diff --git a/codex-rs/core/src/config.rs b/codex-rs/core/src/config.rs
index e2a68d07dc..d4a365aa1a 100644
--- a/codex-rs/core/src/config.rs
+++ b/codex-rs/core/src/config.rs
@@ -535,7 +535,8 @@ impl Config {
         let mut model_providers = built_in_model_providers();
         // Merge user-defined providers into the built-in list.
         for (key, provider) in cfg.model_providers.into_iter() {
-            model_providers.entry(key).or_insert(provider);
+            // Override the built-in provider if the same key is present ib config.toml
+            model_providers.insert(key, provider);
         }
 
         let model_provider_id = model_provider
diff --git a/codex-rs/core/tests/model_provider_override.rs b/codex-rs/core/tests/model_provider_override.rs
new file mode 100644
index 0000000000..c568785b21
--- /dev/null
+++ b/codex-rs/core/tests/model_provider_override.rs
@@ -0,0 +1,50 @@
+#![expect(clippy::unwrap_used)]
+
+use std::collections::HashMap;
+
+use codex_core::config::{Config, ConfigOverrides, ConfigToml};
+use codex_core::model_provider_info::{ModelProviderInfo, WireApi};
+use tempfile::TempDir;
+
+#[test]
+fn user_defined_provider_overrides_builtin() {
+    let tmp = TempDir::new().unwrap();
+
+    let mut cfg = ConfigToml::default();
+    cfg.model_provider = Some("oss".to_string());
+    cfg.model = Some("gpt-oss:20b".to_string());
+
+    let mut providers = HashMap::new();
+    providers.insert(
+        "oss".to_string(),
+        ModelProviderInfo {
+            name: "Custom".into(),
+            base_url: Some("https://example.com/v1".into()),
+            env_key: None,
+            env_key_instructions: None,
+            wire_api: WireApi::Chat,
+            query_params: None,
+            http_headers: None,
+            env_http_headers: None,
+            request_max_retries: None,
+            stream_max_retries: None,
+            stream_idle_timeout_ms: None,
+            requires_openai_auth: false,
+        },
+    );
+    cfg.model_providers = providers;
+
+    let config = Config::load_from_base_config_with_overrides(
+        cfg,
+        ConfigOverrides::default(),
+        tmp.path().to_path_buf(),
+    )
+    .unwrap();
+
+    assert_eq!(config.model_provider.name, "Custom");
+    assert_eq!(
+        config.model_provider.base_url.as_deref(),
+        Some("https://example.com/v1")
+    );
+}
+