# 0003 — postcard over FlatBuffers

**Status:** accepted
**Date:** 2026-06-18

## Context

Need a binary serialization format for the TCP protocol between server and ESP32 clients. Must be compact (bandwidth-constrained), ideally `no_std` compatible for future bare-metal targets.

## Decision

Use `postcard` with `serde` derives instead of FlatBuffers.

## Alternatives considered

**FlatBuffers** — zero-copy reads, schema-driven. But requires external codegen toolchain (flatc), `no_std` support is limited, and zero-copy is overkill for our small messages (a handful of widgets).

**Custom binary protocol** — maximum control, zero deps. But significant implementation effort for serialize/deserialize code, error-prone, and postcard already solves this.

## Consequences

- `no_std` protocol crate out of the box — future-proof for bare-metal targets.
- No codegen step — types are normal Rust structs with serde derives.
- Compact wire format via varint encoding.
- No zero-copy reads — messages are deserialized into owned types. Acceptable for our message sizes.
- Adds `serde` and `postcard` as dependencies to the protocol crate.