From 9a9b215f0f83b15be8dc32fc66ef8897a7900243 Mon Sep 17 00:00:00 2001
From: Sichan Yoo <chanyoo@amazon.com>
Date: Wed, 13 May 2026 12:24:10 -0700
Subject: [PATCH 1/5] Initial changes

---
 .../lib/aws-sdk-code-generator.rb             |   2 +
 .../aws-sdk-code-generator/code_builder.rb    |  16 +-
 .../rbs/input_type_alias_collector.rb         | 156 ++++++++++++++++++
 .../rbs/keyword_argument_builder.rb           |  16 +-
 .../views/rbs/client_class.rb                 |   3 +
 .../views/rbs/params.rb                       |  43 +++++
 .../templates/rbs/params.mustache             |  11 ++
 7 files changed, 240 insertions(+), 7 deletions(-)
 create mode 100644 build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
 create mode 100644 build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
 create mode 100644 build_tools/aws-sdk-code-generator/templates/rbs/params.mustache

diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator.rb
index f724b685700..5a8e67ac0cf 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator.rb
@@ -83,6 +83,7 @@
 require_relative 'aws-sdk-code-generator/rbs/error_list'
 require_relative 'aws-sdk-code-generator/rbs/method_signature'
 require_relative 'aws-sdk-code-generator/rbs/keyword_argument_builder'
+require_relative 'aws-sdk-code-generator/rbs/input_type_alias_collector'
 require_relative 'aws-sdk-code-generator/rbs/resource_action'
 require_relative 'aws-sdk-code-generator/rbs/resource_association'
 require_relative 'aws-sdk-code-generator/rbs/resource_batch_action'
@@ -90,6 +91,7 @@
 require_relative 'aws-sdk-code-generator/rbs/waiter'
 require_relative 'aws-sdk-code-generator/views/rbs/async_client_class'
 require_relative 'aws-sdk-code-generator/views/rbs/client_class'
+require_relative 'aws-sdk-code-generator/views/rbs/params'
 require_relative 'aws-sdk-code-generator/views/rbs/errors_module'
 require_relative 'aws-sdk-code-generator/views/rbs/resource_class'
 require_relative 'aws-sdk-code-generator/views/rbs/root_resource_class'
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
index 1dcfcb6fa61..c6041c512b6 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
@@ -114,8 +114,17 @@ def rbs_files(options = {})
         prefix = options.fetch(:prefix, '')
         codegenerated_plugins = codegen_plugins(prefix)
         unless @service.h2_required_setting?
-          client_class = client_class_rbs(codegenerated_plugins)
+          collector = RBS::InputTypeAliasCollector.new(api: @service.api)
+          aliased_shapes = collector.shapes_to_alias
+          client_class = client_class_rbs(codegenerated_plugins, aliased_shapes)
           y.yield("#{prefix}/client.rbs", client_class.render)
+          if aliased_shapes.any?
+            y.yield("#{prefix}/params.rbs", Views::RBS::Params.new(
+              service_name: @service.name,
+              api: @service.api,
+              aliased_shapes: aliased_shapes
+            ).render)
+          end
           y.yield("#{prefix}/resource.rbs", Views::RBS::RootResourceClass.new(
             service_name: @service.name,
             client_class: client_class,
@@ -197,7 +206,7 @@ def client_class(codegenerated_plugins)
       ).render
     end
 
-    def client_class_rbs(codegenerated_plugins)
+    def client_class_rbs(codegenerated_plugins, aliased_shapes)
       Views::RBS::ClientClass.new(
         service_name: @service.name,
         codegenerated_plugins: codegenerated_plugins,
@@ -209,7 +218,8 @@ def client_class_rbs(codegenerated_plugins)
         protocol: @service.protocol,
         add_plugins: @service.add_plugins,
         remove_plugins: @service.remove_plugins,
-        protocol_settings: @service.protocol_settings
+        protocol_settings: @service.protocol_settings,
+        aliased_shapes: aliased_shapes
       )
     end
 
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
new file mode 100644
index 00000000000..024e524760e
--- /dev/null
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
@@ -0,0 +1,156 @@
+# frozen_string_literal: true
+
+module AwsSdkCodeGenerator
+  module RBS
+    # Collects structure shapes referenced more than once in input shape.
+    class InputTypeAliasCollector
+      def initialize(api:)
+        @api = api
+        @shape_usage_count = Hash.new(0)
+        @size_cache = {}
+      end
+      # Returns a topologically sorted array of shape names to render as type aliases.
+      # Leaf dependencies come first so aliases can reference each other.
+      # params.rbs uses aliases in this list to define RBS type aliases.
+      # client_class.rbs uses aliases in this list to deduplicate content.
+      def shapes_to_alias
+        count_shape_usage
+        aliased = @shape_usage_count.select do |shape_name, count|
+          shape = @api['shapes'][shape_name]
+          shape['type'] == 'structure' &&
+            count > 1 &&
+            rendered_rbs_line_count_heuristic(shape_name) > 5
+        end.keys.to_set
+        topological_sort(aliased)
+      end
+
+      private
+
+      def topological_sort(shape_names)
+        sorted = []
+        visited = Set.new
+        shape_names.each { |name| topo_visit(name, shape_names, visited, sorted) }
+        sorted
+      end
+
+      def topo_visit(name, shape_names, visited, sorted)
+        return if visited.include?(name)
+
+        visited << name
+        shape = @api['shapes'][name]
+        shape['members']&.each_value do |ref|
+          dep = ref['shape']
+          topo_visit(dep, shape_names, visited, sorted) if shape_names.include?(dep)
+        end
+        sorted << name
+      end
+
+      def count_shape_usage
+        @api['operations'].each_value do |op|
+          input_shape_name = op.dig('input', 'shape')
+          next unless input_shape_name
+
+          walk_shape(input_shape_name, Set.new)
+        end
+      end
+
+      def walk_shape(shape_name, ancestors)
+        return if ancestors.include?(shape_name)
+
+        ancestors += [shape_name]
+
+        shape = @api['shapes'][shape_name]
+        return unless shape && shape['type'] == 'structure'
+
+        shape['members']&.each_value do |member_ref|
+          member_shape_name = member_ref['shape']
+          member_shape = @api['shapes'][member_shape_name]
+          next unless member_shape
+
+          case member_shape['type']
+          when 'structure'
+            @shape_usage_count[member_shape_name] += 1
+            walk_shape(member_shape_name, ancestors)
+          when 'list'
+            walk_list(member_shape, ancestors)
+          when 'map'
+            walk_map(member_shape, ancestors)
+          end
+        end
+      end
+
+      def walk_list(list_shape, ancestors)
+        member_ref = list_shape['member']
+        member_shape = @api['shapes'][member_ref['shape']]
+        return unless member_shape
+
+        case member_shape['type']
+        when 'structure'
+          @shape_usage_count[member_ref['shape']] += 1
+          walk_shape(member_ref['shape'], ancestors)
+        when 'list'
+          walk_list(member_shape, ancestors)
+        when 'map'
+          walk_map(member_shape, ancestors)
+        end
+      end
+
+      def walk_map(map_shape, ancestors)
+        value_ref = map_shape['value']
+        value_shape = @api['shapes'][value_ref['shape']]
+        return unless value_shape
+
+        case value_shape['type']
+        when 'structure'
+          @shape_usage_count[value_ref['shape']] += 1
+          walk_shape(value_ref['shape'], ancestors)
+        when 'list'
+          walk_list(value_shape, ancestors)
+        when 'map'
+          walk_map(value_shape, ancestors)
+        end
+      end
+
+      def rendered_rbs_line_count_heuristic(shape_name, visited = Set.new)
+        return 0 if visited.include?(shape_name)
+
+        # Cache results to deduplicate calculation for nested structure types that get used multiple times in the model.
+        @size_cache ||= {}
+        return @size_cache[shape_name] if @size_cache.key?(shape_name)
+
+        visited += [shape_name]
+        shape = @api['shapes'][shape_name]
+        return @size_cache[shape_name] = 1 unless shape['members']
+
+        @size_cache[shape_name] = shape['members'].sum do |_, ref|
+          member_line_count(ref, visited)
+        end
+      end
+
+      def member_line_count(ref, visited)
+        child = @api['shapes'][ref['shape']]
+        case child['type']
+          # If it's a structure, add 2 lines then recurse (one line each for '{' and '}' in rendered RBS)
+        when 'structure' then 2 + rendered_rbs_line_count_heuristic(ref['shape'], visited)
+        when 'list'
+          member_shape = @api['shapes'][child['member']['shape']]
+          # If it's a list with structure member, add 2 lines & recurse.
+          #   (one line each for 'Hash[::String, {' and '}]' in rendered RBS)
+          # If it's a list with primitive member, add 1 and return
+          #   (e.g., renders as 1-liner, like 'list: Array[::String]')
+          member_shape['type'] == 'structure' ? 2 +
+            rendered_rbs_line_count_heuristic(child['member']['shape'], visited) : 1
+        when 'map'
+          value_shape = @api['shapes'][child['value']['shape']]
+          # If it's a map with structure member, add 2 lines & recurse.
+          #   (one line each for 'Hash[::String, {' and '}]' in rendered RBS)
+          # If it's a map with primitive member, add 1 and return
+          #   (e.g., renders as 1-liner, like 'map: Hash[::String, ::String]')
+          value_shape['type'] == 'structure' ? 2 +
+            rendered_rbs_line_count_heuristic(child['value']['shape'], visited) : 1
+        else 1
+        end
+      end
+    end
+  end
+end
\ No newline at end of file
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
index 87b0843aeff..f9ac1dacdcc 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
@@ -8,11 +8,13 @@ class KeywordArgumentBuilder
 
       attr_reader :newline
 
-      def initialize(api:, shape:, newline:, options: {})
+      def initialize(api:, shape:, newline:, options: {}, aliased_shapes: Set.new, alias_namespace: nil)
         @api = api
         @shape = shape
         @newline = newline
         @options = options
+        @aliased_shapes = aliased_shapes
+        @alias_namespace = alias_namespace
       end
 
       def format(indent: '')
@@ -69,10 +71,16 @@ def struct_member(struct, member_name, member_ref, i, visited, keyword:)
       def ref_value(ref, i, visited)
         if visited.include?(ref['shape'])
           return "untyped"
-        else
-          visited  = visited + [ref['shape']]
         end
 
+        # If this shape should be aliased, emit the alias reference
+        if @aliased_shapes.include?(ref['shape'])
+          alias_name = Underscore.underscore(ref['shape'])
+          return @alias_namespace ? "#{@alias_namespace}::#{alias_name}" : alias_name
+        end
+
+        visited  = visited + [ref['shape']]
+
         s = shape(ref)
         case s['type']
         when 'structure'
@@ -115,7 +123,7 @@ def scalar_list(member_ref, i, visited)
 
       def complex_list(member_ref, i, visited)
         newline_indent = newline ? "\n#{i}" : ""
-        "Array[#{newline_indent}#{more_indent}#{ref_value(member_ref, i + more_indent, visited)},#{newline_indent}]"
+        "Array[#{newline_indent}#{more_indent}#{ref_value(member_ref, i + more_indent, visited)}#{newline_indent}]"
       end
 
       def complex?(ref)
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb
index 778719cfd1a..dc81ba70c50 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb
@@ -24,6 +24,7 @@ def initialize(options)
           @codegenerated_plugins = options.fetch(:codegenerated_plugins)
           @waiters = AwsSdkCodeGenerator::RBS::Waiter.build_list(api: @api, waiters:options.fetch(:waiters))
           @protocol_settings = options.fetch(:protocol_settings, {})
+          @aliased_shapes = options.fetch(:aliased_shapes, Set.new).to_set
         end
 
         # @return [String|nil]
@@ -60,6 +61,8 @@ def operations
                 api: @api,
                 shape: input_shape,
                 newline: true,
+                aliased_shapes: @aliased_shapes,
+                alias_namespace: 'Params',
               )
               arguments = builder.format(indent: indent)
               include_required = input_shape["required"]&.empty?&.!
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
new file mode 100644
index 00000000000..40a3ac97ee0
--- /dev/null
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+module AwsSdkCodeGenerator
+  module Views
+    module RBS
+      class Params < View
+        include Helper
+
+        def initialize(options)
+          @service_name = options.fetch(:service_name)
+          @api = options.fetch(:api)
+          @aliased_shapes = options.fetch(:aliased_shapes)
+        end
+
+        def generated_src_warning
+          GENERATED_SRC_WARNING
+        end
+
+        def service_name
+          @service_name
+        end
+
+        def type_aliases
+          aliased_set = @aliased_shapes.to_set
+          @aliased_shapes.map do |shape_name|
+            shape = @api['shapes'][shape_name]
+            builder = AwsSdkCodeGenerator::RBS::KeywordArgumentBuilder.new(
+              api: @api,
+              shape: shape,
+              newline: true,
+              aliased_shapes: aliased_set - [shape_name],
+              alias_namespace: 'Params',
+            )
+            {
+              'name' => Underscore.underscore(shape_name),
+              'definition' => builder.struct(shape, '      ', []),
+            }
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/build_tools/aws-sdk-code-generator/templates/rbs/params.mustache b/build_tools/aws-sdk-code-generator/templates/rbs/params.mustache
new file mode 100644
index 00000000000..d6a81432d3a
--- /dev/null
+++ b/build_tools/aws-sdk-code-generator/templates/rbs/params.mustache
@@ -0,0 +1,11 @@
+{{generated_src_warning}}
+module Aws
+  module {{service_name}}
+    module Params
+      {{#type_aliases}}
+      type {{name}} = {{{definition}}}
+
+      {{/type_aliases}}
+    end
+  end
+end

From 25ccb4ee1768c1ae85fe24ba40c53e8b345b50f9 Mon Sep 17 00:00:00 2001
From: Sichan Yoo <chanyoo@amazon.com>
Date: Wed, 13 May 2026 13:23:50 -0700
Subject: [PATCH 2/5] Unit tests for InputTypeAliasCollector

---
 .../input_type_alias_collector_spec.rb        | 313 ++++++++++++++++++
 1 file changed, 313 insertions(+)
 create mode 100644 build_tools/aws-sdk-code-generator/spec/aws-sdk-code-generator/input_type_alias_collector_spec.rb

diff --git a/build_tools/aws-sdk-code-generator/spec/aws-sdk-code-generator/input_type_alias_collector_spec.rb b/build_tools/aws-sdk-code-generator/spec/aws-sdk-code-generator/input_type_alias_collector_spec.rb
new file mode 100644
index 00000000000..1ba543ed3ea
--- /dev/null
+++ b/build_tools/aws-sdk-code-generator/spec/aws-sdk-code-generator/input_type_alias_collector_spec.rb
@@ -0,0 +1,313 @@
+# frozen_string_literal: true
+
+require_relative '../spec_helper'
+
+module AwsSdkCodeGenerator
+  module RBS
+    describe InputTypeAliasCollector do
+      def api_with(shapes:, operations:)
+        { 'shapes' => shapes, 'operations' => operations }
+      end
+
+      def large_structure
+        {
+          'type' => 'structure',
+          'members' => {
+            'A' => { 'shape' => 'StringShape' },
+            'B' => { 'shape' => 'StringShape' },
+            'C' => { 'shape' => 'StringShape' },
+            'D' => { 'shape' => 'StringShape' },
+            'E' => { 'shape' => 'StringShape' },
+            'F' => { 'shape' => 'StringShape' }
+          }
+        }
+      end
+
+      def string_shape
+        { 'type' => 'string' }
+      end
+
+      describe '#shapes_to_alias' do
+        it 'returns empty when no shapes are reused' do
+          api = api_with(
+            shapes: {
+              'InputShape' => {
+                'type' => 'structure',
+                'members' => { 'Name' => { 'shape' => 'StringShape' } }
+              },
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'InputShape' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          expect(collector.shapes_to_alias).to eq([])
+        end
+
+        it 'returns empty when a structure is reused but too small' do
+          api = api_with(
+            shapes: {
+              'Input1' => {
+                'type' => 'structure',
+                'members' => { 'Cfg' => { 'shape' => 'SmallStruct' } }
+              },
+              'Input2' => {
+                'type' => 'structure',
+                'members' => { 'Cfg' => { 'shape' => 'SmallStruct' } }
+              },
+              'SmallStruct' => {
+                'type' => 'structure',
+                'members' => { 'X' => { 'shape' => 'StringShape' } }
+              },
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'Input1' } },
+              'Op2' => { 'input' => { 'shape' => 'Input2' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          expect(collector.shapes_to_alias).to eq([])
+        end
+
+        it 'detects a structure used more than once with sufficient size' do
+          api = api_with(
+            shapes: {
+              'Input1' => {
+                'type' => 'structure',
+                'members' => { 'Config' => { 'shape' => 'LargeStruct' } }
+              },
+              'Input2' => {
+                'type' => 'structure',
+                'members' => { 'Config' => { 'shape' => 'LargeStruct' } }
+              },
+              'LargeStruct' => large_structure,
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'Input1' } },
+              'Op2' => { 'input' => { 'shape' => 'Input2' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          expect(collector.shapes_to_alias).to eq(['LargeStruct'])
+        end
+
+        it 'detects structures nested inside lists' do
+          api = api_with(
+            shapes: {
+              'Input1' => {
+                'type' => 'structure',
+                'members' => { 'Items' => { 'shape' => 'ItemList' } }
+              },
+              'Input2' => {
+                'type' => 'structure',
+                'members' => { 'Items' => { 'shape' => 'ItemList' } }
+              },
+              'ItemList' => { 'type' => 'list', 'member' => { 'shape' => 'LargeStruct' } },
+              'LargeStruct' => large_structure,
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'Input1' } },
+              'Op2' => { 'input' => { 'shape' => 'Input2' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          expect(collector.shapes_to_alias).to eq(['LargeStruct'])
+        end
+
+        it 'detects structures nested inside maps' do
+          api = api_with(
+            shapes: {
+              'Input1' => {
+                'type' => 'structure',
+                'members' => { 'Data' => { 'shape' => 'DataMap' } }
+              },
+              'Input2' => {
+                'type' => 'structure',
+                'members' => { 'Data' => { 'shape' => 'DataMap' } }
+              },
+              'DataMap' => {
+                'type' => 'map',
+                'key' => { 'shape' => 'StringShape' },
+                'value' => { 'shape' => 'LargeStruct' }
+              },
+              'LargeStruct' => large_structure,
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'Input1' } },
+              'Op2' => { 'input' => { 'shape' => 'Input2' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          expect(collector.shapes_to_alias).to eq(['LargeStruct'])
+        end
+
+        it 'counts diamond references correctly' do
+          api = api_with(
+            shapes: {
+              'InputShape' => {
+                'type' => 'structure',
+                'members' => {
+                  'B' => { 'shape' => 'StructB' },
+                  'C' => { 'shape' => 'StructC' }
+                }
+              },
+              'StructB' => {
+                'type' => 'structure',
+                'members' => { 'D' => { 'shape' => 'LargeStruct' } }
+              },
+              'StructC' => {
+                'type' => 'structure',
+                'members' => { 'D' => { 'shape' => 'LargeStruct' } }
+              },
+              'LargeStruct' => large_structure,
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'InputShape' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          expect(collector.shapes_to_alias).to include('LargeStruct')
+        end
+
+        it 'does not infinite loop on recursive shapes' do
+          api = api_with(
+            shapes: {
+              'Input1' => {
+                'type' => 'structure',
+                'members' => { 'Node' => { 'shape' => 'RecursiveStruct' } }
+              },
+              'Input2' => {
+                'type' => 'structure',
+                'members' => { 'Node' => { 'shape' => 'RecursiveStruct' } }
+              },
+              'RecursiveStruct' => {
+                'type' => 'structure',
+                'members' => {
+                  'A' => { 'shape' => 'StringShape' },
+                  'B' => { 'shape' => 'StringShape' },
+                  'C' => { 'shape' => 'StringShape' },
+                  'D' => { 'shape' => 'StringShape' },
+                  'E' => { 'shape' => 'StringShape' },
+                  'F' => { 'shape' => 'StringShape' },
+                  'Children' => { 'shape' => 'RecursiveList' }
+                }
+              },
+              'RecursiveList' => { 'type' => 'list', 'member' => { 'shape' => 'RecursiveStruct' } },
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'Input1' } },
+              'Op2' => { 'input' => { 'shape' => 'Input2' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          expect(collector.shapes_to_alias).to include('RecursiveStruct')
+        end
+      end
+
+      describe 'topological ordering' do
+        it 'places leaf dependencies before their parents' do
+          api = api_with(
+            shapes: {
+              'Input1' => {
+                'type' => 'structure',
+                'members' => {
+                  'Parent' => { 'shape' => 'ParentStruct' },
+                  'Leaf' => { 'shape' => 'LeafStruct' }
+                }
+              },
+              'Input2' => {
+                'type' => 'structure',
+                'members' => {
+                  'Parent' => { 'shape' => 'ParentStruct' },
+                  'Leaf' => { 'shape' => 'LeafStruct' }
+                }
+              },
+              'ParentStruct' => {
+                'type' => 'structure',
+                'members' => {
+                  'Child' => { 'shape' => 'LeafStruct' },
+                  'A' => { 'shape' => 'StringShape' },
+                  'B' => { 'shape' => 'StringShape' },
+                  'C' => { 'shape' => 'StringShape' },
+                  'D' => { 'shape' => 'StringShape' },
+                  'E' => { 'shape' => 'StringShape' }
+                }
+              },
+              'LeafStruct' => large_structure,
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'Input1' } },
+              'Op2' => { 'input' => { 'shape' => 'Input2' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          result = collector.shapes_to_alias
+          expect(result.index('LeafStruct')).to be < result.index('ParentStruct')
+        end
+
+        it 'handles multi-level dependency chains' do
+          api = api_with(
+            shapes: {
+              'Input1' => {
+                'type' => 'structure',
+                'members' => {
+                  'Top' => { 'shape' => 'TopStruct' },
+                  'Mid' => { 'shape' => 'MidStruct' },
+                  'Bot' => { 'shape' => 'BotStruct' }
+                }
+              },
+              'Input2' => {
+                'type' => 'structure',
+                'members' => {
+                  'Top' => { 'shape' => 'TopStruct' },
+                  'Mid' => { 'shape' => 'MidStruct' },
+                  'Bot' => { 'shape' => 'BotStruct' }
+                }
+              },
+              'TopStruct' => {
+                'type' => 'structure',
+                'members' => {
+                  'Mid' => { 'shape' => 'MidStruct' },
+                  'A' => { 'shape' => 'StringShape' },
+                  'B' => { 'shape' => 'StringShape' },
+                  'C' => { 'shape' => 'StringShape' },
+                  'D' => { 'shape' => 'StringShape' },
+                  'E' => { 'shape' => 'StringShape' }
+                }
+              },
+              'MidStruct' => {
+                'type' => 'structure',
+                'members' => {
+                  'Bot' => { 'shape' => 'BotStruct' },
+                  'A' => { 'shape' => 'StringShape' },
+                  'B' => { 'shape' => 'StringShape' },
+                  'C' => { 'shape' => 'StringShape' },
+                  'D' => { 'shape' => 'StringShape' },
+                  'E' => { 'shape' => 'StringShape' }
+                }
+              },
+              'BotStruct' => large_structure,
+              'StringShape' => string_shape
+            },
+            operations: {
+              'Op1' => { 'input' => { 'shape' => 'Input1' } },
+              'Op2' => { 'input' => { 'shape' => 'Input2' } }
+            }
+          )
+          collector = InputTypeAliasCollector.new(api: api)
+          result = collector.shapes_to_alias
+          expect(result.index('BotStruct')).to be < result.index('MidStruct')
+          expect(result.index('MidStruct')).to be < result.index('TopStruct')
+        end
+      end
+    end
+  end
+end

From 006919e67312d733066879aa54306f34c1f9bcd3 Mon Sep 17 00:00:00 2001
From: Sichan Yoo <chanyoo@amazon.com>
Date: Thu, 14 May 2026 14:40:26 -0700
Subject: [PATCH 3/5] Address Juli's PR review comments

---
 .../aws-sdk-code-generator/code_builder.rb    | 30 +++++++++++--------
 .../rbs/input_type_alias_collector.rb         |  1 -
 .../rbs/keyword_argument_builder.rb           | 18 ++++++-----
 .../views/rbs/async_client_class.rb           |  3 +-
 .../views/rbs/client_class.rb                 |  3 +-
 .../views/rbs/params.rb                       |  5 ++--
 6 files changed, 33 insertions(+), 27 deletions(-)

diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
index c6041c512b6..ed9f8de5e48 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
@@ -113,18 +113,18 @@ def rbs_files(options = {})
       Enumerator.new do |y|
         prefix = options.fetch(:prefix, '')
         codegenerated_plugins = codegen_plugins(prefix)
+        type_alias_collector = RBS::InputTypeAliasCollector.new(api: @service.api)
+        aliased_shapes = type_alias_collector.shapes_to_alias
+        if aliased_shapes.any?
+          y.yield("#{prefix}/params.rbs", Views::RBS::Params.new(
+            service_name: @service.name,
+            api: @service.api,
+            options: { aliased_shapes: aliased_shapes }
+          ).render)
+        end
         unless @service.h2_required_setting?
-          collector = RBS::InputTypeAliasCollector.new(api: @service.api)
-          aliased_shapes = collector.shapes_to_alias
           client_class = client_class_rbs(codegenerated_plugins, aliased_shapes)
           y.yield("#{prefix}/client.rbs", client_class.render)
-          if aliased_shapes.any?
-            y.yield("#{prefix}/params.rbs", Views::RBS::Params.new(
-              service_name: @service.name,
-              api: @service.api,
-              aliased_shapes: aliased_shapes
-            ).render)
-          end
           y.yield("#{prefix}/resource.rbs", Views::RBS::RootResourceClass.new(
             service_name: @service.name,
             client_class: client_class,
@@ -133,7 +133,12 @@ def rbs_files(options = {})
             paginators: @service.paginators
           ).render)
         end
-        y.yield("#{prefix}/async_client.rbs", async_client_class_rbs(codegenerated_plugins).render) if @service.h2_setting?
+        if @service.h2_setting?
+          y.yield("#{prefix}/async_client.rbs", async_client_class_rbs(
+            codegenerated_plugins,
+            aliased_shapes
+          ).render)
+        end
         y.yield("#{prefix}/errors.rbs", Views::RBS::ErrorsModule.new(service: @service).render)
         if @waiters
           y.yield("#{prefix}/waiters.rbs", Views::RBS::WaitersModule.new(
@@ -243,7 +248,7 @@ def async_client_class(codegenerated_plugins)
       ).render
     end
 
-    def async_client_class_rbs(codegenerated_plugins)
+    def async_client_class_rbs(codegenerated_plugins, aliased_shapes)
       Views::RBS::AsyncClientClass.new(
         service_name: @service.name,
         codegenerated_plugins: codegenerated_plugins,
@@ -255,7 +260,8 @@ def async_client_class_rbs(codegenerated_plugins)
         add_plugins: @service.add_plugins,
         remove_plugins: @service.remove_plugins,
         protocol_settings: @service.protocol_settings,
-        async_client: true
+        async_client: true,
+        aliased_shapes: aliased_shapes
       )
     end
 
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
index 024e524760e..a8e7688b517 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
@@ -115,7 +115,6 @@ def rendered_rbs_line_count_heuristic(shape_name, visited = Set.new)
         return 0 if visited.include?(shape_name)
 
         # Cache results to deduplicate calculation for nested structure types that get used multiple times in the model.
-        @size_cache ||= {}
         return @size_cache[shape_name] if @size_cache.key?(shape_name)
 
         visited += [shape_name]
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
index f9ac1dacdcc..bc4c437b0fd 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
@@ -8,13 +8,11 @@ class KeywordArgumentBuilder
 
       attr_reader :newline
 
-      def initialize(api:, shape:, newline:, options: {}, aliased_shapes: Set.new, alias_namespace: nil)
+      def initialize(api:, shape:, newline:, options: {})
         @api = api
         @shape = shape
         @newline = newline
         @options = options
-        @aliased_shapes = aliased_shapes
-        @alias_namespace = alias_namespace
       end
 
       def format(indent: '')
@@ -26,6 +24,12 @@ def format(indent: '')
         result.join(joint)
       end
 
+      def format_as_alias(indent: '')
+        struct(@shape, indent, [])
+      end
+
+      private
+
       def struct(struct_shape, i, visited)
         members_str = struct_members(struct_shape, i, visited, keyword: false)
         result = ["{"]
@@ -69,14 +73,12 @@ def struct_member(struct, member_name, member_ref, i, visited, keyword:)
       end
 
       def ref_value(ref, i, visited)
-        if visited.include?(ref['shape'])
-          return "untyped"
-        end
+        return "untyped" if visited.include?(ref['shape'])
 
         # If this shape should be aliased, emit the alias reference
-        if @aliased_shapes.include?(ref['shape'])
+        if @options[:aliased_shapes]&.include?(ref['shape'])
           alias_name = Underscore.underscore(ref['shape'])
-          return @alias_namespace ? "#{@alias_namespace}::#{alias_name}" : alias_name
+          return "Params::#{alias_name}"
         end
 
         visited  = visited + [ref['shape']]
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/async_client_class.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/async_client_class.rb
index 68857bbfed8..c6dd618db75 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/async_client_class.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/async_client_class.rb
@@ -25,6 +25,7 @@ def initialize(options)
           @plugins = PluginList.new(options)
           @codegenerated_plugins = options.fetch(:codegenerated_plugins)
           @protocol_settings = options.fetch(:protocol_settings, {})
+          @aliased_shapes = options.fetch(:aliased_shapes, Set.new).to_set
         end
 
         # @return [String|nil]
@@ -68,7 +69,7 @@ def operations
                 api: @api,
                 shape: input_shape,
                 newline: true,
-                options: options
+                options: options.merge(aliased_shapes: @aliased_shapes)
               )
               arguments = builder.format(indent: indent)
               include_required = input_shape['required']&.empty?&.!
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb
index dc81ba70c50..2177f4db9dc 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/client_class.rb
@@ -61,8 +61,7 @@ def operations
                 api: @api,
                 shape: input_shape,
                 newline: true,
-                aliased_shapes: @aliased_shapes,
-                alias_namespace: 'Params',
+                options: { aliased_shapes: @aliased_shapes }
               )
               arguments = builder.format(indent: indent)
               include_required = input_shape["required"]&.empty?&.!
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
index 40a3ac97ee0..b267ad4afbf 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
@@ -28,12 +28,11 @@ def type_aliases
               api: @api,
               shape: shape,
               newline: true,
-              aliased_shapes: aliased_set - [shape_name],
-              alias_namespace: 'Params',
+              options: { aliased_shapes: aliased_set - [shape_name] }
             )
             {
               'name' => Underscore.underscore(shape_name),
-              'definition' => builder.struct(shape, '      ', []),
+              'definition' => builder.format_as_alias(indent: '    '),
             }
           end
         end

From 77bbfa66ca60952a3bbc9a07b6fc18f84ae7b02d Mon Sep 17 00:00:00 2001
From: Sichan Yoo <chanyoo@amazon.com>
Date: Thu, 14 May 2026 15:16:13 -0700
Subject: [PATCH 4/5] Address CodeQL scan warnings & compile error

---
 .../lib/aws-sdk-code-generator/code_builder.rb                  | 2 +-
 .../lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb  | 2 +-
 .../lib/aws-sdk-code-generator/views/rbs/params.rb              | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
index ed9f8de5e48..95e54308c9f 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/code_builder.rb
@@ -119,7 +119,7 @@ def rbs_files(options = {})
           y.yield("#{prefix}/params.rbs", Views::RBS::Params.new(
             service_name: @service.name,
             api: @service.api,
-            options: { aliased_shapes: aliased_shapes }
+            aliased_shapes: aliased_shapes
           ).render)
         end
         unless @service.h2_required_setting?
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
index bc4c437b0fd..475da3d3c0e 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/keyword_argument_builder.rb
@@ -77,7 +77,7 @@ def ref_value(ref, i, visited)
 
         # If this shape should be aliased, emit the alias reference
         if @options[:aliased_shapes]&.include?(ref['shape'])
-          alias_name = Underscore.underscore(ref['shape'])
+          alias_name = underscore(ref['shape'])
           return "Params::#{alias_name}"
         end
 
diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
index b267ad4afbf..a4be3b20c42 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/views/rbs/params.rb
@@ -31,7 +31,7 @@ def type_aliases
               options: { aliased_shapes: aliased_set - [shape_name] }
             )
             {
-              'name' => Underscore.underscore(shape_name),
+              'name' => underscore(shape_name),
               'definition' => builder.format_as_alias(indent: '    '),
             }
           end

From cad391a807ba08b38b5d06352ac171863454e84b Mon Sep 17 00:00:00 2001
From: Sichan Yoo <chanyoo@amazon.com>
Date: Tue, 19 May 2026 09:56:50 -0700
Subject: [PATCH 5/5] Use constant for min lines threshold for alias & add
 newline at the end of file.

---
 .../rbs/input_type_alias_collector.rb                      | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
index a8e7688b517..420ff10f852 100644
--- a/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
+++ b/build_tools/aws-sdk-code-generator/lib/aws-sdk-code-generator/rbs/input_type_alias_collector.rb
@@ -4,11 +4,14 @@ module AwsSdkCodeGenerator
   module RBS
     # Collects structure shapes referenced more than once in input shape.
     class InputTypeAliasCollector
+      MIN_LINES_THRESHOLD_FOR_ALIAS = 5
+
       def initialize(api:)
         @api = api
         @shape_usage_count = Hash.new(0)
         @size_cache = {}
       end
+
       # Returns a topologically sorted array of shape names to render as type aliases.
       # Leaf dependencies come first so aliases can reference each other.
       # params.rbs uses aliases in this list to define RBS type aliases.
@@ -19,7 +22,7 @@ def shapes_to_alias
           shape = @api['shapes'][shape_name]
           shape['type'] == 'structure' &&
             count > 1 &&
-            rendered_rbs_line_count_heuristic(shape_name) > 5
+            rendered_rbs_line_count_heuristic(shape_name) > MIN_LINES_THRESHOLD_FOR_ALIAS
         end.keys.to_set
         topological_sort(aliased)
       end
@@ -152,4 +155,4 @@ def member_line_count(ref, visited)
       end
     end
   end
-end
\ No newline at end of file
+end