diff --git a/packages/mcp/package.json b/packages/mcp/package.json
index aaadac8a..13cadbb3 100644
--- a/packages/mcp/package.json
+++ b/packages/mcp/package.json
@@ -59,6 +59,7 @@
   },
   "dependencies": {
     "@modelcontextprotocol/sdk": "^1.29.0",
+    "@pascal-app/lingo": "^0.1.0",
     "zod": "^4.3.5"
   },
   "devDependencies": {
diff --git a/packages/mcp/src/tools/construction-tools.ts b/packages/mcp/src/tools/construction-tools.ts
index 1488e57f..ff9aba36 100644
--- a/packages/mcp/src/tools/construction-tools.ts
+++ b/packages/mcp/src/tools/construction-tools.ts
@@ -14,6 +14,7 @@ import {
 import { z } from 'zod'
 import type { SceneOperations } from '../operations'
 import { publishLiveSceneSnapshot } from './live-sync'
+import { measurement } from './measurement'
 import { NodeIdSchema, Vec2Schema, Vec3Schema } from './schemas'
 
 const ROOF_TYPES = ['hip', 'gable', 'shed', 'gambrel', 'dutch', 'mansard', 'flat'] as const
@@ -22,12 +23,14 @@ const RAILING_MODES = ['none', 'left', 'right', 'both'] as const
 export const createStoryShellInput = {
   levelId: NodeIdSchema,
   footprint: z.array(Vec2Schema).min(3),
-  wallHeight: z.number().positive().default(2.8),
-  wallThickness: z.number().positive().default(0.16),
+  wallHeight: measurement('length', 'm', { min: 0, description: 'Wall height.' }).default(2.8),
+  wallThickness: measurement('length', 'm', { min: 0, description: 'Wall thickness.' }).default(
+    0.16,
+  ),
   createSlab: z.boolean().default(true),
   createCeiling: z.boolean().default(true),
-  slabElevation: z.number().default(0.1),
-  ceilingHeight: z.number().positive().optional(),
+  slabElevation: measurement('length', 'm', { description: 'Slab elevation.' }).default(0.1),
+  ceilingHeight: measurement('length', 'm', { min: 0, description: 'Ceiling height.' }).optional(),
   namePrefix: z.string().optional(),
   wallMaterialPreset: z.string().optional(),
   slabMaterialPreset: z.string().optional(),
@@ -47,16 +50,25 @@ export const createRoofInput = {
   roofLevelId: NodeIdSchema.optional(),
   useDedicatedRoofLevel: z.boolean().default(true),
   roofLevelLabel: z.string().default('Roof'),
-  roofLevelElevation: z.number().optional(),
-  roofLevelHeight: z.number().positive().optional(),
+  roofLevelElevation: measurement('length', 'm', {
+    description: 'Roof level elevation.',
+  }).optional(),
+  roofLevelHeight: measurement('length', 'm', {
+    min: 0,
+    description: 'Roof level height.',
+  }).optional(),
   center: Vec3Schema.optional(),
-  width: z.number().positive(),
-  depth: z.number().positive(),
+  width: measurement('length', 'm', { min: 0, description: 'Roof width.' }),
+  depth: measurement('length', 'm', { min: 0, description: 'Roof depth.' }),
   roofType: z.enum(ROOF_TYPES).default('hip'),
-  pitch: z.number().min(0).max(85).default(35),
-  wallHeight: z.number().min(0).default(0.35),
-  wallThickness: z.number().positive().default(0.16),
-  overhang: z.number().min(0).default(0.45),
+  pitch: measurement('angle', 'deg', { min: 0, max: 85, description: 'Roof pitch.' }).default(35),
+  wallHeight: measurement('length', 'm', { min: 0, description: 'Knee-wall height.' }).default(
+    0.35,
+  ),
+  wallThickness: measurement('length', 'm', { min: 0, description: 'Wall thickness.' }).default(
+    0.16,
+  ),
+  overhang: measurement('length', 'm', { min: 0, description: 'Eave overhang.' }).default(0.45),
   materialPreset: z.string().optional(),
   name: z.string().optional(),
 }
@@ -73,21 +85,31 @@ export const createStairBetweenLevelsInput = {
   fromLevelId: NodeIdSchema,
   toLevelId: NodeIdSchema,
   position: Vec3Schema,
-  rotation: z.number().default(0),
-  width: z.number().positive().default(1),
-  runLength: z.number().positive().default(3),
-  totalRise: z.number().positive().default(2.8),
+  rotation: measurement('angle', 'rad', { description: 'Y-axis rotation.' }).default(0),
+  width: measurement('length', 'm', { min: 0, description: 'Stair width.' }).default(1),
+  runLength: measurement('length', 'm', { min: 0, description: 'Horizontal run length.' }).default(
+    3,
+  ),
+  totalRise: measurement('length', 'm', { min: 0, description: 'Total vertical rise.' }).default(
+    2.8,
+  ),
   stepCount: z.number().int().positive().default(14),
   railingMode: z.enum(RAILING_MODES).default('both'),
   destinationSlabId: NodeIdSchema.optional(),
   sourceCeilingId: NodeIdSchema.optional(),
   createDestinationSlabOpening: z.boolean().default(true),
   createSourceCeilingOpening: z.boolean().default(true),
-  openingWidth: z.number().positive().optional(),
-  openingLength: z.number().positive().optional(),
-  openingOffset: z.number().min(0).default(0),
+  openingWidth: measurement('length', 'm', {
+    min: 0,
+    description: 'Floor opening width.',
+  }).optional(),
+  openingLength: measurement('length', 'm', {
+    min: 0,
+    description: 'Floor opening length.',
+  }).optional(),
+  openingOffset: measurement('length', 'm', { min: 0, description: 'Opening offset.' }).default(0),
   openingCenter: Vec2Schema.optional(),
-  openingRotation: z.number().optional(),
+  openingRotation: measurement('angle', 'rad', { description: 'Opening rotation.' }).optional(),
   materialPreset: z.string().optional(),
   name: z.string().optional(),
 }
diff --git a/packages/mcp/src/tools/create-level.ts b/packages/mcp/src/tools/create-level.ts
index ade635b3..784297c4 100644
--- a/packages/mcp/src/tools/create-level.ts
+++ b/packages/mcp/src/tools/create-level.ts
@@ -5,12 +5,16 @@ import { z } from 'zod'
 import type { SceneOperations } from '../operations'
 import { ErrorCode, throwMcpError } from './errors'
 import { publishLiveSceneSnapshot } from './live-sync'
+import { measurement } from './measurement'
 import { NodeIdSchema } from './schemas'
 
 export const createLevelInput = {
   buildingId: NodeIdSchema,
   elevation: z.number().optional(),
-  height: z.number().optional(),
+  height: measurement('length', 'm', {
+    min: 0,
+    description: 'Level height (stored in metadata).',
+  }).optional(),
   label: z.string().optional(),
 }
 
diff --git a/packages/mcp/src/tools/create-wall.test.ts b/packages/mcp/src/tools/create-wall.test.ts
index 4d8c49ef..1322b5ff 100644
--- a/packages/mcp/src/tools/create-wall.test.ts
+++ b/packages/mcp/src/tools/create-wall.test.ts
@@ -42,6 +42,34 @@ describe('create_wall', () => {
     expect((created as { thickness?: number }).thickness).toBe(0.15)
   })
 
+  test('accepts a natural-language thickness and canonicalizes to meters', async () => {
+    const level = Object.values(bridge.getNodes()).find((n) => n.type === 'level')!
+    const result = await client.callTool({
+      name: 'create_wall',
+      arguments: {
+        levelId: level.id,
+        start: [0, 0],
+        end: [4, 0],
+        thickness: '6 in',
+        height: '2.5m',
+      },
+    })
+    expect(result.isError).toBeFalsy()
+    const parsed = JSON.parse((result.content as Array<{ type: string; text: string }>)[0]!.text)
+    const created = bridge.getNode(parsed.wallId) as { thickness?: number; height?: number }
+    expect(created.thickness).toBeCloseTo(0.1524, 6)
+    expect(created.height).toBeCloseTo(2.5, 6)
+  })
+
+  test('rejects an out-of-unit-family value', async () => {
+    const level = Object.values(bridge.getNodes()).find((n) => n.type === 'level')!
+    const result = await client.callTool({
+      name: 'create_wall',
+      arguments: { levelId: level.id, start: [0, 0], end: [4, 0], thickness: 'banana' },
+    })
+    expect(result.isError).toBe(true)
+  })
+
   test('publishes a live scene snapshot when bound to a saved scene', async () => {
     const now = new Date().toISOString()
     const savedMeta: SceneMeta = {
diff --git a/packages/mcp/src/tools/create-wall.ts b/packages/mcp/src/tools/create-wall.ts
index 66d6ee5e..ae9b6794 100644
--- a/packages/mcp/src/tools/create-wall.ts
+++ b/packages/mcp/src/tools/create-wall.ts
@@ -5,14 +5,15 @@ import { z } from 'zod'
 import type { SceneOperations } from '../operations'
 import { ErrorCode, throwMcpError } from './errors'
 import { publishLiveSceneSnapshot } from './live-sync'
+import { measurement } from './measurement'
 import { NodeIdSchema, Vec2Schema } from './schemas'
 
 export const createWallInput = {
   levelId: NodeIdSchema,
   start: Vec2Schema,
   end: Vec2Schema,
-  thickness: z.number().positive().optional(),
-  height: z.number().positive().optional(),
+  thickness: measurement('length', 'm', { min: 0, description: 'Wall thickness.' }).optional(),
+  height: measurement('length', 'm', { min: 0, description: 'Wall height.' }).optional(),
 }
 
 export const createWallOutput = {
diff --git a/packages/mcp/src/tools/cut-opening.ts b/packages/mcp/src/tools/cut-opening.ts
index 86c0cc6b..b03f12f7 100644
--- a/packages/mcp/src/tools/cut-opening.ts
+++ b/packages/mcp/src/tools/cut-opening.ts
@@ -6,14 +6,15 @@ import type { SceneOperations } from '../operations'
 import { ErrorCode, throwMcpError } from './errors'
 import { wallLength, wallLocalXFromT } from './geometry'
 import { publishLiveSceneSnapshot } from './live-sync'
+import { measurement } from './measurement'
 import { NodeIdSchema } from './schemas'
 
 export const cutOpeningInput = {
   wallId: NodeIdSchema,
   type: z.enum(['door', 'window']),
   position: z.number().min(0).max(1),
-  width: z.number().positive(),
-  height: z.number().positive(),
+  width: measurement('length', 'm', { min: 0, description: 'Opening width.' }),
+  height: measurement('length', 'm', { min: 0, description: 'Opening height.' }),
 }
 
 export const cutOpeningOutput = {
diff --git a/packages/mcp/src/tools/measurement.test.ts b/packages/mcp/src/tools/measurement.test.ts
new file mode 100644
index 00000000..ff0f9b0b
--- /dev/null
+++ b/packages/mcp/src/tools/measurement.test.ts
@@ -0,0 +1,62 @@
+import { describe, expect, test } from 'bun:test'
+import { z } from 'zod'
+import { measurement } from './measurement'
+
+describe('measurement()', () => {
+  test('parses natural-language length to meters', () => {
+    const m = measurement('length', 'm', { min: 0 })
+    expect(m.parse('6 in')).toBeCloseTo(0.1524, 6)
+    expect(m.parse('180cm')).toBeCloseTo(1.8, 6)
+    expect(m.parse('1m80')).toBeCloseTo(1.8, 6)
+    expect(m.parse(`5'11"`)).toBeCloseTo(1.8034, 4)
+    expect(m.parse('2 ft 3 in')).toBeCloseTo(0.6858, 6)
+  })
+
+  test('passes numbers through unchanged (backward compatible)', () => {
+    const m = measurement('length', 'm', { min: 0 })
+    expect(m.parse(0.15)).toBe(0.15)
+    expect(m.parse(3)).toBe(3)
+  })
+
+  test('reads a bare numeric string as the field unit', () => {
+    expect(measurement('length', 'm').parse('6')).toBe(6)
+    expect(measurement('angle', 'deg').parse('45')).toBe(45)
+  })
+
+  test('canonicalizes angles to the field unit', () => {
+    expect(measurement('angle', 'deg').parse('45°')).toBe(45)
+    expect(measurement('angle', 'deg').parse('1.57rad')).toBeCloseTo(89.954, 3)
+    expect(measurement('angle', 'rad').parse('90deg')).toBeCloseTo(Math.PI / 2, 6)
+    expect(measurement('angle', 'rad').parse('0.25 turn')).toBeCloseTo(Math.PI / 2, 6)
+  })
+
+  test('enforces min/max bounds in the field unit', () => {
+    const m = measurement('length', 'm', { min: 0, max: 3 })
+    expect(m.safeParse('-1').success).toBe(false)
+    expect(m.safeParse('900ft').success).toBe(false)
+    expect(m.safeParse('2m').success).toBe(true)
+    const tooBig = m.safeParse('900ft')
+    expect(tooBig.success).toBe(false)
+    if (!tooBig.success) expect(tooBig.error.issues[0]?.message).toContain('at most 3 m')
+  })
+
+  test('rejects unparseable input with a model-readable message', () => {
+    const r = measurement('length', 'm').safeParse('banana')
+    expect(r.success).toBe(false)
+    if (!r.success) expect(r.error.issues[0]?.message.toLowerCase()).toContain('number')
+  })
+
+  test('emits a number|string JSON schema advertising natural language', () => {
+    const schema = z.toJSONSchema(
+      measurement('length', 'm', { min: 0, description: 'Wall thickness.' }),
+      {
+        io: 'input',
+      },
+    )
+    const json = JSON.stringify(schema)
+    expect(json).toContain('anyOf')
+    expect(json).toContain('number')
+    expect(json).toContain('string')
+    expect((schema as { description?: string }).description).toContain('natural-language')
+  })
+})
diff --git a/packages/mcp/src/tools/measurement.ts b/packages/mcp/src/tools/measurement.ts
new file mode 100644
index 00000000..1767ad06
--- /dev/null
+++ b/packages/mcp/src/tools/measurement.ts
@@ -0,0 +1,98 @@
+import { type Kind, parseQuantity } from '@pascal-app/lingo'
+import { z } from 'zod'
+
+/**
+ * A zod field for a measurement tool argument that a model may emit as a bare
+ * number OR as natural language. `measurement('length', 'm')` accepts `0.15`,
+ * `"6 in"`, `"180cm"`, `"2 ft 3 in"`; `measurement('angle', 'deg')` accepts
+ * `45`, `"45°"`, `"1.57rad"`, `"0.25 turn"`. The value is canonicalized (via
+ * `@pascal-app/lingo`) to a number in `unit` — the exact unit the tool handler
+ * already expects — so no handler change is needed. `min`/`max` (in `unit`)
+ * reject out-of-range values with a model-readable message.
+ *
+ * The emitted JSON Schema is `number | string`, so the model is free to answer
+ * in whatever unit it is thinking in; AI SDK v6 applies the transform and
+ * forwards the canonical number to the tool executor.
+ */
+
+export interface MeasurementOptions {
+  /** Lower bound, in `unit`. */
+  min?: number
+  /** Upper bound, in `unit`. */
+  max?: number
+  /** Semantic description (e.g. "Wall thickness"). The natural-language note is appended. */
+  description?: string
+}
+
+function unitNoun(unit: string): string {
+  switch (unit) {
+    case 'm':
+      return 'meters'
+    case 'deg':
+      return 'degrees'
+    case 'rad':
+      return 'radians'
+    default:
+      return unit
+  }
+}
+
+function naturalLanguageNote(kind: Kind, unit: string): string {
+  const examples =
+    kind === 'angle'
+      ? unit === 'rad'
+        ? '45, "45°", "1.57rad", "0.25 turn"'
+        : '45, "45°", "1.57rad", "0.25 turn"'
+      : '0.9, "6 ft", "180cm", "2 ft 3 in"'
+  return `Accepts a number (${unitNoun(unit)}) or a natural-language string; other units are converted. e.g. ${examples}.`
+}
+
+function boundsNote(unit: string, min?: number, max?: number): string {
+  if (min !== undefined && max !== undefined) return ` Range ${min}–${max} ${unit}.`
+  if (min !== undefined) return ` Minimum ${min} ${unit}.`
+  if (max !== undefined) return ` Maximum ${max} ${unit}.`
+  return ''
+}
+
+export function measurement(kind: Kind, unit: string, opts: MeasurementOptions = {}) {
+  const { min, max } = opts
+  const description = [
+    opts.description,
+    naturalLanguageNote(kind, unit) + boundsNote(unit, min, max),
+  ]
+    .filter(Boolean)
+    .join(' ')
+
+  return z
+    .union([z.number(), z.string()])
+    .transform((val, ctx) => {
+      let value: number
+      if (typeof val === 'number') {
+        value = val
+      } else {
+        const result = parseQuantity(val, { kind, unit, strictness: 'forgiving' })
+        if (!result.ok) {
+          ctx.addIssue({
+            code: 'custom',
+            message: result.issues[0]?.message ?? `Could not read "${val}" as a ${kind}.`,
+          })
+          return z.NEVER
+        }
+        value = result.quantity.to(unit).value
+      }
+      if (!Number.isFinite(value)) {
+        ctx.addIssue({ code: 'custom', message: `Value must be a finite ${kind} in ${unit}.` })
+        return z.NEVER
+      }
+      if (min !== undefined && value < min) {
+        ctx.addIssue({ code: 'custom', message: `Must be at least ${min} ${unit} (got ${value}).` })
+        return z.NEVER
+      }
+      if (max !== undefined && value > max) {
+        ctx.addIssue({ code: 'custom', message: `Must be at most ${max} ${unit} (got ${value}).` })
+        return z.NEVER
+      }
+      return value
+    })
+    .describe(description)
+}
diff --git a/packages/mcp/src/tools/photo-to-scene/photo-to-scene.ts b/packages/mcp/src/tools/photo-to-scene/photo-to-scene.ts
index ad183be5..c2032ab8 100644
--- a/packages/mcp/src/tools/photo-to-scene/photo-to-scene.ts
+++ b/packages/mcp/src/tools/photo-to-scene/photo-to-scene.ts
@@ -13,6 +13,7 @@ import {
 import { z } from 'zod'
 import type { SceneOperations } from '../../operations'
 import { appendLiveSceneEvent } from '../live-sync'
+import { measurement } from '../measurement'
 
 /**
  * Input shape for the `photo_to_scene` orchestrator. `image` matches the
@@ -23,8 +24,14 @@ export const photoToSceneInput = {
   scaleHint: z.string().optional().describe('e.g. "1 cm = 1 m" or "approx 80 m²"'),
   name: z.string().default('Scene from photo'),
   save: z.boolean().default(true),
-  defaultWallThickness: z.number().default(0.2),
-  defaultWallHeight: z.number().default(2.6),
+  defaultWallThickness: measurement('length', 'm', {
+    min: 0,
+    description: 'Default wall thickness.',
+  }).default(0.2),
+  defaultWallHeight: measurement('length', 'm', {
+    min: 0,
+    description: 'Default wall height.',
+  }).default(2.6),
 }
 
 export const photoToSceneOutput = {
diff --git a/packages/mcp/src/tools/place-item.ts b/packages/mcp/src/tools/place-item.ts
index 88691241..1b7da901 100644
--- a/packages/mcp/src/tools/place-item.ts
+++ b/packages/mcp/src/tools/place-item.ts
@@ -7,13 +7,14 @@ import { findCatalogItem } from './asset-catalog'
 import { ErrorCode, throwMcpError } from './errors'
 import { projectWorldPointToWallLocalX, wallLength } from './geometry'
 import { publishLiveSceneSnapshot } from './live-sync'
+import { measurement } from './measurement'
 import { NodeIdSchema, Vec3Schema } from './schemas'
 
 export const placeItemInput = {
   catalogItemId: z.string().min(1),
   targetNodeId: NodeIdSchema,
   position: Vec3Schema,
-  rotation: z.number().optional(),
+  rotation: measurement('angle', 'rad', { description: 'Y-axis rotation.' }).optional(),
 }
 
 export const placeItemOutput = {
diff --git a/packages/mcp/src/tools/room-tools.ts b/packages/mcp/src/tools/room-tools.ts
index 2fca54bd..51aa1dee 100644
--- a/packages/mcp/src/tools/room-tools.ts
+++ b/packages/mcp/src/tools/room-tools.ts
@@ -22,6 +22,7 @@ import {
   wallLocalXFromT,
 } from './geometry'
 import { publishLiveSceneSnapshot } from './live-sync'
+import { measurement } from './measurement'
 import { NodeIdSchema, Vec2Schema } from './schemas'
 
 const ROOM_TYPES = [
@@ -51,8 +52,8 @@ export const createRoomInput = {
   name: z.string().min(1),
   polygon: z.array(Vec2Schema).min(3),
   color: z.string().optional(),
-  wallHeight: z.number().positive().optional(),
-  wallThickness: z.number().positive().optional(),
+  wallHeight: measurement('length', 'm', { min: 0, description: 'Wall height.' }).optional(),
+  wallThickness: measurement('length', 'm', { min: 0, description: 'Wall thickness.' }).optional(),
 }
 
 export const createRoomOutput = {
@@ -67,8 +68,8 @@ export const addDoorInput = {
   wallId: NodeIdSchema,
   t: z.number().min(0).max(1).optional(),
   position: z.number().min(0).max(1).optional(),
-  width: z.number().positive().optional(),
-  height: z.number().positive().optional(),
+  width: measurement('length', 'm', { min: 0, description: 'Door width.' }).optional(),
+  height: measurement('length', 'm', { min: 0, description: 'Door height.' }).optional(),
   hingesSide: z.enum(['left', 'right']).optional(),
   swingDirection: z.enum(['inward', 'outward']).optional(),
 }
@@ -87,9 +88,12 @@ export const addWindowInput = {
   wallId: NodeIdSchema,
   t: z.number().min(0).max(1).optional(),
   position: z.number().min(0).max(1).optional(),
-  width: z.number().positive().optional(),
-  height: z.number().positive().optional(),
-  sillHeight: z.number().min(0).optional(),
+  width: measurement('length', 'm', { min: 0, description: 'Window width.' }).optional(),
+  height: measurement('length', 'm', { min: 0, description: 'Window height.' }).optional(),
+  sillHeight: measurement('length', 'm', {
+    min: 0,
+    description: 'Sill height above floor.',
+  }).optional(),
 }
 
 export const addWindowOutput = {