diff --git a/hermes_cli/web_server.py b/hermes_cli/web_server.py
index 572ee5a38eee7..8a0cc48e179a7 100644
--- a/hermes_cli/web_server.py
+++ b/hermes_cli/web_server.py
@@ -318,6 +318,13 @@ def _apply_ssh_owner_nonce(nonce: Optional[str]) -> None:
 # injection share a single, testable seam.
 _DASHBOARD_EMBEDDED_CHAT_ENABLED = True
 
+# Desktop's file.attach compatibility transport sends a complete base64 data
+# URL in one JSON-RPC frame. Uvicorn defaults to 16 MiB, which rejects files at
+# the preview ceiling before the dispatcher sees them. Keep the gateway
+# finite while allowing the 256 MiB raw Desktop attach cap plus base64/JSON
+# overhead.
+_DESKTOP_ATTACHMENT_WS_MAX_BYTES = 384 * 1024 * 1024
+
 # Simple rate limiter for the reveal endpoint
 _reveal_timestamps: List[float] = []
 _REVEAL_MAX_PER_WINDOW = 5
@@ -20290,6 +20297,7 @@ def start_server(
         # reaped via the WebSocketDisconnect → disconnect/reap path.
         ws_ping_interval=None if _is_loopback else 20.0,
         ws_ping_timeout=None if _is_loopback else 20.0,
+        ws_max_size=_DESKTOP_ATTACHMENT_WS_MAX_BYTES,
     )
     server = uvicorn.Server(config)
 
diff --git a/tests/test_web_server.py b/tests/test_web_server.py
index eee5973879e98..e14d3151d916d 100644
--- a/tests/test_web_server.py
+++ b/tests/test_web_server.py
@@ -107,6 +107,20 @@ def test_start_server_disables_ws_ping_on_loopback(monkeypatch):
     assert captured["ws_ping_timeout"] is None
 
 
+def test_start_server_accepts_base64_desktop_attachments_above_preview_limit(monkeypatch):
+    """The gateway frame cap must fit the Desktop attachment default after
+    base64 expansion and JSON framing; uvicorn's 16 MiB default would reject
+    the request before ``file.attach`` can stage it.
+    """
+    captured = _stub_uvicorn(monkeypatch)
+
+    web_server.start_server(host="127.0.0.1", port=0, open_browser=False)
+
+    raw_attachment_bytes = 256 * 1024 * 1024
+    base64_bytes = ((raw_attachment_bytes + 2) // 3) * 4
+    assert captured["ws_max_size"] > base64_bytes
+
+
 def test_start_server_enables_ws_ping_for_half_open_detection(monkeypatch):
     """Non-loopback (public) binds MUST keep the ws ping enabled so half-open
     connections (reverse-proxy 524, dropped Cloudflare Tunnel) raise
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index b88d77b4fbe6c..a6faccf7d87c0 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -4528,7 +4528,8 @@ def _current_profile_name() -> str:
 # v2: adds the file.attach RPC (remote-gateway non-image file upload).
 # v3: adds approvals.mode config RPCs and session.info reconciliation.
 # v4: session.create fast=false is an explicit per-session normal-tier override.
-DESKTOP_BACKEND_CONTRACT = 4
+# v5: uvicorn ws_max_size raised for one-shot base64 file.attach frames (>16 MiB).
+DESKTOP_BACKEND_CONTRACT = 5
 
 
 def _session_usage_snapshot(session: dict | None) -> dict: