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/tui/src/cli.rs

@@ -17,6 +17,12 @@ pub struct Cli {
     #[arg(long, short = 'm')]
     pub model: Option<String>,
 
+    /// Convenience flag to select the local open source model provider.
+    /// Equivalent to -c model_provider=oss; verifies a local Ollama server is
+    /// running.
+    #[arg(long = "oss", default_value_t = false)]
+    pub oss: bool,
+
     /// Configuration profile from config.toml to specify default options.
     #[arg(long = "profile", short = 'p')]
     pub config_profile: Option<String>,

codex-rs/tui/src/lib.rs

@@ -3,6 +3,7 @@
 // alternate‑screen mode starts; that file opts‑out locally via `allow`.
 #![deny(clippy::print_stdout, clippy::print_stderr)]
 use app::App;
+use codex_core::BUILT_IN_OSS_MODEL_PROVIDER_ID;
 use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
 use codex_core::config_types::SandboxMode;
@@ -70,18 +71,35 @@ pub async fn run_main(
         )
     };
 
+    let model_provider_override = if cli.oss {
+        Some(BUILT_IN_OSS_MODEL_PROVIDER_ID.to_owned())
+    } else {
+        None
+    };
     let config = {
         // Load configuration and support CLI overrides.
         let overrides = ConfigOverrides {
-            model: cli.model.clone(),
+            // When using `--oss`, let the bootstrapper pick the model
+            // (defaulting to gpt-oss:20b) and ensure it is present locally.
+            model: if cli.oss {
+                Some(
+                    codex_ollama::ensure_oss_ready(cli.model.clone())
+                        .await
+                        .map_err(|e| std::io::Error::other(format!("OSS setup failed: {e}")))?,
+                )
+            } else {
+                cli.model.clone()
+            },
             approval_policy,
             sandbox_mode,
             cwd: cli.cwd.clone().map(|p| p.canonicalize().unwrap_or(p)),
-            model_provider: None,
+            model_provider: model_provider_override,
             config_profile: cli.config_profile.clone(),
             codex_linux_sandbox_exe,
             base_instructions: None,
             include_plan_tool: Some(true),
+            default_disable_response_storage: cli.oss.then_some(true),
+            default_show_raw_agent_reasoning: cli.oss.then_some(true),
         };
         // Parse `-c` overrides from the CLI.
         let cli_kv_overrides = match cli.config_overrides.parse_overrides() {