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Codex Integration

Codex uses lspmux-cc as a plain MCP server. It does not use the LSP plugin path; editors can keep using their own LSP clients as long as they point at the same lspmux server.

Install

Manual checkout:

./setup core
./setup host codex

Nix:

nix build
nix build .#rust-analyzer-nightly

Use the built result/bin/lspmux-cc-mcp or place the flake packages in your environment. For a Claude-plugin-shaped Nix output, use nix build .#plugin; Codex only needs the MCP binary and environment.

Primary Config Path

Codex uses TOML config:

  • User-level: ~/.codex/config.toml
  • Project-level: .codex/config.toml

TCP loopback example:

[mcp_servers.lspmux-rust-analyzer]
command = "/absolute/path/to/lspmux-cc/bin/lspmux-cc-mcp"
args = []

[mcp_servers.lspmux-rust-analyzer.env]
WORKSPACE_ROOT = "/absolute/path/to/workspace"
LSPMUX_BOOTSTRAP = "auto"
LSPMUX_CONNECT = "tcp://127.0.0.1:27631"
LSPMUX_CLIENT_KIND = "codex_mcp"
LSPMUX_CLIENT_HOST = "codex"

For a Nix-built default package, set command to the built binary:

command = "/absolute/path/to/result/bin/lspmux-cc-mcp"

If you want the pinned rust-analyzer from this flake, export or configure:

export RUST_ANALYZER_PATH=/absolute/path/to/rust-analyzer-nightly/bin/rust-analyzer

Runtime Contract

Variable Recommended value Notes
WORKSPACE_ROOT Absolute Rust workspace path Used for rust-analyzer initialization.
LSPMUX_BOOTSTRAP auto Reuses a reachable daemon or starts one on demand.
LSPMUX_CONNECT tcp://127.0.0.1:27631 Preferred sandbox-friendly endpoint.
LSPMUX_PATH Optional absolute path Needed only if lspmux is not on PATH.
RUST_ANALYZER_PATH Optional absolute path Use this for pinned Nix rust-analyzer or a custom binary.
LSPMUX_CONFIG_PATH Optional config path Defaults to the platform lspmux config path.
LSPMUX_CLIENT_KIND codex_mcp Telemetry identity.
LSPMUX_CLIENT_HOST codex Telemetry host.
LSPMUX_SESSION_ID Any stable session id Optional; generated if omitted.

LSPMUX_SOCKET_PATH is still accepted as a compatibility alias for older configs, but use LSPMUX_CONNECT for new Codex setups.

Transport

For TCP loopback, set the lspmux config:

listen = "tcp://127.0.0.1:27631"
connect = "tcp://127.0.0.1:27631"

For Unix sockets, omit LSPMUX_CONNECT or set it to the absolute socket path. TCP is usually simpler in sandboxed Codex sessions.

Verification

In a Codex session, call:

rust_server_status

Healthy signals:

  • server_status is running
  • workspace_root is the configured workspace
  • readiness.health becomes ok after indexing

Then run rust_diagnostics on an absolute Rust file path to verify the full MCP-to-LSP path.