When the Hermes Docker image runs an stdio MCP server configured with an
explicit env.PATH that omits /usr/local/bin (a common pattern when users
hand-author PATH for sandboxing), the MCP env-filter passes that narrow
PATH straight through to the subprocess. _resolve_stdio_command's
fallback for bare 'npx' / 'npm' / 'node' commands only checked
$HERMES_HOME/node/bin/ and ~/.local/bin/, so execvp() failed with
'[Errno 2] No such file or directory: npx' on every Node-based stdio
MCP server (Railway, Anthropic, GitHub Copilot, etc.).
The naive workaround — symlink /usr/local/bin/npx into the user's PATH —
fails one layer deeper because npx's shebang re-execs /usr/bin/env node
and node also lives at /usr/local/bin/node.
Fix: add /usr/local/bin/<cmd> as a third candidate in the fallback list.
This is the canonical install location for Node on:
- Linux from-source builds
- the upstream node:bookworm-slim image, which the Hermes Docker
image copies node + npm + corepack from since #4977 (the Node 22 LTS
refactor that exposed this)
- macOS Homebrew on Intel
Because the resolver already calls _prepend_path(resolved_env, command_dir)
after locating the command, /usr/local/bin gets prepended to the env's
PATH automatically, which also fixes the second-layer shebang failure
(npx-cli.js can now find node).
Scope is intentionally narrow: the fix activates only when the bare
command isn't otherwise locatable through the user's PATH. Users who
explicitly narrowed PATH for a non-Node MCP server see no change in
behavior.
Tested:
- tests/tools/test_mcp_tool_issue_948.py: new test
test_resolve_stdio_command_falls_back_to_usr_local_bin (mirrors the
existing hermes-node-bin fallback test)
- Full MCP test suite: 254/254 pass across 7 test files
- E2E against a freshly-built Docker image: reproduced the original
failure mode (env.PATH=/opt/data/bin:/usr/bin:/bin), confirmed the
resolver returns /usr/local/bin/npx and prepends /usr/local/bin to
PATH; subprocess.run of the resolved command prints '10.9.8' and
exits 0 with empty stderr
- Negative E2E on the host (where Node is already on PATH via mise):
resolver still hits the mise install dir, /usr/local/bin candidate
is not consulted, PATH is unchanged
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