Hermes runs stdio MCP servers on its own packaged Node only. A server whose
native addon was compiled by the user's Node (typically through the ~/.npm/_npx
cache the user's npx shares with ours) dies at startup with NODE_MODULE_VERSION /
ERR_DLOPEN_FAILED; the SDK only saw "Connection closed", Hermes retried three
times and parked the server, and the cause lived only in logs/mcp-stderr.log.
The stdio child's stderr now goes through a pipe that still copies every byte
into the shared log but keeps the last 16 KB readable. When the session fails
and that tail shows an addon load failure, the error becomes a
NodeAbiMismatchError naming the addon, both ABI versions and the remedy with
Hermes's own paths: delete the npx cache entry (Hermes's npx reinstalls it) or
`PATH=<managed node dir>:"$PATH" <managed npm> rebuild <pkg> --prefix <root>`.
It is classified permanent, so the server parks at once and self-probes back
after the rebuild. The message reaches every MCP status surface through
_format_connect_error / str(exc): startup banner, `hermes mcp test`, the TUI
and Desktop probes and the dashboard.
The hosts E2E cell that asserted the user's Node wins now asserts the ruling:
the managed Node runs the server and the ABI failure surfaces the remedy.
Refs #124264
