diff --git a/AGENTS.md b/AGENTS.md
index f4f5bef06..ea25d3bfd 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -123,6 +123,10 @@ When adding filtering predicates or aggregation functions:
 3. **Query Translation**: Implement GraphQL → datastore query translation with unit, integration, and acceptance tests
 4. **Documentation**: Add user-facing docs with working examples to `config/site/`
 
+### Verification
+
+After completing a chunk of work, run `script/run_specs` and `script/type_check` to verify correctness across all gems and type signatures.
+
 ### Test Strategy
 
 Three layers of testing:
@@ -186,6 +190,10 @@ Custom gems can be added via `Gemfile-custom` (see `Gemfile-custom.example`), th
 
 ## Important Patterns
 
+### Schema Definition
+
+- When referencing derived type names (e.g. filter input types), never hardcode names like `"StringFilterInput"`. Always use `schema_def_state.type_ref("String").as_filter_input.name` (or similar `as_*` methods on type references). Hardcoded names break when schema element names are customized (e.g. camelCase schemas).
+
 ### Schema Artifacts
 After schema definition changes, always run:
 ```bash
diff --git a/config/schema/artifacts/runtime_metadata.yaml b/config/schema/artifacts/runtime_metadata.yaml
index 46005bf2f..ac8ea8f25 100644
--- a/config/schema/artifacts/runtime_metadata.yaml
+++ b/config/schema/artifacts/runtime_metadata.yaml
@@ -3820,6 +3820,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  DistributionChannelFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   DistributionChannelGroupedBy:
     graphql_fields_by_name:
       active:
@@ -4182,6 +4186,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  InventorFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   InventorGroupedBy:
     graphql_fields_by_name:
       name:
@@ -5098,6 +5106,8 @@ object_types_by_name:
           name: object_without_lookahead
   NamedEntityFilterInput:
     graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
       amount_cents2:
         name_in_index: amount_cents
       created_at2:
@@ -5422,6 +5432,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  NamedInventorFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   NamedInventorGroupedBy:
     graphql_fields_by_name:
       name:
@@ -5630,6 +5644,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  PartFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   PartGroupedBy:
     graphql_fields_by_name:
       created_at:
@@ -6195,6 +6213,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  RetailFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   RetailGroupedBy:
     graphql_fields_by_name:
       active:
@@ -6506,6 +6528,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  StoreFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   StoreGroupedBy:
     graphql_fields_by_name:
       active:
@@ -7540,6 +7566,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  WholesaleFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   WholesaleGroupedBy:
     graphql_fields_by_name:
       active:
@@ -8869,6 +8899,8 @@ object_types_by_name:
           name: object_without_lookahead
   WidgetOrAddressFilterInput:
     graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
       amount_cents2:
         name_in_index: amount_cents
       created_at2:
diff --git a/config/schema/artifacts/schema.graphql b/config/schema/artifacts/schema.graphql
index 024e43ebe..c34a4db96 100644
--- a/config/schema/artifacts/schema.graphql
+++ b/config/schema/artifacts/schema.graphql
@@ -2694,6 +2694,14 @@ Input type used to specify filters on `DistributionChannel` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input DistributionChannelFilterInput {
+  """
+  Filters `DistributionChannel` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -3968,6 +3976,14 @@ Input type used to specify filters on `Inventor` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input InventorFilterInput {
+  """
+  Filters `Inventor` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -6255,6 +6271,14 @@ Input type used to specify filters on `NamedEntity` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input NamedEntityFilterInput {
+  """
+  Filters `NamedEntity` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -7636,6 +7660,14 @@ Input type used to specify filters on `NamedInventor` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input NamedInventorFilterInput {
+  """
+  Filters `NamedInventor` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -7994,6 +8026,14 @@ Input type used to specify filters on `Part` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input PartFilterInput {
+  """
+  Filters `Part` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -11397,6 +11437,14 @@ Input type used to specify filters on `Retail` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input RetailFilterInput {
+  """
+  Filters `Retail` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -12494,6 +12542,14 @@ Input type used to specify filters on `Store` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input StoreFilterInput {
+  """
+  Filters `Store` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -15388,6 +15444,14 @@ Input type used to specify filters on `Wholesale` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input WholesaleFilterInput {
+  """
+  Filters `Wholesale` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -17708,6 +17772,14 @@ Input type used to specify filters on `WidgetOrAddress` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input WidgetOrAddressFilterInput {
+  """
+  Filters `WidgetOrAddress` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
diff --git a/config/schema/artifacts_with_apollo/runtime_metadata.yaml b/config/schema/artifacts_with_apollo/runtime_metadata.yaml
index dfb191604..458a3f6b3 100644
--- a/config/schema/artifacts_with_apollo/runtime_metadata.yaml
+++ b/config/schema/artifacts_with_apollo/runtime_metadata.yaml
@@ -3922,6 +3922,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  DistributionChannelFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   DistributionChannelGroupedBy:
     graphql_fields_by_name:
       active:
@@ -4284,6 +4288,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  InventorFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   InventorGroupedBy:
     graphql_fields_by_name:
       name:
@@ -5221,6 +5229,8 @@ object_types_by_name:
           name: object_without_lookahead
   NamedEntityFilterInput:
     graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
       amount_cents2:
         name_in_index: amount_cents
       created_at2:
@@ -5545,6 +5555,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  NamedInventorFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   NamedInventorGroupedBy:
     graphql_fields_by_name:
       name:
@@ -5753,6 +5767,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  PartFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   PartGroupedBy:
     graphql_fields_by_name:
       created_at:
@@ -6324,6 +6342,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  RetailFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   RetailGroupedBy:
     graphql_fields_by_name:
       active:
@@ -6635,6 +6657,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  StoreFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   StoreGroupedBy:
     graphql_fields_by_name:
       active:
@@ -7669,6 +7695,10 @@ object_types_by_name:
       node:
         resolver:
           name: object_without_lookahead
+  WholesaleFilterInput:
+    graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
   WholesaleGroupedBy:
     graphql_fields_by_name:
       active:
@@ -8998,6 +9028,8 @@ object_types_by_name:
           name: object_without_lookahead
   WidgetOrAddressFilterInput:
     graphql_fields_by_name:
+      _typename:
+        name_in_index: __typename
       amount_cents2:
         name_in_index: amount_cents
       created_at2:
diff --git a/config/schema/artifacts_with_apollo/schema.graphql b/config/schema/artifacts_with_apollo/schema.graphql
index 1418fd751..0a24a785f 100644
--- a/config/schema/artifacts_with_apollo/schema.graphql
+++ b/config/schema/artifacts_with_apollo/schema.graphql
@@ -2961,6 +2961,14 @@ Input type used to specify filters on `DistributionChannel` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input DistributionChannelFilterInput {
+  """
+  Filters `DistributionChannel` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -4250,6 +4258,14 @@ Input type used to specify filters on `Inventor` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input InventorFilterInput {
+  """
+  Filters `Inventor` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -6537,6 +6553,14 @@ Input type used to specify filters on `NamedEntity` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input NamedEntityFilterInput {
+  """
+  Filters `NamedEntity` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -7918,6 +7942,14 @@ Input type used to specify filters on `NamedInventor` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input NamedInventorFilterInput {
+  """
+  Filters `NamedInventor` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -8276,6 +8308,14 @@ Input type used to specify filters on `Part` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input PartFilterInput {
+  """
+  Filters `Part` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
@@ -11720,6 +11760,14 @@ Input type used to specify filters on `Retail` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input RetailFilterInput {
+  """
+  Filters `Retail` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -12817,6 +12865,14 @@ Input type used to specify filters on `Store` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input StoreFilterInput {
+  """
+  Filters `Store` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -15711,6 +15767,14 @@ Input type used to specify filters on `Wholesale` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input WholesaleFilterInput {
+  """
+  Filters `Wholesale` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Used to filter on the `active` field.
 
@@ -18031,6 +18095,14 @@ Input type used to specify filters on `WidgetOrAddress` fields.
 Will match all documents if passed as an empty object (or as `null`).
 """
 input WidgetOrAddressFilterInput {
+  """
+  Filters `WidgetOrAddress` records by concrete type. Only concrete type names are valid values —
+  filtering on an abstract type name will match nothing, since records only have concrete
+  type names for their `__typename` value.
+  Analogous to the `__typename` return field.
+  """
+  _typename: StringFilterInput
+
   """
   Matches records where all of the provided sub-filters evaluate to true. This works just like an AND operator in SQL.
 
diff --git a/elasticgraph-apollo/spec/unit/elastic_graph/apollo/schema_definition_spec.rb b/elasticgraph-apollo/spec/unit/elastic_graph/apollo/schema_definition_spec.rb
index e5b1f8171..558f1cb7a 100644
--- a/elasticgraph-apollo/spec/unit/elastic_graph/apollo/schema_definition_spec.rb
+++ b/elasticgraph-apollo/spec/unit/elastic_graph/apollo/schema_definition_spec.rb
@@ -1357,6 +1357,7 @@ def expect_identifiable_type_tagging_of_token(&type_def_for)
           # the tagging of those source fields. That's why `name`, `options1`, etc are tagged with `public` below.
           expect(type_def_for.call("IdentifiableFilterInput")).to eq(<<~EOS.strip)
             input IdentifiableFilterInput {
+              #{schema_elements._typename}: StringFilterInput
               #{schema_elements.all_of}: [IdentifiableFilterInput!]
               #{schema_elements.any_of}: [IdentifiableFilterInput!]
               id: IDFilterInput
diff --git a/elasticgraph-graphql/spec/acceptance/search_spec.rb b/elasticgraph-graphql/spec/acceptance/search_spec.rb
index 910471b7b..7c19ab980 100644
--- a/elasticgraph-graphql/spec/acceptance/search_spec.rb
+++ b/elasticgraph-graphql/spec/acceptance/search_spec.rb
@@ -799,6 +799,38 @@ module ElasticGraph
           .dig("data", case_correctly("store_aggregations"), "nodes", 0, case_correctly("count"))
         expect(store_agg_count).to eq(expected_store_typenames.size)
 
+        # `_typename` filter allows querying by concrete subtype across multiple indexes and
+        # branches of the type hierarchy. `DirectWholesaler` is in the shared `distribution_channels`
+        # index; `PhysicalStore` has a dedicated index using `constant_keyword` for `__typename`.
+        typename_key = case_correctly("_typename").to_sym
+        wholesaler_or_physical = list_distribution_channels_with(
+          *all_channel_fragments,
+          filter: {typename_key => {equal_to_any_of: ["DirectWholesaler", "PhysicalStore"]}}
+        )
+        expect(wholesaler_or_physical.map { |c| c["__typename"] }).to contain_exactly("DirectWholesaler", "PhysicalStore", "PhysicalStore")
+
+        # `_typename` filter interacts correctly with automatic `__typename` scoping at a sub-interface level.
+        # Filtering `retailers` to `OnlineStore OR PhysicalStore` returns all retailers (the full set),
+        # confirming both the shared-index and dedicated-index subtypes are matched correctly.
+        all_retailers = list_retailers_with(
+          *store_fragments,
+          filter: {typename_key => {equal_to_any_of: ["OnlineStore", "PhysicalStore"]}}
+        )
+        expect(all_retailers.map { |r| r["__typename"] }).to contain_exactly(*expected_store_typenames)
+
+        # `_typename` filter also works on aggregations, including across indexes.
+        wholesaler_or_physical_agg_count = call_graphql_query(<<~QUERY)
+          query {
+            #{case_correctly("distribution_channel_aggregations")}(filter: {
+              #{typename_key}: { #{case_correctly("equal_to_any_of")}: ["DirectWholesaler", "PhysicalStore"] }
+            }) {
+              nodes { #{case_correctly("count")} }
+            }
+          }
+        QUERY
+          .dig("data", case_correctly("distribution_channel_aggregations"), "nodes", 0, case_correctly("count"))
+        expect(wholesaler_or_physical_agg_count).to eq(wholesaler_or_physical.size)
+
         # all_highlights resolves against the concrete type (OnlineStore), not the abstract root
         # (DistributionChannel). OnlineStore.name is absent from DistributionChannel — without
         # __typename-aware type resolution the highlight would be silently dropped.
@@ -1401,6 +1433,16 @@ def expect_error_from(filter, *error_snippets)
             filter: {inventor: {stock_ticker_key => {not: {equal_to_any_of: [widget2.fetch(:inventor).fetch(stock_ticker_key)]}}}}
           )).to contain_exactly(expected_widget1)
 
+          typename_key = case_correctly("_typename").to_sym
+          # ...or on the subfield's _typename (single underscore, since `__` prefix is reserved by the GraphQL spec)
+          expect(list_widgets_with_options_and_inventor(
+            filter: {inventor: {typename_key => {equal_to_any_of: ["Company"]}}}
+          )).to contain_exactly(expected_widget2)
+
+          expect(list_widgets_with_options_and_inventor(
+            filter: {inventor: {typename_key => {equal_to_any_of: ["Person"]}}}
+          )).to contain_exactly(expected_widget1)
+
           # On interfaces you can filter on a subfield that is present on all subtypes...
           expect(list_widgets_with_options_and_inventor(
             filter: {named_inventor: {name: {equal_to_any_of: [widget1.fetch(:inventor).fetch(:name)]}}}
@@ -1420,12 +1462,14 @@ def expect_error_from(filter, *error_snippets)
             filter: {named_inventor: {stock_ticker_key => {not: {equal_to_any_of: [widget2.fetch(:inventor).fetch(stock_ticker_key)]}}}}
           )).to contain_exactly(expected_widget1)
 
-          # ...or on `__typename`. Well, you could if the GraphQL spec allowed input fields
-          # named `__typename`, but it does not (see http://spec.graphql.org/June2018/#sec-Input-Objects)
-          # so we do not yet support it.
-          # expect(list_widgets_with_options_and_inventor(
-          #   filter: { inventor: { __typename: { equal_to_any_of: ["Company"] } } }
-          # )).to contain_exactly(expected_widget2)
+          # ...or on the subfield's _typename (single underscore, since `__` prefix is reserved by the GraphQL spec)
+          expect(list_widgets_with_options_and_inventor(
+            filter: {named_inventor: {typename_key => {equal_to_any_of: ["Company"]}}}
+          )).to contain_exactly(expected_widget2)
+
+          expect(list_widgets_with_options_and_inventor(
+            filter: {named_inventor: {typename_key => {equal_to_any_of: ["Person"]}}}
+          )).to contain_exactly(expected_widget1)
         end
       end
 
diff --git a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_adapter/filters_spec.rb b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_adapter/filters_spec.rb
index 4b095961b..dd752083c 100644
--- a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_adapter/filters_spec.rb
+++ b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/query_adapter/filters_spec.rb
@@ -234,6 +234,25 @@ class QueryAdapter
           expect(query.internal_filters).to be_empty
         end
 
+        it "translates `_typename` filter field on an abstract type to `__typename` in the datastore query" do
+          query = datastore_query_for(:Query, :widgets, <<~QUERY)
+            query {
+              widgets(filter: {inventor: {_typename: {equal_to_any_of: ["Company"]}}}) {
+                edges {
+                  node {
+                    id
+                  }
+                }
+              }
+            }
+          QUERY
+
+          expect(query.client_filters).to contain_exactly({
+            "inventor" => {"__typename" => {"equal_to_any_of" => ["Company"]}}
+          })
+          expect(query.internal_filters).to be_empty
+        end
+
         it "translates `count` on a list field to `#{LIST_COUNTS_FIELD}` while leaving a `count` schema field unchanged" do
           query = datastore_query_for(:Query, :widgets, <<~QUERY)
             query {
diff --git a/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rb b/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rb
index 07932f95d..316da3382 100644
--- a/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rb
+++ b/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rb
@@ -128,7 +128,7 @@ module CamelCaseConverter
           extend self
 
           def normalize_case(name)
-            name.gsub(/_(\w)/) { $1.upcase }
+            name.gsub(/(?<=\w)_(\w)/) { $1.upcase }
           end
         end
 
@@ -141,7 +141,7 @@ def normalize_case(name)
       # @private
       SchemaElementNames = SchemaElementNamesDefinition.new(
         # Filter arg and operation names:
-        :filter,
+        :filter, :_typename,
         :equal_to_any_of, :gt, :gte, :lt, :lte, :matches_phrase, :matches_query, :matches_query_with_prefix, :any_of, :all_of, :not,
         :time_of_day, :any_satisfy, :contains, :starts_with, :all_substrings_of, :any_substring_of, :ignore_case, :any_prefix_of,
         # Directives
diff --git a/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rbs b/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rbs
index d12b861b7..255e2d8c0 100644
--- a/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rbs
+++ b/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names.rbs
@@ -17,6 +17,7 @@ module ElasticGraph
 
         def canonical_name_for: (::String | ::Symbol) -> ::Symbol
         attr_reader filter: ::String
+        attr_reader _typename: ::String
         attr_reader equal_to_any_of: ::String
         attr_reader gt: ::String
         attr_reader gte: ::String
diff --git a/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names_spec.rb b/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names_spec.rb
index e7277743b..b719ecc50 100644
--- a/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names_spec.rb
+++ b/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_element_names_spec.rb
@@ -12,12 +12,12 @@ module ElasticGraph
   module SchemaArtifacts
     module RuntimeMetadata
       ExampleElementNames = SchemaElementNamesDefinition.new(
-        :foo, :multi_word_snake, :multiWordCamel
+        :foo, :multi_word_snake, :multiWordCamel, :_leading_underscore
       )
 
       RSpec.describe SchemaElementNamesDefinition do
         it "exposes the set of element names via an `ELEMENT_NAMES` constant" do
-          expect(ExampleElementNames::ELEMENT_NAMES).to eq [:foo, :multi_word_snake, :multiWordCamel]
+          expect(ExampleElementNames::ELEMENT_NAMES).to eq [:foo, :multi_word_snake, :multiWordCamel, :_leading_underscore]
         end
 
         it "exposes camelCase element names when so configured, via snake case attributes" do
@@ -26,7 +26,8 @@ module RuntimeMetadata
           expect(names).to have_attributes(
             foo: "foo",
             multi_word_snake: "multiWordSnake",
-            multi_word_camel: "multiWordCamel"
+            multi_word_camel: "multiWordCamel",
+            _leading_underscore: "_leadingUnderscore"
           )
         end
 
@@ -36,7 +37,8 @@ module RuntimeMetadata
           expect(names).to have_attributes(
             foo: "foo",
             multi_word_snake: "multi_word_snake",
-            multi_word_camel: "multi_word_camel"
+            multi_word_camel: "multi_word_camel",
+            _leading_underscore: "_leading_underscore"
           )
         end
 
@@ -54,6 +56,7 @@ module RuntimeMetadata
           expect(names.normalize_case("foo_bar")).to eq "fooBar"
           expect(names.normalize_case("fooBar")).to eq "fooBar"
           expect(names.normalize_case("FooBar")).to eq "FooBar"
+          expect(names.normalize_case("_typename")).to eq "_typename"
         end
 
         it "allows overrides" do
diff --git a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/supports_filtering_and_aggregation.rb b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/supports_filtering_and_aggregation.rb
index a438b6fe9..5a67084fe 100644
--- a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/supports_filtering_and_aggregation.rb
+++ b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/supports_filtering_and_aggregation.rb
@@ -87,6 +87,17 @@ def to_input_filters
           return [] if does_not_support?(&:filterable?)
 
           schema_def_state.factory.build_standard_filter_input_types_for_index_object_type(name) do |t|
+            if abstract?
+              t.field schema_def_state.schema_elements._typename, schema_def_state.type_ref("String").as_filter_input.name, name_in_index: "__typename" do |f|
+                f.documentation <<~EOS
+                  Filters `#{name}` records by concrete type. Only concrete type names are valid values —
+                  filtering on an abstract type name will match nothing, since records only have concrete
+                  type names for their `__typename` value.
+                  Analogous to the `__typename` return field.
+                EOS
+              end
+            end
+
             graphql_fields_by_name.values.each do |field|
               if field.filterable?
                 t.graphql_fields_by_name[field.name] = field.to_filter_field(parent_type: t)
diff --git a/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/graphql_schema/filters_spec.rb b/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/graphql_schema/filters_spec.rb
index 0997a7ed2..d3181745e 100644
--- a/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/graphql_schema/filters_spec.rb
+++ b/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/graphql_schema/filters_spec.rb
@@ -1148,15 +1148,14 @@ module SchemaDefinition
               end
             end
 
-            # Note: we would like to support filtering on `__typename` but this is invalid according to the
-            # GraphQL Spec: http://spec.graphql.org/June2018/#sec-Input-Objects
-            # > For each input field of an Input Object type:
-            # > 2. The input field must not have a name which begins with the characters "__" (two underscores).
+            # Uses `_typename` (single underscore) since the GraphQL spec prohibits `__` prefix on input fields:
+            # http://spec.graphql.org/June2018/#sec-Input-Objects
             expect(filter_type_from(result, "Inventor")).to eq(<<~EOS.strip)
               input InventorFilterInput {
                 #{schema_elements.any_of}: [InventorFilterInput!]
                 #{schema_elements.all_of}: [InventorFilterInput!]
                 #{schema_elements.not}: InventorFilterInput
+                #{schema_elements._typename}: StringFilterInput
                 name: StringFilterInput
                 nationality: StringFilterInput
                 stock_ticker: StringFilterInput
@@ -1222,6 +1221,7 @@ module SchemaDefinition
                 #{schema_elements.any_of}: [ClothingItemFilterInput!]
                 #{schema_elements.all_of}: [ClothingItemFilterInput!]
                 #{schema_elements.not}: ClothingItemFilterInput
+                #{schema_elements._typename}: StringFilterInput
                 size: SizeFilterInput
                 shirt_color: StringFilterInput
                 pants_color: StringFilterInput
@@ -1279,6 +1279,7 @@ module SchemaDefinition
                 #{schema_elements.any_of}: [InventorFilterInput!]
                 #{schema_elements.all_of}: [InventorFilterInput!]
                 #{schema_elements.not}: InventorFilterInput
+                #{schema_elements._typename}: StringFilterInput
                 name: StringFilterInput
                 stock_ticker: StringFilterInput
               }
@@ -1340,6 +1341,7 @@ def link_supertype_to_subtypes(interface_type, *subtype_names)
                 #{schema_elements.any_of}: [InventorFilterInput!]
                 #{schema_elements.all_of}: [InventorFilterInput!]
                 #{schema_elements.not}: InventorFilterInput
+                #{schema_elements._typename}: StringFilterInput
                 name: StringFilterInput
                 nationality: StringFilterInput
                 stock_ticker: StringFilterInput