diff --git a/apps/api/src/modules/inventory/domain/inventory-image.js b/apps/api/src/modules/inventory/domain/inventory-image.js
index b9eba29..93a9410 100644
--- a/apps/api/src/modules/inventory/domain/inventory-image.js
+++ b/apps/api/src/modules/inventory/domain/inventory-image.js
@@ -42,4 +42,17 @@ export function validateImageSignature(data, mimeType) {
   }
 
   return true;
-}
\ No newline at end of file
+}
+export function validateImageSortOrder(sortOrder) {
+  if (!Number.isInteger(sortOrder) || sortOrder < 0 || sortOrder > 2147483647) {
+    throw createApplicationError('IMAGE_ORDER_INVALID', 'Image order must be an integer between 0 and 2147483647.', 400);
+  }
+}
+
+export function normalizeImageAltText(altText) {
+  if (altText === null) return null;
+  if (typeof altText !== 'string' || altText.trim().length > 1000) {
+    throw createApplicationError('IMAGE_ALT_TEXT_INVALID', 'Alt text must be text up to 1,000 characters or null.', 400);
+  }
+  return altText.trim();
+}
diff --git a/apps/api/src/modules/inventory/routes/admin-image-routes.js b/apps/api/src/modules/inventory/routes/admin-image-routes.js
index f1c63a1..64d7400 100644
--- a/apps/api/src/modules/inventory/routes/admin-image-routes.js
+++ b/apps/api/src/modules/inventory/routes/admin-image-routes.js
@@ -1,5 +1,5 @@
 import { toAdminInventoryImageDto } from '@pokemon-inventory/contracts/image';
-import { createInventoryImageSchema, moveInventoryImageSchema } from '@pokemon-inventory/validation/image';
+import { createInventoryImageSchema, moveInventoryImageSchema, updateImageOrderSchema, updateImageAltTextSchema } from '@pokemon-inventory/validation/image';
 import { randomUUID } from 'node:crypto';
 
 import { createApplicationError } from '../../../shared/errors/application-error.js';
@@ -110,11 +110,13 @@ export async function registerAdminImageRoutes(app, options) {
     return { data: toAdminInventoryImageDto(image) };
   });
 
-  app.patch('/images/:id/order', async (request) => {
+  app.patch('/images/:id/order', async (request, reply) => {
     const admin = await authorizeAdminMutation(request);
+    const result = updateImageOrderSchema.safeParse(request.body);
+    if (!result.success) return validationError(reply, result);
     const image = await inventoryImageService.updateOrder({
       imageId: request.params.id,
-      sortOrder: request.body?.sortOrder,
+      sortOrder: result.data.sortOrder,
       sellerId: admin.sellerId,
     });
     return { data: toAdminInventoryImageDto(image) };
@@ -129,11 +131,13 @@ export async function registerAdminImageRoutes(app, options) {
     return { data: toAdminInventoryImageDto(image) };
   });
 
-  app.patch('/images/:id/alt-text', async (request) => {
+  app.patch('/images/:id/alt-text', async (request, reply) => {
     const admin = await authorizeAdminMutation(request);
+    const result = updateImageAltTextSchema.safeParse(request.body);
+    if (!result.success) return validationError(reply, result);
     const image = await inventoryImageService.updateAltText({
       imageId: request.params.id,
-      altText: request.body?.altText ?? null,
+      altText: result.data.altText,
       sellerId: admin.sellerId,
     });
     return { data: toAdminInventoryImageDto(image) };
diff --git a/apps/api/src/modules/inventory/services/inventory-image-service.js b/apps/api/src/modules/inventory/services/inventory-image-service.js
index c3807aa..e11d48c 100644
--- a/apps/api/src/modules/inventory/services/inventory-image-service.js
+++ b/apps/api/src/modules/inventory/services/inventory-image-service.js
@@ -1,6 +1,6 @@
 import { validateImageMove } from '../domain/image-order.js';
 import { createApplicationError } from '../../../shared/errors/application-error.js';
-import { validateImageSignature, validateInventoryImageMetadata } from '../domain/inventory-image.js';
+import { normalizeImageAltText, validateImageSortOrder, validateImageSignature, validateInventoryImageMetadata } from '../domain/inventory-image.js';
 
 export function createInventoryImageService({ inventoryImageRepository, imageDerivativeJobRepository, imageStorage }) {
   return {
@@ -49,6 +49,7 @@ export function createInventoryImageService({ inventoryImageRepository, imageDer
     },
 
     async updateOrder({ imageId, sortOrder, sellerId }) {
+      validateImageSortOrder(sortOrder);
       return requireOwnedImage(inventoryImageRepository.updateOrderForSeller(imageId, sortOrder, sellerId));
     },
 
@@ -57,7 +58,7 @@ export function createInventoryImageService({ inventoryImageRepository, imageDer
     },
 
     async updateAltText({ imageId, altText, sellerId }) {
-      return requireOwnedImage(inventoryImageRepository.updateAltTextForSeller(imageId, altText, sellerId));
+      return requireOwnedImage(inventoryImageRepository.updateAltTextForSeller(imageId, normalizeImageAltText(altText), sellerId));
     },
 
     async delete({ imageId, sellerId }) {
diff --git a/apps/api/tests/application/inventory-image-management.test.js b/apps/api/tests/application/inventory-image-management.test.js
index e5f54e8..d3064bb 100644
--- a/apps/api/tests/application/inventory-image-management.test.js
+++ b/apps/api/tests/application/inventory-image-management.test.js
@@ -48,3 +48,19 @@ test('moves an image through the owned atomic repository operation', async () =>
   await assert.rejects(() => service.move({ imageId: 'missing', direction: 'down', sellerId: 'seller-1' }), { code: 'IMAGE_NOT_FOUND' });
   await assert.rejects(() => service.move({ imageId: 'owned', direction: 'left', sellerId: 'seller-1' }), { code: 'IMAGE_MOVE_INVALID' });
 });
+
+test('validates image updates before persistence and normalizes valid alt text', async () => {
+  const { service, calls } = createService();
+  for (const sortOrder of [undefined, null, -1, 0.5, '1', NaN, Infinity, 2147483648]) {
+    await assert.rejects(() => service.updateOrder({ imageId: 'image-1', sortOrder, sellerId: 'seller-1' }), { code: 'IMAGE_ORDER_INVALID' });
+  }
+  for (const altText of [undefined, 123, {}, [], 'x'.repeat(1001)]) {
+    await assert.rejects(() => service.updateAltText({ imageId: 'image-1', altText, sellerId: 'seller-1' }), { code: 'IMAGE_ALT_TEXT_INVALID' });
+  }
+  assert.deepEqual(calls, []);
+  await service.updateOrder({ imageId: 'image-1', sortOrder: 2147483647, sellerId: 'seller-1' });
+  for (const [input, expected] of [['  Front  ', 'Front'], ['   ', ''], [null, null], ['x'.repeat(1000), 'x'.repeat(1000)]]) {
+    const result = await service.updateAltText({ imageId: 'image-1', altText: input, sellerId: 'seller-1' });
+    assert.equal(result.altText, expected);
+  }
+});
diff --git a/apps/api/tests/http/image-update-validation.test.js b/apps/api/tests/http/image-update-validation.test.js
new file mode 100644
index 0000000..eb8eb33
--- /dev/null
+++ b/apps/api/tests/http/image-update-validation.test.js
@@ -0,0 +1,51 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { buildApp } from '../../src/app/build-app.js';
+import { createInventoryImageService } from '../../src/modules/inventory/services/inventory-image-service.js';
+
+test('rejects malformed image updates without writes and accepts normalized boundary values', async (context) => {
+  const writes = [];
+  const app = buildApp({
+    authenticateAdmin: async () => ({ sellerId: 'seller-1' }),
+    inventoryImageService: createInventoryImageService({ inventoryImageRepository: {
+      async updateOrderForSeller(id, sortOrder, sellerId) { writes.push({ id, sortOrder, sellerId }); return { id, sortOrder }; },
+      async updateAltTextForSeller(id, altText, sellerId) { writes.push({ id, altText, sellerId }); return { id, altText }; },
+    } }),
+  });
+  context.after(() => app.close());
+  const patch = (path, payload) => app.inject({ method: 'PATCH', url: `/api/v1/admin/images/image-1/${path}`, payload });
+  for (const payload of [{}, { sortOrder: -1 }, { sortOrder: 0.2 }, { sortOrder: '1' }, { sortOrder: null }, { sortOrder: 2147483648 }, { sortOrder: 0, sellerId: 'other' }]) {
+    const response = await patch('order', payload);
+    assert.equal(response.statusCode, 400);
+    assert.equal(response.json().error.code, 'VALIDATION_ERROR');
+  }
+  for (const payload of [{}, { altText: 1 }, { altText: {} }, { altText: [] }, { altText: 'x'.repeat(1001) }, { altText: 'Front', private: true }]) {
+    assert.equal((await patch('alt-text', payload)).statusCode, 400);
+  }
+  assert.deepEqual(writes, []);
+  for (const sortOrder of [0, 2147483647]) assert.equal((await patch('order', { sortOrder })).json().data.sortOrder, sortOrder);
+  for (const [altText, expected] of [[' Front ', 'Front'], ['   ', ''], [null, null], ['x'.repeat(1000), 'x'.repeat(1000)]]) {
+    const response = await patch('alt-text', { altText });
+    assert.equal(response.statusCode, 200);
+    assert.equal(response.json().data.altText, expected);
+  }
+  assert.ok(writes.every(({ sellerId }) => sellerId === 'seller-1'));
+});
+
+test('image update validation does not bypass authentication or CSRF', async (context) => {
+  for (const signedIn of [false, true]) {
+    const app = buildApp({
+      authenticateAdmin: async () => signedIn ? { sellerId: 'seller-1' } : null,
+      authorizeUnsafeRequest: async () => { throw Object.assign(new Error('Forbidden'), { statusCode: 403 }); },
+      inventoryImageService: {
+        async updateOrder() { assert.fail('blocked request reached order service'); },
+        async updateAltText() { assert.fail('blocked request reached alt-text service'); },
+      },
+    });
+    context.after(() => app.close());
+    for (const path of ['order', 'alt-text']) {
+      const response = await app.inject({ method: 'PATCH', url: `/api/v1/admin/images/image-1/${path}`, payload: {} });
+      assert.equal(response.statusCode, signedIn ? 403 : 401);
+    }
+  }
+});
diff --git a/apps/web/src/components/organisms/AdminImageManager.stories.js b/apps/web/src/components/organisms/AdminImageManager.stories.js
index 9fe84f6..1fb784e 100644
--- a/apps/web/src/components/organisms/AdminImageManager.stories.js
+++ b/apps/web/src/components/organisms/AdminImageManager.stories.js
@@ -55,3 +55,33 @@ export const DisabledAfterRefreshFailure = {
     }
   },
 };
+
+export const AltTextLimit = {
+  args: { images: [{ ...images[0], altText: 'card '.repeat(200) }] },
+  play: async ({ canvasElement }) => {
+    const canvas = within(canvasElement);
+    await userEvent.click(canvas.getByRole('button', { name: 'Edit alt text' }));
+    const input = canvas.getByLabelText('Alt text');
+    await expect(input).toHaveAttribute('maxlength', '1000');
+    await expect(input).toHaveAccessibleDescription(/up to 1,000 characters/);
+    await userEvent.type(input, 'extra');
+    await expect(input).toHaveValue('card '.repeat(200));
+    await userEvent.clear(input);
+    await userEvent.type(input, '  Front of card  ');
+    await userEvent.click(canvas.getByRole('button', { name: 'Save alt text' }));
+    await expect(input).toHaveValue('Front of card');
+  },
+};
+export const OversizedExistingAltText = {
+  args: { images: [{ ...images[0], altText: 'card '.repeat(201) }] },
+  play: async ({ canvasElement }) => {
+    const canvas = within(canvasElement);
+    await userEvent.click(canvas.getByRole('button', { name: 'Edit alt text' }));
+    await expect(canvas.getByRole('alert')).toHaveTextContent('Alt text must be 1,000 characters or fewer.');
+    await expect(canvas.getByLabelText('Alt text')).toHaveAttribute('aria-invalid', 'true');
+    await expect(canvas.getByRole('button', { name: 'Save alt text' })).toBeDisabled();
+    await userEvent.clear(canvas.getByLabelText('Alt text'));
+    await expect(canvas.getByRole('button', { name: 'Save alt text' })).toBeEnabled();
+    await expect(canvas.queryByRole('alert')).toBeNull();
+  },
+};
diff --git a/apps/web/src/components/organisms/AdminImageManager.vue b/apps/web/src/components/organisms/AdminImageManager.vue
index 80faa6b..edd0c40 100644
--- a/apps/web/src/components/organisms/AdminImageManager.vue
+++ b/apps/web/src/components/organisms/AdminImageManager.vue
@@ -23,7 +23,10 @@ function beginEditAltText(image) {
 }
 
 function saveAltText(imageId) {
-  emit('alt-text', { id: imageId, altText: altTextDrafts[imageId] ?? '' });
+  const altText = (altTextDrafts[imageId] ?? '').trim();
+  if (props.isSaving || props.disabled || altText.length > 1000) return;
+  altTextDrafts[imageId] = altText;
+  emit('alt-text', { id: imageId, altText });
 }
 
 watch(() => props.images, (images) => {
@@ -85,11 +88,14 @@ function handleDrop(event) {
 
         <div class="admin-image-manager__row">
           <label class="fixture-label" :for="`admin-alt-text-${image.id}`">Alt text</label>
-          <input :id="`admin-alt-text-${image.id}`" v-model="altTextDrafts[image.id]" class="fixture-input" :disabled="isSaving || disabled || editingImageId !== image.id" />
+          <input :id="`admin-alt-text-${image.id}`" v-model="altTextDrafts[image.id]" class="fixture-input" maxlength="1000" :aria-describedby="`admin-alt-help-${image.id}${(altTextDrafts[image.id] ?? '').trim().length > 1000 ? ` admin-alt-error-${image.id}` : ''}`" :aria-invalid="(altTextDrafts[image.id] ?? '').trim().length > 1000" :disabled="isSaving || disabled || editingImageId !== image.id" />
           <button v-if="editingImageId !== image.id" class="button button--secondary" type="button" :disabled="isSaving || disabled" @click="beginEditAltText(image)">Edit alt text</button>
-          <button v-else class="button button--primary" type="button" :disabled="isSaving || disabled" @click="saveAltText(image.id)">Save alt text</button>
+          <button v-else class="button button--primary" type="button" :disabled="isSaving || disabled || (altTextDrafts[image.id] ?? '').trim().length > 1000" @click="saveAltText(image.id)">Save alt text</button>
         </div>
 
+        <p :id="`admin-alt-help-${image.id}`">Describe the image in up to 1,000 characters. Clear the field to remove its alt text.</p>
+        <p :id="`admin-alt-error-${image.id}`" v-if="(altTextDrafts[image.id] ?? '').trim().length > 1000" class="state state--error" role="alert">Alt text must be 1,000 characters or fewer.</p>
+
         <div class="admin-image-manager__row">
           <button class="button button--secondary" type="button" :disabled="isSaving || disabled || image.isPrimary" @click="emit('primary', image.id)">Make primary</button>
           <button class="button button--secondary" type="button" :disabled="isSaving || disabled || index === 0" @click="emit('move', { id: image.id, direction: 'up' })">Move up</button>
diff --git a/docs/API.md b/docs/API.md
index daee988..606b51d 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -207,3 +207,14 @@ successful no-op. Missing/unowned images return `404 IMAGE_NOT_FOUND`; invalid
 bodies return `400 VALIDATION_ERROR`. The response uses the existing admin image
 DTO. The legacy absolute `/order` endpoint remains available, but UI moves use
 this new operation exclusively.
+
+Image update payloads are strict:
+- `PATCH /api/v1/admin/images/:id/order` requires exactly `sortOrder`, an integer
+  from 0 through 2147483647 (the stored PostgreSQL integer range).
+- `PATCH /api/v1/admin/images/:id/alt-text` requires exactly `altText`, either
+  trimmed text up to 1,000 characters or explicit `null`. An empty string clears
+  the text; omission no longer implicitly clears it.
+
+Malformed, missing, mistyped, oversized, or extra fields return
+`400 VALIDATION_ERROR` without a persistence write. The service also enforces
+these constraints for non-HTTP callers. Existing ownership and CSRF checks apply.
diff --git a/docs/IMAGES.md b/docs/IMAGES.md
index ac15d26..759ac30 100644
--- a/docs/IMAGES.md
+++ b/docs/IMAGES.md
@@ -129,3 +129,11 @@ contiguous zero-based positions, resolving tied legacy/upload positions, within
 one PostgreSQL transaction. Any failure rolls back all changes. Concurrent moves
 within one item serialize; primary selection and image metadata are unchanged.
 Boundary moves are no-ops. No database migration is required.
+
+## Image update validation
+
+Alt-text edits use the same trimmed 1,000-character limit as image creation.
+Explicit empty text or null may clear it; missing fields are invalid. The editor
+shows the limit, prevents typing past it, and flags oversized pre-existing drafts
+until corrected. Legacy absolute sort-order updates require a non-negative
+PostgreSQL integer; ordinary Move controls continue using the atomic move API.
diff --git a/docs/IMPLEMENTATION-STATUS.md b/docs/IMPLEMENTATION-STATUS.md
index e30954c..4062199 100644
--- a/docs/IMPLEMENTATION-STATUS.md
+++ b/docs/IMPLEMENTATION-STATUS.md
@@ -120,7 +120,7 @@ docker context use orbstack
 | HTTP | Product and inventory creation, validation, authentication/recovery, CSRF, rate limiting, and session management |
 | Integration | PostgreSQL catalog/session/audit/password recovery persistence and Redis-shared login throttling |
 
-Verified locally: `npm test` passes 171 tests with no failures (24 domain, 80 application, and 67 HTTP); worker tests, web tests, web build, Storybook build, Storybook interaction/accessibility checks, integration tests, and authenticated E2E workflows also pass when their required local services and credentials are available.
+Verified locally: `npm test` passes 174 tests with no failures (24 domain, 81 application, and 69 HTTP); worker tests, web tests, web build, Storybook build, Storybook interaction/accessibility checks, integration tests, and authenticated E2E workflows also pass when their required local services and credentials are available.
 
 ### Public Seller Description
 
@@ -177,14 +177,21 @@ Verified locally: `npm test` passes 171 tests with no failures (24 domain, 80 ap
 - Dark-theme coverage also fixed destructive-button contrast by separating its solid background token from error-text color.
 - Validation: 167 API tests, 11 web tests, production web build, Storybook build, and 32 Storybook suites / 124 tests. No API or persistence changes.
 
-### Latest Vertical Slice: Atomic Image Reordering
+### Atomic Image Reordering
 
 - Move up/down now calls a single authenticated, CSRF-protected `POST /api/v1/admin/images/:id/move` operation. The server derives the adjacent image from current persisted order.
 - The repository locks the owning Inventory Item and its images, applies the domain move rule, and commits contiguous positions in one transaction. Ties are resolved deterministically; boundary moves are no-ops; primary state is preserved.
 - Tests cover invalid requests, ownership, CSRF, missing images, boundary moves, tied positions, rollback after the first write, concurrent moves, and UI success/failure recovery.
 - Validation: 171 API tests, 26 isolated PostgreSQL/Redis integration tests, 11 web tests, production web build, Storybook build, and 32 Storybook suites / 126 tests. No migration required.
 
-Next bounded slice: image mutation payload validation. The legacy absolute-order and alt-text endpoints currently pass unchecked request fields into the service. Add strict schemas and domain validation for these updates, provide clear UI validation feedback, and test malformed/oversized requests without changing valid image workflows.
+### Latest Vertical Slice: Image Update Validation
+
+- Legacy order updates require a non-negative PostgreSQL integer; alt-text updates require trimmed text up to 1,000 characters or explicit null. Missing/extra/mistyped fields are rejected before persistence.
+- Domain validation protects non-HTTP service callers as well. Authentication, CSRF and seller ownership remain unchanged.
+- The alt-text editor shows its limit, trims saved drafts, prevents oversized input and gives associated accessible feedback for oversized pre-existing text.
+- Validation: 174 API tests, 26 isolated integration tests, 11 web tests, production web build, Storybook build, and 32 Storybook suites / 128 tests. No migration required.
+
+Next bounded slice: server-derived upload dimensions. `docs/IMAGES.md` identifies that upload dimensions are still supplied by the client. Decode image metadata behind an injected image-inspection interface, enforce dimension limits on actual bytes, and test forged dimensions and malformed images through upload validation.
 
 ## Next Milestone
 
diff --git a/packages/validation/image.js b/packages/validation/image.js
index 7f29edf..4f3236f 100644
--- a/packages/validation/image.js
+++ b/packages/validation/image.js
@@ -14,3 +14,11 @@ export const createInventoryImageSchema = z.object({
 export const moveInventoryImageSchema = z.object({
   direction: z.enum(['up', 'down']),
 }).strict();
+
+export const updateImageOrderSchema = z.object({
+  sortOrder: z.int().min(0).max(2147483647),
+}).strict();
+
+export const updateImageAltTextSchema = z.object({
+  altText: z.string().trim().max(1000).nullable(),
+}).strict();