feat(rivetkit): JSON_COMPAT_UINT8_ARRAY byte-payload encoding parity with TS (encode + decode + cross-language tests)

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Stack for rivet-dev/rivet

• feat(rivetkit-agent-os): phase 3 process — 11 arms (exec/spawn/wait/kill/stop/list/all/tree/get/writeStdin/closeStdin) + ExecResultDto #5197 unkrsszt · 2026-06-08 18:29:29
• test(rivetkit-agent-os): partial-failure driver test for readFiles batch DTO (Option<content>, Option<error>) #5196 pzzzpmtr · 2026-06-08 18:29:28
• feat(rivetkit-agent-os): phase 3 filesystem — mkdir/readdir/exists/move/deleteFile/readFiles/writeFiles/readdirRecursive arms + batch DTOs #5195 onyykwlk · 2026-06-08 18:29:27
• test(rivetkit-agent-os): config parsing tests + documented JS↔Rust software shape mismatch (phase 3 prep) #5194 ouyvrlxt · 2026-06-08 18:29:26
• feat(rivetkit-agent-os): phase 2 — stat action arm + structured-payload wrapper tests for cross-encoding parity #5193 slkvonmt · 2026-06-08 18:29:25
• feat(rivetkit): phase 1c — agentOs() native ladder + engine driver branch + driver-suite e2e green #5192 nxzrussy · 2026-06-08 18:29:24
• feat(rivetkit-napi): phase 1b — NapiActorFactory.fromAgentOs static constructor + writeFile arm + e2e factory round-trip #5191 mtlnunov · 2026-06-08 18:29:22
• feat(rivetkit-agent-os): phase 1a — rust crate skeleton + readFile arm + dispatcher e2e against real sidecar #5190 wrxkrssv · 2026-06-08 18:29:21
• feat(rivetkit): JSON_COMPAT_UINT8_ARRAY byte-payload encoding parity with TS (encode + decode + cross-language tests) #5189 ukssumln · 2026-06-08 18:29:20 👈
• chore: phase 0 prereqs (rivetkit workspace member, serde_bare align, wasm skip.agentOs default) #5188 qvnkvkxw · 2026-06-08 18:29:19
• docs: PLAN2 + TODOLIST + RIVETKIT_RUST_FIX (clean slate, post-wipe) #5187 yqrwozvn · 2026-06-08 18:29:18
• main

Check out this stack: forklift get https://github.com/rivet-dev/rivet/pull/5189
Pull/update this stack: forklift sync
Publish local edits: forklift submit
Merge when ready: forklift merge

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Code Review

Overview

This PR adds byte-payload encoding/decoding parity between the Rust and TypeScript RivetKit implementations. It introduces a serde serializer adapter that intercepts serialize_bytes calls and wraps them as ["$Uint8Array", base64] 2-element tuples (matching the TS encodeJsonCompatValue convention), and a corresponding revival pass on the client side. The change is applied to action responses, workflow history, and workflow replay paths.

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Functional Issues

Missing is_human_readable() forwarding (potential bug)

JsonCompatSerializer does not override is_human_readable(), so it always returns the serde default of true. ciborium returns false. Types that branch on this (e.g. uuid::Uuid serializes as a hyphenated string when true, as 16 raw bytes when false) will serialize incorrectly when wrapped in JsonCompatSerializer. The fix is to add:

fn is_human_readable(&self) -> bool {
    self.inner.is_human_readable()
}

skip_field not delegated in compound serializers

JsonCompatSerializeStruct and JsonCompatSerializeTupleStruct do not implement skip_field. The serde trait default is a no-op, but ciborium needs to stay in sync with the len hint passed to serialize_struct. Structs with #[serde(skip_serializing_if = "...")] fields could produce malformed CBOR. Each compound wrapper should delegate skip_field to self.inner.skip_field(key).

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Dependency cleanup

• serde_bytes in [dependencies]: rivetkit/src/encoding.rs does not import serde_bytes; it only appears in tests. Move serde_bytes from [dependencies] to [dev-dependencies] only.
• Redundant base64 in [dev-dependencies]: base64 is already a regular dependency; the [dev-dependencies] entry is redundant.

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CLAUDE.md convention violations

Module doc comments in both rivetkit/src/encoding.rs and client/src/encoding.rs contain em dashes. CLAUDE.md says not to use em dashes; use periods to separate sentences instead.

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Tests

• Tests correctly live in tests/ (not inline), satisfying CLAUDE.md.
• Good coverage: bare ByteBuf, nested struct, Option<ByteBuf>, Vec<ByteBuf>, plain Vec<u8> staying as an integer array, primitives, 3-element arrays not falsely matching.
• encoding_roundtrip.rs correctly verifies the full encode-to-decode path.
• The cross-language fixture approach (encoding_fixtures.rs + byte-encoding-parity.test.ts) is solid. Worth adding a comment in byte-encoding-parity.test.ts noting fixtures must be regenerated via cargo test -p rivetkit --test encoding_fixtures if the Rust encoding changes.
• All four .json fixture files are missing trailing newlines. Fix: append a newline after serde_json::to_string_pretty in write_fixture.

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Design (informational)

• The documented caveat that revive_json_compat leaves base64 strings in serde_json::Value is correct and well-explained.
• Applying revive_json_compat to all three encoding paths in decode_http_action_response is correct since both Rust and TS actors share the same wire convention.
• Scoping to $Uint8Array only is appropriate for current consumers.