diff --git a/lib/proto_plugin.rb b/lib/proto_plugin.rb
index 67f2134..f07f206 100644
--- a/lib/proto_plugin.rb
+++ b/lib/proto_plugin.rb
@@ -6,8 +6,12 @@ module ProtoPlugin
 
 require_relative "proto_plugin/utils"
 require_relative "proto_plugin/context"
+require_relative "proto_plugin/commentable"
 require_relative "proto_plugin/file_descriptor"
 require_relative "proto_plugin/enum_descriptor"
+require_relative "proto_plugin/enum_value_descriptor"
+require_relative "proto_plugin/field_descriptor"
+require_relative "proto_plugin/oneof_descriptor"
 require_relative "proto_plugin/message_descriptor"
 require_relative "proto_plugin/service_descriptor"
 require_relative "proto_plugin/method_descriptor"
diff --git a/lib/proto_plugin/commentable.rb b/lib/proto_plugin/commentable.rb
new file mode 100644
index 0000000..6229444
--- /dev/null
+++ b/lib/proto_plugin/commentable.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+module ProtoPlugin
+  # A mixin providing access to the comments associated with a descriptor via
+  # its file's `SourceCodeInfo`.
+  #
+  # Including classes must respond to `#file` (returning the {FileDescriptor}
+  # the element belongs to) and `#descriptor` (returning the raw descriptor
+  # proto the comments are keyed against).
+  #
+  # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L1213
+  #   Google::Protobuf::SourceCodeInfo::Location
+  module Commentable
+    # The `SourceCodeInfo::Location` associated with this element, if source
+    # info was included in the request.
+    #
+    # @return [Google::Protobuf::SourceCodeInfo::Location]
+    # @return [nil] if no location is available
+    def source_location
+      file&.location_for(descriptor)
+    end
+
+    # The comment block appearing directly above this element.
+    #
+    # @return [String] the leading comment, as provided by `protoc`
+    # @return [nil] if there is no leading comment
+    def leading_comments
+      presence(source_location&.leading_comments)
+    end
+
+    # The comment appearing directly after this element on the same or
+    # following line.
+    #
+    # @return [String] the trailing comment, as provided by `protoc`
+    # @return [nil] if there is no trailing comment
+    def trailing_comments
+      presence(source_location&.trailing_comments)
+    end
+
+    # The comments attached to this element, in source order: the leading
+    # comment followed by the trailing comment. Absent blocks are omitted.
+    #
+    # Detached comments (blocks the author separated from the element with a
+    # blank line) are not exposed. Per protoc they appear before "but [are]
+    # not connected to" the element, so they are file organization rather than
+    # documentation of any element.
+    #
+    # @example
+    #   field.comments.join("\n").strip
+    #
+    # @return [Array<String>]
+    def comments
+      [leading_comments, trailing_comments].compact
+    end
+
+    private
+
+    def presence(value)
+      value unless value.nil? || value.empty?
+    end
+  end
+end
diff --git a/lib/proto_plugin/enum_descriptor.rb b/lib/proto_plugin/enum_descriptor.rb
index 6a016cc..b09d3d3 100644
--- a/lib/proto_plugin/enum_descriptor.rb
+++ b/lib/proto_plugin/enum_descriptor.rb
@@ -10,6 +10,8 @@ module ProtoPlugin
   #
   # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L336
   class EnumDescriptor < SimpleDelegator
+    include Commentable
+
     # @return [Google::Protobuf::EnumDescriptorProto]
     attr_reader :descriptor
 
@@ -28,6 +30,25 @@ def initialize(descriptor, parent)
       @parent = parent
     end
 
+    # The file descriptor this enum belongs to.
+    #
+    # @return [FileDescriptor]
+    def file
+      parent.file
+    end
+
+    # The values defined for this enum.
+    #
+    # @return [Array<EnumValueDescriptor>]
+    #
+    # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L343
+    #   Google::Protobuf::EnumDescriptorProto#value
+    def values
+      @values ||= @descriptor.value.map do |v|
+        EnumValueDescriptor.new(v, self)
+      end
+    end
+
     # The full name of the enum, including parent namespace.
     #
     # @example
diff --git a/lib/proto_plugin/enum_value_descriptor.rb b/lib/proto_plugin/enum_value_descriptor.rb
new file mode 100644
index 0000000..e2aa264
--- /dev/null
+++ b/lib/proto_plugin/enum_value_descriptor.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require "delegate"
+
+module ProtoPlugin
+  # A wrapper class around `Google::Protobuf::EnumValueDescriptorProto`
+  # which provides helpers and more idiomatic Ruby access patterns.
+  #
+  # Any method not defined directly is delegated to the descriptor the wrapper was initialized with.
+  #
+  # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L356
+  #   Google::Protobuf::EnumValueDescriptorProto
+  class EnumValueDescriptor < SimpleDelegator
+    include Commentable
+
+    # @return [Google::Protobuf::EnumValueDescriptorProto]
+    attr_reader :descriptor
+
+    # The enum descriptor this value was defined within.
+    #
+    # @return [EnumDescriptor]
+    attr_reader :enum
+
+    # @param descriptor [Google::Protobuf::EnumValueDescriptorProto]
+    # @param enum [EnumDescriptor] The enum this value was defined within.
+    def initialize(descriptor, enum)
+      super(descriptor)
+      @descriptor = descriptor
+      @enum = enum
+    end
+
+    # The file descriptor this enum value belongs to.
+    #
+    # @return [FileDescriptor]
+    def file
+      enum.file
+    end
+
+    # The full name of the enum value, including parent namespace.
+    #
+    # @example
+    #   "My::Ruby::Package::EnumName::VALUE_NAME"
+    #
+    # @return [String]
+    def full_name
+      @full_name ||= "#{enum.full_name}::#{name}"
+    end
+  end
+end
diff --git a/lib/proto_plugin/field_descriptor.rb b/lib/proto_plugin/field_descriptor.rb
new file mode 100644
index 0000000..20159ca
--- /dev/null
+++ b/lib/proto_plugin/field_descriptor.rb
@@ -0,0 +1,136 @@
+# frozen_string_literal: true
+
+require "delegate"
+
+module ProtoPlugin
+  # A wrapper class around `Google::Protobuf::FieldDescriptorProto`
+  # which provides helpers and more idiomatic Ruby access patterns.
+  #
+  # Any method not defined directly is delegated to the descriptor the wrapper was initialized with.
+  #
+  # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L242
+  #   Google::Protobuf::FieldDescriptorProto
+  class FieldDescriptor < SimpleDelegator
+    include Commentable
+
+    # @return [Google::Protobuf::FieldDescriptorProto]
+    attr_reader :descriptor
+
+    # The message descriptor this field was defined within.
+    #
+    # @return [MessageDescriptor]
+    attr_reader :message
+
+    # @param descriptor [Google::Protobuf::FieldDescriptorProto]
+    # @param message [MessageDescriptor] The message this field was defined within.
+    # @param context [Context]
+    def initialize(descriptor, message, context)
+      super(descriptor)
+      @descriptor = descriptor
+      @message = message
+      @context = context
+    end
+
+    # The file descriptor this field belongs to.
+    #
+    # @return [FileDescriptor]
+    def file
+      message.file
+    end
+
+    # Resolves the message or enum descriptor referenced by this field.
+    #
+    # Only message, enum, and group fields reference another type. For scalar
+    # fields (or when the referenced type was not included in the request),
+    # `nil` is returned.
+    #
+    # @return [MessageDescriptor] if the field is a message or group type
+    # @return [EnumDescriptor] if the field is an enum type
+    # @return [nil] if the field is a scalar type or the type was not found
+    def type_descriptor
+      return if scalar?
+
+      @context.type_by_proto_name(type_name)
+    end
+
+    # Returns true if the field is a message type.
+    #
+    # @return [Boolean]
+    def message?
+      type == :TYPE_MESSAGE
+    end
+
+    # Returns true if the field is an enum type.
+    #
+    # @return [Boolean]
+    def enum?
+      type == :TYPE_ENUM
+    end
+
+    # Returns true if the field is a group type.
+    #
+    # @return [Boolean]
+    def group?
+      type == :TYPE_GROUP
+    end
+
+    # Returns true if the field is a scalar type (i.e. not a message, enum, or group).
+    #
+    # @return [Boolean]
+    def scalar?
+      !message? && !enum? && !group?
+    end
+
+    # Returns true if the field has the `repeated` label.
+    #
+    # @return [Boolean]
+    def repeated?
+      label == :LABEL_REPEATED
+    end
+
+    # Returns true if the field has the `required` label (proto2 only).
+    #
+    # @return [Boolean]
+    def required?
+      label == :LABEL_REQUIRED
+    end
+
+    # Returns true if the field has the `optional` label.
+    #
+    # @note In proto3 all singular fields carry the `optional` label internally.
+    #   Use {#proto3_optional?} to detect fields with explicit presence tracking.
+    #
+    # @return [Boolean]
+    def optional?
+      label == :LABEL_OPTIONAL
+    end
+
+    # Returns true if the field was declared with proto3 explicit presence,
+    # i.e. an `optional` keyword in a proto3 file.
+    #
+    # @return [Boolean]
+    def proto3_optional?
+      descriptor.proto3_optional
+    end
+
+    # Returns true if the field is a member of a oneof.
+    #
+    # @note Fields declared with the proto3 `optional` keyword are backed by a
+    #   synthetic oneof. Those are not considered oneof members here.
+    #
+    # @return [Boolean]
+    def oneof?
+      descriptor.has_oneof_index? && !proto3_optional?
+    end
+
+    # The oneof this field is a member of, if any.
+    #
+    # @return [OneofDescriptor] if the field is a member of a oneof
+    # @return [nil] otherwise
+    def oneof
+      return unless oneof?
+
+      message.oneofs[descriptor.oneof_index]
+    end
+  end
+end
diff --git a/lib/proto_plugin/file_descriptor.rb b/lib/proto_plugin/file_descriptor.rb
index 6515547..74d3eab 100644
--- a/lib/proto_plugin/file_descriptor.rb
+++ b/lib/proto_plugin/file_descriptor.rb
@@ -11,6 +11,8 @@ module ProtoPlugin
   # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L97
   #   Google::Protobuf::FileDescriptorProto
   class FileDescriptor < SimpleDelegator
+    include Commentable
+
     # @return [Google::Protobuf::FileDescriptorProto]
     attr_reader :descriptor
 
@@ -22,6 +24,26 @@ def initialize(context, descriptor)
       @descriptor = descriptor
     end
 
+    # The file descriptor this element belongs to.
+    #
+    # For a `FileDescriptor` this is the descriptor itself. Defined so that
+    # {Commentable} can resolve comments uniformly across all descriptor types.
+    #
+    # @return [FileDescriptor]
+    def file
+      self
+    end
+
+    # Returns the `SourceCodeInfo::Location` for a given raw descriptor proto
+    # defined within this file, if source info was included in the request.
+    #
+    # @param proto [Object] a raw descriptor proto contained in this file
+    # @return [Google::Protobuf::SourceCodeInfo::Location]
+    # @return [nil] if no matching location is available
+    def location_for(proto)
+      source_locations[proto]
+    end
+
     # The enums defined as children of this file.
     #
     # @return [Array<EnumDescriptor>]
@@ -42,7 +64,7 @@ def enums
     #   Google::Protobuf::DescriptorProto#message_type
     def messages
       @messages ||= @descriptor.message_type.map do |m|
-        MessageDescriptor.new(m, self)
+        MessageDescriptor.new(m, self, @context)
       end
     end
 
@@ -85,5 +107,75 @@ def services
         ServiceDescriptor.new(s, self, @context)
       end
     end
+
+    private
+
+    # Field numbers of the relevant repeated fields within their parent
+    # descriptor proto, used to construct `SourceCodeInfo` paths.
+    #
+    # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto
+    FILE_MESSAGE = 4
+    FILE_ENUM = 5
+    FILE_SERVICE = 6
+    MESSAGE_FIELD = 2
+    MESSAGE_NESTED = 3
+    MESSAGE_ENUM = 4
+    MESSAGE_ONEOF = 8
+    ENUM_VALUE = 2
+    SERVICE_METHOD = 2
+    private_constant :FILE_MESSAGE,
+      :FILE_ENUM,
+      :FILE_SERVICE,
+      :MESSAGE_FIELD,
+      :MESSAGE_NESTED,
+      :MESSAGE_ENUM,
+      :MESSAGE_ONEOF,
+      :ENUM_VALUE,
+      :SERVICE_METHOD
+
+    # Builds a map from each raw descriptor proto within this file to its
+    # `SourceCodeInfo::Location`, keyed by object identity.
+    #
+    # The `SourceCodeInfo` locations are addressed by a numeric path into the
+    # `FileDescriptorProto` tree. This walks that tree in the same order,
+    # reconstructing each path and associating it with the descriptor found
+    # there.
+    #
+    # @return [Hash]
+    def source_locations
+      @source_locations ||= begin
+        by_path = (@descriptor.source_code_info&.location || []).each_with_object({}) do |loc, hash|
+          hash[loc.path.to_a] = loc
+        end
+
+        index = {}.compare_by_identity
+        assign = ->(proto, path) { (loc = by_path[path]) && index[proto] = loc }
+
+        assign.call(@descriptor, [])
+
+        visit_enum = ->(enum, path) {
+          assign.call(enum, path)
+          enum.value.each_with_index { |v, i| assign.call(v, path + [ENUM_VALUE, i]) }
+        }
+
+        visit_message = ->(message, path) {
+          assign.call(message, path)
+          message.field.each_with_index { |f, i| assign.call(f, path + [MESSAGE_FIELD, i]) }
+          message.oneof_decl.each_with_index { |o, i| assign.call(o, path + [MESSAGE_ONEOF, i]) }
+          message.enum_type.each_with_index { |e, i| visit_enum.call(e, path + [MESSAGE_ENUM, i]) }
+          message.nested_type.each_with_index { |n, i| visit_message.call(n, path + [MESSAGE_NESTED, i]) }
+        }
+
+        @descriptor.message_type.each_with_index { |m, i| visit_message.call(m, [FILE_MESSAGE, i]) }
+        @descriptor.enum_type.each_with_index { |e, i| visit_enum.call(e, [FILE_ENUM, i]) }
+        @descriptor.service.each_with_index do |s, i|
+          sp = [FILE_SERVICE, i]
+          assign.call(s, sp)
+          s["method"].each_with_index { |m, j| assign.call(m, sp + [SERVICE_METHOD, j]) }
+        end
+
+        index
+      end
+    end
   end
 end
diff --git a/lib/proto_plugin/message_descriptor.rb b/lib/proto_plugin/message_descriptor.rb
index 88770f1..23c0814 100644
--- a/lib/proto_plugin/message_descriptor.rb
+++ b/lib/proto_plugin/message_descriptor.rb
@@ -11,6 +11,8 @@ module ProtoPlugin
   # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L134
   #   Google::Protobuf::DescriptorProto
   class MessageDescriptor < SimpleDelegator
+    include Commentable
+
     # @return [Google::Protobuf::DescriptorProto]
     attr_reader :descriptor
 
@@ -23,10 +25,43 @@ class MessageDescriptor < SimpleDelegator
     # @param descriptor [Google::Protobuf::DescriptorProto]
     # @param parent [FileDescriptorFileDescriptorProto, MessageDescriptor]
     #   The file or message descriptor this message was defined within.
-    def initialize(descriptor, parent)
+    # @param context [Context]
+    def initialize(descriptor, parent, context)
       super(descriptor)
       @descriptor = descriptor
       @parent = parent
+      @context = context
+    end
+
+    # The file descriptor this message belongs to.
+    #
+    # @return [FileDescriptor]
+    def file
+      parent.file
+    end
+
+    # The fields defined on this message.
+    #
+    # @return [Array<FieldDescriptor>]
+    #
+    # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L138
+    #   Google::Protobuf::DescriptorProto#field
+    def fields
+      @fields ||= @descriptor.field.map do |f|
+        FieldDescriptor.new(f, self, @context)
+      end
+    end
+
+    # The oneofs defined on this message.
+    #
+    # @return [Array<OneofDescriptor>]
+    #
+    # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L145
+    #   Google::Protobuf::DescriptorProto#oneof_decl
+    def oneofs
+      @oneofs ||= @descriptor.oneof_decl.each_with_index.map do |o, i|
+        OneofDescriptor.new(o, self, i)
+      end
     end
 
     # The enums defined as children of this message.
@@ -49,7 +84,7 @@ def enums
     #   Google::Protobuf::DescriptorProto#nested_type
     def messages
       @nested_messages ||= @descriptor.nested_type.map do |m|
-        MessageDescriptor.new(m, self)
+        MessageDescriptor.new(m, self, @context)
       end
     end
 
diff --git a/lib/proto_plugin/method_descriptor.rb b/lib/proto_plugin/method_descriptor.rb
index 199e85f..ddef1ea 100644
--- a/lib/proto_plugin/method_descriptor.rb
+++ b/lib/proto_plugin/method_descriptor.rb
@@ -10,6 +10,8 @@ module ProtoPlugin
   #
   # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L381
   class MethodDescriptor < SimpleDelegator
+    include Commentable
+
     # @return [Google::Protobuf::MethodDescriptorProto]
     attr_reader :descriptor
 
@@ -28,6 +30,13 @@ def initialize(descriptor, service, context)
       @context = context
     end
 
+    # The file descriptor this method belongs to.
+    #
+    # @return [FileDescriptor]
+    def file
+      service.file
+    end
+
     # Returns the `MessageDescriptor` of the method's input type.
     #
     # @return [MessageDescriptor]
diff --git a/lib/proto_plugin/oneof_descriptor.rb b/lib/proto_plugin/oneof_descriptor.rb
new file mode 100644
index 0000000..474b072
--- /dev/null
+++ b/lib/proto_plugin/oneof_descriptor.rb
@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require "delegate"
+
+module ProtoPlugin
+  # A wrapper class around `Google::Protobuf::OneofDescriptorProto`
+  # which provides helpers and more idiomatic Ruby access patterns.
+  #
+  # Any method not defined directly is delegated to the descriptor the wrapper was initialized with.
+  #
+  # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L349
+  #   Google::Protobuf::OneofDescriptorProto
+  class OneofDescriptor < SimpleDelegator
+    include Commentable
+
+    # @return [Google::Protobuf::OneofDescriptorProto]
+    attr_reader :descriptor
+
+    # The message descriptor this oneof was defined within.
+    #
+    # @return [MessageDescriptor]
+    attr_reader :message
+
+    # The index of this oneof within its message's `oneof_decl` list.
+    #
+    # @return [Integer]
+    attr_reader :index
+
+    # @param descriptor [Google::Protobuf::OneofDescriptorProto]
+    # @param message [MessageDescriptor] The message this oneof was defined within.
+    # @param index [Integer] The index of this oneof within the message.
+    def initialize(descriptor, message, index)
+      super(descriptor)
+      @descriptor = descriptor
+      @message = message
+      @index = index
+    end
+
+    # The file descriptor this oneof belongs to.
+    #
+    # @return [FileDescriptor]
+    def file
+      message.file
+    end
+
+    # The fields that are members of this oneof.
+    #
+    # @return [Array<FieldDescriptor>]
+    def fields
+      @fields ||= message.fields.select do |field|
+        field.oneof? && field.oneof_index == index
+      end
+    end
+  end
+end
diff --git a/lib/proto_plugin/service_descriptor.rb b/lib/proto_plugin/service_descriptor.rb
index b4a53a6..c7f9dab 100644
--- a/lib/proto_plugin/service_descriptor.rb
+++ b/lib/proto_plugin/service_descriptor.rb
@@ -10,6 +10,8 @@ module ProtoPlugin
   #
   # @see https://github.com/protocolbuffers/protobuf/blob/v28.2/src/google/protobuf/descriptor.proto#L373
   class ServiceDescriptor < SimpleDelegator
+    include Commentable
+
     # @return [Google::Protobuf::ServiceDescriptorProto]
     attr_reader :descriptor
 
@@ -28,6 +30,13 @@ def initialize(descriptor, parent, context)
       @context = context
     end
 
+    # The file descriptor this service belongs to.
+    #
+    # @return [FileDescriptor]
+    def file
+      parent.file
+    end
+
     # The full name of the service, including parent namespace.
     #
     # @example
diff --git a/test/fixtures/blog.cgr b/test/fixtures/blog.cgr
index 32d5e69..a591532 100644
Binary files a/test/fixtures/blog.cgr and b/test/fixtures/blog.cgr differ
diff --git a/test/fixtures/blog.fds b/test/fixtures/blog.fds
index 844cefc..8fcdff2 100644
Binary files a/test/fixtures/blog.fds and b/test/fixtures/blog.fds differ
diff --git a/test/fixtures/blog/article.proto b/test/fixtures/blog/article.proto
index bb9521b..d936a63 100644
--- a/test/fixtures/blog/article.proto
+++ b/test/fixtures/blog/article.proto
@@ -6,10 +6,12 @@ import "google/protobuf/timestamp.proto";
 
 import "comment.proto";
 
+// An article published on the blog.
 message Article {
+    // The unique identifier for the article.
     uint64 id = 1;
-    
-    string title = 2;
+
+    string title = 2; // The article's headline.
 
     Author author = 3;
 
@@ -19,7 +21,9 @@ message Article {
 
     repeated Comment comments = 6;
 
+    // The lifecycle status of an article.
     enum Status {
+        // The article is a work in progress.
         DRAFT   = 0;
         PUBLISHED = 1;
         DELETED   = 2;
diff --git a/test/fixtures/blog/category.proto b/test/fixtures/blog/category.proto
index 5e8e236..bebebfb 100644
--- a/test/fixtures/blog/category.proto
+++ b/test/fixtures/blog/category.proto
@@ -2,6 +2,7 @@ syntax = "proto3";
 
 package proto_plugin.fixtures;
 
+// A top-level content category.
 enum Category {
     CATEGORY_UNSPECIFIED = 0;
     CATEGORY_ANNOUNCEMENT = 1;
diff --git a/test/fixtures/blog/comment.proto b/test/fixtures/blog/comment.proto
index b98ff0e..7f2dd5b 100644
--- a/test/fixtures/blog/comment.proto
+++ b/test/fixtures/blog/comment.proto
@@ -23,3 +23,14 @@ message Comment {
         DELETED   = 4;
     }
 }
+
+message CommentEvent {
+    uint64 comment_id = 1;
+
+    // Describes what happened to the comment.
+    oneof payload {
+        string created = 2;
+        string edited = 3;
+        bool deleted = 4;
+    }
+}
diff --git a/test/fixtures/blog/service.proto b/test/fixtures/blog/service.proto
index 82cbcce..11c29a0 100644
--- a/test/fixtures/blog/service.proto
+++ b/test/fixtures/blog/service.proto
@@ -4,7 +4,9 @@ package proto_plugin.fixtures;
 
 import "article.proto";
 
+// Provides access to articles.
 service ArticlesService {
+    // Fetches a single article by id.
     rpc GetArticle(GetArticleRequest) returns (GetArticleResponse);
 
     rpc GetArticles(GetArticlesRequest) returns (GetArticlesResponse);
diff --git a/test/proto_plugin/commentable_test.rb b/test/proto_plugin/commentable_test.rb
new file mode 100644
index 0000000..bfd13dc
--- /dev/null
+++ b/test/proto_plugin/commentable_test.rb
@@ -0,0 +1,86 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module ProtoPlugin
+  # Exercises comment resolution (via SourceCodeInfo) across every descriptor type.
+  class CommentableTest < Minitest::Test
+    def setup
+      @context = Context.new(request: load_request_fixture)
+      @article = @context.type_by_proto_name(".proto_plugin.fixtures.Article")
+    end
+
+    def test_message_comment
+      assert_equal(" An article published on the blog.\n", @article.leading_comments)
+    end
+
+    def test_field_leading_comment
+      id = @article.fields.find { |f| f.name == "id" }
+      assert_equal(" The unique identifier for the article.\n", id.leading_comments)
+    end
+
+    def test_field_trailing_comment
+      title = @article.fields.find { |f| f.name == "title" }
+      assert_equal(" The article's headline.\n", title.trailing_comments)
+    end
+
+    def test_nested_enum_comment
+      status = @article.enums.first
+      assert_equal(" The lifecycle status of an article.\n", status.leading_comments)
+    end
+
+    def test_enum_value_comment
+      draft = @article.enums.first.values.first
+      assert_equal(" The article is a work in progress.\n", draft.leading_comments)
+    end
+
+    def test_file_level_enum_comment
+      category = @context.type_by_proto_name(".proto_plugin.fixtures.Category")
+      assert_equal(" A top-level content category.\n", category.leading_comments)
+    end
+
+    def test_service_comment
+      service = @context.file_by_filename("service.proto").services.first
+      assert_equal(" Provides access to articles.\n", service.leading_comments)
+    end
+
+    def test_method_comment
+      method = @context.file_by_filename("service.proto").services.first.rpc_methods.first
+      assert_equal(" Fetches a single article by id.\n", method.leading_comments)
+    end
+
+    def test_oneof_comment
+      event = @context.type_by_proto_name(".proto_plugin.fixtures.CommentEvent")
+      assert_equal(" Describes what happened to the comment.\n", event.oneofs.first.leading_comments)
+    end
+
+    def test_comments_aggregates_leading_and_trailing
+      title = @article.fields.find { |f| f.name == "title" }
+      assert_equal([" The article's headline.\n"], title.comments)
+
+      id = @article.fields.find { |f| f.name == "id" }
+      assert_equal([" The unique identifier for the article.\n"], id.comments)
+    end
+
+    def test_comments_empty_when_absent
+      author = @article.fields.find { |f| f.name == "author" }
+      assert_empty(author.comments)
+    end
+
+    def test_absent_comments_are_nil
+      author = @article.fields.find { |f| f.name == "author" }
+
+      assert_nil(author.leading_comments)
+      assert_nil(author.trailing_comments)
+    end
+
+    def test_file_resolves_to_owning_descriptor
+      status = @article.enums.first
+
+      assert_instance_of(FileDescriptor, @article.file)
+      assert_equal("article.proto", @article.file.name)
+      assert_equal(@article.file, status.file)
+      assert_equal(@article.file, status.values.first.file)
+    end
+  end
+end
diff --git a/test/proto_plugin/enum_descriptor_test.rb b/test/proto_plugin/enum_descriptor_test.rb
index e097158..7d799da 100644
--- a/test/proto_plugin/enum_descriptor_test.rb
+++ b/test/proto_plugin/enum_descriptor_test.rb
@@ -18,5 +18,16 @@ def test_full_name_of_message_enum
       enum = @article.enums.first
       assert_equal("ProtoPlugin::Fixtures::Article::Status", enum.full_name)
     end
+
+    def test_values
+      assert_equal(
+        ["CATEGORY_UNSPECIFIED", "CATEGORY_ANNOUNCEMENT", "CATEGORY_PRODUCT_RELEASE"],
+        @category.values.map(&:name),
+      )
+
+      @category.values.each do |v|
+        assert_instance_of(EnumValueDescriptor, v)
+      end
+    end
   end
 end
diff --git a/test/proto_plugin/enum_value_descriptor_test.rb b/test/proto_plugin/enum_value_descriptor_test.rb
new file mode 100644
index 0000000..87f1c5b
--- /dev/null
+++ b/test/proto_plugin/enum_value_descriptor_test.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module ProtoPlugin
+  class EnumValueDescriptorTest < Minitest::Test
+    def setup
+      @context = Context.new(request: load_request_fixture)
+      @category = @context.type_by_proto_name(".proto_plugin.fixtures.Category")
+      @article = @context.type_by_proto_name(".proto_plugin.fixtures.Article")
+    end
+
+    def test_name_and_number
+      value = @category.values.first
+
+      assert_instance_of(EnumValueDescriptor, value)
+      assert_equal("CATEGORY_UNSPECIFIED", value.name)
+      assert_equal(0, value.number)
+    end
+
+    def test_enum
+      value = @category.values.first
+      assert_equal(@category, value.enum)
+    end
+
+    def test_full_name_of_file_enum_value
+      value = @category.values.last
+      assert_equal("ProtoPlugin::Fixtures::Category::CATEGORY_PRODUCT_RELEASE", value.full_name)
+    end
+
+    def test_full_name_of_message_enum_value
+      status = @article.enums.first
+      value = status.values.first
+      assert_equal("ProtoPlugin::Fixtures::Article::Status::DRAFT", value.full_name)
+    end
+  end
+end
diff --git a/test/proto_plugin/field_descriptor_test.rb b/test/proto_plugin/field_descriptor_test.rb
new file mode 100644
index 0000000..fa3326d
--- /dev/null
+++ b/test/proto_plugin/field_descriptor_test.rb
@@ -0,0 +1,112 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module ProtoPlugin
+  class FieldDescriptorTest < Minitest::Test
+    def setup
+      @context = Context.new(request: load_request_fixture)
+      @article = @context.type_by_proto_name(".proto_plugin.fixtures.Article")
+      @fields = @article.fields.each_with_object({}) do |field, hash|
+        hash[field.name] = field
+      end
+    end
+
+    def test_message
+      assert_equal(@article, @fields["title"].message)
+    end
+
+    def test_scalar_type_predicates
+      title = @fields["title"]
+
+      assert(title.scalar?)
+      refute(title.message?)
+      refute(title.enum?)
+      refute(title.group?)
+    end
+
+    def test_message_type_predicates
+      author = @fields["author"]
+
+      assert(author.message?)
+      refute(author.scalar?)
+      refute(author.enum?)
+    end
+
+    def test_type_descriptor_for_scalar
+      assert_nil(@fields["title"].type_descriptor)
+    end
+
+    def test_type_descriptor_for_message
+      author = @fields["author"].type_descriptor
+
+      assert_instance_of(MessageDescriptor, author)
+      assert_equal("ProtoPlugin::Fixtures::Article::Author", author.full_name)
+    end
+
+    def test_type_descriptor_for_repeated_message
+      comment = @fields["comments"].type_descriptor
+
+      assert_instance_of(MessageDescriptor, comment)
+      assert_equal("ProtoPlugin::Fixtures::Comment", comment.full_name)
+    end
+
+    def test_type_descriptor_for_imported_message
+      timestamp = @fields["published_at"].type_descriptor
+
+      assert_instance_of(MessageDescriptor, timestamp)
+      assert_equal("Timestamp", timestamp.name)
+    end
+
+    def test_type_descriptor_returns_nil_for_unindexed_type
+      field = FieldDescriptor.new(
+        Google::Protobuf::FieldDescriptorProto.new(
+          name: "mystery",
+          type: :TYPE_MESSAGE,
+          type_name: ".does.not.Exist",
+        ),
+        @article,
+        @context,
+      )
+
+      assert(field.message?)
+      assert_nil(field.type_descriptor)
+    end
+
+    def test_cardinality
+      assert(@fields["comments"].repeated?)
+      refute(@fields["comments"].optional?)
+
+      assert(@fields["title"].optional?)
+      refute(@fields["title"].repeated?)
+      refute(@fields["title"].required?)
+    end
+
+    def test_proto3_optional
+      refute(@fields["title"].proto3_optional?)
+    end
+
+    def test_oneof_membership
+      event = @context.type_by_proto_name(".proto_plugin.fixtures.CommentEvent")
+      fields = event.fields.each_with_object({}) do |field, hash|
+        hash[field.name] = field
+      end
+
+      created = fields["created"]
+      assert(created.oneof?)
+      assert_instance_of(OneofDescriptor, created.oneof)
+      assert_equal("payload", created.oneof.name)
+
+      comment_id = fields["comment_id"]
+      refute(comment_id.oneof?)
+      assert_nil(comment_id.oneof)
+    end
+
+    def test_delegates_to_descriptor
+      title = @fields["title"]
+
+      assert_equal("title", title.name)
+      assert_equal(2, title.number)
+    end
+  end
+end
diff --git a/test/proto_plugin/message_descriptor_test.rb b/test/proto_plugin/message_descriptor_test.rb
index 4c566bd..c4e6759 100644
--- a/test/proto_plugin/message_descriptor_test.rb
+++ b/test/proto_plugin/message_descriptor_test.rb
@@ -31,6 +31,31 @@ def test_messages
       assert_equal(@message, child_two.parent)
     end
 
+    def test_fields
+      assert_equal(6, @message.fields.count)
+
+      @message.fields.each do |f|
+        assert_instance_of(FieldDescriptor, f)
+        assert_equal(@message, f.message)
+      end
+
+      assert_equal(
+        ["id", "title", "author", "content", "published_at", "comments"],
+        @message.fields.map(&:name),
+      )
+    end
+
+    def test_oneofs
+      event = @context.type_by_proto_name(".proto_plugin.fixtures.CommentEvent")
+
+      assert_equal(1, event.oneofs.count)
+
+      oneof = event.oneofs.first
+      assert_instance_of(OneofDescriptor, oneof)
+      assert_equal("payload", oneof.name)
+      assert_equal(0, oneof.index)
+    end
+
     def test_full_name
       child_one = @message.messages[0]
       child_two = @message.messages[1]
diff --git a/test/proto_plugin/oneof_descriptor_test.rb b/test/proto_plugin/oneof_descriptor_test.rb
new file mode 100644
index 0000000..d92c46c
--- /dev/null
+++ b/test/proto_plugin/oneof_descriptor_test.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module ProtoPlugin
+  class OneofDescriptorTest < Minitest::Test
+    def setup
+      @context = Context.new(request: load_request_fixture)
+      @message = @context.type_by_proto_name(".proto_plugin.fixtures.CommentEvent")
+      @oneof = @message.oneofs.first
+    end
+
+    def test_name_and_index
+      assert_instance_of(OneofDescriptor, @oneof)
+      assert_equal("payload", @oneof.name)
+      assert_equal(0, @oneof.index)
+    end
+
+    def test_message
+      assert_equal(@message, @oneof.message)
+    end
+
+    def test_fields
+      assert_equal(
+        ["created", "edited", "deleted"],
+        @oneof.fields.map(&:name),
+      )
+
+      @oneof.fields.each do |f|
+        assert_instance_of(FieldDescriptor, f)
+      end
+    end
+
+    def test_fields_excludes_non_members
+      refute_includes(@oneof.fields.map(&:name), "comment_id")
+    end
+  end
+end