Instead of including the xd:// device inventory in the system-prompt signature, mount/unmount events now inject a steered `xdev-mount-notice` message so the system prompt (and its provider cache prefix) stays byte-stable across MCP connects and disconnects. Full device docs are picked up opportunistically on the next unrelated rebuild.
Also caps external (dynamic-mount) device descriptions to 200 chars in `docsAll` to prevent server-controlled prose from consuming prompt budget; built-ins keep their full curated docs, and `read xd://` always returns the untruncated text.
Legacy `discoveryMode: "off"` → `tools.xdev: false` migration is removed; the setting keeps its own default without inference from the deprecated key.
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