diff --git a/src/ASDF.jl b/src/ASDF.jl
index f2894b6..17d031c 100644
--- a/src/ASDF.jl
+++ b/src/ASDF.jl
@@ -2,7 +2,7 @@ module ASDF
 
 using ChunkCodecLibBlosc: BloscCodec, BloscEncodeOptions
 using ChunkCodecLibBzip2: BZ2Codec, BZ2EncodeOptions
-using ChunkCodecLibLz4: LZ4FrameCodec, LZ4FrameEncodeOptions
+using ChunkCodecLibLz4: LZ4BlockCodec, LZ4FrameCodec, LZ4BlockEncodeOptions, LZ4FrameEncodeOptions
 using ChunkCodecLibZlib: ZlibCodec, ZlibEncodeOptions
 using ChunkCodecLibZstd: ZstdCodec, ZstdEncodeOptions, decode, encode
 using CodecXz: XzCompressor, XzDecompressor
@@ -53,6 +53,7 @@ struct BlockHeader
     used_size::UInt64
     data_size::UInt64
     checksum::AbstractVector{UInt8} # length 16
+    validate_checksum::Bool
 end
 
 mutable struct LazyBlockHeaders
@@ -90,6 +91,8 @@ big2native_U8(bytes::AbstractVector{UInt8}) = bytes[1]
 big2native_U16(bytes::AbstractVector{UInt8}) = (UInt16(bytes[1]) << 8) | bytes[2]
 big2native_U32(bytes::AbstractVector{UInt8}) = (UInt32(big2native_U16(@view bytes[1:2])) << 16) | big2native_U16(@view bytes[3:4])
 big2native_U64(bytes::AbstractVector{UInt8}) = (UInt64(big2native_U32(@view bytes[1:4])) << 32) | big2native_U32(@view bytes[5:8])
+# Read a 4-byte little-endian UInt32 (used for lz4.block store_size prefix)
+little2native_U32(bytes::AbstractVector{UInt8}) = (UInt32(bytes[1])) | (UInt32(bytes[2]) << 8) | (UInt32(bytes[3]) << 16) | (UInt32(bytes[4]) << 24)
 
 native2big_U8(val::UInt8) = UInt8[val]
 native2big_U16(val::UInt16) = UInt8[(val >>> 0x08) & 0xff, (val >>> 0x00) & 0xff]
@@ -111,7 +114,7 @@ native2big_U16(val::Integer) = native2big_U16(UInt16(val))
 native2big_U32(val::Integer) = native2big_U32(UInt32(val))
 native2big_U64(val::Integer) = native2big_U64(UInt64(val))
 
-function read_block_header(io::IO, position::Int64)
+function read_block_header(io::IO, position::Int64; validate_checksum::Bool)
     # Read block header
     max_header_size = 6 + 48
     header = Array{UInt8}(undef, max_header_size)
@@ -145,14 +148,14 @@ function read_block_header(io::IO, position::Int64)
         error("ASDF file header incorrectly specifies amount of space to use")
     end
 
-    return BlockHeader(io, position, token, header_size, flags, compression, allocated_size, used_size, data_size, checksum)
+    return BlockHeader(io, position, token, header_size, flags, compression, allocated_size, used_size, data_size, checksum, validate_checksum)
 end
 
-function find_all_blocks(io::IO, pos::Int64=Int64(0))
+function find_all_blocks(io::IO, pos::Int64=Int64(0); validate_checksum::Bool)
     headers = BlockHeader[]
     pos = find_next_block(io, pos)
     while pos !== nothing
-        header = read_block_header(io, pos)
+        header = read_block_header(io, pos; validate_checksum)
         push!(headers, header)
         pos = Int64(header.position + 6 + header.header_size + header.allocated_size)
         pos = find_next_block(io, pos)
@@ -170,7 +173,7 @@ function read_block(header::BlockHeader)
     end
 
     # Check checksum
-    if any(!iszero, header.checksum)
+    if header.validate_checksum && any(!iszero, header.checksum)
         actual_checksum = md5(data)
         if any(actual_checksum != header.checksum)
             error("Checksum mismatch in ASDF file header")
@@ -184,13 +187,13 @@ function read_block(header::BlockHeader)
         # do nothing, the block is uncompressed
     elseif compression == C_Xz
         data = transcode(XzDecompressor, data)
+    elseif compression == C_Lz4
+        data = decode_Lz4(data)
     else
         if compression == C_Blosc
             codec = BloscCodec()
         elseif compression == C_Bzip2
             codec = BZ2Codec()
-        elseif compression == C_Lz4
-            codec = LZ4FrameCodec()
         elseif compression == C_Zlib
             codec = ZlibCodec()
         elseif compression == C_Zstd
@@ -209,6 +212,46 @@ function read_block(header::BlockHeader)
     return data
 end
 
+function decode_Lz4(data)
+    if (# LZ4 Frame magic bytes 04 22 4D 18
+        length(data) >= 4 &&
+        data[1] == 0x04 && data[2] == 0x22 &&
+        data[3] == 0x4D && data[4] == 0x18)
+        return decode(LZ4FrameCodec(), data)
+    else
+        # If the data was originally created from Python's ASDF, then it will be in block instead of frame layout,
+        # where each chunk is:
+        #
+        #   [4 bytes, big-endian]    compressed chunk size (the ASDF envelope)
+        #   [4 bytes, little-endian] uncompressed chunk size (lz4.block store_size=True prefix)
+        #   [N bytes]                raw LZ4 block payload
+        #
+        # lz4.block.compress() defaults to store_size=True, which prepends the
+        # uncompressed size as a little-endian uint32. LZ4BlockCodec expects only
+        # the raw block, so both the outer BE envelope and the inner LE prefix must
+        # be stripped, with the LE value used as the uncompressed_size hint.
+
+        out = UInt8[]
+        pos = 1
+
+        while pos <= length(data)
+            # Outer ASDF envelope: big-endian compressed chunk size
+            compressed_chunk_size = Int(big2native_U32(@view data[pos:pos+3]))
+            pos += 4
+            # Inner lz4.block store_size=True prefix: little-endian uncompressed size
+            uncompressed_chunk_size = Int(little2native_U32(@view data[pos:pos+3]))
+            pos += 4
+            # Raw LZ4 block payload (compressed_chunk_size includes the 4-byte LE prefix)
+            payload_len = compressed_chunk_size - 4
+            payload = @view data[pos:pos+payload_len-1]
+            pos += payload_len
+            append!(out, decode(LZ4BlockCodec(), payload; max_size = uncompressed_chunk_size, size_hint = uncompressed_chunk_size))
+        end
+
+        return out
+    end
+end
+
 ################################################################################
 
 """
@@ -561,11 +604,25 @@ end
 
 ################################################################################
 
-asdf_constructors = copy(YAML.default_yaml_constructors)
-asdf_constructors["tag:stsci.edu:asdf/core/asdf-1.1.0"] = asdf_constructors["tag:yaml.org,2002:map"]
-asdf_constructors["tag:stsci.edu:asdf/core/software-1.0.0"] = asdf_constructors["tag:yaml.org,2002:map"]
+function load_file(filename::AbstractString; extensions = false, validate_checksum = true)
+    asdf_constructors = copy(YAML.default_yaml_constructors)
+    asdf_constructors["tag:stsci.edu:asdf/core/asdf-1.1.0"] = asdf_constructors["tag:yaml.org,2002:map"]
+    asdf_constructors["tag:stsci.edu:asdf/core/software-1.0.0"] = asdf_constructors["tag:yaml.org,2002:map"]
+    asdf_constructors["tag:stsci.edu:asdf/core/extension_metadata-1.0.0"] = asdf_constructors["tag:yaml.org,2002:map"]
+
+    if extensions
+        # Use fallbacks for now
+        asdf_constructors[nothing] = (constructor, node) -> begin
+            if node isa YAML.MappingNode
+                return YAML.construct_mapping(constructor, node)
+            elseif node isa YAML.SequenceNode
+                return YAML.construct_sequence(constructor, node)
+            else
+                return YAML.construct_scalar(constructor, node)
+            end
+        end
+    end
 
-function load_file(filename::AbstractString)
     io = open(filename, "r")
     lazy_block_headers = LazyBlockHeaders()
     construct_yaml_ndarray = make_construct_yaml_ndarray(lazy_block_headers)
@@ -574,12 +631,13 @@ function load_file(filename::AbstractString)
 
     asdf_constructors′ = copy(asdf_constructors)
     asdf_constructors′["tag:stsci.edu:asdf/core/ndarray-1.0.0"] = construct_yaml_ndarray
+    asdf_constructors′["tag:stsci.edu:asdf/core/ndarray-1.1.0"] = construct_yaml_ndarray
     asdf_constructors′["tag:stsci.edu:asdf/core/ndarray-chunk-1.0.0"] = construct_yaml_ndarray_chunk
     asdf_constructors′["tag:stsci.edu:asdf/core/chunked-ndarray-1.0.0"] = construct_yaml_chunked_ndarray
 
     metadata = YAML.load(io, asdf_constructors′)
     # lazy_block_headers.block_headers = find_all_blocks(io, position(io))
-    lazy_block_headers.block_headers = find_all_blocks(io)
+    lazy_block_headers.block_headers = find_all_blocks(io; validate_checksum)
     return ASDFFile(filename, metadata, lazy_block_headers)
 end
 
@@ -603,9 +661,10 @@ struct NDArrayWrapper
     array::AbstractArray
     compression::Compression
     inline::Bool
+    lz4_layout::Symbol
 end
-function NDArrayWrapper(array::AbstractArray; compression::Compression=C_Bzip2, inline::Bool=false)
-    return NDArrayWrapper(array, compression, inline)
+function NDArrayWrapper(array::AbstractArray; compression::Compression=C_Bzip2, inline::Bool=false, lz4_layout::Symbol=:block)
+    return NDArrayWrapper(array, compression, inline, lz4_layout)
 end
 Base.getindex(val::NDArrayWrapper) = val.array
 
@@ -655,6 +714,40 @@ function YAML._print(io::IO, val::NDArrayWrapper, level::Int=0, ignore_level::Bo
     YAML._print(io, ndarray, level, ignore_level)
 end
 
+function encode_Lz4_block(input::AbstractVector{UInt8}; chunk_size::Int = 1024 * 1024 * 8)
+    out = UInt8[]
+    offset = 1
+    while offset <= length(input)
+        chunk_end = min(offset + chunk_size - 1, length(input))
+        chunk = @view input[offset:chunk_end]
+
+        # Compress the raw chunk with LZ4 block codec
+        # LZ4BlockEncodeOptions does NOT prepend the uncompressed size,
+        # so we must prepend the LE uint32 ourselves to match Python's
+        # lz4.block.compress(store_size=True) behaviour.
+        compressed_payload = encode(LZ4BlockEncodeOptions(), chunk)
+        uncompressed_size = UInt32(length(chunk))
+        compressed_chunk_size = UInt32(4 + length(compressed_payload))  # LE prefix + raw payload
+
+        # Outer ASDF envelope: big-endian compressed chunk size (includes the 4-byte LE prefix)
+        append!(out, native2big_U32(compressed_chunk_size))
+
+        # Inner lz4.block store_size=True prefix: little-endian uncompressed size
+        append!(out, [
+            (uncompressed_size >>> 0x00) & 0xff,
+            (uncompressed_size >>> 0x08) & 0xff,
+            (uncompressed_size >>> 0x10) & 0xff,
+            (uncompressed_size >>> 0x18) & 0xff,
+        ])
+
+        # Raw LZ4 block payload
+        append!(out, compressed_payload)
+        offset = chunk_end + 1
+    end
+
+    return out
+end
+
 function write_file(filename::AbstractString, document::Dict{Any,Any})
     # Set up block descriptors
     global blocks
@@ -723,12 +816,15 @@ function write_file(filename::AbstractString, document::Dict{Any,Any})
             # TODO: Don't copy input
             input = input isa Vector ? input : Vector(input)
             data = transcode(XzCompressor, input)
+        elseif array.compression == C_Lz4 && array.lz4_layout == :block
+            data = encode_Lz4_block(input)
+            #data = encode(LZ4BlockEncodeOptions(), input) # Not compatible with Python asdf
         else
             if array.compression == C_Blosc
                 encode_options = BloscEncodeOptions(; clevel=9, doshuffle=2, typesize=sizeof(eltype(array.array)), compressor="zstd")
             elseif array.compression == C_Bzip2
                 encode_options = BZ2EncodeOptions(; blockSize100k=9)
-            elseif array.compression == C_Lz4
+            elseif array.compression == C_Lz4 && array.lz4_layout == :frame
                 encode_options = LZ4FrameEncodeOptions(; compressionLevel=12, blockSizeID=7)
             elseif array.compression == C_Zlib
                 encode_options = ZlibEncodeOptions(; level=9)
diff --git a/test/runtests.jl b/test/runtests.jl
index 145877a..28d5d15 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -62,7 +62,7 @@ const init_code = quote
         ))
         write(io, data_bytes)
         seekstart(io)
-        return ASDF.read_block_header(io, Int64(0))
+        return ASDF.read_block_header(io, Int64(0); validate_checksum = true)
     end
 end
 
diff --git a/test/test-blocks.jl b/test/test-blocks.jl
index 3f529bf..8ae339e 100644
--- a/test/test-blocks.jl
+++ b/test/test-blocks.jl
@@ -1,6 +1,6 @@
 function test_read_block_header(msg; kwargs...)
     io = make_raw_header(; kwargs...) |> IOBuffer
-    @test_throws msg ASDF.read_block_header(io, Int64(0))
+    @test_throws msg ASDF.read_block_header(io, Int64(0); validate_checksum = true)
 end
 
 function test_read_block(msg; kwargs...)
diff --git a/test/test-read_fallbacks.jl b/test/test-read_fallbacks.jl
new file mode 100644
index 0000000..d30cbe7
--- /dev/null
+++ b/test/test-read_fallbacks.jl
@@ -0,0 +1,120 @@
+function write_asdf(dir, body)
+    path = joinpath(dir, "temp.asdf")
+    open(path, "w") do io
+        print(io,
+            """
+            #ASDF 1.0.0
+            #ASDF_STANDARD 1.2.0
+            # This is an ASDF file <https://asdf-standard.readthedocs.io/>
+            %YAML 1.1
+            %TAG ! tag:stsci.edu:asdf/
+            ---
+            !core/asdf-1.1.0
+            $(body)
+            ...
+            """
+        )
+    end
+    return path
+end
+
+load_tag(tag; kwargs...) = mktempdir() do dir
+    path = write_asdf(dir, tag)
+    af = ASDF.load_file(path; kwargs...)
+end
+
+@testset "unknown mapping" begin
+    tag_unknown_mapping = """
+    known_key: hello
+    custom_obj: !<tag:example.org:mylib/widget-1.0.0>
+      width: 42
+      height: 7
+    """
+
+    # The default `extensions = false` case
+    @test_throws Exception load_tag(tag_unknown_mapping; extensions = false)
+    @test_throws Exception load_tag(tag_unknown_sequence; extensions = false)
+    @test_throws Exception load_tag(tag_unknown_scalar; extensions = false)
+
+    # Should fall back to an `AbstractDict`
+    af = load_tag(tag_unknown_mapping; extensions = true)
+    obj = af.metadata["custom_obj"]
+    @test obj isa AbstractDict
+    @test obj["width"]  == 42
+    @test obj["height"] == 7
+
+    # Known key still parsed as normal
+    @test af.metadata["known_key"] == "hello"
+
+    # And loading the file multiple times does not mutate any shared/global state
+    af1 = load_tag(tag_unknown_mapping; extensions = true)
+    af2 = load_tag(tag_unknown_mapping; extensions = true)
+    @test af1.metadata["custom_obj"]["width"] == af2.metadata["custom_obj"]["width"]
+end
+
+@testset "unknown sequence" begin
+    tag_unknown_sequence = """
+    known_key: hello
+    custom_list: !<tag:example.org:mylib/series-1.0.0>
+      - alpha
+      - beta
+      - gamma
+    """
+
+    # Should fall back to an `AbstractVector`
+    af = load_tag(tag_unknown_sequence; extensions = true)
+    list = af.metadata["custom_list"]
+
+    @test list isa AbstractVector
+    @test length(list) == 3
+    @test list[1] == "alpha"
+    @test list[2] == "beta"
+    @test list[3] == "gamma"
+
+end
+
+@testset "unknown scalar" begin
+    tag_unknown_scalar = """
+    known_key: hello
+    custom_value: !<tag:example.org:mylib/quantity-1.0.0> 3.14
+    """
+
+    # Should fall back to an `AbstractString`
+    af = load_tag(tag_unknown_scalar; extensions = true)
+    value = af.metadata["custom_value"]
+
+    @test value isa AbstractString
+    @test value == "3.14"
+end
+
+@testset "unknown all" begin
+    tag_unknown_all = """
+    known_key: hello
+    mapping_node: !<tag:example.org:mylib/widget-1.0.0>
+      width: 42
+      height: 7
+    sequence_node: !<tag:example.org:mylib/series-1.0.0>
+      - alpha
+      - beta
+    scalar_node: !<tag:example.org:mylib/quantity-1.0.0> 3.14
+    """
+
+    # Fallbacks should also work if all unknowns are present in the same file.
+    af = load_tag(tag_unknown_all; extensions = true)
+    md = af.metadata
+
+    # Mapping branch
+    @test md["mapping_node"] isa AbstractDict
+    @test md["mapping_node"]["width"] == 42
+
+    # Sequence branch
+    @test md["sequence_node"] isa AbstractVector
+    @test md["sequence_node"][1] == "alpha"
+
+    # Scalar branch
+    @test md["scalar_node"] isa AbstractString
+    @test md["scalar_node"] == "3.14"
+
+    # Known key unaffected
+    @test md["known_key"] == "hello"
+end
diff --git a/test/test-write.jl b/test/test-write.jl
index 57d935a..4960107 100644
--- a/test/test-write.jl
+++ b/test/test-write.jl
@@ -10,10 +10,11 @@
             "element1" => ASDF.NDArrayWrapper(array; compression=ASDF.C_None),
             "element2" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Blosc),
             "element3" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Bzip2),
-            "element4" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Lz4),
-            "element5" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Xz),
-            "element6" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Zlib),
-            "element7" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Zstd),
+            "element4" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Lz4, lz4_layout=:block),
+            "element5" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Lz4, lz4_layout=:frame),
+            "element6" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Xz),
+            "element7" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Zlib),
+            "element8" => ASDF.NDArrayWrapper(array; compression=ASDF.C_Zstd),
         ),
     )
     ASDF.write_file(filename, doc)
@@ -33,7 +34,7 @@
     @test size(data2′) == size(data2)
     @test data2′ == data2
 
-    for n in 1:7
+    for n in 1:8
         element = doc["group"]["element$n"][]
         element′ = doc′.metadata["group"]["element$n"][]
         @test eltype(element′) == eltype(element)