Flet protocol framing upgraded for DataChannel support

:::note This guide is accurate as of Flet 0.86.0. Later releases might add new APIs or additional migration paths.

The breaking changes and deprecations index lists the guides created for each release. :::

Summary

Flet 0.86.0 introduces dedicated data channels for widgets that need to move bulk binary data (image frames, audio buffers, ML tensors) between Dart and Python without going through the MsgPack control protocol.

Enabling this required a one-time upgrade of the Flet protocol's wire format:

• Stream-oriented transports (flet run dev mode over UDS / TCP) now use 4-byte little-endian length-prefixed framing. The previous streaming msgpack.Unpacker.feed decode is gone.
• Every transport (sockets, WebSocket, dart_bridge FFI, Pyodide postMessage) now puts a 1-byte type discriminator at the head of each packet:
  • 0x00 — MsgPack-encoded Flet control frame (widget state, events) — same contents as before, just with a type byte in front.
  • 0x01 — raw DataChannel frame ([channel_id:u32 LE][payload]).

The new format is not backwards-compatible: a Flet 0.85 client cannot talk to a Flet 0.86 server, and vice versa. The flet-cli dev server and the in-process Python runtime were upgraded together in this release.

Background

Flet's previous wire format on UDS / TCP relied on msgpack.Unpacker.feed to re-assemble messages from arbitrary byte-stream chunks. That works for MsgPack-only traffic but doesn't compose with a second logical stream of raw bytes — you can't tell a partial MsgPack value from the start of a raw frame without an out-of-band marker.

Adding the 1-byte type byte + 4-byte length prefix solves this in the smallest possible way:

• Receivers read [length][type][...], fan out by type byte, no streaming decoder state to maintain.
• Message-oriented transports (WebSocket, postMessage, dart_bridge) drop the length prefix since each message is already one packet; they keep the type byte.
• Per-frame overhead is 5 bytes (message-oriented) or 9 bytes (stream-oriented) — under 1% at any payload size that motivates a data channel.

Migration guide

Most users — nothing to change

If you only use flet build artifacts or run flet run with the matching flet package version (the default install pulls both at once), there's nothing to do. The CLI and the runtime are version-locked.

Users running flet-cli and the flet Python package from different installs

Make sure both come from the same release. The common gotcha is a global flet install plus a project-local flet in .venv at different versions — upgrade or pin both to ≥ 0.86.0 (or both to ≤ 0.85.x).

A version mismatch will surface as a connection failure during flet run with a parse error in the dev-server log.

Users with custom backends or sidecars speaking the Flet protocol

The wire format changed. Update your encoder/decoder:

• Inbound on a stream transport (UDS / TCP): read 4 bytes (length, u32 LE), read that many bytes, the first byte is the type discriminator, the rest is the payload.
• Inbound on a message transport (WebSocket, postMessage): each message's first byte is the type discriminator, the rest is the payload.
• Outbound: same shape, mirrored.

Treat 0x01 (raw DataChannel) frames as opaque if you don't use data channels — forward them along or drop them, never feed them to your MsgPack decoder.

Code that depended on StreamingMsgpackDeserializer

The Dart-side class package:flet/src/transport/streaming_msgpack_deserializer.dart is removed. There are no public consumers — it was an internal helper for stream-transport framing that's no longer needed now that every packet is length-delimited. If you imported it (you shouldn't have), decode each inbound buffer with one-shot msgpack.deserialize(bytes) instead.

Custom widgets that override MatplotlibChartCanvas

MatplotlibChartCanvas now transports its apply_full / apply_diff / clear frames through a [DataChannel] rather than _invoke_method arguments. The Python-facing method signatures are unchanged (apply_full(image_bytes: bytes), etc.), and they remain the documented API. But if you subclassed the canvas and overrode the Dart-side _invokeMethod handler, that override no longer fires — the Dart side now consumes a 1-byte opcode + PNG payload from the channel directly.

Timeline

• Changed in: 0.86.0

References

• API: flet.DataChannel (Python) and FletBackend.openDataChannel (Dart) — see the module docstrings in flet/data_channel.py; dedicated reference pages will be added in a follow-up.
• Release notes: Flet 0.86.0