diff --git a/agent/context_references.py b/agent/context_references.py
index 6f428bf87badd..a44b90d90704e 100644
--- a/agent/context_references.py
+++ b/agent/context_references.py
@@ -555,6 +555,30 @@ def _rg_files(path: Path, cwd: Path, limit: int) -> list[Path] | None:
     return files[:limit]
 
 
+def _agent_visible_path(path: Path) -> str:
+    """Map a host path to the path the agent's tools can read in the active backend.
+
+    Under a container backend (docker) the gateway host path dangles inside the
+    sandbox — the container has its own filesystem and the host path is not
+    mounted. Files staged into an auto-mounted cache dir (``images/``,
+    ``attachments/``, ...) are translated to their in-container path via the
+    existing ``tools.credential_files`` machinery (#76577). Falls back to the
+    host path when the backend is local or translation is unavailable.
+    """
+    try:
+        # Desktop/in-process gateways may not have bridged ``terminal.*``
+        # config into ``TERMINAL_ENV`` at startup; run the idempotent bridge so
+        # the credential_files translation gate sees the active backend.
+        from tools.terminal_tool import _ensure_terminal_env_bridged
+
+        _ensure_terminal_env_bridged()
+        from tools.credential_files import to_agent_visible_cache_path
+
+        return to_agent_visible_cache_path(str(path))
+    except Exception:
+        return str(path)
+
+
 def _binary_reference_block(ref: ContextReference, path: Path) -> str:
     mime, _ = mimetypes.guess_type(path.name)
     mime = mime or "application/octet-stream"
@@ -564,7 +588,7 @@ def _binary_reference_block(ref: ContextReference, path: Path) -> str:
         size = "unknown size"
     return (
         f"📎 {ref.raw} ({mime}, {size}) — binary file, not inlined as text. "
-        f"It is available on disk at `{path}`. Use your tools to work with it "
+        f"It is available on disk at `{_agent_visible_path(path)}`. Use your tools to work with it "
         f"(read or convert it, extract its text, or view/render it as needed); "
         f"do not tell the user the file type is unsupported."
     )
diff --git a/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts b/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts
index 7ae6943cbc974..b0d6a0f57de24 100644
--- a/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts
+++ b/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts
@@ -59,6 +59,7 @@ import {
   setCurrentUsage,
   setMessages,
   setSessions,
+  setTerminalBackend,
   setTurnStartedAt,
   setWorkspaceCwdOwner,
   setYoloActive
@@ -464,6 +465,10 @@ export function useGatewayEventHandler(deps: GatewayEventDeps) {
             setCurrentBranch(payload.branch)
           }
 
+          if (typeof payload?.terminal_backend === 'string') {
+            setTerminalBackend(payload.terminal_backend)
+          }
+
           if (typeof payload?.personality === 'string') {
             setCurrentPersonality(normalizePersonalityValue(payload.personality))
           }
diff --git a/apps/desktop/src/app/session/hooks/use-prompt-actions/index.test.tsx b/apps/desktop/src/app/session/hooks/use-prompt-actions/index.test.tsx
index fc810946e3bb4..c8e2ef1d5e060 100644
--- a/apps/desktop/src/app/session/hooks/use-prompt-actions/index.test.tsx
+++ b/apps/desktop/src/app/session/hooks/use-prompt-actions/index.test.tsx
@@ -16,6 +16,7 @@ import {
   $currentUsage,
   $messages,
   $sessions,
+  $terminalBackend,
   $turnStartedAt,
   setCurrentUsage,
   setMessages,
@@ -2535,6 +2536,56 @@ describe('usePromptActions file attachment sync', () => {
     expect(uploaded.path).toBe('/root/tmp/photo.jpg')
   })
 
+  it('uploads file bytes when the terminal backend is a container (docker)', async () => {
+    // Container backends have their own filesystem: the host drop path would
+    // dangle inside the sandbox, so the bytes must cross via file.attach's
+    // data_url pipeline and be staged into a bind-mounted cache dir (#76577).
+    $connection.set({ mode: 'local' } as never)
+    $terminalBackend.set('docker')
+    const readFileDataUrl = vi.fn(async () => 'data:text/plain;base64,aGVsbG8=')
+    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,
+          path: '/root/.hermes/attachments/report.txt',
+          ref_text: '@file:/root/.hermes/attachments/report.txt',
+          uploaded: true
+        } as never
+      }
+
+      return {} as never
+    })
+
+    let handle: HarnessHandle | null = null
+    await actRender(
+      <Harness onReady={h => (handle = h)} refreshSessions={async () => undefined} requestGateway={requestGateway} />
+    )
+
+    expect(await handle!.submitText('summarize', { attachments: [fileAttachment()] })).toBe(true)
+    expect(readFileDataUrl).toHaveBeenCalledWith('/Users/alice/Downloads/report.txt')
+    expect(calls[0]).toEqual({
+      method: 'file.attach',
+      params: {
+        data_url: 'data:text/plain;base64,aGVsbG8=',
+        name: 'report.txt',
+        path: '/Users/alice/Downloads/report.txt',
+        session_id: RUNTIME_SESSION_ID
+      }
+    })
+
+    // Don't leak the container-backend state into later tests.
+    $terminalBackend.set('')
+  })
+
   it('passes a path-less @file: ref straight through (no path = nothing to upload)', async () => {
     // Submit-layer contract: only attachments that carry a `path` are upload
     // candidates. A path-less ref (an @-mention/context ref or pasted text)
diff --git a/apps/desktop/src/app/session/hooks/use-prompt-actions/index.ts b/apps/desktop/src/app/session/hooks/use-prompt-actions/index.ts
index 478c868b0bacb..9811c678fa0a0 100644
--- a/apps/desktop/src/app/session/hooks/use-prompt-actions/index.ts
+++ b/apps/desktop/src/app/session/hooks/use-prompt-actions/index.ts
@@ -27,6 +27,7 @@ import {
   $connection,
   $currentCwd,
   $messages,
+  $terminalBackend,
   setActiveSessionId,
   setAwaitingResponse,
   setBusy,
@@ -80,11 +81,29 @@ interface HandoffResult {
 const WINDOWS_ABSOLUTE_PATH_RE = /^(?:[A-Za-z]:[\\/]|\\\\)/
 const POSIX_ABSOLUTE_PATH_RE = /^\/(?!\/)/
 
+// Terminal backends whose execution environment has its own filesystem
+// (docker/ssh/singularity/modal/...) cannot see the desktop's host paths —
+// they must be crossed as bytes, like remote attachments. Mirrors the
+// container_backend set in tools/terminal_tool.py::_get_env_config.
+const CONTAINER_TERMINAL_BACKENDS = new Set([
+  'docker',
+  'ssh',
+  'singularity',
+  'modal',
+  'daytona',
+  'vercel_sandbox'
+])
+
 // `mode: local` means the gateway was launched locally, not necessarily that
 // Electron and the gateway share a filesystem. Windows Desktop can front a
 // WSL/Docker backend whose cwd is POSIX, so a Windows host path must cross the
-// boundary as bytes just like a remote attachment.
-function attachmentPathNeedsUpload(path: string, backendCwd?: null | string): boolean {
+// boundary as bytes just like a remote attachment. Container terminal backends
+// (docker, ssh, ...) always need bytes: the sandbox has its own filesystem and
+// the host path would dangle inside it (#76577).
+function attachmentPathNeedsUpload(path: string, backendCwd?: null | string, terminalBackend?: string): boolean {
+  if (CONTAINER_TERMINAL_BACKENDS.has((terminalBackend || '').trim().toLowerCase())) {
+    return true
+  }
   return WINDOWS_ABSOLUTE_PATH_RE.test(path.trim()) && POSIX_ABSOLUTE_PATH_RE.test(backendCwd?.trim() || '')
 }
 
@@ -107,12 +126,13 @@ export async function uploadComposerAttachment(
     storedSessionId?: null | string
     /** Called when the attach recovered onto a fresh live id. */
     onSessionRecovered?: (sessionId: string) => void
+    terminalBackend?: string
   }
 ): Promise<ComposerAttachment> {
-  const { backendCwd, remote, requestGateway, storedSessionId, onSessionRecovered } = opts
+  const { backendCwd, remote, requestGateway, storedSessionId, onSessionRecovered, terminalBackend } = opts
   const path = attachment.path ?? ''
   const label = attachment.label || pathLabel(path)
-  const uploadBytes = remote || attachmentPathNeedsUpload(path, backendCwd)
+  const uploadBytes = remote || attachmentPathNeedsUpload(path, backendCwd, terminalBackend)
 
   // Read bytes/paths ONCE, outside the retry. Only the session-scoped RPC is
   // replayed on recovery — re-reading a multi-MB file to retry a dead session
@@ -364,7 +384,8 @@ export function usePromptActions({
             requestGateway,
             sessionId: liveSessionId,
             storedSessionId,
-            onSessionRecovered
+            onSessionRecovered,
+            terminalBackend: $terminalBackend.get()
           })
 
           // Update-only: never resurrect a chip the user removed mid-upload.
@@ -409,7 +430,8 @@ export function usePromptActions({
             backendCwd: $currentCwd.get(),
             remote,
             requestGateway,
-            sessionId
+            sessionId,
+            terminalBackend: $terminalBackend.get()
           })
         )
       } catch (err) {
diff --git a/apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx b/apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx
index dbb90459ccee8..c7af4d56e050a 100644
--- a/apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx
+++ b/apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx
@@ -74,7 +74,7 @@ import { Loader2Icon } from '@/lib/icons'
 import { cn } from '@/lib/utils'
 import type { ComposerAttachment } from '@/store/composer'
 import { notifyError } from '@/store/notifications'
-import { $connection } from '@/store/session'
+import { $connection, $terminalBackend } from '@/store/session'
 import { notifyThreadEditClose } from '@/store/thread-scroll'
 
 interface UserEditComposerProps {
@@ -427,7 +427,7 @@ export const UserEditComposer: FC<UserEditComposerProps> = ({ cwd, gateway, sess
         try {
           const uploaded = await uploadComposerAttachment(
             { detail: path, id: attachmentId(kind, path), kind, label: pathLabel(path), path },
-            { backendCwd: cwd, remote, requestGateway, sessionId }
+            { backendCwd: cwd, remote, requestGateway, sessionId, terminalBackend: $terminalBackend.get() }
           )
 
           const ref = attachmentDisplayText(uploaded)
diff --git a/apps/desktop/src/lib/chat-messages.ts b/apps/desktop/src/lib/chat-messages.ts
index 1e0bd1b0a4548..f58cd9d16f27e 100644
--- a/apps/desktop/src/lib/chat-messages.ts
+++ b/apps/desktop/src/lib/chat-messages.ts
@@ -61,6 +61,7 @@ export type GatewayEventPayload = {
   running?: boolean
   cwd?: string
   branch?: string
+  terminal_backend?: string
   credential_warning?: string
   install_warning?: string
   personality?: string
diff --git a/apps/desktop/src/store/session.ts b/apps/desktop/src/store/session.ts
index 24a0b8beff619..953745f8112d6 100644
--- a/apps/desktop/src/store/session.ts
+++ b/apps/desktop/src/store/session.ts
@@ -527,6 +527,12 @@ export const $currentCwd = atom(getRememberedWorkspaceCwd())
 // would collapse the workspace panes and drop file-tree state on every switch,
 // so the path stays put and is simply marked as not-yet-owned.
 export const $workspaceCwdOwner = atom<null | string>(null)
+
+// Terminal execution backend (local | docker | ssh | ...) mirrored from the
+// gateway (session.info). Drives attachment upload decisions: container
+// backends have their own filesystem, so a dropped host path must be uploaded
+// as bytes and staged into a bind-mounted cache dir (#76577).
+export const $terminalBackend = atom('')
 export const $newChatWorkspaceTarget = atom<NewChatWorkspaceTarget>(undefined)
 export const $newChatWorkspaceTargetGeneration = atom(0)
 export const $currentBranch = atom('')
@@ -648,6 +654,8 @@ export const setCurrentCwd = (next: Updater<string>) => {
   persistString(workspaceCwdKey(), $currentCwd.get().trim() || null)
 }
 
+export const setTerminalBackend = (next: Updater<string>) => updateAtom($terminalBackend, next)
+
 export const setCurrentCwdTransient = (next: Updater<string>) => updateAtom($currentCwd, next)
 
 // Released-ownership marker: the live path belongs to no conversation. `null`
diff --git a/tests/agent/test_context_references.py b/tests/agent/test_context_references.py
index aac7c251849b2..b58623584b8ff 100644
--- a/tests/agent/test_context_references.py
+++ b/tests/agent/test_context_references.py
@@ -121,6 +121,60 @@ def test_missing_file_becomes_warning(sample_repo: Path):
     assert "not found" in result.message.lower()
 
 
+def test_binary_reference_block_maps_host_attachment_to_container_path(tmp_path: Path, monkeypatch):
+    """Docker backend: a staged binary attachment's host path is rendered as the
+    bind-mounted in-container path so the agent's tools can read it.
+
+    Regression test for #76577 — the container has its own filesystem, so the
+    gateway host path would dangle inside the sandbox.
+    """
+    from agent.context_references import preprocess_context_references
+
+    hermes_home = tmp_path / ".hermes"
+    attachments = hermes_home / "attachments"
+    attachments.mkdir(parents=True)
+    payload = attachments / "archive.zip"
+    payload.write_bytes(b"PK\x03\x04binary-zip-bytes")
+
+    monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+    monkeypatch.setenv("TERMINAL_ENV", "docker")
+
+    result = preprocess_context_references(
+        f"Read the attachment @file:{payload}",
+        cwd=tmp_path,
+        context_length=100_000,
+    )
+
+    assert result.expanded
+    # Default container base for the docker backend is /root/.hermes.
+    assert "/root/.hermes/attachments/archive.zip" in result.message
+    assert "binary file, not inlined" in result.message
+
+
+def test_binary_reference_block_keeps_host_path_on_local_backend(tmp_path: Path, monkeypatch):
+    """Local backend: no translation — the agent's tools run on the host."""
+    from agent.context_references import preprocess_context_references
+
+    hermes_home = tmp_path / ".hermes"
+    attachments = hermes_home / "attachments"
+    attachments.mkdir(parents=True)
+    payload = attachments / "archive.zip"
+    payload.write_bytes(b"PK\x03\x04binary-zip-bytes")
+
+    monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+    monkeypatch.setenv("TERMINAL_ENV", "local")
+
+    result = preprocess_context_references(
+        f"Read the attachment @file:{payload}",
+        cwd=tmp_path,
+        context_length=100_000,
+    )
+
+    assert result.expanded
+    assert str(payload) in result.message
+    assert "/root/.hermes/attachments/" not in result.message
+
+
 
 
 
diff --git a/tests/test_tui_gateway_server.py b/tests/test_tui_gateway_server.py
index 27aa067236a88..c3d8e5d313510 100644
--- a/tests/test_tui_gateway_server.py
+++ b/tests/test_tui_gateway_server.py
@@ -8048,15 +8048,20 @@ def test_image_attach_accepts_unquoted_screenshot_path_with_spaces(monkeypatch):
 
 
 def test_file_attach_uploads_remote_file_into_session_workspace(monkeypatch, tmp_path):
-    """Remote case: client path doesn't exist on gateway → decode data_url bytes."""
+    """Remote case: client path doesn't exist on gateway → decode data_url bytes.
+
+    Staged into the session home's ``attachments/`` dir (bind-mounted into
+    container backends) rather than the workspace (#76577).
+    """
     workspace = tmp_path / "workspace"
     workspace.mkdir()
+    home = tmp_path / "home"
     fake_cli = types.ModuleType("cli")
     fake_cli._detect_file_drop = lambda raw: None
     fake_cli._split_path_input = lambda raw: (raw, "")
     fake_cli._resolve_attachment_path = lambda raw: None
 
-    server._sessions["sid"] = _session(cwd=str(workspace))
+    server._sessions["sid"] = _session(cwd=str(workspace), profile_home=str(home))
     monkeypatch.setitem(sys.modules, "cli", fake_cli)
 
     try:
@@ -8073,11 +8078,11 @@ def test_file_attach_uploads_remote_file_into_session_workspace(monkeypatch, tmp
             }
         )
 
-        stored = workspace / ".hermes" / "desktop-attachments" / "report.txt"
+        stored = home / "attachments" / "report.txt"
         assert resp["result"]["attached"] is True
         assert resp["result"]["uploaded"] is True
         assert resp["result"]["path"] == str(stored)
-        assert resp["result"]["ref_text"] == "@file:.hermes/desktop-attachments/report.txt"
+        assert resp["result"]["ref_text"] == f"@file:{stored}"
         assert stored.read_text(encoding="utf-8") == "hello world"
     finally:
         server._sessions.pop("sid", None)
@@ -8087,6 +8092,7 @@ def test_file_attach_copies_gateway_visible_file_outside_workspace(monkeypatch,
     """Local case: gateway can see the file but it's outside the workspace → copy in."""
     workspace = tmp_path / "workspace"
     workspace.mkdir()
+    home = tmp_path / "home"
     source = tmp_path / "outside.txt"
     source.write_text("outside workspace", encoding="utf-8")
     fake_cli = types.ModuleType("cli")
@@ -8094,7 +8100,7 @@ def test_file_attach_copies_gateway_visible_file_outside_workspace(monkeypatch,
     fake_cli._split_path_input = lambda raw: (raw, "")
     fake_cli._resolve_attachment_path = lambda raw: source
 
-    server._sessions["sid"] = _session(cwd=str(workspace))
+    server._sessions["sid"] = _session(cwd=str(workspace), profile_home=str(home))
     monkeypatch.setitem(sys.modules, "cli", fake_cli)
 
     try:
@@ -8106,10 +8112,10 @@ def test_file_attach_copies_gateway_visible_file_outside_workspace(monkeypatch,
             }
         )
 
-        stored = workspace / ".hermes" / "desktop-attachments" / "outside.txt"
+        stored = home / "attachments" / "outside.txt"
         assert resp["result"]["attached"] is True
         assert resp["result"]["uploaded"] is True
-        assert resp["result"]["ref_text"] == "@file:.hermes/desktop-attachments/outside.txt"
+        assert resp["result"]["ref_text"] == f"@file:{stored}"
         assert stored.read_text(encoding="utf-8") == "outside workspace"
     finally:
         server._sessions.pop("sid", None)
@@ -8141,8 +8147,10 @@ def test_file_attach_uses_in_workspace_file_without_copying(monkeypatch, tmp_pat
         assert resp["result"]["attached"] is True
         assert resp["result"]["uploaded"] is False
         assert resp["result"]["ref_text"] == "@file:data/exam.csv"
-        # No copy: nothing staged under desktop-attachments.
+        # No copy: nothing staged under desktop-attachments or the home
+        # attachments dir.
         assert not (workspace / ".hermes" / "desktop-attachments").exists()
+        assert not (tmp_path / "home" / "attachments").exists()
     finally:
         server._sessions.pop("sid", None)
 
@@ -8183,7 +8191,7 @@ def test_file_attach_quotes_ref_with_spaces(monkeypatch, tmp_path):
     fake_cli._split_path_input = lambda raw: (raw, "")
     fake_cli._resolve_attachment_path = lambda raw: None
 
-    server._sessions["sid"] = _session(cwd=str(workspace))
+    server._sessions["sid"] = _session(cwd=str(workspace), profile_home=str(tmp_path / "home"))
     monkeypatch.setitem(sys.modules, "cli", fake_cli)
 
     try:
@@ -8199,8 +8207,10 @@ def test_file_attach_quotes_ref_with_spaces(monkeypatch, tmp_path):
             }
         )
 
+        stored = tmp_path / "home" / "attachments" / "my exam schedule.csv"
         assert resp["result"]["attached"] is True
-        assert resp["result"]["ref_text"] == "@file:`.hermes/desktop-attachments/my exam schedule.csv`"
+        assert resp["result"]["ref_text"] == f"@file:`{stored}`"
+        assert stored.read_text(encoding="utf-8") == "a,b\n"
     finally:
         server._sessions.pop("sid", None)
 
diff --git a/tools/credential_files.py b/tools/credential_files.py
index e57af0bbb4679..a0d5edb889332 100644
--- a/tools/credential_files.py
+++ b/tools/credential_files.py
@@ -400,6 +400,11 @@ _CACHE_DIRS: list[tuple[str, str]] = [
     # reach uploads inside sandbox containers (#69575). No legacy alias exists,
     # so both tuple slots are ``images``.
     ("images", "images"),
+    # Desktop non-image file attachments (tui_gateway ``file.attach`` staging)
+    # land in the flat top-level ``attachments/`` dir. Mount it so the agent's
+    # file tools can read dropped binaries (zip/pdf/...) from inside sandbox
+    # containers instead of dangling host paths (#76577).
+    ("attachments", "attachments"),
 ]
 
 
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index 0b776f03547db..1a6f84d04aac0 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -2527,6 +2527,27 @@ def _is_local_terminal_backend() -> bool:
     return not backend or backend == "local"
 
 
+def _effective_terminal_backend() -> str:
+    """Active terminal backend name (``local``, ``docker``, ``ssh``, ...).
+
+    ``TERMINAL_ENV`` is authoritative when set (launchers bridge
+    ``terminal.backend`` into env at startup). Desktop/TUI in-process gateways
+    skip that bridge, so fall back to the ``terminal.backend`` config key —
+    the same rule ``_terminal_task_cwd`` uses.
+    """
+    backend = (os.environ.get("TERMINAL_ENV") or "").strip().lower()
+    if not backend or backend == "local":
+        try:
+            terminal_cfg = _load_cfg().get("terminal", {})
+            if isinstance(terminal_cfg, dict):
+                cfg_backend = str(terminal_cfg.get("backend") or "").strip().lower()
+                if cfg_backend and cfg_backend != "local":
+                    backend = cfg_backend
+        except Exception:
+            pass
+    return backend or "local"
+
+
 def _display_session_cwd(session: dict | None) -> str:
     """Session cwd for display/probe surfaces, healed past deleted worktrees.
 
@@ -5171,6 +5192,7 @@ def _session_info(agent, session: dict | None = None) -> dict:
         "cwd": cwd,
         "branch": _git_branch_for_cwd(cwd),
         "project": _project_info_for_cwd(cwd),
+        "terminal_backend": _effective_terminal_backend(),
         "personality": str(personality or ""),
         "running": bool((session or {}).get("running")),
         "title": _session_live_title(session or {}, session_key) if session_key else "",
@@ -10492,7 +10514,19 @@ def _attachment_ref_path(session: dict, target: Path) -> str:
 
 
 def _desktop_attachment_dir(session: dict) -> Path:
-    root = Path(_session_cwd(session)).resolve() / ".hermes" / "desktop-attachments"
+    """Resolve the file-attachment staging dir against the session's effective home.
+
+    Anchored on the session profile's ``attachments/`` dir (same rule as
+    ``_session_images_dir``): ``file.attach`` runs BEFORE ``prompt.submit``
+    installs the session's profile HERMES_HOME override, while the docker/ssh
+    sandbox mounts are resolved against the *session profile's* home at run
+    time — so the staged file must land where the bind mount points, or the
+    container can never see it (#76577). ``attachments/`` is registered in
+    ``tools.credential_files._CACHE_DIRS`` and auto-mounted into containers.
+    """
+    profile_home = session.get("profile_home")
+    base = Path(profile_home) if profile_home else _hermes_home
+    root = base / "attachments"
     root.mkdir(parents=True, exist_ok=True)
     return root
 
@@ -10572,10 +10606,11 @@ def _stage_session_file_attachment(
       1. The path resolves to a file already INSIDE the session workspace — use
          it as-is (no copy, ``uploaded=False``).
       2. The path resolves to a gateway-visible file OUTSIDE the workspace — copy
-         it into ``.hermes/desktop-attachments/`` so the ``@file:`` ref resolves.
+         it into the session home's ``attachments/`` dir (bind-mounted into
+         container backends) so the ``@file:`` ref resolves inside the sandbox.
       3. The path doesn't exist on the gateway (the common remote case: it's a
          path on the CLIENT's disk) — decode the uploaded ``data_url`` bytes and
-         write them into ``.hermes/desktop-attachments/``.
+         write them into the session home's ``attachments/`` dir.
 
     Returns ``(stored_path, uploaded)``.
     """