Documented source build fails on macOS arm64: sandboxed V8 asset returns 404 and `codex-code-mode-host` is not built

[noscripter]

Issue body

What version of Codex CLI is running?

Source checkout of main at bb5054fe47abe73ecbbd454751066a28c89f4bb9 (codex-cli 0.0.0).

What subscription do you have?

N/A — this is a source-build failure before any API request or model invocation.

Which model were you using?

N/A.

What platform is your computer?

Darwin 25.6.0 arm64 arm
macOS 26.6 (25G72)
rustc 1.95.0 (59807616e 2026-04-14)
cargo 1.95.0 (f2d3ce0bd 2026-03-21)
toolchain: 1.95.0-aarch64-apple-darwin (repository override)

What terminal emulator and version are you using?

Not relevant; this reproduces during a non-interactive Cargo build.

Codex doctor report

Not available before applying the workaround because the documented source build does not produce a runnable local package.

What issue are you seeing?

The documented source-build flow in docs/install.md says to run:

cd codex/codex-rs
cargo build
cargo run --bin codex -- "explain this codebase to me"

This no longer works on a clean Apple Silicon checkout of current main.

There are two connected failures:

1. Building only codex-cli produces target/debug/codex, but not the now-required sibling executable target/debug/codex-code-mode-host. Running the CLI then reports:

   /path/to/codex/codex-rs/target/debug/codex-code-mode-host: No such file or directory

2. Building the host explicitly fails while compiling v8 v150.4.0:

   error: failed to run custom build command for v8 v150.4.0

   static lib URL: https://github.com/denoland/rusty_v8/releases/download/v150.4.0/librusty_v8_ptrcomp_sandbox_release_aarch64-apple-darwin.a.gz
   HTTP Error 404: Not Found

   Failed to download V8 prebuilt archive from
   https://github.com/denoland/rusty_v8/releases/download/v150.4.0/librusty_v8_ptrcomp_sandbox_release_aarch64-apple-darwin.a.gz

The failing filename is the sandbox-enabled ptrcomp_sandbox_release variant selected by v8_enable_sandbox. The Codex rusty-v8-v150.4.0 release contains the corresponding OpenAI-built archive and binding, and .github/actions/setup-rusty-v8/action.yml already downloads them and supplies:

RUSTY_V8_ARCHIVE
RUSTY_V8_SRC_BINDING_PATH

However, the documented direct Cargo/Just development path does not perform this setup. The repository's package builder contains similar download-and-verification logic, but it is not used by cargo build, just codex, or the source-build instructions.

This is related to, but different from, #31906. That issue concerns a Homebrew package missing the sidecar. This report concerns a clean build from source and the sandboxed V8 dependency setup.

The likely regression window is the combination of:

• 97576b1794 — run code mode exclusively through the standalone host; and
• 2e32d95894 — enable sandboxed V8 for code mode.

What steps can reproduce the bug?

Starting from a clean checkout:

git clone https://github.com/openai/codex.git
cd codex/codex-rs

The previously sufficient command builds the CLI but not its required host.

cargo build -p codex-cli --bin codex
./target/debug/codex

Attempt to build the missing host.

cargo build -p codex-code-mode-host --bin codex-code-mode-host

The last command fails on the Deno rusty_v8 404 shown above. Running the documented plain cargo build also reaches the same V8 failure.

What is the expected behavior?

The documented build-from-source command should produce a runnable local Codex installation from a clean checkout.

If codex-code-mode-host is mandatory, the supported local build command should:

• build both codex and codex-code-mode-host;
• place them where runtime discovery expects them;
• obtain and checksum-verify the correct sandbox-enabled V8 archive and Rust binding automatically; and
• avoid requiring contributors to discover CI-only environment variables.

Suggested fix

Preferred approach:

1. Extract/reuse the verified V8 artifact setup already implemented by scripts/codex_package and .github/actions/setup-rusty-v8 as a first-class local developer helper.

2. Add a supported just build recipe that runs that helper and then performs one grouped Cargo build for:

   cargo build
   -p codex-cli --bin codex
   -p codex-code-mode-host --bin codex-code-mode-host

3. Make just codex ensure the host has been built with the same V8 configuration before launching the CLI.

4. Update docs/install.md to use this supported command instead of raw cargo build / cargo run --bin codex.

5. Add a clean-checkout macOS arm64 CI smoke test that executes the documented source-build command and verifies both sibling binaries exist and start.

At minimum, the documentation should explain the two required binaries and show how to download and verify the Codex-hosted V8 pair before setting RUSTY_V8_ARCHIVE and RUSTY_V8_SRC_BINDING_PATH.

Confirmed workaround

Downloading the three assets below from the rusty-v8-v150.4.0 Codex release, verifying the supplied checksum file, and exporting the two paths makes the grouped build succeed:

librusty_v8_ptrcomp_sandbox_release_aarch64-apple-darwin.a.gz
src_binding_ptrcomp_sandbox_release_aarch64-apple-darwin.rs
rusty_v8_ptrcomp_sandbox_release_aarch64-apple-darwin.sha256

export RUSTY_V8_ARCHIVE=/path/to/librusty_v8_ptrcomp_sandbox_release_aarch64-apple-darwin.a.gz
export RUSTY_V8_SRC_BINDING_PATH=/path/to/src_binding_ptrcomp_sandbox_release_aarch64-apple-darwin.rs

cargo build
-p codex-cli --bin codex
-p codex-code-mode-host --bin codex-code-mode-host

./target/debug/codex

With those overrides, both arm64 Mach-O binaries build successfully and the CLI resolves the host beside itself.