diff --git a/gems/smithy-cbor/lib/smithy-cbor/builder.rb b/gems/smithy-cbor/lib/smithy-cbor/builder.rb
index 76443366f..bee2d7c4b 100644
--- a/gems/smithy-cbor/lib/smithy-cbor/builder.rb
+++ b/gems/smithy-cbor/lib/smithy-cbor/builder.rb
@@ -61,7 +61,7 @@ def structure(shape, values)
           member_shape = members[member_name]
           next unless member_shape
 
-          data[@extension.wire_name(member_shape)] = build_shape(member_shape, value)
+          data[@extension.legacy_wire_name(member_shape)] = build_shape(member_shape, value)
         end
       end
 
@@ -71,12 +71,12 @@ def union(shape, values) # rubocop:disable Metrics/AbcSize
         data = {}
         if values.is_a?(Schema::Union)
           _name, member_shape = shape.target.member_by_type(values.class)
-          data[@extension.wire_name(member_shape)] = build_shape(member_shape, values.value)
+          data[@extension.legacy_wire_name(member_shape)] = build_shape(member_shape, values.value)
         else
           key, value = values.first
           if shape.target.member?(key)
             member_shape = shape.target.member(key)
-            data[@extension.wire_name(member_shape)] = build_shape(member_shape, value)
+            data[@extension.legacy_wire_name(member_shape)] = build_shape(member_shape, value)
           end
         end
         data
diff --git a/gems/smithy-cbor/lib/smithy-cbor/parser.rb b/gems/smithy-cbor/lib/smithy-cbor/parser.rb
index 85cf46015..b5e8d8216 100644
--- a/gems/smithy-cbor/lib/smithy-cbor/parser.rb
+++ b/gems/smithy-cbor/lib/smithy-cbor/parser.rb
@@ -54,7 +54,7 @@ def map(shape, values, result = nil)
 
       def structure(shape, values, result = nil)
         result = shape.target.type.new if result.nil?
-        index = @extension.member_index(shape.target)
+        index = @extension.wire_index(shape.target)
         values.each do |wire_name, value|
           next if value.nil?
 
@@ -68,7 +68,7 @@ def structure(shape, values, result = nil)
       end
 
       def union(shape, values, result = nil) # rubocop:disable Metrics/AbcSize
-        index = @extension.member_index(shape.target)
+        index = @extension.wire_index(shape.target)
         values.each do |wire_name, value|
           next if value.nil?
 
diff --git a/gems/smithy-json/lib/smithy-json/builder.rb b/gems/smithy-json/lib/smithy-json/builder.rb
index 8bbf36e70..99d880eed 100644
--- a/gems/smithy-json/lib/smithy-json/builder.rb
+++ b/gems/smithy-json/lib/smithy-json/builder.rb
@@ -9,7 +9,8 @@ class Builder
       include Smithy::Schema::Shapes
 
       def initialize(options = {})
-        @extension = options[:json_name] ? Smithy::Json::Extension : Smithy::Schema::Extension
+        @json_name = options[:json_name] || false
+        @extension = @json_name ? Smithy::Json::Extension : Smithy::Schema::Extension # TODO: removal
       end
 
       def build(shape, data)
@@ -73,7 +74,7 @@ def structure(shape, values)
           member_shape = members[member_name]
           next unless member_shape
 
-          data[@extension.wire_name(member_shape)] = build_shape(member_shape, value)
+          data[wire_name(member_shape)] = build_shape(member_shape, value)
         end
       end
 
@@ -94,16 +95,24 @@ def union(shape, values) # rubocop:disable Metrics/AbcSize
         data = {}
         if values.is_a?(Schema::Union)
           _name, member_shape = shape.target.member_by_type(values.class)
-          data[@extension.wire_name(member_shape)] = build_shape(member_shape, values.value)
+          data[wire_name(member_shape)] = build_shape(member_shape, values.value)
         else
           key, value = values.first
           if shape.target.member?(key)
             member_shape = shape.target.member(key)
-            data[@extension.wire_name(member_shape)] = build_shape(member_shape, value)
+            data[wire_name(member_shape)] = build_shape(member_shape, value)
           end
         end
         data
       end
+
+      def wire_name(member_shape)
+        if @json_name
+          @extension.wire_name(member_shape)
+        else
+          @extension.legacy_wire_name(member_shape)
+        end
+      end
     end
   end
 end
diff --git a/gems/smithy-json/lib/smithy-json/oj_engine.rb b/gems/smithy-json/lib/smithy-json/oj_engine.rb
index 3061929be..fc6232b23 100644
--- a/gems/smithy-json/lib/smithy-json/oj_engine.rb
+++ b/gems/smithy-json/lib/smithy-json/oj_engine.rb
@@ -14,7 +14,9 @@ def load(json)
         end
 
         def dump(value)
-          Oj.dump(value, mode: :compat)
+          # Avoid Oj's JSON mode calling Time#to_json with keywords rejected by
+          # recent json gem versions while preserving the existing time format.
+          Oj.dump(value, { mode: :custom, time_format: :ruby })
         end
       end
     end
diff --git a/gems/smithy-json/lib/smithy-json/parser.rb b/gems/smithy-json/lib/smithy-json/parser.rb
index 3dc25ada2..dcf6a2b1b 100644
--- a/gems/smithy-json/lib/smithy-json/parser.rb
+++ b/gems/smithy-json/lib/smithy-json/parser.rb
@@ -9,7 +9,8 @@ class Parser
       include Smithy::Schema::Shapes
 
       def initialize(options = {})
-        @extension = options[:json_name] ? Smithy::Json::Extension : Smithy::Schema::Extension
+        @json_name = options[:json_name] || false
+        @extension = @json_name ? Smithy::Json::Extension : Smithy::Schema::Extension # TODO: removal
       end
 
       def parse(shape, bytes, result = nil)
@@ -68,7 +69,7 @@ def structure(shape, values, result = nil)
         return if values.nil?
 
         result = shape.target.type.new if result.nil?
-        index = @extension.member_index(shape.target)
+        index = member_index(shape.target)
         values.each do |wire_name, value|
           next if value.nil?
 
@@ -95,7 +96,7 @@ def timestamp(value)
       end
 
       def union(shape, values, result = nil) # rubocop:disable Metrics/AbcSize
-        index = @extension.member_index(shape.target)
+        index = member_index(shape.target)
         values.each do |wire_name, value|
           next if value.nil?
 
@@ -115,6 +116,14 @@ def union(shape, values, result = nil) # rubocop:disable Metrics/AbcSize
       def sparse?(shape)
         @extension.sparse?(shape)
       end
+
+      def member_index(shape)
+        if @json_name
+          @extension.member_index(shape)
+        else
+          @extension.wire_index(shape)
+        end
+      end
     end
   end
 end
diff --git a/gems/smithy-schema/lib/smithy-schema/document_utils/deserializer.rb b/gems/smithy-schema/lib/smithy-schema/document_utils/deserializer.rb
index 28e7dbb75..e25692e14 100644
--- a/gems/smithy-schema/lib/smithy-schema/document_utils/deserializer.rb
+++ b/gems/smithy-schema/lib/smithy-schema/document_utils/deserializer.rb
@@ -114,7 +114,7 @@ def union(shape, values, result = nil) # rubocop:disable Metrics/AbcSize
         end
 
         def wire_name(member_shape)
-          Smithy::Schema::Extension.wire_name(member_shape)
+          Smithy::Schema::Extension.legacy_wire_name(member_shape)
         end
       end
     end
diff --git a/gems/smithy-schema/lib/smithy-schema/extension.rb b/gems/smithy-schema/lib/smithy-schema/extension.rb
index c9ba91086..5d0a2c281 100644
--- a/gems/smithy-schema/lib/smithy-schema/extension.rb
+++ b/gems/smithy-schema/lib/smithy-schema/extension.rb
@@ -2,43 +2,353 @@
 
 module Smithy
   module Schema
-    # Lookup helpers for protocol-agnostic serde using modeled member names.
+    # Cached schema metadata shared by Smithy protocol codecs.
     #
     # Raw Smithy trait data remains on +shape.traits+ and +member.traits+ with
-    # string keys. This module only provides generic modeled-name lookup
-    # helpers and memoizes shape-level indexes in metadata when that
-    # meaningfully avoids rebuilding them.
+    # string keys. This module resolves generic shape classification and
+    # modeled-member indexes. Protocol extensions own wire-specific metadata.
     # @api private
+    # rubocop:disable-next Metrics/ModuleLength
     module Extension
-      extend ExtensionHelpers
+      KEY = :schema
+
+      SHAPE_LIST = 1
+      SHAPE_MAP = 2
+      SHAPE_STRUCTURE = 3
+      SHAPE_UNION = 4
+      SHAPE_BLOB = 5
+      SHAPE_FLOAT = 6
+      SHAPE_TIMESTAMP = 7
+      SHAPE_BIG_DECIMAL = 8
+      SHAPE_BOOLEAN = 9
+      SHAPE_DOCUMENT = 10
+      SHAPE_ENUM = 11
+      SHAPE_INTEGER = 12
+      SHAPE_INT_ENUM = 13
+      SHAPE_STRING = 14
+
+      SHAPE_REF_BY_CLASS = {
+        Shapes::ListShape => SHAPE_LIST,
+        Shapes::MapShape => SHAPE_MAP,
+        Shapes::StructureShape => SHAPE_STRUCTURE,
+        Shapes::UnionShape => SHAPE_UNION,
+        Shapes::BlobShape => SHAPE_BLOB,
+        Shapes::FloatShape => SHAPE_FLOAT,
+        Shapes::TimestampShape => SHAPE_TIMESTAMP,
+        Shapes::BigDecimalShape => SHAPE_BIG_DECIMAL,
+        Shapes::BooleanShape => SHAPE_BOOLEAN,
+        Shapes::DocumentShape => SHAPE_DOCUMENT,
+        Shapes::EnumShape => SHAPE_ENUM,
+        Shapes::IntegerShape => SHAPE_INTEGER,
+        Shapes::IntEnumShape => SHAPE_INT_ENUM,
+        Shapes::StringShape => SHAPE_STRING
+      }.freeze
 
       class << self
-        # Returns the modeled member lookup index cached on the shape as
-        # +shape[:member_index]+.
+        # Returns the complete cached Schema metadata payload.
+        #
+        # Example:
+        #   Extension.fetch(shape)
+        #   # => { target_shape: Extension::SHAPE_STRUCTURE, ... }
+        def fetch(shape)
+          return shape[KEY] if shape.key?(KEY)
+
+          shape[KEY] =
+            case shape
+            when Shapes::OperationShape
+              build_operation_metadata(shape)
+            when Shapes::StructureShape, Shapes::UnionShape
+              build_aggregate_metadata(shape)
+            when Shapes::MemberShape
+              build_member_metadata(shape)
+            else
+              build_shape_metadata(shape)
+            end
+        end
+
+        # Returns the modeled wire-name lookup used by existing serde
+        # consumers. The index maps modeled member name to
+        # [ruby_member_name, member_shape, target_shape_ref].
         #
-        # The index maps:
-        # - modeled member name
-        # - to [ruby_member_name, member_shape]
+        # Example:
+        #   Extension.wire_index(shape)
+        #   # => { 'wireName' => [:ruby_name, member, Extension::SHAPE_STRING] }
+        def wire_index(shape)
+          fetch(shape)[:wire_index]
+        end
+
+        # Returns the canonical build lookup index. The index maps Ruby member
+        # name to [modeled_member_name, member_shape, target_shape_ref].
+        #
+        # Example:
+        #   Extension.member_index(shape)
+        #   # => { ruby_name: ['wireName', member, Extension::SHAPE_STRING] }
         def member_index(shape)
-          shape[:member_index] ||= build_member_index(shape)
+          fetch(shape)[:member_index]
+        end
+
+        # Returns a normalized reference for the target shape of +shape+.
+        # Bare shapes reference themselves, while member shapes reference
+        # their modeled target.
+        #
+        # Example:
+        #   Extension.target_shape(member)
+        #   # => Extension::SHAPE_STRING
+        def target_shape(shape)
+          fetch(shape)[:target_shape]
+        end
+
+        # Returns [member_shape, target_shape_ref, sparse] for a list.
+        #
+        # Example:
+        #   Extension.list_member(list)
+        #   # => [member, Extension::SHAPE_STRING, true]
+        def list_member(shape)
+          fetch(shape)[:list_member]
+        end
+
+        # Returns [member_shape, target_shape_ref] for a map key.
+        #
+        # Example:
+        #   Extension.map_key_member(map)
+        #   # => [member, Extension::SHAPE_STRING]
+        def map_key_member(shape)
+          fetch(shape)[:map_key_member]
+        end
+
+        # Returns [member_shape, target_shape_ref, sparse] for a map value.
+        #
+        # Example:
+        #   Extension.map_value_member(map)
+        #   # => [member, Extension::SHAPE_STRING, false]
+        def map_value_member(shape)
+          fetch(shape)[:map_value_member]
+        end
+
+        # Returns the modeled media type, when present.
+        #
+        # Example:
+        #   Extension.media_type(shape)
+        #   # => 'application/octet-stream'
+        def media_type(shape)
+          fetch(shape)[:media_type]
+        end
+
+        # Returns whether the sensitive trait is present.
+        def sensitive?(shape)
+          fetch(shape)[:sensitive]
+        end
+
+        # Returns whether the streaming trait is present.
+        def streaming?(shape)
+          fetch(shape)[:streaming]
+        end
+
+        # Returns whether the requires-length trait is present.
+        def requires_length?(shape)
+          fetch(shape)[:requires_length]
+        end
+
+        def endpoint_host_prefix(operation)
+          fetch(operation)[:endpoint_host_prefix]
+        end
+
+        def request_compression_encodings(operation)
+          fetch(operation)[:request_compression_encodings]
+        end
+
+        def checksum_required?(operation)
+          fetch(operation)[:checksum_required]
+        end
+
+        def long_polling?(operation)
+          fetch(operation)[:long_polling]
+        end
+
+        def unsigned_payload?(operation)
+          fetch(operation)[:unsigned_payload]
+        end
+
+        def required_members(shape)
+          fetch(shape).fetch(:required_members, [].freeze)
+        end
+
+        def host_label_index(shape)
+          fetch(shape).fetch(:host_label_index, {}.freeze)
+        end
+
+        def idempotency_token_member(shape)
+          fetch(shape)[:idempotency_token_member]
+        end
+
+        def streaming_member(shape)
+          fetch(shape)[:streaming_member]
+        end
+
+        def streaming_member_unknown_length(shape)
+          fetch(shape)[:streaming_member_unknown_length]
+        end
+
+        def event_stream_member(shape)
+          fetch(shape)[:event_stream_member]
+        end
+
+        # Returns the effective timestamp format, or +:default+ when the
+        # model does not select one.
+        #
+        # Example:
+        #   Extension.timestamp_format(member)
+        #   # => 'date-time'
+        def timestamp_format(shape)
+          fetch(shape).fetch(:timestamp_format, :default)
         end
 
-        # Returns the modeled member name for schema lookup.
-        def wire_name(member)
+        # Returns a modeled union's unknown-member type when present.
+        #
+        # Example:
+        #   Extension.unknown_member_type(union)
+        #   # => Types::Unknown
+        def unknown_member_type(shape)
+          fetch(shape)[:unknown_member_type]
+        end
+
+        # Iterates modeled members with separate Ruby name and member-shape
+        # arguments. With no block, returns the underlying enumerator.
+        #
+        # Example:
+        #   Extension.each_member(shape) { |name, member| ... }
+        def each_member(shape, &block)
+          return shape.members.each unless block
+
+          shape.members.each { |name, member| block.call(name, member) }
+        end
+
+        # TODO: Remove after the remaining legacy deserializer migrates to a
+        # resolved extension index.
+        def legacy_wire_name(member)
           member.name
         end
 
+        # Returns whether a collection may retain nil values.
+        #
+        # Example:
+        #   Extension.sparse?(list)
+        #   # => true
+        def sparse?(shape)
+          shape.traits.key?('smithy.api#sparse')
+        end
+
         private
 
-        def build_member_index(shape)
-          index = {}
-          shape.members.each do |name, member|
-            wire_name = wire_name(member)
-            next unless wire_name
+        def build_operation_metadata(operation)
+          traits = operation.traits
+          {
+            endpoint_host_prefix: traits.dig('smithy.api#endpoint', 'hostPrefix'),
+            request_compression_encodings: traits.dig('smithy.api#requestCompression', 'encodings'),
+            checksum_required: traits.key?('smithy.api#httpChecksumRequired') || nil,
+            long_polling: traits.key?('smithy.api#longPoll') || nil,
+            unsigned_payload: traits.key?('aws.auth#unsignedPayload') || nil
+          }.compact.freeze
+        end
+
+        def build_shape_metadata(shape)
+          target = shape.target
+          target_shape = SHAPE_REF_BY_CLASS[target.class]
+          metadata = { target_shape: target_shape }.compact
+          add_collection_metadata(metadata, shape) if target.equal?(shape)
+          add_media_type_metadata(metadata, shape)
+          add_boolean_trait_metadata(metadata, shape)
+          add_timestamp_metadata(metadata, shape)
+          metadata[:unknown_member_type] = shape.member_type(:unknown) if
+            target_shape == SHAPE_UNION && shape.member_type?(:unknown)
+          metadata.freeze
+        end
 
-            index[wire_name] = [name, member]
+        def build_member_metadata(member)
+          target_shape = SHAPE_REF_BY_CLASS[member.target.class] if member.target
+          metadata = { target_shape: target_shape }.compact
+          add_media_type_metadata(metadata, member)
+          add_boolean_trait_metadata(metadata, member)
+          add_timestamp_metadata(metadata, member)
+          metadata.freeze
+        end
+
+        # rubocop:disable-next Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/MethodLength, Metrics/PerceivedComplexity
+        def build_aggregate_metadata(shape)
+          metadata = build_shape_metadata(shape).dup
+          wire_index = {}
+          member_index = {}
+          required_members = []
+          host_label_index = {}
+
+          shape.members.each do |ruby_name, member|
+            modeled_name = member.name
+            next unless modeled_name
+
+            target_shape = fetch(member)[:target_shape]
+            wire_index[modeled_name] = [ruby_name, member, target_shape].freeze
+            member_index[ruby_name] = [modeled_name, member, target_shape].freeze
+            if member.traits.key?('smithy.api#required') &&
+               !member.traits.key?('smithy.api#clientOptional')
+              required_members << ruby_name
+            end
+            host_label_index[modeled_name] = ruby_name if member.traits.key?('smithy.api#hostLabel')
+            metadata[:idempotency_token_member] ||= ruby_name if member.traits.key?('smithy.api#idempotencyToken')
+            next unless streaming_trait?(member.target)
+
+            metadata[:streaming_member] ||= member
+            metadata[:event_stream_member] ||= member if target_shape == SHAPE_UNION
+            metadata[:streaming_member_unknown_length] ||= member unless requires_length_trait?(member.target)
           end
-          index.freeze
+
+          metadata[:wire_index] = wire_index.freeze
+          metadata[:member_index] = member_index.freeze
+          metadata[:required_members] = required_members.freeze
+          metadata[:host_label_index] = host_label_index.freeze
+          metadata.freeze
+        end
+
+        def add_collection_metadata(metadata, shape)
+          case shape
+          when Shapes::ListShape
+            metadata[:list_member] = member_metadata(shape.member, sparse?(shape))
+          when Shapes::MapShape
+            metadata[:map_key_member] = member_metadata(shape.key)
+            metadata[:map_value_member] = member_metadata(shape.value, sparse?(shape))
+          end
+        end
+
+        def add_timestamp_metadata(metadata, shape)
+          return unless metadata[:target_shape] == SHAPE_TIMESTAMP
+
+          metadata[:timestamp_format] =
+            shape.traits['smithy.api#timestampFormat'] ||
+            shape.target.traits['smithy.api#timestampFormat'] ||
+            :default
+        end
+
+        def add_media_type_metadata(metadata, shape)
+          media_type = shape.traits['smithy.api#mediaType']
+          metadata[:media_type] = media_type if media_type
+        end
+
+        def add_boolean_trait_metadata(metadata, shape)
+          metadata[:sensitive] = true if shape.traits.key?('smithy.api#sensitive')
+          metadata[:streaming] = true if streaming_trait?(shape)
+          metadata[:requires_length] = true if requires_length_trait?(shape)
+        end
+
+        def streaming_trait?(shape)
+          shape.traits.key?('smithy.api#streaming')
+        end
+
+        def requires_length_trait?(shape)
+          shape.traits.key?('smithy.api#requiresLength')
+        end
+
+        def member_metadata(member, sparse = nil)
+          target_shape = target_shape(member) if member
+          [member, target_shape, sparse].compact.freeze
         end
       end
     end
diff --git a/gems/smithy-schema/lib/smithy-schema/shapes.rb b/gems/smithy-schema/lib/smithy-schema/shapes.rb
index 75fcedee5..ece5abeee 100644
--- a/gems/smithy-schema/lib/smithy-schema/shapes.rb
+++ b/gems/smithy-schema/lib/smithy-schema/shapes.rb
@@ -32,6 +32,12 @@ def [](key)
           @metadata[key]
         end
 
+        # @param [Symbol] key
+        # @return [Boolean]
+        def key?(key)
+          @metadata.key?(key)
+        end
+
         # @param [Symbol] key
         # @param [Object] value
         def []=(key, value)
@@ -62,6 +68,12 @@ def [](key)
           @metadata[key]
         end
 
+        # @param [Symbol] key
+        # @return [Boolean]
+        def key?(key)
+          @metadata.key?(key)
+        end
+
         # @param [Symbol] key
         # @param [Object] value
         def []=(key, value)
@@ -212,6 +224,10 @@ def member(name)
       class ListShape < Shape
         # @return [MemberShape]
         attr_accessor :member
+
+        def add_member(_name, member_shape)
+          @member = member_shape
+        end
       end
 
       # Represents a Map shape.
@@ -221,6 +237,13 @@ class MapShape < Shape
 
         # @return [MemberShape]
         attr_accessor :value
+
+        def add_member(name, member_shape)
+          case name
+          when :key then @key = member_shape
+          when :value then @value = member_shape
+          end
+        end
       end
 
       # Represents a String shape.
diff --git a/gems/smithy-schema/sig/smithy-schema/extension.rbs b/gems/smithy-schema/sig/smithy-schema/extension.rbs
index 49f76edcc..3b79cac15 100644
--- a/gems/smithy-schema/sig/smithy-schema/extension.rbs
+++ b/gems/smithy-schema/sig/smithy-schema/extension.rbs
@@ -1,9 +1,33 @@
 module Smithy
   module Schema
     module Extension
-      def self.member_index: (untyped shape) -> Hash[String, [Symbol, Shapes::MemberShape]]
-      def self.wire_name: (Shapes::MemberShape member) -> String?
-      def self.sparse?: (untyped shape) -> bool
+      def self.fetch: ((Shapes::Shape | Shapes::MemberShape) shape) -> Hash[Symbol, untyped]
+      def self.wire_index: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Hash[String, [Symbol, Shapes::MemberShape, Integer?]]
+      def self.member_index: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Hash[Symbol, [String, Shapes::MemberShape, Integer?]]
+      def self.target_shape: ((Shapes::Shape | Shapes::MemberShape) shape) -> Integer?
+      def self.list_member: (Shapes::ListShape shape) -> Array[untyped]
+      def self.map_key_member: (Shapes::MapShape shape) -> Array[untyped]
+      def self.map_value_member: (Shapes::MapShape shape) -> Array[untyped]
+      def self.media_type: ((Shapes::Shape | Shapes::MemberShape) shape) -> String?
+      def self.sensitive?: ((Shapes::Shape | Shapes::MemberShape) shape) -> bool?
+      def self.streaming?: ((Shapes::Shape | Shapes::MemberShape) shape) -> bool?
+      def self.requires_length?: ((Shapes::Shape | Shapes::MemberShape) shape) -> bool?
+      def self.endpoint_host_prefix: (Shapes::OperationShape operation) -> String?
+      def self.request_compression_encodings: (Shapes::OperationShape operation) -> Array[String]?
+      def self.checksum_required?: (Shapes::OperationShape operation) -> bool?
+      def self.long_polling?: (Shapes::OperationShape operation) -> bool?
+      def self.unsigned_payload?: (Shapes::OperationShape operation) -> bool?
+      def self.required_members: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Array[Symbol]
+      def self.host_label_index: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Hash[String, Symbol]
+      def self.idempotency_token_member: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Symbol?
+      def self.streaming_member: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Shapes::MemberShape?
+      def self.streaming_member_unknown_length: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Shapes::MemberShape?
+      def self.event_stream_member: ((Shapes::StructureShape | Shapes::UnionShape) shape) -> Shapes::MemberShape?
+      def self.timestamp_format: ((Shapes::Shape | Shapes::MemberShape) shape) -> (String | Symbol)
+      def self.unknown_member_type: (Shapes::UnionShape shape) -> untyped
+      def self.each_member: ((Shapes::StructureShape | Shapes::UnionShape) shape) ?{ (Symbol, Shapes::MemberShape) -> void } -> untyped
+      def self.legacy_wire_name: (Shapes::MemberShape member) -> String?
+      def self.sparse?: ((Shapes::Shape | Shapes::MemberShape) shape) -> bool
     end
   end
 end
diff --git a/gems/smithy-schema/sig/smithy-schema/shapes.rbs b/gems/smithy-schema/sig/smithy-schema/shapes.rbs
index bedf1d5fe..38ed6e212 100644
--- a/gems/smithy-schema/sig/smithy-schema/shapes.rbs
+++ b/gems/smithy-schema/sig/smithy-schema/shapes.rbs
@@ -9,6 +9,7 @@ module Smithy
         attr_accessor name: String
         attr_accessor traits: Hash[String, untyped]
         def []: (Symbol) -> Object
+        def key?: (Symbol) -> bool
         def []=: (Symbol, Object) -> void
       end
 
@@ -19,6 +20,7 @@ module Smithy
         attr_accessor name: String?
         attr_accessor traits: Hash[String, untyped]
         def []: (Symbol) -> Object
+        def key?: (Symbol) -> bool
         def []=: (Symbol, Object) -> void
       end
 
@@ -71,12 +73,14 @@ module Smithy
       end
 
       class ListShape < Shape
-        attr_accessor member: MemberShape
+        attr_accessor member: MemberShape?
+        def add_member: (Symbol, MemberShape) -> MemberShape
       end
 
       class MapShape < Shape
-        attr_accessor key: MemberShape
-        attr_accessor value: MemberShape
+        attr_accessor key: MemberShape?
+        attr_accessor value: MemberShape?
+        def add_member: (Symbol, MemberShape) -> MemberShape?
       end
 
       class StringShape < Shape
diff --git a/gems/smithy-schema/spec/smithy-schema/extension_spec.rb b/gems/smithy-schema/spec/smithy-schema/extension_spec.rb
index ea18aa533..0b554ed40 100644
--- a/gems/smithy-schema/spec/smithy-schema/extension_spec.rb
+++ b/gems/smithy-schema/spec/smithy-schema/extension_spec.rb
@@ -5,31 +5,43 @@
 module Smithy
   module Schema
     describe Extension do
-      describe '.member_index' do
+      describe '.wire_index' do
         let(:shape) { Shapes::StructureShape.new }
         let(:member) { Shapes::MemberShape.new(target: Shapes::StringShape.new, name: 'wireName') }
 
         it 'returns a frozen member index keyed by member name' do
           shape.add_member(:some_member, member)
 
-          expect(described_class.member_index(shape)).to eq('wireName' => [:some_member, member])
-          expect(described_class.member_index(shape)).to be_frozen
+          expected_values = [:some_member, member, described_class::SHAPE_STRING]
+          expect(described_class.wire_index(shape)).to eq('wireName' => expected_values)
+          expect(described_class.wire_index(shape)).to be_frozen
         end
 
         it 'ignores members that do not have a modeled member name' do
           shape.add_member(:missing_name, Shapes::MemberShape.new(target: Shapes::StringShape.new))
 
-          expect(described_class.member_index(shape)).to eq({})
+          expect(described_class.wire_index(shape)).to eq({})
         end
 
         it 'memoizes the index on the shape metadata' do
           shape.add_member(:some_member, member)
 
-          expect(described_class.member_index(shape)).to be(described_class.member_index(shape))
+          expect(described_class.wire_index(shape)).to be(described_class.wire_index(shape))
+        end
+      end
+
+      describe '.member_index' do
+        it 'returns a frozen member index keyed by Ruby member name' do
+          member = Shapes::MemberShape.new(target: Shapes::StringShape.new, name: 'wireName')
+          shape = Shapes::StructureShape.new
+          shape.add_member(:some_member, member)
+
+          expected_values = ['wireName', member, described_class::SHAPE_STRING]
+          expect(described_class.member_index(shape)).to eq(some_member: expected_values)
         end
       end
 
-      describe '.wire_name' do
+      describe '.legacy_wire_name' do
         it 'returns the model name' do
           member = Shapes::MemberShape.new(
             target: Shapes::StringShape.new,
@@ -37,19 +49,59 @@ module Schema
             traits: { 'smithy.api#jsonName' => 'jsonWireName' }
           )
 
-          expect(described_class.wire_name(member)).to eq('wireName')
+          expect(described_class.legacy_wire_name(member)).to eq('wireName')
         end
       end
 
       describe '.sparse?' do
-        it 'returns true when the sparse trait is present' do
-          shape = Shapes::ListShape.new(traits: { 'smithy.api#sparse' => {} })
+        it 'returns whether the sparse trait is present' do
+          expect(described_class.sparse?(Shapes::ListShape.new)).to be(false)
+          expect(described_class.sparse?(Shapes::ListShape.new(traits: { 'smithy.api#sparse' => {} }))).to be(true)
+        end
+      end
 
-          expect(described_class.sparse?(shape)).to be(true)
+      describe 'generic shape metadata' do
+        it 'classifies target shapes' do
+          expect(described_class.target_shape(Shapes::BlobShape.new)).to eq(described_class::SHAPE_BLOB)
+          expect(described_class.target_shape(Shapes::FloatShape.new)).to eq(described_class::SHAPE_FLOAT)
+          expect(described_class.target_shape(Shapes::ListShape.new)).to eq(described_class::SHAPE_LIST)
         end
 
-        it 'returns false when the sparse trait is absent' do
-          expect(described_class.sparse?(Shapes::ListShape.new)).to be(false)
+        it 'caches collection members and sparse metadata' do
+          list = Shapes::ListShape.new(traits: { 'smithy.api#sparse' => {} })
+          member = Shapes::MemberShape.new(target: Shapes::StringShape.new)
+          list.add_member(:member, member)
+
+          expect(described_class.list_member(list)).to eq([member, described_class::SHAPE_STRING, true])
+        end
+
+        it 'resolves a member timestamp format before its target format' do
+          timestamp = Shapes::TimestampShape.new(
+            traits: { 'smithy.api#timestampFormat' => 'date-time' }
+          )
+          member = Shapes::MemberShape.new(
+            target: timestamp,
+            traits: { 'smithy.api#timestampFormat' => 'http-date' }
+          )
+
+          expect(described_class.timestamp_format(member)).to eq('http-date')
+          expect(described_class.timestamp_format(Shapes::TimestampShape.new)).to eq(:default)
+        end
+
+        it 'caches a modeled media type using the Smithy trait key' do
+          shape = Shapes::BlobShape.new(
+            traits: { 'smithy.api#mediaType' => 'application/custom' }
+          )
+
+          expect(described_class.media_type(shape)).to eq('application/custom')
+        end
+
+        it 'caches an unknown union member type when present' do
+          union = Shapes::UnionShape.new
+          unknown_type = Class.new
+          union.add_member(:unknown, unknown_type, Shapes::MemberShape.new)
+
+          expect(described_class.unknown_member_type(union)).to be(unknown_type)
         end
       end
     end
diff --git a/gems/smithy-schema/spec/spec_helper.rb b/gems/smithy-schema/spec/spec_helper.rb
index a42d47ea6..1cc21da28 100644
--- a/gems/smithy-schema/spec/spec_helper.rb
+++ b/gems/smithy-schema/spec/spec_helper.rb
@@ -2,12 +2,12 @@
 
 require 'simplecov'
 SimpleCov.start do
-  add_filter '/spec/'
-  add_filter 'gems/smithy/'
-  add_filter 'gems/smithy-cbor/'
-  add_filter 'gems/smithy-client/'
-  add_filter 'gems/smithy-json/'
-  add_filter 'gems/smithy-xml/'
+  skip '/spec/'
+  skip 'gems/smithy/'
+  skip 'gems/smithy-cbor/'
+  skip 'gems/smithy-client/'
+  skip 'gems/smithy-json/'
+  skip 'gems/smithy-xml/'
 end
 
 require 'smithy'
diff --git a/gems/smithy/lib/smithy/templates/client/schema.erb b/gems/smithy/lib/smithy/templates/client/schema.erb
index 4c4bd1251..8627474de 100644
--- a/gems/smithy/lib/smithy/templates/client/schema.erb
+++ b/gems/smithy/lib/smithy/templates/client/schema.erb
@@ -17,10 +17,10 @@ module <%= module_name %>
     <%= shape.name %>.add_member(:<%= member.ruby_name %>, <%= member.initializer %>)
 <% end -%>
 <% when 'list' -%>
-    <%= shape.name %>.member = <%= shape.member.initializer %>
+    <%= shape.name %>.add_member(:member, <%= shape.member.initializer %>)
 <% when 'map' -%>
-    <%= shape.name %>.key = <%= shape.key.initializer %>
-    <%= shape.name %>.value = <%= shape.value.initializer %>
+    <%= shape.name %>.add_member(:key, <%= shape.key.initializer %>)
+    <%= shape.name %>.add_member(:value, <%= shape.value.initializer %>)
 <% when 'structure' -%>
 <% shape.members.each do |member| -%>
     <%= shape.name %>.add_member(:<%= member.ruby_name %>, <%= member.initializer %>)
diff --git a/projections/shapes/lib/shapes/schema.rb b/projections/shapes/lib/shapes/schema.rb
index 6a807dec5..464aa2e09 100644
--- a/projections/shapes/lib/shapes/schema.rb
+++ b/projections/shapes/lib/shapes/schema.rb
@@ -30,9 +30,9 @@ module Schema
 
     Enum.add_member(:foo, ::Smithy::Schema::Shapes::MemberShape.new(target: ::Smithy::Schema::Shapes::Prelude::Unit, name: "FOO", traits: {"smithy.api#enumValue" => "bar"}))
     IntEnum.add_member(:baz, ::Smithy::Schema::Shapes::MemberShape.new(target: ::Smithy::Schema::Shapes::Prelude::Unit, name: "BAZ", traits: {"smithy.api#enumValue" => 1}))
-    List.member = ::Smithy::Schema::Shapes::MemberShape.new(target: String, name: "member", traits: {"smithy.ruby.tests#shape" => {}})
-    Map.key = ::Smithy::Schema::Shapes::MemberShape.new(target: String, name: "key", traits: {"smithy.ruby.tests#shape" => {}})
-    Map.value = ::Smithy::Schema::Shapes::MemberShape.new(target: String, name: "value", traits: {"smithy.ruby.tests#shape" => {}})
+    List.add_member(:member, ::Smithy::Schema::Shapes::MemberShape.new(target: String, name: "member", traits: {"smithy.ruby.tests#shape" => {}}))
+    Map.add_member(:key, ::Smithy::Schema::Shapes::MemberShape.new(target: String, name: "key", traits: {"smithy.ruby.tests#shape" => {}}))
+    Map.add_member(:value, ::Smithy::Schema::Shapes::MemberShape.new(target: String, name: "value", traits: {"smithy.ruby.tests#shape" => {}}))
     OperationInput.add_member(:blob, ::Smithy::Schema::Shapes::MemberShape.new(target: Blob, name: "blob"))
     OperationInput.add_member(:boolean, ::Smithy::Schema::Shapes::MemberShape.new(target: Boolean, name: "boolean"))
     OperationInput.add_member(:string, ::Smithy::Schema::Shapes::MemberShape.new(target: String, name: "string"))
diff --git a/projections/weather/lib/weather/schema.rb b/projections/weather/lib/weather/schema.rb
index 40c7c2248..cb1a1a078 100644
--- a/projections/weather/lib/weather/schema.rb
+++ b/projections/weather/lib/weather/schema.rb
@@ -22,7 +22,7 @@ module Schema
     CityCoordinates.add_member(:latitude, ::Smithy::Schema::Shapes::MemberShape.new(target: ::Smithy::Schema::Shapes::Prelude::Float, name: "latitude", traits: {"smithy.api#required" => {}}))
     CityCoordinates.add_member(:longitude, ::Smithy::Schema::Shapes::MemberShape.new(target: ::Smithy::Schema::Shapes::Prelude::Float, name: "longitude", traits: {"smithy.api#required" => {}}))
     CityCoordinates.type = Types::CityCoordinates
-    CitySummaries.member = ::Smithy::Schema::Shapes::MemberShape.new(target: CitySummary, name: "member")
+    CitySummaries.add_member(:member, ::Smithy::Schema::Shapes::MemberShape.new(target: CitySummary, name: "member"))
     CitySummary.add_member(:city_id, ::Smithy::Schema::Shapes::MemberShape.new(target: CityId, name: "cityId", traits: {"smithy.api#required" => {}}))
     CitySummary.add_member(:name, ::Smithy::Schema::Shapes::MemberShape.new(target: ::Smithy::Schema::Shapes::Prelude::String, name: "name", traits: {"smithy.api#required" => {}}))
     CitySummary.type = Types::CitySummary
diff --git a/tasks/smithy-cbor.rake b/tasks/smithy-cbor.rake
index f20517ff0..4c824fb5f 100644
--- a/tasks/smithy-cbor.rake
+++ b/tasks/smithy-cbor.rake
@@ -13,7 +13,7 @@ namespace 'smithy-cbor' do
 
   desc 'Run RBS validation.'
   task 'rbs:validate' do
-    sh('bundle exec rbs -I gems/smithy-cbor/sig -I gems/smithy-schema/sig validate')
+    sh('bundle exec rbs -r delegate -I gems/smithy-cbor/sig -I gems/smithy-schema/sig validate')
   end
 
   desc 'Run RBS spy tests on all unit tests.'
@@ -22,7 +22,7 @@ namespace 'smithy-cbor' do
       'RUBYOPT' => '-r bundler/setup -r rbs/test/setup',
       'RBS_TEST_RAISE' => 'true',
       'RBS_TEST_LOGLEVEL' => 'error',
-      'RBS_TEST_OPT' => '-I gems/smithy-cbor/sig -I gems/smithy-schema/sig',
+      'RBS_TEST_OPT' => '-r delegate -I gems/smithy-cbor/sig -I gems/smithy-schema/sig',
       'RBS_TEST_TARGET' => '"Smithy,Smithy::*,Smithy::Cbor,Smithy::Cbor::*"',
       'RBS_TEST_DOUBLE_SUITE' => 'rspec'
     }
diff --git a/tasks/smithy-client.rake b/tasks/smithy-client.rake
index 20185ce9c..c99bf6442 100644
--- a/tasks/smithy-client.rake
+++ b/tasks/smithy-client.rake
@@ -14,7 +14,7 @@ namespace 'smithy-client' do
 
   desc 'Run RBS validation.'
   task 'rbs:validate' do
-    sh('bundle exec rbs -I gems/smithy-client/sig -I gems/smithy-schema/sig validate')
+    sh('bundle exec rbs -r delegate -r uri -I gems/smithy-client/sig -I gems/smithy-schema/sig validate')
   end
 
   desc 'Run RBS spy tests on all unit tests.'
@@ -23,7 +23,7 @@ namespace 'smithy-client' do
       'RUBYOPT' => '-r bundler/setup -r rbs/test/setup',
       'RBS_TEST_RAISE' => 'true',
       'RBS_TEST_LOGLEVEL' => 'error',
-      'RBS_TEST_OPT' => '-I gems/smithy-client/sig -I gems/smithy-schema/sig',
+      'RBS_TEST_OPT' => '-r delegate -r uri -I gems/smithy-client/sig -I gems/smithy-schema/sig',
       'RBS_TEST_TARGET' => '"Smithy,Smithy::*,Smithy::Client,Smithy::Client::*"',
       'RBS_TEST_DOUBLE_SUITE' => 'rspec'
     }
diff --git a/tasks/smithy-json.rake b/tasks/smithy-json.rake
index a959d4568..82c45d08c 100644
--- a/tasks/smithy-json.rake
+++ b/tasks/smithy-json.rake
@@ -13,7 +13,7 @@ namespace 'smithy-json' do
 
   desc 'Run RBS validation.'
   task 'rbs:validate' do
-    sh('bundle exec rbs -I gems/smithy-json/sig -I gems/smithy-schema/sig validate')
+    sh('bundle exec rbs -r delegate -I gems/smithy-json/sig -I gems/smithy-schema/sig validate')
   end
 
   desc 'Run RBS spy tests on all unit tests.'
@@ -22,7 +22,7 @@ namespace 'smithy-json' do
       'RUBYOPT' => '-r bundler/setup -r rbs/test/setup',
       'RBS_TEST_RAISE' => 'true',
       'RBS_TEST_LOGLEVEL' => 'error',
-      'RBS_TEST_OPT' => '-I gems/smithy-json/sig -I gems/smithy-schema/sig',
+      'RBS_TEST_OPT' => '-r delegate -I gems/smithy-json/sig -I gems/smithy-schema/sig',
       'RBS_TEST_TARGET' => '"Smithy,Smithy::*,Smithy::Json,Smithy::Json::*"',
       'RBS_TEST_DOUBLE_SUITE' => 'rspec'
     }
diff --git a/tasks/smithy-schema.rake b/tasks/smithy-schema.rake
index c43092dae..7c852131a 100644
--- a/tasks/smithy-schema.rake
+++ b/tasks/smithy-schema.rake
@@ -13,7 +13,7 @@ namespace 'smithy-schema' do
 
   desc 'Run RBS validation.'
   task 'rbs:validate' do
-    sh('bundle exec rbs -I gems/smithy-schema/sig validate')
+    sh('bundle exec rbs -r delegate -I gems/smithy-schema/sig validate')
   end
 
   desc 'Run RBS spy tests on all unit tests.'
diff --git a/tasks/smithy-xml.rake b/tasks/smithy-xml.rake
index 0cd0febc5..08bbb2ab1 100644
--- a/tasks/smithy-xml.rake
+++ b/tasks/smithy-xml.rake
@@ -13,7 +13,7 @@ namespace 'smithy-xml' do
 
   desc 'Run RBS validation.'
   task 'rbs:validate' do
-    sh('bundle exec rbs -I gems/smithy-xml/sig -I gems/smithy-schema/sig validate')
+    sh('bundle exec rbs -r delegate -I gems/smithy-xml/sig -I gems/smithy-schema/sig validate')
   end
 
   desc 'Run RBS spy tests on all unit tests.'
@@ -22,7 +22,7 @@ namespace 'smithy-xml' do
       'RUBYOPT' => '-r bundler/setup -r rbs/test/setup',
       'RBS_TEST_RAISE' => 'true',
       'RBS_TEST_LOGLEVEL' => 'error',
-      'RBS_TEST_OPT' => '-I gems/smithy-xml/sig -I gems/smithy-schema/sig',
+      'RBS_TEST_OPT' => '-r delegate -I gems/smithy-xml/sig -I gems/smithy-schema/sig',
       'RBS_TEST_TARGET' => '"Smithy,Smithy::*,Smithy::Xml,Smithy::Xml::*"',
       'RBS_TEST_DOUBLE_SUITE' => 'rspec'
     }