diff --git a/docs/src/reference.md b/docs/src/reference.md
index 610e23f..7a490af 100644
--- a/docs/src/reference.md
+++ b/docs/src/reference.md
@@ -42,6 +42,7 @@ Base.position(::NoopStream)
 ```@docs
 TranscodingStreams.Codec
 TranscodingStreams.expectedsize
+TranscodingStreams.pledgeinsize
 TranscodingStreams.minoutsize
 TranscodingStreams.initialize
 TranscodingStreams.finalize
diff --git a/src/codec.jl b/src/codec.jl
index d6cff34..75d03bd 100644
--- a/src/codec.jl
+++ b/src/codec.jl
@@ -14,6 +14,7 @@ Transcoding proceeds by calling some functions in a specific way. We call this
 
 There are six functions for a codec to implement:
 - `expectedsize`: return the expected size of transcoded data
+- `pledgeinsize`: tell the codec the total input size
 - `minoutsize`: return the minimum output size of `process`
 - `initialize`: initialize the codec
 - `finalize`: finalize the codec
@@ -22,7 +23,7 @@ There are six functions for a codec to implement:
 
 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`,
+others are optional.  `expectedsize`, `minoutsize`, `pledgeinsize`, `initialize`, `finalize`,
 and `startproc` have a default implementation.
 
 Your codec type is denoted by `C` and its object by `codec`.
@@ -39,6 +40,18 @@ used as a hint to determine the size of a data buffer when `transcode` is
 called. A good hint will reduce the number of buffer resizing and hence result
 in better performance.
 
+### `pledgeinsize`
+
+The `pledgeinsize(codec::C, insize::Int64, error::Error)::Symbol` method is used
+when `transcode` is called to tell the `codec` the total input size.
+This is called after `startproc` and before `process`. Some
+compressors can add this total input size to a header, making `expectedsize`
+accurate during later decompression. By default this just returns `:ok`.
+If there is an error, the return code must be `:error` and the `error` argument
+must be set to an exception object. Setting an inaccurate `insize` may cause the
+codec to error later on while processing data. A negative `insize` means unknown
+content size.
+
 ### `minoutsize`
 
 The `minoutsize(codec::C, input::Memory)::Int` method takes `codec` and `input`,
@@ -71,10 +84,11 @@ the stream will become the close mode for safety.
 ### `startproc`
 
 The `startproc(codec::C, mode::Symbol, error::Error)::Symbol` method takes
-`codec`, `mode` and `error`, and returns a status code. This is called just
-before the stream starts reading or writing data. `mode` is either `:read` or
-`:write` and then the stream starts reading or writing, respectively.  The
-return code must be `:ok` if `codec` is ready to read or write data.  Otherwise,
+`codec`, `mode`, and `error`, and returns a status code. This resets the state
+of the codec and is called before the stream starts processing data.
+After a call to `startproc`, `pledgeinsize` can be optionally called.
+`mode` is either `:read` or `:write`. The
+return code must be `:ok` if `codec` is ready to process data.  Otherwise,
 it must be `:error` and the `error` argument must be set to an exception object.
 
 ### `process`
@@ -112,6 +126,17 @@ function expectedsize(codec::Codec, input::Memory)::Int
     return input.size
 end
 
+"""
+    pledgeinsize(codec::Codec, insize::Int64, error::Error)::Symbol
+
+Tell the codec the total input size.
+
+The default method does nothing and returns `:ok`.
+"""
+function pledgeinsize(codec::Codec, insize::Int64, error::Error)::Symbol
+    return :ok
+end
+
 """
     minoutsize(codec::Codec, input::Memory)::Int
 
diff --git a/src/transcode.jl b/src/transcode.jl
index fca3373..ab27803 100644
--- a/src/transcode.jl
+++ b/src/transcode.jl
@@ -147,6 +147,9 @@ function unsafe_transcode!(
     if code === :error
         @goto error
     end
+    if pledgeinsize(codec, Int64(buffersize(input)), error) === :error
+        @goto error
+    end
     n = GC.@preserve input minoutsize(codec, buffermem(input))
     @label process
     makemargin!(output, n)
@@ -168,6 +171,9 @@ function unsafe_transcode!(
             if startproc(codec, :write, error) === :error
                 @goto error
             end
+            if pledgeinsize(codec, Int64(buffersize(input)), error) === :error
+                @goto error
+            end
             n = GC.@preserve input minoutsize(codec, buffermem(input))
             @goto process
         end
diff --git a/test/codecdoubleframe.jl b/test/codecdoubleframe.jl
index c004986..0d85b37 100644
--- a/test/codecdoubleframe.jl
+++ b/test/codecdoubleframe.jl
@@ -20,9 +20,11 @@ struct DoubleFrameEncoder <: TranscodingStreams.Codec
     opened::Base.RefValue{Bool}
     stopped::Base.RefValue{Bool}
     got_stop_msg::Base.RefValue{Bool}
+    pledged_in_size::Base.RefValue{Int64}
+    in_size_count::Base.RefValue{Int64}
 end
 
-DoubleFrameEncoder() = DoubleFrameEncoder(Ref(false), Ref(false), Ref(false))
+DoubleFrameEncoder() = DoubleFrameEncoder(Ref(false), Ref(false), Ref(false), Ref(Int64(-1)), Ref(Int64(0)))
 
 function TranscodingStreams.process(
         codec     :: DoubleFrameEncoder,
@@ -30,6 +32,7 @@ function TranscodingStreams.process(
         output    :: TranscodingStreams.Memory,
         error_ref :: TranscodingStreams.Error,
     )
+    pledged = codec.pledged_in_size[]
     if input.size == 0
         codec.got_stop_msg[] = true
     end
@@ -45,26 +48,59 @@ function TranscodingStreams.process(
         return 0, 0, :error
     elseif !codec.opened[]
         output[1] = UInt8('[')
-        output[2] = UInt8(' ')
+        if pledged ∈ (0:9)
+            output[2] = UInt8('0'+pledged)
+        else
+            output[2] = UInt8(' ')
+        end
         codec.opened[] = true
         return 0, 2, :ok
     elseif codec.got_stop_msg[]
+        # check in_size_count against pledged
+        if pledged ∈ (0:9)
+            if pledged > codec.in_size_count[]
+                error_ref[] = ErrorException("pledged in size was too big")
+                return 0, 0, :error
+            end
+        end
         output[1] = UInt8(' ')
         output[2] = UInt8(']')
         codec.stopped[] = true
         return 0, 2, :end
     else
         i = j = 0
+        # check input.size against pledged
+        if pledged ∈ (0:9)
+            if input.size > pledged || pledged - input.size < codec.in_size_count[]
+                error_ref[] = ErrorException("pledged in size was too small")
+                return 0, 0, :error
+            end
+        end
         while i + 1 ≤ lastindex(input) && j + 2 ≤ lastindex(output)
             b = input[i+1]
             i += 1
             output[j+1] = output[j+2] = b
             j += 2
         end
+        codec.in_size_count[] += i
         return i, j, :ok
     end
 end
 
+function TranscodingStreams.pledgeinsize(
+        codec::DoubleFrameEncoder,
+        insize::Int64,
+        error::Error,
+    )::Symbol
+    if codec.opened[]
+        error[] = ErrorException("pledgeinsize called after opening")
+        return :error
+    else
+        codec.pledged_in_size[] = insize
+        return :ok
+    end
+end
+
 function TranscodingStreams.expectedsize(
               :: DoubleFrameEncoder,
         input :: TranscodingStreams.Memory)
@@ -81,6 +117,8 @@ function TranscodingStreams.startproc(codec::DoubleFrameEncoder, ::Symbol, error
     codec.opened[] = false
     codec.got_stop_msg[] = false
     codec.stopped[] = false
+    codec.pledged_in_size[] = -1
+    codec.in_size_count[] = 0
     return :ok
 end
 
@@ -149,7 +187,7 @@ function TranscodingStreams.process(
         codec.a[] != UInt8('[') && error("expected [")
         @label state2
         do_read(codec.a) || return (codec.state[]=2; (Δin, Δout, :ok))
-        codec.a[] != UInt8(' ') && error("expected space")
+        codec.a[] ∉ (UInt8(' '), UInt8('0'):UInt8('9')...) && error("expected space or size")
         while true
             @label state3
             do_read(codec.a) || return (codec.state[]=3; (Δin, Δout, :ok))
@@ -189,12 +227,14 @@ DoubleFrameDecoderStream(stream::IO; kwargs...) = TranscodingStream(DoubleFrameD
 
 
 @testset "DoubleFrame Codecs" begin
-    @test transcode(DoubleFrameEncoder, b"") == b"[  ]"
-    @test transcode(DoubleFrameEncoder, b"a") == b"[ aa ]"
-    @test transcode(DoubleFrameEncoder, b"ab") == b"[ aabb ]"
-    @test transcode(DoubleFrameEncoder(), b"") == b"[  ]"
-    @test transcode(DoubleFrameEncoder(), b"a") == b"[ aa ]"
-    @test transcode(DoubleFrameEncoder(), b"ab") == b"[ aabb ]"
+    @test transcode(DoubleFrameEncoder, b"") == b"[0 ]"
+    @test transcode(DoubleFrameEncoder, b"a") == b"[1aa ]"
+    @test transcode(DoubleFrameEncoder, b"ab") == b"[2aabb ]"
+    @test transcode(DoubleFrameEncoder(), b"") == b"[0 ]"
+    @test transcode(DoubleFrameEncoder(), b"a") == b"[1aa ]"
+    @test transcode(DoubleFrameEncoder(), b"ab") == b"[2aabb ]"
+    @test transcode(DoubleFrameEncoder(), ones(UInt8,9)) == [b"[9"; ones(UInt8,18); b" ]";]
+    @test transcode(DoubleFrameEncoder(), ones(UInt8,10)) == [b"[ "; ones(UInt8,20); b" ]";]
 
     @test_throws Exception transcode(DoubleFrameDecoder, b"")
     @test_throws Exception transcode(DoubleFrameDecoder, b" [")