Handle model refusal in ACP threads (#37383)

If the model refuses a prompt, we now:
* Show an error if it was a user prompt (and truncate it out of the
history)
* Respond with a failed tool call if the refusal was for a tool call

<img width="607" height="260" alt="Screenshot 2025-09-02 at 5 11 45 PM"
src="https://github.com/user-attachments/assets/070b5ee7-6ad6-4a63-8395-f9a5093cc40e"
/>
<img width="607" height="265" alt="Screenshot 2025-09-02 at 5 11 38 PM"
src="https://github.com/user-attachments/assets/98862586-390b-494e-b1f8-71d8341c8d9d"
/>



Release Notes:

- Improve handling of model refusals in ACP threads

Author: rtfeldman

crates/acp_thread/src/acp_thread.rs

@@ -804,6 +804,7 @@ pub enum AcpThreadEvent {
     Error,
     LoadError(LoadError),
     PromptCapabilitiesUpdated,
+    Refusal,
 }
 
 impl EventEmitter<AcpThreadEvent> for AcpThread {}
@@ -1569,15 +1570,42 @@ impl AcpThread {
                             this.send_task.take();
                         }
 
-                        // Truncate entries if the last prompt was refused.
+                        // Handle refusal - distinguish between user prompt and tool call refusals
                         if let Ok(Ok(acp::PromptResponse {
                             stop_reason: acp::StopReason::Refusal,
                         })) = result
-                            && let Some((ix, _)) = this.last_user_message()
                         {
-                            let range = ix..this.entries.len();
-                            this.entries.truncate(ix);
-                            cx.emit(AcpThreadEvent::EntriesRemoved(range));
+                            if let Some((user_msg_ix, _)) = this.last_user_message() {
+                                // Check if there's a completed tool call with results after the last user message
+                                // This indicates the refusal is in response to tool output, not the user's prompt
+                                let has_completed_tool_call_after_user_msg =
+                                    this.entries.iter().skip(user_msg_ix + 1).any(|entry| {
+                                        if let AgentThreadEntry::ToolCall(tool_call) = entry {
+                                            // Check if the tool call has completed and has output
+                                            matches!(tool_call.status, ToolCallStatus::Completed)
+                                                && tool_call.raw_output.is_some()
+                                        } else {
+                                            false
+                                        }
+                                    });
+
+                                if has_completed_tool_call_after_user_msg {
+                                    // Refusal is due to tool output - don't truncate, just notify
+                                    // The model refused based on what the tool returned
+                                    cx.emit(AcpThreadEvent::Refusal);
+                                } else {
+                                    // User prompt was refused - truncate back to before the user message
+                                    let range = user_msg_ix..this.entries.len();
+                                    if range.start < range.end {
+                                        this.entries.truncate(user_msg_ix);
+                                        cx.emit(AcpThreadEvent::EntriesRemoved(range));
+                                    }
+                                    cx.emit(AcpThreadEvent::Refusal);
+                                }
+                            } else {
+                                // No user message found, treat as general refusal
+                                cx.emit(AcpThreadEvent::Refusal);
+                            }
                         }
 
                         cx.emit(AcpThreadEvent::Stopped);
@@ -2681,6 +2709,187 @@ mod tests {
         assert_eq!(fs.files(), vec![Path::new(path!("/test/file-0"))]);
     }
 
+    #[gpui::test]
+    async fn test_tool_result_refusal(cx: &mut TestAppContext) {
+        use std::sync::atomic::AtomicUsize;
+        init_test(cx);
+
+        let fs = FakeFs::new(cx.executor());
+        let project = Project::test(fs, None, cx).await;
+
+        // Create a connection that simulates refusal after tool result
+        let prompt_count = Arc::new(AtomicUsize::new(0));
+        let connection = Rc::new(FakeAgentConnection::new().on_user_message({
+            let prompt_count = prompt_count.clone();
+            move |_request, thread, mut cx| {
+                let count = prompt_count.fetch_add(1, SeqCst);
+                async move {
+                    if count == 0 {
+                        // First prompt: Generate a tool call with result
+                        thread.update(&mut cx, |thread, cx| {
+                            thread
+                                .handle_session_update(
+                                    acp::SessionUpdate::ToolCall(acp::ToolCall {
+                                        id: acp::ToolCallId("tool1".into()),
+                                        title: "Test Tool".into(),
+                                        kind: acp::ToolKind::Fetch,
+                                        status: acp::ToolCallStatus::Completed,
+                                        content: vec![],
+                                        locations: vec![],
+                                        raw_input: Some(serde_json::json!({"query": "test"})),
+                                        raw_output: Some(
+                                            serde_json::json!({"result": "inappropriate content"}),
+                                        ),
+                                    }),
+                                    cx,
+                                )
+                                .unwrap();
+                        })?;
+
+                        // Now return refusal because of the tool result
+                        Ok(acp::PromptResponse {
+                            stop_reason: acp::StopReason::Refusal,
+                        })
+                    } else {
+                        Ok(acp::PromptResponse {
+                            stop_reason: acp::StopReason::EndTurn,
+                        })
+                    }
+                }
+                .boxed_local()
+            }
+        }));
+
+        let thread = cx
+            .update(|cx| connection.new_thread(project, Path::new("/test"), cx))
+            .await
+            .unwrap();
+
+        // Track if we see a Refusal event
+        let saw_refusal_event = Arc::new(std::sync::Mutex::new(false));
+        let saw_refusal_event_captured = saw_refusal_event.clone();
+        thread.update(cx, |_thread, cx| {
+            cx.subscribe(
+                &thread,
+                move |_thread, _event_thread, event: &AcpThreadEvent, _cx| {
+                    if matches!(event, AcpThreadEvent::Refusal) {
+                        *saw_refusal_event_captured.lock().unwrap() = true;
+                    }
+                },
+            )
+            .detach();
+        });
+
+        // Send a user message - this will trigger tool call and then refusal
+        let send_task = thread.update(cx, |thread, cx| {
+            thread.send(
+                vec![acp::ContentBlock::Text(acp::TextContent {
+                    text: "Hello".into(),
+                    annotations: None,
+                })],
+                cx,
+            )
+        });
+        cx.background_executor.spawn(send_task).detach();
+        cx.run_until_parked();
+
+        // Verify that:
+        // 1. A Refusal event WAS emitted (because it's a tool result refusal, not user prompt)
+        // 2. The user message was NOT truncated
+        assert!(
+            *saw_refusal_event.lock().unwrap(),
+            "Refusal event should be emitted for tool result refusals"
+        );
+
+        thread.read_with(cx, |thread, _| {
+            let entries = thread.entries();
+            assert!(entries.len() >= 2, "Should have user message and tool call");
+
+            // Verify user message is still there
+            assert!(
+                matches!(entries[0], AgentThreadEntry::UserMessage(_)),
+                "User message should not be truncated"
+            );
+
+            // Verify tool call is there with result
+            if let AgentThreadEntry::ToolCall(tool_call) = &entries[1] {
+                assert!(
+                    tool_call.raw_output.is_some(),
+                    "Tool call should have output"
+                );
+            } else {
+                panic!("Expected tool call at index 1");
+            }
+        });
+    }
+
+    #[gpui::test]
+    async fn test_user_prompt_refusal_emits_event(cx: &mut TestAppContext) {
+        init_test(cx);
+
+        let fs = FakeFs::new(cx.executor());
+        let project = Project::test(fs, None, cx).await;
+
+        let refuse_next = Arc::new(AtomicBool::new(false));
+        let connection = Rc::new(FakeAgentConnection::new().on_user_message({
+            let refuse_next = refuse_next.clone();
+            move |_request, _thread, _cx| {
+                if refuse_next.load(SeqCst) {
+                    async move {
+                        Ok(acp::PromptResponse {
+                            stop_reason: acp::StopReason::Refusal,
+                        })
+                    }
+                    .boxed_local()
+                } else {
+                    async move {
+                        Ok(acp::PromptResponse {
+                            stop_reason: acp::StopReason::EndTurn,
+                        })
+                    }
+                    .boxed_local()
+                }
+            }
+        }));
+
+        let thread = cx
+            .update(|cx| connection.new_thread(project, Path::new(path!("/test")), cx))
+            .await
+            .unwrap();
+
+        // Track if we see a Refusal event
+        let saw_refusal_event = Arc::new(std::sync::Mutex::new(false));
+        let saw_refusal_event_captured = saw_refusal_event.clone();
+        thread.update(cx, |_thread, cx| {
+            cx.subscribe(
+                &thread,
+                move |_thread, _event_thread, event: &AcpThreadEvent, _cx| {
+                    if matches!(event, AcpThreadEvent::Refusal) {
+                        *saw_refusal_event_captured.lock().unwrap() = true;
+                    }
+                },
+            )
+            .detach();
+        });
+
+        // Send a message that will be refused
+        refuse_next.store(true, SeqCst);
+        cx.update(|cx| thread.update(cx, |thread, cx| thread.send(vec!["hello".into()], cx)))
+            .await
+            .unwrap();
+
+        // Verify that a Refusal event WAS emitted for user prompt refusal
+        assert!(
+            *saw_refusal_event.lock().unwrap(),
+            "Refusal event should be emitted for user prompt refusals"
+        );
+
+        // Verify the message was truncated (user prompt refusal)
+        thread.read_with(cx, |thread, cx| {
+            assert_eq!(thread.to_markdown(cx), "");
+        });
+    }
+
     #[gpui::test]
     async fn test_refusal(cx: &mut TestAppContext) {
         init_test(cx);
@@ -2744,8 +2953,8 @@ mod tests {
             );
         });
 
-        // Simulate refusing the second message, ensuring the conversation gets
-        // truncated to before sending it.
+        // Simulate refusing the second message. The message should be truncated
+        // when a user prompt is refused.
         refuse_next.store(true, SeqCst);
         cx.update(|cx| thread.update(cx, |thread, cx| thread.send(vec!["world".into()], cx)))
             .await

crates/agent_ui/src/acp/model_selector_popover.rs

@@ -36,6 +36,14 @@ impl AcpModelSelectorPopover {
     pub fn toggle(&self, window: &mut Window, cx: &mut Context<Self>) {
         self.menu_handle.toggle(window, cx);
     }
+
+    pub fn active_model_name(&self, cx: &App) -> Option<SharedString> {
+        self.selector
+            .read(cx)
+            .delegate
+            .active_model()
+            .map(|model| model.name.clone())
+    }
 }
 
 impl Render for AcpModelSelectorPopover {