diff --git a/codex-rs/codex-api/tests/models_endpoint.rs b/codex-rs/codex-api/tests/models_endpoint.rs
new file mode 100644
index 0000000000..7bb7d2ff91
--- /dev/null
+++ b/codex-rs/codex-api/tests/models_endpoint.rs
@@ -0,0 +1,103 @@
+#![allow(clippy::expect_used)]
+
+use async_trait::async_trait;
+use codex_api::ModelsClient;
+use codex_api::auth::AuthProvider;
+use codex_api::provider::Provider;
+use codex_api::provider::RetryConfig;
+use codex_api::provider::WireApi;
+use codex_client::HttpTransport;
+use codex_client::Request;
+use codex_client::Response;
+use codex_client::StreamResponse;
+use codex_client::TransportError;
+use codex_protocol::openai_models::ModelsResponse;
+use http::HeaderMap;
+use http::StatusCode;
+use std::sync::Arc;
+use std::sync::Mutex;
+use std::time::Duration;
+
+#[derive(Clone, Default)]
+struct DummyAuth;
+
+impl AuthProvider for DummyAuth {
+    fn bearer_token(&self) -> Option<String> {
+        None
+    }
+}
+
+#[derive(Clone)]
+struct CapturingTransport {
+    last_request: Arc<Mutex<Option<Request>>>,
+    body: Arc<ModelsResponse>,
+}
+
+#[async_trait]
+impl HttpTransport for CapturingTransport {
+    async fn execute(&self, req: Request) -> Result<Response, TransportError> {
+        *self.last_request.lock().expect("lock poisoned") = Some(req);
+        let body = serde_json::to_vec(&*self.body).expect("serialization should succeed");
+        Ok(Response {
+            status: StatusCode::OK,
+            headers: HeaderMap::new(),
+            body: body.into(),
+        })
+    }
+
+    async fn stream(&self, _req: Request) -> Result<StreamResponse, TransportError> {
+        Err(TransportError::Build("stream should not run".to_string()))
+    }
+}
+
+fn provider(base_url: &str) -> Provider {
+    Provider {
+        name: "test".to_string(),
+        base_url: base_url.to_string(),
+        query_params: None,
+        wire: WireApi::Responses,
+        headers: HeaderMap::new(),
+        retry: RetryConfig {
+            max_attempts: 1,
+            base_delay: Duration::from_millis(1),
+            retry_429: false,
+            retry_5xx: true,
+            retry_transport: true,
+        },
+        stream_idle_timeout: Duration::from_secs(1),
+    }
+}
+
+#[tokio::test]
+async fn builds_correct_url() {
+    let response = ModelsResponse { models: Vec::new() };
+    let transport = CapturingTransport {
+        last_request: Arc::new(Mutex::new(None)),
+        body: Arc::new(response),
+    };
+
+    let client = ModelsClient::new(
+        transport.clone(),
+        provider("https://example.com/api/codex"),
+        DummyAuth,
+    );
+
+    client
+        .list_models("0.1.2", HeaderMap::new())
+        .await
+        .expect("list_models should succeed");
+
+    let url = transport
+        .last_request
+        .lock()
+        .expect("lock poisoned")
+        .as_ref()
+        .expect("request recorded")
+        .url
+        .clone();
+
+    assert_eq!(
+        url,
+        "https://example.com/api/codex/models?client_version=0.1.2"
+    );
+}