LM Studio OSS Support (#2312)

## Overview

Adds LM Studio OSS support. Closes #1883


### Changes
This PR enhances the behavior of `--oss` flag to support LM Studio as a
provider. Additionally, it introduces a new flag`--local-provider` which
can take in `lmstudio` or `ollama` as values if the user wants to
explicitly choose which one to use.

If no provider is specified `codex --oss` will auto-select the provider
based on whichever is running.

#### Additional enhancements 
The default can be set using `oss-provider` in config like:

```
oss_provider = "lmstudio"
```

For non-interactive users, they will need to either provide the provider
as an arg or have it in their `config.toml`

### Notes
For best performance, [set the default context
length](https://lmstudio.ai/docs/app/advanced/per-model) for gpt-oss to
the maximum your machine can support

---------

Co-authored-by: Matt Clayton <matt@lmstudio.ai>
Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/core/src/config/mod.rs

@@ -25,7 +25,9 @@ use crate::git_info::resolve_root_git_project_for_trust;
 use crate::model_family::ModelFamily;
 use crate::model_family::derive_default_model_family;
 use crate::model_family::find_family_for_model;
+use crate::model_provider_info::LMSTUDIO_OSS_PROVIDER_ID;
 use crate::model_provider_info::ModelProviderInfo;
+use crate::model_provider_info::OLLAMA_OSS_PROVIDER_ID;
 use crate::model_provider_info::built_in_model_providers;
 use crate::openai_model_info::get_model_info;
 use crate::project_doc::DEFAULT_PROJECT_DOC_FILENAME;
@@ -466,6 +468,48 @@ pub fn set_project_trust_level(
         .apply_blocking()
 }
 
+/// Save the default OSS provider preference to config.toml
+pub fn set_default_oss_provider(codex_home: &Path, provider: &str) -> std::io::Result<()> {
+    // Validate that the provider is one of the known OSS providers
+    match provider {
+        LMSTUDIO_OSS_PROVIDER_ID | OLLAMA_OSS_PROVIDER_ID => {
+            // Valid provider, continue
+        }
+        _ => {
+            return Err(std::io::Error::new(
+                std::io::ErrorKind::InvalidInput,
+                format!(
+                    "Invalid OSS provider '{provider}'. Must be one of: {LMSTUDIO_OSS_PROVIDER_ID}, {OLLAMA_OSS_PROVIDER_ID}"
+                ),
+            ));
+        }
+    }
+    let config_path = codex_home.join(CONFIG_TOML_FILE);
+
+    // Read existing config or create empty string if file doesn't exist
+    let content = match std::fs::read_to_string(&config_path) {
+        Ok(content) => content,
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => String::new(),
+        Err(e) => return Err(e),
+    };
+
+    // Parse as DocumentMut for editing while preserving structure
+    let mut doc = content.parse::<DocumentMut>().map_err(|e| {
+        std::io::Error::new(
+            std::io::ErrorKind::InvalidData,
+            format!("failed to parse config.toml: {e}"),
+        )
+    })?;
+
+    // Set the default_oss_provider at root level
+    use toml_edit::value;
+    doc["oss_provider"] = value(provider);
+
+    // Write the modified document back
+    std::fs::write(&config_path, doc.to_string())?;
+    Ok(())
+}
+
 /// Apply a single dotted-path override onto a TOML value.
 fn apply_toml_override(root: &mut TomlValue, path: &str, value: TomlValue) {
     use toml::value::Table;
@@ -663,6 +707,8 @@ pub struct ConfigToml {
     pub experimental_use_rmcp_client: Option<bool>,
     pub experimental_use_freeform_apply_patch: Option<bool>,
     pub experimental_sandbox_command_assessment: Option<bool>,
+    /// Preferred OSS provider for local models, e.g. "lmstudio" or "ollama".
+    pub oss_provider: Option<String>,
 }
 
 impl From<ConfigToml> for UserSavedConfig {
@@ -851,6 +897,34 @@ pub struct ConfigOverrides {
     pub additional_writable_roots: Vec<PathBuf>,
 }
 
+/// Resolves the OSS provider from CLI override, profile config, or global config.
+/// Returns `None` if no provider is configured at any level.
+pub fn resolve_oss_provider(
+    explicit_provider: Option<&str>,
+    config_toml: &ConfigToml,
+    config_profile: Option<String>,
+) -> Option<String> {
+    if let Some(provider) = explicit_provider {
+        // Explicit provider specified (e.g., via --local-provider)
+        Some(provider.to_string())
+    } else {
+        // Check profile config first, then global config
+        let profile = config_toml.get_config_profile(config_profile).ok();
+        if let Some(profile) = &profile {
+            // Check if profile has an oss provider
+            if let Some(profile_oss_provider) = &profile.oss_provider {
+                Some(profile_oss_provider.clone())
+            }
+            // If not then check if the toml has an oss provider
+            else {
+                config_toml.oss_provider.clone()
+            }
+        } else {
+            config_toml.oss_provider.clone()
+        }
+    }
+}
+
 impl Config {
     /// Meant to be used exclusively for tests: `load_with_overrides()` should
     /// be used in all other cases.
@@ -3265,6 +3339,41 @@ trust_level = "trusted"
         Ok(())
     }
 
+    #[test]
+    fn test_set_default_oss_provider() -> std::io::Result<()> {
+        let temp_dir = TempDir::new()?;
+        let codex_home = temp_dir.path();
+        let config_path = codex_home.join(CONFIG_TOML_FILE);
+
+        // Test setting valid provider on empty config
+        set_default_oss_provider(codex_home, OLLAMA_OSS_PROVIDER_ID)?;
+        let content = std::fs::read_to_string(&config_path)?;
+        assert!(content.contains("oss_provider = \"ollama\""));
+
+        // Test updating existing config
+        std::fs::write(&config_path, "model = \"gpt-4\"\n")?;
+        set_default_oss_provider(codex_home, LMSTUDIO_OSS_PROVIDER_ID)?;
+        let content = std::fs::read_to_string(&config_path)?;
+        assert!(content.contains("oss_provider = \"lmstudio\""));
+        assert!(content.contains("model = \"gpt-4\""));
+
+        // Test overwriting existing oss_provider
+        set_default_oss_provider(codex_home, OLLAMA_OSS_PROVIDER_ID)?;
+        let content = std::fs::read_to_string(&config_path)?;
+        assert!(content.contains("oss_provider = \"ollama\""));
+        assert!(!content.contains("oss_provider = \"lmstudio\""));
+
+        // Test invalid provider
+        let result = set_default_oss_provider(codex_home, "invalid_provider");
+        assert!(result.is_err());
+        let error = result.unwrap_err();
+        assert_eq!(error.kind(), std::io::ErrorKind::InvalidInput);
+        assert!(error.to_string().contains("Invalid OSS provider"));
+        assert!(error.to_string().contains("invalid_provider"));
+
+        Ok(())
+    }
+
     #[test]
     fn test_untrusted_project_gets_workspace_write_sandbox() -> anyhow::Result<()> {
         let config_with_untrusted = r#"
@@ -3295,6 +3404,85 @@ trust_level = "untrusted"
         Ok(())
     }
 
+    #[test]
+    fn test_resolve_oss_provider_explicit_override() {
+        let config_toml = ConfigToml::default();
+        let result = resolve_oss_provider(Some("custom-provider"), &config_toml, None);
+        assert_eq!(result, Some("custom-provider".to_string()));
+    }
+
+    #[test]
+    fn test_resolve_oss_provider_from_profile() {
+        let mut profiles = std::collections::HashMap::new();
+        let profile = ConfigProfile {
+            oss_provider: Some("profile-provider".to_string()),
+            ..Default::default()
+        };
+        profiles.insert("test-profile".to_string(), profile);
+        let config_toml = ConfigToml {
+            profiles,
+            ..Default::default()
+        };
+
+        let result = resolve_oss_provider(None, &config_toml, Some("test-profile".to_string()));
+        assert_eq!(result, Some("profile-provider".to_string()));
+    }
+
+    #[test]
+    fn test_resolve_oss_provider_from_global_config() {
+        let config_toml = ConfigToml {
+            oss_provider: Some("global-provider".to_string()),
+            ..Default::default()
+        };
+
+        let result = resolve_oss_provider(None, &config_toml, None);
+        assert_eq!(result, Some("global-provider".to_string()));
+    }
+
+    #[test]
+    fn test_resolve_oss_provider_profile_fallback_to_global() {
+        let mut profiles = std::collections::HashMap::new();
+        let profile = ConfigProfile::default(); // No oss_provider set
+        profiles.insert("test-profile".to_string(), profile);
+        let config_toml = ConfigToml {
+            oss_provider: Some("global-provider".to_string()),
+            profiles,
+            ..Default::default()
+        };
+
+        let result = resolve_oss_provider(None, &config_toml, Some("test-profile".to_string()));
+        assert_eq!(result, Some("global-provider".to_string()));
+    }
+
+    #[test]
+    fn test_resolve_oss_provider_none_when_not_configured() {
+        let config_toml = ConfigToml::default();
+        let result = resolve_oss_provider(None, &config_toml, None);
+        assert_eq!(result, None);
+    }
+
+    #[test]
+    fn test_resolve_oss_provider_explicit_overrides_all() {
+        let mut profiles = std::collections::HashMap::new();
+        let profile = ConfigProfile {
+            oss_provider: Some("profile-provider".to_string()),
+            ..Default::default()
+        };
+        profiles.insert("test-profile".to_string(), profile);
+        let config_toml = ConfigToml {
+            oss_provider: Some("global-provider".to_string()),
+            profiles,
+            ..Default::default()
+        };
+
+        let result = resolve_oss_provider(
+            Some("explicit-provider"),
+            &config_toml,
+            Some("test-profile".to_string()),
+        );
+        assert_eq!(result, Some("explicit-provider".to_string()));
+    }
+
     #[test]
     fn test_untrusted_project_gets_unless_trusted_approval_policy() -> std::io::Result<()> {
         let codex_home = TempDir::new()?;