diff --git a/src/features/animation/__tests__/sequences-test.ts b/src/features/animation/__tests__/sequences-test.ts
index 863ba33..684d5be 100644
--- a/src/features/animation/__tests__/sequences-test.ts
+++ b/src/features/animation/__tests__/sequences-test.ts
@@ -3,6 +3,7 @@ import {
   createInitialSequences,
   duplicateSequence,
   getSequenceSpring,
+  NEUTRAL_EXPRESSION_ID,
   normalizeSequencesForExpressions,
   parseSequences,
   remapSequencesAfterExpressionDelete,
@@ -88,6 +89,21 @@ describe('editable avatar sequences', () => {
     expect(normalized.steps[0].expressionId).toBe(initialExpressions[0].id)
   })
 
+  it('preserves the system neutral target during normalization', () => {
+    const sequence = createInitialSequences()[0]
+    const [normalized] = normalizeSequencesForExpressions(
+      [
+        {
+          ...sequence,
+          steps: [{ ...sequence.steps[0], expressionId: NEUTRAL_EXPRESSION_ID }],
+        },
+      ],
+      []
+    )
+
+    expect(normalized.steps[0].expressionId).toBe(NEUTRAL_EXPRESSION_ID)
+  })
+
   it('maps transition styles and durations to distinct spring dynamics', () => {
     const smooth = getSequenceSpring('smooth', 900, 7)
     const snappy = getSequenceSpring('snappy', 250, 7)
diff --git a/src/features/animation/components/SequenceWorkspace.tsx b/src/features/animation/components/SequenceWorkspace.tsx
index 32c8cc1..6bf0f84 100644
--- a/src/features/animation/components/SequenceWorkspace.tsx
+++ b/src/features/animation/components/SequenceWorkspace.tsx
@@ -25,7 +25,8 @@ import { ControlSection, InspectorCard, PanelTitle } from '@/app/components/comm
 import { NumericField } from '@/app/components/controls'
 import {
   createSequenceStep,
-  findExpressionIndex,
+  NEUTRAL_EXPRESSION_ID,
+  resolveSequenceExpression,
   type AvatarSequence,
   type SequenceStep,
 } from '@/features/animation/sequences'
@@ -37,6 +38,7 @@ import {
 import { type BodyNode } from '@/features/avatar/body'
 import { ExpressionPreview } from '@/features/avatar/components/ExpressionWorkspace'
 import { type Expression } from '@/features/avatar/geometry'
+import { defaultExpression } from '@/features/avatar/presets'
 import { type SurfaceConfig } from '@/features/avatar/surfaces'
 export function SequenceWorkspace({
   editing,
@@ -268,9 +270,8 @@ export function SequenceWorkspace({
             {editing.draft.steps.length ? (
               <div className="sequence-timeline">
                 {editing.draft.steps.map((step, position) => {
-                  const expressionIndex = findExpressionIndex(expressions, step.expressionId)
-                  const preset = expressions[expressionIndex]
-                  if (!preset) return null
+                  const resolved = resolveSequenceExpression(expressions, step.expressionId)
+                  if (!resolved.expression) return null
                   const card = (
                     <Button
                       variant="outline"
@@ -280,11 +281,15 @@ export function SequenceWorkspace({
                         onSelectedStepChange(step.id)
                         onPreviewStep(step)
                       }}
-                      onDoubleClick={() => onEditExpression(expressionIndex, preset)}
+                      onDoubleClick={
+                        resolved.neutral
+                          ? undefined
+                          : () => onEditExpression(resolved.index, resolved.expression)
+                      }
                     >
                       <GripVertical className="sequence-grip" />
                       <ExpressionPreview
-                        expression={preset}
+                        expression={resolved.expression}
                         surface={surface}
                         bodyNodes={bodyNodes}
                         colors={colors}
@@ -292,7 +297,11 @@ export function SequenceWorkspace({
                         renderStyle={renderStyle}
                         id={`sequence-${editing.draft.id}-${step.id}`}
                       />
-                      <span>{String(expressionIndex).padStart(2, '0')}</span>
+                      <span>
+                        {resolved.neutral
+                          ? t('Apparence neutre')
+                          : String(resolved.index).padStart(2, '0')}
+                      </span>
                       <small>{position + 1}</small>
                     </Button>
                   )
@@ -318,16 +327,20 @@ export function SequenceWorkspace({
                         draggedStepId.current = null
                       }}
                     >
-                      <ContextMenu>
-                        <ContextMenuTrigger render={card} />
-                        <ContextMenuContent>
-                          <ContextMenuItem
-                            onClick={() => onEditExpression(expressionIndex, preset)}
-                          >
-                            <Pencil /> {t('Modifier')}
-                          </ContextMenuItem>
-                        </ContextMenuContent>
-                      </ContextMenu>
+                      {resolved.neutral ? (
+                        card
+                      ) : (
+                        <ContextMenu>
+                          <ContextMenuTrigger render={card} />
+                          <ContextMenuContent>
+                            <ContextMenuItem
+                              onClick={() => onEditExpression(resolved.index, resolved.expression)}
+                            >
+                              <Pencil /> {t('Modifier')}
+                            </ContextMenuItem>
+                          </ContextMenuContent>
+                        </ContextMenu>
+                      )}
                     </motion.div>
                   )
                 })}
@@ -408,6 +421,28 @@ export function SequenceWorkspace({
               subtitle="Sélectionne un preset pour l’ajouter à la fin de la timeline."
             />
             <div className="expression-grid sequence-expression-library">
+              <Button
+                className="expression-card"
+                variant="outline"
+                type="button"
+                onClick={() => {
+                  const step = createSequenceStep(NEUTRAL_EXPRESSION_ID)
+                  onChange({ ...editing.draft, steps: [...editing.draft.steps, step] })
+                  onSelectedStepChange(step.id)
+                  onPreviewStep(step)
+                }}
+              >
+                <ExpressionPreview
+                  expression={defaultExpression}
+                  surface={surface}
+                  bodyNodes={bodyNodes}
+                  colors={colors}
+                  avatarEyes={avatarEyes}
+                  renderStyle={renderStyle}
+                  id="sequence-library-neutral"
+                />
+                <span>{t('Apparence neutre')}</span>
+              </Button>
               {expressions.map((preset, index) => (
                 <Button
                   className="expression-card"
diff --git a/src/features/animation/sequences.ts b/src/features/animation/sequences.ts
index 5c05261..daf3a37 100644
--- a/src/features/animation/sequences.ts
+++ b/src/features/animation/sequences.ts
@@ -1,4 +1,5 @@
 import {
+  defaultExpression,
   getStatePlaybackConfig,
   initialExpressions,
   stateGroups,
@@ -9,6 +10,7 @@ import type { Expression } from '../avatar/geometry'
 
 export type SequencePlaybackMode = 'loop' | 'once' | 'pingPong'
 export type SequenceTransition = 'spring' | 'smooth' | 'snappy'
+export const NEUTRAL_EXPRESSION_ID = defaultExpression.id
 
 export type SequenceStep = {
   id: string
@@ -170,13 +172,16 @@ export const normalizeSequencesForExpressions = (
   expressions: Expression[]
 ) => {
   const availableIds = new Set(expressions.map(expression => expression.id))
-  const fallbackId = expressions[0]?.id ?? initialExpressions[0].id
+  const fallbackId = expressions[0]?.id ?? NEUTRAL_EXPRESSION_ID
   return sequences.map(sequence => ({
     ...sequence,
     steps: (sequence.steps.length ? sequence.steps : [createSequenceStep(fallbackId)]).map(
       step => ({
         ...step,
-        expressionId: availableIds.has(step.expressionId) ? step.expressionId : fallbackId,
+        expressionId:
+          step.expressionId === NEUTRAL_EXPRESSION_ID || availableIds.has(step.expressionId)
+            ? step.expressionId
+            : fallbackId,
       })
     ),
   }))
@@ -185,6 +190,17 @@ export const normalizeSequencesForExpressions = (
 export const findExpressionIndex = (expressions: Expression[], expressionId: string) =>
   expressions.findIndex(expression => expression.id === expressionId)
 
+export const resolveSequenceExpression = (expressions: Expression[], expressionId: string) => {
+  if (expressionId === NEUTRAL_EXPRESSION_ID) {
+    return { expression: defaultExpression, index: null, neutral: true } as const
+  }
+  const index = findExpressionIndex(expressions, expressionId)
+  const expression = expressions[index]
+  return expression
+    ? ({ expression, index, neutral: false } as const)
+    : ({ expression: undefined, index: null, neutral: false } as const)
+}
+
 export const readSequenceClock = () => performance.now()
 
 export const groupSequences = (sequences: AvatarSequence[]) => {
diff --git a/src/features/avatar/__tests__/avatars-test.ts b/src/features/avatar/__tests__/avatars-test.ts
index 3ca19b3..a2331fd 100644
--- a/src/features/avatar/__tests__/avatars-test.ts
+++ b/src/features/avatar/__tests__/avatars-test.ts
@@ -1,5 +1,5 @@
 import { defaultExpression } from '@/features/avatar/presets'
-import { createInitialSequences } from '@/features/animation/sequences'
+import { createInitialSequences, NEUTRAL_EXPRESSION_ID } from '@/features/animation/sequences'
 import {
   applyAvatarEyeDefaults,
   cloneAvatarBehavior,
@@ -7,6 +7,7 @@ import {
   createUnkeyedExpressionCopy,
   defaultAvatarEyes,
   parseAvatarEyeDefaults,
+  parseAvatarLibrary,
   parseAvatarRenderStyle,
   resolveAvatarBehavior,
 } from '@/features/avatar/avatars'
@@ -84,4 +85,30 @@ describe('avatar behavior library', () => {
     expect(behavior.sequences[0].steps).not.toBe(base.sequences[0].steps)
     expect(behavior.sequences[0].blink).not.toBe(base.sequences[0].blink)
   })
+
+  it('preserves an owned neutral-only behavior library on reload', () => {
+    const avatar = createAvatar('Neutral only')
+    const sequence = createInitialSequences()[0]
+    avatar.behavior = {
+      expressions: [],
+      sequences: [
+        {
+          ...sequence,
+          steps: [{ ...sequence.steps[0], expressionId: NEUTRAL_EXPRESSION_ID }],
+        },
+      ],
+    }
+    const fallbackAvatar = createAvatar('Fallback')
+
+    const parsed = parseAvatarLibrary(
+      { activeAvatarId: avatar.id, avatars: [avatar] },
+      { activeAvatarId: fallbackAvatar.id, avatars: [fallbackAvatar] },
+      base
+    )
+
+    expect(parsed.avatars[0].behavior?.expressions).toEqual([])
+    expect(parsed.avatars[0].behavior?.sequences[0].steps[0].expressionId).toBe(
+      NEUTRAL_EXPRESSION_ID
+    )
+  })
 })
diff --git a/src/features/avatar/__tests__/import-avatar-definition-test.ts b/src/features/avatar/__tests__/import-avatar-definition-test.ts
index 348964d..e5ac1ee 100644
--- a/src/features/avatar/__tests__/import-avatar-definition-test.ts
+++ b/src/features/avatar/__tests__/import-avatar-definition-test.ts
@@ -1,8 +1,14 @@
+import { type AvatarDefinition } from '@bible-strong/avatar-core'
 import { describe, expect, it } from 'vitest'
 
 import strobi from '../../../../examples/react-vite-consumer/src/strobi.avatar.json'
+import { NEUTRAL_EXPRESSION_ID } from '../../animation/sequences'
 import { createAvatarDefinition } from '../avatarDefinition'
-import { isAvatarDefinitionSource, studioAvatarFromDefinition } from '../importAvatarDefinition'
+import {
+  isAvatarDefinitionSource,
+  studioAvatarFromDefinition,
+  studioAvatarFromDefinitionSource,
+} from '../importAvatarDefinition'
 
 describe('studioAvatarFromDefinition', () => {
   it('recognises a definition and rejects a studio project', () => {
@@ -39,5 +45,101 @@ describe('studioAvatarFromDefinition', () => {
     expect(result.value.expressionOrder).toEqual(strobi.expressionOrder)
     expect(result.value.animationOrder).toEqual(strobi.animationOrder)
     expect(result.value.expressions).toEqual(strobi.expressions)
+    expect(result.value.animations).toEqual(strobi.animations)
+  })
+
+  it('inverts customized neutral eyes instead of applying them twice', () => {
+    const customized = structuredClone(strobi) as AvatarDefinition
+    Object.values(customized.expressions).forEach(item => {
+      item.eyes.left.width += 10
+      item.eyes.right.width += 10
+      item.eyes.spacing += 6
+      item.eyes.left.y -= 4
+      item.eyes.right.y -= 4
+    })
+
+    const imported = studioAvatarFromDefinition(customized)
+    const result = createAvatarDefinition({
+      avatar: imported.avatar,
+      behavior: { expressions: imported.expressions, sequences: imported.sequences },
+    })
+
+    expect(result.ok).toBe(true)
+    if (!result.ok) return
+    expect(result.value.expressions.neutral).toEqual(customized.expressions.neutral)
+    Object.keys(customized.expressions).forEach(key => {
+      expect(result.value.expressions[key].eyes.left.width).toBeCloseTo(
+        customized.expressions[key].eyes.left.width,
+        10
+      )
+      expect(result.value.expressions[key].eyes.spacing).toBeCloseTo(
+        customized.expressions[key].eyes.spacing,
+        10
+      )
+      expect(result.value.expressions[key].eyes.left.y).toBeCloseTo(
+        customized.expressions[key].eyes.left.y,
+        10
+      )
+    })
+  })
+
+  it('preserves animation steps targeting the neutral appearance', () => {
+    const customized = structuredClone(strobi) as AvatarDefinition
+    customized.animations.sleeping.steps[0].expression = 'neutral'
+
+    const imported = studioAvatarFromDefinition(customized)
+    const sleeping = imported.sequences.find(sequence => sequence.semanticKey === 'sleeping')
+    expect(sleeping?.steps[0].expressionId).toBe(NEUTRAL_EXPRESSION_ID)
+
+    const result = createAvatarDefinition({
+      avatar: imported.avatar,
+      behavior: { expressions: imported.expressions, sequences: imported.sequences },
+    })
+    expect(result.ok).toBe(true)
+    if (!result.ok) return
+    expect(result.value.animations.sleeping).toEqual(customized.animations.sleeping)
+  })
+
+  it('creates unique Studio ids for repeated imports', () => {
+    const first = studioAvatarFromDefinition(strobi)
+    const second = studioAvatarFromDefinition(strobi)
+
+    expect(first.avatar.id).not.toBe(second.avatar.id)
+    expect(first.expressions[0].id).not.toBe(second.expressions[0].id)
+    expect(first.sequences[0].id).not.toBe(second.sequences[0].id)
+  })
+
+  it('rejects neutral properties that the Studio cannot represent', () => {
+    const customized = structuredClone(strobi) as AvatarDefinition
+    customized.expressions.neutral.head.x = 1
+
+    expect(() => studioAvatarFromDefinition(customized)).toThrow(
+      'The neutral expression must use zero head rotation'
+    )
+  })
+
+  it('rejects eye dimensions that the Studio would clamp during re-export', () => {
+    const customized = structuredClone(strobi) as AvatarDefinition
+    customized.expressions['upward-side-glance'].eyes.left.width = 9
+
+    expect(() => studioAvatarFromDefinition(customized)).toThrow(
+      'uses an eye dimension below the Studio minimum of 10'
+    )
+  })
+
+  it('uses bounded parsing for untrusted definition text', () => {
+    expect(() => studioAvatarFromDefinitionSource('{')).toThrow('Unterminated object')
+    expect(() =>
+      studioAvatarFromDefinitionSource(JSON.stringify({ ...strobi, schemaVersion: 2 }))
+    ).toThrow()
+
+    const duplicateKey = JSON.stringify(strobi).replace(
+      '"schemaVersion":1',
+      '"schemaVersion":1,"schemaVersion":1'
+    )
+    expect(() => studioAvatarFromDefinitionSource(duplicateKey)).toThrow('Duplicate object member')
+    expect(() => studioAvatarFromDefinitionSource(' '.repeat(262_145))).toThrow(
+      'JSON input exceeds'
+    )
   })
 })
diff --git a/src/features/avatar/avatarDefinition.ts b/src/features/avatar/avatarDefinition.ts
index ecea833..a125e6c 100644
--- a/src/features/avatar/avatarDefinition.ts
+++ b/src/features/avatar/avatarDefinition.ts
@@ -13,6 +13,7 @@ import {
 } from '@bible-strong/avatar-core'
 import type { SurfaceType } from '@bible-strong/avatar-core'
 
+import { NEUTRAL_EXPRESSION_ID } from '../animation/sequences'
 import { applyAvatarEyeDefaults, type AvatarBehaviorLibrary, type StudioAvatar } from './avatars'
 import { defaultExpression } from './presets'
 import type { Expression } from './geometry'
@@ -124,7 +125,10 @@ export const createAvatarDefinition = ({
     )
     if (error) errors.push(error)
     sequence.steps.forEach((step, stepIndex) => {
-      if (!expressionKeyById.has(step.expressionId)) {
+      if (
+        step.expressionId !== NEUTRAL_EXPRESSION_ID &&
+        !expressionKeyById.has(step.expressionId)
+      ) {
         errors.push({
           path: `/studio/animations/${index}/steps/${stepIndex}/expressionId`,
           code: 'unresolved_expression_reference',
@@ -149,7 +153,10 @@ export const createAvatarDefinition = ({
       {
         playbackMode: sequence.playbackMode,
         steps: sequence.steps.map(step => ({
-          expression: expressionKeyById.get(step.expressionId)!,
+          expression:
+            step.expressionId === NEUTRAL_EXPRESSION_ID
+              ? 'neutral'
+              : expressionKeyById.get(step.expressionId)!,
           holdMs: step.holdMs,
           transitionMs: step.transitionMs,
           transition: step.transition,
diff --git a/src/features/avatar/avatars.ts b/src/features/avatar/avatars.ts
index 5a9d491..9c731d1 100644
--- a/src/features/avatar/avatars.ts
+++ b/src/features/avatar/avatars.ts
@@ -229,8 +229,8 @@ const parseAvatarBehavior = (
 ): AvatarBehaviorLibrary | undefined => {
   if (!value || typeof value !== 'object') return undefined
   const candidate = value as Partial<AvatarBehaviorLibrary>
-  if (!Array.isArray(candidate.expressions) || !candidate.expressions.length) return undefined
-  const expressions = parseExpressions(candidate.expressions)
+  if (!Array.isArray(candidate.expressions)) return undefined
+  const expressions = candidate.expressions.length ? parseExpressions(candidate.expressions) : []
   return restoreLegacyBehaviorSemanticKeys(
     {
       expressions,
diff --git a/src/features/avatar/importAvatarDefinition.ts b/src/features/avatar/importAvatarDefinition.ts
index 257ff55..341d869 100644
--- a/src/features/avatar/importAvatarDefinition.ts
+++ b/src/features/avatar/importAvatarDefinition.ts
@@ -1,12 +1,17 @@
 import {
+  parseAvatarDefinition,
   validateAvatarDefinition,
   type AvatarAnimationDefinition,
   type AvatarDefinition,
   type AvatarExpressionDefinition,
 } from '@bible-strong/avatar-core'
 
-import type { AvatarSequence, SequenceStep } from '../animation/sequences'
-import type { StudioAvatar } from './avatars'
+import {
+  NEUTRAL_EXPRESSION_ID,
+  type AvatarSequence,
+  type SequenceStep,
+} from '../animation/sequences'
+import { defaultAvatarEyes, type StudioAvatar } from './avatars'
 import type { Expression } from './geometry'
 
 /**
@@ -33,27 +38,82 @@ const slugify = (value: string) =>
     .replace(/[^a-z0-9]+/g, '-')
     .replace(/^-+|-+$/g, '') || 'avatar'
 
+const createImportId = (slug: string) =>
+  `${slug}-${
+    typeof crypto !== 'undefined' && crypto.randomUUID
+      ? crypto.randomUUID()
+      : `${Date.now()}-${Math.random().toString(36).slice(2)}`
+  }`
+
+const relativeEyeValue = (value: number, neutral: number, fallback: number) =>
+  value - neutral + fallback
+
 const toExpression = (
-  slug: string,
+  importId: string,
   semanticKey: string,
-  expression: AvatarExpressionDefinition
+  expression: AvatarExpressionDefinition,
+  neutral: AvatarExpressionDefinition
 ): Expression => ({
-  id: `expression-${slug}-${semanticKey}`,
+  id: `expression-${importId}-${semanticKey}`,
   semanticKey,
   headX: expression.head.x,
   headY: expression.head.y,
   headZ: expression.head.z,
-  widthLeft: expression.eyes.left.width,
-  widthRight: expression.eyes.right.width,
-  heightLeft: expression.eyes.left.height,
-  heightRight: expression.eyes.right.height,
-  spacing: expression.eyes.spacing,
-  positionXLeft: expression.eyes.left.x,
-  positionXRight: expression.eyes.right.x,
-  positionYLeft: expression.eyes.left.y,
-  positionYRight: expression.eyes.right.y,
-  leftAngle: expression.eyes.left.angle,
-  rightAngle: expression.eyes.right.angle,
+  widthLeft: relativeEyeValue(
+    expression.eyes.left.width,
+    neutral.eyes.left.width,
+    defaultAvatarEyes.widthLeft
+  ),
+  widthRight: relativeEyeValue(
+    expression.eyes.right.width,
+    neutral.eyes.right.width,
+    defaultAvatarEyes.widthRight
+  ),
+  heightLeft: relativeEyeValue(
+    expression.eyes.left.height,
+    neutral.eyes.left.height,
+    defaultAvatarEyes.heightLeft
+  ),
+  heightRight: relativeEyeValue(
+    expression.eyes.right.height,
+    neutral.eyes.right.height,
+    defaultAvatarEyes.heightRight
+  ),
+  spacing: relativeEyeValue(
+    expression.eyes.spacing,
+    neutral.eyes.spacing,
+    defaultAvatarEyes.spacing
+  ),
+  positionXLeft: relativeEyeValue(
+    expression.eyes.left.x,
+    neutral.eyes.left.x,
+    defaultAvatarEyes.positionXLeft
+  ),
+  positionXRight: relativeEyeValue(
+    expression.eyes.right.x,
+    neutral.eyes.right.x,
+    defaultAvatarEyes.positionXRight
+  ),
+  positionYLeft: relativeEyeValue(
+    expression.eyes.left.y,
+    neutral.eyes.left.y,
+    defaultAvatarEyes.positionYLeft
+  ),
+  positionYRight: relativeEyeValue(
+    expression.eyes.right.y,
+    neutral.eyes.right.y,
+    defaultAvatarEyes.positionYRight
+  ),
+  leftAngle: relativeEyeValue(
+    expression.eyes.left.angle,
+    neutral.eyes.left.angle,
+    defaultAvatarEyes.leftAngle
+  ),
+  rightAngle: relativeEyeValue(
+    expression.eyes.right.angle,
+    neutral.eyes.right.angle,
+    defaultAvatarEyes.rightAngle
+  ),
   perspective: expression.perspective,
   eyeMotion: expression.motion.eyes,
   bodyMotion: expression.motion.body,
@@ -62,19 +122,19 @@ const toExpression = (
 })
 
 const toSequence = (
-  slug: string,
+  importId: string,
   semanticKey: string,
   animation: AvatarAnimationDefinition,
   expressionIdByKey: Map<string, string>
 ): AvatarSequence => {
   const steps: SequenceStep[] = []
   animation.steps.forEach((step, index) => {
-    const expressionId = expressionIdByKey.get(step.expression)
-    // A step pointing at an expression the definition never declared cannot be
-    // represented; dropping it keeps the sequence playable.
-    if (!expressionId) return
+    const expressionId =
+      step.expression === 'neutral' ? NEUTRAL_EXPRESSION_ID : expressionIdByKey.get(step.expression)
+    // Validation guarantees that non-neutral references resolve.
+    if (!expressionId) throw new Error(`Unknown expression '${step.expression}'`)
     steps.push({
-      id: `step-${slug}-${semanticKey}-${index}`,
+      id: `step-${importId}-${semanticKey}-${index}`,
       expressionId,
       holdMs: step.holdMs,
       transitionMs: step.transitionMs,
@@ -82,7 +142,7 @@ const toSequence = (
     })
   })
   return {
-    id: `sequence-${slug}-${semanticKey}`,
+    id: `sequence-${importId}-${semanticKey}`,
     semanticKey,
     name: animation.metadata?.label ?? semanticKey,
     group: animation.metadata?.group ?? 'Importé',
@@ -100,39 +160,78 @@ export type ImportedAvatarDefinition = {
   sequences: AvatarSequence[]
 }
 
+const assertRepresentableNeutral = (neutral: AvatarExpressionDefinition) => {
+  const canonical =
+    neutral.head.x === 0 &&
+    neutral.head.y === 0 &&
+    neutral.head.z === 0 &&
+    neutral.perspective === 1 &&
+    neutral.motion.eyes === 'none' &&
+    neutral.motion.body === 'none' &&
+    neutral.colors === undefined
+  if (!canonical) {
+    throw new Error(
+      'The neutral expression must use zero head rotation, perspective 1, no ambient motion and no color overrides.'
+    )
+  }
+}
+
+const assertRepresentableEyeDimensions = (definition: AvatarDefinition) => {
+  Object.entries(definition.expressions).forEach(([key, expression]) => {
+    const dimensions = [
+      expression.eyes.left.width,
+      expression.eyes.right.width,
+      expression.eyes.left.height,
+      expression.eyes.right.height,
+    ]
+    if (dimensions.some(value => value < 10)) {
+      throw new Error(`Expression '${key}' uses an eye dimension below the Studio minimum of 10.`)
+    }
+  })
+}
+
+const formatValidationError = (errors: readonly { path: string; message: string }[]) => {
+  const first = errors[0]
+  return first ? `${first.path}: ${first.message}` : 'Invalid avatar definition'
+}
+
 /** Throws when the file is not a valid v1 avatar definition. */
 export const studioAvatarFromDefinition = (value: unknown): ImportedAvatarDefinition => {
   const result = validateAvatarDefinition(value)
   if (!result.ok) {
-    const first = result.errors[0]
-    throw new Error(first ? `${first.path}: ${first.message}` : 'Invalid avatar definition')
+    throw new Error(formatValidationError(result.errors))
   }
   const definition: AvatarDefinition = result.value
   const slug = slugify(definition.name ?? 'avatar')
+  const importId = createImportId(slug)
+  const neutral = definition.expressions.neutral
+  assertRepresentableNeutral(neutral)
+  assertRepresentableEyeDimensions(definition)
 
   // `neutral` is reserved: the studio does not keep it as an editable expression,
   // it lives on the avatar as eye defaults and is re-emitted on export.
   const keys = definition.expressionOrder.filter(
     key => key !== 'neutral' && definition.expressions[key]
   )
-  const expressions = keys.map(key => toExpression(slug, key, definition.expressions[key]!))
+  const expressions = keys.map(key =>
+    toExpression(importId, key, definition.expressions[key]!, neutral)
+  )
   const expressionIdByKey = new Map(keys.map((key, i) => [key, expressions[i]!.id]))
 
   const sequences = definition.animationOrder
     .filter(key => definition.animations[key])
-    .map(key => toSequence(slug, key, definition.animations[key]!, expressionIdByKey))
+    .map(key => toSequence(importId, key, definition.animations[key]!, expressionIdByKey))
 
   // The studio stores one flat set of eye defaults per avatar; `neutral` is the
   // resting pose, so it is the expression those defaults come from.
-  const neutral = definition.expressions.neutral
   const avatar: StudioAvatar = {
-    id: `avatar-${slug}-${definition.body.nodes.length}-${expressions.length}`,
+    id: `avatar-${importId}`,
     name: definition.name ?? slug,
     body: {
       primary: definition.body.primary,
       // Definition nodes are anonymous; the studio addresses them by id in the editor.
       nodes: definition.body.nodes.map((node, index) => ({
-        id: `shape-${slug}-${index}`,
+        id: `shape-${importId}-${index}`,
         name: `${node.surface.type} ${index + 1}`,
         surface: node.surface,
         position: node.position,
@@ -159,3 +258,10 @@ export const studioAvatarFromDefinition = (value: unknown): ImportedAvatarDefini
 
   return { avatar, expressions, sequences }
 }
+
+/** Parses untrusted `.avatar.json` text with the core size, depth and duplicate-key limits. */
+export const studioAvatarFromDefinitionSource = (source: string): ImportedAvatarDefinition => {
+  const result = parseAvatarDefinition(source)
+  if (!result.ok) throw new Error(formatValidationError(result.errors))
+  return studioAvatarFromDefinition(result.value)
+}
diff --git a/src/features/export/exporter.ts b/src/features/export/exporter.ts
index 2763e16..ed958f3 100644
--- a/src/features/export/exporter.ts
+++ b/src/features/export/exporter.ts
@@ -1,9 +1,10 @@
 import { applyAvatarEyeDefaults, type StudioAvatar } from '../avatar/avatars'
 import { avatarDefinitionFileName, type AvatarDefinition } from '@bible-strong/avatar-core'
 import type { Expression } from '../avatar/geometry'
+import { defaultExpression } from '../avatar/presets'
 import { translateStudioText, type StudioLanguage } from '../../i18n'
 import { proceduralBrowserRuntime } from './proceduralBrowserRuntime'
-import type { AvatarSequence } from '../animation/sequences'
+import { NEUTRAL_EXPRESSION_ID, type AvatarSequence } from '../animation/sequences'
 import { standaloneEngineSource } from './standaloneEngine.generated'
 import { createStoredZip } from './storedZip'
 
@@ -60,7 +61,10 @@ export const createAvatarExportPayload = (
   )
   const exportedExpressions = Object.fromEntries(
     [...referencedIds].flatMap(expressionId => {
-      const expression = expressionById.get(expressionId)
+      const expression =
+        expressionId === NEUTRAL_EXPRESSION_ID
+          ? defaultExpression
+          : expressionById.get(expressionId)
       if (!expression) return []
       const { semanticKey: _semanticKey, ...legacyExpression } = applyAvatarEyeDefaults(
         expression,
diff --git a/src/features/studio/components/StudioInspector.tsx b/src/features/studio/components/StudioInspector.tsx
index 7ae7bf6..89e8e1d 100644
--- a/src/features/studio/components/StudioInspector.tsx
+++ b/src/features/studio/components/StudioInspector.tsx
@@ -66,7 +66,7 @@ import {
   type SnapshotFormat,
 } from '@/app/studio-utils'
 import { SequenceWorkspace } from '@/features/animation/components/SequenceWorkspace'
-import { findExpressionIndex, groupSequences } from '@/features/animation/sequences'
+import { groupSequences, resolveSequenceExpression } from '@/features/animation/sequences'
 import { defaultAvatarEyes } from '@/features/avatar/avatars'
 import {
   ExpressionCard,
@@ -679,9 +679,10 @@ export function StudioInspector({ controller }: { controller: StudioController }
                 setSequenceEditing(current => (current ? { ...current, draft } : current))
               }
               onPreviewStep={step => {
-                const expressionIndex = findExpressionIndex(expressions, step.expressionId)
-                const preset = expressions[expressionIndex]
-                if (preset) transitionToExpression(preset, expressionIndex, step)
+                const resolved = resolveSequenceExpression(expressions, step.expressionId)
+                if (resolved.expression) {
+                  transitionToExpression(resolved.expression, resolved.index, step)
+                }
               }}
               onEditExpression={openExpressionEditor}
               onPlay={() => launchSequence(sequenceEditing.draft, false, false)}
@@ -1065,17 +1066,6 @@ export function StudioInspector({ controller }: { controller: StudioController }
                     <Copy />
                     {t('Dupliquer')}
                   </Button>
-                  <Button
-                    variant="outline"
-                    disabled={!runtimeDefinitionResult.ok}
-                    // downloadAvatarRuntimeDefinition is a no-op on an invalid
-                    // definition, so surface why the button is dead.
-                    title={runtimeExportErrors[0] ?? undefined}
-                    onClick={downloadAvatarRuntimeDefinition}
-                  >
-                    <Download />
-                    {t('Exporter')}
-                  </Button>
                 </div>
                 <Button onClick={saveAvatarEditing}>{t('Enregistrer')}</Button>
               </footer>
@@ -1225,7 +1215,8 @@ export function StudioInspector({ controller }: { controller: StudioController }
                           {group.sequences.map(sequence => {
                             const firstStep = sequence.steps[0]
                             const firstExpression = firstStep
-                              ? expressionById.get(firstStep.expressionId)
+                              ? resolveSequenceExpression(expressions, firstStep.expressionId)
+                                  .expression
                               : undefined
                             const card = (
                               <Button
@@ -1705,7 +1696,8 @@ export function StudioInspector({ controller }: { controller: StudioController }
                           {sequences.map(animation => {
                             const firstStep = animation.steps[0]
                             const firstExpression = firstStep
-                              ? expressionById.get(firstStep.expressionId)
+                              ? resolveSequenceExpression(expressions, firstStep.expressionId)
+                                  .expression
                               : undefined
                             return (
                               <Button
@@ -2017,20 +2009,21 @@ export function StudioInspector({ controller }: { controller: StudioController }
             <div className="state-playback-bar">
               <div className="state-playback-timeline">
                 {activeSequence.steps.map((step, position) => {
-                  const expressionIndex = findExpressionIndex(expressions, step.expressionId)
-                  const preset = expressions[expressionIndex]
-                  if (!preset) return null
+                  const resolved = resolveSequenceExpression(expressions, step.expressionId)
+                  if (!resolved.expression) return null
                   return (
                     <Button
                       className="state-playback-step"
                       variant="outline"
                       type="button"
                       key={step.id}
-                      aria-pressed={activeExpression === expressionIndex}
-                      onClick={() => transitionToExpression(preset, expressionIndex, step)}
+                      aria-pressed={activeExpression === resolved.index}
+                      onClick={() =>
+                        transitionToExpression(resolved.expression, resolved.index, step)
+                      }
                     >
                       <ExpressionPreview
-                        expression={preset}
+                        expression={resolved.expression}
                         surface={surface}
                         bodyNodes={bodyNodes}
                         colors={activeAvatar.colors}
diff --git a/src/features/studio/useStudioController.ts b/src/features/studio/useStudioController.ts
index 292c712..58b6633 100644
--- a/src/features/studio/useStudioController.ts
+++ b/src/features/studio/useStudioController.ts
@@ -36,8 +36,10 @@ import {
   duplicateSequence,
   findExpressionIndex,
   getSequenceSpring,
+  NEUTRAL_EXPRESSION_ID,
   readSequenceClock,
   remapSequencesAfterExpressionDelete,
+  resolveSequenceExpression,
   type AvatarSequence,
   type SequenceStep,
 } from '@/features/animation/sequences'
@@ -66,7 +68,7 @@ import {
 } from '@/features/avatar/avatars'
 import {
   isAvatarDefinitionSource,
-  studioAvatarFromDefinition,
+  studioAvatarFromDefinitionSource,
 } from '@/features/avatar/importAvatarDefinition'
 import {
   bodyPrimitiveTypes,
@@ -1104,16 +1106,15 @@ export function useStudioController() {
       playbackTimeline.current = { ...playbackTimeline.current, stepDueAt: null }
       const step = sequence.steps[playbackTimeline.current.position]
       const availableExpressions = expressionsRef.current
-      const expressionIndex = findExpressionIndex(availableExpressions, step.expressionId)
-      const preset = availableExpressions[expressionIndex]
+      const resolved = resolveSequenceExpression(availableExpressions, step.expressionId)
       const durationMs = (reduceMotion ? 0 : step.transitionMs) + step.holdMs
       setPlaybackVisual(current => ({
         position: playbackTimeline.current.position,
         run: current.run + 1,
         durationMs,
       }))
-      if (preset) {
-        transitionToExpression(preset, expressionIndex, step)
+      if (resolved.expression) {
+        transitionToExpression(resolved.expression, resolved.index, step)
       }
       scheduleAdvance(durationMs)
     }
@@ -1402,15 +1403,18 @@ export function useStudioController() {
           steps: sequence.steps.map(step => ({ ...step })),
           blink: { ...sequence.blink },
         }
-      : createSequence(expressions[activeExpression ?? 0]?.id ?? expressions[0]?.id)
+      : createSequence(
+          expressions[activeExpression ?? 0]?.id ?? expressions[0]?.id ?? NEUTRAL_EXPRESSION_ID
+        )
     setSequenceEditing({ sourceId: sequence?.id ?? null, draft })
     setSelectedSequenceStepId(draft.steps[0]?.id ?? null)
     const firstStep = draft.steps[0]
-    const expressionIndex = firstStep
-      ? findExpressionIndex(expressions, firstStep.expressionId)
-      : -1
-    const preset = expressions[expressionIndex]
-    if (preset) transitionToExpression(preset, expressionIndex, firstStep)
+    const resolved = firstStep
+      ? resolveSequenceExpression(expressions, firstStep.expressionId)
+      : null
+    if (resolved?.expression) {
+      transitionToExpression(resolved.expression, resolved.index, firstStep)
+    }
   }
 
   const cancelSequenceEditing = () => {
@@ -1824,9 +1828,8 @@ export function useStudioController() {
       .then(source => {
         // One picker, two formats: a `.avatar.json` definition adds a single avatar
         // to the current library, a studio project replaces the whole document.
-        const parsed: unknown = JSON.parse(source)
-        if (isAvatarDefinitionSource(parsed)) {
-          const { avatar } = studioAvatarFromDefinition(parsed)
+        try {
+          const { avatar } = studioAvatarFromDefinitionSource(source)
           const document = currentStudioDocument()
           setPendingProjectImport({
             document: {
@@ -1843,13 +1846,19 @@ export function useStudioController() {
             kind: 'avatar',
           })
           return
+        } catch (avatarError) {
+          const parsed: unknown = JSON.parse(source)
+          if (isAvatarDefinitionSource(parsed)) throw avatarError
         }
         const imported = parseImportedStudioDocument(source, currentStudioDocument())
         setPendingProjectImport({ document: imported, fileName: file.name, kind: 'project' })
       })
-      .catch(() => {
+      .catch(error => {
+        const summary = t(
+          'Ce fichier n’est ni un avatar .avatar.json ni un projet Avatar Studio valide.'
+        )
         setProjectImportError(
-          t('Ce fichier n’est ni un avatar .avatar.json ni un projet Avatar Studio valide.')
+          error instanceof Error && error.message ? `${summary} ${error.message}` : summary
         )
       })
   }
diff --git a/src/i18n/index.ts b/src/i18n/index.ts
index 9cd76e8..e93a861 100644
--- a/src/i18n/index.ts
+++ b/src/i18n/index.ts
@@ -76,6 +76,7 @@ const english: Record<string, string> = {
     'Use lowercase letters, numbers, and hyphens, for example happy-smile.',
   'neutral est réservé à l’apparence neutre de l’avatar.':
     'neutral is reserved for the avatar neutral appearance.',
+  'Apparence neutre': 'Neutral appearance',
   'Cette clé est déjà utilisée dans cette bibliothèque.':
     'This key is already used in this library.',
   'L’avatar à gauche affiche cette expression en direct.':
diff --git a/src/i18n/zh.ts b/src/i18n/zh.ts
index 1f97bf2..cd6e91f 100644
--- a/src/i18n/zh.ts
+++ b/src/i18n/zh.ts
@@ -48,6 +48,7 @@ export const chinese: Record<string, string> = {
   'Utilise des lettres minuscules, des chiffres et des tirets, par exemple happy-smile.':
     '请使用小写字母、数字和连字符，例如 happy-smile。',
   'neutral est réservé à l’apparence neutre de l’avatar.': 'neutral 保留用于头像的中性外观。',
+  'Apparence neutre': '中性外观',
   'Cette clé est déjà utilisée dans cette bibliothèque.': '此键已在当前库中使用。',
   'L’avatar à gauche affiche cette expression en direct.': '左侧头像会实时预览此表情。',
   Corps: '身体',