Cleans up the signature for `new_stdio_client()` to more closely mirror
how MCP servers are declared in config files (`command`, `args`, `env`).
Also takes a cue from Claude Code where the MCP server is launched with
a restricted `env` so that it only includes "safe" things like `USER`
and `PATH` (see the `create_env_for_mcp_server()` function introduced in
this PR for details) by default, as it is common for developers to have
sensitive API keys present in their environment that should only be
forwarded to the MCP server when the user has explicitly configured it
to do so.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/831).
* #829
* __->__ #831
This PR introduces an initial `McpClient` that we will use to give Codex
itself programmatic access to foreign MCPs. This does not wire it up in
Codex itself yet, but the new `mcp-client` crate includes a `main.rs`
for basic testing for now.
Manually tested by sending a `tools/list` request to Codex's own MCP
server:
```
codex-rs$ cargo build
codex-rs$ cargo run --bin codex-mcp-client ./target/debug/codex-mcp-server
{
"tools": [
{
"description": "Run a Codex session. Accepts configuration parameters matching the Codex Config struct.",
"inputSchema": {
"properties": {
"approval-policy": {
"description": "Execution approval policy expressed as the kebab-case variant name (`unless-allow-listed`, `auto-edit`, `on-failure`, `never`).",
"enum": [
"auto-edit",
"unless-allow-listed",
"on-failure",
"never"
],
"type": "string"
},
"cwd": {
"description": "Working directory for the session. If relative, it is resolved against the server process's current working directory.",
"type": "string"
},
"disable-response-storage": {
"description": "Disable server-side response storage.",
"type": "boolean"
},
"model": {
"description": "Optional override for the model name (e.g. \"o3\", \"o4-mini\")",
"type": "string"
},
"prompt": {
"description": "The *initial user prompt* to start the Codex conversation.",
"type": "string"
},
"sandbox-permissions": {
"description": "Sandbox permissions using the same string values accepted by the CLI (e.g. \"disk-write-cwd\", \"network-full-access\").",
"items": {
"enum": [
"disk-full-read-access",
"disk-write-cwd",
"disk-write-platform-user-temp-folder",
"disk-write-platform-global-temp-folder",
"disk-full-write-access",
"network-full-access"
],
"type": "string"
},
"type": "array"
}
},
"required": [
"prompt"
],
"type": "object"
},
"name": "codex"
}
]
}
```
