RUDPFramework/openspec/changes/fix-controlled-player-jitter-step1-replay-granularity/specs/client-gameplay-input/spec.md

client-gameplay-input Specification

MODIFIED Requirements

Requirement: Controlled client movement input preserves immediate prediction and explicit stop signaling

The MVP client SHALL capture movement intent for the controlled player in Unity-side input code, apply local movement prediction immediately, and send MoveInput updates through the networking boundary. When movement input transitions from non-zero to idle, the client MUST send one final zero-vector MoveInput so authoritative movement can stop cleanly. When the client reconciles against authoritative state and replays pending MoveInput messages, the replay path MUST apply each pending input in fixed-duration substeps matching the server authoritative movement cadence, so that replay trajectory matches live prediction trajectory for the same input sequence.

Scenario: Controlled player moves locally without waiting for the network

• WHEN the controlled player provides non-zero movement input
• THEN the client applies local movement prediction immediately for presentation
• THEN the client submits a MoveInput carrying the current player id, tick, and planar movement vector through the networking send path

Scenario: Releasing movement emits an explicit stop update

• WHEN the controlled player releases movement input after previously providing non-zero movement
• THEN the client sends exactly one final MoveInput whose movement vector is zero
• THEN local predicted movement also stops immediately without waiting for authoritative correction

Scenario: Replay uses fixed-step substeps matching server cadence

• WHEN the client accepts an authoritative PlayerState and replays pending MoveInput messages
• THEN each PredictedMoveStep is consumed by applying its input in fixed-duration substeps equal to the server authoritative movement cadence
• THEN the replay accumulation shape is identical to the live FixedUpdate prediction path for the same input values
• THEN non-linear trajectories (e.g. simultaneous turn-and-move) produce the same result in both replay and live prediction