diff --git a/codex-rs/common/src/oss.rs b/codex-rs/common/src/oss.rs
index a579f72373..249cc0c45c 100644
--- a/codex-rs/common/src/oss.rs
+++ b/codex-rs/common/src/oss.rs
@@ -14,6 +14,24 @@ pub fn get_default_model_for_oss_provider(provider_id: &str) -> Option<&'static
     }
 }
 
+/// Detect whether the selected Ollama instance supports the responses API and, if not, downgrade
+/// to the chat completions wire API. This should run whenever the Ollama provider is selected,
+/// even when `--oss` is not in use, so older servers remain compatible.
+pub async fn detect_ollama_wire_api_if_needed(config: &mut Config) {
+    if config.model_provider_id != OLLAMA_OSS_PROVIDER_ID
+        || config.model_provider.wire_api != WireApi::Responses
+    {
+        return;
+    }
+
+    match codex_ollama::detect_wire_api(&config.model_provider).await {
+        Ok(Some(detection)) if detection.wire_api == WireApi::Chat => {
+            config.model_provider.wire_api = WireApi::Chat;
+        }
+        Ok(_) | Err(_) => {}
+    }
+}
+
 /// Ensures the specified OSS provider is ready (models downloaded, service reachable).
 pub async fn ensure_oss_provider_ready(
     provider_id: &str,
@@ -26,14 +44,7 @@ pub async fn ensure_oss_provider_ready(
                 .map_err(|e| std::io::Error::other(format!("OSS setup failed: {e}")))?;
         }
         OLLAMA_OSS_PROVIDER_ID => {
-            if config.model_provider.wire_api == WireApi::Responses {
-                match codex_ollama::detect_wire_api(&config.model_provider).await {
-                    Ok(Some(detection)) if detection.wire_api == WireApi::Chat => {
-                        config.model_provider.wire_api = WireApi::Chat;
-                    }
-                    Ok(_) | Err(_) => {}
-                }
-            }
+            detect_ollama_wire_api_if_needed(config).await;
 
             codex_ollama::ensure_oss_ready(config)
                 .await
diff --git a/codex-rs/exec/src/lib.rs b/codex-rs/exec/src/lib.rs
index 5998920a7e..86589315d4 100644
--- a/codex-rs/exec/src/lib.rs
+++ b/codex-rs/exec/src/lib.rs
@@ -13,6 +13,7 @@ pub mod exec_events;
 pub use cli::Cli;
 pub use cli::Command;
 pub use cli::ReviewArgs;
+use codex_common::oss::detect_ollama_wire_api_if_needed;
 use codex_common::oss::ensure_oss_provider_ready;
 use codex_common::oss::get_default_model_for_oss_provider;
 use codex_core::AuthManager;
@@ -204,6 +205,10 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
 
     let mut config = Config::load_with_cli_overrides(cli_kv_overrides, overrides).await?;
 
+    if !oss {
+        detect_ollama_wire_api_if_needed(&mut config).await;
+    }
+
     if let Err(err) = enforce_login_restrictions(&config).await {
         eprintln!("{err}");
         std::process::exit(1);
diff --git a/codex-rs/ollama/src/lib.rs b/codex-rs/ollama/src/lib.rs
index 3412921b8c..f719ed7977 100644
--- a/codex-rs/ollama/src/lib.rs
+++ b/codex-rs/ollama/src/lib.rs
@@ -94,7 +94,10 @@ mod tests {
 
     #[test]
     fn test_wire_api_for_version_dev_zero_keeps_responses() {
-        assert_eq!(wire_api_for_version(&Version::new(0, 0, 0)), WireApi::Responses);
+        assert_eq!(
+            wire_api_for_version(&Version::new(0, 0, 0)),
+            WireApi::Responses
+        );
     }
 
     #[test]
@@ -104,7 +107,13 @@ mod tests {
 
     #[test]
     fn test_wire_api_for_version_at_or_after_cutoff_is_responses() {
-        assert_eq!(wire_api_for_version(&Version::new(0, 13, 4)), WireApi::Responses);
-        assert_eq!(wire_api_for_version(&Version::new(0, 14, 0)), WireApi::Responses);
+        assert_eq!(
+            wire_api_for_version(&Version::new(0, 13, 4)),
+            WireApi::Responses
+        );
+        assert_eq!(
+            wire_api_for_version(&Version::new(0, 14, 0)),
+            WireApi::Responses
+        );
     }
 }
diff --git a/codex-rs/tui/src/lib.rs b/codex-rs/tui/src/lib.rs
index bb2daa8844..2765ab5bb3 100644
--- a/codex-rs/tui/src/lib.rs
+++ b/codex-rs/tui/src/lib.rs
@@ -7,6 +7,7 @@ use additional_dirs::add_dir_warning_message;
 use app::App;
 pub use app::AppExitInfo;
 use codex_app_server_protocol::AuthMode;
+use codex_common::oss::detect_ollama_wire_api_if_needed;
 use codex_common::oss::ensure_oss_provider_ready;
 use codex_common::oss::get_default_model_for_oss_provider;
 use codex_core::AuthManager;
@@ -553,7 +554,10 @@ async fn load_config_or_exit(
 ) -> Config {
     #[allow(clippy::print_stderr)]
     match Config::load_with_cli_overrides(cli_kv_overrides, overrides).await {
-        Ok(config) => config,
+        Ok(mut config) => {
+            detect_ollama_wire_api_if_needed(&mut config).await;
+            config
+        }
         Err(err) => {
             eprintln!("Error loading configuration: {err}");
             std::process::exit(1);
diff --git a/codex-rs/tui2/src/lib.rs b/codex-rs/tui2/src/lib.rs
index de2a7204e2..cd5743129f 100644
--- a/codex-rs/tui2/src/lib.rs
+++ b/codex-rs/tui2/src/lib.rs
@@ -7,6 +7,7 @@ use additional_dirs::add_dir_warning_message;
 use app::App;
 pub use app::AppExitInfo;
 use codex_app_server_protocol::AuthMode;
+use codex_common::oss::detect_ollama_wire_api_if_needed;
 use codex_common::oss::ensure_oss_provider_ready;
 use codex_common::oss::get_default_model_for_oss_provider;
 use codex_core::AuthManager;
@@ -574,7 +575,10 @@ async fn load_config_or_exit(
 ) -> Config {
     #[allow(clippy::print_stderr)]
     match Config::load_with_cli_overrides(cli_kv_overrides, overrides).await {
-        Ok(config) => config,
+        Ok(mut config) => {
+            detect_ollama_wire_api_if_needed(&mut config).await;
+            config
+        }
         Err(err) => {
             eprintln!("Error loading configuration: {err}");
             std::process::exit(1);