diff --git a/packages/mnswpr/core/index.js b/packages/mnswpr/core/index.js
index 6b67c4d..d99384a 100644
--- a/packages/mnswpr/core/index.js
+++ b/packages/mnswpr/core/index.js
@@ -19,7 +19,7 @@ export { mulberry32, randInt } from './session/rng.js'
 
 // Layer 2 — Minesweeper rules & pure board generation
 export { MinesweeperRules } from './minesweeper/rules.js'
-export { generateBoard } from './minesweeper/board.js'
+export { generateBoard, validateLayout } 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 dd1d5a2..65a4789 100644
--- a/packages/mnswpr/core/minesweeper/board.js
+++ b/packages/mnswpr/core/minesweeper/board.js
@@ -83,6 +83,66 @@ export function fillMines(rng, config, exclude, grid) {
   return placed
 }
 
+/**
+ * Assert a plain layout object (as produced by {@link generateBoard}) is
+ * well-formed before it's injected into a game: correct dimensions, cell shape,
+ * a mine count that matches its own cells, and in-bounds mine positions. Throws a
+ * clear error on the first problem so a malformed board can't silently corrupt
+ * win detection or adjacency. Returns the layout for convenient chaining.
+ *
+ * @param {unknown} layout
+ * @returns {Layout}
+ */
+export function validateLayout(layout) {
+  if (layout === null || typeof layout !== 'object') {
+    throw new TypeError(`validateLayout: expected a layout object (got ${layout === null ? 'null' : typeof layout})`)
+  }
+  const { rows, cols, mines, cells, mineLocations } = /** @type {any} */ (layout)
+  if (!Number.isInteger(rows) || !Number.isInteger(cols) || rows < 1 || cols < 1) {
+    throw new RangeError(`validateLayout: rows/cols must be positive integers (got ${rows}x${cols})`)
+  }
+  if (!Array.isArray(cells) || cells.length !== rows) {
+    throw new RangeError(`validateLayout: cells must have ${rows} rows (got ${Array.isArray(cells) ? cells.length : typeof cells})`)
+  }
+  let mineCount = 0
+  for (let r = 0; r < rows; r++) {
+    const row = cells[r]
+    if (!Array.isArray(row) || row.length !== cols) {
+      throw new RangeError(`validateLayout: row ${r} must have ${cols} cells (got ${Array.isArray(row) ? row.length : typeof row})`)
+    }
+    for (let c = 0; c < cols; c++) {
+      const cell = row[c]
+      if (cell === null || typeof cell !== 'object' || typeof cell.mine !== 'boolean' || !Number.isInteger(cell.adjacent)) {
+        throw new TypeError(`validateLayout: cell (${r},${c}) must be { mine: boolean, adjacent: integer }`)
+      }
+      if (cell.mine) mineCount++
+    }
+  }
+  if (!Number.isInteger(mines) || mines < 0 || mines > rows * cols) {
+    throw new RangeError(`validateLayout: mines must be an integer in [0, ${rows * cols}] (got ${mines})`)
+  }
+  if (mineCount !== mines) {
+    throw new RangeError(`validateLayout: mines (${mines}) disagrees with ${mineCount} mined cells`)
+  }
+  // mineLocations is optional, but if present it must agree with the grid — the
+  // "dimensions vs. mine positions" cross-check.
+  if (mineLocations !== undefined) {
+    if (!Array.isArray(mineLocations) || mineLocations.length !== mineCount) {
+      throw new RangeError(`validateLayout: mineLocations must list all ${mineCount} mines (got ${Array.isArray(mineLocations) ? mineLocations.length : typeof mineLocations})`)
+    }
+    for (const loc of mineLocations) {
+      if (!Array.isArray(loc) || loc.length !== 2) {
+        throw new TypeError(`validateLayout: each mineLocation must be a [r, c] pair (got ${JSON.stringify(loc)})`)
+      }
+      const [r, c] = loc
+      if (!Number.isInteger(r) || !Number.isInteger(c) || r < 0 || c < 0 || r >= rows || c >= cols || !cells[r][c].mine) {
+        throw new RangeError(`validateLayout: mineLocation [${r}, ${c}] is out of bounds or not a mined cell`)
+      }
+    }
+  }
+  return /** @type {Layout} */ (layout)
+}
+
 /**
  * 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
diff --git a/packages/mnswpr/core/minesweeper/rules.js b/packages/mnswpr/core/minesweeper/rules.js
index e1b0eea..e68c3f5 100644
--- a/packages/mnswpr/core/minesweeper/rules.js
+++ b/packages/mnswpr/core/minesweeper/rules.js
@@ -1,7 +1,7 @@
 // @ts-check
 import { Grid } from '../grid/grid.js'
 import { eightWay } from '../grid/neighbors.js'
-import { placeMines, excludeAround } from './board.js'
+import { placeMines, excludeAround, validateLayout } from './board.js'
 import { floodReveal, countFlagsAround, allMines } from './reveal.js'
 
 /**
@@ -35,11 +35,16 @@ function reveal(state, r, c) {
 
   // First reveal: generate the board now, excluding this cell's neighborhood, so
   // the opening click is always safe and the seed fully determines the layout.
+  // Injected boards (fromLayout) arrive with minesPlaced already true and skip
+  // this — their opening reveal plays exactly as the layout dictates.
   if (!state.minesPlaced) {
     placeMines(state.seed, state.config, excludeAround(state.config, r, c), state.grid)
     state.minesPlaced = true
-    state.phase = 'active'
   }
+  // fresh → active on the first real reveal, whether the board was just generated
+  // or injected pre-built — decoupled from placement so both paths transition the
+  // same way.
+  if (state.phase === 'fresh') state.phase = 'active'
 
   if (cell.mine) {
     cell.status = 'revealed'
@@ -140,6 +145,36 @@ export const MinesweeperRules = {
     }
   },
 
+  /**
+   * Build a game state from an explicit, pre-built layout (as returned by
+   * `generateBoard`) instead of generating one from a seed. Parallel to
+   * {@link init}: it yields a `State` a `GameSession` can drive identically —
+   * same rules, same transitions — the only difference being that the board is
+   * fixed up front, so the opening reveal is NOT made safe (first-click safety is
+   * a property of internal generation, not of a caller-supplied board). The
+   * layout is validated first and a malformed one throws.
+   *
+   * @param {import('./board.js').Layout} layout
+   * @param {{ seed?: number }} [opts] - seed is metadata only (no generation happens); defaults to 0
+   * @returns {State}
+   */
+  fromLayout(layout, { seed = 0 } = {}) {
+    validateLayout(layout)
+    const { rows, cols, mines } = layout
+    return {
+      seed,
+      config: { rows, cols, mines },
+      grid: new Grid(rows, cols, (r, c) => ({
+        mine: layout.cells[r][c].mine,
+        adjacent: layout.cells[r][c].adjacent,
+        status: 'hidden'
+      })),
+      phase: 'fresh',
+      minesPlaced: true,
+      revealedSafe: 0
+    }
+  },
+
   /** @param {State} state @returns {Phase} */
   status(state) {
     return state.phase
diff --git a/packages/mnswpr/core/session/session.js b/packages/mnswpr/core/session/session.js
index eba12ef..5b0c41d 100644
--- a/packages/mnswpr/core/session/session.js
+++ b/packages/mnswpr/core/session/session.js
@@ -11,13 +11,18 @@
  */
 export class GameSession {
   /**
+   * Start from either `{ seed, config }` (the rules generate the board) or a
+   * pre-built `{ state }` (e.g. a rules factory that injected an explicit board);
+   * `state` wins when both are given. The session stays generic — it just holds
+   * whatever state the rules produced.
+   *
    * @param {Rules} rules
-   * @param {{ seed: number, config: object, clock?: () => number }} opts
+   * @param {{ seed?: number, config?: object, state?: object, clock?: () => number }} opts
    */
-  constructor(rules, { seed, config, clock = () => 0 }) {
+  constructor(rules, { seed, config, state, clock = () => 0 }) {
     this.rules = rules
     this.clock = clock
-    this.state = rules.init(seed, config)
+    this.state = state ?? rules.init(seed, config)
     /** @type {Array<{ move: object, t: number }>} */
     this._log = []
     this._t0 = null
diff --git a/packages/mnswpr/test/core.test.js b/packages/mnswpr/test/core.test.js
index 715b979..3187c26 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, generateBoard, levels
+  MinesweeperRules, generateBoard, validateLayout, levels
 } from '../core/index.js'
 import { placeMines, excludeAround } from '../core/minesweeper/board.js'
 
@@ -247,6 +247,114 @@ describe('rules (Layer 2)', () => {
   })
 })
 
+describe('board injection (fromLayout, Layer 2)', () => {
+  // A hand-built 3x3 with a single mine at (0,0). Adjacency: the mine's three
+  // neighbors (0,1),(1,0),(1,1) are 1; everything else is 0.
+  const knownLayout = () => ({
+    rows: 3,
+    cols: 3,
+    mines: 1,
+    cells: [
+      [{ mine: true, adjacent: 0 }, { mine: false, adjacent: 1 }, { mine: false, adjacent: 0 }],
+      [{ mine: false, adjacent: 1 }, { mine: false, adjacent: 1 }, { mine: false, adjacent: 0 }],
+      [{ mine: false, adjacent: 0 }, { mine: false, adjacent: 0 }, { mine: false, adjacent: 0 }]
+    ],
+    mineLocations: [[0, 0]]
+  })
+
+  it('creates a game instance from an injected layout and scripts moves to known states', () => {
+    const session = new GameSession(MinesweeperRules, { state: MinesweeperRules.fromLayout(knownLayout()) })
+    expect(session.status()).toBe('fresh')
+
+    // Reveal a numbered (non-zero) cell: it activates the game and reveals just
+    // itself, carrying the layout's adjacency — proof the injected board is live.
+    const first = session.applyMove({ type: 'reveal', r: 0, c: 1 })
+    expect(session.status()).toBe('active')
+    expect(first.events[0].cells).toEqual([{ r: 0, c: 1, adjacent: 1 }])
+
+    // Stepping on the injected mine loses and reports it (before any flood wins).
+    const boom = session.applyMove({ type: 'reveal', r: 0, c: 0 })
+    expect(session.status()).toBe('lost')
+    expect(boom.events[0]).toMatchObject({ type: 'explode', r: 0, c: 0 })
+    expect(boom.events[0].mines).toEqual([{ r: 0, c: 0 }])
+  })
+
+  it('a zero cell floods the connected safe region on an injected board', () => {
+    const session = new GameSession(MinesweeperRules, { state: MinesweeperRules.fromLayout(knownLayout()) })
+    // (2,2) is a zero far from the mine; flood opens all 8 safe cells → win.
+    const flood = session.applyMove({ type: 'reveal', r: 2, c: 2 })
+    expect(flood.events[0].cells.length).toBe(8)
+    expect(session.status()).toBe('won')
+  })
+
+  it('an injected board can be played to a win, every safe cell revealed', () => {
+    const session = new GameSession(MinesweeperRules, { state: MinesweeperRules.fromLayout(knownLayout()) })
+    // Reveal all 8 non-mine cells; avoid (0,0).
+    for (let r = 0; r < 3; r++) {
+      for (let c = 0; c < 3; c++) {
+        if (r === 0 && c === 0) continue
+        session.applyMove({ type: 'reveal', r, c })
+      }
+    }
+    expect(session.status()).toBe('won')
+  })
+
+  it('plays identically to a generated board: injecting a generated layout reproduces its play', () => {
+    // Generate a board, then inject that exact layout. Revealing any cell must
+    // report the layout's own adjacency, and mines must sit where the layout says.
+    const layout = generateBoard(9, 9, 10, { seed: 42, safeCell: { r: 4, c: 4 } })
+    const state = MinesweeperRules.fromLayout(layout)
+    const { events } = MinesweeperRules.apply(state, { type: 'reveal', r: 4, c: 4 })
+    expect(state.phase).toBe('active')
+    // (4,4) was forced safe; its revealed adjacency matches the layout.
+    const seen = events[0].cells.find(cell => cell.r === 4 && cell.c === 4)
+    expect(seen.adjacent).toBe(layout.cells[4][4].adjacent)
+    // The mines are exactly the layout's mines.
+    const mines = []
+    state.grid.forEach((cell, r, c) => { if (cell.mine) mines.push([r, c]) })
+    expect(mines.sort()).toEqual([...layout.mineLocations].sort())
+  })
+
+  it('two sessions from the same layout with the same script transition identically', () => {
+    const script = [{ type: 'reveal', r: 2, c: 2 }, { type: 'flag', r: 0, c: 0 }, { type: 'reveal', r: 0, c: 1 }]
+    const run = () => {
+      const s = new GameSession(MinesweeperRules, { state: MinesweeperRules.fromLayout(knownLayout()) })
+      return script.map(m => s.applyMove(m).view)
+    }
+    expect(run()).toEqual(run())
+  })
+
+  it('rejects malformed layouts with a clear error', () => {
+    expect(() => MinesweeperRules.fromLayout(null)).toThrow(TypeError)
+    expect(() => MinesweeperRules.fromLayout('nope')).toThrow(TypeError)
+    // dimensions don't match the cells grid
+    expect(() => MinesweeperRules.fromLayout({ ...knownLayout(), rows: 4 })).toThrow(RangeError)
+    // mine count disagrees with the actual mined cells
+    expect(() => MinesweeperRules.fromLayout({ ...knownLayout(), mines: 2 })).toThrow(RangeError)
+    // a cell missing its shape
+    const badCells = knownLayout()
+    badCells.cells[1][1] = { mine: false } // no `adjacent`
+    expect(() => MinesweeperRules.fromLayout(badCells)).toThrow(TypeError)
+    // mineLocations pointing at a non-mined cell
+    const badLocs = knownLayout()
+    badLocs.mineLocations = [[2, 2]]
+    expect(() => MinesweeperRules.fromLayout(badLocs)).toThrow(RangeError)
+  })
+
+  it('validateLayout accepts a freshly generated board', () => {
+    expect(() => validateLayout(generateBoard(16, 16, 40, { seed: 3 }))).not.toThrow()
+  })
+
+  it('leaves existing seed-based construction unaffected', () => {
+    // No `state` supplied ⇒ the session still generates from seed/config as before.
+    const session = new GameSession(MinesweeperRules, { seed: 3, config: beginner })
+    const { events } = session.applyMove({ type: 'reveal', r: 0, c: 0 })
+    expect(session.status()).toBe('active')
+    expect(session.state.grid.at(0, 0).mine).toBe(false) // first-click safety intact
+    expect(events[0].cells.length).toBeGreaterThan(1)
+  })
+})
+
 describe('session + timing (Layer 1)', () => {
   it('reports authoritative elapsed time from the injected clock', () => {
     const clock = makeClock()