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## Why While investigating `codex exec hi` startup latency, the useful questions were not "is startup slow?" but "which durable bucket is slow in production?" The path we observed has a few distinct stages: 1. `thread/start` creates the session 2. startup prewarm builds the turn context, tools, and prompt 3. startup prewarm warms the websocket 4. the first real turn resolves the prewarm 5. the model produces the first token Before this PR, production telemetry had some of the raw measurements already: - aggregate startup-prewarm duration / age-at-first-turn metrics - TTFT as a metric - websocket request telemetry But there was no coherent production event stream for the startup breakdown itself, and TTFT was metric-only. That made it hard to answer the same latency questions from OpenTelemetry-backed logs without adding one-off local instrumentation. ## What changed Add durable production telemetry on the existing `SessionTelemetry` path: - new `codex.startup_phase` OTel log/trace events plus `codex.startup.phase.duration_ms` - new `codex.turn_ttft` OTel log/trace events while preserving the existing TTFT metric The startup phase event is emitted for the coarse buckets we actually observed while running `exec hi`: - `thread_start_create_thread` - `startup_prewarm_total` - `startup_prewarm_create_turn_context` - `startup_prewarm_build_tools` - `startup_prewarm_build_prompt` - `startup_prewarm_websocket_warmup` - `startup_prewarm_resolve` These phases are intentionally low-cardinality so they remain safe as production telemetry tags. ## Why this shape This keeps the instrumentation on the same production path as the rest of the session telemetry instead of adding a local debug-only trace mode. It also avoids changing startup behavior: - prewarm still runs - no control flow changes - no extra remote calls - no user-visible behavior changes One boundary is intentional: very early process bootstrap that happens before a session exists is not included here, because this PR uses session-scoped production telemetry. The expensive buckets we were trying to understand after `thread/start` are now covered durably. ## Verification - `cargo test -p codex-otel` - `cargo test -p codex-core turn_timing` - `cargo test -p codex-core regular_turn_emits_turn_started_without_waiting_for_startup_prewarm` - `cargo test -p codex-core interrupting_regular_turn_waiting_on_startup_prewarm_emits_turn_aborted` - `cargo test -p codex-app-server thread_start` - `just fix -p codex-otel -p codex-core -p codex-app-server` I also ran `cargo test -p codex-core`; it built successfully and then hit an existing unrelated stack overflow in `tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed`.
Codex CLI (Rust Implementation)
We provide Codex CLI as a standalone executable to ensure a zero-dependency install.
Installing Codex
Today, the easiest way to install Codex is via npm:
npm i -g @openai/codex
codex
You can also install via Homebrew (brew install --cask codex) or download a platform-specific release directly from our GitHub Releases.
Documentation quickstart
- First run with Codex? Start with
docs/getting-started.md(links to the walkthrough for prompts, keyboard shortcuts, and session management). - Want deeper control? See
docs/config.mdanddocs/install.md.
What's new in the Rust CLI
The Rust implementation is now the maintained Codex CLI and serves as the default experience. It includes a number of features that the legacy TypeScript CLI never supported.
Config
Codex supports a rich set of configuration options. Note that the Rust CLI uses config.toml instead of config.json. See docs/config.md for details.
Model Context Protocol Support
MCP client
Codex CLI functions as an MCP client that allows the Codex CLI and IDE extension to connect to MCP servers on startup. See the configuration documentation for details.
MCP server (experimental)
Codex can be launched as an MCP server by running codex mcp-server. This allows other MCP clients to use Codex as a tool for another agent.
Use the @modelcontextprotocol/inspector to try it out:
npx @modelcontextprotocol/inspector codex mcp-server
Use codex mcp to add/list/get/remove MCP server launchers defined in config.toml, and codex mcp-server to run the MCP server directly.
Notifications
You can enable notifications by configuring a script that is run whenever the agent finishes a turn. The notify documentation includes a detailed example that explains how to get desktop notifications via terminal-notifier on macOS. When Codex detects that it is running under WSL 2 inside Windows Terminal (WT_SESSION is set), the TUI automatically falls back to native Windows toast notifications so approval prompts and completed turns surface even though Windows Terminal does not implement OSC 9.
codex exec to run Codex programmatically/non-interactively
To run Codex non-interactively, run codex exec PROMPT (you can also pass the prompt via stdin) and Codex will work on your task until it decides that it is done and exits. If you provide both a prompt argument and piped stdin, Codex appends stdin as a <stdin> block after the prompt so patterns like echo "my output" | codex exec "Summarize this concisely" work naturally. Output is printed to the terminal directly. You can set the RUST_LOG environment variable to see more about what's going on.
Use codex exec --ephemeral ... to run without persisting session rollout files to disk.
Experimenting with the Codex Sandbox
To test to see what happens when a command is run under the sandbox provided by Codex, we provide the following subcommands in Codex CLI:
# macOS
codex sandbox macos [--log-denials] [COMMAND]...
# Linux
codex sandbox linux [COMMAND]...
# Windows
codex sandbox windows [COMMAND]...
# Legacy aliases
codex debug seatbelt [--log-denials] [COMMAND]...
codex debug landlock [COMMAND]...
To try a writable legacy sandbox mode with these commands, pass an explicit config override such
as -c 'sandbox_mode="workspace-write"'.
Selecting a sandbox policy via --sandbox
The Rust CLI exposes a dedicated --sandbox (-s) flag that lets you pick the sandbox policy without having to reach for the generic -c/--config option:
# Run Codex with the default, read-only sandbox
codex --sandbox read-only
# Allow the agent to write within the current workspace while still blocking network access
codex --sandbox workspace-write
# Danger! Disable sandboxing entirely (only do this if you are already running in a container or other isolated env)
codex --sandbox danger-full-access
The same setting can be persisted in ~/.codex/config.toml via the top-level sandbox_mode = "MODE" key, e.g. sandbox_mode = "workspace-write".
In workspace-write, Codex also includes ~/.codex/memories in its writable roots so memory maintenance does not require an extra approval.
Code Organization
This folder is the root of a Cargo workspace. It contains quite a bit of experimental code, but here are the key crates:
core/contains the business logic for Codex. Ultimately, we hope this becomes a library crate that is generally useful for building other Rust/native applications that use Codex.exec/"headless" CLI for use in automation.tui/CLI that launches a fullscreen TUI built with Ratatui.cli/CLI multitool that provides the aforementioned CLIs via subcommands.
If you want to contribute or inspect behavior in detail, start by reading the module-level README.md files under each crate and run the project workspace from the top-level codex-rs directory so shared config, features, and build scripts stay aligned.