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+# 50-Move Rule — Design Spec
+**Branch:** feat/NCS-11
+**Date:** 2026-03-30
+
+---
+
+## Overview
+
+Implement the 50-move rule: a player may claim a draw if no pawn move or capture has occurred in the last 50 half-moves (plies). The rule is **not** enforced automatically — the eligible player must actively claim it via a TUI menu option.
+
+---
+
+## Architecture
+
+### Approach
+Thread `halfMoveClock: Int` explicitly through `processMove` and `gameLoop` (Approach A). This mirrors the existing `halfMoveClock` field in `GameState` (already parsed from FEN) and requires no changes to `GameHistory` or `HistoryMove`.
+
+---
+
+## Section 1: Data & Signatures
+
+### `MoveResult` changes (`GameController.scala`)
+
+```scala
+case class Moved(newBoard: Board, newHistory: GameHistory, captured: Option[Piece],
+                 newHalfMoveClock: Int, newTurn: Color) extends MoveResult
+case class MovedInCheck(newBoard: Board, newHistory: GameHistory, captured: Option[Piece],
+                        newHalfMoveClock: Int, newTurn: Color) extends MoveResult
+case object DrawClaimed extends MoveResult
+```
+
+`Stalemate` remains unchanged (automatic draw, no clock involved).
+
+### `processMove` signature
+
+```scala
+def processMove(board: Board, history: GameHistory, turn: Color,
+                halfMoveClock: Int, raw: String): MoveResult
+```
+
+### `gameLoop` signature
+
+```scala
+def gameLoop(board: Board, history: GameHistory, turn: Color, halfMoveClock: Int): Unit
+```
+
+### Clock update logic (inside `processMove`, after a legal move)
+
+```scala
+val movedPiece = board.pieceAt(from).get
+val isReset = movedPiece.pieceType == PieceType.Pawn || captured.isDefined || isEP
+val newClock = if isReset then 0 else halfMoveClock + 1
+```
+
+---
+
+## Section 2: TUI Menu / `gameLoop` Behaviour
+
+When `halfMoveClock >= 50` at the start of a player's turn, `gameLoop` shows a special prompt **before** asking for a move:
+
+```
+[50-move rule] You may claim a draw, or continue playing.
+  1. Claim draw
+  2. Continue
+```
+
+- **Option 1:** `processMove` is called with `raw = "draw"` → returns `DrawClaimed` → prints `"Draw claimed by 50-move rule."` then restarts with the initial board.
+- **Option 2:** Falls through to the normal move prompt for that same turn. The clock is **not** reset by choosing to continue — it only resets on a pawn move or capture.
+- If `halfMoveClock < 50`: no menu shown, normal move prompt only.
+- If `raw = "draw"` arrives in `processMove` with `halfMoveClock < 50`: treated as `InvalidFormat("draw")`.
+
+---
+
+## Section 3: Testing
+
+All tests in `GameControllerTest` (`AnyFunSuite with Matchers`, pure unit tests on `processMove`):
+
+| Test | Expected result |
+|------|----------------|
+| `raw = "draw"`, `halfMoveClock = 50` | `DrawClaimed` |
+| `raw = "draw"`, `halfMoveClock = 49` | `InvalidFormat("draw")` |
+| After a pawn move | `newHalfMoveClock == 0` |
+| After a capture | `newHalfMoveClock == 0` |
+| After an en-passant capture | `newHalfMoveClock == 0` |
+| After a quiet piece move, clock = 10 | `newHalfMoveClock == 11` |
+| `Moved` result carries updated clock | clock present in result |
+| `MovedInCheck` result carries updated clock | clock present in result |
+
+No changes needed to `GameRulesTest`, `FenParserTest`, or `FenExporterTest`.
+
+---
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala` | Add `halfMoveClock` param to `processMove` and `gameLoop`; add clock update logic; add draw claim handling; update `MoveResult` cases |
+| `modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala` | Add tests for all clock and draw claim scenarios |