Rust: ~/.codex/config.toml model_providers overrides ignored (regression between rust-v0.23.0-alpha and -alpha.5)

Thank you for your work on Codex! This is a regression in the Rust CLI configuration handling.

What version of Codex is running?

• codex --version: rust-v0.23.0-alpha.5 (regression appears between rust-v0.23.0-alpha and rust-v0.23.0-alpha.5)

Which model were you using?

• Any; reproduces with built-in provider ids such as openai and oss.

What platform is your computer?

• e.g., macOS (uname -mprs), Linux, or Windows. Please fill in.

What steps can reproduce the bug?

1. Create/update ~/.codex/config.toml with a user override for a built-in model provider key:

   model_provider = "oss"

   [model_providers.oss]
   name = "Open Source"
   base_url = "http://my-ollama.example.com:11434/v1"

1. Run Codex (Rust CLI), e.g.:

• codex -m gpt-oss:20b --oss or codex with defaults

1. Observe that requests still use the built-in provider configuration instead of the user override (e.g., base_url is not changed).

What is the expected behavior?

• User-defined entries under [model_providers.<id>] in ~/.codex/config.toml should override the built-in provider with the same <id>, as documented (“override

and amend the default set of model providers”).

What do you see instead?

• The built-in provider entry wins; the user override is ignored when <id> collides with a built-in key.

Additional information

• Root cause:
• codex-rs/core/src/config.rs merges built-ins with user-defined providers using:

for (key, provider) in cfg.model_providers.into_iter() {
model_providers.entry(key).or_insert(provider);
}

• or_insert prevents overriding an existing (built-in) entry with the same key, so user overrides for keys like openai, azure, ollama, or oss are ignored.
• Background and related changes:
• Introduced in the work to read model_provider/model_providers from config.toml and later work adding chat completions support and profiles; docs were

updated to encourage overriding providers.

• This behavior diverges from docs and breaks common use cases (e.g., overriding [model_providers.oss] base_url).
• Proposed fix:
• Replace or_insert with insert so user-defined providers replace built-ins on the same key:

for (key, provider) in cfg.model_providers.into_iter() {
model_providers.insert(key, provider);
}

• This aligns with the documentation (“override and amend”).
• Suggested regression test:
• Given model_provider = "oss" and a custom [model_providers.oss].base_url, assert that Config.model_provider.base_url equals the custom value.

Thank you!