Enforce errors on overriding built-in model providers (#12024)

We receive bug reports from users who attempt to override one of the
three built-in model providers (openai, ollama, or lmstuio). Currently,
these overrides are silently ignored. This PR makes it an error to
override them.

## Summary
- add validation for `model_providers` so `openai`, `ollama`, and
`lmstudio` keys now produce clear configuration errors instead of being
silently ignored

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

@@ -47,6 +47,7 @@ use crate::model_provider_info::LMSTUDIO_OSS_PROVIDER_ID;
 use crate::model_provider_info::ModelProviderInfo;
 use crate::model_provider_info::OLLAMA_CHAT_PROVIDER_REMOVED_ERROR;
 use crate::model_provider_info::OLLAMA_OSS_PROVIDER_ID;
+use crate::model_provider_info::OPENAI_PROVIDER_ID;
 use crate::model_provider_info::built_in_model_providers;
 use crate::path_utils::normalize_for_native_workdir;
 use crate::project_doc::DEFAULT_PROJECT_DOC_FILENAME;
@@ -139,6 +140,11 @@ pub(crate) const DEFAULT_AGENT_JOB_MAX_RUNTIME_SECONDS: Option<u64> = None;
 
 pub const CONFIG_TOML_FILE: &str = "config.toml";
 const OPENAI_BASE_URL_ENV_VAR: &str = "OPENAI_BASE_URL";
+const RESERVED_MODEL_PROVIDER_IDS: [&str; 3] = [
+    OPENAI_PROVIDER_ID,
+    OLLAMA_OSS_PROVIDER_ID,
+    LMSTUDIO_OSS_PROVIDER_ID,
+];
 
 fn resolve_sqlite_home_env(resolved_cwd: &Path) -> Option<PathBuf> {
     let raw = std::env::var(codex_state::SQLITE_HOME_ENV).ok()?;
@@ -367,7 +373,7 @@ pub struct Config {
     /// to 127.0.0.1 (using `mcp_oauth_callback_port` when provided).
     pub mcp_oauth_callback_url: Option<String>,
 
-    /// Combined provider map (defaults merged with user-defined overrides).
+    /// Combined provider map (defaults plus user-defined providers).
     pub model_providers: HashMap<String, ModelProviderInfo>,
 
     /// Maximum number of bytes to include from an AGENTS.md project doc file.
@@ -1262,8 +1268,9 @@ pub struct ConfigToml {
     /// to 127.0.0.1 (using `mcp_oauth_callback_port` when provided).
     pub mcp_oauth_callback_url: Option<String>,
 
-    /// User-defined provider entries that extend/override the built-in list.
-    #[serde(default)]
+    /// User-defined provider entries that extend the built-in list. Built-in
+    /// IDs cannot be overridden.
+    #[serde(default, deserialize_with = "deserialize_model_providers")]
     pub model_providers: HashMap<String, ModelProviderInfo>,
 
     /// Maximum number of bytes to include from an AGENTS.md project doc file.
@@ -1890,6 +1897,37 @@ pub struct ConfigOverrides {
     pub additional_writable_roots: Vec<PathBuf>,
 }
 
+fn validate_reserved_model_provider_ids(
+    model_providers: &HashMap<String, ModelProviderInfo>,
+) -> Result<(), String> {
+    let mut conflicts = model_providers
+        .keys()
+        .filter(|key| RESERVED_MODEL_PROVIDER_IDS.contains(&key.as_str()))
+        .map(|key| format!("`{key}`"))
+        .collect::<Vec<_>>();
+    conflicts.sort_unstable();
+    if conflicts.is_empty() {
+        Ok(())
+    } else {
+        Err(format!(
+            "model_providers contains reserved built-in provider IDs: {}. \
+Built-in providers cannot be overridden. Rename your custom provider (for example, `openai-custom`).",
+            conflicts.join(", ")
+        ))
+    }
+}
+
+fn deserialize_model_providers<'de, D>(
+    deserializer: D,
+) -> Result<HashMap<String, ModelProviderInfo>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    let model_providers = HashMap::<String, ModelProviderInfo>::deserialize(deserializer)?;
+    validate_reserved_model_provider_ids(&model_providers).map_err(serde::de::Error::custom)?;
+    Ok(model_providers)
+}
+
 /// 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(
@@ -2011,6 +2049,8 @@ impl Config {
         codex_home: PathBuf,
         config_layer_stack: ConfigLayerStack,
     ) -> std::io::Result<Self> {
+        validate_reserved_model_provider_ids(&cfg.model_providers)
+            .map_err(|message| std::io::Error::new(std::io::ErrorKind::InvalidInput, message))?;
         // Ensure that every field of ConfigRequirements is applied to the final
         // Config.
         let ConfigRequirements {