Add ModelSelector capability to AgentConnection

- Add ModelSelector trait to acp_thread crate with list_models, select_model, and selected_model methods
- Extend AgentConnection trait with optional model_selector() method returning Option<Rc<dyn ModelSelector>>
- Implement ModelSelector for agent2's AgentConnection using LanguageModelRegistry
- Make selected_model field mandatory on Thread struct
- Update Thread::new to require a default_model parameter
- Update agent2 to fetch default model from registry when creating threads
- Fix prompt method to use the thread's selected model directly
- All methods use &mut AsyncApp for async-friendly operations

crates/acp_thread/Cargo.toml

@@ -26,6 +26,7 @@ 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/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>,
@@ -23,6 +71,14 @@ pub trait AgentConnection {
     fn prompt(&self, params: acp::PromptRequest, cx: &mut App) -> Task<Result<()>>;
 
     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)]