# Preserve PATH precedence & fix Windows MCP env propagation
## Problem & intent
Preserve user PATH precedence and reduce Windows setup friction for MCP
servers by avoiding PATH reordering and ensuring Windows child processes
receive essential env vars.
- Addresses: #4180#5225#2945#3245#3385#2892#3310#3457#4370
- Supersedes: #4182, #3866, #3828 (overlapping/inferior once this
merges)
- Notes: #2626 / #2646 are the original PATH-mutation sources being
corrected.
---
## Before / After
**Before**
- PATH was **prepended** with an `apply_patch` helper dir (Rust + Node
wrapper), reordering tools and breaking virtualenvs/shims on
macOS/Linux.
- On Windows, MCP servers missed core env vars and often failed to start
without explicit per-server env blocks.
**After**
- Helper dir is **appended** to PATH (preserves user/tool precedence).
- Windows MCP child env now includes common core variables and mirrors
`PATH` → `Path`, so typical CLIs/plugins work **without** per-server env
blocks.
---
## Scope of change
### `codex-rs/arg0/src/lib.rs`
- Append temp/helper dir to `PATH` instead of prepending.
### `codex-cli/bin/codex.js`
- Mirror the same append behavior for the Node wrapper.
### `codex-rs/rmcp-client/src/utils.rs`
- Expand Windows `DEFAULT_ENV_VARS` (e.g., `COMSPEC`, `SYSTEMROOT`,
`PROGRAMFILES*`, `APPDATA`, etc.).
- Mirror `PATH` → `Path` for Windows child processes.
- Small unit test; conditional `mut` + `clippy` cleanup.
---
## Security effects
No broadened privileges. Only environment propagation for well-known
Windows keys on stdio MCP child processes. No sandbox policy changes and
no network additions.
---
## Testing evidence
**Static**
- `cargo fmt`
- `cargo clippy -p codex-arg0 -D warnings` → **clean**
- `cargo clippy -p codex-rmcp-client -D warnings` → **clean**
- `cargo test -p codex-rmcp-client` → **13 passed**
**Manual**
- Local verification on Windows PowerShell 5/7 and WSL (no `unused_mut`
warnings on non-Windows targets).
---
## Checklist
- [x] Append (not prepend) helper dir to PATH in Rust and Node wrappers
- [x] Windows MCP child inherits core env vars; `PATH` mirrored to
`Path`
- [x] `cargo fmt` / `clippy` clean across touched crates
- [x] Unit tests updated/passing where applicable
- [x] Cross-platform behavior preserved (macOS/Linux PATH precedence
intact)
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