diff --git a/packages/abalone-engine/package.json b/packages/abalone-engine/package.json
index 3b9bf127..e00f5bba 100644
--- a/packages/abalone-engine/package.json
+++ b/packages/abalone-engine/package.json
@@ -19,9 +19,11 @@
     "./types": "./src/types.ts"
   },
   "scripts": {
-    "test": "vitest run"
+    "test": "vitest run",
+    "test:coverage": "vitest run --coverage"
   },
   "devDependencies": {
+    "@vitest/coverage-v8": "4.1.12",
     "typescript": "5.9.3",
     "vitest": "4.1.12"
   }
diff --git a/packages/abalone-engine/src/game-state.test.ts b/packages/abalone-engine/src/game-state.test.ts
index b5414059..b927c331 100644
--- a/packages/abalone-engine/src/game-state.test.ts
+++ b/packages/abalone-engine/src/game-state.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest"
 import type { SetupKey } from "#abalone-engine/board-setups"
-import type { GameState } from "#abalone-engine/game-state"
+import type { GameState, MoveDetails } from "#abalone-engine/game-state"
 import {
   canNextMove,
   canPrevMove,
@@ -107,6 +107,14 @@ describe("isValidSelection", () => {
     expect(isValidSelection(state, "0,0")).toBe(false)
     expect(isValidSelection(resignGame(state), "4,0")).toBe(false)
   })
+
+  it("treats a bot game with no colour recorded as the human playing black", () => {
+    const unset = {
+      ...createGameState("standard", "white", "ai"),
+      playerColor: undefined as unknown as GameState["playerColor"],
+    }
+    expect(isValidSelection(unset, "4,0")).toBe(true)
+  })
 })
 
 describe("makeMove", () => {
@@ -236,6 +244,29 @@ describe("walking through history", () => {
     expect(goToMove(played, -1)).toBe(played)
     expect(goToMove(played, 99)).toBe(played)
   })
+
+  //only makeMove writes history, so these entries are hand-edited: the board
+  //must still restore and simply skip a highlight it cannot work out
+  it("restores an entry whose move it cannot draw", () => {
+    const withDetails = (patch: Partial<MoveDetails>): GameState => {
+      const history = [...played.moveHistory]
+      const details = history[1].moveDetails as MoveDetails
+      history[1] = { ...history[1], moveDetails: { ...details, ...patch } }
+      return { ...played, moveHistory: history }
+    }
+
+    const noMarbles = goToMove(withDetails({ marbles: [] }), 1)
+    expect(noMarbles.black.has("1,0")).toBe(true)
+    expect(noMarbles.lastMove).toBeNull()
+
+    const farDestination = withDetails({ destination: "-4,0" })
+    expect(goToMove(farDestination, 1).lastMove).toBeNull()
+
+    const noShoved = withDetails({
+      shovedMarbles: undefined as unknown as string[],
+    })
+    expect(goToMove(noShoved, 1).lastMove?.shovedMarbles).toEqual([])
+  })
 })
 
 describe("truncateToMove", () => {
@@ -278,6 +309,15 @@ describe("undoTargetIndex", () => {
     expect(undoTargetIndex(ai)).toBe(2)
   })
 
+  it("takes back as black when the bot game has no colour recorded", () => {
+    const ai = play(createGameState("standard", "white", "ai"), SHUFFLE)
+    const unset = {
+      ...ai,
+      playerColor: undefined as unknown as GameState["playerColor"],
+    }
+    expect(undoTargetIndex(unset)).toBe(2)
+  })
+
   it("has nothing to take back at the start or before the player has moved", () => {
     expect(undoTargetIndex(createGameState())).toBe(-1)
     const botOpened = play(
diff --git a/packages/abalone-engine/src/moves.test.ts b/packages/abalone-engine/src/moves.test.ts
index e03f7c0d..ca891a2f 100644
--- a/packages/abalone-engine/src/moves.test.ts
+++ b/packages/abalone-engine/src/moves.test.ts
@@ -85,6 +85,16 @@ describe("destinationsFor — plain moves", () => {
     expect(new Set(backward)).toEqual(new Set(forward))
   })
 
+  it("offers a line across the rows in one order whichever end leads", () => {
+    //the step from "1,0" to "0,0" climbs a row, which CANONICAL_HEADING flips,
+    //so this reading takes the other branch from the one above
+    const black = ["0,0", "1,0"]
+    const downward = destinations(black, [], ["0,0", "1,0"])
+    const upward = destinations(black, [], ["1,0", "0,0"])
+    expect(upward).toEqual(downward)
+    expect(upward).toHaveLength(6)
+  })
+
   it("blocks a broadside when any landing square is taken", () => {
     const found = destinations(["0,0", "0,1"], ["1,0"], ["0,0", "0,1"])
     expect(found).not.toContain("1,1")
diff --git a/packages/abalone-engine/src/rules.test.ts b/packages/abalone-engine/src/rules.test.ts
index 5f2b3c1e..5740352c 100644
--- a/packages/abalone-engine/src/rules.test.ts
+++ b/packages/abalone-engine/src/rules.test.ts
@@ -73,6 +73,15 @@ describe("selectMarble", () => {
       selectMarble(board, ["0,0", "0,1", "0,2"], "1,0", "black"),
     ).toEqual(["1,0"])
   })
+
+  it("grows a white line and starts over on black", () => {
+    const whites = boardOf(["1,1"], ["0,3", "0,4"])
+    expect(selectMarble(whites, ["0,3"], "0,4", "white")).toEqual([
+      "0,3",
+      "0,4",
+    ])
+    expect(selectMarble(whites, ["0,3"], "1,1", "black")).toEqual(["1,1"])
+  })
 })
 
 describe("deselectMarble", () => {
@@ -104,6 +113,14 @@ describe("selectRun", () => {
     expect(selectRun(board, "1,1", "0,0")).toBeNull()
     expect(selectRun(board, "0,0", "9,9")).toBeNull()
   })
+
+  it("returns null when the run would leave the board on the way", () => {
+    //from the rim the walk runs off the board on several headings before it
+    //finds the reach, and must give up on each rather than wrap around
+    const rim = boardOf(["0,4", "0,3", "0,2"], [])
+    expect(selectRun(rim, "0,4", "0,2")).toEqual(["0,4", "0,3", "0,2"])
+    expect(selectRun(rim, "0,4", "-4,4")).toBeNull()
+  })
 })
 
 describe("getPossibleMoves", () => {
diff --git a/packages/abalone-engine/vitest.config.ts b/packages/abalone-engine/vitest.config.ts
index a19f7b0d..65c8dede 100644
--- a/packages/abalone-engine/vitest.config.ts
+++ b/packages/abalone-engine/vitest.config.ts
@@ -13,5 +13,20 @@ export default defineConfig({
   },
   test: {
     environment: "node",
+    coverage: {
+      provider: "v8",
+      include: ["src/**/*.ts"],
+      exclude: ["src/**/*.test.ts"],
+      reporter: ["text", "json-summary"],
+      //today's numbers rounded down: the engine holds every rule of the game,
+      //so a change that ships an untested branch here fails the CI gate, which
+      //runs this script instead of the plain test one
+      thresholds: {
+        lines: 99,
+        branches: 98,
+        functions: 100,
+        statements: 99,
+      },
+    },
   },
 })
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index e3dc9838..a3b2b10f 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -229,6 +229,9 @@ importers:
 
   packages/abalone-engine:
     devDependencies:
+      '@vitest/coverage-v8':
+        specifier: 4.1.12
+        version: 4.1.12(vitest@4.1.12)
       typescript:
         specifier: 5.9.3
         version: 5.9.3