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 e7dfe9d7da5b4..90fd85ec33670 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
@@ -859,4 +859,60 @@ describe('uploadComposerAttachment remote read failures', () => {
       )
     ).rejects.toThrow('ENOENT: no such file')
   })
+
+  it('hosted web client attaches a non-image file by path only (no client read, no data_url)', async () => {
+    // The web SPA has no local disk: the .md was already uploaded to
+    // ~/.hermes/uploads by the web shim, so we attach by PATH and let the gateway
+    // read it. The shim's readFileDataUrl only caches image previews, so for a
+    // .md it returns '' — the old code then threw "Could not read <name>" even
+    // though the upload had succeeded.
+    const win = window as unknown as { __HERMES_WEB_CLIENT__?: boolean }
+    win.__HERMES_WEB_CLIENT__ = true
+    const readFileDataUrl = vi.fn(async () => '') // web-shim cache miss for non-images
+
+    try {
+      Object.defineProperty(window, 'hermesDesktop', {
+        configurable: true,
+        value: { readFileDataUrl }
+      })
+
+      const calls: { method: string; params?: Record<string, unknown> }[] = []
+      const requestGateway = vi.fn(async (method: string, params?: Record<string, unknown>) => {
+        calls.push({ method, params })
+        if (method === 'file.attach') {
+          return {
+            attached: true,
+            ref_text: '@file:.hermes/desktop-attachments/notes.md',
+            uploaded: true
+          } as never
+        }
+        return {} as never
+      })
+
+      const result = await uploadComposerAttachment(
+        {
+          id: 'file:notes',
+          kind: 'file',
+          label: 'notes.md',
+          path: '/home/hermes/.hermes/uploads/20260613/071447-79fff2b5-notes.md'
+        },
+        { remote: true, requestGateway, sessionId: RUNTIME_SESSION_ID }
+      )
+
+      // Attaches by path; never reads bytes client-side; sends NO data_url.
+      expect(readFileDataUrl).not.toHaveBeenCalled()
+      expect(calls).toHaveLength(1)
+      expect(calls[0]?.method).toBe('file.attach')
+      expect(calls[0]?.params).toMatchObject({
+        session_id: RUNTIME_SESSION_ID,
+        path: '/home/hermes/.hermes/uploads/20260613/071447-79fff2b5-notes.md',
+        name: 'notes.md'
+      })
+      expect(calls[0]?.params).not.toHaveProperty('data_url')
+      expect(result.refText).toBe('@file:.hermes/desktop-attachments/notes.md')
+      expect(result.attachedSessionId).toBe(RUNTIME_SESSION_ID)
+    } finally {
+      delete win.__HERMES_WEB_CLIENT__
+    }
+  })
 })
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 b09d86ffd10b3..6779ad2a46752 100644
--- a/apps/desktop/src/app/session/hooks/use-prompt-actions.ts
+++ b/apps/desktop/src/app/session/hooks/use-prompt-actions.ts
@@ -150,6 +150,17 @@ async function readFileDataUrlForAttach(filePath: string): Promise<string | null
   return dataUrl || null
 }
 
+// The hosted web SPA (lib/web-shim.ts) sets `window.__HERMES_WEB_CLIENT__`. That
+// build runs in a plain browser with NO local filesystem: every file path it
+// holds is already gateway-resident — non-image drops are uploaded to
+// ~/.hermes/uploads via the web shim, and in-app refs are workspace paths. So it
+// cannot (and need not) read file bytes client-side for a path attach; the
+// gateway's file.attach resolves the path directly. Reading would hit the web
+// shim's image-only data-URL cache and wrongly fail for .md/text/pdf.
+function isHostedWebClient(): boolean {
+  return Boolean((window as unknown as { __HERMES_WEB_CLIENT__?: boolean }).__HERMES_WEB_CLIENT__)
+}
+
 // 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
@@ -235,7 +246,15 @@ export async function uploadComposerAttachment(
   // Non-image file.
   let dataUrl: string | null = null
 
-  if (remote) {
+  // Native (Electron) remote mode reads the file's bytes here because the path
+  // lives on the CLIENT's disk, invisible to the gateway — they ride along as a
+  // data_url. The hosted web client is the opposite: it has no local disk and
+  // the file is ALREADY on the gateway (uploaded to ~/.hermes/uploads, or an
+  // in-app workspace ref), so attach by path only and let the gateway read it.
+  // Reading here would hit the web shim's image-only cache and throw
+  // "Could not read <name>" for every .md/text/pdf even though the upload that
+  // produced this attachment already succeeded.
+  if (remote && !isHostedWebClient()) {
     try {
       dataUrl = await readFileDataUrlForAttach(path)
     } catch (err) {