Module step 

The per-message coordination step — the pure, I/O-free kernel invariants.

Every accepted MACP message passes the same per-message invariants: dedup (RFC-MACP-0001 §8 idempotency), mode-binding, TTL, and the monotonic OPEN gate (§7.2/§7.3), then mode validation, then commit. Historically these lived welded into the gRPC server’s process_message, so any other consumer of the coordination core (e.g. an embedding library) had to re-implement them and risk drift. This module hosts them once — synchronous and free of tokio, storage, transport, and the wall clock (the caller injects now_ms).

Two ways to drive it:

• step — all-in-one, for in-memory consumers that do not interpose durable storage between validation and commit.
• check_preconditions + validate_message + commit — the phases, for a durable consumer (the runtime) that must write the message to its append-only log between validation and commit, so a failed write never consumes a dedup slot.

Enums

Precheck

Outcome of the mode-independent precondition checks.

StepOutcome

Outcome of step.

Functions

check_preconditions

Mode-independent per-message invariants. Pure: no mutation, no I/O, no clock.

commit

Commit a validated message into the session: consume the dedup slot, record participant activity, and apply the mode response. Returns the resulting session state.

step

All-in-one per-message step for in-memory consumers: preconditions → mode validation → commit, mirroring the runtime’s external contract. Expiry marks the session Expired and returns MacpError::TtlExpired; a duplicate is reported as StepOutcome::Duplicate; any other rejection returns its error without consuming a dedup slot or applying state.

validate_message

Mode-dependent validation: sender authorization + mode rules. Pure — returns the ModeResponse to apply and mutates nothing. Call only after check_preconditions returns Precheck::Proceed.