Differentiate model-switch compaction failures from oversize input

codex-rs/core/tests/suite/compact.rs

@@ -1800,6 +1800,130 @@ async fn pre_sampling_compact_runs_on_switch_to_smaller_context_model() {
     );
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn pre_sampling_compact_context_window_failure_surfaces_compact_task_error() {
+    skip_if_no_network!();
+
+    let server = MockServer::start().await;
+    let previous_model = "gpt-5.2-codex";
+    let next_model = "gpt-5.1-codex-max";
+
+    let models_mock = mount_models_once(
+        &server,
+        ModelsResponse {
+            models: vec![
+                model_info_with_context_window(previous_model, 273_000),
+                model_info_with_context_window(next_model, 125_000),
+            ],
+        },
+    )
+    .await;
+
+    let request_log = mount_sse_sequence(
+        &server,
+        vec![
+            sse(vec![
+                ev_assistant_message("m1", "before switch"),
+                ev_completed_with_tokens("r1", 120_000),
+            ]),
+            sse_failed(
+                "compact-failed",
+                "context_length_exceeded",
+                CONTEXT_LIMIT_MESSAGE,
+            ),
+        ],
+    )
+    .await;
+
+    let mut model_provider = non_openai_model_provider(&server);
+    model_provider.stream_max_retries = Some(0);
+    let mut builder = test_codex()
+        .with_auth(CodexAuth::create_dummy_chatgpt_auth_for_testing())
+        .with_model(previous_model)
+        .with_config(move |config| {
+            config.model_provider = model_provider;
+            set_test_compact_prompt(config);
+            config.features.enable(Feature::RemoteModels);
+        });
+    let test = builder.build(&server).await.expect("build test codex");
+
+    test.codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "before switch".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: test.cwd.path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::DangerFullAccess,
+            model: previous_model.to_string(),
+            effort: None,
+            summary: ReasoningSummary::Auto,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await
+        .expect("submit first user turn");
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    test.codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "after switch".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: test.cwd.path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::DangerFullAccess,
+            model: next_model.to_string(),
+            effort: None,
+            summary: ReasoningSummary::Auto,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await
+        .expect("submit second user turn");
+
+    let error_message = wait_for_event_match(&test.codex, |event| match event {
+        EventMsg::Error(err) => Some(err.message.clone()),
+        _ => None,
+    })
+    .await;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    assert_eq!(models_mock.requests().len(), 1);
+    assert!(
+        error_message.contains("Error running local compact task"),
+        "expected local compact-task failure prefix, got {error_message}"
+    );
+    assert!(
+        !error_message.contains(
+            "Incoming user message and/or turn context is too large to fit in context window"
+        ),
+        "model-switch pre-sampling compaction failure should not be misclassified as incoming-input oversize: {error_message}"
+    );
+
+    let requests = request_log.requests();
+    assert_eq!(
+        requests.len(),
+        2,
+        "expected first user turn and one pre-sampling compaction request"
+    );
+    let compact_request_body = requests[1].body_json().to_string();
+    assert!(
+        body_contains_text(&compact_request_body, SUMMARIZATION_PROMPT),
+        "second request should be the pre-sampling compaction request"
+    );
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn pre_sampling_compact_runs_after_resume_and_switch_to_smaller_model() {
     skip_if_no_network!();