diff --git a/sig/plugins/multipart.rbs b/sig/plugins/multipart.rbs
index 89126add..62399621 100644
--- a/sig/plugins/multipart.rbs
+++ b/sig/plugins/multipart.rbs
@@ -3,18 +3,41 @@ module HTTPX
     module Multipart
       def self.load_dependencies: (singleton(Session)) -> void
       def self.configure: (*untyped) -> void
-      def self?.encode: (untyped) -> Encoder
+      def self?.encode: (untyped) -> (Encoder | Transcoder::Form::Encoder)
+
+      type multipart_value = string | Pathname | File | _Reader
+
+      type record_multipart_value = multipart_value |
+                                    { content_type: String, filename: String, body: multipart_value } |
+                                    { content_type: String, body: multipart_value }
+
+      type multipart_nested_value = multipart_value | _ToAry[multipart_value] | _ToHash[string, multipart_value]
+
+      type multipart_input = Enumerable[[string, multipart_value], untyped]
 
       class Encoder
         include Transcoder::_Encoder
-        include _ToS
         include _Reader
 
+        def content_type: () -> String
+
         private
 
-        def initialize: (Hash[Symbol | string, HTTP::FormData::Part | string] multipart_data) -> untyped
+        def initialize: (Hash[Symbol | string, multipart_nested_value] multipart_data) -> untyped
 
-        def multipart?: (top) -> bool
+        def header_part: (string key, String content_type, String? filename) -> StringIO
+
+        def read_chunks: (String buffer, Integer? length) -> void
+
+        def read_from_part: (Integer? max_length) -> void
+      end
+
+      module Part
+        def self?.call: (multipart_nested_value) -> ([_Reader, String, String?] | [_Reader, String])
+      end
+
+      module MimeTypeDetector
+        def self?.call: (IO file, ?String filename) -> String?
       end
     end
   end
diff --git a/sig/transcoder.rbs b/sig/transcoder.rbs
index e151c902..84785d48 100644
--- a/sig/transcoder.rbs
+++ b/sig/transcoder.rbs
@@ -4,8 +4,8 @@ module HTTPX
   module Transcoder
     extend HTTPX::Registry[String, Class]
 
-    def self.normalize_keys: (string | Symbol, top) { (string, top) -> void } -> void
-                           | (string | Symbol, top) { (string) -> void } -> void
+    def self.normalize_keys: (string | Symbol, top, ?Proc?) { (string, top) -> void } -> void
+                           | (string | Symbol, top, ?Proc?) { (string) -> void } -> void
 
     interface _Encoder
       def bytesize: () -> Numeric
diff --git a/sig/transcoder/body.rbs b/sig/transcoder/body.rbs
index 1969f3fd..4af5a0c9 100644
--- a/sig/transcoder/body.rbs
+++ b/sig/transcoder/body.rbs
@@ -7,6 +7,8 @@ module HTTPX
 
         @raw: bodyIO
 
+        def content_type: () -> String
+
         private
 
         def initialize: (untyped body ) -> untyped
diff --git a/sig/transcoder/form.rbs b/sig/transcoder/form.rbs
index 7981be20..cd7b5fec 100644
--- a/sig/transcoder/form.rbs
+++ b/sig/transcoder/form.rbs
@@ -1,5 +1,9 @@
 module HTTPX::Transcoder
-  type urlencoded_input = Enumerable[[string, string], untyped]
+  type form_value = string
+
+  type form_nested_value = form_value | _ToAry[form_value] | _ToHash[string, form_value]
+
+  type urlencoded_input = Enumerable[[string, form_nested_value], untyped]
 
   module Form
     def self?.encode: (urlencoded_input form) -> Encoder
@@ -8,6 +12,8 @@ module HTTPX::Transcoder
       include _Encoder
       include _ToS
 
+      def content_type: () -> String
+
       private
 
       def initialize: (urlencoded_input form) -> untyped
diff --git a/sig/transcoder/json.rbs b/sig/transcoder/json.rbs
index c1f14770..c9b51412 100644
--- a/sig/transcoder/json.rbs
+++ b/sig/transcoder/json.rbs
@@ -6,9 +6,11 @@ module HTTPX::Transcoder
       include _Encoder
       include _ToS
 
+      def content_type: () -> String
+
       private
 
-      def initalize: (_ToJson json) -> untyped
+      def initialize: (_ToJson json) -> untyped
     end
   end
 end
diff --git a/test/support/ci/build.sh b/test/support/ci/build.sh
index c3b2c4f4..5287e92e 100755
--- a/test/support/ci/build.sh
+++ b/test/support/ci/build.sh
@@ -35,7 +35,7 @@ if [[ ${RUBY_VERSION:0:1} = "3" ]]; then
   export RUBYOPT="$RUBYOPT -rbundler/setup -rrbs/test/setup"
   export RBS_TEST_RAISE=true
   export RBS_TEST_LOGLEVEL=error
-  export RBS_TEST_OPT="-Isig -ruri -rjson -ripaddr -rhttp-2-next"
+  export RBS_TEST_OPT="-Isig -ruri -rjson -ripaddr -rpathname -rhttp-2-next"
   export RBS_TEST_TARGET="HTTP*"
 fi