diff --git a/packages/mnswpr/core/index.js b/packages/mnswpr/core/index.js
index c0e4672..6b67c4d 100644
--- a/packages/mnswpr/core/index.js
+++ b/packages/mnswpr/core/index.js
@@ -17,8 +17,9 @@ export { GameSession } from './session/session.js'
 export { replay } from './session/replay.js'
 export { mulberry32, randInt } from './session/rng.js'
 
-// Layer 2 — Minesweeper rules
+// Layer 2 — Minesweeper rules & pure board generation
 export { MinesweeperRules } from './minesweeper/rules.js'
+export { generateBoard } from './minesweeper/board.js'
 
 // Shared level presets (also consumed by the DOM client)
 export { levels } from '../levels.js'
diff --git a/packages/mnswpr/core/minesweeper/board.js b/packages/mnswpr/core/minesweeper/board.js
index 47a5d29..6a3202b 100644
--- a/packages/mnswpr/core/minesweeper/board.js
+++ b/packages/mnswpr/core/minesweeper/board.js
@@ -1,12 +1,18 @@
 // @ts-check
+import { Grid } from '../grid/grid.js'
 import { mulberry32, randInt } from '../session/rng.js'
 import { eightWay } from '../grid/neighbors.js'
 
 /**
  * @typedef {{ rows: number, cols: number, mines: number, id?: string }} Config
  * @typedef {{ mine: boolean, adjacent: number, status: 'hidden' | 'flagged' | 'revealed' }} Cell
+ * @typedef {{ mine: boolean, adjacent: number }} LayoutCell
+ * @typedef {{ rows: number, cols: number, mines: number, cells: LayoutCell[][], mineLocations: [number, number][] }} Layout
  */
 
+/** Shared empty exclude set for generation without first-click safety. */
+const NO_EXCLUDE = new Set()
+
 /**
  * The set of cells kept mine-free for first-click safety: the clicked cell, plus
  * its 8 neighbors when the board has room for all mines outside that 3x3. Falls
@@ -34,6 +40,7 @@ export function excludeAround(config, r, c) {
 /**
  * Deterministically place mines and compute adjacency counts, mutating the grid
  * in place. Pure function of (seed, config, exclude) — same inputs, same board.
+ * Thin convenience wrapper over {@link fillMines} that builds the RNG from a seed.
  *
  * @param {number} seed
  * @param {Config} config
@@ -42,7 +49,21 @@ export function excludeAround(config, r, c) {
  * @returns {Set<number>} the mined coordinate keys
  */
 export function placeMines(seed, config, exclude, grid) {
-  const rng = mulberry32(seed)
+  return fillMines(mulberry32(seed), config, exclude, grid)
+}
+
+/**
+ * The injected-RNG seam under {@link placeMines}: place mines and compute
+ * adjacency counts, mutating the grid in place. Takes an rng function (any
+ * `() => [0, 1)`), so callers own determinism — same rng sequence, same board.
+ *
+ * @param {() => number} rng
+ * @param {Config} config
+ * @param {Set<number>} exclude - coordinate keys never to mine (first-click safety)
+ * @param {import('../grid/grid.js').Grid<Cell>} grid
+ * @returns {Set<number>} the mined coordinate keys
+ */
+export function fillMines(rng, config, exclude, grid) {
   const { rows, cols, mines } = config
   const placed = new Set()
   while (placed.size < mines) {
@@ -61,3 +82,50 @@ export function placeMines(seed, config, exclude, grid) {
   })
   return placed
 }
+
+/**
+ * Pure, Node-runnable board generation: given a size, a mine count, and an
+ * injected RNG, produce a plain layout object — no DOM, no I/O, no `Grid` class
+ * leaking out. This is the headless entry point behind `@ayo-run/mnswpr/core`;
+ * the DOM client reaches the same generator lazily through `MinesweeperRules`.
+ *
+ * The injected `rng` is the determinism seam: the same rng sequence always
+ * yields the same layout. `seed` is a convenience — when no `rng` is given it is
+ * wrapped with {@link mulberry32}, keeping generation reproducible and free of
+ * `Math.random` (invariant #4).
+ *
+ * @param {number} rows - number of rows (board height)
+ * @param {number} cols - number of columns (board width)
+ * @param {number} mines - number of mines to place
+ * @param {{ rng?: () => number, seed?: number, exclude?: Set<number> }} [options]
+ * @returns {Layout} a plain, serializable layout object
+ */
+export function generateBoard(rows, cols, mines, { rng, seed = 0, exclude = NO_EXCLUDE } = {}) {
+  if (!Number.isInteger(rows) || !Number.isInteger(cols) || rows < 1 || cols < 1) {
+    throw new RangeError(`generateBoard: rows/cols must be positive integers (got ${rows}x${cols})`)
+  }
+  const capacity = rows * cols - exclude.size
+  if (!Number.isInteger(mines) || mines < 0 || mines > capacity) {
+    throw new RangeError(`generateBoard: mines must be an integer in [0, ${capacity}] (got ${mines})`)
+  }
+
+  const config = { rows, cols, mines }
+  const grid = new Grid(rows, cols, () => ({ mine: false, adjacent: 0, status: 'hidden' }))
+  fillMines(rng ?? mulberry32(seed), config, exclude, grid)
+
+  /** @type {LayoutCell[][]} */
+  const cells = []
+  /** @type {[number, number][]} */
+  const mineLocations = []
+  for (let r = 0; r < rows; r++) {
+    /** @type {LayoutCell[]} */
+    const row = []
+    for (let c = 0; c < cols; c++) {
+      const cell = grid.at(r, c)
+      row.push({ mine: cell.mine, adjacent: cell.adjacent })
+      if (cell.mine) mineLocations.push([r, c])
+    }
+    cells.push(row)
+  }
+  return { rows, cols, mines, cells, mineLocations }
+}
diff --git a/packages/mnswpr/test/core.test.js b/packages/mnswpr/test/core.test.js
index 7a37855..a93505e 100644
--- a/packages/mnswpr/test/core.test.js
+++ b/packages/mnswpr/test/core.test.js
@@ -6,7 +6,7 @@ import { dirname, join } from 'node:path'
 import {
   Grid, eightWay, orthogonal,
   GameSession, replay, mulberry32,
-  MinesweeperRules, levels
+  MinesweeperRules, generateBoard, levels
 } from '../core/index.js'
 import { placeMines, excludeAround } from '../core/minesweeper/board.js'
 
@@ -95,6 +95,59 @@ describe('board generation (Layer 2)', () => {
   })
 })
 
+describe('generateBoard (Layer 2, pure)', () => {
+  it('is callable in plain Node and returns a plain layout object', () => {
+    // Non-square (rows ≠ cols) pins the rows-first orientation.
+    const layout = generateBoard(4, 7, 5, { seed: 42 })
+    expect(layout.rows).toBe(4)
+    expect(layout.cols).toBe(7)
+    expect(layout.mines).toBe(5)
+    expect(layout.cells).toHaveLength(4)
+    expect(layout.cells.every(row => row.length === 7)).toBe(true)
+    // plain data only — no Grid instance, no class methods leaking out
+    expect(layout.cells[0][0]).toEqual({ mine: expect.any(Boolean), adjacent: expect.any(Number) })
+    expect(layout).toEqual(JSON.parse(JSON.stringify(layout)))
+    // places exactly `mines` mines
+    expect(layout.mineLocations).toHaveLength(5)
+    let mineCount = 0
+    for (const row of layout.cells) for (const cell of row) if (cell.mine) mineCount++
+    expect(mineCount).toBe(5)
+  })
+
+  it('same injected RNG (seed) → identical layout', () => {
+    const a = generateBoard(16, 16, 40, { rng: mulberry32(123) })
+    const b = generateBoard(16, 16, 40, { rng: mulberry32(123) })
+    expect(a).toEqual(b)
+    // and the seed convenience wrapper matches an explicitly injected mulberry32
+    expect(generateBoard(16, 16, 40, { seed: 123 })).toEqual(a)
+    // a different seed diverges
+    expect(generateBoard(16, 16, 40, { seed: 124 })).not.toEqual(a)
+  })
+
+  it('computes adjacency as the 8-way mine count', () => {
+    const layout = generateBoard(9, 9, 10, { seed: 99 })
+    const g = new Grid(9, 9, (r, c) => layout.cells[r][c])
+    layout.cells.forEach((row, r) => row.forEach((cell, c) => {
+      if (cell.mine) return
+      let n = 0
+      for (const [nr, nc] of eightWay(g, r, c)) if (g.at(nr, nc).mine) n++
+      expect(cell.adjacent).toBe(n)
+    }))
+  })
+
+  it('honors an exclude set (e.g. first-click safety)', () => {
+    const exclude = new Set([0]) // key 0 == cell (0,0)
+    const layout = generateBoard(9, 9, 10, { seed: 7, exclude })
+    expect(layout.cells[0][0].mine).toBe(false)
+  })
+
+  it('rejects impossible dimensions and mine counts', () => {
+    expect(() => generateBoard(0, 9, 1)).toThrow(RangeError)
+    expect(() => generateBoard(3, 3, 10)).toThrow(RangeError) // more mines than cells
+    expect(() => generateBoard(3, 3, -1)).toThrow(RangeError)
+  })
+})
+
 describe('rules (Layer 2)', () => {
   it('first reveal is never a mine and opens a region', () => {
     const s = MinesweeperRules.init(3, beginner)