diff --git a/apps/desktop/src/app/session/hooks/use-prompt-actions.test.tsx b/apps/desktop/src/app/session/hooks/use-prompt-actions.test.tsx
index 145a397d87f..96af1e8400e 100644
--- a/apps/desktop/src/app/session/hooks/use-prompt-actions.test.tsx
+++ b/apps/desktop/src/app/session/hooks/use-prompt-actions.test.tsx
@@ -7,7 +7,7 @@ import { $composerAttachments, type ComposerAttachment } from '@/store/composer'
 import { $connection, $sessions, setSessions } from '@/store/session'
 import type { SessionInfo } from '@/types/hermes'
 
-import { usePromptActions } from './use-prompt-actions'
+import { uploadComposerAttachment, usePromptActions } from './use-prompt-actions'
 
 vi.mock('@/hermes', () => ({
   getProfiles: vi.fn(async () => ({ profiles: [] })),
@@ -703,3 +703,52 @@ describe('usePromptActions eager attachment upload (drop-time)', () => {
   })
 })
 
+describe('uploadComposerAttachment remote read failures', () => {
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('turns the raw 16MB IPC cap error into a friendly remote-gateway message', async () => {
+    // electron/hardening.cjs rejects the readFileDataUrl IPC with this exact
+    // shape when a file exceeds DATA_URL_READ_MAX_BYTES.
+    Object.defineProperty(window, 'hermesDesktop', {
+      configurable: true,
+      value: {
+        readFileDataUrl: vi.fn(async () => {
+          throw new Error('File preview failed: file is too large (20971520 bytes; limit 16777216 bytes).')
+        })
+      }
+    })
+
+    const requestGateway = vi.fn(async () => ({}) as never)
+
+    await expect(
+      uploadComposerAttachment(
+        { id: 'file:big', kind: 'file', label: 'huge.csv', path: '/abs/huge.csv' },
+        { remote: true, requestGateway, sessionId: RUNTIME_SESSION_ID }
+      )
+    ).rejects.toThrow('huge.csv is too large to upload to the remote gateway (max 16 MB).')
+
+    // The cap is hit before any gateway round-trip.
+    expect(requestGateway).not.toHaveBeenCalled()
+  })
+
+  it('passes non-cap read errors through unchanged', async () => {
+    Object.defineProperty(window, 'hermesDesktop', {
+      configurable: true,
+      value: {
+        readFileDataUrl: vi.fn(async () => {
+          throw new Error('ENOENT: no such file')
+        })
+      }
+    })
+
+    await expect(
+      uploadComposerAttachment(
+        { id: 'file:gone', kind: 'file', label: 'gone.csv', path: '/abs/gone.csv' },
+        { remote: true, requestGateway: vi.fn(async () => ({}) as never), sessionId: RUNTIME_SESSION_ID }
+      )
+    ).rejects.toThrow('ENOENT: no such file')
+  })
+})
+
diff --git a/apps/desktop/src/app/session/hooks/use-prompt-actions.ts b/apps/desktop/src/app/session/hooks/use-prompt-actions.ts
index 63e15f05185..167f0d3224f 100644
--- a/apps/desktop/src/app/session/hooks/use-prompt-actions.ts
+++ b/apps/desktop/src/app/session/hooks/use-prompt-actions.ts
@@ -120,6 +120,27 @@ async function readFileDataUrlForAttach(filePath: string): Promise<string | null
   return dataUrl || null
 }
 
+// The readFileDataUrl IPC base64-loads the whole file into memory and is
+// hard-capped (DATA_URL_READ_MAX_BYTES, 16 MB) in electron/hardening.cjs, which
+// rejects with a raw "file is too large (N bytes; limit M bytes)" string. In
+// remote mode every attachment's bytes go through that read, so a big file
+// surfaces that internal message verbatim in the failure toast. Translate it
+// into a friendly "too large to upload to the remote gateway" line, parsing the
+// limit out of the message so it tracks the real cap. Non-cap errors pass
+// through unchanged.
+function friendlyRemoteAttachError(err: unknown, label: string): Error {
+  const message = err instanceof Error ? err.message : String(err)
+
+  if (!/too large/i.test(message)) {
+    return err instanceof Error ? err : new Error(message)
+  }
+
+  const limitBytes = Number(message.match(/limit (\d+) bytes/)?.[1])
+  const cap = Number.isFinite(limitBytes) && limitBytes > 0 ? ` (max ${Math.floor(limitBytes / (1024 * 1024))} MB)` : ''
+
+  return new Error(`${label} is too large to upload to the remote gateway${cap}.`)
+}
+
 type GatewayRequest = <T>(method: string, params?: Record<string, unknown>) => Promise<T>
 
 /**
@@ -142,7 +163,13 @@ export async function uploadComposerAttachment(
     let result: ImageAttachResponse
 
     if (remote) {
-      const payload = await readImageForRemoteAttach(path)
+      let payload: Awaited<ReturnType<typeof readImageForRemoteAttach>>
+
+      try {
+        payload = await readImageForRemoteAttach(path)
+      } catch (err) {
+        throw friendlyRemoteAttachError(err, label)
+      }
 
       if (!payload) {
         throw new Error(`Could not read ${label}`)
@@ -176,10 +203,18 @@ export async function uploadComposerAttachment(
   }
 
   // Non-image file.
-  const dataUrl = remote ? await readFileDataUrlForAttach(path) : null
+  let dataUrl: string | null = null
 
-  if (remote && !dataUrl) {
-    throw new Error(`Could not read ${label}`)
+  if (remote) {
+    try {
+      dataUrl = await readFileDataUrlForAttach(path)
+    } catch (err) {
+      throw friendlyRemoteAttachError(err, label)
+    }
+
+    if (!dataUrl) {
+      throw new Error(`Could not read ${label}`)
+    }
   }
 
   const result = await requestGateway<FileAttachResponse>('file.attach', {