diff --git a/src/app/models/flow.ts b/src/app/models/flow.ts
index df9fb6f..c09044c 100644
--- a/src/app/models/flow.ts
+++ b/src/app/models/flow.ts
@@ -205,12 +205,20 @@ export type FlowValueKind = {
   multiple: boolean;
 };
 
+/** Client-facing hints for a file input. The server validates the actual bytes again. */
+export type FileInputConstraints = {
+  acceptedMediaTypes?: string[];
+  maxFiles?: number | null;
+  maxTotalBytes?: number | null;
+};
+
 export type FlowPort = {
   name: string;
   type: string;
   multiple: boolean;
   valueKinds?: FlowValueKind[];
   valueSchema?: Record<string, unknown> | null;
+  fileConstraints?: FileInputConstraints | null;
 };
 
 export type FlowBlockConnection = {
diff --git a/src/app/models/task-execution.ts b/src/app/models/task-execution.ts
index 37182a4..93ec750 100644
--- a/src/app/models/task-execution.ts
+++ b/src/app/models/task-execution.ts
@@ -1,4 +1,12 @@
-import { FlowBlockConnection, FlowData, FlowNode, FlowNodeDependency, FlowPort, LLMDescriptor } from './flow';
+import {
+  FileInputConstraints,
+  FlowBlockConnection,
+  FlowData,
+  FlowNode,
+  FlowNodeDependency,
+  FlowPort,
+  LLMDescriptor
+} from './flow';
 import { BiasExecutionContext } from './bias-impact';
 
 export type TaskExecutionStatus = 'CREATED' | 'READY' | 'RUNNING' | 'WAITING' | 'SUSPENDED' | 'SUCCESS' | 'ERROR' | 'CANCELLED';
@@ -151,6 +159,7 @@ export type TaskExecutionGlobalInputDescriptor = {
   description?: string | null;
   cleanupPolicy?: string | null;
   multiple?: boolean;
+  fileConstraints?: FileInputConstraints | null;
 };
 
 export type TaskExecutionStep = {
diff --git a/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.html b/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.html
index a944d4e..9145b84 100644
--- a/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.html
+++ b/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.html
@@ -207,9 +207,13 @@
     <input
       type="file"
       class="block w-full text-sm text-slate-700"
+      [attr.accept]="fileAccept(executionInput)"
       [attr.multiple]="isMultipleInput(executionInput) ? '' : null"
       [disabled]="readOnly()"
       (change)="onFileInputChange(executionInput, $event)" />
+    @if (fileInputHint(executionInput); as hint) {
+    <div class="mt-1 text-[11px] text-slate-500">{{ hint }}</div>
+    }
     } @else if (isMultipleInput(executionInput)) {
     @if (itemsOpen(executionInput)) {
     <div class="inputs-panel-items">
diff --git a/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.spec.ts b/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.spec.ts
index f194667..a139204 100644
--- a/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.spec.ts
+++ b/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.spec.ts
@@ -1,7 +1,12 @@
 import { TestBed } from '@angular/core/testing';
 import { vi } from 'vitest';
 
-import { EditableExecutionInput, TaskExecutionInputsPanelComponent, parseJsonArrayInput } from './task-execution-inputs-panel';
+import {
+  EditableExecutionInput,
+  TaskExecutionInputsPanelComponent,
+  parseJsonArrayInput,
+  validateFileSelection
+} from './task-execution-inputs-panel';
 
 function makeInput(overrides: Partial<EditableExecutionInput> = {}): EditableExecutionInput {
   const input: EditableExecutionInput = {
@@ -50,6 +55,55 @@ async function build(inputs: EditableExecutionInput[], options: {
 describe('TaskExecutionInputsPanelComponent', () => {
   afterEach(() => TestBed.resetTestingModule());
 
+  it('validates MCP image constraints before upload', () => {
+    const imageInput = makeInput({
+      type: 'FILE',
+      multiple: true,
+      fileConstraints: {
+        acceptedMediaTypes: ['image/png', 'image/jpeg', 'image/webp'],
+        maxFiles: 4,
+        maxTotalBytes: 15_000_000
+      }
+    });
+    const png = new File(['image'], 'reference.png', { type: 'image/png' });
+    const gif = new File(['image'], 'reference.gif', { type: 'image/gif' });
+
+    expect(validateFileSelection(imageInput, [png])).toBeNull();
+    expect(validateFileSelection(imageInput, [png, png, png, png, png])).toContain('At most 4 files');
+    expect(validateFileSelection(imageInput, [gif])).toContain('PNG, JPEG, WebP');
+  });
+
+  it('validates MCP PDF aggregate budget before upload', () => {
+    const documentInput = makeInput({
+      type: 'FILE',
+      multiple: true,
+      fileConstraints: {
+        acceptedMediaTypes: ['application/pdf'],
+        maxFiles: 4,
+        maxTotalBytes: 32_000_000
+      }
+    });
+    const tooLarge = new File([new Uint8Array(32_000_001)], 'specification.pdf', { type: 'application/pdf' });
+
+    expect(validateFileSelection(documentInput, [tooLarge])).toContain('32,000,000 bytes');
+  });
+
+  it('renders the MIME filter and server-provided upload hint', async () => {
+    const fixture = await build([makeInput({
+      type: 'FILE',
+      multiple: true,
+      fileConstraints: {
+        acceptedMediaTypes: ['application/pdf'],
+        maxFiles: 4,
+        maxTotalBytes: 32_000_000
+      }
+    })]);
+
+    const picker = fixture.nativeElement.querySelector('input[type="file"]') as HTMLInputElement;
+    expect(picker.accept).toBe('application/pdf');
+    expect(fixture.nativeElement.textContent).toContain('Allowed: PDF; up to 4 files; total up to 32,000,000 bytes.');
+  });
+
   it('counts every input, node ones included, since all of them are required', async () => {
     // A step without its manual input never reaches READY, so a node input blocks the start just
     // as a global one does and must be part of the tally.
diff --git a/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.ts b/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.ts
index 6905a40..329f9f9 100644
--- a/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.ts
+++ b/src/app/shared/task-execution-inputs-panel/task-execution-inputs-panel.ts
@@ -8,6 +8,7 @@ import { ModalShellComponent } from '@shared/modal-shell/modal-shell';
 import { MatFormFieldModule } from '@angular/material/form-field';
 import { MatInputModule } from '@angular/material/input';
 import { TaskExecutionAuthorizationRequirement } from '@models/task-execution';
+import { FileInputConstraints } from '@models/flow';
 
 export type JsonArrayParseResult =
   | { values: string[]; error: null }
@@ -72,6 +73,7 @@ export type EditableExecutionInput = {
   subtitle: string;
   type: string;
   multiple: boolean;
+  fileConstraints?: FileInputConstraints | null;
   value: string | string[];
   /**
    * Whether a value is actually stored in the execution - unsaved edits do not count. Node inputs
@@ -80,6 +82,68 @@ export type EditableExecutionInput = {
   provided: boolean;
 };
 
+const FILE_EXTENSION_BY_MEDIA_TYPE: Record<string, string[]> = {
+  'image/png': ['.png'],
+  'image/jpeg': ['.jpg', '.jpeg'],
+  'image/webp': ['.webp'],
+  'application/pdf': ['.pdf']
+};
+
+function formatBytes(bytes: number): string {
+  return `${new Intl.NumberFormat('en-US').format(bytes)} bytes`;
+}
+
+function mediaTypeLabel(mediaTypes: string[]): string {
+  const labels = mediaTypes.map((mediaType) => {
+    if (mediaType === 'image/png') return 'PNG';
+    if (mediaType === 'image/jpeg') return 'JPEG';
+    if (mediaType === 'image/webp') return 'WebP';
+    if (mediaType === 'application/pdf') return 'PDF';
+    return mediaType;
+  });
+  return labels.join(', ');
+}
+
+function inferredMediaType(file: File, acceptedMediaTypes: string[]): string | null {
+  const browserType = file.type.trim().toLowerCase();
+  if (acceptedMediaTypes.includes(browserType)) return browserType;
+  const lowerName = file.name.toLowerCase();
+  return acceptedMediaTypes.find((mediaType) =>
+    (FILE_EXTENSION_BY_MEDIA_TYPE[mediaType] ?? []).some((extension) => lowerName.endsWith(extension))) ?? null;
+}
+
+/**
+ * Validates an upload before it crosses the network. This mirrors the public constraints on the
+ * descriptor; the MCP executor still validates byte signatures and totals authoritatively.
+ */
+export function validateFileSelection(input: EditableExecutionInput, files: File[]): string | null {
+  if (!files.length) return 'Choose at least one file.';
+  if (!input.multiple && files.length !== 1) return 'This input accepts one file.';
+
+  const constraints = input.fileConstraints;
+  if (!constraints) return null;
+  const maxFiles = input.multiple
+    ? constraints.maxFiles ?? Number.MAX_SAFE_INTEGER
+    : 1;
+  if (files.length > maxFiles) {
+    return `At most ${maxFiles} ${maxFiles === 1 ? 'file is' : 'files are'} allowed.`;
+  }
+
+  const acceptedMediaTypes = constraints.acceptedMediaTypes ?? [];
+  for (const file of files) {
+    if (file.size === 0) return `${file.name} is empty.`;
+    if (acceptedMediaTypes.length && !inferredMediaType(file, acceptedMediaTypes)) {
+      return `${file.name} must be ${mediaTypeLabel(acceptedMediaTypes)}.`;
+    }
+  }
+
+  const totalBytes = files.reduce((sum, file) => sum + file.size, 0);
+  if (constraints.maxTotalBytes != null && totalBytes > constraints.maxTotalBytes) {
+    return `The selected files total ${formatBytes(totalBytes)}; the limit is ${formatBytes(constraints.maxTotalBytes)}.`;
+  }
+  return null;
+}
+
 @Component({
   selector: 'app-task-execution-inputs-panel',
   imports: [CommonModule, FormsModule, MatButtonModule, MatFormFieldModule, MatIconModule, MatInputModule, MatTooltipModule, ModalShellComponent],
@@ -309,6 +373,22 @@ export class TaskExecutionInputsPanelComponent {
     return input.multiple;
   }
 
+  fileAccept(input: EditableExecutionInput): string | null {
+    const mediaTypes = input.fileConstraints?.acceptedMediaTypes ?? [];
+    return mediaTypes.length ? mediaTypes.join(',') : null;
+  }
+
+  fileInputHint(input: EditableExecutionInput): string | null {
+    const constraints = input.fileConstraints;
+    if (!constraints) return null;
+    const parts: string[] = [];
+    const mediaTypes = constraints.acceptedMediaTypes ?? [];
+    if (mediaTypes.length) parts.push(`Allowed: ${mediaTypeLabel(mediaTypes)}`);
+    if (constraints.maxFiles != null) parts.push(`up to ${constraints.maxFiles} files`);
+    if (constraints.maxTotalBytes != null) parts.push(`total up to ${formatBytes(constraints.maxTotalBytes)}`);
+    return parts.length ? `${parts.join('; ')}.` : null;
+  }
+
   textValues(input: EditableExecutionInput): string[] {
     if (Array.isArray(input.value)) {
       return input.value;
diff --git a/src/app/shared/task-execution-viewer/task-execution-viewer.ts b/src/app/shared/task-execution-viewer/task-execution-viewer.ts
index 5229630..028d992 100644
--- a/src/app/shared/task-execution-viewer/task-execution-viewer.ts
+++ b/src/app/shared/task-execution-viewer/task-execution-viewer.ts
@@ -32,6 +32,7 @@ import { ExecutionVaultCredential, LlmProviderCapability } from '@models/llm-pro
 import { VaultSecret } from '@models/assistant';
 import {
   EditableExecutionInput,
+  validateFileSelection,
   InputCopySource,
   TaskExecutionInputsPanelComponent
 } from '@shared/task-execution-inputs-panel/task-execution-inputs-panel';
@@ -814,6 +815,7 @@ export class TaskExecutionViewerComponent implements OnDestroy {
         subtitle: inputName,
         type: String(descriptor?.kind ?? 'TEXT').toUpperCase(),
         multiple: Boolean(descriptor?.multiple),
+        fileConstraints: descriptor?.fileConstraints ?? null,
         // The backend's own answer, and it reports what is stored - so an unsaved edit does not
         // make an input look satisfied.
         provided: !missingGlobalInputNames.has(inputName),
@@ -843,6 +845,7 @@ export class TaskExecutionViewerComponent implements OnDestroy {
           subtitle: inputName,
           type: String(input.descriptor?.type ?? 'TEXT').toUpperCase(),
           multiple: Boolean(input.descriptor?.multiple),
+          fileConstraints: input.descriptor?.fileConstraints ?? null,
           // No per-input flag from the backend here, so judge the stored value, ignoring pending.
           provided: hasStoredValue(rawValue),
           value: pendingValue ?? normalizeEditableInputValue(rawValue, Boolean(input.descriptor?.multiple))
@@ -1265,6 +1268,12 @@ export class TaskExecutionViewerComponent implements OnDestroy {
     const executionId = this.execution()?.id;
     if (!executionId || !files.length) return;
 
+    const validationError = validateFileSelection(input, files);
+    if (validationError) {
+      this.setInputError(input.key, validationError);
+      return;
+    }
+
     this.setInputSaving(input.key, true);
     const request$ = input.scope === 'global'
       ? (