_NUMERIC_PLACEHOLDER
-            expect(schema.type_named("JsonSafeLong").grouping_missing_value_placeholder).to eq MISSING_NUMERIC_PLACEHOLDER
           end
 
           it "returns nil for object types" do
diff --git a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema_spec.rb b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema_spec.rb
index af1a8465e..2327ee1ad 100644
--- a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema_spec.rb
+++ b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema_spec.rb
@@ -16,7 +16,6 @@ class GraphQL
         define_schema do |schema|
           schema.scalar_type "_FieldSet" do |t|
             t.mapping type: "keyword"
-            t.json_schema type: "string"
           end
 
           schema.object_type "Widget" do |t|
diff --git a/elasticgraph-health_check/spec/unit/elastic_graph/health_check/health_checker_spec.rb b/elasticgraph-health_check/spec/unit/elastic_graph/health_check/health_checker_spec.rb
index f1ee584da..806dc7357 100644
--- a/elasticgraph-health_check/spec/unit/elastic_graph/health_check/health_checker_spec.rb
+++ b/elasticgraph-health_check/spec/unit/elastic_graph/health_check/health_checker_spec.rb
@@ -467,6 +467,7 @@ def build_health_checker(health_check:, latest: {}, index_definitions: nil, sche
           clients_by_name: datastore_clients_by_name,
           clock: class_double(::Time, now: now),
           schema_definition: schema_definition,
+          schema_definition_extension_modules: [],
           index_definitions: index_definitions,
           extension_settings: {"health_check" => health_check_settings}.compact
         )
diff --git a/elasticgraph-health_check/spec/unit/envoy_extension_spec.rb b/elasticgraph-health_check/spec/unit/envoy_extension_spec.rb
index acd06b7b7..b4f1c4782 100644
--- a/elasticgraph-health_check/spec/unit/envoy_extension_spec.rb
+++ b/elasticgraph-health_check/spec/unit/envoy_extension_spec.rb
@@ -145,7 +145,7 @@ def process(http_method, url, with_configured_path_segment:, body: nil, cluster_
 
         def build_graphql_for_path(http_path_segment)
           config = {http_path_segment: http_path_segment}.compact
-          schema_artifacts = generate_schema_artifacts do |schema|
+          schema_artifacts = generate_schema_artifacts(extension_modules: []) do |schema|
             schema.register_graphql_extension(EnvoyExtension, defined_at: "elastic_graph/health_check/envoy_extension", **config)
             schema.object_type "Widget" do |t|
               t.field "id", "ID"
diff --git a/elasticgraph-indexer/spec/support/multiple_version_support.rb b/elasticgraph-indexer/spec/support/multiple_version_support.rb
index f385d8cd4..9831a2174 100644
--- a/elasticgraph-indexer/spec/support/multiple_version_support.rb
+++ b/elasticgraph-indexer/spec/support/multiple_version_support.rb
@@ -6,6 +6,7 @@
 #
 # frozen_string_literal: true
 
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
 require "elastic_graph/spec_support/schema_definition_helpers"
 
 module ElasticGraph
@@ -15,7 +16,12 @@ class Indexer
 
       def build_indexer_with_multiple_schema_versions(schema_versions:)
         results_by_version = schema_versions.to_h do |json_schema_version, prior_def|
-          results = define_schema(schema_element_name_form: :snake_case, json_schema_version: json_schema_version, &prior_def)
+          results = define_schema(
+            schema_element_name_form: :snake_case,
+            json_schema_version: json_schema_version,
+            extension_modules: [JSONIngestion::SchemaDefinition::APIExtension],
+            &prior_def
+          )
           [json_schema_version, results]
         end
 
diff --git a/elasticgraph-indexer/spec/unit/elastic_graph/indexer/record_preparer_spec.rb b/elasticgraph-indexer/spec/unit/elastic_graph/indexer/record_preparer_spec.rb
index 1ecc94fc3..bfea42d70 100644
--- a/elasticgraph-indexer/spec/unit/elastic_graph/indexer/record_preparer_spec.rb
+++ b/elasticgraph-indexer/spec/unit/elastic_graph/indexer/record_preparer_spec.rb
@@ -7,6 +7,7 @@
 # frozen_string_literal: true
 
 require "elastic_graph/indexer/record_preparer"
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
 require "elastic_graph/spec_support/schema_definition_helpers"
 require "support/multiple_version_support"
 
diff --git a/elasticgraph-json_ingestion/spec/support/json_ingestion_schema_definition_helpers.rb b/elasticgraph-json_ingestion/spec/support/json_ingestion_schema_definition_helpers.rb
new file mode 100644
index 000000000..1cae87905
--- /dev/null
+++ b/elasticgraph-json_ingestion/spec/support/json_ingestion_schema_definition_helpers.rb
@@ -0,0 +1,28 @@
+# Copyright 2024 - 2026 Block, Inc.
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+#
+# frozen_string_literal: true
+
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
+require "elastic_graph/spec_support/schema_definition_helpers"
+
+# Extends the shared "SchemaDefinitionHelpers" context to automatically apply this gem's
+# `APIExtension` to every defined schema, since every spec in this gem exercises behavior
+# provided by that extension. Additional extension modules can still be passed via
+# `extension_modules:` and will be applied alongside it.
+::RSpec.shared_context "JSONIngestionSchemaDefinitionHelpers" do
+  include_context "SchemaDefinitionHelpers"
+
+  def define_schema_with_schema_elements(schema_elements, extension_modules: [], output: nil, **options, &block)
+    super(
+      schema_elements,
+      extension_modules: [::ElasticGraph::JSONIngestion::SchemaDefinition::APIExtension] | extension_modules,
+      output: output || log_device,
+      **options,
+      &block
+    )
+  end
+end
diff --git a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/json_schema_with_metadata_spec.rb b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/json_schema_with_metadata_spec.rb
index 021d71793..a116267d2 100644
--- a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/json_schema_with_metadata_spec.rb
+++ b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/json_schema_with_metadata_spec.rb
@@ -7,13 +7,13 @@
 # frozen_string_literal: true
 
 require "elastic_graph/json_ingestion/schema_definition/indexing/json_schema_with_metadata"
-require "elastic_graph/spec_support/schema_definition_helpers"
+require "support/json_ingestion_schema_definition_helpers"
 
 module ElasticGraph
   module JSONIngestion::SchemaDefinition
     module Indexing
       ::RSpec.describe JSONSchemaWithMetadata do
-        include_context "SchemaDefinitionHelpers"
+        include_context "JSONIngestionSchemaDefinitionHelpers"
 
         it "ignores derived indexed types that do not show up in the JSON schema" do
           v1_json_schema = dump_versioned_json_schema do |schema|
@@ -1056,7 +1056,6 @@ def metadata_for(json_schema, type, field)
         def define_schema(&schema_definition)
           super(
             schema_element_name_form: "snake_case",
-            extension_modules: [APIExtension],
             &schema_definition
           )
         end
diff --git a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/wrappers_spec.rb b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/wrappers_spec.rb
index feb265303..8d1ed751a 100644
--- a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/wrappers_spec.rb
+++ b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/indexing/wrappers_spec.rb
@@ -11,13 +11,13 @@
 require "elastic_graph/json_ingestion/schema_definition/indexing/field"
 require "elastic_graph/json_ingestion/schema_definition/indexing/field_reference"
 require "elastic_graph/json_ingestion/schema_definition/indexing/field_type/object"
-require "elastic_graph/spec_support/schema_definition_helpers"
+require "support/json_ingestion_schema_definition_helpers"
 require "support/json_schema_matcher"
 
 module ElasticGraph
   module JSONIngestion::SchemaDefinition
     ::RSpec.describe "JSON schema indexing wrappers" do
-      include_context "SchemaDefinitionHelpers"
+      include_context "JSONIngestionSchemaDefinitionHelpers"
 
       # `FieldReference#resolve` is a lazy reference: the referenced type need not exist when a field is
       # defined, only when artifacts are dumped. These two specs drive both outcomes (resolves / never
@@ -45,7 +45,11 @@ module JSONIngestion::SchemaDefinition
           })
         end
 
-        it "raises a clear error (rather than blowing up internally) for a field whose type never resolves" do
+        # `:dont_validate_graphql_schema` matters here: with `VALIDATE_GRAPHQL_SCHEMAS=1` (as on CI), the
+        # eager SDL validation would raise this error during GraphQL schema generation, before the JSON
+        # schema generation path exercises the wrapped `FieldReference#resolve` nil return that this
+        # example exists to cover.
+        it "raises a clear error (rather than blowing up internally) for a field whose type never resolves", :dont_validate_graphql_schema do
           # When a field references a type that is never defined, the wrapped `FieldReference#resolve`
           # returns `nil`. The schema definition machinery relies on that `nil` to detect the unresolvable
           # type and surface a helpful error instead of crashing.
diff --git a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_field_metadata_spec.rb b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_field_metadata_spec.rb
index 0ebfeafe0..b06ac7980 100644
--- a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_field_metadata_spec.rb
+++ b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_field_metadata_spec.rb
@@ -7,12 +7,12 @@
 # frozen_string_literal: true
 
 require "elastic_graph/json_ingestion/schema_definition/indexing/json_schema_field_metadata"
-require "elastic_graph/spec_support/schema_definition_helpers"
+require "support/json_ingestion_schema_definition_helpers"
 
 module ElasticGraph
   module JSONIngestion::SchemaDefinition
     ::RSpec.describe "JSON schema field metadata generation" do
-      include_context "SchemaDefinitionHelpers"
+      include_context "JSONIngestionSchemaDefinitionHelpers"
 
       it "generates no field metadata for built-in scalar and enum types" do
         metadata_by_type_and_field_name = dump_metadata
@@ -143,7 +143,6 @@ def dump_metadata(&schema_definition)
       def define_schema(&schema_definition)
         super(
           schema_element_name_form: "snake_case",
-          extension_modules: [APIExtension],
           &schema_definition
         )
       end
diff --git a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_pruner_spec.rb b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_pruner_spec.rb
index c8327661f..9a6b878c6 100644
--- a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_pruner_spec.rb
+++ b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_pruner_spec.rb
@@ -8,12 +8,12 @@
 
 require "elastic_graph/constants"
 require "elastic_graph/json_ingestion/schema_definition/json_schema_pruner"
-require "elastic_graph/spec_support/schema_definition_helpers"
+require "support/json_ingestion_schema_definition_helpers"
 
 module ElasticGraph
   module JSONIngestion::SchemaDefinition
     RSpec.describe JSONSchemaPruner do
-      include_context "SchemaDefinitionHelpers"
+      include_context "JSONIngestionSchemaDefinitionHelpers"
 
       describe ".prune" do
         subject { described_class.prune(schema) }
diff --git a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_spec.rb b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_spec.rb
index 1c964801d..507eaef43 100644
--- a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_spec.rb
+++ b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/json_schema_spec.rb
@@ -8,14 +8,13 @@
 
 require "elastic_graph/constants"
 require "elastic_graph/errors"
-require "elastic_graph/json_ingestion/schema_definition/api_extension"
-require "elastic_graph/spec_support/schema_definition_helpers"
+require "support/json_ingestion_schema_definition_helpers"
 require "support/json_schema_matcher"
 
 module ElasticGraph
   module JSONIngestion::SchemaDefinition
     ::RSpec.describe "JSON schema generation" do
-      include_context "SchemaDefinitionHelpers"
+      include_context "JSONIngestionSchemaDefinitionHelpers"
       json_schema_id = {"allOf" => [{"$ref" => "#/$defs/ID"}, {"maxLength" => DEFAULT_MAX_KEYWORD_LENGTH}]}
       json_schema_float = {"$ref" => "#/$defs/Float"}
       json_schema_integer = {"$ref" => "#/$defs/Int"}
@@ -113,10 +112,7 @@ module JSONIngestion::SchemaDefinition
         it "configures built-in scalar JSON schema before user schema blocks are evaluated" do
           json_schema_options_in_schema_block = nil
 
-          define_schema(
-            schema_element_name_form: "snake_case",
-            extension_modules: [JSONIngestion::SchemaDefinition::APIExtension]
-          ) do |s|
+          define_schema(schema_element_name_form: "snake_case") do |s|
             json_schema_options_in_schema_block = s.state.scalar_types_by_name.fetch("String").json_schema_options.dup
 
             s.object_type "Widget" do |t|
diff --git a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/schema_elements/scalar_type_extension_spec.rb b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/schema_elements/scalar_type_extension_spec.rb
index 364dfe2ee..8497479c6 100644
--- a/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/schema_elements/scalar_type_extension_spec.rb
+++ b/elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/schema_elements/scalar_type_extension_spec.rb
@@ -8,15 +8,16 @@
 
 require "elastic_graph/constants"
 require "elastic_graph/errors"
+require "elastic_graph/json_ingestion/schema_definition/api_extension"
 require "elastic_graph/json_ingestion/schema_definition/schema_elements/scalar_type_extension"
-require "elastic_graph/spec_support/schema_definition_helpers"
+require "support/json_ingestion_schema_definition_helpers"
 
 module ElasticGraph
   module JSONIngestion
     module SchemaDefinition
       module SchemaElements
         RSpec.describe ScalarTypeExtension do
-          include_context "SchemaDefinitionHelpers"
+          include_context "JSONIngestionSchemaDefinitionHelpers"
 
           it "requires custom scalar types to declare their JSON schema representation" do
             expect {
diff --git a/elasticgraph-query_interceptor/spec/unit/elastic_graph/query_interceptor/graphql_extension_spec.rb b/elasticgraph-query_interceptor/spec/unit/elastic_graph/query_interceptor/graphql_extension_spec.rb
index 8407e57c4..cf61aa6ba 100644
--- a/elasticgraph-query_interceptor/spec/unit/elastic_graph/query_interceptor/graphql_extension_spec.rb
+++ b/elasticgraph-query_interceptor/spec/unit/elastic_graph/query_interceptor/graphql_extension_spec.rb
@@ -170,7 +170,7 @@ def expect_configured_interceptors(graphql)
       end
 
       def generate_schema_artifacts
-        super do |schema|
+        super(extension_modules: []) do |schema|
           yield schema
 
           # Ensure there's at least one indexed type defined to avoid GraphQL validation errors.
diff --git a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/test_support.rb b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/test_support.rb
index 9dcc2533a..5a2402501 100644
--- a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/test_support.rb
+++ b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/test_support.rb
@@ -79,8 +79,10 @@ def define_schema_with_schema_elements(
         # exists when an extension that implements JSON schema generation (such as
         # `elasticgraph-json_ingestion`) is loaded; without one there is no JSON schema version to set.
         if !json_schema_version.nil? && api.respond_to?(:json_schema_version)
+          # :nocov: -- only entered when a JSON schema extension is loaded, which is not the case in elasticgraph-schema_definition's tests.
           versioned_api = api # : untyped
           versioned_api.json_schema_version(json_schema_version) if versioned_api.state.json_schema_version.nil?
+          # :nocov:
         end
 
         # :nocov: -- the else branch and code past this aren't used by tests in elasticgraph-schema_definition.
diff --git a/spec_support/lib/elastic_graph/spec_support/builds_datastore_core.rb b/spec_support/lib/elastic_graph/spec_support/builds_datastore_core.rb
index bca599c27..0edb7c041 100644
--- a/spec_support/lib/elastic_graph/spec_support/builds_datastore_core.rb
+++ b/spec_support/lib/elastic_graph/spec_support/builds_datastore_core.rb
@@ -29,6 +29,7 @@ def build_datastore_core(
       clusters: nil,
       schema_artifacts_directory: nil,
       schema_artifacts: nil,
+      schema_definition_extension_modules: [],
       datastore_backend: nil,
       reload_schema_artifacts: false,
       **config_overrides,
@@ -59,6 +60,7 @@ def build_datastore_core(
             schema_element_name_overrides: schema_element_name_overrides,
             derived_type_name_formats: derived_type_name_formats,
             enum_value_overrides_by_type: enum_value_overrides_by_type,
+            extension_modules: schema_definition_extension_modules,
             reload_schema_artifacts: reload_schema_artifacts,
             &schema_definition
           )
diff --git a/spec_support/lib/elastic_graph/spec_support/builds_indexer.rb b/spec_support/lib/elastic_graph/spec_support/builds_indexer.rb
index 23fa72092..33ae7fbf3 100644
--- a/spec_support/lib/elastic_graph/spec_support/builds_indexer.rb
+++ b/spec_support/lib/elastic_graph/spec_support/builds_indexer.rb
@@ -21,11 +21,18 @@ def build_indexer(
       datastore_router: nil,
       clock: nil,
       monotonic_clock: nil,
+      schema_definition_extension_modules: nil,
       **datastore_core_options,
       &customize_datastore_config
     )
+      datastore_core ||= build_datastore_core(
+        schema_definition_extension_modules: schema_definition_extension_modules || default_indexer_schema_definition_extension_modules,
+        **datastore_core_options,
+        &customize_datastore_config
+      )
+
       Indexer.new(
-        datastore_core: datastore_core || build_datastore_core(**datastore_core_options, &customize_datastore_config),
+        datastore_core: datastore_core,
         config: Indexer::Config.new(
           latency_slo_thresholds_by_timestamp_in_ms: latency_slo_thresholds_by_timestamp_in_ms,
           skip_derived_indexing_type_updates: skip_derived_indexing_type_updates
@@ -35,6 +42,17 @@ def build_indexer(
         monotonic_clock: monotonic_clock
       )
     end
+
+    private
+
+    # The indexer can only ingest JSON events today, so artifacts generated for an indexer must
+    # include the JSON schemas; this is the one spec builder that opts into the extension by default.
+    # The require is lazy so that gems whose bundles do not include `elasticgraph-json_ingestion`
+    # can still use this builder with an externally built `datastore_core:`.
+    def default_indexer_schema_definition_extension_modules
+      require "elastic_graph/json_ingestion/schema_definition/api_extension"
+      [JSONIngestion::SchemaDefinition::APIExtension]
+    end
   end
 
   RSpec.configure do |c|
diff --git a/spec_support/lib/elastic_graph/spec_support/schema_definition_helpers.rb b/spec_support/lib/elastic_graph/spec_support/schema_definition_helpers.rb
index d279fd460..de63f4e7d 100644
--- a/spec_support/lib/elastic_graph/spec_support/schema_definition_helpers.rb
+++ b/spec_support/lib/elastic_graph/spec_support/schema_definition_helpers.rb
@@ -8,49 +8,22 @@
 
 require "elastic_graph/schema_definition/test_support"
 
-module ElasticGraph
-  module SpecSupport
-    DEFAULT_SCHEMA_DEFINITION_EXTENSION_MODULES = begin
-      require "elastic_graph/json_ingestion/schema_definition/api_extension"
-      [::ElasticGraph::JSONIngestion::SchemaDefinition::APIExtension]
-    rescue LoadError => e
-      # :nocov: -- per-gem spec bundles may not include the optional `elasticgraph-json_ingestion` gem.
-      raise unless e.path == "elastic_graph/json_ingestion/schema_definition/api_extension"
-
-      []
-      # :nocov:
-    end.freeze
-  end
-end
-
 # Combines `:capture_logs` with `ElasicGraph::SchemaDefinition::TestSupport` in order
 # to silence log output and fail if any tests result in logged warnings.
 ::RSpec.shared_context "SchemaDefinitionHelpers", :capture_logs do
   include ::ElasticGraph::SchemaDefinition::TestSupport
 
-  # Defaults `extension_modules` and `output` for tests; all other options are forwarded to
-  # `TestSupport` unchanged. `output` must be handled with `||` (rather than a keyword default)
-  # because `TestSupport#define_schema` passes `output: nil` explicitly when no output is given.
-  def define_schema(extension_modules: default_schema_definition_extension_modules, output: nil, **options, &block)
-    super(
-      extension_modules: extension_modules,
-      output: output || log_device,
-      **options,
-      &block
-    )
-  end
-
-  def define_schema_with_schema_elements(schema_elements, extension_modules: default_schema_definition_extension_modules, output: nil, **options, &block)
-    super(
-      schema_elements,
-      extension_modules: extension_modules,
-      output: output || log_device,
-      **options,
-      &block
-    )
+  # Defaults `output` for tests; all other options are forwarded to `TestSupport` unchanged.
+  # `output` must be handled with `||` (rather than a keyword default) because
+  # `TestSupport#define_schema` passes `output: nil` explicitly when no output is given.
+  #
+  # Note: schema definition extension modules (such as the one from `elasticgraph-json_ingestion`)
+  # are intentionally not defaulted; specs that need an extension must opt in explicitly.
+  def define_schema(output: nil, **options, &block)
+    super(output: output || log_device, **options, &block)
   end
 
-  def default_schema_definition_extension_modules
-    ::ElasticGraph::SpecSupport::DEFAULT_SCHEMA_DEFINITION_EXTENSION_MODULES.dup
+  def define_schema_with_schema_elements(schema_elements, output: nil, **options, &block)
+    super(schema_elements, output: output || log_device, **options, &block)
   end
 end
diff --git a/spec_support/spec_helper.rb b/spec_support/spec_helper.rb
index 6de600e91..86191369f 100644
--- a/spec_support/spec_helper.rb
+++ b/spec_support/spec_helper.rb
@@ -356,7 +356,10 @@ def without_vcr
     end
     # :nocov:
 
+    # `extension_modules` intentionally defaults to none: specs must opt in explicitly to schema
+    # definition extensions (such as the one from `elasticgraph-json_ingestion`).
     def generate_schema_artifacts(
+      extension_modules: [],
       schema_element_name_form: :snake_case,
       schema_element_name_overrides: {},
       derived_type_name_formats: {},
@@ -364,16 +367,22 @@ def generate_schema_artifacts(
       reload_schema_artifacts: false
     )
       require "elastic_graph/schema_definition/test_support"
-      require "elastic_graph/spec_support/schema_definition_helpers"
       require "stringio"
 
+      unless block_given?
+        # When no block is given we load the repository's main test schema (`config/schema.rb`),
+        # which uses the JSON ingestion schema definition DSL, so it requires this extension.
+        require "elastic_graph/json_ingestion/schema_definition/api_extension"
+        extension_modules += [JSONIngestion::SchemaDefinition::APIExtension]
+      end
+
       output = ::StringIO.new # to silence warnings.
       ::ElasticGraph::SchemaDefinition::TestSupport.define_schema(
         schema_element_name_form: schema_element_name_form,
         schema_element_name_overrides: schema_element_name_overrides,
         derived_type_name_formats: derived_type_name_formats,
         enum_value_overrides_by_type: enum_value_overrides_by_type,
-        extension_modules: ::ElasticGraph::SpecSupport::DEFAULT_SCHEMA_DEFINITION_EXTENSION_MODULES.dup,
+        extension_modules: extension_modules,
         reload_schema_artifacts: reload_schema_artifacts,
         output: output
       ) do |schema|