Gear Builtin Actors

Goal

Treat builtin actors as a distinct call target: runtime-provided pseudo-programs at hardcoded ActorIds, reached via the normal msg::send_for_reply family but with request and reply types defined in the gbuiltin-* helper crates, not in a Sails IDL.

Inputs

• ../../references/gear-builtin-actors.md — actor catalog, ActorId derivation, per-actor request/response shapes, calling pattern
• ../../references/gear-messaging-and-replies.md — reply semantics for send_for_reply
• ../../references/gear-gas-reservations-and-waitlist.md — gas budgeting and waitlist interaction when awaiting builtin replies
• ../../references/gear-gstd-api-and-syscalls.md — the underlying gstd::msg API family

Route Here When

• a feature needs staking, nominating, proxy management, BLS12-381 pairings, or ETH bridging from inside a Sails program
• a send_for_reply targets a hardcoded non-program ActorId and the reply does not decode as a Sails route
• a reply bytes stream is expected to decode as a gbuiltin_*::Response rather than a Sails-routed payload
• a builder is unsure whether to call a builtin directly from a service handler or wrap it behind a dedicated broker service
• a gtest or local-smoke run fails at the builtin call site with gas or ED errors
• a runtime upgrade changes the set of registered builtins and existing hardcoded ActorId constants need revalidation

Working Model

1. Confirm the actor is registered in the target runtime. For Vara, the authoritative tuple is BuiltinActors in gear/vara/runtime/vara/src/lib.rs (IDs 1 = BLS12-381, 2 = staking, 3 = ETH bridge, 4 = proxy). IDs are runtime-version-bound.
2. Pick the correct gbuiltin-* helper crate: gbuiltin-staking, gbuiltin-proxy, gbuiltin-bls381, gbuiltin-eth-bridge. Import its Request (and Response where defined).
3. Derive the ActorId from the builtin ID via hash((b"built/in", id).encode()), or take the stable hex from the reference. Store as a const ActorId in the module that owns the call.
4. Call from an async handler: msg::send_bytes_for_reply(ACTOR_ID, &payload[..], value, reply_deposit)?.await. Value and reply deposit follow Substrate ED rules for the builtin's underlying pallet.
5. Classify the reply: success reply decodes as gbuiltin_*::Response (where one is defined) or is empty for fire-and-forget variants; an error reply surfaces as the .await Err arm and carries a BuiltinActorError payload.
6. Budget gas against the builtin's declared max_gas() weight; block allowance failures manifest as error replies, not panics.
7. For first-time interactions with a new builtin address, confirm ED is minted (runtime migrations handle this for shipped builtins — only relevant on fresh chains or new actors).
8. When a service wraps a builtin (broker pattern), keep the wrapping service responsible for idempotency, reply routing, and state reconciliation — the builtin itself has no memory, no read_state, and no code.

Guardrails

• Do not hardcode ActorId literals without citing the runtime file or using hash((b"built/in", id).encode()) — IDs are runtime-version-bound and change with registration changes.
• Do not expect Sails Header framing on builtin replies. Decode with Response::decode(&mut &bytes[..]) from the matching gbuiltin-* crate, not with a generated Sails client.
• Do not call builtins from a sync handler that cannot await; use #[gstd::async_main] or an async service method.
• Do not treat builtin calls as free. Gas is charged against the sender's block allowance, and ED rules apply to any value transferred.
• Do not confuse builtin IDs with actor IDs in broker examples; the ID is a u64 registration key, the ActorId is its hashed account.
• If the task is really about adding a new builtin actor to the runtime itself, stop — that is runtime-maintenance work inside the gear repo, not Sails application work covered by this pack.
• If the target chain is Vara.eth or ethexe, stop — the builtin-actor set differs and routes through a different pack.