fix(rivetkit): treat empty actor input bytes as absent

[abcxff]

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[abcxff]

Stack for rivet-dev/rivet

• #5557 docs(container-runner): sync deploy docs with runner source
• #5556 fix(container-runner): read cgroup v1 memory usage under gvi …
• #5548 feat(container-runner): log version and git sha at actor sta …
• #5549 feat(container-runner): setup instance memory and cpu usage …
• #5546 fix(rivetkit): keep actor event loop alive through shutdown …
• #5539 feat(container-runner): report actors as crashed on unexpect …
• #5536 feat(container-runner): log unexpected platform SIGTERM as a …
• #5530 fix(rivetkit-core): cancel driver alarm before sqlite teardo …
• #5531 feat(container-runner): keep instance warm instead of self-e …
• #5533 chore(container-runner): default stop grace to 10s
• #5528 fix(rivetkit): log and surface actor startup failures
• #5529 fix(rivetkit): treat empty actor input bytes as absent 👈
• #5558 fix(sdks): drop rivet-util metrics dep from protocol crates …
• main

Get stack: forklift get 5529
Push local edits: forklift submit
Merge when ready: forklift merge 5529

_{change ouvoqtns}

[railway-app]

🚅 Deployed to the rivet-pr-5529 environment in rivet-frontend

Service	Status	Web	Updated (UTC)
frontend-cloud	😴 Sleeping (View Logs)	Web	Aug 7, 2026 at 1:13 am
website	😴 Sleeping (View Logs)	Web	Aug 2, 2026 at 7:07 pm
kitchen-sink	😴 Sleeping (View Logs)	Web	Aug 2, 2026 at 7:58 am
frontend-inspector	😴 Sleeping (View Logs)	Web	Aug 2, 2026 at 7:30 am
ladle	✅ Success (View Logs)	Web	Jul 31, 2026 at 4:13 am
mcp-hub	✅ Success (View Logs)	Web	Jul 31, 2026 at 4:13 am

[claude]

Review

Small, well-scoped fix. Input::decode()/decode_or() now normalize a zero-length byte buffer to "absent" via a new present_bytes() helper, mirroring the existing decode_conn_params (bytes.is_empty() || bytes == [0xf6]) and Snapshot::decode patterns already in this file. Since a zero-length buffer is never valid CBOR on its own, this does not lose any real information: an empty payload could never have decoded successfully before this change, so treating it as "missing" instead of surfacing a CBOR decode error is a strict improvement. Good test coverage for both the unit-input and Default-input cases, plus a regression test confirming genuinely invalid (non-empty, malformed) CBOR still fails start.

One thing worth a look:

• Input::is_present() (rivetkit-rust/packages/rivetkit/src/start.rs:44) still does self.bytes.is_some() and was not updated alongside present_bytes(). After this change, is_present() can return true for the same zero-length buffer that decode()/decode_or() now treat as absent. Input is a public type (pub mod start), so a caller doing if input.is_present() { input.decode() } could still hit MissingInput (or silently default via decode_or) despite is_present() reporting true. The two methods now disagree about what "present" means. Consider either routing is_present() through present_bytes() too, or adding a doc comment on is_present() clarifying that it does not filter empty buffers, so the asymmetry is intentional and discoverable rather than surprising.

Otherwise no correctness, performance, or security concerns. This is an internal, in-process CBOR decode path with no untrusted-boundary implications beyond what already existed.