diff --git a/codex-rs/core/tests/prompt_caching.rs b/codex-rs/core/tests/prompt_caching.rs
index e528cb7a..9f5829e1 100644
--- a/codex-rs/core/tests/prompt_caching.rs
+++ b/codex-rs/core/tests/prompt_caching.rs
@@ -6,8 +6,8 @@ use codex_core::protocol::EventMsg;
 use codex_core::protocol::InputItem;
 use codex_core::protocol::Op;
 use codex_core::protocol::SandboxPolicy;
-use codex_core::protocol_config_types::ReasoningEffort as ReasoningEffortConfig;
-use codex_core::protocol_config_types::ReasoningSummary as ReasoningSummaryConfig;
+use codex_core::protocol_config_types::ReasoningEffort;
+use codex_core::protocol_config_types::ReasoningSummary;
 use codex_login::CodexAuth;
 use core_test_support::load_default_config_for_test;
 use core_test_support::load_sse_fixture_with_id;
@@ -197,8 +197,8 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() {
                 exclude_slash_tmp: true,
             }),
             model: Some("o3".to_string()),
-            effort: Some(ReasoningEffortConfig::High),
-            summary: Some(ReasoningSummaryConfig::Detailed),
+            effort: Some(ReasoningEffort::High),
+            summary: Some(ReasoningSummary::Detailed),
         })
         .await
         .unwrap();
@@ -256,3 +256,107 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() {
     );
     assert_eq!(body2["input"], expected_body2);
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn per_turn_overrides_keep_cached_prefix_and_key_constant() {
+    use pretty_assertions::assert_eq;
+
+    let server = MockServer::start().await;
+
+    let sse = sse_completed("resp");
+    let template = ResponseTemplate::new(200)
+        .insert_header("content-type", "text/event-stream")
+        .set_body_raw(sse, "text/event-stream");
+
+    // Expect two POSTs to /v1/responses
+    Mock::given(method("POST"))
+        .and(path("/v1/responses"))
+        .respond_with(template)
+        .expect(2)
+        .mount(&server)
+        .await;
+
+    let model_provider = ModelProviderInfo {
+        base_url: Some(format!("{}/v1", server.uri())),
+        ..built_in_model_providers()["openai"].clone()
+    };
+
+    let cwd = TempDir::new().unwrap();
+    let codex_home = TempDir::new().unwrap();
+    let mut config = load_default_config_for_test(&codex_home);
+    config.cwd = cwd.path().to_path_buf();
+    config.model_provider = model_provider;
+    config.user_instructions = Some("be consistent and helpful".to_string());
+
+    let conversation_manager = ConversationManager::default();
+    let codex = conversation_manager
+        .new_conversation_with_auth(config, Some(CodexAuth::from_api_key("Test API Key")))
+        .await
+        .expect("create new conversation")
+        .conversation;
+
+    // First turn
+    codex
+        .submit(Op::UserInput {
+            items: vec![InputItem::Text {
+                text: "hello 1".into(),
+            }],
+        })
+        .await
+        .unwrap();
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
+
+    // Second turn using per-turn overrides via UserTurn
+    let new_cwd = TempDir::new().unwrap();
+    let writable = TempDir::new().unwrap();
+    codex
+        .submit(Op::UserTurn {
+            items: vec![InputItem::Text {
+                text: "hello 2".into(),
+            }],
+            cwd: new_cwd.path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::WorkspaceWrite {
+                writable_roots: vec![writable.path().to_path_buf()],
+                network_access: true,
+                exclude_tmpdir_env_var: true,
+                exclude_slash_tmp: true,
+            },
+            model: "o3".to_string(),
+            effort: ReasoningEffort::High,
+            summary: ReasoningSummary::Detailed,
+        })
+        .await
+        .unwrap();
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
+
+    // Verify we issued exactly two requests, and the cached prefix stayed identical.
+    let requests = server.received_requests().await.unwrap();
+    assert_eq!(requests.len(), 2, "expected two POST requests");
+
+    let body1 = requests[0].body_json::<serde_json::Value>().unwrap();
+    let body2 = requests[1].body_json::<serde_json::Value>().unwrap();
+
+    // prompt_cache_key should remain constant across per-turn overrides
+    assert_eq!(
+        body1["prompt_cache_key"], body2["prompt_cache_key"],
+        "prompt_cache_key should not change across per-turn overrides"
+    );
+
+    // The entire prefix from the first request should be identical and reused
+    // as the prefix of the second request.
+    let expected_user_message_2 = serde_json::json!({
+        "type": "message",
+        "id": serde_json::Value::Null,
+        "role": "user",
+        "content": [ { "type": "input_text", "text": "hello 2" } ]
+    });
+    let expected_body2 = serde_json::json!(
+        [
+            body1["input"].as_array().unwrap().as_slice(),
+            [expected_user_message_2].as_slice(),
+        ]
+        .concat()
+    );
+    assert_eq!(body2["input"], expected_body2);
+}