diff --git a/Project.toml b/Project.toml
index e02dee0e..d4ddee32 100644
--- a/Project.toml
+++ b/Project.toml
@@ -2,7 +2,7 @@ name = "TranscodingStreams"
 uuid = "3bb67fe8-82b1-5028-8e26-92a6c54297fa"
 license = "MIT"
 authors = ["Kenta Sato <bicycle1885@gmail.com>"]
-version = "0.10.8"
+version = "0.10.9"
 
 [deps]
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
diff --git a/docs/src/reference.md b/docs/src/reference.md
index 610e23fa..6a748d80 100644
--- a/docs/src/reference.md
+++ b/docs/src/reference.md
@@ -45,7 +45,9 @@ TranscodingStreams.expectedsize
 TranscodingStreams.minoutsize
 TranscodingStreams.initialize
 TranscodingStreams.finalize
+TranscodingStreams.startproc2
 TranscodingStreams.startproc
+TranscodingStreams.process2
 TranscodingStreams.process
 ```
 
diff --git a/src/codec.jl b/src/codec.jl
index d6cff34e..dfb6d289 100644
--- a/src/codec.jl
+++ b/src/codec.jl
@@ -18,12 +18,16 @@ There are six functions for a codec to implement:
 - `initialize`: initialize the codec
 - `finalize`: finalize the codec
 - `startproc`: start processing with the codec
+- `startproc2`: start processing with the codec
 - `process`: process data with the codec.
+- `process2`: process data with the codec.
 
 These are defined in the `TranscodingStreams` and a new codec type must extend
 these methods if necessary.  Implementing a `process` method is mandatory but
 others are optional.  `expectedsize`, `minoutsize`, `initialize`, `finalize`,
-and `startproc` have a default implementation.
+`startproc`, `startproc2`, and `process2` have a default implementation.
+
+Methods ending with a 2 allow extra keyword arguments, for future hints.
 
 Your codec type is denoted by `C` and its object by `codec`.
 
@@ -156,6 +160,17 @@ function startproc(codec::Codec, mode::Symbol, error::Error)::Symbol
     return :ok
 end
 
+"""
+    startproc2(codec::Codec, mode::Symbol, error::Error; kwargs...)::Symbol
+
+Start data processing with `codec` of `mode`.
+
+The default method does nothing and returns `:ok`.
+"""
+function startproc2(codec::Codec, mode::Symbol, error::Error; kwargs...)::Symbol
+    startproc(codec, mode, error)
+end
+
 """
     process(codec::Codec, input::Memory, output::Memory, error::Error)::Tuple{Int,Int,Symbol}
 
@@ -167,3 +182,14 @@ function process(codec::Codec, input::Memory, output::Memory, error::Error)::Tup
     # no default method
     throw(MethodError(process, (codec, input, output, error)))
 end
+
+"""
+    process2(codec::Codec, input::Memory, output::Memory, error::Error; kwargs...)::Tuple{Int,Int,Symbol}
+
+Do data processing with `codec`.
+
+There is no default method.
+"""
+function process2(codec::Codec, input::Memory, output::Memory, error::Error; kwargs...)::Tuple{Int,Int,Symbol}
+    process(codec, input, output, error)
+end
\ No newline at end of file
diff --git a/src/stream.jl b/src/stream.jl
index 22ef593f..3da0794f 100644
--- a/src/stream.jl
+++ b/src/stream.jl
@@ -675,7 +675,7 @@ end
 # Call `startproc` with epilogne.
 function callstartproc(stream::TranscodingStream, mode::Symbol)
     state = stream.state
-    state.code = startproc(stream.codec, mode, state.error)
+    state.code = startproc2(stream.codec, mode, state.error)
     if state.code == :error
         changemode!(stream, :panic)
     end
@@ -687,7 +687,7 @@ function callprocess(stream::TranscodingStream, inbuf::Buffer, outbuf::Buffer)
     state = stream.state
     input = buffermem(inbuf)
     GC.@preserve inbuf makemargin!(outbuf, minoutsize(stream.codec, input))
-    Δin, Δout, state.code = GC.@preserve inbuf outbuf process(stream.codec, input, marginmem(outbuf), state.error)
+    Δin, Δout, state.code = GC.@preserve inbuf outbuf process2(stream.codec, input, marginmem(outbuf), state.error)
     @debug(
         "called process()",
         code = state.code,
@@ -781,7 +781,7 @@ function changemode!(stream::TranscodingStream, newmode::Symbol)
         throw(state.error[])
     elseif mode == :idle
         if newmode == :read || newmode == :write
-            state.code = startproc(stream.codec, newmode, state.error)
+            state.code = startproc2(stream.codec, newmode, state.error)
             if state.code == :error
                 changemode!(stream, :panic)
             end
diff --git a/src/transcode.jl b/src/transcode.jl
index e5a3bfb1..823edfb8 100644
--- a/src/transcode.jl
+++ b/src/transcode.jl
@@ -143,14 +143,16 @@ function unsafe_transcode!(
     input::Buffer,
 )
     error = Error()
-    code = startproc(codec, :write, error)
+    # Note: pledged_input_size is currently experimental
+    code = startproc2(codec, :write, error; pledged_input_size=buffersize(input))
     if code === :error
         @goto error
     end
     n = minoutsize(codec, buffermem(input))
     @label process
     makemargin!(output, n)
-    Δin, Δout, code = process(codec, buffermem(input), marginmem(output), error)
+    # Note: all_input is an experimental hint that all input is available for the current frame.
+    Δin, Δout, code = process2(codec, buffermem(input), marginmem(output), error; all_input=true)
     @debug(
         "called process()",
         code = code,
@@ -165,7 +167,7 @@ function unsafe_transcode!(
         @goto error
     elseif code === :end
         if buffersize(input) > 0
-            if startproc(codec, :write, error) === :error
+            if startproc2(codec, :write, error) === :error
                 @goto error
             end
             n = minoutsize(codec, buffermem(input))
diff --git a/test/codecdoubleframe.jl b/test/codecdoubleframe.jl
index 1ebeee02..b5d7540a 100644
--- a/test/codecdoubleframe.jl
+++ b/test/codecdoubleframe.jl
@@ -22,11 +22,13 @@ end
 
 DoubleFrameEncoder() = DoubleFrameEncoder(Ref(false), Ref(false), Ref(false))
 
-function TranscodingStreams.process(
+TranscodingStreams.process(c::DoubleFrameEncoder, i, o, e) = TranscodingStreams.process2(c, i, o, e)
+function TranscodingStreams.process2(
         codec     :: DoubleFrameEncoder,
         input     :: TranscodingStreams.Memory,
         output    :: TranscodingStreams.Memory,
-        error_ref :: TranscodingStreams.Error,
+        error_ref :: TranscodingStreams.Error;
+        kwargs...,
     )
     if input.size == 0
         codec.got_stop_msg[] = true
@@ -75,7 +77,8 @@ function TranscodingStreams.minoutsize(
     return 2
 end
 
-function TranscodingStreams.startproc(codec::DoubleFrameEncoder, ::Symbol, error::Error)
+TranscodingStreams.startproc(c::DoubleFrameEncoder, s, e) = TranscodingStreams.startproc2(c, s, e)
+function TranscodingStreams.startproc2(codec::DoubleFrameEncoder, ::Symbol, error::Error; kwargs...)
     codec.opened[] = false
     codec.got_stop_msg[] = false
     codec.stopped[] = false
@@ -91,11 +94,13 @@ end
 
 DoubleFrameDecoder() = DoubleFrameDecoder(Ref(1), Ref(0x00), Ref(0x00))
 
-function TranscodingStreams.process(
+TranscodingStreams.process(c::DoubleFrameDecoder, i, o, e) = TranscodingStreams.process2(c, i, o, e)
+function TranscodingStreams.process2(
         codec     :: DoubleFrameDecoder,
         input     :: TranscodingStreams.Memory,
         output    :: TranscodingStreams.Memory,
-        error_ref :: TranscodingStreams.Error,
+        error_ref :: TranscodingStreams.Error;
+        kwargs...
     )
     Δin::Int = 0
     Δout::Int = 0
@@ -172,7 +177,8 @@ function TranscodingStreams.process(
     end
 end
 
-function TranscodingStreams.startproc(codec::DoubleFrameDecoder, ::Symbol, error::Error)
+TranscodingStreams.startproc(c::DoubleFrameDecoder, x::Symbol, e::Error) = TranscodingStreams.startproc2(c, x, e)
+function TranscodingStreams.startproc2(codec::DoubleFrameDecoder, ::Symbol, error::Error; kwargs...)
     codec.state[] = 1
     codec.a[] = 0x00
     codec.b[] = 0x00