diff --git a/packages/mcp/src/tools/construction-tools.ts b/packages/mcp/src/tools/construction-tools.ts
index ff9aba36..a8e43e92 100644
--- a/packages/mcp/src/tools/construction-tools.ts
+++ b/packages/mcp/src/tools/construction-tools.ts
@@ -23,14 +23,20 @@ const RAILING_MODES = ['none', 'left', 'right', 'both'] as const
 export const createStoryShellInput = {
   levelId: NodeIdSchema,
   footprint: z.array(Vec2Schema).min(3),
-  wallHeight: measurement('length', 'm', { min: 0, description: 'Wall height.' }).default(2.8),
-  wallThickness: measurement('length', 'm', { min: 0, description: 'Wall thickness.' }).default(
-    0.16,
+  wallHeight: measurement('length', 'm', { positive: true, description: 'Wall height.' }).default(
+    2.8,
   ),
+  wallThickness: measurement('length', 'm', {
+    positive: true,
+    description: 'Wall thickness.',
+  }).default(0.16),
   createSlab: z.boolean().default(true),
   createCeiling: z.boolean().default(true),
   slabElevation: measurement('length', 'm', { description: 'Slab elevation.' }).default(0.1),
-  ceilingHeight: measurement('length', 'm', { min: 0, description: 'Ceiling height.' }).optional(),
+  ceilingHeight: measurement('length', 'm', {
+    positive: true,
+    description: 'Ceiling height.',
+  }).optional(),
   namePrefix: z.string().optional(),
   wallMaterialPreset: z.string().optional(),
   slabMaterialPreset: z.string().optional(),
@@ -50,24 +56,24 @@ export const createRoofInput = {
   roofLevelId: NodeIdSchema.optional(),
   useDedicatedRoofLevel: z.boolean().default(true),
   roofLevelLabel: z.string().default('Roof'),
-  roofLevelElevation: measurement('length', 'm', {
-    description: 'Roof level elevation.',
-  }).optional(),
+  // A level ordinal (story index), not a length — kept numeric.
+  roofLevelElevation: z.number().optional(),
   roofLevelHeight: measurement('length', 'm', {
-    min: 0,
+    positive: true,
     description: 'Roof level height.',
   }).optional(),
   center: Vec3Schema.optional(),
-  width: measurement('length', 'm', { min: 0, description: 'Roof width.' }),
-  depth: measurement('length', 'm', { min: 0, description: 'Roof depth.' }),
+  width: measurement('length', 'm', { positive: true, description: 'Roof width.' }),
+  depth: measurement('length', 'm', { positive: true, description: 'Roof depth.' }),
   roofType: z.enum(ROOF_TYPES).default('hip'),
   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,
-  ),
+  wallThickness: measurement('length', 'm', {
+    positive: true,
+    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(),
@@ -86,13 +92,15 @@ export const createStairBetweenLevelsInput = {
   toLevelId: NodeIdSchema,
   position: Vec3Schema,
   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,
-  ),
+  width: measurement('length', 'm', { positive: true, description: 'Stair width.' }).default(1),
+  runLength: measurement('length', 'm', {
+    positive: true,
+    description: 'Horizontal run length.',
+  }).default(3),
+  totalRise: measurement('length', 'm', {
+    positive: true,
+    description: 'Total vertical rise.',
+  }).default(2.8),
   stepCount: z.number().int().positive().default(14),
   railingMode: z.enum(RAILING_MODES).default('both'),
   destinationSlabId: NodeIdSchema.optional(),
@@ -100,11 +108,11 @@ export const createStairBetweenLevelsInput = {
   createDestinationSlabOpening: z.boolean().default(true),
   createSourceCeilingOpening: z.boolean().default(true),
   openingWidth: measurement('length', 'm', {
-    min: 0,
+    positive: true,
     description: 'Floor opening width.',
   }).optional(),
   openingLength: measurement('length', 'm', {
-    min: 0,
+    positive: true,
     description: 'Floor opening length.',
   }).optional(),
   openingOffset: measurement('length', 'm', { min: 0, description: 'Opening offset.' }).default(0),
diff --git a/packages/mcp/src/tools/create-wall.ts b/packages/mcp/src/tools/create-wall.ts
index ae9b6794..70679d15 100644
--- a/packages/mcp/src/tools/create-wall.ts
+++ b/packages/mcp/src/tools/create-wall.ts
@@ -12,8 +12,11 @@ export const createWallInput = {
   levelId: NodeIdSchema,
   start: Vec2Schema,
   end: Vec2Schema,
-  thickness: measurement('length', 'm', { min: 0, description: 'Wall thickness.' }).optional(),
-  height: measurement('length', 'm', { min: 0, description: 'Wall height.' }).optional(),
+  thickness: measurement('length', 'm', {
+    positive: true,
+    description: 'Wall thickness.',
+  }).optional(),
+  height: measurement('length', 'm', { positive: true, 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 b03f12f7..e5d8e2c4 100644
--- a/packages/mcp/src/tools/cut-opening.ts
+++ b/packages/mcp/src/tools/cut-opening.ts
@@ -13,8 +13,8 @@ export const cutOpeningInput = {
   wallId: NodeIdSchema,
   type: z.enum(['door', 'window']),
   position: z.number().min(0).max(1),
-  width: measurement('length', 'm', { min: 0, description: 'Opening width.' }),
-  height: measurement('length', 'm', { min: 0, description: 'Opening height.' }),
+  width: measurement('length', 'm', { positive: true, description: 'Opening width.' }),
+  height: measurement('length', 'm', { positive: true, description: 'Opening height.' }),
 }
 
 export const cutOpeningOutput = {
diff --git a/packages/mcp/src/tools/measurement.test.ts b/packages/mcp/src/tools/measurement.test.ts
index ff0f9b0b..deb90c0c 100644
--- a/packages/mcp/src/tools/measurement.test.ts
+++ b/packages/mcp/src/tools/measurement.test.ts
@@ -46,6 +46,19 @@ describe('measurement()', () => {
     if (!r.success) expect(r.error.issues[0]?.message.toLowerCase()).toContain('number')
   })
 
+  test('positive rejects zero and negatives (restores .positive() behavior)', () => {
+    const m = measurement('length', 'm', { positive: true })
+    expect(m.safeParse(0).success).toBe(false)
+    expect(m.safeParse('0m').success).toBe(false)
+    expect(m.safeParse(-1).success).toBe(false)
+    expect(m.parse(0.1)).toBe(0.1)
+  })
+
+  test('rejects ambiguous separators instead of a silent 1000x reading', () => {
+    const r = measurement('length', 'm').safeParse('1,234')
+    expect(r.success).toBe(false)
+  })
+
   test('emits a number|string JSON schema advertising natural language', () => {
     const schema = z.toJSONSchema(
       measurement('length', 'm', { min: 0, description: 'Wall thickness.' }),
diff --git a/packages/mcp/src/tools/measurement.ts b/packages/mcp/src/tools/measurement.ts
index 1767ad06..6df60b1e 100644
--- a/packages/mcp/src/tools/measurement.ts
+++ b/packages/mcp/src/tools/measurement.ts
@@ -13,15 +13,21 @@ import { z } from 'zod'
  * 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.
+ *
+ * Tool-boundary safety: genuinely ambiguous separators (`"1,234"`, which could
+ * mean 1234 or 1.234) are rejected rather than silently absorbed, so a European
+ * decimal never becomes a 1000× value.
  */
 
 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
+  /** Upper bound, in `unit`. */
+  max?: number
+  /** Inclusive lower bound, in `unit`. */
+  min?: number
+  /** Require the value to be strictly greater than 0 (for non-zero dimensions). */
+  positive?: boolean
 }
 
 function unitNoun(unit: string): string {
@@ -38,16 +44,29 @@ function unitNoun(unit: string): string {
 }
 
 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"'
+  let examples: string
+  if (kind === 'angle') {
+    // Lead with an example in the field's own unit so a bare number is read
+    // the way the model intends (a bare "45" in a radians field is 45 rad).
+    examples =
+      unit === 'rad' ? '1.5708, "90°", "1.57rad", "0.25 turn"' : '45, "45°", "1.57rad", "0.25 turn"'
+  } else {
+    examples = '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 {
+function boundsNote(
+  unit: string,
+  min: number | undefined,
+  max: number | undefined,
+  positive: boolean,
+): string {
+  if (positive) {
+    return max === undefined
+      ? ` Must be greater than 0 ${unit}.`
+      : ` Greater than 0, up to ${max} ${unit}.`
+  }
   if (min !== undefined && max !== undefined) return ` Range ${min}–${max} ${unit}.`
   if (min !== undefined) return ` Minimum ${min} ${unit}.`
   if (max !== undefined) return ` Maximum ${max} ${unit}.`
@@ -55,10 +74,10 @@ function boundsNote(unit: string, min?: number, max?: number): string {
 }
 
 export function measurement(kind: Kind, unit: string, opts: MeasurementOptions = {}) {
-  const { min, max } = opts
+  const { min, max, positive = false } = opts
   const description = [
     opts.description,
-    naturalLanguageNote(kind, unit) + boundsNote(unit, min, max),
+    naturalLanguageNote(kind, unit) + boundsNote(unit, min, max, positive),
   ]
     .filter(Boolean)
     .join(' ')
@@ -70,7 +89,14 @@ export function measurement(kind: Kind, unit: string, opts: MeasurementOptions =
       if (typeof val === 'number') {
         value = val
       } else {
-        const result = parseQuantity(val, { kind, unit, strictness: 'forgiving' })
+        const result = parseQuantity(val, {
+          kind,
+          unit,
+          strictness: 'forgiving',
+          // A genuinely ambiguous separator ("1,234") must not silently become
+          // a 1000× value at a tool boundary — fail so the model self-corrects.
+          escalate: { AMBIGUOUS_NUMBER: 'error' },
+        })
         if (!result.ok) {
           ctx.addIssue({
             code: 'custom',
@@ -84,6 +110,10 @@ export function measurement(kind: Kind, unit: string, opts: MeasurementOptions =
         ctx.addIssue({ code: 'custom', message: `Value must be a finite ${kind} in ${unit}.` })
         return z.NEVER
       }
+      if (positive && value <= 0) {
+        ctx.addIssue({ code: 'custom', message: `Must be greater than 0 ${unit} (got ${value}).` })
+        return z.NEVER
+      }
       if (min !== undefined && value < min) {
         ctx.addIssue({ code: 'custom', message: `Must be at least ${min} ${unit} (got ${value}).` })
         return z.NEVER
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 c2032ab8..96d2d2b1 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
@@ -25,11 +25,11 @@ export const photoToSceneInput = {
   name: z.string().default('Scene from photo'),
   save: z.boolean().default(true),
   defaultWallThickness: measurement('length', 'm', {
-    min: 0,
+    positive: true,
     description: 'Default wall thickness.',
   }).default(0.2),
   defaultWallHeight: measurement('length', 'm', {
-    min: 0,
+    positive: true,
     description: 'Default wall height.',
   }).default(2.6),
 }
diff --git a/packages/mcp/src/tools/room-tools.ts b/packages/mcp/src/tools/room-tools.ts
index 51aa1dee..2a36fdc1 100644
--- a/packages/mcp/src/tools/room-tools.ts
+++ b/packages/mcp/src/tools/room-tools.ts
@@ -52,8 +52,14 @@ export const createRoomInput = {
   name: z.string().min(1),
   polygon: z.array(Vec2Schema).min(3),
   color: z.string().optional(),
-  wallHeight: measurement('length', 'm', { min: 0, description: 'Wall height.' }).optional(),
-  wallThickness: measurement('length', 'm', { min: 0, description: 'Wall thickness.' }).optional(),
+  wallHeight: measurement('length', 'm', {
+    positive: true,
+    description: 'Wall height.',
+  }).optional(),
+  wallThickness: measurement('length', 'm', {
+    positive: true,
+    description: 'Wall thickness.',
+  }).optional(),
 }
 
 export const createRoomOutput = {
@@ -68,8 +74,8 @@ export const addDoorInput = {
   wallId: NodeIdSchema,
   t: z.number().min(0).max(1).optional(),
   position: z.number().min(0).max(1).optional(),
-  width: measurement('length', 'm', { min: 0, description: 'Door width.' }).optional(),
-  height: measurement('length', 'm', { min: 0, description: 'Door height.' }).optional(),
+  width: measurement('length', 'm', { positive: true, description: 'Door width.' }).optional(),
+  height: measurement('length', 'm', { positive: true, description: 'Door height.' }).optional(),
   hingesSide: z.enum(['left', 'right']).optional(),
   swingDirection: z.enum(['inward', 'outward']).optional(),
 }
@@ -88,8 +94,8 @@ export const addWindowInput = {
   wallId: NodeIdSchema,
   t: z.number().min(0).max(1).optional(),
   position: z.number().min(0).max(1).optional(),
-  width: measurement('length', 'm', { min: 0, description: 'Window width.' }).optional(),
-  height: measurement('length', 'm', { min: 0, description: 'Window height.' }).optional(),
+  width: measurement('length', 'm', { positive: true, description: 'Window width.' }).optional(),
+  height: measurement('length', 'm', { positive: true, description: 'Window height.' }).optional(),
   sillHeight: measurement('length', 'm', {
     min: 0,
     description: 'Sill height above floor.',