diff --git a/src/main/runtime/rpc/methods/files.test.ts b/src/main/runtime/rpc/methods/files.test.ts
index d3c4edaea5a..70187103315 100644
--- a/src/main/runtime/rpc/methods/files.test.ts
+++ b/src/main/runtime/rpc/methods/files.test.ts
@@ -391,6 +391,10 @@ describe('file RPC methods', () => {
     [
       'non-string content',
       { worktree: 'id:wt-1', relativePath: 'assets/logo.png', contentBase64: 0 }
+    ],
+    [
+      'malformed content',
+      { worktree: 'id:wt-1', relativePath: 'assets/logo.png', contentBase64: '!!!!' }
     ]
   ])('rejects a base64 write with %s', async (_name, params) => {
     const runtime = {
@@ -456,6 +460,15 @@ describe('file RPC methods', () => {
         contentBase64: 0,
         append: true
       }
+    ],
+    [
+      'malformed content',
+      {
+        worktree: 'id:wt-1',
+        relativePath: 'assets/video.mov',
+        contentBase64: '!!!!',
+        append: true
+      }
     ]
   ])('rejects a base64 chunk write with %s (inherits the schema)', async (_name, params) => {
     const runtime = {
diff --git a/src/main/runtime/rpc/methods/files.ts b/src/main/runtime/rpc/methods/files.ts
index fbcf31936de..b4ba19db69f 100644
--- a/src/main/runtime/rpc/methods/files.ts
+++ b/src/main/runtime/rpc/methods/files.ts
@@ -4,6 +4,13 @@ import { defineMethod, defineStreamingMethod, type RpcAnyMethod } from '../core'
 import { createFileWatchEventBatcher } from './file-watch-event-batcher'
 
 let filesWatchSubscriptionSeq = 0
+const RUNTIME_FILE_BASE64_PATTERN = /^[A-Za-z0-9+/]*={0,2}$/
+
+function isValidRuntimeFileBase64(value: unknown): value is string {
+  return (
+    typeof value === 'string' && value.length % 4 !== 1 && RUNTIME_FILE_BASE64_PATTERN.test(value)
+  )
+}
 
 const WorktreeSelector = z.object({
   worktree: z
@@ -43,6 +50,9 @@ const FileWriteBase64 = FileOpen.extend({
   contentBase64: z
     .unknown()
     .refine((v): v is string => typeof v === 'string', { message: 'Missing file content' })
+    // Why: Buffer.from(..., 'base64') accepts malformed input by dropping
+    // invalid bytes, which can silently create empty or corrupt uploaded files.
+    .refine(isValidRuntimeFileBase64, 'File content must be base64')
 })
 
 const FileWriteBase64Chunk = FileWriteBase64.extend({