ACP champagne (#35609)

cherry pick changes from #35510 onto latest main

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Lukas Wirth <lukas@zed.dev>

crates/acp_thread/Cargo.toml

@@ -25,6 +25,7 @@ futures.workspace = true
 gpui.workspace = true
 itertools.workspace = true
 language.workspace = true
+language_model.workspace = true
 markdown.workspace = true
 project.workspace = true
 serde.workspace = true

crates/acp_thread/src/acp_thread.rs

@@ -656,6 +656,10 @@ impl AcpThread {
         &self.entries
     }
 
+    pub fn session_id(&self) -> &acp::SessionId {
+        &self.session_id
+    }
+
     pub fn status(&self) -> ThreadStatus {
         if self.send_task.is_some() {
             if self.waiting_for_tool_confirmation() {
@@ -1377,6 +1381,9 @@ mod tests {
                                 cx,
                             )
                             .unwrap();
+                    })?;
+                    Ok(acp::PromptResponse {
+                        stop_reason: acp::StopReason::EndTurn,
                     })
                 }
                 .boxed_local()
@@ -1449,7 +1456,9 @@ mod tests {
                         .unwrap()
                         .await
                         .unwrap();
-                    Ok(())
+                    Ok(acp::PromptResponse {
+                        stop_reason: acp::StopReason::EndTurn,
+                    })
                 }
                 .boxed_local()
             },
@@ -1522,7 +1531,9 @@ mod tests {
                         })
                         .unwrap()
                         .unwrap();
-                    Ok(())
+                    Ok(acp::PromptResponse {
+                        stop_reason: acp::StopReason::EndTurn,
+                    })
                 }
                 .boxed_local()
             }
@@ -1632,7 +1643,9 @@ mod tests {
                         })
                         .unwrap()
                         .unwrap();
-                    Ok(())
+                    Ok(acp::PromptResponse {
+                        stop_reason: acp::StopReason::EndTurn,
+                    })
                 }
                 .boxed_local()
             }
@@ -1686,7 +1699,7 @@ mod tests {
                         acp::PromptRequest,
                         WeakEntity<AcpThread>,
                         AsyncApp,
-                    ) -> LocalBoxFuture<'static, Result<()>>
+                    ) -> LocalBoxFuture<'static, Result<acp::PromptResponse>>
                     + 'static,
             >,
         >,
@@ -1713,7 +1726,7 @@ mod tests {
                 acp::PromptRequest,
                 WeakEntity<AcpThread>,
                 AsyncApp,
-            ) -> LocalBoxFuture<'static, Result<()>>
+            ) -> LocalBoxFuture<'static, Result<acp::PromptResponse>>
             + 'static,
         ) -> Self {
             self.on_user_message.replace(Rc::new(handler));
@@ -1755,7 +1768,11 @@ mod tests {
             }
         }
 
-        fn prompt(&self, params: acp::PromptRequest, cx: &mut App) -> Task<gpui::Result<()>> {
+        fn prompt(
+            &self,
+            params: acp::PromptRequest,
+            cx: &mut App,
+        ) -> Task<gpui::Result<acp::PromptResponse>> {
             let sessions = self.sessions.lock();
             let thread = sessions.get(&params.session_id).unwrap();
             if let Some(handler) = &self.on_user_message {
@@ -1763,7 +1780,9 @@ mod tests {
                 let thread = thread.clone();
                 cx.spawn(async move |cx| handler(params, thread, cx.clone()).await)
             } else {
-                Task::ready(Ok(()))
+                Task::ready(Ok(acp::PromptResponse {
+                    stop_reason: acp::StopReason::EndTurn,
+                }))
             }
         }
 

crates/acp_thread/src/connection.rs

@@ -1,13 +1,61 @@
-use std::{error::Error, fmt, path::Path, rc::Rc};
+use std::{error::Error, fmt, path::Path, rc::Rc, sync::Arc};
 
 use agent_client_protocol::{self as acp};
 use anyhow::Result;
 use gpui::{AsyncApp, Entity, Task};
+use language_model::LanguageModel;
 use project::Project;
 use ui::App;
 
 use crate::AcpThread;
 
+/// Trait for agents that support listing, selecting, and querying language models.
+///
+/// This is an optional capability; agents indicate support via [AgentConnection::model_selector].
+pub trait ModelSelector: 'static {
+    /// Lists all available language models for this agent.
+    ///
+    /// # Parameters
+    /// - `cx`: The GPUI app context for async operations and global access.
+    ///
+    /// # Returns
+    /// A task resolving to the list of models or an error (e.g., if no models are configured).
+    fn list_models(&self, cx: &mut AsyncApp) -> Task<Result<Vec<Arc<dyn LanguageModel>>>>;
+
+    /// Selects a model for a specific session (thread).
+    ///
+    /// This sets the default model for future interactions in the session.
+    /// If the session doesn't exist or the model is invalid, it returns an error.
+    ///
+    /// # Parameters
+    /// - `session_id`: The ID of the session (thread) to apply the model to.
+    /// - `model`: The model to select (should be one from [list_models]).
+    /// - `cx`: The GPUI app context.
+    ///
+    /// # Returns
+    /// A task resolving to `Ok(())` on success or an error.
+    fn select_model(
+        &self,
+        session_id: acp::SessionId,
+        model: Arc<dyn LanguageModel>,
+        cx: &mut AsyncApp,
+    ) -> Task<Result<()>>;
+
+    /// Retrieves the currently selected model for a specific session (thread).
+    ///
+    /// # Parameters
+    /// - `session_id`: The ID of the session (thread) to query.
+    /// - `cx`: The GPUI app context.
+    ///
+    /// # Returns
+    /// A task resolving to the selected model (always set) or an error (e.g., session not found).
+    fn selected_model(
+        &self,
+        session_id: &acp::SessionId,
+        cx: &mut AsyncApp,
+    ) -> Task<Result<Arc<dyn LanguageModel>>>;
+}
+
 pub trait AgentConnection {
     fn new_thread(
         self: Rc<Self>,
@@ -20,9 +68,18 @@ pub trait AgentConnection {
 
     fn authenticate(&self, method: acp::AuthMethodId, cx: &mut App) -> Task<Result<()>>;
 
-    fn prompt(&self, params: acp::PromptRequest, cx: &mut App) -> Task<Result<()>>;
+    fn prompt(&self, params: acp::PromptRequest, cx: &mut App)
+    -> Task<Result<acp::PromptResponse>>;
 
     fn cancel(&self, session_id: &acp::SessionId, cx: &mut App);
+
+    /// Returns this agent as an [Rc<dyn ModelSelector>] if the model selection capability is supported.
+    ///
+    /// If the agent does not support model selection, returns [None].
+    /// This allows sharing the selector in UI components.
+    fn model_selector(&self) -> Option<Rc<dyn ModelSelector>> {
+        None // Default impl for agents that don't support it
+    }
 }
 
 #[derive(Debug)]