Introduce --oss flag to use gpt-oss models (#1848)

This adds support for easily running Codex backed by a local Ollama
instance running our new open source models. See
https://github.com/openai/gpt-oss for details.

If you pass in `--oss` you'll be prompted to install/launch ollama, and
it will automatically download the 20b model and attempt to use it.

We'll likely want to expand this with some options later to make the
experience smoother for users who can't run the 20b or want to run the
120b.

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/exec/src/cli.rs

@@ -14,6 +14,9 @@ pub struct Cli {
     #[arg(long, short = 'm')]
     pub model: Option<String>,
 
+    #[arg(long = "oss", default_value_t = false)]
+    pub oss: bool,
+
     /// Select the sandbox policy to use when executing model-generated shell
     /// commands.
     #[arg(long = "sandbox", short = 's')]

codex-rs/exec/src/lib.rs

@@ -9,6 +9,7 @@ use std::path::PathBuf;
 use std::sync::Arc;
 
 pub use cli::Cli;
+use codex_core::BUILT_IN_OSS_MODEL_PROVIDER_ID;
 use codex_core::codex_wrapper::CodexConversation;
 use codex_core::codex_wrapper::{self};
 use codex_core::config::Config;
@@ -35,6 +36,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     let Cli {
         images,
         model,
+        oss,
         config_profile,
         full_auto,
         dangerously_bypass_approvals_and_sandbox,
@@ -114,6 +116,24 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         sandbox_mode_cli_arg.map(Into::<SandboxMode>::into)
     };
 
+    // When using `--oss`, let the bootstrapper pick the model (defaulting to
+    // gpt-oss:20b) and ensure it is present locally. Also, force the built‑in
+    // `oss` model provider.
+    let model_provider_override = if oss {
+        Some(BUILT_IN_OSS_MODEL_PROVIDER_ID.to_owned())
+    } else {
+        None
+    };
+    let model = if oss {
+        Some(
+            codex_ollama::ensure_oss_ready(model.clone())
+                .await
+                .map_err(|e| anyhow::anyhow!("OSS setup failed: {e}"))?,
+        )
+    } else {
+        model
+    };
+
     // Load configuration and determine approval policy
     let overrides = ConfigOverrides {
         model,
@@ -123,10 +143,12 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         approval_policy: Some(AskForApproval::Never),
         sandbox_mode,
         cwd: cwd.map(|p| p.canonicalize().unwrap_or(p)),
-        model_provider: None,
+        model_provider: model_provider_override,
         codex_linux_sandbox_exe,
         base_instructions: None,
         include_plan_tool: None,
+        default_disable_response_storage: oss.then_some(true),
+        default_show_raw_agent_reasoning: oss.then_some(true),
     };
     // Parse `-c` overrides.
     let cli_kv_overrides = match config_overrides.parse_overrides() {